Session: K16-03: HEAT TRANSFER IN ELECTRONIC EQUIPMENT III

Paper Number: 138458

138458 - Prediction of Transient Heat Pipe Dryout and Rewetting in Response to Pulse Loads Beyond the Capillary Limit 

Abstract: 

Heat pipes, recognized as passive thermal management devices, provide efficient transport of heat by leveraging phase change of an internal working fluid. The passive functionality of heat pipes hinges on capillary pumping of the internal liquid within a porous wick structure.  A key operational limit of heat pipes is determined by the maximum capillary pressure head that the wick can provide. Namely, the capillary limit represents the threshold for the maximum heat input at which this capillary pressure can overcome the pressure drop in the wick and the vapor core. Operating heat pipes above the capillary limit at steady state results in dryout at the evaporator, leading to drastically reduced heat transport performance. While traditional design of thermal management solutions using heat pipes has focused on their steady-state operation, recent studies have shown that heat pipes can handle transient power pulses even exceeding the capillary limit for short durations without the occurrence of dryout. As the utilization of heat pipes in electronic systems experiencing transient power inputs continues to rise due to typical user activities, it becomes key to understand their transient behavior in response to heat loads beyond the capillary limit. This is especially crucial because over-designing heat pipes to continuously operate at the peak transient loads becomes impractical in such scenarios.

Our recent work experimentally characterized the transient heat pipe response to power pulses exceeding the capillary limit. These observations revealed that the heat pipe underwent dryout only when the pulse load duration exceeded a specific characteristic time interval (time-to-dryout). Additionally, it was discovered that post-transient dryout, the thermal resistance did not necessarily return to its pre-dryout performance, even when the power was reduced below the capillary limit—a phenomenon referred to as thermal hysteresis. This hysteresis results from contact angle hysteresis (wetting hysteresis) at the three-phase contact line of the wick-liquid interface. Therefore, throttling the power below the capillary limit for a sufficient duration (time-to-rewet) can recover the performance. This presentation reports a transient heat pipe model based on this proposed mechanism to predict pulse-load-induced dryout and recovery from dryout. The model is validated through experiments involving diverse commercial heat pipe samples varying in sizes and wick types. In addition to outlining the model, this presentation will focus on demonstrating the intended use of the developed model for mapping the operational regions of a heat pipe based on characteristics of interest such as the time-to-dryout, time-to-rewet, or transient evaporator temperature. The discussion will underscore how these operational maps serve as a potent tool for promptly assessing the impact of modifications to heat pipe design on the transient performance in response to pulse loads.

Presenting Author: Shrutika Singh Purdue University

Presenting Author Biography: Shrutika Singh is a PhD student pursuing advanced studies in the Mechanical Engineering department at Purdue University. Under the guidance of Dr. Justin A. Weibel, Shrutika's research focuses on heat pipes and vapor chambers. Through her doctoral studies, Shrutika aims to contribute valuable insights and advancements in the understanding and application of heat pipes and vapor chambers, showcasing a passion for pushing the boundaries of knowledge in this specialized field of thermal management.

Authors:

Shrutika Singh Purdue University
Justin Weibel Purdue University