Influence of orientation and roughness of heater surface on critical heat flux and pool boiling heat transfer coefficient of nanofluid

Applied Thermal Engineering 124 (2017) 353–361

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Influence of orientation and roughness of heater surface on critical heat flux and pool boiling heat transfer coefficient of nanofluid Mina Dadjoo, Nasrin Etesami ⇑, Mohsen Nasr Esfahany Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Pool boiling of silica nanofluid over an

Water input

inclined surface was investigated.
Heater orientation has a dramatic effect on pool boiling of nanofluid.
Effect of initial roughness of surface on characteristics of boiling was studied.
Boiling heat transfer was enhanced over a nanocoated surface.

10 90

T1 13

i n f o

Article history: Received 1 April 2017 Revised 19 May 2017 Accepted 7 June 2017 Available online 10 June 2017 Keywords: Nanofluid Pool boiling Inclined heater surface CHF Heat transfer coefficient (BHTC) Atomic force microscope (AFM)

6

T2

9

7

4 5 3

14

Firebrick

2 1 15

11

a r t i c l e

Water output

8 0

12

a b s t r a c t Pool boiling of SiO2/water nanofluid over a copper flat plate heater at various inclinations of the heater surface was investigated experimentally. In this work, the effect of heater surface orientation on changes in surface roughness and on the characteristics of nanofluid boiling was studied. We examined pool boiling of silica nanofluid at various concentrations (<0.1 vol.%) and various heater orientations from a horizontal state (0°) to a vertical state (90°). The results showed that in nanofluid boiling, increasing the inclination angle of the heater surface from 0° to 90° increases the critical heat flux (CHF) and decreases the boiling heat transfer coefficient (BHTC), while in boiling DI water, both for CHF and BHTC, decreases with the angle of the heater surface. Atomic force microscope images from the heater surface which has been boiled in nanofluids illustrated that surface roughness varies with the orientation of the surface. It was found that deposition of nanoparticles and bubble movements have important effects on nanofluid boiling over inclined surface. In addition, the performance of the boiling of DI water on a nanocoated surface was examined. The results showed that CHF and BHTC increase in comparison with the boiling of DI water on a bare heater. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Nucleate boiling plays a basic role in many types of heat transfer equipment. In industrial heat transfer equipment such as cooling nuclear reactors and electronic chips, boilers, etc., achieving the ⇑ Corresponding author. E-mail address: [email protected] (N. Etesami). http://dx.doi.org/10.1016/j.applthermaleng.2017.06.025 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.

highest heat transfer rate is of great significance [1]. Since critical heat flux (CHF) limits heat transfer, CHF enhancement has been the purpose of many recent studies. Using nanofluid is a new way to improve the boiling heat transfer coefficient (BHTC) and CHF. You et al. [2] surveyed CHF enhancement by using Al2O3-water nanofluid, and reported a 200% increase in comparison with pure water. Several researchers later used different nanofluids in boiling experiments. Kim et al. [3] observed that

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Nomenclature BHTC CHF Cp C sf H hf g q00 Pr T V I

boiling heat transfer coefficient critical heat flux specific heat (J/kg k) fluid-surface combination coefficient heat transfer coefficient latent heat of vaporization (J/kg) heat flux (kW/m2) Prandtl number temperature (°C) voltage ampere

