Boiling has received a lot of attention across the world because of its incredible ability to transfer heat, making it a crucial process in industrial use. The study and sizing of sorption machine evaporators for example are based on the prediction of the boiling heat transfer coefficient at low pressure. Their advantages are that they meet new directives and regulations for the protection of the environment. However, experimental studies investigating the dependence of the phenomena of boiling heat transfer on varying pressure levels are highly scarce in the open literature, an aspect that forms the motivation of the present work. In an effort to bridge this gap, we report an experimental study on understanding the plausible influence of varying pressure conditions on bubble dynamic parameters and heat transfer rates under nucleate pool boiling conditions. Experiments have been conducted for a given heat flux level in a specially-designed boiling chamber that allows the realization of sub-atmospheric conditions through integrating it with a vacuum chamber. One of the gradientsbased imaging techniques, namely rainbow schlieren deflectometry, has been employed to simultaneously map the bubble dynamics parameters and the associated thermal distribution around the growing vapor bubble.
The observations revealed that as the pressure decreased, several changes occurred. The size of the bubbles, contact radius, growth time, and waiting time all increased, while the bubble growth rate decreased. The bubble growth is primarily influenced by heat diffusion, with two distinct stages: rapid growth initially and slower growth until departure. Lower pressures extended the duration of the initial growth stage.
The analysis of Schlieren images allowed for the derivation of temperature distributions. At 0.6 bar pressure, boiling had a pronounced impact on temperature distribution, resulting in increased natural convection heat transfer despite higher wall superheat. As pressure decreased, the rates attributed to bubble growth decreased.