Influence of nanofluid on heat transfer in a loop heat

International Communications in Heat and Mass Transfer 47 (2013) 82–91

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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt

Influence of nanofluid on heat transfer in a loop heat pipe☆ P. Gunnasegaran a, M.Z. Abdullah b,⁎, N.H. Shuaib c a b c

Centre for Advanced Computational Engineering (CACE), College of Engineering, Universiti Tenaga Nasional, Putrajaya Campus, Jalan IKRAM-UNITEN, 43000 Kajang, Malaysia School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia TNB Research Sdn. Bhd., Tenaga Nasional Berhad, Jalan Ayer-Hitam, 43000 Kajang, Malaysia

a r t i c l e

i n f o

Available online 16 July 2013 Keywords: Loop heat pipe Heat transfer coefficient Nanofluid Thermal resistance Effective thermal conductivity

a b s t r a c t Experiments are conducted to investigate heat transfer characteristics of using nanofluid in a Loop Heat Pipe (LHP) as a working medium for heat input range from 20 W to 100 W. The experiments are carried out by manufacturing the LHP, in which the setup consists of a water tank with pump, a flat evaporator, condenser installed with two pieces of fans, two transportation lines (vapor and liquid lines), copper pipe sections for attachment of the thermocouples and power supply. The uniqueness of the current experimental setup is the vapor and liquid lines of LHP which are made of transparent plastic tube to visualize the fluid flow patterns. In this study, the LHP performance using silica (SiO2–H2O) nanofluid with particle volume fraction of 3% which was used as a coolant is examined. The experimental results are verified by simulation using Finite Element Method (FEM). The LHP performance is evaluated in terms of transient temperature distribution and total thermal resistance (Rt). Rt is estimated for both LHP using SiO2–H2O nanofluid and pure water cases under a steady state condition. The results reveal the average decrease of 28%–44% at heat input ranging from 20 W to 100 W in total thermal resistance of LHP using SiO2–H2O nanofluid as compared with pure water. Therefore, the presence of nanoparticles could greatly enhance the cooling of LHP. The experimental and simulation results are found in good agreement. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Loop heat pipes (LHPs) are very reliable and versatile two phase heat transfer devices that are based on the capillary driven loop principle. LHPs possess all the main advantages of conventional heat pipes and are additionally capable to transport large heat to a long distance with small temperature difference by phase change of the working fluid. LHPs are expected to be applied widely in the aerospace and the electronic cooling. A typical LHP consists of five parts, including the evaporator, the compensation chamber, the vapor line, the condenser, and the liquid line. The traditional shape of the evaporator is cylinder, and it is thought that a flat evaporator can reduce the thermal resistance between the evaporator and the heat source [1]. Since the present study focuses on LHP, the related previous works are reviewed and presented as follows. Maidanik et al. [2] has introduced the first patent related to a loop heat pipe (LHP) in the United States of America in 1985. LHPs were used in space for thermal management purposes, especially on satellites. After successfully demonstrating the heat transport capability and reliability in space applications, LHPs started gaining worldwide attention in the late 1980s. Since then, numerous studies focusing

☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (M.Z. Abdullah). 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.07.003

on improving the efficiency of the system and understanding its operating characteristics have been conducted. The LHP is known for its high pumping capability and robust operation because it uses fine-pored metal wicks and the integral evaporator/reservoir design. It is the baseline design for thermal control of several spacecraft and commercial satellites. Tsai et al. [3] developed a novel LHP of no energy consumption using chaotic motion of bubbles, which is named as bubble-driven heat transport device (BD-HTD) and shows better performance than the conventional heat pipes because the convection of bubbly flow accomplishes effective heat transport with the latent heat [4]. In recent years, LHPs are necessary and widely utilized not only in space but also in ground applications such as electronic cooling and computers. As highly efficient heat transfer devices, LHP presents several unique cooling opportunities and hold a significant promise for cooling electronics [5]. Due to the abovementioned merits for LHPs in electronic cooling, Maydanik [6] reported a meticulous review of the history and developments up to the year 2000, of the application of LHP technology for cooling of electronics and computers. Computer technology, in particular PC “Notebook”, is a new sphere of LHP application, which is revealed owing to the appearance of miniature and fairly efficient devices [7,8]. The first experience in this direction was obtained in 2001, when a number of compact coolers for CPU with a mass of about 50 g were created on the basis of LHPs by Maydanik et al. [9]. It dissipated a heat flow at the level of 25– 30 W. By predictions of experts, in the near future one can expect

