Abstract
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-phase 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 analysis 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 dilute suspensions is attributed to the effects of bubble–wall collisions. The more gradual subsequent hindering of bubble motion is in qualitative agreement with the predictions of Spelt & Sangani (1998) for the effects of potential-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 conditions Spelt & Sangani predict that the homogeneous suspension will be unstable and clustering into horizontal rafts will take place. Evidence for bubble 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 fluctuations that are slow compared with the time for the passage of an individual bubble. These observations provide further evidence for bubble clustering.
Highlights
Hydrodynamic interactions and direct collisions between particles, drops, or bubbles can have a dramatic effect on particulate and multiphase flows
The results show that the mean bubble velocity decreases as the bubble concentration increases, at a rate comparable to that predicted by Spelt & Sangani (1998)
We have presented the first experimental results concerning the averaged behaviour of bubble suspensions that satisfy the conditions of large Reynolds number and moderately small Weber number necessary for comparison with theories based on potential flow interactions among the bubbles
Summary
Hydrodynamic interactions and direct collisions between particles, drops, or bubbles can have a dramatic effect on particulate and multiphase flows. Through the comparison of careful experimental measurements with analytical theories and numerical simulations, considerable progress has been made in the understanding of suspensions in which viscous forces dominate on the particle length scale. The current understanding of the effects of particle interactions on inertial suspensions is much more limited. One type of inertial suspension, for which extensive theoretical work has been conducted, is a suspension of spherical, high-Reynolds-number bubbles. There is a dearth of experimental measurements with which these theories can be compared.
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