An Experimental Investigation on Heat Transfer ... - Springer Link

Journal of Thermal Science Vol.22, No.5 (2013) 484490

DOI: 10.1007/s11630-013-0652-8

Article ID: 1003-2169(2013)05-0484-07

An Experimental Investigation on Heat Transfer Performance of Nanofluid Pulsating Heat Pipe Hongwei Jia1,2, Li Jia1,*, Zetao Tan1,2 1. Institute of Thermal Energy Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China 2. Jiangsu Key Laboratory of Process Enhancement & New Energy Equipment Technology Nanjing University of Technology © Science Press and Institute of Engineering Thermophysics, CAS and Springer-Verlag Berlin Heidelberg 2013

The effect of SiO2 particles on heat transfer performance of a pulsating heat pipe (PHP) was investigated experimentally. DI water was used as the base fluid and contrast medium for the PHP. In order to study and measure the character, there are SiO2/H2O nanofluids with different concentration and applying with various heating powers during the experiment investigation. According to the experimental result, the high fraction of SiO2/H2O will deteriorate the performance of PHP compared with DI water, i.e. the thermal resistance and the temperature of evaporation section increases. It is in contrary in the case of low fraction of SiO2/H2O. Finally, the comparison of the thermal performances between the normal operation system and the static settlement system is given. It is found that both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after standing for a period, and it is the same trend for the temperature fluctuation at the identical heating power for PHP.

Keywords: Nanofluid; Pulsating heat pipe; Heat transfer characteristics; Suspension characteristics

Introduction The pulsating heat pipe (PHP) which also is referred to as self-excited oscillation heat pipe was initially presented in 1990s by a Japanese scholar named Akachi [1]. The PHP is known as a new and unique member with great potential within the heat pipe family. During the past ten years, nanofluids attracted more and more attention because of its great thermal characteristics [2-4]. Inspired by the informed research in these two fields, some studies on the PHP with nanofluids were investigated recently. Tsai et al. [5] investigated a cooper heat pipe charged with gold nanofluids. It is found that the

thermal resistance of the heat pipe with nanofluid is 0.17 ºC/W, and it decreases about 37 % compared with distilled water. In 2006, Ma et al. [6] charged the nanofluids with nano-diamond concentration of 1.0 vol% in a PHP and found that the nanofluids can significantly enhance the heat transport capability of the PHP. The thermal resistance decreases to 0.03 ºC/W at a power input of 336 W. In the investigation, it is found that the nanoparticles suspension is excited by the oscillation motion of the working fluid. Li et al. [7] conducted a visual experimental investigation to study the flow behavior of the nanofluid in a pulsating heat pipe. Their results showed that the critical temperature is increased when PHP is charged

Received: December 2012 Li Jia: Professor This paper is supported by NSFC (No. 51176008), National Key Technology R&D Program (No. 2012 BAB12B02) and Jiangsu Key Laboratory of Process Enhancement & New Energy Equipment Technology (Nanjing University of Technology). www.springerlink.com

Hongwei Jia et al.

An Experimental Investigation on Heat Transfer Performance of Nanofluid Pulsating Heat Pipe

with water-based silicon dioxide particles. There are many flow patterns of nanofluid in the tube, including bubble flow, slug flow and annular flow. In order to improve the heat transfer performance, especially the limit heat flux, Lin et al. [8] investigated a copper PHP with an inner diameter of 2.4mm charged with Ag/H2O nanofluids at concentrations of 0.1 % and 1.45 %. It is found that there are an optimal filling ratio and concentration. With the filling ratio of 60%, the thermal resistance of PHP charged with 0.1 vol% Ag/H2O nanofluids could be 0.092 ºC/W. Qu et al. [9] studied the heat transfer performance of the PHP charged with SiO2/H2O and Al2O3/ H2O nanofluids. They found that the precipitation of nanoparticles influences the performance of PHP directly, for example, the change of the surface condition at the evaporator and condenser due to different nano-particles deposition behavior. Ji et al. [10] investigated experimentally the Al2O3 particle effect on the heat transfer performance of a pulsating heat pipe. Four size particles with diameters of 50 nm, 80 nm, 2.2 μm, and 20 μm, were studied in the experiment. It is found that the heat transfer performance of PHP is significantly affected when Al2O3 particles is added in.

