Shapes and Rising Velocities of Single Bubbles rising through an Inner Subchannel

Abstract

Shapes and velocities of single air bubbles rising through stagnant and flowing waters in an inner subchannel are measured by making use of fluorocarbon tubes. It is confirmed that (1) bubble shapes and motions in the subchannel are by far different from those in simple geometry, and they depend on the ratio λ of the bubble diameter to the subchannel hydraulic diameter, (2) when λ>0.9, a part of a bubble intrudes into neighboring subchannels, and thereby a kind of void drift takes place even with a single bubble, (3) the terminal velocity V_T of a small bubble (λ<0.9) is accurately predicted by a theoretical model proposed by Tomiyama et al., (4) an empirical correlation of V_T for cell-Taylor bubbles (λ>0.9) is presented, and (5) the rising velocity V_B in laminar and turbulent flow conditions are well evaluated by substituting the proposed V_T models and the ratio of the maximum liquid velocity to the mean liquid velocity into the Nicklin correlation.

KEYWORDS:

• bubble
• subchannel
• terminal velocity
• drag coefficient
• shape
• aspect ratio
• rising velocity
• wall effect