Abstract: With the extension of application field, the loop heat pipes （LHPs） may operate under various attitudes, including gravity-assisted attitude. The steady-state operating characteristics of LHPs under gravity-assisted attitude are experimentally studied and compared with those under horizontal and adverse elevation attitudes. The experimental results show that there exist two driving modes when LHPs operate under gravity-assisted attitude: gravitydriven mode and capillary force and gravity co-driven mode. When the heat load is small, the LHPs operate in gravity-driven mode, the fluid in the vapor transport line is in the two-phase state, and the operating temperature is obviously lower than that under horizontal and adverse elevation attitudes, due to the strengthen of cooling effect of the return liquid on the compensation chamber. When the heat load is bigger than the critical value, the LHPs operate in capillary force and gravity co-driven mode, the fluid in the vapor transport line is in the single-phase vapor state, and the operating temperature is approximately the same as that under horizontal and adverse elevation attitudes. In the end, the experimental phenomena are analyzed and explained based on the pressure balance in the system and the energy balance of the compensation chamber.

Key words: loop heat pipe, gravity-assisted, operating characteristic, energy balance