Effect of modified surfaces on bubble dynamics and pool boiling heat transfer enhancement: A review

Journal Pre-proofs Effect of modified surfaces on bubble dynamics and pool boiling heat transfer enhancement: A review Afsaneh Mehralizadeh, Seyed Reza Shabanian, Gholamreza Bakeri PII: DOI: Reference:

S2451-9049(19)30256-2 https://doi.org/10.1016/j.tsep.2019.100451 TSEP 100451

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Thermal Science and Engineering Progress

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13 July 2019 5 November 2019 12 November 2019

Please cite this article as: A. Mehralizadeh, S. Reza Shabanian, G. Bakeri, Effect of modified surfaces on bubble dynamics and pool boiling heat transfer enhancement: A review, Thermal Science and Engineering Progress (2019), doi: https://doi.org/10.1016/j.tsep.2019.100451

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Effect of modified surfaces on bubble dynamics and pool boiling heat transfer enhancement: A review Afsaneh Mehralizadeh, Seyed Reza Shabanian1, Gholamreza Bakeri Department of Chemical Engineering, Babol Noshirvani University of Technology, Shariati Ave., Babol, 47148-71167, Iran

Abstract The vast application of boiling phenomena in industrial processes has led scientists to seek methods to increase boiling heat transfer. This requires a precise prediction of the boiling heat transfer coefficient (HTC) between the heater surface and the boiling fluid. Considering the complexity of the heat transfer mechanism, the prediction of this phenomenon under different conditions can be difficult. In pool boiling process, a liquid is heated to its boiling point through a heating surface and phase transition from the liquid state to the vapor state will occur. Therefore, the physical and chemical properties of the liquid, vapor and surface are important in this process." In this paper, a review of methods for increasing the boiling heat transfer carried out in three main parts: 1) Surface textures 2) surface characteristics 3) Surface structures. The effect of modification of these parts on the boiling process parameters, especially the bubbles dynamics, the heat transfer coefficient and critical heat flux (CHF) were investigated which are the most important criteria for evaluating the boiling. The purpose of this study is to obtain a relatively comprehensive knowledge of the enhancement techniques in pool boiling heat transfer in recent years. Empirical correlations and models containing surface specifications are also reviewed and presented for the prediction of boiling characteristics. Keywords: Boiling heat transfer enhancement; Surface modification; Bubble dynamics; Surface textures; Critical heat flux; Wettability Contents 1 Introduction ..........................................................................................................................................3 2

3

Surface texture......................................................................................................................................8 2.1

Surface morphology......................................................................................................................8

2.2

Surface topography.....................................................................................................................12

Surface characteristics........................................................................................................................15 3.1

Heating surface size and thickness .............................................................................................15

3.2

Heating surface thermophysical properties ...............................................................................16

Email address: [email protected], [email protected]

3.3 4

Heating surface orientation........................................................................................................18

Surface structure ................................................................................................................................20 4.1

Surface coating ...........................................................................................................................21

4.1.1

Porous coating ....................................................................................................................22

4.1.2

Nanostructure coating ........................................................................................................24

4.2

Nanowire ....................................................................................................................................26

4.3

Channels/tunnels........................................................................................................................28

4.4

Reentrant cavity/ channels.........................................................................................................29

4.5

Honeycomb.................................................................................................................................30

4.6

Pin-fin..........................................................................................................................................31

5

Other surface enhancement techniques ............................................................................................31

6

Deposition of nanoparticles................................................................................................................33

7

Correlations, models and simulations ................................................................................................35

8

Conclusion ..........................................................................................................................................45

9

Acknowledgment................................................................................................................................46

References .................................................................................................................................................46

Nomenclature A

base area

Greek symbol

Af

fin area

α

mouth radius

β

cone angle

a

heater surface area (cm2) contacted area of HPP with heater surface constant in Eq. (4)

B b

AH

η°

overall fin efficiency

ηf

fin efficiency

constant in Eq. (14)

θ

constant in Eq. (14)

θ∗

Cp

specific heat (kJ/kgK)

δh

solid-liquid contact angle (°) apparent contact angle on roughened surfaces height of honeycomb porous plate

Csf

constant in Eq. (1)

γ

material parameter of the heater

constant in Eq. (14)

λd

Taylor unstable wavelength

Aw

c

Dh hydraulic diameter dV ( dt)t = 0 initial volume flow rate (m3/s)

µ

liquid viscosity (kg/ms) ρ

density (Kg/m3)

σ

surface tension (N/m)

g

coalesced bubble departure frequency (1/s) gravitational acceleration (m/s2)

φ

tilt angle of boiling surface (°)

Hb

height of bubble

ϕ

cavity side angle (°)

h

heat transfer coefficient

Subscripts

specific latent heat (kJ/kg)

l

liquid

Ja

Jakob number

sat

saturation

K

permeability

sub

subcooling

k

thermal conductivity (W/mK)

TME

thermal management of electronics

L

heater length (m)

t

total

m

v

vapor

w

heater surface

N

number of micro-pillars active nucleation site density (site/mm) average cavity density

Nj

number of escaping vapor jets

Abbreviations AFM

atomic force microscope

Nux,HTC

local Nusselt number

BHT

boiling heat transfer

n

constant in Eq. (16)

CHF

critical heat flux

P

Absolute pressure (kPa)

CNT

carbon nanotube

Prandtl number

HF

heat flux

q

heat flux (W/cm2)

HPP

honeycomb porous plate

Ra

surface roughness (µm)

HT

heat transfer

r

surface roughness factor

HTC

heat transfer coefficient (W/cm2K)

effective pore radius

LBM

lattice Boltzmann method

S

thermal activity parameter

ONB

onset of nucleate boiling

s

constant in Eq. (1)

SEM

scanning electron microscopy

T

temperature (K)

ΔTsat

x*

wall superheat (K) distance from the entrance region of the flow non-dimensional x

z

constant in Eq. (1)

f

hlv

N

Pr

reff

x

1

Introduction

The boiling process takes place in many industrial and daily living areas. It is necessary to achieve enough knowledge of the boiling phenomenon. Boiling is evaporation at the solid-liquid interface. When the heater surface temperature exceeds the saturation temperature of the liquid (at a defined pressure) boiling happens. The pool boiling curve (Fig. 1a) which relates the heat flux from the heating surface to its wall superheat is typically observed in most boiling processes and can be classified into four distinct regimes, namely, free convection, nucleate boiling, transition boiling and film boiling. These four regimes are demarcated by three important transition points: (i) onset of boiling (or incipient boiling) corresponding to first bubble formation on the surface, (ii) critical heat flux (CHF), where bubble nucleation in nucleate boiling is replaced by localized vapor blankets merging together across the surface, and (iii) minimum heat flux (or Leidenfrost point), corresponding to the onset of breakup of the continuous vapor blanket in film boiling when decreasing wall superheat [1]. Fig. 1b demonstrates the bubble nucleation in the different heat transfer regimes. Generally, the pool boiling experimental setup consisted of a pool boiling chamber with a heating surface at the bottom, a Power supplier to the heater, a data acquisition instrument, and thermocouples to monitor the temperature of the heating surface and liquid bulk, as shown in Fig. 2. In this process, a large amount of heat transfer takes place due to the phase change phenomenon at a low-temperature difference. The nucleate boiling is a complex physical process that involves interactions between heating surface and boiling fluid.

a

b Fig. 1. (a) Pool boiling curve, (b) schematic of bubble nucleation in different regimes.

Fig. 2. Schematic of pool boiling experiment setup.

In the boiling process, a liquid receives heat from the heater surface and its phase changes from liquid to vapor along with bubbles generation. Understanding the Formation, growth and detachment mechanisms of bubbles from the surface can lead to the recognition of the heat transfer mechanism. Bubble dynamics parameters are nucleation site density, bubble departure diameter, bubble waiting period, bubble growth period and bubble departure frequency. The boiling heat transfer coefficient can be calculated from correlations developed based on bubble dynamics parameters [2]. Due to the relative simplicity of implementation, it is desirable to obtain high heat transfer coefficients (HTC) and critical heat flux (CHF) with immersion pool boiling without the use of forced convective methods. High HTC increases heat transfer and high CHF increases the upper limit of heat load for safe operation. Surface modification techniques have been used over the past few decades to enhance heat transfer performance by providing larger surface area, higher density of nucleation sites, and potentially smaller superheat for the phase change heat transfer [3]. Marto et al. [4] investigated liquid nitrogen boiling on copper and nickel mirror finishes and copper surfaces that were roughened, grease coated, Teflon-coated, and copper mirror finishes with artificial cavities. Their results showed that due to better thermal properties, copper has a higher heat transfer rate than nickel. They also concluded that grease coatings have a negative impact, Teflon coatings are ineffective and artificial cavities have a positive effect on boiling performance. Marto and Lepere [5] continued experiments by changing heating surface to Plain copper tubes and three copper-enhanced surfaces: a union carbide high flux surface, a Hitachi Thermoexcel-E surface and a Wieland Gewa-T surface and announced that enhanced surfaces multiply the heat transfer coefficient compared to plain tubes.

Wang and Dhir [6] changed the wettability of the surface using cavities with oxidation degree control. The results of boiling in the saturated water pool showed that with increasing wettability, the number of cavities was reduced. Regardless of surface wettability, the density of active nucleation sites varies approximately with q2. In recent years, new and various methods for improving the surface properties of the heaters were studied. Chen and Wang [7] added CuO and SiO2 nanoparticles to pure water to make nanofluid and boiled on copper beads packed porous structure. Copper bead acts like extended surfaces and increases nucleation sites. There is an optimal amount for the nanofluid concentration, which further increasing causes the aggregation and deposition of nanoparticles on the porous surface and reduces heat transfer. Interestingly, this optimal amount of nanofluid concentrations is much higher for boiling on the smooth surface. A surface covered with a layer of indium tin oxide was used as a heater by Seo et al. [8] They made holes in different sizes and patterns using a microelectromechanical method on the heating surface. As the number of holes increased, the CHF and HTC increased compared to the plain surface. CHF enhancement can be attributed to the additional water supply in holepatterned regions. Zhao et al. [9] utilized an enhanced surface modified by the combination of microstructures and wetting properties. The performance of mixed hydrophilic and hydrophobic microstructures is better than the hydrophilic microstructures only. The enhancement values in heat transfer are different for various microstructure heights and widths. The heat transfer increasing techniques can be divided into active and passive methods. Passive methods such as enhanced surfaces, porous surfaces, micro-channel surfaces or additives such as nanoparticles or surfactants to fluids do not require external forces to increase heat transfer, and active methods such as the electrostatic field or surface and fluid vibrations require direct force. Sathyabhama and Dinesh [10] employed surface with grooves and vibration of surface to improve HTC. Combining these two techniques causes more increase in HTC compared to using each method separately. The compound technique increases the bubble nucleation over the grooved surface and increases the bubble frequency due to the vibration. In Table 1, a number of other studies which are about the surface modification are reported.

