Experimental study of nucleate pool boiling heat transfer of water by surface functionalization with crystalline TiO2 nanostructure

Accepted Manuscript Research Paper Experimental study of nucleate pool boiling heat transfer of water by surface functionalization with crystalline TiO2 nanostructure S. Das, B. Saha, S. Bhaumik PII: DOI: Reference:

S1359-4311(16)33460-3 http://dx.doi.org/10.1016/j.applthermaleng.2016.11.135 ATE 9546

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

31 January 2016 31 October 2016 18 November 2016

Please cite this article as: S. Das, B. Saha, S. Bhaumik, Experimental study of nucleate pool boiling heat transfer of water by surface functionalization with crystalline TiO2 nanostructure, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng.2016.11.135

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Experimental study of nucleate pool boiling heat transfer of water by surface functionalization with crystalline TiO2 nanostructure S. Dasa, c*, B. Sahab and S. Bhaumikc a

Department of Chemical Engineering, National Institute of Technology Agartala, Jirania, West Tripura 799046, India. b Department of Physics, National Institute of Technology Agartala, Jirania, West Tripura 799046, India. c Department of Mechanical Engineering, National Institute of Technology Agartala, Jirania, West Tripura 799046, India.

Abstract The recent astonishing development in nanoscience created a profound space of works for state-of-the-art technology in conserving the energy by reducing the dissipation in energy conversion devices. An enhancement in boiling heat transfer is reported in this article through surface modification of Cu substrate by crystalline TiO 2 nanostructure coating. The TiO2 nanostructure coating was done by electron beam evaporation technique on copper substrate at different thickness.

The coated surfaces were studied by X-ray diffraction, field emission

scanning electron microscopic measurement, transmission electron microscopic measurement and atomic force microscope measurement. Crystalline TiO2 nanostructure modified surfaces have been employed for the studies on nucleate pool boiling and an excellent enhancement was observed in heat transfer coefficient. The mechanism of enhancement of heat transfer coefficient in nucleate pool boiling with TiO2 nanostructure modified surface involves several effects, which includes enhancement in effective surface, facilitating generation of larger nucleation sites and enhanced surface wettability. The effect of the thickness of TiO 2 nanostructure coating on heat transfer during nucleate pool boiling was extensively studied and this indicates that the boiling heat transfer coefficient increases significantly with increase of TiO 2 coating thickness. The 1

nanostructured surfaces in repeated test runs for the heat transfer process was found to remain stable with almost same heat transfer coefficient after three test runs. Key words: TiO2, nanostructure, pool boiling, heat transfer coefficient *Corresponding author’s Email: [email protected] 1. Introduction Improvement in boiling heat transfer is one of the most significant approaches in the recent activities of research with energy conversion devices in order to reduce the energy consumptions. The enhancement of the efficiency of the systems (engine, re-boiler, evaporator, heat exchanger, micro and nanoelectronic device) involving the phase change heat transfer mechanism attracts intense attention of the researchers. There are plenty of opportunities to bring an excellent control over the thermodynamic processes by introducing crystalline and nanostructured surfaces at the boiling interface of liquid and solid. Recently organic and inorganic phase change materials have obtained extensive attention because of their higher latent heat, suitable phase-transition temperature and stable physical and chemical properties [1–3]. Many reports on the enhancement of heat transfer by using newly developed materials or structures combined with pool boiling are coming up [4-11]. Surface structure has been recognized to play a critical role in the enhancement of nucleate boiling heat transfer [12-14]. Generally, the boiling enhanced surfaces are created to have more micro-cavities in order to increase the vapor/gas entrapment and active nucleation site density. Thus developing an economic and sustainable process of surface engineering to improve heat transfer performance based on crystalline nano material creates a potential platform of research in this field. The aim of this work was to investigate the effect of surface functionalization by introducing a TiO2 thin film layer on copper surface over the pool boiling heat transfer 2

performance. Oxide materials are much stable compound and therefore find significant importance as a coating material for boiler or such energy conversion devices. Different oxide materials such as Al2O3, TiO2, SiO2, ZnO and CuO [15-20] have been used as coating for surface functionalization. The TiO2 micro/nanostructured surface is highly stable structure in hydrothermal environment [21, 22]. Secondly TiO2 is an excellent oxide material which crystallizes easily and has better impressing mechanical properties making it suitable for heat transfer and boiling applications [23] Different deposition techniques such as DC sputtering [24] radio frequency magnetron sputtering [25-28], pulsed laser deposition technique [29, 30], microwave assisted deposition [31], spray pyrolysed deposition [32]

and electron beam

evaporation technique [33-34] are usually employed to prepare the nanocrystalline thin film of oxides. In this approach thin layer of TiO2 was deposited by electron beam evaporation technique over the copper surface. TiO2 nanostructure was deposited at different thickness, as it was an objective to investigate the effect of coating thickness on pool boiling. The effect of the crystalline nanostructured TiO2 surface and its thickness on the pool boiling were investigated systematically and significant increase in the heat transfer coefficient and boiling performance have been reported. This kind of surface engineered material through nanostructure coating with an objective to reduce the energy dissipation from energy conversion devices must find its significance in the recent context of conservation of energy. 2. Experimental details 2.1 Deposition of thin film The thin film of TiO2 was deposited on pure copper substrate through electron beam evaporation technique. TiO2 target material (MTI, USA) of high purity (99.999%) was used for 3

