On the morphology of elongated bubbles during their formation at submerged orifices

Abstract

Bubbly flow is a widely observed phenomenon in various industrial applications and has remained a topic of sustained interest over decades, owing to the complex multi-physics that govern the multiphase phenomenon. The bubble geometrical morphology and dynamics critically depend on various parameters such as flow rate, orifice diameter, working fluid properties, and height of the fluid above the orifice. Accordingly, experiments are conducted in water, acetone, and Glycerol-water solutions (10% w/w and 20% w/w) using a sub-millimeter submerged orifice of diameter 0.4 mm, 0.6 mm, and 1 mm. The volume flow rate is varied in the range of 100–300 ml per min and experiments are carried out at varying orifice depths of 4 – 12 cm. The dynamics of bubble formation is recorded by high-speed cinematography and is processed using MATLAB script. This paper presents the bubble rise and coalescence morphology and the effect of operating parameters on it. The gas injection and bubble geometrical parameters can be expressed in terms of non-dimensional numbers such as Reynolds number, Weber number, Froude number, and Eotvos number. It is found that simple scaling laws can be established among Reynolds number with Weber number, Froude number, and their ratio, which can effectively predict the bubble geometry. It is found that the relation between the ratio of Weber and Froude number to Reynolds number depends on the height of fluid above the orifice. The present work also quantifies the interconnection between Reynolds number and Eotvos number and demonstrates a relationship that is incumbent on the gas superficial velocity. In particular, it is observed that the slope of the Re-Eo curve decreases with the increase in the gas superficial velocity.