Simultaneous Mapping of Bubbling Regimes and Thermal Layer During Direct Contact Condensation of Steam Using Gradients-Based Approach

Abstract

Direct contact condensation experiments of steam in subcooled water are carried out for subcooling levels of ΔTsub= 20 °C, 30 °C, and 40 °C with a steam injection mass flux of 20 kg/m2 s. Rainbow schlieren deflectometry is employed to visualize the thermal gradients around the condensing steam bubble in a nonintrusive manner. For the chosen flow and subcooling conditions, the bubbling regimes observed are the steam bubble growth stage, the bubble receding stage, and the bubble collapse stage. Two-dimensional images captured during the process of bubble condensation using rainbow schlieren images are presented. The redistribution of color captured through the recorded images directly reflects the thermal gradients present 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 the complete breakup of thermal gradient layer at the bubble collapse stage. The local profiles of hue distribution in the direction normal to the thermal gradient layer indicate high temperature gradients in this narrow region. The hue values and the average thickness of the thermal gradient layer were found to be maximum for 40 °C subcooling level compared to the other cases. 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, this 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.