Reference Patil and Kandlikar [11] Dong et al. [12] Kim et al. [13] Sarafraz et al. [14]

Table 1. Effect of surface modification on boiling heat transfer characteristics in references. Surface Fabrication Remark Test fluid Enhancement reasons modification technique (compare to the bare surface) Bubbles leaving the fin Microporous tops induce strong Optimal CHF enhancement, and wall Water coatings on the tops localized liquid circulation electrodeposition superheat reduction of microchannels currents in the microchannels Micro Micro/NanoHF enhancement, and wall Increasing active Ethanol electromechanical structures nucleation site density superheat reduction systems Micro Microstructure Water electromechanical CHF and HTC enhancement Larger heat transfer area surfaces systems Aqueous Concentric circular Larger heat transfer area, CNC micro alumina micro-structured HTC enhancement higher number of active machinery nanofluid surface nucleation sites and

Mori et al. [15]

Water

Honeycomb porous plate and/or nanoparticle deposited surface

Quan et al. [16]

R113

Smooth and ribs surface under an electric field

Zupancic et al. [17]

Doubledistilled water

Hydrophobic polydimethyl siloxane-silica coating- biphilic surfaces

Pulsed Nd: YAG laser

HTC and CHF enhancement

Saeidi et al. [18]

Deionize d water

Aluminized copper surface

Aluminizing

HTC and CHF enhancement

Park et al. [19]

Deionized

Photolithography

CHF and BHT reduction

water

Serpentine-pattern thin-film platinum heater Micro-fin structure and micro-fins with sintered perforated foil (MFP) Sprayed reduced graphene oxide onto a copper substrate Triangular crosssection copper nanoparticles lines Micro nano biporous copper surface with optimal cavity size

Commercially honeycomb porous plate

CHF enhancement

HT enhancement

Pastuszko and Wójcik [20]

Water and FC-72

An et al.[21]

FC-72

Jo et al. [22]

HFE7100

Wang et al. [23]

Water

Dehshali et al. [24]

Distilled water

Installed Twisted Tape Fins on surface

Laser-cut

HTC enhancement

Zhang et al. [25]

Water

3D thin wall grid structures

Selective laser melting

CHF enhancement

rigorous bubble formation Capillary supply of liquid through the microstructure and the release of vapor through the channels effect of Electroconvection and electrohydrodynamic on bubble growth and departure Smaller bubble diameter, higher density of active nucleation sites and nucleation frequency, delaying the dry-out high thermal conductivity of copper and relatively high CHF of aluminum surfaces

HF enhancement

Increasing the number of nucleation sites

Supersonic spraycoating

HTC and CHF enhancement

More nucleation sites and increasing wettability and roughness

Supersonic spraycoating

HTC and CHF enhancement

More nucleation sites and Pathways of escaping bubbles

Electrodeposition

HTC enhancement

Reducing bubble growth periods Separating bubble-liquid pathways and inducing more chaotic bubbly rising flow Grid structure’s ‘‘partition effect” that inhibited Helmholtz instability, confined bubble, and hot spot expansion

In this paper, a review of recent methods for increasing the pool boiling heat transfer carried out in three main parts: 1) Surface textures 2) surface characteristics 3) Surface structures. The effect of these methods on the boiling process parameters, especially the heat transfer coefficient (HTC) and critical heat flux (CHF) was elaborated. Also, other parameters such as the onset of nucleate boiling and bubble dynamics features (active nucleation site density, bubbles departure diameter, and bubble departure frequency) were reviewed. Finally, empirical correlations and

models containing surface specifications are also reviewed and presented for the prediction of the boiling characteristics. The purpose of this study is to obtain a relatively comprehensive knowledge of the enhancement techniques in pool boiling heat transfer in recent years.

2

Surface texture

2.1 Surface morphology Surface morphology is three-dimensional surface shapes or qualitative descriptive of what the surface is. It is evaluated using imaging techniques such as optical microscopy or scanning electron microscopy (SEM). Surface wettability is a morphological characteristic of surface and one of the key factors in increasing heat transfer. Wettability usually obtains by changing surface chemistry. The contact angle is usually measured to study wettability.

Fig. 3. The contact angle of a liquid and a solid.

As can be seen in Fig. 3, the small contact angle (<90) indicates a high wettability and a large contact angle (> 90) indicating low wettability. Also, the contact angle is used to calculate the important parameter of surface tension. Teodori et al. [26] studied the boiling of water on surfaces with extreme wetting regimes, such as superhydrophobic surfaces and hydrophilic ones. As shown in Fig. 4c, the heat flux-wall superheat curve for the superhydrophobic surface is linear, and the line slope is much lower than the one for the hydrophilic surface. This phenomenon is due to the formation of a vapor film on the entire surface which is due to immediate coalescence of the bubbles generated on the hydrophobic surfaces. This phenomenon is called "quasi-Leidenfrost". Hsu et al. [27] investigated horizontal copper cylinder surfaces as heating sources in pool boiling process. The surfaces were a combination of superhydrophilic and hydrophobic types of wettability. The results showed that more bubbles were generated on interlaced lines. The more interlaced lines reduce wall superheat values and thus increase the heat transfer performance under the same heat flux. As depicted in Fig. 5, when the area ratio of the superhydrophilic surface is greater, the amount of wall superheat is lower.

Fig. 4. High-speed images of bubble dynamics for (a) hydrophilic and (b) superhydrophobic raw surfaces at 40 K wall superheats. (c) Boiling curves for water on hydrophilic and superhydrophobic surfaces. Adapted from Teodori et al. [26].

Fig. 5. Surface nucleation (a) on surfaces with different numbers of interlaced wettability (b) on surfaces with different area ratios. Adapted from Hsu et al. [27].

Compared to the heterogeneous and homogeneous wettable surfaces, Kumar et al. [28] found that surfaces with heterogeneous wettability due to the presence of hydrophobic patterns provide faster onset of nucleate boiling and more density of nucleation in comparison to homogeneous wettable surfaces. Heterogeneous wettable surfaces have a higher HTC, which this increasing percentage decreases by increasing heat flux. Chang et al. [29] found the best size and pitch of hydrophobic squares on the surface with the aim of achieving maximum bubble coalescence behavior and also a rewetting phenomenon. As the results of Chang and Kumar's research showed, putting hydrophilic and hydrophobic patterns together would increase the heat transfer performance. Shen et al. [30] also benefited from this advantage, along with the benefits of using enhanced heat transfer surfaces. Hybrid wetting surfaces were made by pillars in millimeters, which were fabricated by chemical deposition. As expected, the heat transfer coefficient of hybrid wetting surfaces, regardless of hybrid modes, to be more than spatially uniform wetting surfaces. Zhang et al. [31] measured the water contact angle (WCA) of the TiO2 nanotube arrayed surface (TNAS). TNAS was made by an anodic oxidation technique on the titanium substrate. With WCA reducing, the boiling curves move to the right, which means reducing the heat transfer coefficient and decreasing the active nucleation site density. On the other hand, CHF increases, which can be due to having the thicker macro layer and promoting the rewetting process on the surface. Quan et al. [32] investigated the effect of nanoparticle wettability on nanofluid boiling by visualization study of bubble dynamics and morphology analysis of nanoparticle deposition layers. Moderately hydrophilic nanoparticles perform better than

strongly hydrophilic nanoparticles. Moderately hydrophilic nanoparticles prevent bubbles from coalescence and have a rough deposition, leading more nucleation sites. Usually change in surface wettability is associated with a change in surface topography. However, Bordon et al. [33] changed the surface wettability without changing its topography using the grafting different monolayers on the surface. The aim of this study was to evaluate the effect of wettability change on the onset of boiling. As the contact angle increases, superheat temperature at the onset of pool boiling decreases. The density of nucleation sites increases. The mobility and thus probability to form a nanobubble of gas also increase. For the hydrophilic surfaces, CHF is much higher than hydrophobic surfaces. However, it can be said that by placing hydrophilic and Hydrophobic points side by side, more heat transfer occurs compared to only hydrophilic or Hydrophobic. Hydrophobic surfaces easily initiate bubble nucleation while hydrophilic ones facilitate bubbles detachment [34]. Generally, the heterogeneous surface performs better. This is hydrophilic surface where hydrophobic points are located. The size of these hydrophobic points, the ratio between the hydrophilic surface and hydrophobic points give different results. This can be related to the operating conditions of the experiments.

2.2

Surface topography

CHF is highly dependent on surface roughness and roughness improves boiling performance. The roughness is defined as the ratio of total surface area to the projected surface area. The roughness of surface could increase the length of liquid-solid contact line and capillary wicking ability. The increase in capillary wicking prevents the dry out of the heated surface and causes augmentation of the CHF. The effectiveness of surface roughness on CHF enhancement depends on the surface conditions such as wettability and porosity. A droplet located on a hydrophilic rough surface is mostly in the Wenzel state (Fig. 6b) and the surface wetting is completely occurred. However, on a hydrophobic rough surface, the droplet will be in Cassie state (Fig. 6a) and a slight wetting is happened. For the hydrophilic high roughness surface, the droplet will be in Sunny side up state. (Fig. 6c). In addition, for the biphilic surfaces, the topography has an important role in the wettability of heating surfaces. Betz et al. [35] investigated the effect of topography of biphilic and superbiphilic surfaces on HTC and CHF. They found that the optimal surface topography is depends on operating condition such as wall superheat. Also, the surface roughness could increase the density of active nucleation sites, number of cavities, and evaporation rate in liquid-vapor interface and therefore increase the HTC. SEM images in Fig. 7 clearly show the different structures of the copper surfaces which were polished again with various sandpapers having 2000, 600, 220, and 80 grit. The height of the scratches increased with increasing Ra, whereas the number of scratches decreases from the smooth surface to the rough surface [36].

Cassie state

Wenzel state

Sunny-side up state

(a)

(b)

(C)

Fig. 6. Droplet wetting models in different states for rough surfaces.

Fig. 7. SEM images of surfaces having different roughness (×1000). Adapted from Kim et al. [36].

Jones et al. [37] investigated the effect of surface roughness on pool boiling heat transfer for polished and EDM surfaces and two different fluids of water and FC-77. The range of roughness for polished surfaces was between 0.027 µm and 0.038 µm and for EDM surfaces was from 1.08 µm to 10.0 µm. For both of fluids, the HTC increases with increasing the surface roughness. The effect of roughness on HTC for FC-77 is higher than that for water. The enhancement of HTC using the roughest surface with respect to the polished surface was 210% and 100% for FC-77

and water, respectively. Although roughness and wettability are two different properties of the surface, surface wettability could be controlled by incorporating microstructures that introduce roughness [38], so the study of roughness is usually associated with the measurement of the contact angle of the surface. Kim et al. [39] studied the water pool boiling on the roughed copper surface over the entire regime from nucleate boiling to film boiling. With the increase of heat flux, for rough surface, HTC initially has a large amount and then rapidly decreases, resulting in a smaller CHF. For a smoother surface, HTC slowly increases. After reaching a maximum value, it decreases. This difference is due to the difference in the number of nucleation sites and vapor blanketing on the boiling surface, which is related to both roughness and contact angle. Kim et al. [40] had previously investigated the superhydrophilic aluminum heating surface. The heat transfer coefficient increases with the same boiling process on rough copper heating surface, but the improvement of wettability will degrade the boiling heat transfer performance. Improvement of wettability caused flooding of nucleation sites. Fan et al. [41] prepared a superhydrophilic surface by spray coating silica nanoparticles on the stainless steel spheres to increase surface roughness at the nanoscale. Quenching performance improved on this surface, and a value of 78% for the enhancement of the CHF was observed. In the use of Cr-sputtered surfaces that exhibit superhydrophilic properties, Son et al [42] showed that the CHF increases with increasing the roughness. The main reason for the increase of CHF on rough hydrophilic/superhydrophilic surfaces is capillary wicking, which increases fluid suction into the dry spots and increases bubble growth. The capillary wicking also was investigated by Kwak et al. [43] using a heater with top-opened rectangular microchannels. For a certain value of channel width, the liquidwicking capability increases with the increasing of the microchannel height. This can be due to the balance between the capillary-pressure potential and the viscous friction of the channel walls. This capillary wicking causes additional liquid supply to the dry spot, followed by an increase in CHF and HTC. One of the topics that have been considered by scientists in recent years is to investigate the effect of nanofluids boiling and subsequently the deposition of nanoparticles on the surfaces with different roughness, which causes some changes in active nucleation sites. Dareh et al. [44] studied the boiling of nanoparticles on two polished and machined surfaces. It was observed that for the machined surface where the surface roughness was greater than the size of the nanoparticles, the increase in nucleation sites resulted in a modest increase in HTC compared to pure water. In contrast, for the polished surfaces due to the equality of surface roughness with the size of nanoparticles, the nanoparticles will fill the surface cavities and prevent bubble nucleation, and therefore, the HTC is less than the pure water. Ham et al. [45] studied the simultaneous effects of nanofluid concentration and surface roughness. As the concentration of nanofluid increases up to 0.05%, the CHF increases. The enhancement in CHF is lower for the higher value of surface roughness. As the concentration increases, the HTC decreases. The decreasing percentage of HTC for the higher value of roughness is higher than that for the lower value of roughness. This can be due to the increasing in the number of the cavities with increasing the deposition of nanoparticles for the surface with lower values of roughness.