P. Gunnasegaran et al. / International Communications in Heat and Mass Transfer 47 (2013) 82–91

Nomenclature A Ab Af Ah Cp hnf hpw keff keff, e keff, l keff, v Lc Le Ll Lv Q q Q⁎ R Rb Rc Re Rv Rl Rt T t Ta Tb Tc Te Tv Tl x, y, z

Area, m2 Area of base plate, m2 Surface area of aluminum rectangular fin, m2 Cross-sectional area of LHP, m2 Specific heat, J/kg · K Heat transfer coefficient of nanofluid, W/m2 · K Heat transfer coefficient of pure water, W/m2 · K Effective thermal conductivity, W/m · K Effective thermal conductivity of evaporator, W/m · K Effective thermal conductivity of liquid line, W/m · K Effective thermal conductivity of vapor line, W/m · K Condenser section length, mm Evaporator section length, mm Liquid line length, mm Vapor line length, mm Heat input, W Heat flux, W/m2 Coolant flow rate, m3/s Thermal resistance, °C/W Base thermal resistance, °C/W Convective thermal resistance, °C/W Evaporator thermal resistance, °C/W Vapor line thermal resistance, °C/W Liquid line thermal resistance, °C/W Total thermal resistance, °C/W Temperature, °C Time, s Ambient temperature, °C Base plate temperature, °C Condenser temperature, °C Evaporator temperature, °C Vapor line temperature, °C Liquid line temperature, °C Space coordinates

Greek symbols μ Dynamic viscosity, Ns/m2 ρ Density, kg/m3

Subscripts s Solid w Wall

the appearance of more powerful processors dissipating 100 W and more, for which LHPs may prove to be the only alternative [6]. However, one of the limitations of LHPs' performance which does not cause a failure but bounds the heat transfer rate is the limitation owing to transport properties of the working fluid such as its thermal conductivity. One of the most advanced methods to improve the thermal conductivity of heat pipes' working fluid is the dispersion of nanoscale solid particles into plane fluid (hence the name “Nanofluids”). Since the thermal conductivity of solid materials is higher than fluids, therefore the mixture will have a higher overall thermal conductivity. Xuan and Li [10] classify the possible reasons of thermal conductivity improvement of nanofluids, which are due to an increase in effective surface area of heat flux absorbent, an increase in effective thermal conductivity of the fluid, particles' random interaction and collision and unstructured flow geometry due to higher turbulence possibility in presence of nanoparticles. Few experimental and analytical

