Experiments and Analyses of Flat Miniature Heat Pipes

Abstract

Two flat copper-water axially grooved miniature heat pipes are fabricated employing the electric-discharge-machining (EDM) wire-cutting method. Because of the advantage of the EDM method, axial grooves are provided around the entire interior perimeters of the miniature heat pipes. The two miniature heat pipes are tested under different heat inputs, cooling temperatures, and orientations. The maximum heat transfer rate for the heat pipes tested is about 40 W, and the maximum heat flux achieved is about 20 W/cm 2 , based on the outer surface of the evaporator. The effective thermal conductance of the heat pipe is on the order of 40 times that of copper based on the external cross-sectional area of the miniature heat pipe. If the effective thermal conductance is evaluated based on the interior cross-sectional area of the heat pipe, it can be 100 times higher than the thermal conductivity of copper. Analyses for heat-pipe limitations are also performed based on certain analytical relations. It is found that the capillary limit is the dominant heat transfer limitation for the miniature heat pipes tested in this paper. To improve the accuracy of the analytical model, the hydraulic radius in the capillary limit is corrected using the exact two-dimensional solutions found in the literature, and the expression for the friction factor that takes into account the shear stress at the liquid/vapor interface is adopted. The analytical results based on these modifications are compared with corresponding experimental results with good agreement.