3 Surface characteristics 3.1 Heating surface size and thickness Rainy and You [46] studied the pool boiling process for plain and microporous coated surfaces with three different sizes (1 × 1, 2 × 2 and 5 × 5 cm2). For both types of surfaces, the CHF values were higher for smaller heaters. So the relationship between the heating surface size and the CHF is inversely proportional. They explained this observation via the fluid rewetting resistance in large surfaces. Kwak et al. [47] investigated the effect of surface size on CHF for two kinds of nanocoated and uncoated surfaces. The results showed that the CHF decreased for both surfaces when the heater size increased from (0.75 cm × 0.75 cm) to (2 cm × 2 cm). A long path of fluid from the side which is used for cooling the hot spots of the heater could be one reason for decreasing the CHF. Wang et al. [48] studied the heater size effect on subcooled pool boiling. They used a 2 × 2 cm2 silicon chip as a boiling surface and compared the results with a 1 × 1 cm2 silicon chip. Their observations also showed that for a larger surface, the departure of the coalesced bubble occurs in a wider range of input heat flux. By increasing the heater size in the same heat flux, the bubble departure radius is larger, but the performance of nucleate boiling heat transfer performance deteriorated. It is predictable that the first studies on the effect of surface characteristics on the heat transfer function are related to the impact of surface size. Therefore, fewer scientists have focused on this study in recent years. Instead of a lab study, Mesoscale simulations have been used to determine the impact of a heater size. In simulation, more detail and more precision are shown in comparison to the experiments. Zhang et al. [49] simulated the effect of the heater size on the boiling. They said that the size of the heater had little effect on the onset of boiling nucleation and heat flux in the nucleate boiling regime. By reducing the heater size, the boiling HTC increases. Despite the relatively small effect of the heater size on the nuclear boiling regime, its effect is significant in transition boiling and film boiling regimes. Large heaters do not affect their boiling curves, for medium heaters with increasing size, CHF decreases and disappears for small heaters in the transition boiling regime. Fang et al. [50] investigated the effect of heater size on pool boiling CHF for different pressures and gravities. They used platinum heaters with length of 26 mm and two diameters of 30 μm and 50 μm. A decreasing trend in CHF was observed with increasing the heater diameter. Weaker surface tension and higher frequency of departing bubbles on small heaters can be some reasons for enhancing the HTC and CHF. Ma et al [50] also simulated the effect of a heater size on boiling curves in microgravity and normal gravity. Simulation showed that in both situations if the surface area is smaller than a certain amount, the transition boiling regime and the apparent CHF do not occur. Also, reducing the heater size will shorten the transition boiling regime and increase the minimum heat flux. Various studies have been performed about the effect of heater thickness on the HTC and CHF parameters. Some studies reported that the effect of thickness on boiling heat transfer is significant [51], and some other studies showed that the thickness effect is negligible [52]. Ma

and Cheng [52] used the phase-change lattice Boltzmann method to 3D simulation of pool boiling to investigate the effect of heater thickness on the CHF and boiling curves. The boiling curves for five different thicknesses collapsed into one, except in the transition boiling regime. Also in the transition boiling regime, for the heaters with sufficient thickness, the effect of the thickness on the boiling curve is small. It is found that heater thickness has almost no effect on the CHF values. Liang and Mudawar [53] illustrated that the CHF increases with increasing heater thickness. They explained that there is a limitation for the increasing of CHF which is called asymptotic value. Increasing the thickness beyond the value corresponding to the CHF asymptotic value has no effect on the CHF. Raghopati and Kandlikar [51] conducted the pool boiling experiments with a thick heater in order to investigate the effect of thermophysical properties of heater substrate on CHF. They found that density and specific heat are important parameters in changing the CHF for a thick heater. Margini and Nanei [54] studied the effect of heater thickness on HTC for surfaces that made of copper, silver, zinc, nickel and tin. For nickel, tin and zinc, the HTC increases with decreasing the thickness. However, for the thicknesses larger than a certain value (limiting thickness), varying the thickness, will not change the HTC. The limiting values of thickness for the nickel, tin and zinc surfaces are 15 μm, 15 μm and 70 μm, respectively. The effect of heater thickness on HTC for the copper and silver surfaces is negligible. The influence of thickness becomes higher when the thermal conductivity of heating material is smaller. The influence of heater thickness on HTC depends on the experimental range of thickness changing and more importantly the heater material and its thermophysical properties.

3.2 Heating surface thermophysical properties The heater surface material and the thermophysical property of material have a great influence on heat transfer pool boiling performance. A comparison of the different material's performance as the heater surface in operating conditions can help to understand the details of this influence. Bombardieri and Manfletti [55] explored the impact of heater material thermal conductivity which is one of the surface characteristics that influence the boiling heat transfer. The effect of three different materials (copper, aluminum, stainless steel) is evaluated. The highest slope of the boiling curve is obtained for aluminum, while stainless steel shows the lowest value. At the same wall temperature, the results indicate that copper has the highest heat flux, while aluminum has lower heat fluxes than stainless steel despite its higher thermal conductivity. The CHF derived from the experimental results is also the highest for copper. Raghupathi and Kandlikar [51] to examine the effect of the thermophysical property of the heater material compared to the CHF on seven different heater types: aluminum, brass, copper, carbon steel, monel 400, silver, and silicon. They said that the silicon and carbon steel had the highest and lowest CHF, respectively. The density and specific heat of a material, in contrast to thermal conductivity, are effective parameters in the amount of CHF. Kim et al. [56] compared the pool boiling on a TiO2-coated surface (TCS) with a SiO2-surface coated (SCS). The results showed that HTC and CHF are both higher for TCS. At the same time, the contact angle of both surfaces was measured after heat treatment, it was found that the TCS wettability increased and SCS wettability remained

unchanged. This contact angle change after heating suggests that the wetting transition of TCS is a key factor in the enhancement of both HTC and CHF in boiling. TCS is hydrophobic at a low wall temperature and becomes hydrophilic as the wall temperature increases. This suggests that this TCS thermally-induced wetting transition could be a major driver for HTC and CHF increasing. TiO2 can be considered a “smart” surface. In recent years, new materials are known to have special thermal properties when used as a heater in pool boiling, these special properties can be better for heat transfer. The surfaces that are grafted with stimuli-responsive polymers are one of those surfaces. Switchable or thermoresponsive polymers exhibit drastic variations in their physical properties around specific narrow temperature variations. These surfaces around this specific temperature the transition hydrophilic to hydrophobic will be totally reversible. This surface combines the benefits of hydrophobic and hydrophilic surfaces within nucleate boiling conditions [34]. Hao et al. [57] used shape-memoryalloys material that changes the geometry according to the heat transfer condition to achieve optimal geometry. A variety of alloys exhibits the shape memory effect such as Ti-Ni, Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni and so forth. The alloy has studied by Hao et al. was Ni49.5Ti50.5 (at. %), which was constructed in three different types: open tunnels, closed tunnels, and Recoverable fins. The third type, like the second type, had bent fins, but these fins can return to the original straight configuration when the surface is heated to a temperature higher than the transformation point, as shown in Fig. 8. The purpose of using these three structures was to examine the benefits of each of them. Finally, the results showed that the recoverable fins in high heat flux have the highest HTC. These surfaces also have the advantage of forming nucleation sites of the closed-tunnel structure, as well as the bubble growth and departure of open-tunnel structure.

Fig. 8. Repeated thermal-mechanical training of deformable structure. Adapted from Hao et al. [57].

3.3 Heating surface orientation Another problem that can affect the boiling process is the orientation of the heater surface and the fluid. The heater surface can be top of the fluid and heat transfer downward, or the heater can be down of the fluid, and the heat transfer is upward (such as the bottom of the pool boiling, which is more common). The difference between these two modes plays a very important role in the bubbles dynamics. surface orientation is a mechanism to quickly remove vapor from the surface and increase the amount of HTC [58]. In most cases, bubbles are coalesced to the downward surface and can hardly be separated from the surface. Nishikawa et al. [59] investigated the effect of orientation of heater copper surface in the nucleate pool boiling of water at atmospheric pressure. The inclination angle was varied from 0° to 175° with the horizontal plane. At low heat fluxes, the HTC increases with increasing the inclination angle. The effect of heater surface configuration on HTC at high amount of heat fluxes is not significant. For the inclined surface downward facing, two mechanisms of latent heat transport and sensible heat transport were proposed. The latent heat transport is due to the evaporation of the thin film of liquid under the rising bubbles. The second one is related to the removal of the thermal layer by the bubble rising along the heating surface. The analytical analysis showed that the latent heat

transport has more important role in heat transfer of downward facing surfaces. Hsu et al. [27] used a horizontal copper cylinder as a boiling surface. The isolated bubbles accumulate together on the lower surface due to the surface blockage and large bubbles detach from the surface easier than the upper surface. Also due to the natural convection conditions, the bottom surface HTC was higher than the upper surface. The heat transfer performance is very different when the orientation is changed from the horizontal upward to the vertical, and when it changes from the vertical to the downward surface. Zhong [60] increased the inclination angle from 5 to 90. With an increasing inclination angle, CHF increased. Dadjoo et al. [61] used water and SiO2/water in pool boiling experiments. The heater orientation increased from zero to 90 degrees, and the results showed that for both the fluid, HTC is reduced due to the coalescing bubbles and the formation of vapor film on the surface that increased heat transfer resistance. As regards DI water, CHF also decreases for the same reasons and reduces wettability. While for the boiling of nanofluid, CHF increases, which is due to the deposition of the nanoparticles when increasing the inclination angle, which leads to increased surface wettability. Dadjoo et al. results were in contrast to Zhong for DI water and plain surface, as shown in Fig. 9. The effect of surface orientation is also highly dependent on the surface structure. When surface modification techniques are used, surface orientation can have effects on the pool boiling heat transfer performance, which varies with the effect of orientation on the smooth surface. Jun et al. [62] investigate pool boiling heat transfer of saturated water using a durable high-temperature thermally-conductive microporous coating copper. The CHF values from 0 to 90 degrees remained at a constant value, then linearly decreased by varying the inclination angle from 90 to 180 degrees. They suggested the reduction of CHF because increasing the vapor residence time on the surface and preventing liquid access to the heated surface at downward inclination angles. Kim et al. [63] achieved a similarly positive result in 2D- and 3D-graphene-coated surfaces orientation experiments, but the effect of changing the orientation angle on the HTC was insignificant. On the downward-facing surface, bubbles were deformed against the wall by normal components of buoyancy forces from the wall, so that they became thinner and wider with lower escape speeds. They stated 2D- and 3D-graphene-coated surfaces could be a good choice for downstream heat exchangers. Azman et al. [64] observed that as the heater orientation changed from upward- to downward-facing, the CHF decreases. However, installing a porous honeycomb plate on the heater surface and adding nanoparticles to the fluid increases the CHF. So that the CHF of the downward-facing surface is approximately 12 times that of a flat downward-facing surface without nanoparticle deposition. But the CHF of the upward-facing is still greater than the downward-facing surface. Figure 8 demonstrates that increasing the angle to 90 degrees usually has a positive effect on the increase of CHF. However, a further increase in the angle reduces the CHF.

3000

CHF (kW/m2)

water, plain surface, Aznam et al. 2500

0.1 wt% nanofluid, honeycomb porous plate, Aznam et al. DI water, plain surface, Dadjoo et al.

2000

0.005 vol.%, plain surface, Dadjoo et al. water, plain surface, Gong et al.

1500

water, microporous coating, Jun et al.

1000

subcooled dodecane, plain surface, Pi et al. DI water, plain surface, Zhong et al. DI water, cavity structures, Zhong et al

500

0 0

30

60

90

120

150

180

 (degrees)

Fig. 9. Relationship between CHF and surface orientation.

Gong et al. [65] investigated stainless steel surface inclination in two bare and porous honeycomb structures. For both surface structures, with increasing surface inclination, the bubble film departure frequency (correspondingly water supply capacity) and bubble removal capacity, and subsequently CHF, increase. This increase is higher for porous honeycomb surfaces. However, Zhong [60] does not consider the downward-facing surfaces with cavity structures to increase the CHF more than flat downward-facing surfaces. In pool boiling, the heater does not necessarily have flat or curved surfaces. A thin wire can also be used as a heater. Pi and Rangwala [66] examine thin wire heater orientation (ϕ=0° to ϕ=90°) influence on nucleate boiling in subcooled dodecane. As the ϕ increased, CHF decreased as the number and size of the bubble decreased. The highest HTC was found at ϕ=22°.

4 Surface structure The most varied section of the studies conducted in order to surface modification is the change in surface structure. Enhanced surfaces such as micro pin-fin and channel/tunnel, porous surfaces, coated surfaces, surfaces with nanowire, reentrant cavities, honeycomb surfaces, etc. are only

part of them. In this research, several commonly used techniques are mentioned. A simple scheme of integrated surface structures is shown in Fig. 10.

