Experimental investigation on the impact of pressure head of evaporation during the loop heat pipe operation

Abstract

The capillary force is normally considered as the main driving force for loop heat pipes. It has been theoretically proven that the pressure head of evaporation cannot be ignored during the LHP operation. To quantify the impact of the pressure head of evaporation, this work experimentally studied a LHP with two designs of the evaporator: in LHP-A, the wick is separated from the heating surface by a steam chamber and the evaporation mainly occurs in the steam chamber. In LHP-B, the wick gets touched on the heating surface and the evaporation occurs in the wick. Moreover, different wick materials are also tested, including stainless steel (LHP-B1) and copper (LHP-B2). The impact of pressure head of evaporation was comprehensively explored under both stable start-up form and overshoot start-up form. Results showed the performance of stable start-up form was better than that of overshoot start-up form. In the stable start-up form, LHP-A, which was enhanced by pressure head of evaporation, could decrease the operating temperature by 14.5 °C/17.5 °C, the start-up time by 5.7%/25%, and thermal resistance by 44.9%/63.8% at 55 W/220 W (2.8 W/cm2/11.2 W/cm2), respectively. Moreover, the wick with low thermal conductivity could decrease the operating temperature and thermal resistance especially at high heat load; but increase the start-up time. For the overshoot start-up form, LHP-A also had the best operating performance. What’s more, the usage of pressure head of evaporation was helpful to decrease the temperature oscillation during the LHP operation, especially at high heat load.