synthesis of crystalline TiO2 thin film. Proper cleaning of the copper substrate was done using electronic grade acetone and 18 MΩ DI water. The deposition was carried out at a base pressure of 2×10−5 mbar inside the e-beam evaporator chamber achieved by rotary and diffusion pump combination. A low deposition rate of 1.2 Å/s was kept constant. During deposition of the TiO2 thin film, a digital thickness monitor was used to maintain desired thickness and films of different thickness around 250 nm, 500 nm 750 nm and 1000 nm were prepared. The substrates were used at a constant azimuthal rotation of 120 rpm and orientations of 85° with respect to the perpendicular line between the metal source and the planer substrate holder. 2.2 Characterization The TiO2 thin films deposited on copper substrate were characterized by employing various

advanced techniques

for

their

structural,

morphological and

compositional

investigations. The structural investigations were carried out by X-ray diffraction (XRD) measurements using (Bruker, D–8 Advance) and transmission electron microscopic (TEM) measurements (model: Philips- CM200), while the morphological studies were done by employing scanning electron microscopic (SEM) measurements (model: Jeol, JSM-7600F) and atomic force microscopic (AFM) measurements (model: Bruker, Multi Mode-8). The energy dispersive X-ray measurements (model: Hitachi S-4800) were done for compositional investigations.

Contact angle for deionized water at different surfaces were measured by

Microscopic contact angle meter (MCA 3, Kyowa Interface Science). 2.3 Experimental Facility The boiling heat transfer coefficient was measured for each of the TiO2 coated surfaces along with normal and treated Cu surfaces. The polishing of Cu surface was performed using water emery paper grits-2000. The boiling heat transfer coefficient measurements were 4

performed through nucleate pool boiling system. A schematic representation of the nucleate pool boiling experimental apparatus and the actual photographic view of the system are shown in Fig. 1. It mainly consisted of a test boiling chamber, heating block, ceramic insulator block, condenser, liquid-vapor separator and surge tank. A copper heat transfer block embedded with a heater unit at the lower section was built and submerged in the test fluid (water), which was then heated, up to its saturation temperature 100o C at 1 atm. The test surface (circular diameter 0.009 m) is fitted on the top of the copper block and it was directly placed in contact with test fluid, while the other surfaces were protected from heat loss with Teflon insulation. The experiment was conducted in steady state condition, while the outer surface temperature of the insulated surface was found to be very close to room temperature. This confirms that no significant radial heat flow was practically occurred. The reduced diameter (0.009 m) in the sample substrate was kept only to create provision of tightening by Teflon and screw, ensuring tight contact between copper heating block and sample substrate without any air gap at bottom. The cartridge heater was placed on the bottom of the copper block substrate and could supply heat up to 1000 W. A variable transformer was used to control the heat flux by regulating the input voltage during the experiments. Three k-type thermocouples with a diameter of 0.001 m were embedded inside the copper block to measure the temperature of heating copper block at three different positions (T1, T2, and T3) as shown in Fig 1(d). The linear temperature profile (ΔT/Δx) was observed, confirming the uniform axial flow of heat. Using one dimensional heat conduction equation, the temperatures Tx, Ty and hence the surface temperature Ts, at the interface of test liquid and the TiO2 nanostructured surface was calculated and the system is depicted in Fig. 1(e). The saturation temperature of the test fluid (T L), was recorded by a thermocouple located about 0.009 m from the boiling surface in the pool. In order to observe the status of bubbles on the boiling 5

surface, a window of diameter 0.1 m was installed on both side of the stainless steel vessel. An internal and external condenser was used inside and on the top of the boiling pool to maintain the system under atmospheric pressure. Before starting the boiling experiment and related measurements the whole boiling vessel was heated by an external auxiliary heater surrounding the vessel to ensure that the bulk test fluid was kept at a temperature just below the boiling temperature. The auxiliary heater is turned off when the primary heater is started for measuring heat flux and other heat transfer related data. 2.3.1 Data Reduction One dimensional Fourier law of heat conduction was employed to obtain the heat flux and surface temperature. The corresponding resistance diagram is depicted in Fig. 1(d). The rate of heat transfer (Q) in section (I), section (x) and section (y) are calculated by using the Fourier heat conduction equation 1, equation 2 and equation 3 respectively

QI =

QX =

Qy =

k 3x ×A3x (T3 -Tx ) Δz3x

(1)

kxy ×A xy ×(Tx -Ty )

(2)

Δz xy k ys ×A ys ×(Ty -Ts )

(3)

Δz ys

The equation from 1 to 3 is used to obtain the surface temperature of the copper substrate T s with the following expression.

Ts =T3 -

VI  Z3 x  K  A3 x

  Z xy      Axy

  Z ys      Ays

 VI    T3 - δ K   6

(4)

Where, V= Volt; I= Current; Ts = Surface temperature; T3 = Temperature at location 3; ΔZ3x ,ΔZxy ,ΔZys = Thickness at different section of I, X, and Y respectively and

A3x ,A xy ,A ys = Cross sectional areas of I, X, and Y sections respectively

The heat flux provided to the system, q, was calculated from

q=

Q AP A HS

(5)

The boiling heat transfer coefficient, h, was calculated by using Newton law of cooling,

h

q q  T Ts  TL

(6)

2.3.2 Uncertainty and error analysis The uncertainty of heat flux and heat transfer coefficient was calculated by two different methods proposed by Schultz and Cole [35] and Kline and McClintock [36]. To determine the uncertainty the dependent variable ‘y’ is expressed as a function of independently measured quantities (x1, x2, x3,…..xn). Mathematically, y = f(x1, x2, x3,…..xn);

(7)

The uncertainty, uy associated with the variable x is defined as the value of the maximum expected deviation from the experimental values and expressed as 2 2 2  y  y    y  u y   ux 1    ux 2   ................   ux n   x1   x2   xn 

7

1

2  

(8)

Where u x 1 , u x 2 , u x 3 , ......., u x n are uncertainty in the independent variables x1, x2, x3, ……. , xn respectively. The uncertainty in heat flux and heat transfer coefficient was calculated using the uncertainties (Table. 1) in the different parameter. The estimated uncertainties of power input, heat flux, and heat transfer coefficient using both the methods mentioned above [35, 36] were found within 5.2-6.5 %, 7.3-8.5 % and 9.5-10.7 % respectively. Further the possible error introduced due to calculation of heat flux using voltage and current relation over that obtained from series thermal resistances has been computed and found as 2.03%.