Honeycomb

Pin-fin

Coating

Channels

Fig. 10. A simple scheme of integrated surface structures.

4.1

Surface coating

The surface coatings can add other materials to a substrate or create a coating on the substrate and are generally used to provide a porous surface. Kano [67] used the electrically co-deposition technique to construct a microstructure surface with 5 metal layers and diamond particles. These layers included copper for thermal conduction, nickel for bonding diamond particles and boron to oxidation resistance. He studied the effect of diamond particle size on boiling performance. By increasing the particle size, due to the cavities around the diamond particles, the amount of entrapped vapor increases and the superheat decreases. Jaikumar et al. [68] used Graphene and Graphene oxide for its excellent thermal properties to coating the copper chips. They have four different surfaces by increasing dip-coating durations with solutions. They found that the amount of CHF and HTC surface with the shortest coating time increased by 42% and 47%, respectively, compared to plain surface. This increase is due to the roughness effect, rather than wettability. Sarangi et al. [69] used two free particles and sintered particle coatings on a copper substrate. The wall superheat of sintered coatings surface is much less than the surface with a layer of free particles. Although the free particle allows the escape of trapped vapor, because of entrainment of free particles into the liquid, the active nucleation sites are reduced, so porous sintered particle coatings have higher heat transfer. In the following section, a number of studies on porous surfaces are mentioned.

4.1.1 Porous coating The main advantage of porous surfaces is the increase of nucleation sites and thus HTC. It is also one of the best ways to increase CHF. But these surfaces may also cause a large increase in wall superheat and physical damage. Thiagarajan et al. [3] performed their experiments on a thermally conductive microporous copper surface in different thicknesses, porosity, and cavity sizes, and compared them with plain copper surfaces. The micro-porous surface has a lower boiling incipience temperature due to the presence of cavities in large numbers. due to much larger nucleate site density than the plain surfaces, HTC increased by about 50-270% and the critical thermal flux increased by 33-60%. Xu and Qin [70] investigated the pool boiling of water at atmospheric pressure on gradient metal foams with double layers made of uniform copper foam and nickel foam. The results showed that the heat transfer performance is better when the copper foam is placed under the nickel foam. When the copper foam layer is lower, due to more thermal conductivity, nucleation sites are more activated quantitatively and more bubbles are generated. Seo et al. [71] compared boiling heat transfer for bare indium tin oxide (ITO) surface, a nonporous few-layered graphene-deposited ITO surface, a nonporous (SiC) layer-deposited ITO surface, and porous graphene and silicon carbide SiC layer-deposited ITO surfaces. The CHF Increasing values for surface with porous graphene layers and SiC layers are higher than the nonporous graphene layers and SiC layers. This difference can be due to the effect of porosity and permeability on hydrodynamic limits and Capillary pumping limits. The porous surfaces had microcavities that easily activated bubble nucleation. The nonporous SiC layer has a better performance than the nonporous graphene layer. However, for porous layers surfaces, the performance of porous graphene layers was better than porous SiC layers because of the difference in the thermal properties of graphene and SiC and The porous graphene heater exhibited higher porosity and permeability values than the porous SiC heater. A problem with porous coatings is that in the high heat flux, a vapor layer grows in the porous coatings causes' thermal resistance which is responsible for the large surface superheat. Once the vapor layer grows to fill the porous structure, transition to film boiling occurs and CHF is reached. By disrupting the formation of this vapor layer through the fabrication of channels/microchannel to allow vapor escape, an enhancement in the CHF and HTC was observed [72]. Zhang et al. [73] studied the heat transfer performance of solid interconnected microchannel nets (SIMN) that was fabricated using wire electric discharge machining and porous interconnected microchannel nets (PIMN) that was fabricated by sintering copper powder. PIMN exhibits a lower wall superheat at the onset of nucleate boiling (ONB) and a higher HTC than that of the SIMN. Also, the bubble departure diameter for the PIMN is shown to be smaller than that for the SIMN. Deng et al. [74] in similar studies with Zhang et al, compared the solid reentrant channels (SRCs) and porous coating reentrant channels (PRCs). Again, the porous coating surface introduced a lower wall superheat at ONB. The PRCs clearly increased heat transfer and promoted the boiling nucleation significantly by providing the nucleation sites.

Porosity can range from macroscopic to micro/nanoscales. For example, Kruse et al. [75] examined the deionized water pool boiling on multiscale (micro/nano) functionalized stainless steel surfaces made using the femtosecond laser surface Process. CHF increased due to improved wettability and capillary function. Due to the increased surface area ratio, the structure peak-tovalley height, and the active nucleation site, HTC also increases. Porous coating, due to the creation of more nucleation sites, leads to earlier phase-change heat transfer and improves bubble dynamics [3, 70, 74, 76-80]. In Table 2, further studies have been carried out that emphasize the effects of porous coating on bubble dynamics and heat transfer. Reference Niu and Li [77]

Pratik et al. [78]

Gheitaghy et al. [79] Gheitaghy et al. [81] Jun et al. [82] Surtaev et al. [83] Lee et al. [84] Lu et al. [85] Tang et al. [86]

Table 2. Effect of porous coating on boiling heat transfer characteristics Test fluid Test surface Fabrication technique Remark Al2O3 and Copper porous coating and Sintering copper micro- Heat transfer performance CuO nanopolished surface powder enhancement fluid Silicon surfaces modified CHF enhancement using with planar or foam-like Chemical vapor Foam-modified surfaces hierarchical hexagonal boron Deionized deposition process onto Compared to control planar hnitride (h-BN) nanomaterials water the surfaces of openBN and surfaces with only plus adhesion layer, and a cell nickel foam adhesion layer silicon sample with only the adhesion layer Wire Electric Copper mesochannel, CHF and HTC enhancement Discharge Machine, microstructured porous using combination of Water two-stage coating, and combination of microporous copper on electrodeposition of them microchannel surface copper boiling incipience temperature Distilled Microporous surfaces Electrodeposition reduction, CHF and HTC water enhancement Brazing copper Water Microporous coating particles onto a copper BHT and CHF enhancement surface Boiling heat transfer depends Liquid Three-dimensional capillaryPlasma spraying on the coating thickness and nitrogen porous coating parameters of microstructures Layer-by-layer assembled Deionized polyethyleneimine- multiSolution processing HTC and CHF enhancement water walled carbon nanotube coatings HTC enhancement, higher Deionized Surface alloying and Nanoporous copper bubble departure frequency, water dealloying smaller bubble diameter Lower wall-superheat at the Deionized Porous interconnected Copper powder ONB and a higher HTC than water microchannel nets sintering and WEDM the solid interconnected microchannel net

Shojaeian and Kosar [87] reviewed the experimental investigations performed on pool and flow boiling on nano/microporous surfaces. Types of techniques used to fabricate these surfaces and

the properties of surfaces have been studied. In most of the literature, porous coatings have increased the heated surface area and nucleation site. 4.1.2 Nanostructure coating In the surface coating method, as in many scientific fields in the last century, scientists are interested in using nanotechnology to achieve better results. Further examples of nanostructure coatings are mentioned. Nanotexture: Micro/Nanotextured coatings have enabled the manipulation of thermal characteristics in pool boiling heat transfer such as HTC and CHF because of the ability to optimize bubble formations and departures [84]. Joe et al. [88] used Nickel-chrome that was electroplated with copper and then annealed. This process leads to the formation of a hydrophobic nanotextured. As the electroplating time continues the hydrophobic nanotextured surfaces changes to hydrophilic. Hydrophobicity increases the bubble dynamics and leads to a decrease in surface temperature of the wire, in turn, increase superheat and effective HTC. The hydrophilic surface decorated with numerous nucleation sites enhanced overall heat removal, and therefore the CHF. Another nanotextured surface that Joe et al. [89] experimented with was nanopillars bismuth vanadate (BiVO4) formed by electrostatic spray deposition. Increasing the spraying time, increased the height of the nanopillar structure and provided numerous bubble nucleation sites because of increased surface area. Subsequently, CHF and HTC increased. Electrospraying has an optimal time, and it further increases the nanopillar height through trapping of bubbles. Ray et al. [90] used the electron beam evaporator to fabricate the TiO2 thin film coating on the copper surface. HTC of coated thin film surface was higher than the uncoated surface due to the augmented roughness and increment of dynamic nucleation site density. Dewangan et al. [91] covered a copper tube with a powder flame spraying technique. The effect of coating thickness, pore diameter and porosity on HTC for the two refrigerants was compared. HTC of coated copper surfaces was about 1.1-2 times higher than that of plain copper. The coating thickness and the diameter of the pores are the most important parameters affecting the growth of the bubbles. Nazari et al. [92] used the anodizing technique for electrochemical coating. If the anodizing process is carried out in the presence of a strong acid, the coating on the surface will be porous. They used nanostructure coating aluminum oxide on the surface of the aluminum substrate. The best coating (the smallest pores diameter of coating and most wettability) was obtained in the presence of sulfuric acid, which increased the HTC and CHF relative to the non-anodized surfaces. Souza et al. [93] attribute HTC to the ratio between the surface roughness and the nanoparticle diameter. The addition of nanostructures in different amounts of this ratio does not necessarily mean that HTC is rising. For example, for values close to one, HTC will be reduced. This is due to the blockage of nucleation sites and the need for a greater superheating in order to activate the nucleation sites. Nanofibers: Sahu et al. [80] studied pool boiling of ethanol, water, and their binary mixtures on bare copper and nano-textured metal-plated surfaces. For nanotextured surfaces, the bare copper surface was coated with ultrafine polyacrylonitrile nanofibers. In the same surface superheat,

nanofiber surfaces show heat flux 2-7 times higher than those on the bare copper. The boiling data on these surfaces does not follow the standard boiling curve and shows a sharp deviation. The powerful feature of these surfaces was not to be deteriorated after several cycles of pool boiling experiments. In another study, they used Novec FC 7300 fluid and self-rewetting water– heptanol mixture as a working fluid. The results revealed a significant increase in the heat removal rate up to the CHF and the corresponding HTC on the copper-plated nanofiber surfaces in comparison with bare copper surface. Also, the pool boiling data for the self-rewetting fluid revealed a higher heat removal rate from the nanofiber surfaces than even the one for pure water [94]. Sinha-Ray et al. [95] covered copper surface with supersonically blown or electrospun polymer nanofibers instead of using electroplating. The Novec 7300 pool boiling shows that supersonically-blown polymer nanofibers increase the heat removal rate from the heater surface compared with the clean copper or copper coated with electrospun nanofibers. This coating provides a larger number of active nucleation sites and small pores and then facilitate nucleate boiling much stronger. Supersonically-blown nanofibers also have good adhesion to the surface and maintain their position by up to 7.5 hours of boiling. Nanoparticle: Das et al. [96-98] coated the copper by Electron beam physical vapor evaporation with SiO2 nanoparticles. The surface characterizations Determine in terms of dynamic contact angle, surface roughness, topography, and morphology. Compared to untreated surfaces, they found about a 36% reduction in the incipience superheat and a 58% increase in HTC, due to increased wettability, increased surface roughness and an increase in the number of small artificial cavities on the surface of the heater. Micro/nanostructures surface act to hold growing bubbles on the surface for a longer time which is considered also an important factor for enhanced heat transfer. They continued experiments on this surface and observed that with the increase in the thickness of the nanoparticle film, HTC will also increase. Such a surface has higher hydrophilicity, more nucleation sites, and higher bubble frequency. Up to 80% enhancement in the nucleate pool boiling heat transfer coefficient has been obtained. They also continued their research with the replacement of crystalline TiO2 instead of SiO2 nanoparticles in various thicknesses. The TiO2 nanoparticle layer increased HTC. The reason for this increase was an enhancement in an effective surface, facilitating the generation of larger nucleation sites and enhanced surface wettability. Nanotube: When the nanotube word is heard, the first matter that comes to mind is the carbon nanotube (CNT). In FC-72 under atmospheric condition, Ho et al. [99] tested bare silicon, fully coated CNT surface and interlaced patterned CNT surface. The fully coated CNT surface and interlaced patterned CNT surface increased the HTC as compared to bare silicon. This increase is due to the large number of submicron cavities provided by the CNT structures. With larger cavities located along the coating perimeter, optimizing the coating perimeter with interlaced surface allowed sufficient nucleation cavities to be compensated at reduced coating area such that similar heat transfer performance as the fully coated CNT surface was achieved. Dharmendra et al. [100] performed pool boiling heat transfer experiments with a bare copper

surface and carbon nanotube coated copper substrates. The results showed that due to the relative high roughness, the more effective surface area and the hydrophobicity for the CNT coated samples, nucleation sites density increased. Therefore the earlier onset of boiling and higher CHF is obtained compared with the bare copper. Kumar et al. [101] like the study of Dharmendra et al. [100] used copper substrates. However, using the plasma enhanced chemical vapor deposition (PECVD) technique, they first covered it with a thin layer of graphene and then developed CNT on it. An increase in CHF and HTC and a decrease in the boiling incipience superheat were observed for Gr/CNT heterostructures compared to bare copper surface. The reasons for the boiling performance variations, were the increase in nucleation sites, the capillary effect, the lateral transfer through the graphene layer, and the improvement of the bubbles dynamics. Park et al. [102] performed an experimental study on the spray-depositing oxidized multi-wall carbon nanotubes onto heater samples. As the spray deposition time increased, the contact angle decreased linearly which resulted in an increased CHF. In the surfaces that reviewed in this section, the nanoparticle was fabricated on the heater substrate. While deposition of suspended nanoparticles in nanofluids, which causes changes in heater surface properties, is also widely used which will be mentioned? In the next section, another popular nanostructured surface is examined, that is the surfaces covered with nanowires.

