Abstract
Liquid-vapor phase change heat transfer on nanoporous membranes in facing horizontal upwards, downwards and vertically was investigated. Anodic aluminum oxide (AAO) membranes were as the nanoporous substrates. Water was supplied through nanoporous membranes by pressure difference. Heat transfer characteristics and temperature instability were analyzed and compared among three orientations. Results showed that negative slope and “special points” appeared on boiling curves. The interaction between liquid film thickness and heat transfer coefficient (HTC) was proposed and verified to be the main mechanism that leading to this phenomenon. Because the rapid replenishment of liquid and removal of heated liquid under gravity, horizontal downward surface always had the best heat transfer performance which exhibited maximal enhancement of HTC was 170% and 70% at heat flux of 39 W/cm2 over horizontal upward and vertical surfaces. Temperature instability in horizontal downward and vertical surfaces had the similar trend and became more pronounced at higher heat fluxes. In horizontal upward surface, temperature was stable at the heat flux range of 40– 55 W/cm2 and “thin film evaporation” was realized. In addition, the maximum heat flux was calculated by liquid transport limit and kinetic theory limit for evaporation and a comprehensive understanding was established.
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