3. Result and discussions The results of experimental investigation of nucleate pool boiling characteristics with normal and treated Cu surfaces and TiO2 nanostructure coated surfaces under atmospheric pressure condition have been discussed in this section.

3.1.1 Structural characterizations The structural characterization of the TiO2 thin film coating was done from the XRD measurements and is shown in Fig .2. The TiO2 coating was found crystalline with occurrence of the peak at 2θ = 38.9° corresponding to (112) Miller plane of TiO2. The other three prominent peaks occurred at 2θ value of 43.2°, 50.3° and 73.9° correspond to the Cu substrate. All the films of different thickness are found well crystalline with the occurrence of same plane. The crystalline phase of the coating layer stands improving the thermal conductivity of the layer by 8

phonon transport. The SEM images of the TiO2 coatings of different thickness are shown in Fig. 3. The SEM images depicts that the TiO2 films have fractal structure leading to surface cavities. The cavity depth increases with the increase in the thickness of the films and such cavities are ideal as active sites for bubble formation during boiling. The samples were further investigated from TEM measurements which indicate that the TiO2 crystals were grown with a rod like structure as shown in Fig. 4. This rod like structures facilitates the formation of deep cavities between such rods. The surface morphology of the films was investigated from the AFM images as shown in Fig. 5. The AFM images show that the film surfaces are uniformly distributed with TiO2 nanostructure. The sizes of the nanostructure show slight variation with film thickness. These surface cavities with distributed nanostructure over the surface controls the bubble dynamics leading towards high bubble frequency. 3.1.2. Compositional studies The TiO2 coated surfaces were studied through energy dispersive X-ray (EDX) for compositional investigations. A typical EDX spectrum of the TiO2 coated Cu surfaces was shown in Fig. 6. It is observed from the EDX spectra that each of films contains titanium, oxygen and copper. The peak of copper appears from the Cu substrate. The appearance of strong peak of oxygen confirms the formation of oxides at the surface. 3.1.3 Wettability Characterization To examine the hydrophilic properties and its variation with TiO 2 nanostructure coating thickness on the Cu surface the wettability measurement was done. The wettability characterization was carried out through contact angle measurement. Using water as test fluid at room temperature (25°C) and at atmospheric pressure contact angle measurement was done by sessile drop method. Microscopic Contact Angle Meter was used for this measurement. The 9

dynamic contact angle values for different time interval with images are shown in the Table .2. The time intervals selected in this studies of contact angle are t = 0 ms, 50 ms, 100 ms, 150 ms, 200 ms and 250 ms. From the Table 2, one can observe that the TiO2 coating surfaces had the higher dynamic contact angle, with a trend of increase, with increasing coating thickness. It was also observed that the normal surface had taken the maximum time to completely wet the surface, whereas amongst the TiO2 coating surfaces, the time tends to decrease, as the thickness of the coating increases. 3.2 Boiling curve analysis of heat transfer surfaces The boiling curves and heat transfer coefficients of saturated water pool boiling using normal and treated Cu surface and different TiO2 coated surfaces are shown in Fig. 7(a) and Fig. 7(b) at various heat flux conditions. Fig.7 (a) shows that, as heat flux increases, the wall superheat reduces for nanostructure coated surfaces as compared to normal surface. The reduction in wall superheats occurs chronologically with the increase of TiO2 coating thickness at the same heat flux. The maximum reduction (42.5%) of wall superheat is obtained for 1000 nm TiO2 coated surface at high flux of 2357 kWm-2 . It is observed from Fig.7 (b) that, with the increase of heat flux, there is a slight decrease in heat transfer coefficient up to the heat flux of 500 kWm-2 and then it starts increasing sharply. The heat transfer coefficient was found higher for higher thickness of nanostructure coated surfaces. The maximum enhancement of heat transfer coefficient is obtained for 1000 nm TiO2 coated surface at high heat flux values. Thus, remarkable improvements in the heat transfer coefficient have been brought with TiO2 coating and its thickness variation. The improvement of the nucleate boiling heat transfer coefficient in the TiO2 coated surfaces are due to an increase in the number of active nucleation sites. Further this enhancement of the heat transfer coefficient with TiO 2 coating may be 10