the CHF of low concentration nanofluid (<0.1 vol.%) in pool boiling increased with the concentration, and in high levels afterwards, it remained constant. Kim et al. [4] clarified that nanoparticles deposited on heater surfaces during nanofluid pool boiling led to his suggesting a hypothesis for the decomposition of nanoparticles. The nanocoated layer improves surface wettability by reducing the static contact angle on surfaces which have been boiled in nanofluid compared with surface boiled in pure water. Kwark et al. [5] studied nucleate boiling of low concentrations of Al2O3, CuO and diamond nanofluids over a flat plate to determine the mechanism of adhering nanoparticles on heater surfaces by experimental means. They compared the nanoparticle coatings formed when boiling on the heater with nanoparticle coatings formed when caused by gravity, natural convection and electric field. They reported that only the layer of nanoparticles formed during the boiling process has significant effects on CHF. Shahmoradi et al. [6] found that using alumina nanofluid (<0.1 vol.%) in pool boiling on a flat plate heater enhanced CHF and deteriorated BHTC. In their work, AFM images of heater surfaces before and after nanofluid boiling showed that the deposition of nanoparticles on the surface increased the roughness and CHF consequently. However increasing the thickness of the nanolayer enhances thermal resistance and deteriorates the heat transfer coefficient. It can be seen that some of the results obtained in nanofluid pool boiling are controversial. Most researchers considered some of the parameters involved in pool boiling of nanofluids such as nanoparticle concentration, nanoparticle material and size, heater characteristics and size, system pressure, using of additive, etc. [7]. Another parameter which affects the boiling mechanism is heater orientation, which plays an essential role in the formation, growth and movement of bubbles over the surface of the heater. Pool boiling of dilute CuO/water on the surface of a horizontal cylindrical heater was investigated by Sarafraz and Hormozi [8]. They focused on the region of nucleate boiling of nanofluid. They observed that the deterioration of the BHTC of nanofluids for all concentrations (<0.4 wt.%) while using the surfactants helps improve the heat transfer coefficients of nanofluid due to decreases in surface tension. They also reported that the roughness of the heating section and the number of nucleation sites were reduced when the concentration of nanofluid filled the micro-cavities of the surface with nanoparticles during boiling. A recent study of the effect of surface on boiling heat transfer is Wen et al.’s work [9]. They observed that boiling heat transfer is dependent upon the relative size between nanoparticles and the heating surface and their interactions. Researchers have accurately studied some important parameters that can be affected surface roughness and the boiling heat transfer of nanofluids, but they did not investigate the effect of orientation of heating surfaces on changes in surface roughness and the boiling behavior of nanofluid.

A DT

area of boiling surface wall super heat (°C)

Greek symbols q density (kg/m3) r surface tension (N/m) h inclination angle (°) a static contact angle

In addition, their experiments were carried out in low heat fluxes below CHF. Practically speaking, boiling surfaces can occur at various orientations such as boiler walls, tubes and curved walls of vessels involving boiling. In these cases, the orientation of surface affects boiling mechanisms [10,11]. Most previous studies that investigated this parameter used common fluids [12–15], not nanofluids. Narayan et al. [16] have studied the effects of surface orientation on alumina nanofluid pool boiling performance and mechanisms. Their experimental set up was a tubular heater (diameter 33 mm and length 170 mm) at various inclination (0°,45° and 90°) when the range of heat flux was 10–70 kW/m2, lower than CHF. Their results showed that maximum BHTC occurred at horizontal configurations of heaters. Kwark et al. [17] have performed water pool boiling experiments over alumina nanoparticle coated heaters in order to observe the effects of nanocoated heaters, pressure, size and orientations of heaters on CHF and BHTC. They indicated that in lower heat fluxes (100 kW/m2), when heater orientation varies from 0° to 180°, BHTC increases. In higher heat fluxes, increase in the inclination angle of the heater does not have an appreciable effect on BHTC. In addition, they concluded that CHF significantly increases when heater orientations are beyond 90°, although CHF almost remained constant for lower inclination angles. The capacity of graphene-oxide in CHF enhancement during external reactor vessel cooling (ERVC) was investigated by Park et al. [18]. Their experimental results showed that CHF was enhanced about 40% and 200% at vertical and horizontal orientations of thin-wire heater, respectively. Real applications of surface boiling at various orientations, and the lack of studies of nanofluid boiling over inclined flat plates for realizing the effects of heater orientation on CHF and BHTC, motivated this work. Some of these studies were carried out in low heat fluxes (below CHF). The present study investigates the effect of heater orientation on characteristics of pool boiling of silica/water nanofluid. In this manner, some important questions will be answered: can the orientation of the heater surface affect conditions which influence nanoparticles deposition on the heater surface and roughness of surface? What are the variations of the CHF and BHTC of nanofluid at various inclination angles of heater surfaces? For these purposes, nanofluid boiling at different concentrations was carried out over a flat plate heater at various inclination angles. Boiling curves were then analyzed up to CHF. AFM analysis is used to characterize surface roughness before and after the boiling processes. In order to investigate the effect of the initial roughness of the surface on the performance of boiling, a series of experiments utilizing different surfaces (sanded and polished) under the nanofluid boiling were designed and AFM images were analyzed.

