Heat transfer enhancement of pool boiling for a horizontal U-tube ...

Journal of Mechanical Science and Technology 28 (12) (2014) 5197~5204 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-014-1143-x

Heat transfer enhancement of pool boiling for a horizontal U-tube using TiO2-R141b nanofluid† Rong-Horng Chen and Tong-Bou Chang* Department of Mechanical and Energy Engineering, National Chiayi University, Chiayi City, Taiwan (Manuscript Received February 1, 2014; Revised June 17, 2014; Accepted August 15, 2014) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract An experimental investigation is performed into the pool boiling heat transfer performance of TiO2-R141b nanofluid containing 0 vol%, 0.0001 vol%, 0.001 vol%, and 0.01 vol% TiO2 nanoparticles. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) observations reveal that some of the TiO2 nanoparticles adhere to the heated surface during the boiling process. As a result, the heat transfer performance is poorer than that obtained using pure R141b as the working fluid. Accordingly, a further investigation is performed in which the heated surface is vibrated ultrasonically. It is shown that the ultrasonic vibration creates an acoustic cavitation effect, which inhibits the formation of the nano-sorption layer and improves the heat transfer performance as a result. Keywords: Nanofluids; Pool boiling; Ultrasonic vibration ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction With the continuing trend toward device miniaturization in the electronics and optical component fields, the cooling of high-powered microelectronic devices and high-brightness optical components has emerged as a major concern. Consequently, the heat transfer performance of nanofluids has attracted significant interest in recent years. Nanofluids, comprising nano-scale metal particles or metal oxides suspended in a base liquid, were originally proposed by Choi and Eastman [1]. The metal particles increase the effective thermal conductivity of the working fluid, and thus the heat transfer performance is significantly improved. Furthermore, the nanoparticles have an extremely large surface area, and thus both the heat transfer performance and the suspension stability are significantly enhanced. The improvement in the overall heat transfer performance results in significant energy and cost savings, and supports the development of smaller and lighter heat exchange systems. Lee et al. [2] showed that the addition of 4.0 vol% copper oxide nanoparticles to ethylene glycol improved the heat transfer performance by around 20%. Similarly, Eastman et al. [3] found that the heat transfer performance of ethylene glycol could be improved by up to 40% via the addition of 0.3 vol% copper nanoparticles with a mean diameter of less than 10 nm. Early heat exchange systems were based on natural convec*

Corresponding author. Tel.: +886 5 2717565, Fax.: +886 5 2717561 E-mail address: [email protected] † Recommended by Associate Editor Ji Hwan Jeong © KSME & Springer 2014

tion or forced convection cooling methods. However, such methods only have limited effect when applied to today’s high-powered electronic components. Accordingly, many researchers have suggested the use of pool boiling or spray cooling methods. Das et al. [4] analyzed the pool boiling heat transfer performance of Al2O3-water nanofluid with nanoparticle additions ranging from 1 to 4 vol%. The experimental results showed that some of the nanoparticles were deposited on the heated surface during boiling, thereby reducing the boiling heat transfer performance. Moreover, it was found that the more nanoparticles were added the worse the heat transfer performance would be. You et al. [5] found that the addition of very small amounts of alumina oxide nanoparticles (< 0.001%) to base fluid increased the critical heat flux in the pool boiling mode by around 200%. Bang and Chang [6] studied the boiling heat transfer performance of Al2O3-water nanofluid on a horizontal heated surface given a mean nanoparticle size of 47 nm and nanoparticle additions of 0.5 to 4 vol%. Consistent with Eastman’s work [3], these results demonstrated that the nucleate pool boiling performance obtained using the nanofluids was poorer than that obtained using a pure working fluid due to the deposition of nanoparticles on the heated surface [6]. Kim et al. [7] investigated the pool boiling heat transfer performance of three different types of nanofluid (Al2O3-water, ZrO2-water and SiO2-water) for particle volume fractions of 0.001%, 0.01% and 0.1%. The experimental results showed that a thin nano-porous layer was formed on the heated surface for all three nanofluids. These layers not only enhanced the wettability of the surface, but