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investigations on using nanofluids as a working medium in heat pipes have been reported in the literature. In 2004, this is the first time that the gold nanoparticle suspended in working fluid (gold nanofluid) filled meshed heat pipe was used. The circular meshed heat pipe had a length of 170 mm and an outer diameter of 6 mm. The heat pipe thermal resistance ranged from 0.17 to 0.215 °C/W. The measured results show that the thermal resistance of the heat pipes with nanofluids is lower than pipes containing pure water [11]. Kang et al. [12] investigated the effect of silver nanofluid with a concentration of 1 mg/l to 100 mg/l and with a particle size of 35 nm, on the performance of a circular grooved heat pipe. Temperature distribution on evaporator and condenser section and subsequently the thermal resistance of the heat pipe using nanofluid and DI-water as working fluid was compared. Their results show that for the input power range of 30 to 60 W the thermal resistance decreased by 10– 80%. Kang et al. [13] employed the same experimental test that they did in 2006 to investigate the effect of silver nanofluid with the same particle concentration and dimension on a sintered heat pipe. Their results indicate that the temperature difference in evaporator and condenser sections drops to 0.56–0.65 °C. Furthermore, the pipe filled with silver nanofluids has the ability to transfer heat up to 70 W while its counterpart with DI-water as working fluid is limited to 20 W. Wang et al. [14] studied the CuO nanofluid effect both in the steady operation process and start up conditions. According to their achievement, in the start-up conditions nanofluid implementation drastically reduces the response time of the heat pipe. Besides the heat transfer capacity increases by 40% as well as a significant reduction in thermal resistance of the heat pipe by approximately 50%. The effect of a nanofluid on flat heat pipe (FHP) thermal performance using silver nanofluid as the coolant is studied by Chen et al. [15]. An addition of silver nanoparticle solution has lowered the temperature difference and the thermal resistance of the FHP than those with base working fluid (pure water). The enhancements of the thermal performance of the FHP using the nanofluid are due to the significant heat flux improvement by higher wettability and the reduction of the boiling limit. Liu [16] used nanofluid which was a mixture of CuO with DI-water with an average diameter of 50 nm. A horizontal mesh wick of a cylindrical heat pipe was used. According to the outcome of the study, once the heat pipe reached the steady state condition, it is observed that the heat transfer coefficient both in the evaporator and condenser improved, as well as the total transferred heat flux. Few recent analytical investigations on the use of nanofluids in heat pipes include those of Shafahi et al. [17]. They analytically solved a mathematical model of a cylindrical heat pipe using Al2O3, CuO and TiO2 nanofluids. The effect of nanofluid in their analytical study is applied by using the mathematical model of effective heat conductivity, introduced by Yu and Choi [18,19]. Numerous interesting points are dazzling in the analysis outcomes such as the smaller particle size, and the more pronounced effect on the evaporator/condenser temperature difference. Also they have proven that there is an optimum mass concentration which restricts the amount of removed heat load. Lastly the heat pipe thermal resistance falls as the mentioned nanofluids are utilized as the heat pipe working fluid compared to water agrees with previous experimental works. Two years later, Gavtash et al. [20] studied the thermal performance of cylindrical heat pipes to extend the works of [17] by modeling and simulation of the effects of nanofluids using the ANSYS-FLUENT CFD commercial software. The heat pipe outer wall temperature distribution, thermal resistance, liquid pressure and axial velocity in the presence of suspended nanoscale solid particle including Al2O3, Cu and TiO2 within the pure water (base fluid) are reported in their studies. They concluded that the thermal performance of the heat pipe is improved when using nanofluid as the system working fluid which agrees with [17]. Additionally, it is proven that the thermal resistance of the heat pipe drops as the particle concentration level increases and particle radius decreases.

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Computer Data acquisition system

Blower fan

Thermocouples

Condenser

Vapor line

Evaporator

Liquid line

Flow controller valve

Power supply Reservoir

Multi meter

Pump 30A-720A Fig. 1. Schematic diagram of experimental apparatus.

Generally, previous works done by many researchers have revealed that using nanofluids as a working medium in the heat pipes provides greater heat transfer characteristics than conventional pure fluids due to remarkably higher thermal conductivity. It should be noted from the above literature review, however, that LHPs have never used the nanofluid as working fluid and limited studies are available on nanofluid flow and heat transfer characteristics of LHP performance and this has motivated the present study. Thus, the present study deals with experimental investigation to evaluate the thermal performance of LHP using silica (SiO2–H2O) nanofluid with heat input which ranged from 20 W to 100 W and compare the thermal performance with pure water. Thermal analysis is performed under forced convection mode and the results are verified by simulation using FEM (ANSYS 14 software). The results of interests such as total thermal resistance as function of heat input and transient temperature distribution in the LHP are reported to illustrate the effects of nanofluid on LHP performance. 2. Materials and methods 2.1. Description of the loop heat pipe The schematic diagram of experimental setup for LHP under investigation is shown in Fig. 1. The main function of this experiment rig is to determine the thermal performance of LHP by using SiO2– H2O nanofluid as a working medium and compare that with using conventional pure water. The LHP is a closed high pressure tubing system using pure water and SiO2–H2O nanofluid inside the tube as the working medium. The LHP setup is designed of a water tank with pump, condenser installed with two pieces of fans, a flat smooth copper base made evaporator, two transportation lines (vapor and liquid lines), copper pipe section for attachment of the thermocouple

and power supply (W5 Series 30A-720A). The detailed dimensions of the LHP are presented in Table 1. 2.2. The experimental setup The LHP shown in Fig. 1 has a flat evaporator, which is combined with the compensation chamber, with a total dimension of 50 mm × 50 mm × 4 mm. A water tank with 0.75 l glass vessel equipped by drain valve is used as liquid reservoir and connected

Table 1 Specification of LHP. Specification Evaporator Dimension (mm) Reservoir Volume (liter) Dimension (mm) Material Vapor line Outlet diameter (mm) Inlet diameter (mm) Length (mm) Liquid line Outlet diameter (mm) Inlet diameter (mm) Length (mm) Condenser Dimension (mm) Material Pump Performance (liter/hour) Delivery head (mm)

