Film Boiling Heat Transfer on an Isothermal Vertical Surface

Abstract

Natural convection film boiling heat transfer of saturated liquids on an isothermal vertical surface is investigated both experimentally and theoretically. Local heat transfer coefficients are obtained at steady-state conditions on a 6.3-cm-wide and 10.3-cm-high heated surface which was machined from a large block of copper. Effectively isothermal surface conditions are achieved even for wall superheats up to 450 K. Experiments conducted with water at 1 atm pressure show that predictions from existing theoretical models are inadequate and a detailed consideration of the liquid-vapor interface behavior is required. Still and motion pictures of the vapor film are taken and data on vapor film thickness, interfacial wave behavior, and bubble detachment characteristics are obtained to build an analytical model for predicting film boiling heat transfer. This time-dependent model based on laminar flow in the film incorporates the effects of both large-scale and small-scale waves. Results from analysis are compared with experimental data.