Experimental study on a rising air bubble in liquid medium through confinement*

The effect of confining an air bubble in a fluid between two parallel plates is experimentally studied by analyzing the path and shape oscillations of an air bubble rising in Millipore water. The confinement parameter ‘h’ was defined as the ratio of the distance between the two parallel plates (H) to the bubble radius (R). High speed imaging was used to obtain both front and side views of the bubble. This study is representative of a bubble that moves at a high Reynolds number and has a small Weber number.  Results show that the bubble velocity fluctuations due to the interaction with the plates are greater for the confined bubble as compared to a bubble with no confinement. The trajectory of the rising bubble changes from zig-zag (for h = 2.1) to nearly spiral trajectory (for 2.1<h<6). Because of the plates, the bubble is attracted towards one of the plate and repelled continuously and bubble is raised towards one of the plate. The effect of confinement becomes negligible if the distance between the plates is greater than 6.0 times of the radius of the bubble (h>6.0).