attributed to the combined effects of the interaction between active nucleation sites, the increase of bubble generation frequency, and the intensification of bubble interactions. The rough nanostructured surface increases the nucleation site density leading towards high bubble frequency from nanostructured surfaces, which accelerates the heat transfer, reducing the wall superheat. [12-15]. 3.3 Comparisons of our experimental data with other data and correlation from literature 3.3.1. Present experimental results with others experimental data from literature In Fig. 8, the experimental results of this work of pool boiling has been documented and compared with the significant published results by other research groups. It can be seen here that the present experimental results are either similar or better than the previous literature data, for instance those reported by, Shi et al. [8], Tang et al. [14], Forrest et al. [18], Jo et al. [19], Huang et al. [37] Feng et al. [38], Lee et al. [39] and Kim et al. [40]. As a continuous process of scientific development in nucleate pool boiling, the e-beam evaporation technique produced TiO2 layer offers excellent adhesiveness with the Cu surface so that it will be very suitable in the real field application. Secondly bringing control over the surface properties with different parameters of e-beam evaporation provides opportunities to adjust the porosity, wettability, crystallinity etc. Moreover TiO2 nanostructure offers higher nucleation density with higher stability in hydrothermal environment, making it a suitable candidate to be applied in high heat flux energy conversion devices. 3.3.2 Present experimental data with well-known correlations from literature The experimental results of boiling heat transfer coefficients for 1000 nm TiO2 nanostructure coated surface and the normal copper surfaces are compared with the well-known 11

correlations reported by Rohsenow [41], Pioro [42], Kutateladze and Borishanski [43], Labuntsov [44], Kruzhilin [45], Stephan and Abdelsalam [46], and Li et al. [47]. Rohsenow [41] C p T h fg

 q  Csf   h fg

and h 

  g (    g ) 



0.33

 Cp   k   

n

(8)

q , where Csf is the constant, depend upon the nature of the heating surface-fluid T

combination and n is the power. Pioro correlation [42] 2

Nu  Csf* K 3 Pr n 2

 3 hl q  *  Or  Csf  Pr n 0.25  0.5 k  h fg  g  g (    g )  

(9)

Kutateladze and Borishanski correlation [43]:

K f  1104 qP  f h  0.44  l*  g ifg  g  f  f   g

0.7

 0.35  Pr 

(10)

Labuntsov correlation [44]:   g h  0.075 1  10    f  g   

  

0.67

0.33  2  k 0.67   q    (Tsat  273.15)  

Kruzhilin correlation [45]:

12

(11)

Kf  I f gq f h  0.082  l  g (Tsat  273.15)k f  f   g

  

0.7

 Tsat  273.15 c p f  i 2fg  g2i f 

  

0.33

Pr 0.45

(12)

Stephan and Abdelsalam correlation [46] for water:

h

C (Tw  Tsat )3.058

(13)

(Tw  Tsat )

Li et al. [47]:

qw  518503Csf

kl3.03 (hl l )2.03

g (  l   )



Tsat3.03

(14)

All these selected nucleate pool-boiling correlations are well known and widely used in engineering practice. For the normal Cu surface and TiO2 coated surface, the error of Rohsenow [41], Pioro [42], Kutateladze and Borishanski [43], Labuntsov [44], Kruzhilin [45], Stephan and Abdelsalam [46], and Li et al. [47] correlations based on the present experimental results are presented in Table.3, and Fig.9. The Rohsenow [41] correlation is found to be best fitted to present results for both surfaces. The difference between the measured and predicted nucleate boiling heat transfer coefficients at the low heat flux condition was larger than the high heat flux condition due to the augmentation effect. 3.4 Boiling performance analysis of heat transfer surfaces 3.4.1 Effect of surface coating thickness The heat transfer through the TiO2 coated Cu substrate during nucleate pool boiling was studied for different thickness of the coating. It was observed that the boiling heat transfer coefficient respond to the thickness in a significant manner. This dependence of heat transfer coefficient is controlled by a number of parameters which in turn are controlled by the thickness. 13

These parameters are surface cavities, deep cavities, cavity size and cavity spacing. The variation of heat transfer coefficient with different surfaces (normal Cu, treated Cu, TiO2 coated Cu) have been depicted in Fig. 10 which was performed for different heat flux. The figure clearly reveals the increase of heat transfer coefficient with increase in TiO 2 coating thickness. The increased thickness of TiO2 gives formation of deep cavities which controls the bubble formation and release dynamics. The deep cavities lead to the entrapment of vapor which creates high pressure inside the cavity and facilitates the faster release of the bubble. Thus with increasing thickness of TiO2 film, the overall heat transfer coefficient increases. 3.4.2 Effect of surface wettability Interactions between liquids and solids are typically characterized by the wetting angle that a liquid droplet makes on the solid surface. A contact angle less than 90° indicates that wetting of the surface is favorable, and the fluid will spread over a large area on the surface; while contact angles greater than 90° generally means that wetting of the surface is unfavorable so the fluid will minimize its contact with the surface and form a compact liquid droplet. The surface wettability and hence the contact angle plays a significant role in the heat transfer process at the solid liquid interface. In this study, the surface contact angles of DI water on TiO2 coated surfaces have been measured. The results indicate that the surfaces with greater TiO2 film thickness have more hydrophilic property as shown in Fig. 11. The increase in boiling heat transfer coefficient with increasing film thickness is in good agreement with the theoretical model proposed by Dhir and Liaw [48], claiming that the heat transfer coefficient on the boiling surface increases as the surface contact angle decreases. Further the hydrophilic surface reduces bubbles attaching time [49], which increases the bubble frequency and enhances heat transfer. This is reasonable as heat transfer in boiling is dominated by bubble dynamics [48] rather than 14