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also reduced the static contact angle and increased the CHF (critical heat flux). However, the pool boiling heat transfer performance is still lower than that of pure water. Kwark et al. [8] conducted nucleate pool boiling experiments using a lowconcentration of Al2O3 nanofluid under atmospheric pressure conditions. They stated that although a nanoparticle film could increase CHF, relatively thick nanoparticle films actually created an additional thermal resistance and reduced boiling heat transfer coefficient. Oppositely, Wen et al. [9] used Al2O3-water nanofluid as working fluid to investigate the pool boiling heat transfer performance for a rough surface. They showed that there was no nanoparticle film deposited on the heated surface, therefore the boiling heat transfer performance was enhanced by 40%. Peng et al. [10] investigated the nucleate boiling heat transfer performance of R-113/VG68-diamond nanofluid. They demonstrated that since the thermal conductivity of nanoparticle (i.e. diamond) is much higher than the heated surface (i.e. Cu), the boiling heat transfer coefficient could be enhanced by around 60%, even in the presence of a thin nano-diamond film deposition. From the above works [5-10], it can be concluded that if we can prevent the deposition of nanoparticles on the heated surface, the nucleate boiling heat transfer performance can be enhanced by using nanofluid. Wong and Chon [11] examined the effect of ultrasonic vibration on the heat transfer performance of natural convection and pool boiling. The experimental results showed that an eight-fold improvement in the heat transfer coefficient was obtained in natural convection due to the motion of cavitation bubbles on the heated surface. However, a negligible improvement in the heat transfer performance was observed in the nucleate boiling region. Zhou [12] investigated the heat transfer characteristics of copper nanofluids with and without acoustic cavitation, respectively. The results indicated that the copper nanoparticles and acoustic cavitation had a profound and significant effect on the heat transport in the nanofluid. Pool boiling heat transfer is a highly effective cooling method with many applications in the nuclear and microelectronics fields. The individual effects of nanoparticle addition and acoustic cavitation on the heat transfer performance of pool boiling have been examined [5-12]. However, the combined effects of nanoparticle addition and acoustic cavitation have attracted relatively little attention. Accordingly, the present study examines the pool boiling heat transfer performance of TiO2-R141b nanofluid containing 0 vol%, 0.01 vol%, 0.001 vol%, and 0.0001 vol% TiO2 nanoparticles with and without ultrasonic vibration, respectively. The results show that the heat transfer performance of the nanofluids containing 0.0001 vol% or 0.001 vol% TiO2 nanoparticles is better than that of pure R141b.

2. Experimental equipment An experimental system was constructed to measure the shell-side heat transfer coefficients of a U-tube in the pool

Fig. 1. Schematic illustration of experimental system.

boiling mode. The U-tube heat exchanger has a better heat transfer performance than the straight-tube heat exchanger [13]. Thus, for the same heat transfer performance, U-tube heat exchanger would require fewer tubes than the straighttube one. In other words, for the same amount of tubes, Utube heat exchanger is much more compacted and requires less space than does the straight-tube one. As shown in Fig. 1, the experimental system comprised a hot water flow loop, a test section containing a horizontal Utube immersed in nanofluid, a cooling water flow loop, an ultrasonic crusher, and a data acquisition system. The hot water flow loop was designed in such a way as to ensure that the water entered the test section at the desired flow rate and temperature. As the hot water flowed through the U-tube, the nanofluid within the test section evaporated and was then condensed by a coil condenser installed in the upper part of the test section. The resulting condensate dripped off the coil, falling back into the saturated liquid pool. The experimental tests were conducted using R141b refrigerant as the base fluid. R141b has a boiling temperature of just 32ºC under atmospheric pressure conditions. As a result, the test system could be maintained at a low pressure and was therefore safer from an experimental point of view. The test section used in the present study comprised a hollow cylinder made of stainless steel with an internal diameter of 218 mm, a length of 300 mm and a wall thickness of 10 mm. The experimental U-tubes were made of cooper. The diameter, length and thickness were 19.05 mm, 50 cm and 1.5 mm, respectively. The U-tube was rigidly attached to the right-end side plate of the test section, while an ultrasonic crusher was installed on the left-end side plate of the test section with an inclination angle of 45°. Each pool boiling test was performed using a new U-tube which was polished to a surface roughness of 0.5 μm prior to the test. The test section was fitted with two glass ports for observation purposes. The ports were packed with O-rings in order to prevent the escape of the working fluid and / or vapor into the environment. Furthermore, to prevent heat loss during the experiments, the test section was wrapped in heat-insulating polyurethane. Finally the temperature and pressure within the test section were measured using a resistance thermometer and electronic pres-

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where WTiO2

rTiO2 WR141b

r R141b

: mass of TiO2 nanoparticles (kg), : density of TiO2 nanoparticles (kg/m3), : mass of R141b refrigerant (kg), : density of R141b refrigerant (kg/m3).