Dimension/material L50 × W50 × H4 0.75 L149 × W100 × H85 Aluminum faceplate 15 13.5 830 15 13.5 500 L321 × W100 × H1 Aluminum 750 1800


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2.65 g/cm3. The photographic view of the SiO2 nanoparticles as seen by the naked eyes is shown in Fig. 2. 45 g of the SiO2 nanoparticles is used to prepare 1500 ml of SiO2–H2O nanofluid with 3.0% particle mass fraction. Silica nanoparticles are weighted very accurately using a sensitive balance with a 0.1 mg resolution. The particle mass fraction of SiO2–H2O nanofluid in the present study is calculated using Eq. (1) as follows [21]. % mass concentration ¼

W SiO2 W bf

100%

ð1Þ

where, W SiO2 Wbf

Amount of SiO2 nanoparticles in gram Amount of base fluid in gram.

Fig. 2. A photographic view of the SiO2 nanoparticles.

2.4. Thermal analysis to a pump, which is built in low noise and low vibration. To observe the phenomena inside the LHP, both the vapor line and the liquid line are made of transparent plastic tube. The other part of the LHP is made of copper. The internal and external diameters of both vapor and liquid lines are 13.5 mm and 15 mm, respectively. The condenser section is made of 50 aluminum rectangular fins and cooled by installing two pieces of long screwed fans. To maintain steady state cooling conditions in the condenser section, the temperature and flow rate of the cooling liquid are fixed at constant value. The vacuum is maintained in the heat pipe by heating the tube at the evaporator section and the impurities are removed by opening the pressure release valve. To minimize the heat loss, the whole LHP is insulated by using glass wool. A copper block with heat rods inside is used to simulate the heat source, and the contact area between the evaporator and the heat source is 50 mm × 50 mm. The heating power can be changed by adjusting the output voltage of the transformer to the electronic heating rods. In this experiment, the K-type thermocouples are installed on the pipe/wall in different locations of the loop, including the copper base plate (Tb), the evaporator (Te), the vapor line (Tv), the condenser section (Tc) and the liquid line (Tl). The temperatures measured by the thermocouples are collected through a data acquisition (Agilent 34970A) with sample rate of 1 Hz and connected to a PC to collect the data. For both LHP cases using pure water and SiO2–H2O nanofluid, the experiment starts with a heating power of 20 W and increases it up to 100 W by increments of 20 W. The airflow velocity is fixed as 4 m/s and the coolant flow rate is 750 l per hour, controlled by adjusting the flow control valve. In order to study the transient temperature distribution, temperatures at each 500 s from the start of the experiment, are also noted, until the steady state. The specifications of the LHP are listed in Table 1. 2.3. Nanofluid preparation In the present work, Di-water is taken as the base fluid for preparation silica (SiO2–H2O) nanofluid. The (SiO2) nanoparticles that used for investigation have an average size of 12 nm and density of

The objective of the current study is to study the total thermal resistance (Rt) and the temperature distributions of the LHP using pure water and SiO2–H2O nanofluid as working fluids for various heat inputs under steady state and transient conditions. The results obtained from experimental investigation were used to verify by FEM simulation. The type of working fluid and heat input, that yields the minimum Rt are then found out, and the effective thermal conductivity (keff) of evaporator section, vapor line and liquid line is calculated and the various steps to estimate Rt and keff are as follows. The thermal resistance network of the system is shown in Fig. 3. The heat flux q˙ that applied on the bottom of the base plate can be expressed as: ˙ q¼

Q Ab

ð2Þ

where Q denotes the heat input and Ab is the area of base plate. The thermal resistances of the LHP are defined as [22]:The thermal resistance between the copper base plate and the evaporator section (Rb) is: Rb ¼

T b −T e Q

ð3Þ

where Tb denotes the temperature at the copper base plate and Te is the temperature at the evaporator. The thermal resistance of the evaporator section (Re) is: Re ¼

T e −T v Q

ð4Þ

where Tv is the temperature at the vapor line. The thermal resistance of the vapor line (Rv) is: Rv ¼

T v −T c Q

where Tc is the temperature at the condenser section.

Fig. 3. Thermal resistance network of LHP.

