The influence of surface conditions and subcooling on film-transition boiling

Abstract

The influence of surface oxidation, liquid subcooling and surface roughness on pool boiling from a 1.27-cm copper sphere is reported in the present study. The sphere was specially designed to minimize support interference with vapor formation and removal; the surface was carefully prepared so that surface roughness and oxidation could be controlled. Experiments were conducted for four different surface roughness levels and subcooling up to 22°C for Freon-113 and up to 50°C for water. Experiments were also conducted with oxidized spheres in Freon-113. In general, the data and observations show that increased subcooling, roughness, and oxidation increase the heat flux during the stage of film boiling near the minimum heat flux where liquid-solid contact is possible. Such increases carry over into transition boiling as well. Vapor bubble detachment frequencies and diameters are reported. A curious “parent-son” detachment mode was observed. Such behavior is explained by a recirculating liquid flow pattern behind the parent bubble that entrains the son bubble. Oscillation amplitudes and frequencies of the vapor envelope during transition boiling are also reported. The heat flux data for subcooled water exhibit the dual maxima reported by other investigators. An explanation is given that relies on the idea of Witte and Lienhard that a quenching system can choose either a “film-transition” or a “nucleate-transition” curve.