Investigations on vapour blanket formation inside capillary wick of loop heat pipe

Abstract

Deprime in a loop heat pipe (LHP) is a phenomenon of drying out of capillary wick due to penetration of vapour into it. The vapour zone thickness increases with the LHP heat input due to the liquid-vapour interface recession into the wick away from the heating zone. An analytical model referred to as zero recession model is first presented to predict the evaporator wall temperature assuming that the capillary wick is always fully saturated with liquid. Experimental results on an LHP with four working fluids (acetone, methanol, n-pentane and ethanol) for a range of heat inputs till deprime are analyzed. The model predictions reveal that there is an additional resistance offered by the vapour zone of the wick to the heat flow. Subsequently, another model referred to as recession model is presented based on minimization of the difference between the predicted and measured evaporator wall temperatures to predict an equivalent vapour blanket thickness inside the wick with the assumption that the vapour zone thickness inside the wick is uniform. The predictions reveal that the equivalent vapour blanket thickness increases with the applied heat input due to the liquid-vapour interface movement away from the heated surface and approaches the wick thickness at a heat input very close to deprime. It was also found that drop in the evaporative heat transfer coefficient is mainly due to the increase in the equivalent vapour blanket thickness in the wick.