heat conduction. Furthermore, because of the hydrophilic property, the evaporation micro-layer of bubbles spreads larger, resulting in a smaller dry area and higher evaporation efficiency [50]. 3.4.3 Effect of Surface Roughness Atomic Force Microscope (AFM) was used to measure the surface real area, surface roughness, in coated and uncoated surfaces of six test surfaces. AFM images provide the scope of accurate analysis of 2D images of surfaces. In measuring the real area (Aa), all the peaks and valleys are considered, whereas the projected area (Ap) includes only the area of flat surface specimen. The ratios of these two areas for all heating surfaces are listed in Table 4. From Table 4, it is clearly observed that as increases the real area to projected area ratio, the heat transfer coefficient increases for TiO2 coating surfaces with a significant response with the coating thickness [51]. The surface roughness increases with the increasing TiO2 coating thickness because of the creation of more peaks and valleys on surface. This factor plays a vital role in achieving the enhancement of heat transfer coefficient with increasing thickness of the TiO2 coating and consequently we observed maximum heat transfer coefficient for 1000 nm TiO2 coated surface. 3.4.4 Morphological stability with hydrothermal environment In order to study the consequences of repeated runs of boiling with TiO2 coated test surfaces to its heat transfer performance, the system was allowed to undergo three test runs. The boiling curves for three consecutive test runs are shown in Fig. 12(a), where normal cu surface and 1000 nm TiO2 nanostructure coated surface have been selected for this study. As the runs were repeated, the boiling curves for both the surfaces were slightly shifted to the right at all wall superheat condition. Fig. 12 (b) shows the heat flux vs. heat transfer coefficient plot for 15

three successive runs for normal Cu surface and 1000 nm TiO2 coated surface. This shows that with increasing number of repeated tests, the nucleate boiling heat transfer coefficients for both the surfaces decreased. The average decrease of the nucleate boiling heat transfer coefficients between run 1 and run 3 appeared to be approximately 8.9 % and 5.6% for the normal Cu surface and 1000 nm TiO2 nanostructure coated surface respectively. This decrease of heat transfer coefficients becomes insignificant after third run. During first three runs the changes appeared, may be due to the change of micro and nanostructure of test surfaces caused by the boiling operation with saturated water. The FESEM image of the titanium oxide nanostructure surface, before test run and after test runs are shown in Fig. 13 and Fig. 3. After the pool boiling experiment, the surface was greatly transformed from a high ordered nanostructure to blunt structured. As the number of repeated test runs was increased, the structures became more densely populated; the pores between the nano-structures were filled and smoothed. The FESEM images revealed that the changes of the pool boiling heat transfer performance may be due to the change of nanostructure caused by the boiling operation with saturated water.

4. Conclusion The effect of TiO2 nanostructure coating on Cu surface to the nucleate pool boiling was investigated in this article. A significant increase in the boiling heat transfer coefficient has been observed on TiO2 thin film coating. The TiO2 film was deposited on the Cu substrate by electron beam evaporation technique at different thickness controlled by film deposition time. On structural characterization the films were found to be crystalline with the formation of deep cavities as revealed from SEM studies. The TiO2 coated Cu surfaces were investigated by TEM and AFM measurement which reveals the uniform distribution of TiO 2 nanostructure over the 16

surface with deep cavities. Such surface brings a control over the bubble dynamics leading towards high bubble frequency. The EDX measurements further confirms the formation of oxide of TiO2. The crystalline structure of TiO2 on the other hand improved the thermal conductivity of the film facilitated by phonon assisted heat transfer. The effect of TiO2 film thickness on the heat transfer coefficients was investigated systematically and it was found to increase with increasing thickness. This profound improvement in heat transfer is also resulted from the enhanced surface wettability and faster bubble release frequency from the TiO 2 coated surface. Thus this report is expected to bear a great significance owing to the recent global demand of efficient energy conversion and also exploring the possible application of surface engineering with the advent of recent outstanding development in nanoscience and nanotechnology.

Acknowledgments The authors are thankful to SAIF, IIT Bombay for SEM measurement and IIT Patna for EDX, XRD and Microscopic contact angle measurements of all surfaces. The authors are also thankful to NIT Agartala financial support under TEQIP and Central Research Facility (CRF) NIT Agartala for providing AFM measurement facility.

17

Nomenclature V

Voltage

Nu

Nusselt number

I

Ampere

K

Kutateladze number

q

Heat flux (kW m-2)

F

Frequency

k

Thermal conductivity (kW m-1 k-1)

Subscripts

H

Heat transfer coefficient (kW m-2 K-1)

f

Fluid

Q

Heat transfer rate (W)

g

Gas

D

Diameter (m)

in

Inlet

T

Temperature (K)

HS

Heating Surface

G

Gravitational acceleration (m s-2)

s

Surface

Cp

Specific heat(J kg-1 k-1)

AP

Applied power

hfg

Latent heat for phase change (J kg-1)

out

Outlet

Ra

Arithmetic-average roughness

f

Fluid

P

Pressure [Pa]

l

Liquid

Z

Thickness [m]

b

Boiling

t

Time [ms]

CA

Contact angle

Csf

Coefficient in the Rohsenow

sf

Surface–fluid

p

Predicted

m

Measured

cr

Critical

correlation [see equ. (10)]

Csf*

Coefficient in the Pioro correlation[see equ.(11)]

l*

Pool-boiling characteristic dimension [see equ. (12)]

Csf

Coefficient in the Rohsenow 18

correlation [see equ. (10)] Greek Symbols

Abbreviations

ρ

Density (kg m-3)

CHF

Critical heat flux

σ

Surface tension (N m-1)

HTC

Heat transfer coefficient

μ

Viscosity (kg m-1 s -1)

SEM

Scanning electron microscope

ʋ

Kinematic viscosity (m2 s-1)

