Experimental investigation and visual observation of a vapor-liquid separated flat loop heat pipe evaporator

It is known that the circulation driven head of a heat pipe is mainly determined by both the vapor pressure head and the liquid pressure head generated at the phase change interface. In this paper, a unique operation mechanism is proposed; in this mechanism, the circulation of the loop heat pipe (LHP) is primarily driven by the phase change formed at the vapor-liquid interface. To test the new mechanism, a visual flat LHP evaporator prototype and an open experimental system were deliberately designed and assembled to obtain the operation characteristics inside of the evaporation chamber (EC). Three main experimental stages, the pre-boiling, boiling and steady stages, were observed during the start-up process. The evaporation chamber with 5-mm height whose evaporator thermal resistance and LHP thermal resistance are 0.015 °C/W and 0.36 °C/W, respectively, has a better heat transfer performance. The results showed that the circulation driven head formed inside of the EC played an important role in promoting the operation performance, especially when the wick, the vapor-liquid interface and the bottom of the evaporator arrived at a reasonable situation.