Numerical investigation on transient and steady phase change heat transfer in the evaporator of a loop heat pipe

The dynamic evolution of vapor-liquid distribution in a porous wick coupled with the heat conduction in the evaporator casing of a loop heat pipe is numerically solved. The mathematical model is validated based on the experimental results issued from the literature. Both the transient and steady-state thermal behaviors of the evaporator are focused on, with the self-adjusting ability, evaporator temperature, thermal resistance, and parasitic heat leakage analysis as the evaluation indexes. The gradual invasion of the vapor region into the wick over time and the robust response of the evaporator temperature to varying heat loads are tracked. Additionally, the steady-state performance is explored to optimize the structural parameters, namely, the location of grooves, wick material and porosity, casing material, and fin ratio, through which the specific influences of each parameter are figured out. It is found that locating grooves in wick yields a 7.63°C lower evaporator temperature and a more uniform temperature distribution on the evaporator with a difference of 0.75°C than in casing at 140 W. Additionally, a 0.9 mm thick nickel layer with a porosity of 0.4 combined with a 4.1 mm nickel stainless steel layer with a porosity of 0.75 constitutes an optimized layered wick, which exhibits a 7.04°C lower evaporator temperature and a 0.05°C/W smaller thermal resistance than a single-structured stainless steel wick with a porosity of 0.75. Considering the heat transfer and flow pressure drop, the counterbalanced fin ratio range of 0.5~0.6 and the better thermal performance of copper casing are identified. The numerical results are expected to provide references for the evaporator structure optimization, laying a solid foundation for applying loop heat pipes to thermal control fields.