Elsevier

Applied Thermal Engineering

Volume 99, 25 April 2016, Pages 32-41
Applied Thermal Engineering

Research Paper
Effects of vacuuming process parameters on the thermal performance of composite heat pipes

https://doi.org/10.1016/j.applthermaleng.2016.01.035Get rights and content

Highlights

  • Heat pipes with a sintered-grooved composite wick structure are investigated.

  • Effects of various process parameters are examined experimentally.

  • Thermal performances are investigated by conducting both transient-state and steady-state tests.

Abstract

Cylindrical heat pipes with sintered-grooved composite wicks are manufactured by more than 20 processes. Essential to their thermal performances are the working fluid filling and vacuuming processes. In this work, the effects of various process parameters on the thermal performance of a composite heat pipe were examined experimentally by conducting transient and steady-state tests. Under the conditions of the first vacuuming process, the effective working length showed a more remarkable effect on the start-up performance of the heat pipes than the first vacuuming time and filling ratio. The isothermal performance demonstrated sensitivity to the filling ratio. Under the conditions of the second vacuuming process, the second vacuuming temperature showed a remarkable effect on the isothermal performance. The thermal resistances were less than 0.02 K/W at the evaporator and less than 0.09 K/W at the condenser with respect to those less than 0.16 K/W after the first vacuuming process.

Introduction

The development of modern electronic devices is moving toward compactness and miniaturization; along with this change are the problems of high heat flux and limited cooling space, which could seriously affect the performance of electronic devices [1], [2]. As a reliable and efficient heat transfer component, heat pipes have been widely used for the thermal management of electronic devices because of their high heat conductivity, fast thermal response, and good isothermal performance [3], [4]. According to the phase change phenomenon, the working fluids vaporize at the evaporator of heat pipes because of the heat source, which could be transferred to the condenser where the heat is released. The internal wick structures of heat pipes provide the capillary force to drive the working fluid back to the evaporator. Therefore, heat could be constantly transferred without additional power supply.

The wick structures of heat pipes commonly come in groove, mesh, and sintered metal powder forms. However, high capillary force and high permeability are difficult to achieve from a single type of wick structure. Thus, composite wick structures must be investigated. Semenic et al. [5] pointed out that biporous wicks perform better than monoporous wicks because they develop evaporating menisci on top surfaces and within structures [6]. In addition to the wick structure, the internal vacuum condition in heat pipes, which is guaranteed by vacuuming technology, has been proven to be a critical factor in heat pipe performance [7]. Poor vacuum condition indicates the presence of non-condensable gas, which could not condense during the operation, and affects the circulation of the working fluids. Such poor circulation seriously worsens heat pipe performance. In general, two vacuuming processes are employed to ensure the vacuum degree inside heat pipes [8]. Both processes need appropriate parameters to guarantee vacuuming quality and thus, the investigation of process parameters is necessary.

In some experiments, heat pipes are filled with working fluid after the internal vacuum degree reaches a certain level [9]. However, this process is time consuming and the cost of the equipment is high. It takes a long time to reach high vacuum degree inside the pipe even for a vacuum pump with fast pumping speed. Residual working liquid exists in the filling tube during continuous operation, the evaporation of which would also prolong the vacuuming process. Moreover, completing the filling and vacuuming processes by one machine increases the complexity of the manufacturing process, which demands a highly complex and expensive machine. By separating the filling and vacuuming processes, the efficiency is improved because the two processes can be carried out individually at the same time, and automation for individual process can be realized with lower cost. In engineering applications, vacuuming heat pipes after filling the working fluid is already in use as a flexible and economical approach. High vacuum degree can be guaranteed by two vacuuming processes. The working fluid, which is often deionized water, does not need to be degassed before filling using this method. However, controlling the parameters of this process to guarantee heat pipe performance is considerably complicated. Several studies have explored the effects of related process parameters, including the filling ratio and vacuum condition, on heat pipe performance. It is shown that the volume of the working fluid should be equal to the pore volume of the wick structure; hence, the wick structure should be saturated [10], [11]. Chen and Chou [12] examined the effects of liquid filling ratio and leakage on the cooling performance of a flat plate heat pipe. In this study, improper vacuuming and leakage significantly decreased the maximum thermal conductivities from 3150 W/m⋅K to about 200–306 W/m⋅K and 164 W/m⋅K, respectively.