3.2 Experimental data reduction

Fig. 2. TEM image of dispersed TiO2 nanoparticles.

sure gauge installed in the upper part of the test section. The data of interest in the experimental system, i.e., the mass flow rate of the inlet hot water, the inlet and outlet temperatures of the hot water, the saturation temperature and the pressure within the test section, were interfaced to a PC, converted into digitized values and displayed on a monitor. Having waited for approximately 30 minutes for the system to reach steady-state conditions (as indicated by a variation of less than 0.1 °C/min in the saturation temperature), each data point of interest was obtained by averaging a minimum of 20 data-acquisition scans.

3. Experimental procedure and data reduction

To ensure the reliability of the experimental data, each data point was obtained by averaging a minimum of 20 dataacquisition scans acquired under steady-state conditions. In performing the experimental tests, the mass flow rate of the hot water was varied in the range of 0.0329 kg/s to 0.1565 kg/s, while the heat flux was varied from 1.2×104 W/m2 to more than 2.4×104 W/m2. Note that the heat flux of the U-tube (W/m2) was calculated as

Volume of TiO 2 Volume of TiO 2 + Volume of R141b

rTiO2 rTiO2

+

WR141b

r R141b

),

(2)

q'' , LMTD

(3)

where LMTD is the log mean temperature difference between the U-tube inlet and outlet temperatures and the saturation temperature of the test section (°C), Tsat, i.e., LMTD=

( Th,in -Tsat ) - ( Th,out -Tsat ) . é( T -T ) ù ú ln ê h,in sat ( Th,out -Tsat )úû êë

(4)

Gnielinski [15] developed the dimensionless turbulent heat transfer coefficient of the tube, which is defined as 0.4 Nu = 0.012( Re0.87 . di - 280 ) Pr

(5)

The shell-side convection heat transfer coefficient of the Utube, ho, can be calculated using the following procedure: ·

1. Measure the flow rate of the hot water, Q . ·

WTiO2 WTiO2

A0

U0 =

Wen et al. [14] showed that nanoparticles dispersed in a fluid tend to aggregate under the effects of Brownian motion and van der Waals’ forces, and then sink to the bottom of the container due to gravity. Accordingly, in the present study, the various TiO2-R141b nanofluids were prepared using an ultrasonic vibrator and a magnetic stirrer in order to prevent nanoparticle aggregation. Fig. 2 presents a transmission electron microscope (TEM) image of typical TiO2 particles within the nanofluid. From inspection, the average particle diameter is found to be around 50~90 nm. To evaluate the effect of the nanoparticle concentration on the pool boiling heat transfer performance, TiO2-R141b nanofluids with four different TiO2 concentrations were prepared, namely 0 vol%, 0.0001 vol%, 0.001 vol%, and 0.01 vol%. Note that the particle volume fraction (f) was defined as

=

(

& is the mass flow rate of the hot water (kg/s); A0 is where m the outer surface area of the U-tube; Cp is the specific heat of water; and Th,in and Th,out are the temperatures (°C) of the hot water at the inlet and outlet of the test section, respectively. The overall heat transfer coefficient, U0, was determined in accordance with Newton's cooling law, i.e.,

3.1 TiO2-R141b nanofluid preparation

f=

& mCp Th,in - Th,out

q¢¢ =

,

(1)

Q 2. Obtain the hot water velocity by V = . Ac

3. Obtain the Reynolds number of the hot water by Redi =

ρ w Vdi . μw

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4. Obtain the Nusselt number within the U-tube by Eq. (5). 5. Obtain the inner convection heat transfer coefficient of i

di

Predicted single-tube pool boiling(Stephan & Abdelsalam) 2x10

3

.