4.2 Nanowire Shi et al. [103] studied pool boiling heat transfer on copper nanowires arrays with different lengths which were electroplated on bare copper surfaces. As expected, both HTC and CHF increase using integration of copper nanowires arrays on heating surface. As the length of nanowires increased, more enhancements occur. The reason for this improvement is the increase in wettability and the number of nucleation sites. Ray et al. [104] utilized a glancing angle deposition system using an e-beam evaporator to synthesizing vertically oriented TiO2 nanowire arrays in different thicknesses on TiO2 thin film coated copper surfaces. These surfaces in pool boiling tests with refrigerant R134a liquid showed that for all nanowire thicknesses, as HTC improved, the wall superheat also decreased compared to an uncoated surface. This performance is also enhanced by increasing the thickness of the nanowires. The atomic force microscope (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) imaging of the surfaces indicate this improvement in performance due to the increased dynamic nucleation site density. Kumar G et al. [105] illustrated the effect of copper nanowires diameter on pool boiling heat transfer performance. The copper nanowires with different diameters were grown on heating surface using template-based electrodeposition method. The enhancement of both HTC and CHF were observed for surfaces with nanowires compared to the bare copper surfaces. Increase in micron-scale cavity density and cavity size, superhydrophilic nature, capillary effect, and enhanced bubble dynamics parameters were found to be the reasons for this enhancement in heat transfer performance. In examining the various diameters of the nanowires, it was found that an increase in diameter up to 200 nm caused entanglement of the nanowires and reduced the HTC and CHF. Lee et al. [106] found that Nanowire-forested surfaces in applying 30 K subcooled

condition, due to delays in bubble coalescences, reduced spatial variations /Temporal temperature, resulting in improved thermal stability and a 4.3-fold increase in CHF relative to plain surfaces under the saturated condition. The nanowire formation method was metal-assisted chemical etching. Shim et al. [107] also used this method to make aligned silicon nanowires and random silicon nanowires. Their aim was to measure the volumetric wicking rate, which increased its effect on the flow of liquid to the dry area to delay the accumulation of steam and the removal of additional thermal energy, resulting in an increase in CHF. The results showed that the strategic alignment arrangement resulted in a higher volumetric wicking rate. The diameter, pitch, and height of aligned silicon nanowires are also effective in increasing the CHF. The best pool boiling performance results obtained in literature with water in the use of nanostructures on a copper horizontal flat substrate compared in Fig. 11. 2500 nanoparticle coated, Das et al. [88] nanoparticle coated, Das et al. [89] copper-plated nanofibers, Sahu et al. [74]

2000

carbon nanotube coated, Dharmendra et al. [92]

q" (kW/m2)

nanowire arrays coated, Ray et al. [84] nanowire arrays coated, Shi et al. [96]

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nanotextured surfaces, Jo et al. [83] nanoparticle deposited, Mori et al. [15] micro/nano bi-porous copper, Wang et al. [22]

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micro/nanostructured grains on copper, Gheitaghy et al. [75] nanoporous copper, Lu et al. [79] surface with polymer nanofibers, Sinha-Ray et al. [120]

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6 ΔT

8

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Fig. 11. Water boiling performances for employing different nanostructures on a copper horizontal flat substrate.

12

4.3 Channels/tunnels The channels provide three side heating of liquid inside it and therefore nucleation can occur in lower superheat [108]. Jaikumar and Kandlikar [76, 109] investigated the effect of channel width on heat transfer Mechanisms for open microchannel surfaces with three coating configurations: (i) sintered-throughout, (ii) sintered-fin-tops, and (iii) sintered-channels. The effect of selectively coating different regions of the microchannel showed that the highest CHF and HTC were obtained for sintered-throughout channels, as shown in Fig. 12. These channels provide more nucleation sites.

Fig. 12. Heat transfer performance of selectively sintered surfaces using fin top temperature. Adapted from Jaikumar and Kandlikar. [109].

In the study of the effect of channel width, the wider sintered-throughout channels, due to providing additional liquid pathways to feed to the nucleation sites had a higher CHF and HTC compared to a narrower channel. But the highest amount of CHF and HTC was obtained for the thinnest sintered-channel. Bubbles, when departed from the nucleation sites, cause fluid flow to the surface during pool boiling. With alternating the nucleating regions with non-nucleating regions can be separated the liquid-vapor pathways. Using the liquid feeder channels, Jaikumar and Kandlikar [110] led the fluid back to the nucleating regions. This method keeps the separated liquid-vapor pathways even in the high heat fluxes. That way, the CHF and HTC rose, due to the unobstructed surface available for the bubbles to expand over the surface area before departing. Gheitaghy et al. [108] examined the effect of 45 inclination, geometry dimensions and the orthogonal intersection of optimized mini channels. The channels were made using a wire-cut

method on the copper surface. The results showed that the heat transfer increased with increasing channel depth and pitch reduction, and the orthogonal intersection microchannels exhibit the highest amounts of HTC and CHF compared to the plain surface. The improved heat transfer is attributed to bubble dynamics, heat transfer area, bubble slide and scrape, and capillary flow. Halon et al. [111] studied the structure of narrow tunnel structures (NTS). A surface that included horizontal and vertical channels that could be open or covered with perforated copper foil under atmospheric pressure. The results showed that the narrow channel did not differ significantly with the plain surface, but when it was covered with a perforated copper foil and turned into a narrow tunnel, HTC has doubled. This change led to an increase in the bubble departure frequency but did not significantly affect the bubble departure diameter. Halon et al. [112] continued their research by testing the new tunnel structures (TS) and comparing them to narrow tunnel structures. The main difference between these two structures was that the fins in each row were separated from each other for NTS and connected for TS, which led to an increase in the heat transfer area. Walunj and Sathyabhama [113] compared three different geometries of rectangular, parabolic, and stepped open microchannels with plain surfaces. The results showed that all three types of microchannel enhance the heat transfer rate. Bubble departure diameter and bubble frequency have proved a significant performance. The performance of these four surfaces from the worst to the best is the smooth surface, the rectangular, the parabolic and stepped microchannels, respectively. Sarafraz et al. [14] constructed channel-like structures in the form of concentric circular microstructures with different geometric specifications on the copper surface using the micro machinery technique. These microstructures lead to irregular deposition of nanoparticles and create more active nucleation sites compared to the smooth surface. Although smaller microstructures have not particularly impressive in CHF increase, but HTC has risen.

4.4 Reentrant cavity/ channels Deng et al. [74] investigated the pool boiling on the porous structures with reentrant cavities surfaces. The reentrant cavities provided horizontal channels for liquid influx, which facilitated to maintain the liquid replenishment and efficient surface rewetting. This helped to avoid the fast deterioration of heat transfer at moderate to high heat fluxes. The heating surface used by Ji et al. [114] was enhanced tubes. Observations showed that with the increase of heat flux, a reentrant cavity tube increased heat transfer and boiling heat transfer coefficient up to 330% compared to the plain tube. However, with a further increase in heat flux, HTC of the reentrant cavity tube is even lower than the plain tube. Sun et al. [115] employ the orthogonal ploughing/extrusion method to create microgroove surfaces with reentrant cavities (MSRCs) on a pure copper substrate. MSRCs provide a higher HTC and a lower wall superheat at the ONB than the smooth copper sheet. The MSRCs suppress the temperature excursion by lowing the wall superheat at ONB and do not reach CHF during the entire test range. The bubbles on the MSRCs can be divided into two categories of cavity bubble and groove bubble in terms of the nucleation sites. The cavity bubble is smaller than those on the smooth copper sheet and is not particularly dependent on the increase in heat flux. The groove bubble size increase with increasing heat flux,

but decrease sharply when the heat flux increase. The Turbo-ESP surfaces have a Reentrant Cavity structure which is shown in Fig. 13. Kedzierski and Lin [116] compared the boiling performance of R123 and R1336mzz (Z) on the Turbo-ESP surface. The performance of these two refrigerants did not differ statistically. Except in high heat flux, the R123 was slightly better.

Fig. 13. Photograph of the Turbo-ESP surface. Adapted from Kedzierski et al. [116].

4.5

Honeycomb

During the experiments, Gong et al. [65] observed that the honeycomb surface has a higher CHF than the bare surface, due to a larger wetted perimeter offered by the honeycomb structure, which leads to the water supply paths. Also, creating porosity in different sizes on the honeycomb surface leads to an increase in CHF due to providing additional water supply and bubble removal paths. While the pore sizes do not have a significant effect on the CHF. Mori et al. [117] stated that a two-layer structured honeycomb porous plate (HPP) in different thicknesses could improve CHF. First, a thin HPP attached to a heated surface that can reduce the frictional pressure drop caused by internal water flow. This layer should have very fine pores to supply water to the heated surface due to strong capillary action. Then, the other thicker HPP stacked on top of the thin HPP in order to hold enough water to prevent the inside of the HPP from drying out during the bubble hovering period over the plate. Aznam et al. [118] investigated the CHF enhancement in saturated pool boiling of water based TiO2 nanofluid by the attachment of an HPP and a gridded metal structure on a horizontal heater. In addition to the positive effects of placing the honeycomb porous plate as described above, adding a gridded metal structure provides liquid supply on the heater surface and shortens the hovering period of the coalesced bubble. Also, the nanoparticle deposition, improved wettability and capillarity, and simultaneous existence of three factors (honeycomb porous plate, gridded metal structure, and nanoparticle deposition) caused a significant increase in CHF. Therefore, honeycomb surfaces, especially its porous type, due to the separation of the water supply paths entering the heating surface and the bubble removal from the heating surface, reduce the resistance between liquid and vapor, and thereby improve the boiling heat transfer performance.

4.6

Pin-fin

Zhang et al. [119, 120] used the dry etching technique to fabricating micro-pin-finns on silicon chips. Gas-saturated FC-72 nucleate boiling heat transfer in microgravity environment was experimentally investigated. With increasing the heat flux, the bubble detachment radius increases. Micro-pin fins in microgravity have better heat transfer performance compared with smooth surfaces. Ho et al. [121] constructed microstructure using the Selective Laser Melting technique. They found that due to the formation of grooves and cavities, the active nucleation site density and HTCs enhanced. Micro-fin surfaces prevent coalescence of vapor bubbles and increase fluid replenishment paths. For this reason, the occurrence of CHF is delayed. Sarafraz and Hormozi [122] studied the pool boiling heat transfer characteristics of multi-walled carbon nanotube nanofluids on modified surfaces with diamond-shaped micro-finned. Their observations showed that the finned surfaces reduced the rate of fouling compared to the smooth surface. Non-uniform irregular fouling improves the number of active nucleation sites and bubble formation. The bubble formation rate increases, but the bubbles can discreetly depart the surface, which led the HTC to be enhanced due to the local agitation and available heat transfer area. Pastuszko [123] studied the pool boiling of three different fluids on micro fined structures. The HTC from the micro fined surface is up to 6.5 times higher than those for plain surface. The bubbles are generated and grown in the space between the micro fins and departure takes place at the micro-fin tips. In high heat fluxes, micro fins with lower height have better performance. Then he covered micro-fins with a copper wire mesh, which generated larger bubbles and improved heat transfer performance. Kong et al. [124] provided a bistructured surface, with two micro-pin-fin surfaces and a smooth surface adjacent to each other. This structure has the feature that the Micro-Pin-Fin area provides the nucleation sites and small bubbles grow, collide, merge and move rapidly to near smooth areas, which leads to a significant increase in heat transfer performance and CHF improvement.

