Thermal resistance of screen mesh wick heat pipes using the water-based Al2O3 nanofluids

https://doi.org/10.1016/j.ijheatmasstransfer.2010.07.050Get rights and content

Abstract

In the present study, the effect of nanofluids on the thermal performance of heat pipes is experimentally investigated by testing circular screen mesh wick heat pipes using water-based Al2O3 nanofluids with the volume fraction of 1.0 and 3.0 Vol.%. The wall temperature distributions and the thermal resistances between the evaporator and the adiabatic sections are measured and compared with those for the heat pipe using DI water. The averaged evaporator wall temperatures of the heat pipes using the water-based Al2O3 nanofluids are much lower than those of the heat pipe using DI water. The thermal resistance of the heat pipe using the water-based Al2O3 nanofluids with the volume fraction of 3.0 Vol.% is significantly reduced by about 40% at the evaporator-adiabatic section. Also, the experimentally results implicitly show that the water-based Al2O3 nanofluids as the working fluid instead of DI water can enhance the maximum heat transport rate of the heat pipe. Based on the two clear evidences, we conclude that the major reason which can not only improve the maximum heat transport rate but also significantly reduce the thermal resistance of the heat pipe using nanofluids is not the enhancement of the effective thermal conductivity which most of previous researchers presented. Especially, we experimentally first observe the thin porous coating layer formed by nanoparticles suspended in nanofluids at wick structures. Based on the observation, it is first shown that the primary mechanism on the enhancement of the thermal performance for the heat pipe is the coating layer formed by nanoparticles at the evaporator section because the layer can not only extend the evaporation surface with high heat transfer performance but also improve the surface wettability and capillary wicking performance.

Introduction

There has been of interest in the use of heat pipes for thermal management due to increasing heat flux requirements and thermal constraints in many industrial applications. In general, the performance of a heat pipe depends on its geometry, working fluid, capillary wicking material, operating temperature, and applied heat flux [1]. Among them, the selection of the working fluid has essential importance to achieve the enhancement of the thermal performance since the heat pipe utilizes phase change phenomenon of the working fluid. Nanofluids are new kind of heat transfer fluids in which nanoparticles are uniformly and stably dispersed. Due to its unprecedented thermal characteristics, several researchers have used nanofluids as working fluids of heat pipes to improve their heat transfer performance [2], [3], [4], [5].

Chien et al. [2] firstly conducted experiments for applying nanofluids to a disk-shaped aluminous miniature micro-grooved heat pipe. The diameter and the thickness were 9 and 2 mm, respectively. The depth, the width, and the number of rectangular micro-grooves were 0.4 mm 0.35 mm, and 18 mm, respectively. The nanofluid consisted of gold nanoparticles with a diameter of 17 nm and DI water. The total thermal resistance of the heat pipe using nanofluids was compared with that of the heat pipe using DI water. Experimental results showed that the total thermal resistance of the heat pipe using nanofluids was reduced by about 40%, compared with that using DI water at different filling ratios. Wei et al. [3] used a cylindrical micro-grooved heat pipe with the inner diameter and the length of 6 and 200 mm, respectively. The width and the depth of the rectangular groove were 221 and 217 μm, respectively. The working fluid consisted of silver nanoparticles with an average particle size of 10 nm and DI water. The total thermal resistance of the heat pipe using nanofluids could decrease by 28–44% compared with that of the heat pipe using DI water. Kang et al. [4] employed the dilute dispersion of silver nanoparticles with 10 and 35 nm diameters into DI water as a working fluid. They showed that the total thermal resistance decreased 10–80% compared to DI water at a heat input of 30–60 W under the fixed charge volume. Furthermore, they suggested that the heat pipes were properly operated at the higher power compared with the operating power of the heat pipes with DI water. They considered that the improvement of thermal performance is mainly due to the reduction of fluid temperature gradient in nanofluids. Yang et al. [5] carried out an experiment to study the heat transfer performance of a horizontal micro-grooved heat pipe using CuO nanofluids as working fluids. Mass concentration of CuO nanoparticles having the average diameter of 50 nm and the operating pressure vary from 0.5 wt% to 2.0 wt% and from 7.45 kPa to 19.97 kPa, respectively. The experimental results showed that CuO nanofluids can improve the thermal performance of the heat pipe. Under an operating pressure of 7.45 kPa, the heat transfer coefficients of the evaporator can be averagely enhanced by 46% and the CHF can be maximally enhanced by 30% compared with those of the heat pipe using DI water.

