Experimental and numerical investigations of a diffusion bonded flat-type loop heat pipe for passive thermal management

Abstract

Previous diffusion bonded loop heat pipes have been researched due to the benefits of this manufacturing technique, which includes direct sealing and maintaining material properties. The previous single-material diffusion bonded loop heat pipes were made of copper, leading to a high heat leak due to high thermal conductivity, causing a reduction in efficiency. This research used a new approach that separated the evaporator from the loop heat pipe while using diffusion bonding to manufacture a loop heat pipe using two materials, copper and stainless steel. This separation allowed for lower thermal conductivity materials to be used to help reduce the heat leak. Two evaporators with different wicks were tested, one made of copper and the other made of polytetrafluoroethylene. Both designs were tested in multiple orientations and operated for a range of 15 W to 45 W with an evaporator to condenser thermal resistance ranging from 2.18 °C/W to 0.004 °C/W. Additionally, a numerical model was developed to predict the performance of both evaporators, with an average temperature difference ranging from 5.5 °C to 6.6 °C for all thermocouple locations. The heat leak ratio was analyzed using the model, and a heat leak ratio of 7.6% to 8.0% and 13.6% to 16.0% for the copper wick and polytetrafluoroethylene wick loop heat pipe was calculated, respectively. This research demonstrates that diffusion bonding copper with a lower thermal conductivity material and using low thermal conductivity sealing material can help reduce the heat leak within diffusion bonded loop heat pipes.