Wall effects on the rise of single gas bubbles in liquids

We report the results of an extensive experimental investigation on the vel ocity of rise of air bubbles in the size range d(b) = 3 - 80 mm in water. M easurements were made in columns with inside diameters D-T = 0.01, 0.02, 0. 03, 0.051, 0.1, 0.174 and 0.63 m. The column diameter was found to have a s ignificant effect on the rise velocity of the bubbles. When the ratio of th e bubble diameter to the column diameter, d(b)/D-T, is smaller than 0.125 t he influence of the column diameter on the rise velocity is negligible and the rise velocity is described quite accurately by the Mendelson equation. With increasing d(b)/D-T there is a significant reduction of the rise veloc ity, i.e. there is a significant "wall effect". The wall effect for spheric al cap bubbles rising in inviscid flow, obtained for bubble diameters large r than 0.017 m, is described adequately by the Collins relation. The wall e ffect for bubbles smaller than 0.017 m is described by an empirical relatio n suggested by Clift, Grace and Weber. (C) 1999 Elsevier Science Ltd.