Capillary Limit in a Loop Heat Pipe with Dual Evaporators 2002-01-2503

This paper describes a study on the capillary limit of a loop heat pipe (LHP) with two evaporators and two condensers. Both theoretical analysis and experimental investigation are performed. Experimental tests conducted include heat load to one evaporator only, even heat loads to both evaporators, and uneven heat loads to both evaporators. Test results show that after the capillary limit is exceeded, vapor will penetrate through the wick of the weaker evaporator, and the compensation chamber (CC) of that evaporator will control the loop operating temperature regardless of which CC has been in control prior to the event. Because the evaporator can tolerate vapor bubbles, the loop can continue to work after vapor penetration. As the loop operating temperature increases, the system pressure drop actually decreases due to a decrease in liquid and vapor viscosities. Thus, the loop may reach a new steady state at a higher operating temperature after vapor penetration. Furthermore, this process can be repeated with a modest increase in the heat load each time the capillary limit is exceeded until the evaporator temperature exceeds the maximum allowable value. When the heat load is applied to only one of the evaporators, the capillary limit can be identified by a rapid increase in the operating temperature and in the temperature difference between the evaporator and the CC. However, it is more difficult to identify when the capillary limit is exceeded if the heat load is applied to both evaporators. In all cases, the loop can recover by reducing the heat load to the loop.