Effects of liquid height on gas holdup in air–water bubble column

Abstract Effects of the initial liquid height, H 0 , of air–water bubble columns on the total gas holdup, α G , were investigated in this study. Systematic databases of α G in a rectangular and a cylindrical column of the hydraulic diameter D H of 200 mm were obtained. An image processing method was applied to high-speed video images of the liquid height to obtain accurate gas holdup data. Ranges of the superficial gas velocity, J G , and the dimensionless liquid height, H 0 ∗ (= H 0 / D H ), were 0.025 ⩽  J G  ⩽ 0.40 m/s and 1.5 ⩽  H 0 ∗  ⩽ 5.0, respectively. The bubbly flows observed in these ranges could be classified into either the heterogeneous bubbly flow consisting of bubbles much smaller than D H or that with huge bubbles of the column-width scale. The main parameter governing the flow regime transition was J G . The gradient, d α G / dJ G , of α G with respect to J G was of use in flow regime identification. The increase in H 0 ∗ decreased α G because a long bubble residence time at a high H 0 ∗ leads to the increase in the mean bubble diameter due to bubble coalescence. The Froude number using H 0 as a characteristic length well correlates α G at various H 0 . An empirical correlation of α G in terms of the Froude number was then proposed. Comparing the correlation with the α G data showed that the correlation can give good evaluations of α G in the rectangular and cylindrical columns by tuning model parameters.