5

Other surface enhancement techniques

The surface enhancement techniques mentioned have been of great interest to researchers and have been studied several times over the years. But the various techniques and structures used are more diverse than being limited to several categories. There are a number of other interesting studies in this section. Holguin et al. [125] investigated boiling experiments using a dielectric liquid PF-5060 on a binary surface where was a copper surface coated with a non-boiling liquid layer. This non-boiling liquid was water that fills all the micro-/nano- cavities on the surface of the copper and creating solid islands. The bubbles nucleation occurs on these solid islands. This allows early vapor bubble departure, preventing lateral bubble coalescence and facilitating rewetting of the surface. As a result, the maximum heat flux and the average HTC for binary surface increased 2.2 and 7.5 times, respectively, compared with the polished copper surface. Alim Khan et al. [126] used the potential synergistic benefits of combining nano-scale and micro-scale porous structured surfaces into an integrated surface coating fabrication technology. The nucleate pool boiling heat transfer performances of hybrid micro/nano-scale 2-D modulated

porous coated surface (HMPC surface) as well as the 2-D modulated micro-porous surface (MMPC surface) are investigated. The performance of the HMPC surface decreased when compared to the MMPC surface. This reduction is explained by the deterioration of the capillary wicking characteristics. The overall comparison of the plain and structured surfaces showed the highest performance for nanocoated copper surface in terms of HTC and CHF. An important point in the boiling heat transfer process is that, besides improving boiling performance, the stability and uniformity of the surface temperature should be maintained over a long period of time without damage to the surface. The main reason for checking CHF is to prevent physical damage to equipment. Lee et al. [127] showed that micro-nano hybrid surface characteristics can maintain surface thermal uniformity and stability. Their boiling surfaces were made of silicon and had micro-cavities that were completely covered with nanowires. They showed that by combined effects of the confining of the nucleated bubble on nanowires and uniform bubble nucleation on uniformly distributed micro-cavities structures, thermal uniformity/stability increases. Bubble coalescence delays and CHF increases. Seo et al. [128] uses a Layer-by-layer (LbL) assembly technique for multilayers of polyethylenimine and multi-walled carbon nanotubes (MWCNTs) deposition onto a stainless steel surface. The fibrous wires of MWCNTs created a porous structure with random orientations. Surface specification showed that the roughness and wettability of the surface increased. Both roughness and wettability were improved as the bilayer number increased. The visualization results showed that improved rewetting behaviors were observed for all of the LbL-assembled heaters. The maximum CHF enhancement was 94% when compared to the bare heater. The amount of CHF enhancement increased when the number of bilayers increased and its maximum value was 94% which is correspond to LbL-CNT-40bi sample. The maximum values of CHF enhancement were obtained as the bilayer number increased. Pool boiling heat transfer of saturated FC-72 was investigated by Wong and Leong [129] on porous lattice structures that consisted of the Octet-truss unit cell geometry. These porous lattice structures were fabricated using the selective laser melting method. The porous lattice structure increased HTC and delayed the CHF compared to the smooth surface. This increase is due to the increase in surface area, increase nucleation site density and capillary-assisted suction of the porous lattice structure. The pool boiling phenomenon does not always occur at a simple boiling chamber. The boiling heat transfer in micro heat exchangers is generally confined in a very narrow space. In such a case, the heat transfer are different from those of conventional unconfined boiling [130]. Several studies have been conducted for pooling boiling in confined space with various space heights and surface characteristics. Souza et al. [131] presented experimental results of the confined and unconfined nucleate boiling for saturated HFE7100 using nanostructured copper heating surfaces. The HTC increased when the confinement increased, corresponding to a decrease in the gap size, mainly for lower heat flux. Foulkes et al. [132] studied confined subcooled pool boiling on nanoengineered surfaces which enables self-assembly of liquid bridges capable of high heat flux dissipation without external pumping. The pool boiling is the main boiling mechanisms occurring in thermosyphons with a filling ratio of 100% [133]. Chiang et al. [134] investigated

combination effects of microgrooved, sintered, and wickless heat pipes using water based magnetic nanofluids with 0.16%–3.20% volume concentration. The results indicated that combining the wick heat pipes with its optimal nanofluids concentration yielded the optimal promotions: the thermal resistance was reduced by approximately 80% and the critical heat flux was enhanced 2.7-fold compared with general wickless heat pipes filled with deionized water.

6 Deposition of nanoparticles Generally, to increase the boiling heat transfer, the heating surface or boiling fluid properties were changed. One of the most common studies in this field is the use of very small suspended particles. In addition to more heat conducting than pure fluid, the deposition of these particles on the boiling surface is the most important influence of the use of such fluids on the boiling process. However, adding small suspended particles in micrometer to base fluids leads to several other problems, such as poor stability, particle accumulation, pressure drop, channel erosion, high pumping costs, and so on. With the advancement of nanotechnology over the past decades, nanoscale particle synthesis is now feasible. Using the suspension of nanoparticles in base fluids, scientists can often overcome the problems encountered in using microparticles. The deposition of nanoparticles enhances roughness, wettability, and the formation of capillary wicking action within the sediment layers. Filho et al. [135] compared the properties of a neat aluminum surface and the aluminum surface coated with -Al2O3 and SiO2 nanoparticles. In order to coating the clean surface of aluminum, the nanofluid containing -Al2O3 and SiO2 nanoparticles with different concentrations boils on the surface. the pool boil the -Al2O3 and SiO2 nanoparticles in different concentrations on the surface. The results showed that the droplets on the -Al2O3coated samples were spread regularly and the samples showed super wetting behavior. While for SiO2-coated samples, wettability is more than the bare aluminum surface, but super wetting behavior is not necessarily seen. Also, droplets on SiO2 samples have been spread as a nonhomogeneous deposition as preferential. The sedimentation of nanoparticles increased the roughness of all the samples, although no clear correlation was found between roughness and wettability. In this section, pool boiling heat transfer performance of nanofluids and the deposition of nanoparticles on the boiling surface and its effect on the heat transfer are discussed. The aqueous nanoparticles can increase the CHF by about 3-fold compared to pure water. Ciloglu [136] experimentally studied the Nucleate Pool Boiling of SiO2 nanofluids From a hemispherical heater. AFM images indicate nanoparticle deposition on the surface. HTC of nanofluids is lower than deionized water; however, it has a higher CHF. Reducing HTC is due to reduced active nucleation sites and the formation of extra thermal resistance created by blocked bubbles in the porous layer formed on the surface. CHF increased due to increased wettability. Ali et al. [137] investigated the boiling of TiO2/water nanofluids in two mass fractions of 12% and 15% TiO2 nanoparticles at relatively low superheat temperatures. For the 12% and 15% weight concentrations, the heat flux enhancement ratios were 1.89 and 2.22, respectively, when

compared with the pure water. Also, the amounts of HTC enhancement for the 15% concentration were higher than those of 12% concentration. Salimpour et al. [138] also stated that the deposition effect is more pronounced for high concentrations of nanofluid. They studied the boiling of iron oxide/deionized water nanofluid on two smooth and rough copper surfaces. The results showed that for the rough surface in high heat flux and for the flat surface in low heat flux, with the formation of deposition on the heater surface increased boiling heat transfer. Stephen et al. [139] also studied the pool boiling of the iron oxide/water nanofluid on the copper substrate. In addition to variations in surface roughness and nanofluid concentration, the heating mode was also studied. They produced two different experiments to reveal the potential effect of nanoparticle deposition: one by varying the heat flux (VHF) and another by fixing the heat flux (FHF) applied. The difference in the heating mode caused a difference in the deposition of nanoparticles and affected the surface morphology, wettability and thermal resistance of the surface, as shown in Fig. 14. Pure water pool boiling curves on these two surfaces showed that wall superheat in the FHF mode was lower than the VHF mode. Also in relatively low surface superheating, the nanocoated surface with FHF mode can reach a higher heat flux than the nanocoating by VHF mode. Akbari et al. [140] investigated the deposition of silver nanoparticles on the copper substrate. Obviously, the most important factor in this regard is the concentration of nanofluids. They stated that with increasing concentrations, the deposition stability and the diameter of the bubble decreases, but the cluster deposition and hydrophobicity increase and the bubbles appear earlier on the nanocoated surface. Finally, the results showed that at higher concentrations, HTC and CHF increased up to 90% relative to the polished copper.

Fig. 14. SEM images of (1) rough and (2) smooth surfaces, before and after nanofluid pool boiling, with two different mass concentrations and for (a) FHF (b) VHF heating modes. Adapted from Stephen et al. [139].

For a plain surface, a continual fouling layer on the surface created a huge thermal resistance and a significant decrease in HTC. Abdollahi et al. [141] stated that the addition of nanoparticles to base fluid does not necessarily mean an increase in HTC. Their investigation was carried out at

different volume concentrations of Fe3O4/water nanofluid. It was observed that HTC at high concentrations decreased with increasing concentrations, and increased with increasing concentrations at low concentrations. The best HTC was observed at a volume concentration of 0.1%. Nikulin et al. [142] used an Al2O3 nanofluid with a non-aqueous isopropanol base. They observed that the effect of adding nanoparticles to the fluid on HTC depends on both the concentration of nanofluid and boiling temperature and HTC can be reduced or increased. During the test, the nanoparticles are deposited on the heater surface, which can affect the bubble departure diameter, bubble departure frequency, and nucleation site density, but due to the complexity of the phenomenon, there is no clear dependence on the nanofluid concentration and boiling temperature. Sarafraz et al. [143] performed an experimental study on the thermal performance of nanofluids with functionalized carbon nanotube (FCNT) and nonfunctionalized carbon nanotube (CNT). Both of them increase CHF and HTC. The fouling formation for FCNT is negligible compared to CNT, it provides more liquid to be absorbed by the porous fouling layer to the surface due to the increased surface wettability. High concentrations of nanofluids and long boiling time can cause large fouling and a decrease in the number of active nucleation sites. Modified surfaces prevent smooth fouling compared to the smooth surface. Song et al. [144] investigated the pool boiling HTC of nanofluid containing Al2O3 on nanoporous surfaces obtained by an an electrophoretic deposition (EPD) method. The results showed that increasing the concentration of nanofluid caused decreasing the HTC compared to that of base fluid. However, with increasing Al2O3 concentration in the EPD process in fabricating nanoporous surfaces, HTC will increase. This increase is due to an increase in the density of the active nucleation sites of the nanoporous surfaces relative to the smooth surfaces. The positive effect of deposition on the heater surface on the pool boiling performance does not only include the deposition of the nanoparticles. For example, Seawater is available and inexpensive that can be widely used in heat exchangers and phase change. Uesawa et al. [145] tested the effect of salts deposition on boiling heat transfer. Saturated pool nucleate boiling heat transfer experiments with NaCl solution, natural seawater, and artificial seawater as well as distilled water were performed. The formation of secondary coalescent large bubbles were suppressed in the experiments with the NaCl solutions, the natural seawater, and the artificial seawaters, and the primary bubbles seemed to detach from the heat transfer surface. The only salt of calcium sulfate in the artificial seawater was deposited on the surface and, by increasing the thickness of the sediment layer due to increased thermal conduction resistance, slow surface-temperature excursion occurred that if it meant the occurrence of CHF, its amount for artificial seawater was less than distilled water.