6. The shell-side convection heat transfer coefficient of the A ln ( d 0 di ) A 1 = o + 0 U-tube, ho, can thus be derived by U 0 Ai h i 2πkL +

Present data

1 . h0

h(w/m2-k)

the U-tube by h i =

Nu d k

(Kew&Houston)

1x10

3

8x102

6x10

2

working fluid : R141-b operation pressure : 1atm

4. Results and discussion

4x102

1x104

1x104

2x104

2x104

2x104

2x104

2x104

q" (w/m2) Fig. 3. Comparison of present pool boiling data with results obtained by Stephan et al. [17] and Kew et al. [18]. 1300 pure R-141b 0.0001 Vol% 0.001 Vol% 1200

h(W/m2-k)

To ensure the accuracy of the measured data, all the temperature sensors, pressure sensors and flow meters were calibrated prior to usage, and the accuracies of these calibrated devices were found to be ±0.1°C, ±0.1% and ±0.001 l/s, respectively. Since the uncertainty of the dimensionless turbulent heat transfer coefficient of the tube, Eq. (5), has not been provide by Gnielinski [15], the present uncertainty analysis did not take it into consideration. Based on these calibration results, without considering the uncertainty of Eq. (5) and applying the propagation-of-error method presented in Ref. [16], the uncertainty in the experimental heat transfer coefficient measurements was determined to be less than 5.7%. However, if the uncertainty of Eq. (5) has been taken into consideration, the uncertainty of the present experimental heat transfer coefficient should be larger than 5.7%.

0.01 Vol%

1100

1000

900

4.1 Verification of experimental results for pool boiling Fig. 3 illustrates the experimental results obtained for the variation of the pool boiling heat transfer coefficient with the heat flux on the U-tube surface. The results obtained by Stephan et al. [17] and Kew et al. [18] are also shown for comparison purposes. Note that the working fluid is pure R141b in every case. It is evident that the present experimental data fall within the range of values obtained in Stephan et al. [14] and Kew et al. [18]. The roughness of the heater surface used in Stephan et al. [14] was less than 0.1 μm. Meanwhile, that of the heater surface used in Kew et al. [18] was around 1 μm. The surface roughness of the U-tube in the present experimental test section was found to vary in the range of Rp = 0.2~0.8 μm (as measured by an SJ-401 coordinate measurement machine). In other words, the surface roughness is greater than that in Stephan et al. [17], but less than that in Kew et al. [18]. Thus, as shown in Fig. 3, the present results for the pool boiling heat transfer coefficient are higher than those obtained in Stephan et al. [17], but lower than those obtained in Kew et al. [18]. In other words, the results confirm the validity of the experimental setup and measurement system for evaluating the pool boiling performance of the present TiO2 nanofluids.

800 12000

16000

20000

24000

q'' (W/m2)

Fig. 4. Variation of the shell-side convection heat transfer coefficient with heat flux given nanofluids with different levels of TiO2 addition.

4.2 Effect of nanoparticle volume fraction on pool boiling heat transfer performance Fig. 4 shows the variation of the shell-side convection heat transfer coefficient with the heat flux on the U-tube surface given TiO2 nanoparticle additions of 0 vol% (pure R141b), 0.0001 vol%, 0.001 vol% and 0.01 vol%, respectively. During the tests, the hot water inlet temperature and the saturation temperature of R141-b were controlled at 55°C and 28°C, respectively. The hot water outlet temperatures were measured and the values were within 52-53.5°C, which depend on the overall heat transfer rate of each case. This means that the surface temperatures throughout the entire tube are all higher than the saturation temperature, and boiling occurred in every single experiment of the present study. It can be seen that pure R141b yields the optimal heat trans-


(a)

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(b)

(c) Fig. 5. SEM images and EDS analysis results for nano-sorption layers formed on U-tube surface given nanofluids with different levels of TiO2 addition: (a) 0.0001 vol%; (b) 0.001 vol%; (c) 0.01 vol %.

fer performance, followed by the nanofluids with 0.0001 vol%, 0.001 vol%, and 0.01 vol% nanoparticle addition, respectively. Taking the heat transfer performance of pure R-141b as the datum, the heat transfer performance reduces by around 6%, 8.5% and 14% given nanoparticle additions of 0.0001 vol%, 0.001 vol % and 0.01 vol %, respectively. In other words, the reduction in the pool boiling heat transfer performance increases with an increasing level of TiO2 addition.

