Modulated porous wick evaporator for loop heat pipes: Experiment

Abstract The available studies indicate the significantly enhanced pool boiling heat transfer by the modulated porous wick sintered on the heater wall. This paper sinters the modulated porous wick as the primary wick on the evaporator wall for loop heat pipes (LHPs). Three types of evaporators were fabricated to integrate with other LHP components: MWE (microchannel/wick evaporator), MME (modulated monoporous wick evaporator) and MBE (modulated biporous wick evaporator). Experiments were performed with water as the working fluid at various tilt angles. The major findings are (1) MBE LHP significantly shortens start-up time and stabilizes all temperatures during the steady operation; (2) MBE LHP decreased the evaporator wall temperatures by 20∼50 °C at moderate or high heat loads compared with MWE LHP at similar conditions; (3) MBE LHP achieved the evaporator wall temperature of 63 °C at the heat load of 200 W for the anti-gravity operation, under which the heat flux attained 40 W/cm2, which is 1.7∼6.7 times of those reported in references; (4) MBE behaves the nucleate boiling heat transfer at small head loads, and film evaporation heat transfer at moderate or large heat loads; (5) A properly designed MBE LHP achieved better performance when the evaporator is above the condenser; (6) Effect of liquid charge ratios was studied with the best liquid charge ratio of 51.3%. The best geometric parameters for porous stacks and vapor channels, as well as the best particle size were obtained. The MBE multiscale behavior majorly accounts for the performance improvement: small pores (∼μm scale) creating great capillary force for liquid suction, large pores (∼10 μm scale) between clusters increasing surface area for liquid film evaporation, and vapor channels (∼mm scale) for vapor venting. Besides, small contact thermal resistance and reduced heat flow path in the evaporator also improve the LHP performance.

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