XRD

X-ray diffraction



Surface free energy

TEM

Transition electron microscopy

Δ

Difference

EBD

Electron beam deposition

θ

Contact angle (degree)

EDX

Energy dispersive X-ray



Thermal diffusivity (m2s-1 )

MCA

Microscopic contact angle

Dimensionless Numbers Re

Reynolds number

Pr

Prandtl number

19

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[10] E. Demir, T. Izci, A.S. Alagoz, T, Karabacak, A. Kosar, Effect of silicon nanorod length on horizontal nanostructured plates in pool boiling heat transfer with water, Int. J. Thermal. Sci. 82 (2015) 111–121. [11] S. Das, D.S. Kumar, S. Bhaumik, Experimental study of nucleate pool boiling heat transfer of water on silicon oxide nanoparticle coated copper heating surface. Appl. Therm. Eng. 96 (2016) 555–567. [12] A. Sathyanarayana, P. Warrier, Y. Im, Y. Joshi, A.S. Teja, Pool Boiling of HFE 7200–C4H 4F6O

Mixture on Hybrid Micro-Nanostructured Surface, J. Nanotechnol. Eng. Med. 4 (2012)

041004. [13] D. Saeidi, A.A. Alemrajabi, Experimental investigation of pool boiling heat transfer and critical heat flux of nanostructured surfaces, Int. J. Heat Mass Transf. 60 (2013) 440–449. [14] Y. Tang, B. Tang, Q. Li, J. Qing, L. Lu. K. Chen, Pool-boiling enhancement by novel metallic nanoporous surface, Exp. Therm. Fluid Sci. 44 (2013)194–198. [15] K.S. Suganthi, V.L. Vinodhan, K.S. Rajan, Heat transfer performance and transport properties of ZnO–ethylene glycol and ZnO–ethylene glycol–water nanofluid coolants, Appl. Energy 135 (2014)548–559. [16] S. Vemuri, K.J. Kim, Pool boiling of saturated FC-72 on nano-porous surface, Int. Commun. Heat Mass, 32 (2005) 27–31. [17] W. Wu, H. Bostanci, L.C. Chow, Y. Hong, M. Su, J.P. Kizito. Nucleate boiling heat transfer enhancement for water and FC-72 on titanium oxide and silicon oxide surfaces, Int. J. Heat Mass Transf. 53 (2010) 1773–1777.

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[18] E. Forrest, E. Williamson, J. Buongiorno, L.W. Hu, M. Rubner, R. Cohen, Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings, Int. J. Heat Mass Transf. 53 (2010) 58–67. [19] H. Jo, S. Kim, H. Kim, J. Kim, M.H. Kim, Nucleate boiling performance on nano/microstructures with different wetting surfaces, Nanoscale Res. Lett. 7 (2012)1–9. [20] Y. Im, C. Dietz, S.S. Lee, Y. Joshi, Flower-like CuO nanostructures for enhanced boiling, Nanosc. Microsc. Therm. 16 (2012)145–153. [21] W. Yan, W.L Lin, L.M. Yan, Antifouling and enhancing pool boiling by TiO2 coating surface in nanometer scale thickness, J. AIChE. 53 (2007) 3062–3076. [22] W. Wu, C. H. Bostanci, L.C. Chow, Y. Hong, M. Su, J.P. Kizito, Nucleate boiling heat transfer enhancement for water and FC–72 on TiO2 and SiO2 surfaces, Int. J. Heat Mass Transf. 53 (2010) 1773–1777. [23] H. Zhang, J. F. Banfield, Structural characteristics and mechanical and thermodynamic properties of nanocrystalline TiO2, Chem. Rev. 114 (2014) 9613-9644. [24] B. Karunagaran, R.T. Rajendra Kumar, V.S. Kumar, D. Mangalaraj, S.K. Narayandass, G.M. Rao, Structural characterization of DC magnetron-sputtered TiO2 thin films using XRD and Raman scattering studies, Mater. Sci Semicond. Process 6 (2003) 547–550. [25] Saha B, Das S, Chattopadhyay KK. Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering, Sol. Energy Mater. Sol. Cells 91 (2007) 1692–1697. [26] B. Saha, R. Thapa, K.K. Chattopadhyay, Bandgap widening in highly conducting CdO thin film by Ti incorporation through radio frequency magnetron sputtering technique, Solid State Commun. 145 (2008) 33–37. 22

[27] A. Bera, R. Thapa, K.K, Chattopadhyay, B. Saha, In plane conducting channel at the interface of CdO–ZnO isotype thin film heterostructure, J Alloys Compd. 632 (2015) 343–347. [28] B. Saha, N.S. Das, K.K. Chattopadhyay, Combined effect of oxygen deficient point defects and Ni doping in radio frequency magnetron sputtering deposited ZnO thin films, Thin Solid Films 562 (2014) 37–42. [29] A. Ishii, Y. Nakamura, I. Oikawa, A. Kamegawa, H. Takamura Low-temperature preparation of high-n TiO2 thin film on glass by pulsed laser deposition, Appl. Surf. Sci. 347 (2015) 528–534. [30] H. Lin, A.K. Rumaiz, M. Schulz, D. Wang, R. Rock, C.P. Huang, S.I. Shah, Photocatalytic activity of pulsed laser deposited TiO2 thin films, Mater Sci. Eng, B 15 (2008) 133–139. [31] A.N. Kadam, R.S. Dhabbe, M.R. Kokate, Y.B. Gaikwad, K.M. Garadkar, Preparation of N doped TiO2 via microwave-assisted method and its photocatalytic activity for degradation of Malathion, Spectrochim Acta Part A 133 (2014) 669–676. [32] C.S S. Kumar, S. Suresh, Q. Yang, C.R. Aneesh, An experimental investigation on flow boiling heat transfer enhancement using spray pyrolysed alumina porous coatings, Appl. Therm. Eng. 71 (2014) 508–515. [33] S. Das, D.S. Kumar, S. Bhaumik, Experimental study of nucleate pool boiling heat transfer of water on silicon oxide nanoparticle coated copper heating surface, Appl. Therm. Eng. 96 (2016) 555–567. [34] S. Das, S. Bhaumik, Enhancement of Nucleate Pool Boiling Heat Transfer on Titanium Oxide Thin Film Surface, Arab J Sci Eng. 39 (2014) 4997–5006. [35] R.R. Schultz, R. Cole, Uncertainty analysis in boiling nucleation, In AIChE Symp Ser. 75 (1979) 32–38. 23