4.3 Morphology and composition analysis of U-tube surface The morphology and composition of the U-tube surface following each pool boiling experiment were investigated by means of scanning electron microscopy (Model Philips XL40FEG) and energy-dispersive X-ray spectrometry (EDS), respectively. Figs. 5(a)-(c) show the morphology and composition analy-

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R.-H. Chen and T.-B. Chang / Journal of Mechanical Science and Technology 28 (12) (2014) 5197~5204 1400

Table 1. Detailed EDS analysis results for U-tube surface following pool boiling experiments with different nanofluid volume fractions.

0.0001 Vol%, with ultrasonic vibration 0.001 Vol%, with ultrasonic vibration

Volume fraction (vol)

0.0001%

0.001%

0.01%

0.01 Vol%, with ultrasonic vibration pure R-141b, with ultrasonic vibration pure R-141b, without ultrasonic vibration

Ti Wt%

10.03%

23.90%

37.80%

0.0001 Vol%, without ultrasonic vibration 0.001 Vol%, without ultrasonic vibration

Cu Wt%

74.75%

30.98%

4.45%

15%

45.11%

57.76%

sis results given the use of TiO2-R141b nanofluids with TiO2 concentrations of 0.0001 vol%, 0.001 vol% and 0.01 vol %, respectively. It is seen that the U-tube surface contains 10.03%, 23.90% and 37.80% Ti elements, respectively. In other words, an increasing number of TiO2 nanoparticles are deposited on the heated surface as the TiO2 concentration increases. The detailed EDS analysis results for the heated surface following pool boiling with the different nanofluids are presented in Table 1. The results presented in Figs. 5(a)-(c) indicate that an irregular nano-sorption layer is formed on the heated surface during pool boiling. The nano-sorption layer increases the thermal resistance of the heated surface and reduces the number of nucleating points. As a result, the heat transfer performance is lower than that obtained using pure R141b refrigerant. 4.4 Effect of nanoparticle volume fraction on pool boiling heat transfer performance with ultrasonic vibration To inhibit the formation of a nano-sorption layer on the Utube surface, a series of pool boiling heat transfer experiments was performed in which an ultrasonic crusher with a frequency of 20 kHz and an output power of 300 W was applied to the heated surface. To avoid over-heating the crusher, the acoustic vibration was applied with a 5-s cycle. Specifically, the crusher was turned off for the first 4 s of each cycle and was then turned on for 1 s before being turned off and entering the following cycle. Fig. 6 demonstrates the pool boiling heat transfer performance with the application of ultrasonic vibration and nanofluids containing 0% vol%, 0.0001 vol%, 0.001 vol% and 0.01 vol% TiO2 nanoparticles. Note that for comparison and verification purposes, the mean heat transfer coefficients for nanofluid fluids without ultrasonic vibration are also provided. It is seen that the ultrasonic vibration almost has no effect on the heat transfer performance given the use of pure R141b as the working fluid. This observation is consistent with the findings of Wong and Chon [11]. These authors [11] have investigated the effects of ultrasonic vibrations on heat transfer to water by natural convection and by boiling. They found that an eightfold increase in heat transfer coefficient was obtained in natural convection with vibration, but the effects diminished with increased temperature differences and became negligible in the well-developed nucleate boiling region. This is because at sufficiently high surface temperatures, the turbulence generated by the motion of cavitation bubbles was much smaller than that generated from the growth and detachment of vapor

h(W/m2-k)

O Wt%

0.01 Vol%, without ultrasonic vibration

1200

1000

800 12000

16000

20000

24000

q'' (W/m2)

Fig. 6. Variation of the shell-side heat transfer coefficient with heat flux given application of ultrasonic vibration and nanofluids with different levels of TiO2 addition.

