Natural convection film boiling on a vertical surface

Abstract

Experimental results are presented for an investigation of free convection film boiling heat transfer to saturated ethanol, saturated Freon 113, and saturated distilled water from a vertical cylinder. High speed motion pictures of the vapor-liquid interface in film boiling have been obtained. All tests have been conducted at atmospheric pressure over a range of heat flux between 9000 and 37000 BTU/hr ft(2). A study of the stability of the vapor-liquid interface is performed and the results are compared to the photographic data. Parameters influencing the heat transfer process are developed and the experimental data is used to determine their proper relationship. The heat transfer coefficient in natural convection film boiling of saturated liquids on moderately long vertical surfaces at constant heat flux is shown to be insensitive to vertical position, and to be dependent only on the temperature differential and the fluid properties. Large scale waves have been observed on the vapor-liquid interface: however, their effect on the heat transfer is minimal. It appears that intense vaporization at the interface plays an important role in the generation of these waves, while surface tension does not. A strong possibility of liquid droplet entrainment in the vapor flow is indicated by the presence of a rough, wavy interface, a relatively high speed vapor-flow, and by the unexpectedly strong dependence of the heat transfer on the latent heat of vaporization.