Measurements of the average properties of a suspension of bubbles rising in a vertical channel

Experiments were performed in a vertical channel to study the behaviour of a monodisperse bubble suspension for which the dual limit of large Reynolds number and small Weber number was satisfied. Measurements of the liquid-ph ase velocity fluctuations were obtained with a hot-wire anemometer. The gas volume fraction, bubble velocity, bubble velocity fluctuations and bubble collision rate were measured using a dual impedance probe. Digital image an alysis was performed to quantify the small polydispersity of the bubbles as well as the bubble shape. A rapid decrease in bubble velocity with bubble concentration in very dilut e suspensions is attributed to the effects of bubble-wall collisions. The m ore gradual subsequent hindering of bubble motion is in qualitative agreeme nt with the predictions of Spelt & Sangani (1998) for the effects of potent ial-flow bubble-bubble interactions on the mean velocity. The ratio of the bubble velocity variance to the square of the mean is O(0.1). For these con ditions Spelt & Sangani predict that the homogeneous suspension will be uns table and clustering into horizontal rafts will take place. Evidence for bu bble clustering is obtained by analysis of video images. The fluid velocity variance is larger than would be expected for a homogeneous suspension and the fluid velocity frequency spectrum indicates the presence of velocity f luctuations that are slow compared with the time for the passage of an indi vidual bubble. These observations provide further evidence for bubble clust ering.