bubbles as in the well-developed nucleate boiling without vibration, and thus no increase in heat transfer rate was observed. In other words, the application of work from the ultrasonic vibration is much less than the energy of boiling heat transfer, but much higher than the energy of natural convection. Fig. 6 shows that at lower surface heat fluxes (i.e. q¢¢ £ 1.8 ´ 104 W/m 2 ), the nanofluid containing 0.001 vol% TiO2 nanoparticles yields the optimal heat transfer performance, followed by the nanofluids with 0.0001 vol%, 0.01vol%, and 0 vol% TiO2 nanoparticles. However, at higher values of the surface heat flux (i.e. q¢¢ > 1.8 ´ 104 W/m 2 ), the nanofluid containing 0.0001 vol% TiO2 nanoparticles yields the optimal heat transfer performance, followed by the nanofluids with 0.001 vol%, 0 vol% and 0.01 vol% TiO2 nanoparticles. Furthermore, comparing the results with ultrasonic and without ultrasonic vibration, it is found that the use of the ultrasonic crusher increases the pool boiling heat transfer performance of the nanofluids containing 0.0001 vol%, 0.001 vol% and 0.01 vol% TiO2 nanoparticles by 14%, 14.3% and 13.6%, respectively. In other words, the use of ultrasonic vibration suppresses the formation of a nano-sorption layer on the heated surface and improves the pool boiling heat transfer performance as a result. 4.5 Morphology and composition analysis of U-tube surface given ultrasonic vibration The morphology and elemental composition of the U-tube surface following the pool boiling experiments with ultrasonic vibration were examined using SEM and EDS, respectively.

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Table 2. EDS analysis results for U-tube surface following pool boiling experiments given ultrasonic vibration and nanofluid volume fraction of 0.0001 vol%. Piece

11

Ti Wt% 9.37%

21

31

41

51

61

71

7.11%

3.15%

0.68%

5.66%

6.71%

9.48%

Cu Wt% 68.34% 76.18% 86.03% 96.07% 85.29% 74.27% 69.90% O Wt% 22.29% 16.71% 10.82% 3.26% Piece

12

22

Ti Wt% 10.02% 8.68%

9.06% 19.02% 20.62%

32

42

52

5.32%

0.47%

6.63%

62

72

9.99% 10.09%

Cu Wt% 63.01% 73.41% 81.42% 94.85% 80.91% 73.96% 69.33% Fig. 7. Location of numbered test pieces in U-tube.

In investigating the effect of the ultrasonic vibration on the formation of the nano-sorption layer, the U-tube was cut into 14 pieces, as shown in Fig. 7. Each test piece was annotated by a two-digit number, where the first number (i.e., 1 to 7) was assigned in sequence from the inlet to the outlet of the tube, while the second number (i.e., 1 or 2) indicated that the test piece was taken from the inner side of the U-tube or the outer side of the U-tube, respectively. It is noted from Figs. 7 and 1, that test piece #42 is the nearest to the ultrasonic crusher. Tables 2-4 present the EDS analysis results for the U-tube surface following pool boiling with ultrasonic vibration given the use of nanofluids with 0.0001 vol%, 0.001 vol% and 0.01 vol% TiO2 nanoparticles, respectively. In general, it is noted that the Ti content increases with an increasing TiO2 concentration. However, in every case, the Ti content of the heated surface is significantly lower than that of the U-tube used in the pool boiling experiments with no ultrasonic vibration (see Table 1). In other words, the results confirm that the application of ultrasonic vibration to the U-tube surface inhibits the formation of a nano-sorption layer, and improves the heat transfer performance as a result. In addition, it is seen that test piece #42 has the lowest Ti content of all the test pieces and the Ti content increases with an increasing distance from the ultrasonic crusher. Moreover, it is noted that the test pieces on the outer side of the U-tube have a lower Ti content than those on the inner side of the U-tube. In other words, it is inferred that those regions of the U-tube closer to the ultrasonic crusher experience a more powerful vibration effect and are therefore less susceptible to nanoparticle deposition.