7 Correlations, models and simulations The researchers have always been looking for comprehensive and reliable mathematical models of effective parameters in the pool boiling heat transfer process. A model that can accurately predict this phenomenon, and in particular HTC and CHF. But due to the complexity of the boiling mechanism, a complete model is not yet available. Finding the mathematical relation

between the surface-specific characteristics and its effect on the boiling process is one of the first models to be considered. The bubbles dynamics also play a decisive role in determining mathematical models. Most models are written based on a detailed examination of the behavior of a single bubble. One of the oldest models presented for nucleate pool boiling is Rohsenow correlation [146]: 𝐶𝑝𝑙𝑇𝑥 ℎ𝑙𝑣

(

𝑞𝐴 𝐻

= 𝐶𝑠𝑓

𝑔𝑐𝜎

)

𝜇𝑙ℎ𝑙𝑣 𝑔(𝜌𝑙 ― 𝜌𝑣)

𝑧

(1)

𝑃𝑟𝑠

where 𝜇𝑙, ℎ𝑙𝑣, 𝐶𝑝𝑙 and 𝑃𝑟 are respectively liquid viscosity (kg/ms), evaporation latent heat (kJ/kg), liquid specific heat (kJ/kgK) and the Prandtl number (𝑃𝑟 =

𝐶𝑙𝜇𝑙 𝑘𝑙

). The C𝑠𝑓 is a function of

the particular heating surface-fluid combination, which Depends on the fluid, roughness, and material of the surface, and surfaces-fluid interacts. The z and s exponent in the original Rohsenow model were equal to 0.33 and 1.7, respectively. Later, these values have undergone small changes. For example, a few years later, Rohsenow [147] considered the value of s for water equal to 1. Vachon et al. [148] introduced new values for z and C𝑠𝑓 based on changes in surface preparation techniques and surface-fluid combination. They used the pool boiling data of eleven separate studies of the literature. The proposed values of the coefficients were estimated using the least squares method. Kiyomura et al. [149] employed the coefficient values of Vachon et al. [148] study which are C𝑠𝑓 = 0.0147 for smooth and C𝑠𝑓 = 0.0107 for rough surfaces. They reported that their experimental data were in good agreement with Rohsenow correlation, and the mean absolute errors (MAE) for the smooth and the rough surfaces were 4% and 7% ,respectively. Pioro [150] experimentally calculated the constant coefficients of Rohsenow 's correlation. They presented the effect of surface material, heat flux, saturation pressure and thermophysical properties of the fluid on HTC as complete tables of data that can be used as a reference for fluid-surface compounds. Nowadays, many literature validates their experimental data with the Rohsenow's equation [10, 13, 14, 20, 22-24, 36, 44, 45, 57, 58, 61, 79, 81, 85, 99, 100, 121, 123, 145]. Li et al. [151] compared their experimental data with predicted values with a semi-experimental equation similar to the Rohsenow's equation. 𝐶𝑝𝑙∆𝑇𝑠𝑎𝑡 ℎ𝑙𝑣

= 0.013𝐶𝑠―0.33

(

𝑞𝑤

0.33

)

𝜎

𝜇𝑙ℎ𝑙𝑣 𝑔(𝜌𝑙 ― 𝜌𝑣)

𝑃𝑟𝑙

𝐶𝑠 which is a function expression of contact angle (𝜃), surface roughness (Ra) and material influence parameter of the heater (𝛾).

(2)

[

𝐶𝑠 = (1 ― cos 𝜃)0.5 1 +

5.45 2

(𝑅𝑎 ― 3.5) + 2.61

]

𝛾 ―0.04 , 𝜃 = max (𝜃,15°)

(3)

0.5 The material influence parameter of the heater is 𝛾 = (𝑘𝑤𝜌𝑤𝑐𝑝𝑤 𝑘𝑙𝜌𝑙𝑐𝑝𝑙) , where 𝜌𝑙, 𝜌𝑣 and 𝜌𝑤 are densities of liquid, vapor and heater respectively, 𝑐𝑝𝑙 and 𝑐𝑝𝑤 are the special heats of the liquid and heater respectively and 𝑘𝑙 and 𝑘𝑤 are the heat conductivities of the liquid and the heater respectively. The model predictions were acceptable for normal nickel plain surface and chemical treated nickel plain surface. However, bigger errors were found in the heat fluxes close to the CHF. For nano-cone array nickel surface, the predicted values of the model are lower than the experimental measurement values. They rewrote this model by multiplying a (a: the ratio of the actual heat transfer area and its projected area):

(4)

𝐶′𝑠 = 𝑎𝐶𝑠

[

𝐶′𝑠 = 𝑎(1 ― cos 𝜃)0.5 1 +

𝐶𝑝𝑙∆𝑇𝑠𝑎𝑡 ℎ𝑙𝑣

=

0.013𝐶′𝑠― 0.33

(

5.45 2

(𝑅𝑎 ― 3.5) + 2.61

𝑞𝑤

]

(5)

0.33

)

𝜎

𝛾 ―0.04 , 𝜃 = max (𝜃,15°)

𝜇𝑙ℎ𝑙𝑣 𝑔(𝜌𝑙 ― 𝜌𝑣)

𝑃𝑟𝑙

(6)

The new equation can predict HTC for a non-plain surface with a simple convex structure until it is not too large. A well-developed model that describes the effect of boiling surface conditions on the active nucleation site density using its boiling characteristics was presented by Yang and Kim [152] : (7)

𝑁 = 𝑁.𝜑(𝛽).𝜑(𝛼) where 𝑁 is the average cavity density and φ (β) and φ (𝛼) are the particular cavity distribution function, which is a function of the cone angle and the mouth radius, respectively: 𝜑(𝛽) =

∫

𝜃2

𝑓(𝛽)𝑑𝛽 , 𝜑(𝛼) = 0

∫

𝛼𝑚𝑎𝑥

𝑓(𝛼)𝑑𝛼 𝛼𝑚𝑖𝑛

f (𝛼) and f (β) based on the cavities measured by SEM and differential interference contrast microscopy (DIC) were obtained. The boundaries of integrals were obtained using the

(8)

entrainment mechanism. There may be a large number of cavities in a wide range of shapes and sizes on a solid surface, but only a part of them can be as active nucleation sites. Bankoff [153] studied the presence of residual gases in cavities and concluded that bubble nucleation originates from the residual gases in cavities. The trapping of bubbles in surface cavities was investigated using two approaches. The first approach, originally proposed by Bankoff [153], is based on geometrical principles. Fig. 15 shows the schematic of vapor trapping in two different conical cavities. It was found that gas can be trapped when the cone angle of a V shaped groove is smaller than the liquid contact angle and the cavities with this condition can act as nucleation sites. Conversely, when the cone angle is larger than the liquid contact angle, the liquid floods the cavity. Therefore, the geometrical necessity of an active nucleation site is defined as: 𝜃 > (180° ― 2ϕ) where θ and (180° ― ϕ) are contact angle and cavity side angle which are shown in Fig. 16.

Fig. 15. Schematic of vapor trapping during liquid movement over a conical cavity. Adapted from Rykaczewski and Phadnis [154].

(9)

Fig. 16. conditions for gas entrapment in liquid movement across a cavity. Adapted from Bankoff [153].

The second approach, proposed by Johnson and Dettre [155] and expanded by Wang and Dhir [156, 157] is based on the thermodynamic criterion of stability. Fig. 17 shows a cavity with two values of liquid contact angles. It was found that for a contact angle larger than the minimum cavity side angle (𝜙𝑚𝑖𝑛), the Helmholtz free energy has a minimum value on/in cavity. The liquid front will move downward until it reaches the position of minimum energy and the gas/vapor will be entrapped. Otherwise, for a contact angle smaller than or equal to the minimum cavity side angle, the Helmholtz free energy decreases continuously as the liquid front progresses downward into the cavity and gas/vapor cannot be trapped. For spherical and conical cavities, the mouth angle is the minimum side angle.

Fig. 17. liquid-gas interface locations in conical cavities. Adapted from Wang and Dhir [156].

In addition, Lorenz [158] employed the above entrapment condition and determined the volume of trapped vapor and the radius of bubble embryo. He found that the bubble embryo radius, which is a function of the contact angle and the cone angle, is the effective radius for nucleation. By presenting a theoretical model, Wang and Dhir [156] predicted the density of nucleation sites for different wettability: 𝑁𝑎 = 𝑃𝑎𝑠.𝑁𝑎𝑠

(10)

where Nas, is the heater surface cumulative cavity density with cavity mouth angles less than a specified value and Pas, is a function of contact angle and cavity mouth angle.

Wang and Dhir [156] recommended that the minimum wall superheat required to activate a cavity with a mouth diameter of dc is 𝑇𝑊 ― 𝑇𝑠𝑎𝑡 =

4𝜎𝑇𝑠𝑎𝑡

𝐾 ℎ𝑓𝑔𝜌𝑣𝑟𝐶

(11)

Where the coefficient K is correlated to the liquid contact angle by

{

1 𝐾 = sin 𝜃

𝜃 ≤ 90° 𝜃 > 90°

(12)

Despite all these studies, the mechanism of pool boiling is still unknown due to the complexity of this phenomenon, and it cannot be justified in the form of simple mathematical formulas. Therefore, in recent years, scientists have studied the numerical simulation of pool boiling and factors affecting it. The most commonly used models for predicting CHF are hydrodynamic instability models. Lu et al. [159] presented a modified hydrodynamic instability model that could show CHF dependence on nucleation site density and finite heater size. This model is wellmatched with a variety of coated surfaces. The hydrodynamic instability model is the most prominent CHF model, but it fails to explain the evident dependence of CHF on surface properties. Chen et al. [160] cites the fact that the thermal properties of the heater surface have a significant effect on heat transfer and evaporation characteristics of the microlayer. They developed a volume of fluid method based algorithm, in which the experimental results of the microlayer structure are considered, the behavior of a single bubble and microlayer evaporation Simulate. By numerical simulation of the quartz glass, stainless steel, brass, and copper as heater surface, they showed that the evaporation of the microlayer is a major mechanism of boiling heat transfer in nucleation boiling. Marcel et al. [161] used an automata model to take into account the effect of contact angle to simulate pool boiling heat transfer. A model of free bubbles as a population of virtual spheres, whose geometric properties expanded with simple stochastic rules. This model was validated with the pool boiling experimental data of the literature. Also, the dependence of heat flux and the activity of nucleation sites on the wall superheat were also reproduced correctly by the model in a statistical sense. The numerical simulation of a single bubble growth in subcooling liquid under different gravity has been carried out by Yi et al. [162]. The results show that the bubble behavior in the numerical model agrees well with previous experiments conducted in high subcooling liquid under microgravity. The growth period and departure radius both reduce with the increase in gravity level, while the critical subcooling increases slightly. Jiansheng et al. [163] perform a numerical simulation of the pool boiling with heated surfaces in different geometries, including the plain heated surface and the modified heated surfaces in different orientations. It is found that a modified heated surface has a better heat transfer performance compared to that of plain heated surface. Furthermore, the modified heated surface with downward-facing has the best heat transfer performance and the lowest CHF.