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2. Experimental 2.1. Experimental set up

Entry hole for thermocouple

A schematic of the experimental system is shown in Fig. 1. The test heater included three cartridge heaters (8 mm diameter and 80 mm length) with a total maximum of 1500 W (1) which were embedded in a cylindrical copper block and connected to an on/ off switch key (2). The copper block was closured using a firebrick (3) which insulated the lateral surface of the copper block against heat removal. The firebrick was covered by an asbestos sheet insulator to prevent lateral heat loss for better isolation. The heater surface was a circular copper disk (4) that was installed on one of the cross sections of copper block. A high thermal conductivity silicon paste was used between two surfaces to fill the gaps and reduce thermal resistance to the extent possible. The disk was connected to the firebrick using high-temperature glue to insulate. The diameter of the surface heater was 30 mm after pasting. Two K-type thermocouples (5) were placed; one thermocouple was in the mobile heater surface, 1 mm under the surface boiling (T1) and the other thermocouple was in the copper block (T2) to measure the surface temperature. Fig. 1b shows the position of the entry hole in the mobile surface. After each boiling experiment, this surface was removed from the copper block and AFM images were prepared. The thermocouples were connected to a thermostat (6) showing and controlling the surface temperature, and preventing it from exceeding the defined temperature. The boiling vessel (7) was an L-shaped Pyrex glass vessel with circular cross section that was stuck to a firebrick. In order to conduct saturated pool boiling and degas the fluid, a pre-heater (300 W) (8) heated the fluid up to the boiling temperature. A thermometer (9) was used near the surface to measure the bulk liquid temperature. A condenser (10) was used to condense vapor and prevent vapor from escaping. A 5 kW dimmer (11) was set to control the output power by varying the voltage. An ampere meter (12) and a voltmeter (13) were subjected to dimmer measuring amps and volts output. A contactor 3 phase 9 ampere (14) was used to control input current to cartridges and avoid damaging the thermostat. Moreover, there was a mechanical device (15) which inclined the heater on determined inclination angles (h = 0–90°).

Fig. 1b. Mobile boiling surface.

age diameter of 7–14 nm (Plasma Chem GmbH- Germany) was added to deionized water as the base fluid under a mechanical mixer in several stages. The mixture was then placed in an ultrasonicprcessor (UP400S model made by Hielscher Company) for 1 h. The TEM and SEM images of silica nanoparticles are shown in Fig. 2a, b respectively. The images confirmed 7–14 nm nanoparticles sizes. Different concentrations of silica nanofluid, including 0.001, 0.0025, 0.005 and 0.01 vol.%, were prepared. Before each experiment, the boiling vessel was cleaned with deionized water and acetone. Two different surfaces (sanded and polished surface) were used as the heater surface. The polished surface was polished using Buehler polisher (Type 95-2830-250) aided by alumina nanopowder. Sanded surface was prepared using sandpaper sheet (No. 2000). The nanofluid was added into the vessel and was preheated up to saturation temperature and degassed. Afterwards, electric power input was increased by the dimmer gradually and the temperature of the thermocouple was recorded. When the temperature rapidly increased and a mutation was seen in the recorded temperature, CHF had occurred. At this time, cartridge heaters were turned off in order to avoid doing harm to the apparatus. Strong insulation, causes to be neglected the heat loss through radial direction, and it can be used q00 ¼ VAI as heat flux on the surface. Nevertheless the heat flux was calculated and modified by measuring temperatures T1 and T2 with d distance (see Fig. 1) and Fourier’s law. The temperature of the heater surface (Tw) was calculated by extrapolating the method [19,6]:

q00 ¼

q ðT 2  T 1 Þ ¼k A d


 q00 k=d

Tw ¼ T1  2.2. Experimental procedure A two-step method was used to prepare the nanofluid. In this manner, the weighed amount of silica nanoparticles with an aver-

