Measurement of the permeability and porosity for sintered fiber bundle wicks in miniature heat pipe

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. In this study, the effect of the porosity on the effective pore radius and the capillary pressure are discussed. The appropriate method for the porosity investigation was the vertical drop test which the distilled water was used as the working fluid. All the experimental data of the porosity test leaded to the conclusion that the porosity of the sintered fiber bundle wick decreased with the increase of number of fibers. In this study, each single wick fiber wick is packed in a hexagonal array creating a porosity variation. For 50 um fibers packing, the critical pore radius for the packing is found to be 117 um to maintain the pumping force by the capillary action. The appropriate methods of fluid flow characterization for porous wick structure have been developed and tested in order to predict the wick porosity and the permeability. 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 1.26x10-12 m2 which results in an excellent circulation of the working fluid from the condenser to the evaporator section. The effect of wick porosity and pipe flattening on the thermal performance have been experimentally and numerically investigated. The thermal performance of the flat-shaped heat pipe with the sintered fiber bundle wick was experimentally investigated under a horizontal orientation in this study. Considering the effect of pipe flattening, when the heat pipe was flattened at normal region, the overall thermal resistance slightly decreased resulting from the increase of the contact surface in evaporator and condenser sections. It was found that the final thickness decreased approximately 50% from original diameter, the overall thermal resistance reduced about 9%. However, the thermal resistance drastically increased resulting from the increase of pressure drop in vapor core which was observed at a critical region. For original diameter of 2 mm, when heat pipe was flattened into minimum final thickness of 0.45 mm which approximately was 78% from original diameter, the thermal resistance severely increased up to 87% compared with the thermal resistance observed in round- shaped heat pipe. The flattening of heat pipe caused the smaller space of vapor core, therefore the working fluid in vapor phase could not easily transfer heat. Moreover, the new normalized parameter is proposed to represent the appearance of vapor core after pipe flattening which can be used to suggest a desirable normal region of flattened heat pipe with the sintered fiber wick. The critical remaining vapor core height after pipe flattening has been further analyzed to be 0.06 (about 74% from original diameter). For the optimization of miniature heat pipes, the new designed commercial of HPSFWs were completely carried out. Considering the customer demand which the final thickness of heat pipe was needed at 0.45 and 0.8 mm for 2 and 3 mm original diameter of HPSFWs, the maximum net saving was indicated at the porosity of 0.80 and 0.91, respectively.