Experimental investigation on bubble departure diameter in pool boiling under sub-atmospheric pressure

The objective of this research is to compare the nucleate boiling characteristic of the calcium chloride aqueous solution with distilled water at sub-atmospheric pressure by analyzing the growing bubble dynamics in order to provide some data for optimizing the design of the dehumidification system. The experiments were carried out with water and calcium chloride solution on the upper surface of a polished stainless steel plate, under sub-atmospheric pressure ranging from 3.6 kPa to 22.0 kPa. An artificial nucleation site was created in the center of the plate to generate the isolated bubbles. A high-speed camera was used to capture the images of dynamic bubbles, and the relevant parameters on bubble dynamics were measured and calculated by frame-by-frame image treatment.
Experimental results show that bubble diameter tends to increase with the pressure decrease which means the lower vapor density and stronger surface tension force at sub-atmospheric boiling. The influence of superheat and sub-cooling degree were also analyzed. Additionally, a complex boiling regime of calcium chloride solution with irregular bubble dynamic parameters was observed. Finally, bubble growth dynamics under sub-atmospheric were analyzed and the force balance equation were established. It is shown that the dynamic effect especially the inertial force dominated the growth stage under sub-atmospheric boiling. A new bubble departure diameter correlation within ±20% deviation was proposed.