Enhancement of loop heat pipe heat transfer performance with superhydrophilic porous wick

Abstract

A loop heat pipe (LHP) can effectively improve the heat dissipation capacity for a variety of equipment and instruments with high heat flux. A superhydrophilic porous wick was fabricated in a LHP to investigate the effect of wettability and nanostructures on heat transfer performance. These wicks can be achieved by copper powder sintering and surface modification through hydrogen peroxide (H2O2) oxidation. For the LHP with a superhydrophilic wick (SH-LHP), the heat transfer performance was significantly enhanced in comparison with the LHP with a hydrophilic wick (H-LHP). SH-LHP can startup at a lower heat load, and it has smaller temperature fluctuations. At a high heat load, it first enters the normal working mode, and it has better temperature uniformity. When the heat load is 240 W, the temperature in the center of the evaporator is 64.3 °C, which is nearly 8.7 °C below that of H-LHP. Under anti-gravity condition, SH-LHP shows its superior heat transfer performance at a high heat load; the maximum heat transfer coefficient increased by 50% in comparison with the H-LHP. The main mechanism has three aspects. First, superhydrophilic wicks have a large capillary force that provides a sizable driving force and an adequate liquid supply for the circulation and phase transformation of the working fluid. Second, the superhydrophilic wick has multi-scale structures in the nanometer to millimeter range, which effectively regulates the vapor-liquid distribution and bubble formation. Third, the nanostructures on the superhydrophilic wick's surface provide more nucleation sites and a large surface area for phase change heat transfer.