Analysis of the optimum configuration for the capillary rise and the permeability of the fiber wick structure for heat removal in heat pipes

Abstract

The condensed working fluid inside a heat pipe is pumped from the condenser to the evaporator section to complete the operating cycle by using the capillary force generated inside the wick structure. Since the miniature heat pipes have smaller vapor cross-sectional area, the fiber wick structure is suitable for this application. The fiber wick structure can be designed to provide an excellent flow path for the working fluid with an optimum configuration based on a hexagonal fiber arrangement. By a microscopic investigation of the wick structure, the porosity can be related with the physical properties, effective pore radius and the capillary pressure. The effective pore radius and the capillary pressure are important parameters to characterize the permeability because it enables one to predict the flow rate obtainable under a given pressure drop necessary to achieve a specific circulation condition, which affects the heat transfer for the heat pipes. In this study, the effect of the porosity on the effective pore radius and the capillary pressure are discussed. The rate of the test liquid rise has been used to estimate the permeability on several fiber wick samples where the porosity varies. The optimum porosity of the fiber wick structure in miniature heat pipes is found at 0.45 while the permeability approaches the maximum value of 1.26 × 10−12 m2 which results in an excellent circulation of the working fluid from the condenser to the evaporator section. Moreover, the capillary performance (K/reff) of the fiber wick is further investigated for the thermal evaluation of heat pipes. The values of the maximum heat transfer rate due to capillary limit from our prediction reach to 1.81 and 2.68 W for heat pipe diameters of 2 and 3 mm, which contain sintered fiber wick structures under optimum design conditions.