Experimental System and Procedure Preparation of Nanofluids In this study, oxide nanoparticles, SiO2 (Aldrich Co., USA) with the mean diameters of 10 and 20 nm, were chosen as source materials, and DI water was used as the base fluid. The water-based nanofluids were synthesized by the two-step method [11]. No surfactant/dispersant additives were added during the synthesis process because they may affect the thermal physical properties and even deteriorate the boiling heat transfer of heated surface [12]. Instead, the dispersion solution was subse-

Fig. 1

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quently vibrated for about two hours in an ultrasonic bath (KH2200E, Kunshan Equipment Co.) to form the homogeneous suspension. The SiO2/H2O nanofluids (with the SiO2 particle mass concentration of 0.05 %~0.5 %) prepared by the above method could be stably suspended for 12 hours. As soon as the nanofluid was ready-made, it was immediately charged into the PHP. Experimental setup The pulsating heat pipe (PHP) was fabricated by bending red copper tubes with an inner diameter of 2 mm and outer diameter of 3.5 mm. The PHP has 17 turns and three sections: evaporator, condenser and adiabatic sections with lengths of 60 mm, 70 mm, and 30 mm, respectively. The PHP was tested vertically, and the evaporator heated by electric heater on the bottom. The experimental system shown in Fig.1 consists of a PHP, water cold bath, Agilent 34970A data acquisition system, personal computer, electricity heater connected with the AC power supply. Eight T-type thermocouples were fixed on the evaporator and condenser, to measure temperature. The PHP was evacuated by a vacuum pump and then charged with the prepared nanofluids. For the convenience of comparison, filling ratio for each weight fraction nanofluid PHP was set at 50% by volume. The experimental data indicates that it is about 10 minutes for reaching the steady state, when the power input was low. For higher input power, it only take 5 minutes to make the system reach a steady state for the setup used in the experiment. When experiments were finished, the tubes of PHP were cleaned by filling liquid and sweeping repeatedly to ensure no powder residual. Uncertainty analysis The measurement error was inevitable during the ex-

Schematic of experimental set-up

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perimental process due to various objective and subjective factors. As a result, the error analysis of experimental results was vital. Table 1 lists the maximum uncertainties of main parameters for this study. Table 1

Maximum uncertaintie of main parameters

Parameters (unit)

Maximum uncertainties (%) 5.6

T (ºC) Heat input (W)

5.93

Mass flux (g/s)

0.14

Heat transfer rate of the PHP（W） Thermal resistance (ºC)

7.9 11.1

Mass concentration (wt%)

4.5

Results and Discussion In this research, the effect of mass concentration, input power, suspension and temperature of cooling medium on the heat transport capability in the PHP were studied. Due to the heating power limit, heating power is in the range of 25 W and 100 W in the experiments. The thermal resistance (R) is defined as: T T R e c (1) Q Q  qm c p (tout  tin )

(2)

where qm and cp are mass flux and the specific heat of cooling medium. tout and tin in Eq.(2) are based on the average temperature of three thermocouples placed in the inlet and the outlet of the cooling bath. Influence of SiO2 Nanofluid Mass Concentration The evaporator temperature of the PHP charged with SiO2/H2O nanofluid at different heat power is shown in Fig. 2, and the mass concentrations are in the range of 0-0.5wt%. As shown in Fig. 2, the evaporator temperature of the PHP with nanofluid at low concentration (0.05 wt%, 0.1 wt%, 0.3 wt%) increases gradually with the heat input. The average evaporator wall temperature increases with the increasing of the particle concentration from 0.05 wt% to 0.5 wt%. Fig. 3 illustrates the mass concentration effect on the thermal resistance at different heat input. It is indicated that high fraction (0.4 wt%, 0.5 wt%) of SiO2/H2O deteriorates the performance of PHP compared with DI water, the thermal resistance and evaporator temperature (temperature of the evaporator) increasing with the fraction of nano-material. However, the nanofluids with low fractions (0.05 wt%, 0.1 wt%, 0.3 wt%) decrease the thermal resistance and reduce the evaporator temperature. For the concentration of 0.1 wt%, the evaporator temperature and thermal resistance were decreased at most by 5.5ºC (or 6.3%) and 0.15 ºC/W (or 20%) respectively, as compared with those with pure water. Thus, suitable amount of