ð5Þ

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The effective thermal conductivity of the evaporator region is:

Table 2 Material properties. Property

Copper Fins (aluminum) Evaporator Vapor line Liquid line

Thermal conductivity (k) W/m · K Pure water

SiO2–H2O

402 186 2890 44,089 62,368

402 186 3290 46,693 67,381

Specific heat (Cp) J/kg · K

Density (kg/m3)

385 895 385 385 385

8933 2800 8933 8933 8933

keff ;e ¼

Q Le Ah ðT e −T v Þ

ð9Þ

where Q is the heat input, Le denotes the length of evaporator and Ah is the cross-sectional area of the LHP. The effective thermal conductivity of the vapor line is: keff ;v ¼

Q Lv Ah ðT v −T c Þ

ð10Þ

where Lv is the length of vapor line. The effective thermal conductivity of the liquid line is: The convective thermal resistance of the condenser (Rc) is: T −T l Rc ¼ c Q

keff ;l ¼ ð6Þ

h¼ ð7Þ

where Ta is the ambient temperature. According to the thermal resistance network as shown in Fig. 3, the Rt of the system is given by: Rt ¼ Rb þ Re þ Rv þ Rc þ Rl :

ð11Þ

where Ll is the length of liquid line. The heat transfer coefficients (h) for LHP using pure water and SiO2–H2O nanofluid are obtained from Eq. (12) as follows:

where Tl is the temperature at the liquid line. The thermal resistance of the liquid line (Re) is: T −T a Rl ¼ l Q

QLl Ah ðT l −T a Þ

ð8Þ

Different values of effective thermal conductivity, keff are used in three different regions of the LHP including evaporator section, vapor line and liquid line. The values of keff are calculated by using equations shown as below [22]. The keff value for evaporator section, vapor and liquid lines of LHP with pure water and SiO2–H2O nanofluid, calculated using Eqs. (9)–(11) respectively, at heat input of 60 W is shown in Table 2.

Q 100 A f ðT c −T a Þ

ð12Þ

where Af is the surface area of aluminum rectangular fin. 3. FEM simulation The purpose of simulation in the present study is to verify the experimental observations. Accordingly, a three-dimensional (3D) model is designed by assuming the whole LHP as a conducting medium without taking into account the events occurring inside the LHP. The simulation is performed in commercial FEM software package, ANSYS, for both LHP with pure water and SiO2–H2O nanofluid cases. The 3D model of the complex LHP assembly is built in Pro Engineer Wildfire 4.0 and is exported to ANSYS 14. The LHP consists of copper base plate (heat source), evaporator, vapor line, condenser with 50 aluminum rectangular fins and liquid line. The high effective thermal conductivity of LHP is meshed with 1 mm element edge length, whereas the copper base plate and aluminum fins have coarse

Fig. 4. The meshed simulation model.


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a) Bubbly flow at 40 W

Fig. 6. Total thermal resistance versus applied heat load.

b) Slug flow at 60 W 4. Experimental results 4.1. Flow pattern in the vapor line

c) Annular flow at 100 W

Instead of saturated vapor, three different flow patterns are observed in the vapor line using water, as shown in Fig. 5. With a small heating power, there is bubbly flow inside the vapor line, as shown in Fig. 5(a). With the power increasing, slug flow inside the vapor line is observed, as shown in Fig. 5(b). Finally, annular flow appeared inside the vapor line in a larger power, as shown in Fig. 5(c). It is also observed that blurred vision and smaller nucleation size of vapor bubbles appeared in the vapor line for fluid with suspended SiO2 nanoparticles than without them. From the results obtained, the lowest thermal resistance is expected in LHP using SiO2–H2O nanofluid. The reason for reducing the thermal resistance of heat pipe can be explained as follows. A major thermal resistance of heat pipe is caused by the formation of vapor bubbles at the liquid–solid interface. A larger bubble nucleation size creates a higher thermal resistance that prevents the transfer of heat from the solid surface to the liquid [23]. The suspended nanoparticles tend to bombard the vapor bubble during the bubble formation. Therefore, it is expected that the nucleation size of vapor bubble is much smaller for fluid with suspended nanoparticles than that without them as reported by Tsai et al. [11]. 4.2. Thermal resistance analysis

Fig. 5. Different flow patterns inside the vapor line. (a) Bubbly flow at 40 W. (b) Slug flow at 60 W. (c) Annular flow at 100 W.

meshes of 4 mm edge length, forming a total of 205,523 triangular elements. The 3D meshed model of LHP is shown in Fig. 4. The heat flux of 24,000 W/m2 applied on the bottom of the base is calculated by Q/Ab, where Q is 60 W and Ab is the base surface area (0.0025 m2). The initial temperature is taken as 22 °C and the total time steps (thus the number of iterations) are taken as 1800. The heat transfer coefficients of LHP, calculated using Eq. (12) are taken as 142 W/m2 · K and 147 W/m2 · K corresponding to LHP with pure water and SiO2–H2O nanofluid, respectively. The material properties selected are summarized in Table 2. It is to be noted that the value of k for evaporator section, vapor and liquid lines of the LHP with pure water and SiO2–H2O nanofluid in Table 2 is the effective thermal conductivity (keff), calculated using Eqs. (9)–(11) accordingly.

