The Dynamic Capillary Limits of a Heat Pipe: Mapping the Operating Power Ranges Using a Transient Dryout and Rewetting Model

Abstract

This work demonstrates the use of a transient model to map the operational limits of a heat pipe based on the time-to-dryout (TTD), evaporator temperature, and time-to-rewet (TTR) in response to dynamic high-power pulses and subsequent throttling. The model considers dynamic coupling between the local liquid saturation and capillary pressure, as well as incorporates wetting hysteresis to predict pulse-load-induced dryout and power-throttling-assisted recovery. Traditional heat pipe design and performance metrics consider the “steady capillary limit” (maximum power at which a steady heat load can be maintained without dryout), but this steady-state framework does not capture the transient power excursions common in modern electronic systems. This work, therefore, puts forth a framework for mapping the “dynamic capillary limits” of a heat pipe as a function of the transient workload of interest. Specifically, the framework produces design-ready operational maps that quantify the TTD, peak evaporator temperature during a pulse ( $T_{evap}$ ), and TTR during throttling—thereby defining dynamic capillary limits beyond the conventional steady capillary limit. These operational maps are effective as tools for assessing the transient limits of performance and can be generated for different heat pipe designs.