ð1Þ ð2Þ

where T1 is thermocouple temperature under the surface at a distance of d = 1 mm (Fig. 1 (5)) and k is the copper thermal conductivity. BHTC (h) was calculated using:

Water input

10 90

Water output

8 T1 13

6

0

T2

9

5 3

14

Firebrick

2 1 15

11

7

4

12 Fig. 1a. Experimental setup.

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Fig. 2. (a) SEM, (b) TEM image of silica nanopowder.

h¼

q00 T w  T sat

ð3Þ

where T w and T sat are the (wall) surface temperature and saturation temperature, respectively. The error propagation method [20,6] was used to calculate the uncertainties of the parameters. The maximum calculated uncertainties in CHF, wall superheat temperatures and BHTC were 7%, 10% and 11%, respectively.

3. Results and discussion 3.1. Reliability of experimental setup In order to validate the experimental setup, the boiling curve of DI water on bare sanded surface was compared to Rohsenow’s correlation (Eq. (3)) [21] as shown in Fig. 3.

T w  T sat

2 hfg C s;f 4 q00w ¼ C p;l ll hfg

r   g ql  qv

!0:5 31=3 5 pr n

ð4Þ

where hfg (kJ/kg), r (N/m), qg , ql (kg/m3 ), g and pr are the latent heat of the fluid, surface tension, vapor density, liquid density, gravitational acceleration and Prandtl number, respectively. Csf and n are constant values taken 0.013 and 1.7 for pure water boiled on smooth copper surface [22]. Fig. 3 shows good agreement between experimental data of DI water boiling and Rohsenow’s correlation. Repeatability of experiments in boiling of water was checked and error bars have been calculated using standard deviation in our previous work [6]. Maximum error between repetitions of the same nanofluid boiling for boiling curves was under 4%.

3.2. Effects of concentration of nonoparticles Boiling curves of silica-water nanofluid with various concentrations (0.01 vol.%) at horizontal configuration of the sanded heater surface is illustrated in Fig. 4. CHF values can be seen on the end of the curves for all concentrations of nanofluid except 0.01 vol.%. At 0.01 vol.% concentration, we could not achieve CHF and heaters were turned off to prevent overheat of the setup. Fig. 4 shows that CHF increases with nanofluid concentration. Similar results have been reported by other researchers [2–6]. Kim et al. [4] clarified that deposition of nanoparticles on the heater surface causes an increase in the wettability of boiling surface and CHF as a consequence. The wettability of a surface corresponds with the static contact angle of the fluid drop. The static contact angle of water drop (a) on the clear boiling surface and surface boiled under 0.005 vol.% silica nanofluid until reaching CHF are shown in Fig. 5. The contact angle of water drop after boiling (Fig. 5b) is lower than the case before the boiling of nanofluid (Fig. 5a) that indicates increases in the wettability of the boiling surface. Rainho et al. [23] considered the effect of adding nanoparticles (alumina, maghemite and CNTs) to water on the heat transfer coefficient and critical heat flux in a chamber. They also attributed increases in the CHF of nanofluids to decreases in the static contact angle of water drops on copper oxide boiling surface. Fig. 6 reveals the boiling heat transfer coefficients of nanofluids against those of heat flux. It can be seen that for all concentrations of nanofluid, BHTC is higher than is the case for DI water. BHTC increases with nanofluid concentration, but not beyond 0.005 vol. %. It is postulated that deposition of nanoparticles on the heater surface increases nucleation sites and BHTC, but there is an optimal concentration. This conclusion also was reported by Abdollahi et al.