Fig. 6 shows the relationship of total thermal resistance (Rt) of LHP at various applied heat loads. From the graph, it is observed that the Rt of LHP for both working fluids become smaller as the heat load increases. For heat load of 100 W, the total thermal resistances are 1.480 °C/W and 1.304 °C/W for pure water and SiO2–H2O nanofluid, respectively.

Fig. 7. Evaporator temperature versus applied heat load.

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Fig. 8. Transient temperature distribution of LHP using pure water at Q = 60 W.

The Rt that were obtained in LHP with SiO2–H2O nanofluid at all heat loads are lower compared to pure water. This is due to the fact that the suspended nanoparticles in a fluid flow can increase both the thermal conductivity of fluid and convective heat transfer from fluid flow to the wall [24,25], which results in the reduction of total thermal resistance of the LHP with SiO2–H2O nanofluid as discussed in the present study. Based on the obtained results in the present study, the test results indicate that, heat load and nanofluid have significant effects on Rt of the LHP. At the same charge volume, test results showed the average decrease of 28%–44% at heat loads ranging from 20 W to 100 W in Rt of LHP with nanofluid as compared with pure water. 4.3. Temperature analysis Fig. 7 illustrates the temperatures at evaporator versus heat loads. As heat load is applied to the base plate, the temperature of the evaporator increases and formed vapor in the vapor line. In this experiment the reservoir temperature is maintained at ambient temperature. The LHP shows a very good response to the change in the heat load from 20 W to 100 W and reached a steady state within a short time period. It can be seen that the LHP was able to maintain the evaporator temperature within a range of 57.28 °C to 63.181 °C and 55.8 °C to 61.34 °C for pure water and SiO2–H2O nanofluid, respectively. It is evident from the

outcomes in the present study that the LHP exhibited very efficient control over the operating temperature of the evaporator for the entire range of input power. The temperature at the evaporator of the LHP using SiO2–H2O nanofluid is lower than pure water since SiO2–H2O nanofluid has higher thermal conductivity and transfer heat at a high rate compared to pure water. The transient temperature distribution of LHP with pure water and SiO2–H2O nanofluid at heat load of 60 W is shown in Figs. 8 and 9, respectively. It can be seen from Fig. 8 that the temperatures of all points increase gradually until 500 s and become steady thereafter. The temperatures of the base (Tb), evaporator (Te), vapor line (Tv), condenser section (Tc) and liquid line (Tl) are 101.21 °C, 66.26 °C, 59.01 °C, 51.12 °C and 42.36 °C, respectively. LHP with SiO2–H2O nanofluid reached a steady state faster than LHP with pure water as depicted in Fig. 9, which indicates that the temperatures of all points gradually increase until 250 s and remain almost steady thereafter. It is also observed from Fig. 9 that the temperatures at all points diminished due to the enhanced heat removal in the LHP by higher thermal conductivity and convective heat transfer of SiO2–H2O nanofluid. The temperatures of the base (Tb), evaporator (Te), vapor line (Tv), condenser section (Tc) and liquid line (Tl) are decreased from 101.21 °C to 83.06 °C, 66.26 °C to 61.71 °C, 59.01 °C to 55.34 °C, 51.12 °C to 47.89 °C and 42.36 °C to 35.73 °C,

Fig. 9. Transient temperature distribution of LHP using SiO2–H2O nanofluid at Q = 60 W.


Fig. 10. Temperature contour of LHP using pure water at Q = 60 W.

Fig. 11. Temperature contour of LHP using SiO2–H2O nanofluid at Q = 60 W.

Fig. 12. Transient temperature distribution of LHP using pure water at Q = 60 W.

