Wetting hysteresis as the mechanism of heat pipe post-dryout thermal hysteresis and recovery

Abstract

Heat pipes and vapor chambers are passive thermal management devices used for efficient heat transport by phase change. Their passive operation is enabled by capillary pumping of the working fluid in a porous wick, which is operationally limited by the maximum pressure head it can provide. This capillary limit marks the maximum heat input at which the capillary pressure generated can overcome the pressure drop in the wick; operating above the capillary limit at steady state leads to dryout. Heat pipes and vapor chambers are increasingly being used in electronics systems where end-user activity dictates the transient power input which can therefore be highly variable and time-dependent. It was recently shown that heat pipes can withstand a power pulse exceeding the capillary limit for brief time intervals. Under such operating conditions, the heat pipe will experience dryout only if the duration of the pulse load is longer than a certain characteristic time interval. The pulse-load-induced dryout may result in an increased thermal resistance when the power is reduced back down to pre-dryout levels, thus exhibiting a hysteresis in heat pipe thermal performance. In this work, we experimentally characterize the recovery from pulsed-load-induced dryout. We further propose that the observed change in steady-state thermal performance before and after dryout results from contact angle hysteresis at the three-phase contact line of the wick-liquid interface. A model is developed based on this proposed mechanism to predict the nature of recovery from dryout-induced thermal hysteresis, as well as to identify that a given heat pipe has a maximum possible hysteresis. The experiments illustrate the trends inferred from the model for the recovery process and confirm the existence of a “maximum hysteresis line,” which identifies the worst-case scenario for thermal hysteresis after heat pipe dryout. Based on these mechanistic learnings, a new testing protocol is proposed for experimentally characterizing this post-dryout maximum hysteresis signature for a heat pipe.