Capillary evaporator development and qualification for loop heat pipes

In the present paper, design consideration, fabrication techniques and post-assembly qualification tests to guarantee reliable structure of the capillary evaporator for loop heat pipe (LHP) with high heat flux transport capability has been presented. Cylindrical evaporator with nickel as high performance capillary pump, copper as high conductivity containment material and water as superior heat transfer working fluid was designed to transport 500 W heat load up to distance of 250 mm. Nickel powder with average particle diameter of 12 μm was sintered to obtain wick with high porosity (>65%) and fine pore radius <7.2 μm. High volumetric shrinkage (4-50% @750-1000 °C respectively) of the sintered wick results in poor control over the sample dimensions which makes wick integration inside evaporator difficult. Special procedures were conducted to develop machined nickel wick with good dimensional control (±0.15 mm) and high-roughness surface morphology, which provide high evaporation area and high surface porosity. These machined wick were successfully installed inside the evaporator using different integration methods. Qualification tests to validate evaporator structural integrity has been discussed in detail. For the designed loop heat pipe prototype, the minimum value of 0.15 °C/W for total thermal resistance was observed at maximum heat load of 500 W. In summary, capillary evaporator fabrication techniques (including wick development) and structural integrity qualification tests developed in this investigation can be used to fabricate miniature to large-scale loop heat pipes with high heat flux management capabilities.