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90


Fig. 13. Transient temperature distribution of LHP using SiO2–H2O nanofluid at Q = 60 W.

respectively. As a result, the higher thermal performances of the nanofluid have proved its potential as substitute for conventional pure water in the LHP. For both LHP with pure water and SiO2–H2O nanofluid, the temperatures are oscillate, especially the temperatures of vapor line and condenser section. The oscillation observed is coursed by the motion of vapor/liquid interface inside the vapor line and condenser section. 5. Simulation results The simulated temperature distribution in the LHP using pure water at heat transfer coefficient of 142 W/m2.K, and heat input power of 60 W, is presented in Fig. 10. After 1800 s when it comes to steady state, the highest temperature is 102.54 °C (at the base), whereas the lowest is 44.96 °C (liquid line). Fig. 11 shows the predicted temperature contour for LHP using SiO2–H2O nanofluid at a heat transfer coefficient of 147 W/m2 · K. A drop in temperatures is observed, the highest being 83.44 °C and the lowest, 26.14 °C. Fig. 12 shows the predicted transient temperature distribution for LHP using pure water at heat load 60 W. It is observed that the temperatures of all points is increase gradually until 500 s and become steady thereafter. The temperature of the base (Tb), evaporator (Te), vapor line (Tv), condenser section (Tc) and liquid line (Tl) are 102.54 °C, 68.31 °C, 59.07 °C, 52.72 °C and 44.95 °C, respectively. The predicted transient temperature distribution for LHP using SiO2–H2O nanofluid at heat load 60 W is shown in Fig. 13. The temperatures of all points of the LHP increase gradually and the steady state is reached at first 300 s. The temperatures of the base (Tb), evaporator (Te), vapor line (Tv), condenser section (Tc) and liquid line (Tl) are 83.44 °C, 63.89 °C, 56.09 °C, 47.23 °C and 35.81 °C, respectively. As expected, the temperatures obtained at all points in LHP using SiO2–H2O nanofluid are lower and reach their steady state faster than LHP using pure water. 6. Comparison of experimental and simulation results

transient temperature distributions for LHP using water and SiO2–H2O nanofluid in the present simulation (Figs. 12 and 13) are also well matched with the experimentally observed trends (Figs. 8 and 9).

7. Conclusion In the present study, extensive experimental and FEM simulation investigations on the LHP are performed. The LHP which is made of transparent plastic tube is the unique feature of the current study used to visualize the flow patterns. As power increases, three different flow patterns are observed in the vapor line. They are bubbly, slug and annular. The thermal analysis of the LHP using pure water and SiO2–H2O nanofluid at various heat loads is studied. The results of the simulation showed the positive influence of nanofluid utilizing as a heat pipe working fluid on the system thermal performance. It is found that the Rt of LHP decreases when using SiO2–H2O nanofluid. The test result showed the average decrease of 28%–44% at heat load ranging from 20 W to 100 W in Rt of LHP using nanofluid as compared with pure water. Apart from the study on thermal resistance, the transient temperature distributions obtained from experiment and FEM simulation are found in good agreement. The LHP using SiO2–H2O nanofluid yields lower temperature and reaches its steady state faster than LHP using pure water. As the use of nanofluid in heat pipe is gaining attraction in an impetuous development of technology in this sphere and accompanied by a constantly growing amount of heat dissipated by functional components, for instance such as desktop PC CPUs, the present study would definitely open ways for further research. Works are underway to study the thermal performance of the proposed system with different types of nanofluids.

Table 3 Comparison of experimental and simulation temperatures. Temperature (°C)

Table 3 illustrates the comparison of experimental and simulation results at the steady state for LHP using pure water and SiO2–H2O nanofluid. The good agreement indicates the validity of the present methodology for the thermal analysis of the LHP. However, a detailed simulation by incorporating the multi-phase flow within the LHP may yield predictions that are more realistic. It is seen that, the predicted

