Development and Characterization of an Air-Cooled Loop Heat Pipe With a Wick in the Condenser

Abstract

Thermal management of modern electronics is rapidly becoming a critical bottleneck of their computational performance. Air-cooled heat sinks offer ease and flexibility in installation and are currently the most widely used solution for cooling electronics. We report the characterization of a novel loop heat pipe (LHP) with a wick in the condenser, developed for the integration into an air-cooled heat sink. The evaporator and condenser are planar (102 mm  102 mm footprint) and allow for potential integration of multiple, stacked condensers. The condenser wick is used to separate the liquid and vapor phases during condensation by capillary menisci and enables the use of multiple condensers with equal condensation behavior and performance. In this paper, the thermal-fluidic cycle is outlined, and the requirements to generate capillary pressure in the condenser are discussed. The LHP design to fulfill the requirements is then described, and the experimental characterization of a single-condenser version of the LHP is reported. The thermal performance was dependent on the fan speed and the volume of the working fluid; a thermal resistance of 0.177 C/W was demonstrated at a heat load of 200 W, fan speed of 5000 rpm and fluid volume of 67 mL. When the LHP was filled with the working fluid to the proper volume, capillary pressure in the condenser was confirmed for all heat loads tested, with a maximum of 3.5 kPa at 200 W. When overfilled with the working fluid, the condenser was flooded with liquid, preventing the formation of capillary pressure and significantly increasing the LHP thermal resistance. This study provides the detailed thermal-fluidic considerations needed to generate capillary pressure in the condenser for controlling the condensation behavior and serves as the basis of developing multiplecondenser LHPs with low thermal resistance.

Prediction and Characterization of Dry-out Heat Flux in Micropillar Wick Structures

“…, according to equation (7). The exact q dry-out = f(θ r ) is micropillar geometry dependent, as indicated by the difference between the curves for l = 60 µm and l = 20 µm in Figure 7.…”

“…The use of the liquid-to-vapor phase-change process to cool these devices is attractive because it harnesses the latent heat of vaporization with minimal temperature rise [4][5][6] . In particular, capillary-pumped thin-film evaporation has gained increasing attention due to its simple design, stable and self-regulating performance, and minimal pumping power consumption [7][8][9] . These systems generally require a porous wick structure to generate capillary pressure which drives the liquid flow as it evaporates.…”

“…The increasing power densities in high-performance electronic devices such as GaN power amplifiers, concentrated photovoltaics, and laser diodes pose a significant thermal management challenge. − The use of the liquid-to-vapor phase-change process to cool these devices is attractive because it harnesses the latent heat of vaporization with minimal temperature rise. − In particular, capillary-pumped thin-film evaporation has gained increasing attention due to its simple design, stable and self-regulating performance, and minimal pumping power consumption. − These systems generally require a porous wick structure to generate capillary pressure which drives the liquid flow as it evaporates. Among various wick structures investigated including spheres, pyramids, and cylindrical micropillars, the latter have been shown to be a particularly effective wick structure .…”