LIQUID MICROLAYER EVAPORATION DURING NUCLEATE BOILING ON THE SURFACE OF A FLAT COMPOSITE WALL

A transient model is developed to study the evaporation of a liquid mi crolayer under a growing vapor bubble during nucleate boiling on the s urface of a flat composite wall. The wall consists of a thin. volumetr ically heated metallic foil or film and a Pyrex glass substrate or thi n coating. The model predictions are qualitatively in agreement with e xperimental data for a water microlayer evaporation on a SnO2 coated P yrex glass plate. Results for a heated metal foil on a Pyrex glass sub strate show that the local heat flux at the solid-liquid-vapor triple point could be as much as one to two orders of magnitude higher than t he input heat flux. Consequently, the wall temperature at the triple p oint drops rapidly, resulting in a non-uniform wall surface temperatur e. Both the accommodation coefficient of evaporation and lateral heat conduction in the heated wall significantly affects the liquid microla yer evaporation on a thin, highly conductive wall, especially during t he early stage of the bubble growth. As either the thickness or the th ermal conductivity of the heated wall is increased. the evaporation ra te increases due to improved lateral heat conduction, approaching that for an isothermal wall. Conversely, a thin coating of low conductivit y material significantly reduces the evaporation rate of the liquid mi crolayer, whereas the effect of the thermal properties of the heated m etallic substrate is negligible unless the coating is very thin.