An Investigation of the Minimum Film Boiling Temperature on Horizontal Surfaces

The frequency and magnitude of liquid-solid contacts in the film boiling of saturated ethanol and water on horizontal flat stainless steel and copper surfaces are examined with electrical conductance probes. It is observed that, at atmospheric pressure, contacts occur over a wide temperature range and are generally induced by hydrodynamic instabilities. In the ethanol system, these contacts can account for the entire heat transfer rate at the minimum film boiling temperature. The area and duration of contacts are strongly influenced by the dominant nucleation process, and thus, depend on the interface temperature and wettability of the solid. At elevated pressures, direct liquid-solid contacts in film boiling are essentially nonexistent for stainless steel and the first major contact corresponds to quenching of the surface. Under these conditions, spontaneous nucleation upon contact seems to be the controlling mechanism for the minimum film boiling temperature. The minimum point appears to be determined either by a Taylor instability vapor removal limitation or by spontaneous nucleation. The governing mechanism is the one which is stable at the lowest wall temperature.