silica nanoparticles in DI water may improve the thermal performance of the PHP. The nanoparticles suspension is suggested to influence the heat transfer of two-phase oscillation flow due to the following reasons: (1) the presence of nanoparticles in water increases the thermal conductivity of the working fluid. The nanoparticles have changed the structure of the liquid which has an impact on the internal process of energy transfer. The liquid at the interface has a strong interaction with particles, and this interaction makes the interfacial liquid layer a more ordered structure [13]. The liquid in the interracial layer has higher thermal conductivity than in the bulk phase. (2) The addition of nanoparticles to water changes the heat capacity of the work fluid. For the nanofluid having a higher heat capacity than the base water, more heat will be transferred by the fluid if the flow rate remains unchanged based on the convection heat transfer theory. (3) The dynamic viscosity of the fluid is also varied due to the addition of nanoparticles. Because of nanofluid having higher viscosity than base water, high viscosity means lower flow rate, and thus less heat can be transferred by the fluid in the PHP. (4) The nanoparticle suspension was also assumed to enhance the convective heat transfer of the working fluid. The thermal resistance of PHP with low fraction of SiO2/H2O (0.05 wt%, 0.1 wt%, 0.3 wt%) decreases quickly with the increase of power when the power is less than 70 W, seeing Fig. 3. When power is greater than 70 W, thermal resistance changes in a small scale. The change of thermal resistance of PHP with concentration of 0.4 wt% and 0.5 wt% is more distinct in the overall power range in this research. The nanoparticle deposition changes the surface roughness condition. The micro-cavity density is reduced by cramming the particle agglomerates into the surface cavities and the nucleate boiling heat transfer is reduced. Meanwhile, a porous layer constructed by the silica particles is formed and an extra thermal resistance between the liquid and the inner surface of the evaporator is created.

Fig. 2

Mass concentration effect on the evaporator temperature

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As shown, when the PHP was charged with DI water, the startup time reached about 312 s. It is the longest one among six different working fluids including DI water and water mixed with particles at different mass concentration. The startup time was increased by 241 s (or 339 %) compared with SiO2/H2O nanofluids at the concentration of 0.05 wt%. Moreover, the startup temperature of

Fig. 3

Mass concentration effect on the thermal resistance

Fig. 4 and Fig. 5 illustrate the temperature of evaporator at different stages for a given input power (50 W). As shown in Fig .4, the temperature begins increasing and no temperature oscillation is observed in the initial heating stage. When the PHP is in stable operation, the temperature fluctuation is observed. Fig. 5 shows that the amplitude of temperature fluctuation may be more than 10ºC for the nanofluid with a high mass concentration (0.4 wt%, 0.5 wt%). However, for the low concentration (0.05 wt%, 0.1 wt%, 0.3 wt%), the amplitude of evaporator temperature oscillation is smaller. The average evaporator temperature of PHP with low concentration nanofluid is lower than that with the DI water. On the other hand, the average evaporator temperature of PHP with high concentration (0.4 wt%, 0.5 wt%) is greater than that with DI water.

Fig. 4

Fig. 5

Average temperature of evaporator at the power of 50 W (Stabilizing stage)

Fig. 6

Average temperature of evaporator at the power of 80 W (initiating stage).

Fig. 7

Average temperature of evaporator at the power of 80 W (Stabilizing stage)

Average temperature of evaporator at the power of 50 W (initiating stage)

This phenomenon was also presented in other powers, as shown in Figs. 6 and 7. Table 2 shows the mass concentration effect on the startup time and temperature of the PHP charged with SiO2/H2O nanofluids in the case of heating power 50 W.