The lattice Boltzmann method (LBM) has been applied to simulate liquid-vapor phase change. And is a popular model for numerical simulation of a heater surface in pool boiling. Gong and Cheng [164, 165] simulated the effect of wettability on the pool boiling during the entire ebullition cycle beginning from the bubble nucleation process using the phase-change lattice Boltzmann method. The results obtained from this simulation show that at the hydrophilic surface, the bubble is completely departed from the surface with a waiting period in the ebullition cycle, and there is a micro-layer of fluid between the surface and the bubble. At the hydrophobic surface, a residual bubble remains after the bubble departure, and there is no waiting period. Liquid micro-layer is also not formed. They further observed combining wettability simulation. Adding hydrophobic dots on the smooth hydrophilic surface increased bubble nucleation increased heat transfer and reduced nucleation time drastically. Sadeghi et al. [166] used three-dimensional LBM to simulate this phenomenon. This model is able to predict the temperature field inside the bubble. To validate the model, the dynamic behavior of a single bubble was presented. The fit between the bubble departure diameter with gravitational acceleration, surface tension, and the Jacob number was found. They also simulated the departure phenomena of multiple bubbles to demonstrate the functionality of the model. Shin lee and Sang lee [167] considering the fact that the center of the heater has a higher temperature than its surroundings, Patterned wettability In the center of heater are created with a low area fraction of hydrophobic dots and the area fraction gradually increases as the radius from the center increases. A multiphase single component LBM was used for the simulation. They propose was optimized the concentration of hydrophobic dots with respect to CHF and the start of nucleation. Electrohydrodynamics is one of the effective ways to increase CHF. However, the complexity and limitations of the laboratory methods preclude the exact recognition of the effect of the magnetic field on the boiling and the bubble dynamics. Two-dimensional lattice Boltzmann model plus pseudopotential model with phase-change model and an electric field model can validate the pool boiling under the magnetic field. Feng et al. [168] studied the heat transfer details under a uniform electric field using LBM during nucleate boiling and film boiling regimes. Increasing the intensity of the electric field expanded nucleate boiling and increased the CHF and the wall temperature at CHF point. The numerical modeling and simulations of boiling in the previously presented surface structure enhancement techniques such as honey comb, coating, pin-fin and channels is presented in the following paragraphs. Honey comb: Zuber [169] presented a hydrodynamic instability theory of CHF prediction for the infinite horizontal flat plate: 𝐶𝐻𝐹𝑍 ≡

𝜋 14 4 𝜌 ℎ𝑙𝑣 𝑔𝜎(𝜌𝑙 ― 𝜌𝑣) 24 𝑣

(13)

This theory was rational and has been the basis of other studies for many years. Lienhard et al. [170] refined this Zuber expression and introduced a hydrodynamic limit model of CHF prediction for the infinite and finite horizontal flat plate: 𝑁𝑗 𝐶𝐻𝐹 = 1.14 𝐴𝐻 𝜆2 𝐶𝐻𝐹𝑍

(14)

3𝜎 𝑔(𝜌𝑙 ― 𝜌𝑣)

(15)

𝑑

𝜆𝑑 = 2𝜋

where 𝑁𝑗 is the number of escaping vapor jets on a heater of area 𝐴𝐻 and 𝜆𝑑 is the most susceptible Taylor unstable wavelength in a horizontal liquid-vapor interface. Lienhard 's finite experimental plate was an egg-crate structure on a flat heater. Lienhard et al. considered the installation of a gridded structure on the flat heater surface to be the cause of the increase in CHF due to the number of the escaping vapor jets. Mori et al. [117, 171] presented an onedimensional capillary-limit model of CHF prediction for the honeycomb porous plate (HPP): 𝐶𝐻𝐹 =

2𝜎𝐾𝜌𝑙 ℎ𝑙𝑣𝐴𝑤

(16)

𝑟𝑒𝑓𝑓𝜇𝑙𝛿𝑣𝐴𝐻

where 𝐴𝑤 is the contacted area of the honeycomb porous plate with the heated surface, the permeability K and the effective pore radius were determined by experimental measurements ( 𝐾 = 2.4 × 10 ―14 m2, 𝑟𝑒𝑓𝑓=0.13 µm). Actually 𝑟𝑒𝑓𝑓 is obtained from bubbling pressure and Laplace’s equation2𝜎/𝑟𝑒𝑓𝑓. Coating: Arik et al. [172, 173] introduced the thermal management of electronics (TME) correlation that includes the contribution of conduction in the heater together with the hydrodynamics, subcooling, pressure, and length effects: 𝑞𝐶𝐻𝐹,𝑇𝑀𝐸 =

𝑆 𝜋 12 4 𝜌𝑣 ℎ𝑙𝑣 𝑔𝜎(𝜌𝑙 ― 𝜌𝑣) × [1 + 〈𝑐 ― 𝑏 × 𝐿′(𝑃)〉] 𝑆 + 0.1 24 𝜌𝑙 0.75 𝐶𝑝𝑙 ∆𝑇𝑠𝑢𝑏 × 1+𝐵× ( ) 𝜌𝑣 ℎ𝑙𝑣

(

(

where 𝐿′(𝑃) = 𝐿

𝑔(𝜌𝑙 ― 𝜌𝑣) 𝜎

[

)

(17)

] )

and 𝑆 = 𝛿𝑤 𝜌𝑤𝑐𝑝𝑤𝑘𝑤. The first term on the right side of Eq. (17)

represents the classical Kutateladze-Zuber [174, 175] prediction. The second term reflects the

effect of heater thickness and thermal properties. The third term accounts for the influence of the length scale on the CHF and is equal to unity or higher. Quan et al. [176] developed a theoretical model to predict the CHF in pool boiling on a heating surface with micro/nanostructures based on Kandlikar’s CHF model [177] for a smooth heating surface: 𝑞𝐶𝐻𝐹 =

[

1 ℎ𝑙𝑣𝜌𝑣 2

𝜎𝑙𝑣(𝑟 + cos 𝜃 ∗ ) 𝐻𝑏

1 + 𝑔(𝜌𝑙 ― 𝜌𝑣)𝐻𝑏cos 𝜑 2

]

(18)

where 𝐻𝑏 is the height of bubble, which is related to the bubble diameter, and 𝜑 is the tilt angle of the boiling surface. Nabati et al. [178] used computational fluid dynamics (CFD) for modeling of the dynamics of bubbles in water pool boiling on porous coated surfaces. The deposition of MWCNT, CuO and Al2O3 nanoparticles in a water based nanofluid form a porous layer on the heating surface. The CFD results showed that a comprehensive change in the number of bubbles, growth time of bubbles, surface wettability and boiling performance were made by the presence of porous layers. Micro-pin-fin: As shown in Eq. (19), Ho et al. [121] used the Rohsenow's equation with modified values of the constants (C𝑠𝑓, n and s) which varied in accordance to the different conditions to account for the effects of surface-fluid interactions during the boiling process. Using the Gauss-Newton method, the various C𝑠𝑓 and n values were computed for plain, microcavity and the micro-fin structured surfaces by setting s=3.5. By using the C𝑠𝑓 and n values, nonlinear regression analyses were performed and the relationships between C𝑠𝑓, n and r are established as shown in Eqs. (20) and (21). Where r is the surface roughness factor.

[

𝑞" = 𝜇𝑙ℎ𝑙𝑣

𝑔(𝜌𝑙 ― 𝜌𝑣) 𝜎

1 2

][

C𝑠𝑓ℎ𝑙𝑣𝑃𝑟𝑙𝑠

C𝑠𝑓 = (3.21 × 10 ―5).𝑟0.0469 n=1.2163. 𝑟 ―0.0743

]

𝐶𝑝𝑙(𝑇𝑤 ― 𝑇𝑠𝑎𝑡)

1 𝑛

(19)

(20) (21)

Liu et al. [179] proposed a model considering the coalesced bubble departure frequency, Jakob number at CHF and the capillary wicking effects.

𝜌𝑙 ℎ𝑙𝑣 𝐶𝐻𝐹 = 1.57 × 10 ―6. 𝜌𝑙 ℎ𝑙𝑣𝑓 2𝜎 𝑔(𝜌𝑙 ― 𝜌𝑣).𝐽𝑎0.32 + 0.189 (𝑑𝑉 𝑑𝑡)𝑡 = 0 𝐴𝑤

(22)

where 𝐽𝑎 is the Jakob number (𝐽𝑎 = 𝜌𝑙𝐶𝑝𝑙∆𝑇𝑠𝑎𝑡/𝜌𝑣 ℎ𝑙𝑣), (𝑑𝑉 𝑑𝑡)𝑡 = 0 is the initial volume flow rate (m3/s) and 𝑓 is coalesced bubble departure frequency (1/s). To validate the modified model, the experimental data and the predicted CHFs were compared for the micro-pin-finned surfaces with FC-72 as the working fluid. It was found that the predicted results agree well with the experimental data within ±8%. Kim et al. [13] performed a fin analysis for description of the total heat transfer of the structured surface. 𝑞 = ℎ𝐴(𝑟.𝜂°)∆𝑇

𝜂° = 1 ―

𝑚𝐴𝑓 (1 ― 𝜂𝑓) 𝐴𝑡

(23) (24)

where 𝜂° is the overall fin efficiency of the micro-pillar array. However, it is not complete analysis for the boiling heat transfer on the structured surface. In order to find more measureable explanation for the boiling heat transfer, bubble dynamics included analysis would be required. Ma et al. [180] simulated boiling heat transfer from micro-pillar heat sinks with different wettability using the liquid-vapor phase-change LBM. Effects of pillar geometry and wettability on bubble dynamics are studied. The simulation results illustrate that these parameters significantly influence the CHF and the Leidenfrost temperature, resulting in pool boiling curves with extensively different forms. Microchannel:Walunj and Sathyabhama [113] developed a semi-analytical model for prediction of bubble departure diameter from the microchannel geometries considers the unsteady growth force (Fduy), buoyancy force (Fbuy), surface tension (Fst), lift force (FL) and bubble inertia (Fbi). The force balance equation can be written as:

Fduy+ Fst+ Fbi= Fbuy+ FL

(25)

The schematic explanation of the forces acting on the growing bubble is presented in Fig.18. By placing the expression of the different forces in force balance equation, the bubble departure diameter can be calculated. The model predicts their experimental data with mean absolute error of 5.58 %. Patil and Kandlikar [11] developed a liquid microcirculation based theoretical model to predict the local HTC for different locations of a parallel microchannels. The local Nusselt

number (𝑁𝑢𝑥,𝐻𝑇𝐶) under constant heat flux boundary condition was calculated using Eq. (26) from [181]:

[

1 𝑁𝑢𝑥,𝐻𝑇𝐶 = ― 6

∞

∑

]

1 ― exp ( ― 4𝑛2𝜋2𝑥 ∗ )

𝑛=1

𝑛2𝜋2

𝑥

where 𝑥 ∗ , is defined as: 𝑥 ∗ = 𝑃𝑒𝐷ℎ

―1

(26)

(27)

and x is distance from the entrance, 𝑃𝑒 is the Peclet number and 𝐷ℎ is the hydraulic diameter. Using the value of 𝑁𝑢𝑥,𝐻𝑇𝐶, HTC was evaluated using the definition of Nusselt number: 𝑁𝑢𝑥,𝐻𝑇𝐶 =

ℎ𝐷ℎ

(28)

𝑘

Fig. 18. Bubble growth in Rectangular Microchannel. Adapted from Walunj and Sathyabhama [113].

8 Conclusion Knowledge of the boiling phenomenon and the design and optimization of such processes require the precise prediction of the boiling heat transfer coefficient (HTC) between the heating surface and the boiling liquid. The boiling heat transfer is a complex phenomenon and the prediction of this process under different conditions can be difficult. Scientists are looking at empirical methods to increase boiling heat transfer, analyze data and present mathematical

models. This complexity makes it difficult to compare the methods of increasing heat transfer in controlled experimental conditions and introducing a method as a superior method. Therefore, new techniques are always being presented by researchers, and each research has its own strengths and weaknesses. In different cases with the required operational characteristics, due to the disadvantages and advantages of each design, an optimum method can be chosen, for example, a technique that reduces the surface temperature in electronic cooling. When surface modification leads to the generation of smaller bubbles and higher bubble departure frequency, the bubble dynamics actually improves. Surfaces with prominent peaks, sharp edges of pin-fins and channels, and the hydrophilic and hydrophobic regions interfaces, increase nucleation sites and provide more of this opportunity. When the liquid fills the dry spots quickly, the boiling heat transfer is better and the later critical heat flux (CHF) occurs. Interconnecting holes of porous surfaces, interiors of channels and tunnels, reentrant cavities and honeycomb surfaces create the best water supply ways. Improvement techniques for surface modification are very diverse, and it is clear that surface modification plays a more effective role in the boiling process than fluid properties. In order to achieve better heat transfer performance, it is better to use nanofluids instead of the base fluid along with surface modification, since in most cases nanoparticle deposition leads to more favorable results. The nanoparticles deposition makes the smooth surfaces non-uniform and makes the modified surfaces more porous and rough. Such active nuclear nucleation sites increase. An important point in examining heater surfaces is to find methods for improving surface properties that are simple and inexpensive to be used on a large industrial scale. Therefore, the economic benefits of increasing heat transfer are higher than the costs associated with changing the heat transfer surfaces.

9 Acknowledgment The authors acknowledge the funding support of Babol Noshirvani University of Technology through Grant program No. BNUT/388003/97. The authors would also like to thank the National Iranian Oil Engineering & Construction Co. for their financial support of this project.

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The authors declare that no conflict of interest is involved in the present work.

Highlights  This review includes effects of surface modification on pool boiling heat transfer, HTC and CHF.  The modified surfaces are collated and categorized.  Mechanisms of bubble dynamics and boiling heat transfer enhancement by surface modification are explored.  The techniques used to fabricate modified surfaces are mentioned.  Pool Boiling models and simulations of surface characteristics and bubble dynamics are analyzed.