Effect of fabrication parameters on the thermophysical properties of sintered wicks for heat pipe applications

Porous wicks for use in a loop heat pipe were sintered from copper and Monel powder. These wicks are characterized in terms of their porosity, liquid permeability, capillary pressure and thermal conductivity. The effect of fabrication parameters (particle size and sintering conditions) on these properties is studied. The experimentally measured values of permeability and capillary pressure were compared with correlations available in the literature. The Kozeny-Carman correlation was found to overpredict the experimental values of permeability; while the modified Young-Laplace equation was found to predict within 5% of the measured capillary pressure. Additionally, a model for predicting the thermal conductivity of sintered wicks is developed. First, the 'two-sphere model' is used to relate the sintering conditions to the size of the connection (the 'neck') between two particles. Then, a finite element simulation is used to determine the thermal resistance of the bonded particles as a function of the neck between them. This thermal resistance is integrated in a random 3D resistor network as a means to model the multiple connections between spheres in a wick and to calculate the effective thermal conductivity of the wick. Results of the model are compared with experimental measurements of thermal conductivity of sintered copper wicks. Agreement between the model and the experimental measurements is good (within 15%) for sintering temperatures below 550 °C, and within 26% for sintering temperatures up to 950 °C. Finally, a generalized thermal conductivity chart is presented, which can be used to estimate the sintering temperature and time required to achieve the desired thermal conductivity.