488


PHP with DI water is higher than that with other working fluids except the SiO2/H2O nanofluids at the concentration of 0.5 wt%. As shown in Fig. 4 and Fig. 5, the amplitude of temperature fluctuations of the PHP with high concentration nanofluids (0.4 wt%, 0.5 wt%) is larger than others. So, the appropriate concentration of nanofluids will enhance the heat transfer. Table 2 Startup characteristic of PHP charged with SiO2/H2O nanofluids at different mass concentrations for a given input power (50 W) Mass concentration (wt%) 0

Startup time (s)

Startup temperature (℃)

312

76.5

0.05

71

52.31

0.1

186

72.75

0.3

128

67.51

0.4

169

70.58

0.5

242

76.65

Influence of the Input Heat Heating power is an important factor influencing on heat transfer performance of the PHP. The thermal resistance significantly decreases with the heating input (Figs. 2 and 3). Fig. 8 gives the temperature fluctuation of evaporator with different heating power. The fluctuation frequency of the evaporator temperature increases with the heating power. It is due to the driving force increased by the increasing power. Fig. 9 shows the effect of heat input on the startup time of PHP. The startup time reduces with the input power. When the heat input is larger, working fluid would obtain more energy to drive oscillation. In this research, once the power was bigger than 60 W, when the oscillating motion being frequently in the PHP, the further increase of power did not significantly reduce the

Fig. 8

Evaporator temperature curve at 107 point when charged with 0.3 wt%SiO2/H2O

Fig. 9

Start up time as function of heat input

startup time, that means the pulsating motion in the PHP has been developed. Influence of Suspending Characteristics The suspension stability of nanoparticles is important for nanofluid. The suspended state of nanoparticles can change the physical characteristics of nanofluid. The total performance of heat pipe is affected by the thermal conductivity and wettability of the working fluid in PHP. The visualization experiments in Ma et al. [6] showed that the deposition of nanoparticles, which is produced as a result of long standing of working fluid, would suspend again when the vapor-liquid plug starts to oscillate. Fig. 10 shows the thermal conductivity of 0.3 wt% SiO2/H2O nanofluid PHP at two conditions. One is the working fluid suspended well, and the other is that PHP stands for 12 hours. It is apparently that the nanoparticles cannot suspend well when the PHP is in long standing state, hence the heat transfer in PHP is weakened. Meanwhile, the adhesion and the precipitation of the nanoparticles to the wall will cause the increment of wall friction and thermal resistance, which thereby deteriorates the thermal performance of PHP. But the working fluid in PHP oscillates more frequently with the experimental time and the input power, which makes the nanoparticles re-disperse in the nanofluid, and finally the thermal resistance recovers the level of the condition of non-standing. Fig. 11 and Fig. 12 show the relation between evaporator temperature of the PHP and time. It can be seen that the startup temperature and time are increased by 10°C and 150 s, respectively, because of the deposition of nanoparticles. This is mainly due to the increase of wall thermal resistance and friction after the standing of nanofluid. It means that the vapor-liquid plug requires more time to gain enough energy to start moving. In the case of the input power of 50 W, the temperature fluctuation is large but the frequency is low, which means that the heat transfer between the evaporator and condenser sections is

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not in time. However, this deterioration effect can be eased up after the re-suspending of nanoparticles, which is shown in Fig. 13 and Fig. 14.

Fig. 13

Fig. 10

Curves of average evaporator temperature at 80 W

Comparison of the thermal resistance of PHP with 0.3 wt% SiO2/H2O as working fluid before standing and after standing

Fig. 14

Curves of wall temperature of testing point 2 at 80 W

Conclusions Fig. 11

Fig. 12

Curves of average evaporator temperature at 50 W

Curves of wall temperature of testing point 2 at 50 W

This paper investigates the thermal performance of nanofluids with different mass concentrations: (1) In PHP, high fraction of SiO2/H2O will deteriorate the performance of PHP compared with DI water, and the thermal resistance and the temperature of evaporation section increase, but, it is contrary in the case of low fraction of SiO2/H2O; (2) The start-up time is shorter and fluctuation of temperature decreases for all the experiments using nanofluids as working fluids; (3) For nanofluid PHP, the thermal resistance and the start-up time decrease with the heating power, but the operating temperature rises with the heating power increases; (4) Both the thermal resistance of nanofluid PHP and the temperature of the evaporation section increase after PHP is put in quiescence for a period, and it is the same trend for the temperature fluctuation.

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Acknowledgement This paper is supported by NSFC (No. 51176008)， National Key Technology R&D Program (No. 2012 BAB12B02) and Jiangsu Key Laboratory of Process Enhancement & New Energy Equipment Technology (Nanjing University of Technology).

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