Capillary performance of bi-porous sintered metal wicks

The capillary performance (K/Reff) of the wick is a key design parameter for the thermal performance of heat pipes. In this study, the capillary performance of a bi-porous wick is investigated experimentally and analytically. The effects of particle size (40 ⩽ d ⩽ 600 μm) and cluster size (250 ⩽ D ⩽ 1440 μm) on the capillary performance are investigated. The capillary flows in the wicks are visualized using sintered glass powder wicks and a high speed camera. From the experiments, it is shown that the capillary performances of bi-porous wicks are significantly higher than those of mono-porous wicks. The capillary performance of 125/675 μm bi-porous wick is 11 times larger than that of a 125 μm mono-porous wick. In the bi-porous wick, three distinct capillary flow regimes are observed, depending on the ratio between particle and cluster sizes (d/D). A semi-analytic model for predicting the capillary performance of bi-porous wicks is developed for each regime. The model prediction shows excellent agreement with experimental results and indicates that the cluster size should be 4–6 times larger than the particle size for an optimal capillary performance. The effect of the contact angle on the capillary performance of bi-porous wicks is shown to be negligible.