2000 DI water

1800 1200

1600

Rohsenow

1400

q"(kW/m2)

q"(kW/m2)

1000

DI water on copper surface

800 600

0.001 vol.% 0.0025 vol.% 0.005 vol.%

1200

0.01 vol.%

1000 800 600

400

400 200 0

200 0 5

10

15

20 ΔT(°C)

25

30

35

Fig. 3. Boiling curve of DI water boiled on the sanded surface compared to the Rohsenow’s correlation.

0

5

10

15

20

ΔT(°C)

25

30

35

Fig. 4. Boiling curves of nanofluid boiled on sanded heater surface at horizontal configuration.

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Fig. 5. Static contact angle of water drop on (a) clear boiling surafce (b) surafce boiled in 0.005 vol.% silica nanofluid.

90

They stated that the enhancement of BHTC is due to sedimentation of nanoparticles in nucleation sites on the boiling surface and multiplying cavities to smaller sites. Deposition of nanoparticles in boiling of nanofluids increases with the enhancement of concentrations of nanofluid (>0.005 vol. %). Therefore, BHTC begins to reduce compared to 0.005 vol.% due to thickening nanolayers and increases in thermal resistance [6,26].

80 70

50 40

DI water

30

0.001 vol.%

20

0.0025 vol.%

0

3.3. Effect of surface orientation

0.005 vol.%

10

0.01 vol.% 0

500

1000

1500

2000

q"(kW/m2) Fig. 6. BHTC versus heat flux for boiling of various concentration of SiO2/water nanofluids on horizontal flat plate.

[24]. They performed pool boiling of Fe3O4/water nanofluid on a flat circular plate. They observed an optimal concentration of nanofluid. When the case is lower than the optimal value of concentration, BHTC increases with the concentration of nanofluid, and decreases with increasing nanofluid concentration which are higher than the optimal concentration. Two important factors can affect the heat transfer coefficient of nanofluid boiling: (I) changes in nucliation sites on the surface by deposition of nanoparticles on the heater surface and (II) increases in thermal resistance of the formed nanolayer on the surface. Reduction of BHTC shows that beyond 0.005 vol.%, thickening of the nanolayer on the heater surface causes an excess of thermal resistance, and BHTC starts to decrease. In this series of experiments, the ratio of average surface roughness (Ra) to the average diameter of the particles (dp), (Ra/dp), for sanded heater surfaces (Ra = 22.7 nm) is much greater than unity. It can be seen that the BHTC for nanofluid boiled on sanded surface is greater than that for DI water. It shows that the deposition of nanoparticles on the sanded heater surface, when the surface roughness is greater than average diameter of nanoparticles, increases nucleation sites and leads to enhancement of BHTC for nanofluid (0.005 vol.%) boiling. This result is consistent with the reports of Narayan et al. [25]. They reported that for low concentrations (0.5 wt.% of Al2O3) when (Ra/dp) exceeds 1, the enhancement of BHTC is more significant. In cases where (Ra/dp) is near unity, BHTC has minimal value and for Ra/dp  1 the BHTC is better than the one at Ra/dp near unity. Raveshi et al. [19] have also reported the enhancement of BHTC for Al2O3/water-ethylene glycol (<1 vol.%). Their experiments were carried out with Ra/dp  1.

Fig. 7 shows boiling curves of deionized water (DI) at various inclination angles on sanded heater surfaces. It can be seen that in the case of a constant heat flux, excess temperature ðDT ¼ T w  T sat Þ increases with inclination angle. In other words, the boiling curve shifts to the right and BHTC decreases with increasing heater orientation. Fig. 7 shows that there is a small reduction in CHF when increases the heater orientation from 0° to 90°. As regards inclined surface configurations, bubbles move along heater surface, coalesce together and form larger bubbles on the heater surface. Therefore, heat transfer resistance due to the formation of a layer of moving bubbles on the surface increases as the wettability of the surface decreases, after which BHTC and CHF decrease. This phenomena is more profound as the inclination angle increases from 30° to 90°. Similar results have been reported by some researchers [13,15,27]. The boiling of nanofluids in different volume concentrations over a heater surface using various inclination angles was conducted. Boiling curves of 0.001 and 0.005 vol.% SiO2/water

1200

DI water

1000

q"(kW/m2)

h(kW/m2

60

CHF

1015.6 .7

00

.4

330

800

835.5

445 45 5 90

600 400 200 0 5

10

15

20

25

30

35

40

45

ΔT(°C) Fig. 7. Boiling curves of deionized water at various inclinations of sanded heater surface.

