The Influence of End Conditions on Minimum Film Boiling From a Cylinder

Abstract

Upon preparing for an experiment in which liquid-solid contacts were to be measured near the minimum film boiling point in flow boiling, it became clear the thermal condition of the heater ends could exercise undue restrictions on the achievable minimum heat flux and wall superheat. For the experiment in question, the heater was to be heated electrically while suspended horizontally across a flow of Freon-11 (R-11). The heater surface was mounted flush with the inside wall of the test section, bringing into play the interaction with the wall as well as potential axial heat loss from the heater during film boiling. Thus the authors set out to investigate the influence of end conditions using a simpler pool boiling apparatus very similar to the flow boiling apparatus. This paper details how modifications of the end conditions for an electrically heated cylinder influence the minimum film boiling point that is achievable in R-11. The modifications consisted of providing guard heating of the ends along with properly designed heater supports so that the heater temperature does not fall off precipitously near the ends. If the end temperature is not maintained high enough, transition boiling begins at the ends while the majority of themore » heater is immersed in a vapor film. Once the film begins to collapse, it becomes unstable and nucleate boiling ensues.« less