Numerical investigation of characteristics of wick structure and working fluid of U-shape heat pipe for CPU cooling

https://doi.org/10.1016/j.microrel.2013.08.019Get rights and content

Highlights

  • A two-dimensional FEM simulation is performed on U-shape heat pipe.

  • The low difference in evaporator and condenser temperatures enhances heat transfer.

  • The use of screen mesh wick causes a decrease in the pressure drop in liquid-wick region.

  • The use of methanol as working fluid increases the liquid pressure drop.

Abstract

In the current study, the effects of the wick structures and working fluids characteristics on U-shape heat pipe performance were numerically presented. A two-dimensional FEM simulation was performed on U-shape heat pipe to analyze the temperature and the pressure distributions inside the heat pipe. In order to reduce the pressure arising in the area of the wick to increase heat transport capability at the liquid-wick region, a comparative numerical study was made by using methanol and water as working fluids with sintered powder and screen mesh wicks. The simulation was performed by using ANSYS/FLOTRAN software with heat input of 10 W at forced convection. The results showed that the use of sintered copper powder wick resulted in a small temperature difference between the evaporator and condenser compared to screen mesh wick. The use of screen mesh wick caused a decrease in the pressure drop in liquid-wick region while the use of methanol as working fluid increased the liquid pressure drop.

Introduction

The conventional way to dissipate heat from desktop computers was forced convection using a fan with a heat sink directly. However, with increased power as encountered in modern computers, the heat flux at the CPU has significantly increased. Consequently, there has been a growing concern for improved cooling techniques that suit the modern CPU requirements. As alternatives to the traditional heat sinks, two-phase cooling devices such as heat pipe, have been emerged as promising heat transfer devices; the effective thermal conductivity of a heat pipe can be 10–200 times more that of a solid copper rod of the same diameter [1].

Substantial experimental works have been reported on the application of heat pipes for electronic cooling; reviews by [2], [3], [4], [5] provided detailed information on this topic.

Two dimensional models that analyze the liquid and vapor flow and wall temperature along horizontal cylindrical heat pipe are developed by [6], [7], [8]. As well some researcher investigated the flat heat pipes experimentally and numerically such as [9], [10], [11].

The finned U-shape heat pipes have been introduced by Liang and Hung [12] for cooling the high-frequency microprocessors such as Intel Core 2 Duo, Intel Core 2 Quad, AMD Phenom series and AMD Athlon 64 series. Russel et al. [13] studied the effect of orientation on the performance of the U-shaped heat pipe with different wick structures. Subsequently, Elnaggar et al. [14] reported finite element (FE) simulation and experiment on vertically oriented finned U-shape multi heat pipes for desktop PC cooling. It was found that the total thermal resistance decreased with increase in heat input and coolant velocity. Moreover, the vertical mounting demonstrated enhanced thermal performance, compared with the horizontal arrangement; the lowest total thermal resistance achieved was 0.181 °C/W with heat load of 24 W and coolant velocity of 3 m/s. This study was pursued further [15] to determine the optimum heat input and the cooling air velocity for vertical twin U-shape heat pipe with the objective of maximizing the effective thermal conductivity.

Recently, Elnaggar et al. [16] presented a two-dimensional FEM simulation of working fluid behavior inside U-shape heat pipe to predict liquid pressure and the wall temperature by using ANSYS-FLOTRAN software.

Although Elnaggar et al. [16] proved that a good performance of U-shape heat pipe was achieved by employing water as working fluid with sintered copper powder wick, the unfavorable high pressure resulted in low heat transport capability at the liquid-wick region.

Therefore, the present study used other types of working fluid and wick structure to reduce the pressure arising in the area of the wick to increase heat transport capability at the liquid-wick region. Consequently, methanol and water as working fluids with sintered powder wick and screen mesh wick were employed in this simulations.

Section snippets

Mathematical formulation

Table 1 summarizes the specifications of the heat pipe. The complexity of the present heat pipe lies in the fact that it has one horizontal evaporator and two vertical condensers. The horizontal evaporator section directly communicates with the heat source.

In the current study, an attempt was made to extend the work of Elnaggar et al. [16]. Fig. 1 illustrates the detailed boundary conditions of the computational domain. The influence of gravity on the movement of liquid and the vapor inside the

Mesh generation

The analysis function of FLOTRAN CFD in ANSYS program was employed to analyze the two-dimensional fluid flow. The simulation was carried out by using the element FLUID141. The liquid-wick and solid regions were given relatively finer meshing, forming a total of 63,978 triangle elements (48,313 elements for liquid-wick region and 15,665 elements for solid region).

Results and discussion

The simulations are performed on U-shape heat pipe at heat input of 10 W under forced convection (the coolant airflow velocity V = 3 m/s). The physical properties of the working fluids are taken at 303 K as summarized in Table 3.

Conclusion

A two-dimensional FEM simulation of working fluid behavior inside U-shape heat pipe was performed by using ANSYS-FLOTRAN software to study the effects of characteristics of wick and working fluid on U-shape heat pipe performance. The results indicated that the use of sintered copper powder wick resulted in a small temperature difference between the evaporator and condenser but unfavorable high pressure resulted in low heat transport capability at the liquid-wick region. The use of screen mesh

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