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2000

0.001 vol.%(0

q"(kW/m2)

1800

0.001 vol.%(45

1600

0.001 vol.%(

1400

0.005 vol.%(0

1200

0.005 vol.%(45

1000

0.005 vol.%(

800 600 400 200 0

0

10

20

30

40

ΔT(°C) Fig. 8. Boiling curves of nanofluid on sanded heater surface at various surface orientations.

2000

CHF (kW/m 2 )

1800

DI water 0.001 vol.%

1600

the surface increases. According to mechanisms suggested by previous researchers [4,5], by growing bubbles and growing the microsublayer of bubbles as well, the deposition of nanoparticles in the microlayers is enhanced. Finally, the wettability of the upper parts of inclined surfaces is more than that in down part of surface. The wettability of nanocoated surface obtained under boiling of nanofluid on inclined surface heater is greater than that on horizontal surface (Fig. 5a) which produces CHF improvement for inclined surfaces. The difference between the contact angles of upper parts of inclined boiling surfaces and lower parts can reveal different motions of bubbles on horizontal boiling surfaces. When the configuration of heater surfaces approach the vertical, the sliding of bubbles along the heater surfaces are prolong. The deposition of nanoparticles increases and the wettability of surfaces at 45° exceeds that at 0°. The BHTC of nanofluids decreases with inclination angle due to the coalescence of bubbles along the surface and the formation of larger bubbles which increases the heat transfer resistance similar to what happened in the case of pure water [13,15,27]. This observation can be seen in Fig. 8, in which boiling curves shift to the right as the inclination angle increases.

0.0025 vol.% 1400

0.005 vol.%

1200 1000 800 0

20

40

60

80

100

(°) Fig. 9. CHF of nanofluids at different volume concentrations versus inclination angles of the heater surface.

nanofluid over the inclined sanded surface at 0°, 45° and 90° are illustrated in Fig. 8. As can be seen, for both concentrations of nanofluid, the highest CHF was observed at the vertical configuration of the heater (90°). The lowest CHF was at a horizontal configuration (0°). Changes of CHF for nanofluids with various concentrations at different inclination angles are illustrated in Fig. 9. In contrast to DI water, for all concentrations of nanofluid, CHF increases with the inclination angle of the heater surface. Deposition of nanoparticles on the heater surface enhances wettability and CHF as a consequence. The contact angle of water drops on the inclined surface (45°) boiled in 0.005 vol.% silica nanofluid is shown in Fig. 10. Fig. 10a shows water drops on the upper parts of the inclined surface and Fig. 10b shows the same for the lower parts of the inclined surface. The contact angle of water drops on the upper parts of inclined surfaces (Fig. 10a) is lower than is the case for the lower parts (Fig. 10b). Therefore, the wettability of the upper parts of inclined surfaces is greater than for the lower parts. Bubbles generated in the lower parts of the inclined surfaces ascend due to buoyancy forces and coalesce with other bubbles in the upper parts. Therefore, bubbles in the upper parts become larger and their residence time along

