Local temperature fluctuations in saturated pool boiling of pure liquids and binary mixtures

Abstract

A direct experimental verification of the common physical background underlying the various bubble growth theories and Van Stralen's “relaxation microlayer” theory for the mechanism of nucleate boiling is given. For this purpose, instantaneous local temperature measurements with a thin-wired thermocouple have been synchronized with high speed motion pictures of vapour bubbles generated at active nuclei in close proximity to the hot junction. The thermal boundary layer is shown to be pushed away from the wall during rapid initial bubble growth. A subsequent heating of this layer occurs during the waiting time between the detachment of a bubble and the generation of the succeeding bubble. The dew point of vapour in the bubble space is uniform and approximates the saturation temperature at ambient (atmospheric) pressure. The mechanism results in large local temperature fluctuations in pure liquids. In contradistinction to this behaviour, in binary mixtures containing a small fraction of the more volatile component, the temperature dips, which are observed by passage of the hot junction from the superheated liquid into the vapour space, are limited to considerably lower values. Obviously, this phenomenon proves the existence of a decreased “effective superheating”, which is caused by an increase of the dew point in comparison with the boiling point of the liquid of the original composition.