Evaporation/Boiling in Thin Capillary Wicks (l)—Wick Thickness Effects

Abstract

Presented here is the first of a two-part investigation designed to systematically identify and investigate the parameters affecting the evaporation/boiling and critical heat flux (CHF) from thin capillary wicking structures. The evaporation/boiling heat transfer coefficient, characteristics, and CHF were investigated under steady-state conditions for a variety of capillary structures with a range of wick thicknesses, volumetric porosities, and mesh sizes. In Part I of the investigation we describe the wicking fabrication process and experimental test facility and focus on the effects of the capillary wick thickness. In Part II we examine the effects of variations in the volumetric porosity and the mesh size as well as presenting detailed discussions of the evaporation/boiling phenomena from thin capillary wicking structures. An optimal sintering process was developed and employed to fabricate the test articles, which were fabricated using multiple, uniform layers of sintered isotropic copper mesh. This process minimized the interface thermal contact resistance between the heated wall and the capillary wick, as well as enhancing the contact conditions between the layers of copper mesh. Due to the effective reduction in the thermal contact resistance between the wall and capillary wick, both the evaporation/boiling heat transfer coefficient and the critical heat flux (CHF) demonstrated dramatic improvements, with heat transfer coefficients up to 245.5kW∕m2K and CHF values in excess of 367.9W∕cm2, observed. The experimental results indicate that while the evaporation/boiling heat transfer coefficient, which increases with increasing heat flux, is only related to the exposed surface area and is not affected by the capillary wick thickness, the CHF for steady-state operation is strongly dependent on the capillary wick thickness and increases proportionally with increase in the wick thickness. In addition to these observations, the experimental tests and subsequent analysis have resulted in the development of a new evaporation/boiling curve for capillary wicking structures, which provides new physical insights into the unique nature of the evaporation/boiling process in these capillary wicking structures. Sample structures and fabrication processes, as well as the test procedures are described in detail and the experimental results and observations are systematically presented and analyzed.