Effect of tube bending on heat transfer characteristics of miniature heat pipe with sintered porous media

Abstract

The miniature heat pipes have been widely employed in many thermal management applications of electrical equipments, especially in laptop computer. Bending of tube in heat pipe is unavoidable in such small space. However, the numerical codes to simulate the overall heat pipe operation still need to be established to explain the internal phenomena. Therefore, in this study, the heat transfer characteristics of a water-copper sintered-wick bent heat pipe were predicted by the established simulation program and compared with experimental results. The 2D numerical model was initially developed using Finite Element Method (FEM) and validated with the experimental results obtained from Huang et al. (1993) to verify the computational protocols. Although the water-copper screen-wick heat pipes have been studied in their research, their vapor and wall temperature profiles were adapted to compare with our 2D numerical results. The standard deviations of vapor and wall vi temperature between the present simulation and the experimental results are dramatically closed at 0.48 C and 1.78 C, respectively. After the 2D numerical model was justified, the 3D heat transfer and fluid flow in a miniature heat pipe at steady state was subsequently established. The calculated domains of heat pipe consisted of three parts; vapor core, wick, and container wall. In simulation, the vapor was assumed to be incompressible Newtonian fluid. The inertia force of vapor and the gravity effect were neglected. The wick was saturated with the liquid and the convective heat transfer in wick was neglected because the liquid velocity is very low. The governing equations, i.e. continuity, Stokes, and energy equations, and boundary conditions, were solved by using the FEM and the numerical results were compared with the experimental data obtained from our research tests. In the experiment, 27 heat pipes with the outer diameter of 6 mm and the length of 185 mm were used. The evaporator, adiabatic, and condenser section length were 15 mm, 100 mm, and 70 mm, respectively. The straight and bent heat pipes with the bending radius of 15 mm, 21 mm, 27 mm and the bending angle of 30, 60, 90 were constructed and tested at the heat input of 10 W, 20 W, and 30 W. It was found that both results from the experiment and the simulation well agree, the miniature heat pipe with sintered wick have insignificant thermal deterioration in range of bending scope. The percentage of standard deviation of ZB/ZS between the simulation and the experiment are 21.7%. Although, there was insignificant thermal deterioration in range of bending scope, it can further be realized at higher heat load and extremely small tube that, the tube bending affects the thermal performance of heat pipe.