[36] S.J. Kline, F.A. McClintock. Describing uncertainties in single-sample experiments, Mechanical Engineering 75 (1953) 3–8. [37] C.K. Huang, C.W. Lee, C.K. Wang, Boiling enhancement by TiO2 nanoparticle deposition, Int. J Heat Mass Transf. 54 (2011) 4895–4903. [38] B, Feng. K. Weaver, G.P. Peterson, Enhancement of critical heat flux in pool boiling using atomic layer deposition of alumina, Appl. Phy. Lett. 100 (2012) 5312-5320. [39] C.Y. Lee, M.M. Bhuiya, K.J. Kim, Pool boiling heat transfer with nano-porous surface, Int. J Heat Mass Transf. 53 (2010) 4274–4279. [40] B.S. Kim, S. Shin, D. Lee, G. Choi, H. Lee, K.M. Kim, H.H. Cho, Stable and uniform heat dissipation by nucleate-catalytic nanowires for boiling heat transfer, Int. J Heat Mass. Transf. 70 (2014) 23–32. [41] W.M. Rohsenow, A method of correlating heat transfer data for surface boiling of liquids, Transactions of the ASME 74 (1952) 969–976. [42] I. Pioro, Boiling heat transfer characteristics of thin liquid layers in a horizontally flat twophase thermosyphon, Preprints of the 10th International Heat Pipe Conference, Stuttgart, Germany, September (1997) Paper H1-5. [43] S.S. Kutateladze V.M. Borishanskiĭ, A concise encyclopedia of heat transfer, Pergamon Press, (1966) New York) [44] D.A. Labuntsov Heat transfer in film condensation of pure steam on vertical surfaces and horizontal tubes. Thermal Engineering 4 (1957) 72–79. [45] G.N. Kruzhilin, Free-convection transfer of heat from a horizontal plate and boiling liquid, Doklady AN SSSR (Reports of the USSR Academy of Sciences) 58 (1947) 1657–1660.

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[46] K. Stephan, M. Abdelsalam, Heat-transfer correlations for natural convection boiling, Int. J Heat Mass Transf. 23 (1980) 73–87. [47] Y.Y. Li, Y.J. Chen, Z.H. Liu, A uniform correlation for predicting pool boiling heat transfer on plane surface with surface characteristics effect, Int. J Heat Mass Transf. 77 (2014) 809–817. [48] Dhir VK. Boiling heat transfer. Annual review of fluid mechanics 1998;30:365–401. [49] F. Yang, X. Dai, Y. Peles, P. Cheng, J. Khan C. Li, Flow boiling phenomena in a single annular flow regime in microchannels (I): Characterization of flow boiling heat transfer, Int. J Heat Mass Transf. 68 (2014) 703–715. [50] M.G. Cooper, A.J. Lloyd, The microlayer in nucleate pool boiling, Int. J Heat Mass Transf. 12 (1969) 895–913. [51] D. Saeidi, A.A. Alemrajabi, Experimental investigation of pool boiling heat transfer and critical heat flux of nanostructured surfaces, Int. J Heat Mass Transf. 60 (2014) 440–449

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Figure Captions Fig. 1. (a) Schematic Illustration of nucleate pool boiling system, (b) exploded view of test section, (c) complete view test section, (d) resistance diagram for surface temperature calculation, and (e) enlarged and close-up view of TiO2 coated surface with heated meter bar. Fig. 2. X-ray diffraction pattern of TiO2 nanostructure coated on Cu substrates. Fig. 3. FESEM image of TiO2 nanostructure coating of thickness (a) 250 nm (b) 500 nm (c) 750 nm (d) 1000 nm on Cu substrates. Fig. 4. TEM image of TiO2 nanostructure. Fig. 5. AFM image of the TiO2 coated surfaces of thickness (a) 250 nm (b) 500 nm (c) 750 nm (d) 1000 nm on Cu substrates. Fig. 6. EDX spectrum of the TiO2 coated Cu substrate. Fig. 7. (a) Boiling performance as a function of wall superheat (b) Heat transfer coefficient as a function of heat flux for different surfaces. Fig. 8. Comparison of present experimental results with that of other research groups. Fig. 9. Comparison of experimental value with correlation predicted value of heat transfer coefficients. Fig. 10. Variation of heat transfer coefficient with different surfaces. Fig. 11. Variation of contact angle with time for different surfaces. Fig. 12. Boiling curves in three successive test runs (a) Wall superheat vs. heat flux plot (b) Heat flux vs. heat transfer coefficient plot. Fig. 13. FEGSEM images of nanostructure coated surface after boiling test run: (a) 250 nm, (b) 500 nm, (c) 750 nm, and (d) 1000 nm

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Table Captions Table 1 Uncertainties in the various measured parameters. Table 2 Contact angle (in degree) of different heating surfaces with time. Table 3 Investigation of heat transfer coefficient with accuracy of correlations. Table 4 Summary of surface properties with heat transfer coefficients.

