Visualization experimental study on evaporation heat transfer characteristics of an open evaporator with a tower-like porous wick

Abstract

Temperature overshoot often occurs in loop heat pipe, which is caused by the large bubble in the compensation chamber. But it is still uncertain whether this large bubble grew from parasitic bubbles inside porous wick. A multi-layer tower-like porous wick is designed to understand the growth and expansion of parasitic bubbles. The parasitic bubbles are induced to occur inside porous wick by inserting a copper pillar on the evaporator wall, and the multi-layer porous wick is used to eliminate parasitic bubbles. The pumping stagnation occurs under certain heat loads, but temperature overshoot of the evaporator wall no longer occurs in the open evaporator with the tower-like porous wick, which guides the design of evaporator to avoid temperature overshoot. It is found that a lot of small bubbles only appears on the lower surface of porous wick immersed in liquid in the evaporator, while not appears on the upper surface. Three groups of repeated experiments are carried out sequentially and the performance of the evaporation heat transfer gradually deteriorates, which is speculated to be caused by a gap between porous wick and the copper pillar, larger pore size next to copper pillar, or poor wetting caused by copper pillar oxidation. Two different types of the evaporator overheating are discovered, including the overall retreat of the evaporating interface and dry-out region expanded from the internal of porous wick to the external. At high heat load, the evaporating interface expands along the copper pillar, causing an increase in the heat transfer coefficient of the evaporator. The research results indicate that in order to better utilize porous wick for evaporation heat transfer, it is necessary to reliably adhere porous wick to the heating surface and avoid the formation of the large bubble in the compensation chamber.