5. Conclusions Previous studies have shown that the pool boiling heat transfer performance of nanofluids is poorer than that of pure working fluids due to the formation of an irregular nanosorption layer on the heated surface during the boiling / evaporation process. Accordingly, the present study has investigated the effects of ultrasonic vibration in enhancing the pool boiling heat transfer performance given the use of R141b-TiO2

O Wt% 26.97% 17.91% 13.26% 4.67% 12.47% 16.05% 20.58% Table 3. EDS analysis results for U-tube surface following pool boiling experiments given ultrasonic vibration and nanofluid volume fraction of 0.001 vol%. Piece

11

21

31

41

51

61

71

Ti Wt% 20.98% 16.34% 12.30% 11.72% 14.60% 16.54% 21.92% Cu Wt% 41.01% 59.52% 62.23% 70.76% 63.44% 59.20% 36.95% O Wt% 38.01% 21.14% 25.47% 17.52% 21.96% 24.25% 41.13% Piece

12

22

32

42

52

62

72

Ti Wt% 23.54% 19.81% 17.41% 11.79% 17.27% 18.36% 23.89% Cu Wt% 35.03% 45.17% 65.77% 65.34% 60.70% 48.23% 31.76% O Wt% 41.43% 35.02% 16.82% 22.87% 22.03% 33.42% 44.36% Table 4. EDS analysis results for U-tube surface following pool boiling experiments given ultrasonic vibration and nanofluid volume fraction of 0.01 vol%. Piece

11

21

Ti Wt% 32.26% 30.59%

31 25%

41

51

61

71

26.05% 26.35% 39.79% 32.95%

Cu Wt% 2.39% 12.02% 28.51% 30.93% 25.84% 15.57% 1.34% O Wt% 65.36% 57.39% 46.49% 43.01% 47.81% 44.64% 65.71% Piece

12

22

32

42

52

62

72

Ti Wt% 34.27% 31.37% 28.34% 25.03% 29.94% 34.84% 33.08% Cu Wt% 1.42%

8.65% 20.14% 32.06% 16.74% 8.67%

5.51%

O Wt% 64.31% 59.97% 51.52% 42.91% 53.30% 56.49% 61.41%

nanofluids with 0 vol%, 0.0001 vol%, 0.001 vol%, and 0.01 vol% TiO2 nanoparticles, respectively. The experimental results support the following major conclusions: In the pool boiling heat transfer mode, some of the nanoparticles in the nanofluid are deposited on the heated surface. The resulting nano-sorption layer increases the thermal resistance of the heated surface and reduces the number of nucleating points. Consequently, the heat transfer performance is reduced. The reduction in the heat transfer performance increases with an increasing level of nanoparticle addition. Given the application of ultrasonic vibration to the heated surface, the pool boiling heat transfer performance is significantly improved. At lower surface heat fluxes, the nanofluid containing 0.001 vol% TiO2 nanoparticles yields the optimal

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heat transfer performance, followed by the nanofluids with 0.0001 vol%, 0.01 vol%, and 0 vol% TiO2 nanoparticles, respectively. However, at higher values of the surface heat flux (i.e. q¢¢ > 1.8 ´ 104 W/m 2 ), the nanofluid containing 0.0001 vol% TiO2 nanoparticles yields the optimal heat transfer performance, followed by the nanofluids with 0.001 vol%, 0 vol% and 0.01vol% TiO2 nanoparticles, respectively. When ultrasonic vibration is applied to the heated surface, the heat transfer performance of pool boiling using nanofluid as the working fluid is significantly improved. This improvement arises since the ultrasonic vibration induces an acoustic cavitation effect, which causes the nanoparticles to become more active and destroy the formed nano-sorption layer on the heated surface. Also, the heat transfer performance improves with a reducing nanoparticle concentration. The Ti content on the heated surface increases with an increasing distance from the ultrasonic crusher due to a corresponding reduction in the intensity of the ultrasonic vibration.

Acknowledgment This study was supported by the National Science Council of Taiwan under Grant Nos. NSC99-2221-E-218-013 and NSC101-2221-E-218-016.

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Tong-Bou Chang received the Ph.D. in Mechanical Engineering from National Cheng Kung University, Tainan, Taiwan, in 1997. From 1997 to 2001, he was a researcher at Yuloon-Motor Group (Taiwan), whose job function includes design and characterization of the thermal and fluidflow systems for vehicle. Since 2002, he has been as a Professor at the Department of Mechanical Engineering. His current research interests include heat transfer with phase change, energy-system optimization, heat and mass transfer in porous medium, enhancement heat transfer and high performance heat exchangers.