Unlike the heat pipe with the grooved wick structure, there are only a few experimental investigations on the application of nanofluids in the heat pipe with the mesh wick structure, which is to deal with the present study. Tsai et al. [6] performed an experimental investigation for a circular meshed heat pipe using gold nanofluids. They presented that the thermal resistances of the heat pipe with solutions of various-sized gold nanoparticles range from 0.17 to 0.215 °C/W, which is 37% lower than the thermal resistance of the heat pipe using DI water. Chen et al. [7] studied the thermal performance of axially flat mesh wicked heat pipe using water-based silver nanofluids with different nanoparticle concentrations under the input power of 20–40 W. The average diameter of nanoparticles was 35 nm. The height and the length of the heat pipe used in the experiment were 3 and 200 mm, respectively. The total thermal resistance of the heat pipe using nanofluids is reduced compared with that of the heat pipe using DI water. In the volume concentration range tested, the larger the volume concentration of nanoparticles was, the more reduction of the thermal resistance could be. From the previous experimental studies, they concluded that the reasons for the heat transfer enhancement of the heat pipes might be the improvement of the effective liquid conductance due to the increase of the effective thermal conductivity of nanofluids [4], [5], [6], [7].

Most recently, mathematical models have been presented to quantitatively evaluate the thermal performance of heat pipes using nanofluids [8], [9], [10]. Shafahi et al. [8], [9] used a two-dimensional model to simulate the thermal performance of both a cylindrically grooved heat pipe and flat-shaped heat pipe utilizing nanofluids. Three of the most common nanoparticles, Al2O3, CuO, and TiO2 were applied as the working fluids. The simulation found that the nanoparticles within the base liquid enhance the thermal performance of the heat pipe by reducing the thermal resistance while enhancing the maximum heat load. They showed that there exists an optimum nanoparticle mass concentration corresponding to the maximum heat transfer enhancement. In addition, smaller particles have a more pronounced effect on the thermal resistance of the heat pipe. In their study, existing prediction models for the effective thermal conductivity, the viscosity, and the density were used under the assumption that nanoparticles in the base fluid are uniformly dispersed. They mentioned that the change of fluid thermophysical properties was the major mechanism of the heat transfer enhancement when nanofluids were utilized as the working fluid of the heat pipe. Do and Jang [10] developed a mathematical model for quantitatively evaluating the thermal performance of a water-based Al2O3 nanofluid heat pipe with a rectangular grooved wick. They considered the effects of the thermophysical properties of nanofluids as well as the surface characteristics formed by nanoparticles such as a thin porous coating on the thermal performance of the nanofluid heat pipe. The model prediction results showed the feasibility of enhancing the thermal performance up to 100% although water-based Al2O3 nanofluids with the concentration less than 1.0 Vol.% was used as the working fluid. Also, they concluded that the thin porous coating layer formed by nanoparticles suspended in nanofluids was a key effect of the heat transfer enhancement for the heat pipe using nanofluids.

Although many investigators hypothesized the reasons for the heat transfer enhancement based on the results obtained by model predictions or experimental studies, the major heat transfer enhancement mechanism for the heat pipe using nanofluids is not only unclarified but also still poorly understood. So in this paper, the thermal resistances of circular screen mesh wick heat pipes with the water-based Al2O3 nanofluids are measured to understand the effects of nanofluids on the thermal performance. For the purpose, the wall temperature distributions of the heat pipe using the water-based Al2O3 nanofluids are obtained and compared with those of the heat pipe using DI water. Especially, the screen mesh wick used in this experiment is observed using an optical microscope in order to investigate the change of the surface microstructure and topography.

Section snippets

Preparation of water-based Al2O3 nanofluids

The nanoparticles used in these experiments are Al2O3 nanoparticles manufactured and supplied by Nanotechnologies Inc. A two-step method is used to manufacture water-based Al2O3 nanofluids with the volume fraction of 1.0 and 3.0 Vol.% without any surfactant. The nanofluids are oscillated in an ultrasonic homogenizer at sound frequencies of 30–40 kHz to attain uniformly dispersed and stably suspended nanofluids [11]. The Al2O3 nanoparticles are further characterized with a transmission electron

Results and discussion

Fig. 5 shows the wall temperature distributions for heat pipes using DI water and the water-based Al2O3 nanofluids, respectively. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and also increase with input power. As shown in Fig. 5(b) and (c), the wall temperatures at the evaporator section for the heat pipe using water-based Al2O3 nanofluids with 1.0 and 3.0 Vol.% are lower than those for DI water under the fixed

Conclusion

In the present study, the effects of the water-based Al2O3 nanofluids on the thermal performance of heat pipes are experimentally investigated by testing circular screen mesh wick heat pipe using the water-based Al2O3 nanofluids with the volume fraction of 1.0 and 3.0 Vol.%. In order to quantitatively evaluate the thermal performance of the heat pipe using the water-based Al2O3 nanofluids, the wall temperature distributions and the thermal resistances between the evaporator and the adiabatic

Acknowledgement

This work was supported by the Korea Research Foundation Grant by the Korean Government (KRF-2008-331-D00065).

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