Design optimization of gap distance for the capillary limitation of a heat pipe with annular-type wick structure

Abstract

In this study, an experimental investigation was conducted on the rising height and contact angle of fluid in an annular wick-type heat pipe. The annular wick-type heat pipe was characterized by a small gap between the wick structure and tube wall, which compensated for the pressure drop along with the porous media and created additional capillary force. To describe and model the advantage of this gap, the rising of a wetting liquid in the gap between a vertical solid plate and a mesh (with a small angle between them) was experimentally measured and analyzed. An additional experiment was performed to investigate the effect of curvature on the capillary rise using tubes and meshes of varying radii. Resultantly, we confirmed that the linear combination of the contact angles of the solid plate and mesh could be applied to calculate the rising height from the Laplace-Young equation. Furthermore, the effect of curvature on the rising height of the liquid was negligible. These results were extended to the investigation of finding the optimal gap distance for the annular wick-type heat pipe by referring to previous studies. We observed that a gap distance of 1.27 mm provided the largest permeability over the effective pore radius value for a heat pipe with ethanol, which in turn resulted in the highest capillary limitation. For a sodium heat pipe, a gap distance of 0.84 mm resulted in the highest capillary limitation.

Analyses Supporting Heat Pipe Experiments

“…Heat pipe experiments usually focus on the investigation of operating limits [33][34][35][36][37][38], wick development and performance [39], startup transients [40,41], inclination angles [42][43][44], novel instrumentation [16,40,45], two-phase flow characteristics [46], and motion conditions [47]. This subsection discusses some of these different experimental approaches.…”

“…A well-designed wick structure is a crucial component of a heat pipe that has a great influence on its overall performance. Therefore, many researchers have previously investigated the performance and fabrication methods of both conventional and novel wick designs [39, [49][50][51]. In general, the type of wick structure must be chosen specifically based on the application.…”

“…Examples of transients include liquid metal heat pipe frozen startup, power transients during normal operation, recovery from heat pipe limit, the cascading failure of multiple heat pipes, and system level transients. Many works have investigated heat pipe response to changes in evaporator input power or condenser cooling rates [58][59][60], to transients in systems with heat pipes as components [61,62], and heat pipe startup [40,42,[63][64][65].…”

“…Hence, advanced measurement and visualization techniques are currently being developed by multiple institutions [66]. These include fiber optic temperature and strain sensors [16,40,45,67,68], Infrared (IR) thermography, digital image correlation (DIC) [69], high-speed camera flow visualization and pressure measurements of a surrogate fluid [16,45,70,71], radiation based tomography [60], and laser spectroscopy techniques [72].…”

Heat Pipe Cooled Microreactors

“…Experiments usually focus on various aspects of heat pipe operation, such as the investigation of operating limits [33][34][35][36][37][38], wick development and performance [39], working fluid fill ratio [35,[40][41][42], start-up transients [43,44], inclination angles [41,45,46], novel instrumentation [16,43,47], two-phase flow characteristics [48], and motion conditions [49]. This subsection discusses some of these different experimental approaches.…”