Flow around individual Taylor bubbles rising in a vertical column with water: Effect of gas expansion

Abstract

This paper presents an experimental study to evaluate the effect of bubble expansion on the dynamics of an isolated Taylor bubble rising in a vertical column of water by inspection of the liquid flow around the bubble front. An increase in bubble volume results from gas decompression due to hydrostatic pressure drop as the bubble ascends through the liquid. Two reduced pressures (33.3 and 20.0kN/m2) and atmospheric pressure were maintained at the free liquid surface to provide a wide range of expansion rates.PIV coupled with a Shadowgraphy Technique was used to simultaneously obtain the flow field ahead the bubble and a well-defined gas–liquid interface.A very good correlation was found between predicted bubble volume expansion rates considering ideal gas behavior and the upward net volumetric liquid flow rate obtained by integration of the velocity profiles. Above the bubble nose, the liquid velocity at the centerline decreases rapidly from the value of the Taylor bubble velocity at the tip of the bubble. For distances greater than 0.5D, the liquid flow is undisturbed, with a significant axial velocity component for high bubble volume expansion rates due to the displacement of the liquid promoted by gas expansion. The Reynolds number calculated for the liquid flow was less than 2100 for all conditions used. While this suggests a laminar flow regime in the liquid, coefficient C from Nicklin’s equation obtained from data was equal to 1.4. The shapes of the velocity profiles obtained at the undisturbed liquid flow show that for high expansion rates they resemble the power-law velocity profile for turbulent flow. The bubble volume expansion rate modifies continuously during the Taylor bubble rise and a well-defined liquid velocity profile is never attained.