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

• Effects of the initial liquid height on the gas holdup are investigated.
• Wide ranges of the superficial gas velocity and initial liquid height are tested.
• Reliable gas holdup databases are offered for a rectangular and a cylindrical column.
• An accurate gas holdup correlation accounting for the height effect is proposed.
• The functional form of the correlation is validated against other data.

Effects of the initial liquid height, H0, of air–water bubble columns on the total gas holdup, αG, were investigated in this study. Systematic databases of αG in a rectangular and a cylindrical column of the hydraulic diameter DH 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, JG  , and the dimensionless liquid height, H0∗ (= H0/DH), were 0.025 ⩽ JG ⩽ 0.40 m/s and 1.5 ⩽ H0∗ ⩽ 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 DH or that with huge bubbles of the column-width scale. The main parameter governing the flow regime transition was JG. The gradient, dαG/dJG, of αG with respect to JG   was of use in flow regime identification. The increase in H0∗ decreased αG   because a long bubble residence time at a high H0∗ leads to the increase in the mean bubble diameter due to bubble coalescence. The Froude number using H0 as a characteristic length well correlates αG at various H0. An empirical correlation of αG in terms of the Froude number was then proposed. Comparing the correlation with the αG data showed that the correlation can give good evaluations of αG in the rectangular and cylindrical columns by tuning model parameters.