The balance between the capillary pressure provided by the wick in a heat pipe or vapor chamber and the flow resistance to liquid resupply at the evaporator determines the maximum heat load that can be sustained at steady state. This maximum heat load is termed as the capillary limit; operation at steady heat loads above the capillary limit will result in dryout at the evaporator wick. However, different user needs and device workloads can lead to highly transient heat loads in applications ranging from con- sumer electronic devices to server processors. In these instances, the operation of heat pipes must be assessed in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that ex- ceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is demonstrated that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady- state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. This characteristic temperature hysteresis following tran- sient dryout has significant implications for the use of heat pipes for pulsed power dissipation. Further experiments are performed to bound the range of heat loads over which the temperature hysteresis is present following a transient dryout event.
Heat pipe, Vapor chamber, Capillary limit, Pulsed input, Transient dryout, Hysteresis
Date of this Version
K. Baraya, J.A. Weibel and S.V. Garimella, “Heat Pipe Dryout and Temperature Hysteresis in Response to Transient Heat Pulses Exceeding the Capillary Limit,” International Journal of Heat and Mass Transfer, Vol. 148, 119135, 2020.