Bubble dynamics in a subcooled flow boiling of near-critical carbon dioxide

Abstract

Bubbly flow patterns for subcooled flow boiling of carbon dioxide at high reduced pressures were experimentally studied. Development of boiling patterns starting from scant nucleations of bubbles to fully developed boiling was observed. The pressure effect was examined, and it was found that as the critical pressure approached, the bubble diameter and growth rate decreased. Moreover, the bubbles became elongated at reduced pressures above 0.95 as a result of the diminishing surface tension. In addition, bubble diameter, velocity, and shape were measured, and results were compared to the asymptotic solutions of the Rayleigh-Plesset equation. Isolated bubbles and bubble-bubble interactions were considered. It was found that the asymptotic thermally driven solution of the Rayleigh-Plesset equation predicted the bubble dynamics well for isolated bubbles, but deviations were noted when bubbles started interacting. The Mikic model was modified to account for bubble-to-bubble interactions and an additional term was added to the model that is linearly proportional to time (i.e., a·t). A modified model based on experimentally fitted coefficients was developed and used effectively to predict additional set of experiments. The modified model reduced the average error of the diameter from 13.5 μm to 6.6 μm — a reduction of 51%.