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Fig. 1 (a)-(d)

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Fig. 1(e)

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7 (a) and (b)

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Fig. 8

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Fig. 9

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Fig. 10

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Fig. 11

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Fig. 12

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Fig. 13

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Table 1 Uncertainties in the various measured parameters Sl. No.

Parameter

Measuring instrument

Uncertainty

1

Diameter and width of heating surface

Linear scale (L.C. 0.0005 m)

LC = ± 0.00025 m 2

2

Temperature

Thermocouple

± 0.5C

3

Current

Digital Ammeter

0.01A

4

Voltage

Digital Voltmeter

0.01V

5

Coating thickness

Thickness Monitor

0.1nm

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S. Das et al. Table 2 Dynamic contact angle (in degree) of different heating surfaces with time. Normal Cu surfaces t=0 ms, θ=105.3

t=50 ms, θ=95.5

t=100 ms, θ=82.2

t=150 ms, θ =65.6

t=200 ms, θ=44.9

t=250 ms, θ =25.1

t=288 ms θ=0

t=0 ms, θ=103.3

t=100.7 ms, θ=94

t=99 ms, θ=69.5

t=150 ms, θ=70.1

t=200 ms, θ=40.3

t=250 ms, θ=18.3

t=266 ms θ=0

t=0 ms, θ=102.9

t=50 ms, θ=99.9

t=100 ms, θ=61.9

t=150 ms, θ=34.8

t=200 ms, θ=34.5

t=250 ms, θ=1.5

t=258 ms, θ=0

t=0 ms, θ=120.1

t=50 ms, θ=108.7

t=100 ms, θ=84.9

t=150 ms, θ=27.7

t=180 ms, θ =0

t=0 ms, θ=122.9

t=50 ms, θ=109.8

t=100 ms, ϴθ=69.5

t=150 ms, θ=11.6

t=170 ms, θ=0

t=0 ms, θ=126.7

t=50 ms, θ=110.1

t=100 ms, θ=53.6

Treated Cu surface

250 nm nano structured TiO2 coated Surface 500 nm nano structured TiO2 coated Surface 750 nm nano structured TiO2 coated Surface 1000 nm nano structured TiO2 coated Surface

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t=125ms, θ=12.5

t=131 ms, θ=0

S. Das et al. Table 3 Investigation of heat transfer coefficient with accuracy of correlations Fluid– surface

Water / Normal Cu Surface, Ra=0.141μm

qm hm (kw/m2) (kw/m2.k)

314.262357

39.2873.65

hp (kw/m2.k)

Correlation

Error (%)

39.874.65 41.1176.23 43.3283.83

Rohsenow [41] Pioro [42]

0.56 1.56 1.28 4.96 3.2613.82

50.02100.3 29.0467.52 51.392.43 40.5877.65 65.18 130.05 66.11 135.23 70.32 146.83

Water / 1000 nm TiO2 coating on Cu Surface, Ra=0.191μm

314.262357

64.13 128.09

Kutateladze and Borishanski [43] Labuntsov [44] Kruzhilin [45] Stephan and Abdelsalam [46] and Li et al. [47 Rohsenow [41] Pioro [42]

77.02 165.3

Kutateladze and Borishanski [43] Labuntsov [44]

41.04 87.52

Kruzhilin [45]

76.3 146.43

Stephan and Abdelsalam [46] and Li et al. [47

66.3 132.43 44

Mean error (%) 1.31535

RMS error (%) 1.34

3.297888

3.55

7.129419

7.84

23.7236.18 -8.33 to 34.95 10.71 30.6

26.89114

27.14

-23.9989

25.51

19.13923

19.88

0.74 to 5.43 0.18 2.11 1.98 6.56 6.05 14.63

2.267413

2.82

1.42

1.51

4.10

4.37

10.19

10.42

15.62 25. 92 -23.73 to 36.15 9.7123.87

19.92

20.17

-30.63

30.82

14.66

15.11

3.33

3.36

2.06 4.02

S. Das et al. Table 4 Summary of surface properties with heat transfer coefficients.

Test Surface

Roughness μm

Coating Thickness

Real Area, μm2

Projected Area, μm2

Area ratio

Max. HTC, kw/m2.k

Normal Cu Surface

0.141

-

111

100

1.11

73.65

Treated Cu Surface

0.114

-

117

100

1.17

81.27

250 nm nano

0.068

250 nm

121

100

1.21

91.35

0.121

500 nm

134

100

1.34

98.2

0.173

750 nm

148

100

1.48

107.13

0.191

1000 nm

171

100

1.71

128.09

structured TiO2 coated Surface 500 nm nano

structured TiO2 coated Surface 750 nm nano

structured TiO2 coated Surface 1000 nm nano structured TiO2 coated Surface

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Graphical Abstract

46

Highlights



Well-ordered TiO2 nanostructure on Cu metal surface through e-beam evaporator.



Excellent nucleate pool boiling heat transfer through TiO2 nanostructured surfaces.



Formation of deep cavities and nucleation sites on TiO2 surfaces favoring the nucleate boiling.



Remarkable enhancement of boiling heat transfer coefficient with TiO 2 nanostructure coating.



Excellent stability of the nanostructured TiO2 surfaces in hydrothermal environment.

47