Two-phase flow characteristics of a high performance loop heat pipe with flat evaporator under gravity

Aiming to fully investigate the two-phase flow characteristics inside a loop heat pipe (LHP), a visualization study for a compact copper-water loop heat pipe with flat evaporator was carried out under gravity favorable mode in this work. Under forced water cooling condition with the cooling water temperature of 25 °C, the results indicated that the compact LHP could efficiently dissipate a maximum heat load of 550 W with the evaporator temperature of 91.2 °C. A minimum LHP thermal resistance was 0.068 °C/W, which was obtained at 500 W. In addition, the evaporator temperatures at boiling incipience under different heat loads were evaluated, which showed slight dependence on the input heat load. Finally, the flow patterns during the startup stage and the steady stage associating with different heating loads were recorded and analyzed to investigate the internal operation mechanism of the LHP. As a result, various two-phase flow patterns were found to be related to the heat load, and the occurrence of partial dry-out was observed in the evaporator at the maximum heat load of 550 W, corresponding to a heat flux of 88 W/cm2.