3.3.1. AFM analysis AFM images of heater surface before and after boiling (until reaching to CHF) of nanofluids were analyzed to investigate the effect of nanoparticle deposition on the topology of heater surfaces. In order to better investigate changes in surface roughness, a polished surface was selected as a base surface in this series of boiling experiments. Fig. 11A, B, C indicate AFM images of the polished heater surface before the boiling process, after boiling of 0.0025 vol.% nanofluid on a horizontal surface (h = 0°) and on the vertical configuration of the surface (h = 90°), respectively. As can be seen, boiling of nanofluid on the heater surface changes surface roughness. The roughness of the heater surface after boiling of nanofluid for vertical configuration (Ra = 44.7 nm) is greater than is the case for horizontal configuration (Ra = 33.5 nm). The higher degree of roughness results in higher wettability and CHF as a consequence. Nanolayer on the heater surface forms due to evaporation of the microlayer of the liquid near the heater surface, between bubbles and the surface (see [5]). Bubbles leaving the inclined heater surface find it more difficult than is the case for the horizontal heater (see Section 3.2), therefore at h = 90°, the amount of deposited nanoparticles, roughness and, as consequence of the thickness of the nanolayer, are greater than is the case for the horizontal. The thicker nanolayer on the vertical surface results in higher thermal resistance and BHTC deterioration compared to the horizontal surface. 3.4. Effects of heater surface roughness In order to discover the effect of the initial roughness of the heater surface on nanofluid pool boiling, two experiments were conducted. For this purpose, the boiling of 0.0025 vol.% nanofluid was carried out on two types of heater surfaces at the point of horizontal configuration; (i) on sanded surface and, (ii) on polished

Fig. 10. Static contact angle of water drops on upper parts of inclined surfaces; 45° (left) and on lower parts of surfaces (right) boiled in 0.005 vol.% silica nanofluid.

M. Dadjoo et al. / Applied Thermal Engineering 124 (2017) 353–361

359

Fig. 11. Three dimentional AFM images and profiles of polished heater surface before boiling (Ra = 3.76 nm) (A) polished heater surface after boiling of 0.0025 vol.% nanofluid at 0° inclinationangle (Ra = 33.5 nm) (B) polished heater surface after boiling of 0.0025 vol.% nanofluid at 90° inclinationangle (Ra = 44.7 nm) (c).

surface (their AFM images before boiling can be found in Figs. 13A and 11A, respectively). The boiling curves of nanofluid were compared with the boiling curve of DI water on the sanded heater in Fig. 12. The noticeable difference between the two curves of nanofluid shows that surface roughness exerts an important effect on BHTC. The BHTC of nanofluid on a sanded surface is higher than is the case for a polished heater. In order to compare the nanocoating that developed on a sanded heater surface with a polished heater surface, AFM analyses before and after boiling on polished and sanded surfaces were conducted (Figs. 11A & B and 13A & B, respectively). The ratio of average sur-

face roughness to average diameter of particles (Ra/dp) for sanded surface is higher than for the unit (Ra/dp > 1), so the BHTC of nanofluid is higher than is the case for DI water boiling. For polished surfaces, Ra/dpis less than is the case for the unit, and the BHTC of nanofluid is lower than is the case for DI water (see [6,21,25]). Fig. 13 shows that after boiling on sanded surfaces, the average surface roughness achieves Ra = 132.4 nm, whereas for polished surfaces the average surface roughness achieves Ra = 44.7 nm. Increasing in the number of pores and pits for sanded surfaces results in the improved wettability of the surface and the improved capillarity in nanolayer. Therefore CHF enhances [4].

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1800 1400

DI w on uncoated surf.(0 DI W on uncoated surf.( DI W on coated surf.(

2000

DI W on coated surf.(0

1200

0.0025 vol.% (0

1000

q"(kW/m2)

q(kW/m2)

2500

0.0025vol.% on polished surf. 0.0025vol.% on sanded surf. DI water on sanded surf.

1600

800 600 400 200

1500

0.0025 vol.% (

1000

500

0 0

5

10

15

20 ΔT(°C)

25

30

35

40 0 0

Fig. 12. Boiling curves of DI water on sanded surface, 0.0025 vol.% nanofluid on sanded and polished surface at horizontal configuration of the heater surface.

5

10

15

20

25

ΔT(°C)

30

35

40

45

Fig. 14. Boiling curves of DI water on nanocoated and uncoated surfaces and comparing with 0.0025 vol.% nanofluid at various inclination angles of the heater.

