Simultaneous mapping of bubbling regimes and thermal layer during direct contact condensation of steam using gradients-based approach

Direct contact condensation experiments of steam in subcooled water are carried out for varying subcooling levels. Non-intrusive direct visualization of thermal gradients around the condensing bubble is realized using rainbow schlieren deflectometry. The bubbling regimes observed are the steam bubble growth stage, the bubble receding stage, and the bubble collapse stage. Results are presented in the form of two-dimensional rainbow schlieren images captured during the process of bubble condensation. The degree of color re-distribution captured through the recorded images gives a direct measure of thermal gradients in the test section. Qualitative interpretation of the recorded images reveals that the thermal gradient layer thickness around the condensing steam bubble increases during the growth and receding stages, before complete breakup of thermal gradient layer at the bubble collapse stage. The local profiles of hue distribution in the direction normal to thermal gradient layer indicate the presence of high temperature gradients. Hue values and average thickness of thermal gradient layer were found to be maximum for 40 0C subcooling level. The rate of growth and thereby the collapse of the thermal gradient layer is slower for low subcooling levels and increases with higher subcooling values. To the best of the knowledge of the authors, the present work is one of the first attempts to simultaneously capture the dynamical parameters of the condensing steam bubble as well as the associated thermal gradients field using a single imaging technique, thus making the experimental approach relatively simple and cost effective.