Experimental and numerical study of wall phenomena of confined bubble flow in a square channel

Confined bubbly flow, including bubbles of 4.2 and 6.7 mm equivalent diameters rising in a 4 mm square channel, was experimentally and numerically investigated for liquid Reynolds numbers of 0 and 28. Wall shear stress measurements, by means of polarographic method, were used to investigate wall phenomena while the film thickness was estimated using a conductimetric technique. The OpenFOAM package was used for the numerical simulation. The presence of reverse flow in the liquid layer between the bubble and the wall depends on the length of the bubble as well as its velocity. The liquid layer thickness decreases with increasing bubble size. It was shown that the minimal liquid layer thickness between the bubbles and the wall is 10 mu m for an elongated bubble and 36 mu m for a spherical bubble, which is confirmed by numerical simulation. A liquid acceleration zone exists in the center of the channel in the wake of the bubble, and the existence of vortices near the channel walls at some distance from the bubble is also shown.