The transient and steady-state thermal performances of heat pipes have also been investigated under different process parameters. Saad et al. [13] analyzed the effect of non-condensable gas on the transient performance of copper-water wicked heat pipes. The results indicated that non-condensable gas does not significantly affect the transient response of heat pipes during the heat-up phase. Xu et al. [14] sintered the modulated porous wicks for loop heat pipes and found that heat pipes with the modulated biporous wick evaporator significantly shorten start-up time. Li et al. [15] conducted transient and steady performance tests to obtain the maximum heat transport capacity of ultra-thin heat pipes and predicted the thermal resistances by a mathematical model, which were consistent with the experimental results. Kim et al. [16] proposed a mathematical model to investigate the heat transfer characteristics of a grooved heat pipe and determined the heat transport capacity and the overall thermal resistance under various parameters. The predicted maximum heat transport capacity and the overall thermal resistance under steady-state conditions were found to be in close agreement with the experimental results. These results also revealed that the groove wick significantly affects the isothermal performance through thermal resistance.

At present, only a limited number of studies have investigated the filling and vacuuming process parameters of heat pipes, particularly the parameters of the filling–vacuuming process, in which the working fluid is charged into the heat pipes before the vacuuming processes. Tang et al. [17] investigated the effects of the first vacuuming and second vacuuming processes on the isothermal performance of a microgrooved heat pipe. In the experiment, the temperature difference between the evaporator and the condenser generally increased with the increase in the filling ratio, and increasing the first vacuuming time had little effect on the isothermal performance of the heat pipes, whereas the second vacuuming process had remarkable influence. The isothermal performance was studied in detail, but further research must be conducted in addition to isothermal performance. In the present work, we therefore study the filling and vacuuming processes of a composite heat pipe. Both the transient and steady-state performances of the heat pipes are analyzed. The objective is to determine the effects of various process parameters on the start-up performance, isothermal performance, maximum heat transport capacity, and thermal resistance of the composite heat pipes, which would be of help to heat pipe manufacture.

Section snippets

Experimental setup

A composite wick (sintered-grooved wick) is utilized to provide the capillary force for circulating the working fluid. The axially inner grooves are fabricated by employing high-speed ball spinning and drawing processes [18]. Compared with the traditional single wick, the sintered-grooved wick provides an optimum combination of the capillary pressure produced by the powder wick and the permeability of axial grooves [19]. The experimental heat pipes with the composite wick structure are shown in

Effects of vacuuming processes on the start-up and isothermal performances

The start-up performance of the heat pipes with different first vacuuming process parameters is illustrated in Fig. 6. The temperature of the top testing position T1 was selected to evaluate the start-up performance. A good start-up performance indicated that the temperature T1 quickly reached equilibrium. Although the evaporator was heated in the water bath immediately, it took time for the internal working fluid to vaporize and flow through the vapor space. Moreover, the condenser experienced

Conclusions

Various parameters of the filling and vacuuming processes for a composite heat pipe were studied to analyze their effects. Conclusions are summarized as follows:

  • (1)

    Under the first vacuuming conditions, the effective working length had a remarkable effect on the start-up performance. The response time increased from 35 s to 75 s when the effective working length increased from 200 mm to 400 mm.

  • (2)

    The isothermal performance was sensitive to the filling ratio and the second vacuuming temperature. When

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 51175186), Guangdong Provincial Natural Science Foundation of China (Grant No. S2013020012757), and EU project PIIF-GA-2012-332304 (Grant No. ESR332304).

References (20)

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