Decomposition of gas fraction profiles according to bubble size classes

Drag and non-drag forces acting on a bubble depend on the bubble diameter. Recently it was found that the lift force occurring in shear flows changes its sign, when the bubble diameter exceeds a certain critical value. This is probably the main reason for the change from a wall-peaked gas fraction profile to a central-peaked one. The paper presents a new method to obtain bubble size distributions and radial gas fraction profiles decomposed according to bubble size classes. It is based on a wire-mesh sensor, capable in resolving individual bubbles. The sensor delivers time sequences of two-dimensional instantaneous gas fraction distributions in a 51.2 mm diameter pipe with a time resolution of 1200 frames per second. After identifying the bubbles in this data, and after quantifying their effective diameter, the data is decomposed according to given bubble size classes. Partial gas fraction distributions are created by checking each element of the measured data array whether it belongs to a bubble the diameter of which is within a given range, or not. Radial profiles calculated from this partial distributions reflect the quantity of gas carried by the selected bub-ble size class. By varying the axial position of a sensor, the evolution of the bubble size distribution and the decomposed radial profiles in an air-water flow along a vertical pipe was studied. It could be confirmed in a real multi-disperse flow, that bubbles larger than a critical diameter are accumulated in the centre of the pipe, while small bubbles tend to move towards the wall.