AN ANALYTICAL MODEL FOR NEAR-SATURATED POOL BOILING CRITICAL HEAT-FLUX ON VERTICAL SURFACES

Photographic studies of vertical pool boiling for near-saturated condi tions were conducted in order to determine the critical heat flux (CHF ) trigger mechanism. The studies revealed that, for heat fluxes near C HF, vertical pool boiling exhibits vapor behavior similar to that obse rved in flow boiling. At fluxes slightly below CHF, the Kelvin-Helmhol tz instability creates a wavy layer at the liquid-vapor interface. The heater surface is wetted, and thus cooled, only by the wave troughs, referred to as wetting fronts, which repeatedly sweep across the surfa ce. Prior to CHF, the curvature of the wetting front creates a pressur e force which tends to preserve interfacial contact with the wall. Cri tical heat flux occurs when the pressure force due to interfacial curv ature is overcome by the momentum of vapor at the site of the first we tting front, causing the interface to lift away from the wall. Once th e first wetting front lifts off from the surface, the remaining wettin g fronts separate from the surface in succession. It is shown that the interfacial lift-off criterion facilitates accurate theoretical model ing of CHF in vertical pool boiling, and produces an equation which is identical in form, but not mechanism, to traditional horizontal CHF m odels. (C) 1997 Elsevier Science Ltd. All rights reserved.