Bimodal, Thin Wick Structures for High Heat Flux Two-Phase Thermal Control Systems

Modern electronic devices and power conversion systems in space technologies dissipate large amounts of heat through small surface areas, requiring advanced thermal control systems with high heat flux thermal management capabilities. Two-phase thermal control systems, i.e. heat pipes and vapor chambers, offer high heat flux cooling capabilities with reliable operations, and thin wick structures improve their cooling capabilities by assisting liquid supply to the heated surface, thereby avoiding premature surface dryout. Premature surface dryout is related to wicking ability, but it is primarily understood for wick structures with uniform particle-size distribution. The objective of this study is to explore and quantify the enhanced wicking ability of wick structures with bimodal particle-size distributions. This study examines the hypothesis that utilizing bimodal particle sizes in single-layer and multi-layer wick structures improves the wicking ability by creating extra liquid-permeable space from larger particle size and decreasing the effective pore radius from smaller particle size. The wick structures are manufactured using a sintering process with spherical copper particles ranging from 100-200 µm in diameter. To measure wicking ability, the rate-of-rise test is used. One end of the sample is submerged in a liquid acetone, causing the liquid to rise upward into the wick due to capillary suction. The rising liquid front is recorded with a video camera and the resulting video data is processed to analyze the wicking ability and calculate properties such as permeability and effective pore radius. The experimental results show that the wicking ability of the thin bimodal wick structures is significantly higher than that of thin wick structures with uniform particle-size distribution. This knowledge will provide an insight into optimal designs of advanced thermal management systems by discovering the relationship between particle size distributions and wicking ability.