Visualization of vapor–liquid interface and optimization in vapor grooves of loop heat pipe

Loop heat pipes (LHPs), utilizing the capillary force to circulate working fluid internally, have been widely employed as an efficient thermal management device in both aerospace industries and household technologies. Previous studies have proposed various optimization methods for LHPs and have succeeded in improving the device efficiency, while there still exist some difficult problems in dispute. A disagreement is on the optimal groove size and such inconsistency can be owing to the common neglect on the influence imposed by the changing vapor–liquid interface. In this work, therefore, a three-dimensional numerical simulation model and a visualization experiment were used to simultaneously investigate the vapor–liquid distribution and the thermal efficiency of the LHPs with different groove dimensions. The liquid film in the vapor groove gradually attenuates and retracts into the wick as the ratio of the depth and width of vapor groove ($\beta$) increases. The optimal thermal performance is achieved once the liquid interface coincides with the solid interface, namely, neither well above the wick surface nor beneath it, which corresponds to an optimal $\beta$ value. Based on the experimental and numerical studies, such optimal value is around 1 over a wider range of heat loads. On the other hand, the lower the ratio of the width for grooves and fins ($\eta$), the less thermal resistance there is from the wall to the vapor–liquid interface, and thus the lower the temperature of the evaporator. The suitable range of $\eta$ is within 0.7–0.9 exhibiting no clear dependency on the vapor–liquid distribution.