Experimental tests of a stainless steel loop heat pipe with flat evaporator

This paper describes the experimental results showing the thermal characteristics of a loop heat pipe (LHP) with the evaporator in the shape of a flat disk having an active diameter of 50 mm and a thickness of 13 mm. The loop and the wick are made of stainless steel and water is used as working fluid. The system could be used for the thermal control of space application devices, and the goal of the experimental research was to improve and extend the knowledge in this field, adding the results to those obtained with these devices manufactured with different design, architecture and material.Tests carried out at horizontal elevation (orientation), in both configurations, that is with the evaporator below the compensation chamber (favourable) and with the evaporator above the compensation chamber (unfavourable), allow the analysis of the start-up reliability and the operational characteristics of the LHP in a wide range of heat loads. During the testing of the LHP under single step heat load, the thermal response presented by the loop to achieve steady-state is very long implying a long start-up time. With the evaporator above the compensation chamber (unfavourable), thermal and hydraulic oscillations are observed throughout the loop. The effect of these oscillations on the thermal performance of the LHP is not very significant. In the favourable configuration no oscillation occurs and the device is able to transfer a maximum heat load of 75 W with the evaporator temperature below 150 °C. The thermal resistance (RLHP) of the LHP, which is the resistance from the evaporator external surface to the condenser external surface (refrigerant water sink), lies between 3.33 and 50.7 °C/W.