Tb Te Tv Tc Tl

SiO2–H2O

Pure water Experiment

Simulation

Experiment

Simulation

101.21 66.26 59.01 51.12 42.36

102.54 68.31 59.07 52.72 44.95

83.06 61.71 55.34 47.89 35.73

83.44 63.89 56.09 47.23 35.81


Acknowledgment The authors would like to sincerely thank the Ministry of Higher Education (MOHE) of Malaysia for the provision of a grant with code no. 20120228FRGS to support this work. References [1] D.X. Gai, Z.C. Liu, W. Liu, J.G. Yang, Operational characteristics of miniature loop heat pipe with flat evaporator, Heat and Mass Transfer 46 (2) (2009) 267–275. [2] Y. Maidanik, S. Vershinin, V. Kholodov and J. Dolggirev, Heat transfer apparatus, US patent 4515209, May (1985). [3] M.C. Tsai, C.S. Yu, S.W. Kang, Flat plate loop heat pipe with a novel evaporator structure, Proceedings of SEMI-THERM 21 (2005) 187–190. [4] S. Nishio, Study of thermal characteristics of bubble-driven heat transport device, Transactions of the Japan Society of Mechanical Engineers 65 (640) (1999) 4077–4083. [5] Ji Li, D. Wang, G.P. Peterson, Experimental studies on a high performance compact loop heat pipe with a square flat evaporator, Applied Thermal Engineering 30 (2010) 741–752. [6] Y.F. Maydanik, Loop heat pipes, Applied Thermal Engineering 25 (2005) 635–657. [7] C.S. Chang, B.J. Huang, Y.F. Maydanik, Feasibility of a mini LHP for CPU cooling of a Notebook PC, Proc. of 12th Int. Heat Pipe Conference, Moscow, Russia, 2002, pp. 390–393. [8] V.G. Pastukhov, Yu.F. Maydanik, C.V. Vershinin, M.A. Korukov, Miniature loop heat pipes for electronic cooling, Applied Thermal Engineering 23 (2003) 1125–1135. [9] Y.F. Maydanik, S.V. Vershinin, M.A. Korukov, J.M. Ochterbeck, Miniature loop heat pipes — a promising means for cooling electronics, IEEE Transactions on Components and Packaging Technologies 28 (2) (2005) 290–296. [10] Y. Xuan, Q. Li, Heat transfer enhancement of nanofluids, International Journal of Heat and Fluid Transfer 21 (2000) 58–64. [11] C.Y. Tsai, H.T. Chien, B. Chan, P.H. Chen, P.P. Ding, T.Y. Luh, Effect of structural character of gold nanoparticles in nanofluid on heat pipe thermal performance, Materials Letters 58 (2004) 1461–1465.

91

[12] S.W. Kang, W.C. Wei, S.H. Tsai, S.Y. Yang, Experimental investigation of silver nanofluid on heat pipe thermal performance, Applied Thermal Engineering 26 (2006) 2377–2382. [13] S.W. Kang, W.C. Wei, S.H. Tsai, S.Y. Yang, Experimental Investigation of nanofluids on sintered heat pipe thermal performance, Applied Thermal Engineering 29 (2009) 937–979. [14] G.S. Wang, B. Song, Z.H. Liu, Operation characteristics of cylindrical miniature grooved heat pipe using aqueous CuO nanofluids, Experimental Thermal and Fluid Science 34 (2010) 1415–1421. [15] Y. Chen, et al., Effect of nanofluid on flat heat pipe thermal performance, Proceedings, 24th IEEE SEMI-THERM Symposium, San Jose Cal., USA, 2008. [16] Z. Liu, Q. Zhu, Application of aqueous nano fluids in a horizontal mesh heat pipe, Energy Conversion and Management 52 (2011) 292–300. [17] M. Shafahi, B. Vincenzo, K. Vafai, O. Manca, An investigation of thermal performance of cylindrical heat pipes using nanofluids, International Journal of Heat and Mass Transfer 53 (2010) 376–383. [18] W. Yu, S.U.S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovative Maxwell model, Journal of Nanoparticle Research 5 (2003) 167–171. [19] S.K. Das, S.U.S. Choi, W. Yu, T. Pradeep, Nanofluids Science and Technology, John Wiley & Sons, Hoboken, 2008. [20] B. Gavtash, K. Hussain, M. Layeghi, S.S. Lafmejani, Numerical simulation of the effects of nanofluid on a heat pipe thermal performance, World Academy of Science, Engineering and Technology 68 (2012) 549–555. [21] P. Naphon, D. Thongkum, P. Assadamongkol, Heat pipe efficiency enhancement with refrigerant–nanoparticles mixtures, Energy Conversion and Management 50 (2009) (2009) 772–776. [22] G. Franchi, X. Huang, Development of composite wicks for heat pipe performance enhancement, Heat Transfer Engineering 29 (10) (2008) 873–884. [23] J.G. Collier, J.R. Thome, Convective Boiling and Condensation, Clarendon Press, Oxford, 1996. [24] B.X. Wang, H. Li, X.F. Peng, Journal of Thermal Science 11 (3) (2002) 214. [25] Y. Xuan, Q. Li, Journal of Heat Transfer 125 (2003) 151.