Capillary Limit in a Loop Heat Pipe with Dual Evaporators

Abstract

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 conducted.Tests include heat load to one evaporator only, even heat loads to both evaporators and uneven heat load to beth evaporators. 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 may continue to work and reach a new steady state at a higher operating temperature. The loop may even function with a modest increase in the heat load past the capillary limit With a heat load to only one evaporator, the capillary limit can be identified by rapid increases m the operating temperature and in the temperature difference between the evaporator and the CC. However, it is more difficult to tell when the capillary limit is exceeded if heat loads are applied to both evaporators. In all cases, the loop can recover by reducing the heat load to the loop.

Effect of the wick characteristics on the thermal behaviour of a LHP capillary evaporator

“…Another possible cause of this phenomenon is specific to pentane: a progressive dry-out could happen inside the wick just after the capillary limit is reached, which does not hinder the evaporator operation but would lead to lower thermal performance. This phenomenon was also observed by Nagano and Ku [29] and Ku and Birur [30] in heat pipes. It means that the capillary limit would be exceeded for the samples 2, 3, 4 and 8.…”

A review of thermal performance in multiple evaporators loop heat pipe

“…No matter LHP has single or multiple evaporators, the experimental results have proven that when capillary limit is exceeded by a small margin, LHP will still continue to function at a higher temperature. Moreover, LHP can recover from a partial dry-out by reducing the heat load without restart [6,11,[41][42][46][47][48].…”