Non-intrusive experiments on coupled bubble dynamics and heat transfer during nucleate boiling under varying pressure conditions

Abstract

The experimental investigation is concerned with the dynamics of single vapor bubble and heat transfer during nucleate boiling under varying pressure conditions, encompassing both atmospheric and sub-atmospheric levels. Utilizing high-speed rainbow schlieren deflectometry, a non-intrusive imaging technique for visualizing and quantifying the flow field variables in the fluid medium, this research examines the influence of operating pressure on some of the important parameters pertaining to boiling phenomenon such as bubble size, shape, and detachment mechanisms. The observations show that a decrease in pressure leads to the formation of larger bubbles, which, in turn, alters their growth mechanism. These changes in bubble behavior have a significant influence on the various forces acting on the growing bubble as well as on the associated heat transfer rates. The analysis provides insights into the complex interplay between buoyancy, surface tension, growth and contact pressure forces. Change in pressure significantly impacts these forces, influencing bubble dynamics. Moreover, the study presents a quantitative analysis of the whole field temperature distribution and heat transfer rates throughout the boiling process. Results reveal a strong dependence of thermal field on the working pressure, with higher pressures leading to more uniform temperature distributions. Evaporative component of heat transfer from the bubble base and convective heat transfer from the superheated layer are identified as the dominant mechanisms. Natural convection initially dominates, decreases post-nucleation, and increases upon bubble departure due to the fluid quenching effects.