Startup characteristics of an ammonia loop heat pipe with a rectangular evaporator

Flat-plate loop heat pipe (FLHP) is a passive two-phase heat transfer device. Comparing with traditional LHP with a cylindrical evaporator, it can be directly connected to a flat heat source without the employment of a saddle, which can effectively reduce the system thermal resistance and enhance the temperature uniformity. In this work, a stainless steel-ammonia FLHP was developed, and extensive experiments have been conducted to investigate its startup characteristics with the evaporator in the horizontal and vertical positions. Experimental results show that the FLHP exhibits excellent startup performance. It can successfully start up at a small heat load as low as 2 W with no obvious temperature overshoot. In a wide power range of 5–35 W, the FLHP generally starts up in only one situation, much simpler than the startup of a LHP with a cylindrical evaporator. For this rectangular evaporator, the heat leak from the evaporator to the compensation chamber (CC) becomes very small. As a result, the vapor can easily exit the condenser in most cases in the power range of 5–35 W, leading to a 100% utilization efficiency of the condenser and the resultant satisfactory thermal performance of the FLHP. In addition, the startup performance and the system thermal resistance of the FLHP are insensitive to the evaporator orientation, promising great application potential in future electronics cooling.