Evaporation/Boiling in Thin Capillary Wicks (II)—Effects of Volumetric Porosity and Mesh Size

Presented here is the second of a two-part investigation, designed to systematically identify and investigate the parameters affecting the evaporation from and boiling within, thin capillary wicking structures with a range of volumetric porosities and mesh sizes. The experimental studies were investigated under steady-state conditions at atmospheric pressure. Part I of the investigation described the wicking fabrication process and experimental test facility, and focused on the effects of the capillary wick thickness (ASME J. Heat Transfer., 128, pp. 1312–1319). In Part II, we examine the effects of variations in the volumetric porosity and the mesh size. The experimental results presented here indicate that the critical heat flux (CHF) was strongly dependent on both the mesh size and the volumetric porosity; while the evaporation/boiling heat transfer coefficient was significantly affected by mesh size, but not strongly dependent on the volumetric porosity. The experimental results further illustrate that the menisci at the CHF are located in the corners, formed by the wire and the heated wall and between the wires in both the vertical and horizontal directions. The minimum value of these three menisci determined the maximum capillary pressure generated through the capillary wick. The experimental results and observations are systematically presented and analyzed, and the local bubble and liquid vapor interface dynamics are examined theoretically. Based on the relative relationship between the heat flux and superheat, classic nucleate boiling theory, and the visual observations of the phase-change phenomena, as well as by combining the results obtained here with those obtained in Part I of the investigation, the evaporation/boiling heat transfer regimes in these capillary wicking structures are identified and discussed.