3.5. Effects of nanocoated surfaces on boiling of DI water In order to investigate the effect of nanocoated surfaces on the characteristics of DI water boiling, experiments were designed. 0.0025 vol.% nanofluid was boiled on the sanded heater surface to achieve CHF and the nanocoated surface was prepared. After that, DI water was boiled on nanocoated surfaces at 0° and 90° inclination angles of the heater, separately. Fig. 14 indicates the boiling curves of DI water at various inclination angles of nanocoated and uncoated heater surfaces in comparison to boiling curves of 0.0025 vol.% nanofluid on bare sanded heaters. Fig. 14

shows increasing in CHF and deterioration in BHTC as the inclination of heater surfaces increase. This trend is the same as was observed for nanofluid pool boiling in Figs. 4 and 6. Comparing the boiling of DI water on nanocoated heater to bare heaters shows a minor increase in CHF, but not as large as the increase for 0.0025 vol.% nanofluid boiling. Similar results were reported for DI water boiling on vertical cylindrical surfaces in cases of nanocoated and a bare heater by Hegde and Rao [28]. It also can be observed that the BHTCs of DI water on nanocoated surfaces are higher than that of 0.0025 vol.% nanofluid

Fig. 13. Three dimentional AFM images and profiles of: (A) sanded heater surface before boiling (Ra = 22.7 nm) (B) sanded heater surface after boiling of 0.0025 vol.% nanofluid at vertical configuration (Ra = 132.4 nm).

M. Dadjoo et al. / Applied Thermal Engineering 124 (2017) 353–361

and also higher than DI water on uncoated surfaces. It is postulated that the mounted nanoparticles on the coated surfaces prepares the numerous nucliation sites from beginning of the boiling and results in shifting the boiling curve of DI water over coated surfaces to the left, and enhances BHTC. 4. Conclusion Boiling SiO2/water nanofluid at different volume concentrations over a copper flat plate with different orientations of heater surfaces were investigated. The achieved results showed that the deposition of nanoparticles on heater surfaces causes a noticable enhancement in CHF for all concentrations that are less than 0.01 vol.%. BHTC increased with nanofluid concentrations up to 0.005 vol.% due to increases in nucliation sites resulting in the deposition of nanoparticles on the surface with Ra/dp  1. Using higher concentrations results in BHTC starting to decrease due to increases in the thickness of nanolayers on the surface and thermal resistance as a consequence. The heater orientation shows a dramatic effect on pool boiling. For both nanofluid and DI water, BHTC decreased as the heater orientation increased from 0° to 90° due to the coalescing bubbles and the formation of vapor film that increased heat transfer resistance. As regards DI water, CHF decreased slightly with increasing heater orientation. This can be justified by coalescing bubbles over inclined heater surfaces and decreasing the wettability of surfaces. While for boiling of nanofluid, CHF enhances with inclination angles. The deposition of nanoparticles occurs during boiling and becomes more profound as the inclination angle increases. It can help increase surface wettability and CHF accordingly. AFM images confirm these results. We found that using nanocoated heater surfaces improves the boiling heat transfer performance of DI water compared to boiling of DI water on bare heaters. References [1] J.N. Chang, Tailian Chen, Shalabh C. Marcoo, A review of recent progress on nano/micro scale nucleate boiling fundamentals, Front. Heat Mass Transf. 2 (2011) 023004. [2] S.M. You, J.H. Kim, K.H. Kim, Effect of nanoparticle on critical heat flux of water in pool boiling heat transfer, Appl. Phys. Lett. 83 (2003) 3374. [3] M. Kim, H. Kim, Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids, Int. J. Heat Mass Transf. 45 (2009) 991–998. [4] S.J. Kim, I.C. Bang, J. Buongiorno, L.W. Hu, Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux, Int. J. Heat Mass Transf. 50 (2007) 4105–4116. [5] S.M. Kwark, R. Kumar, G. Moreno, J. Yoo, S.M. You, Pool boiling characteristics of low concentration nanofluids, Int. J. Heat Mass Transf. 53 (2010) 972–981.

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