Experimental and numerical investigation of an air pocket immersed and immobilized in a horizontal water duct flow

In this work, we carry out an experimental and numerical study of a horizontal elongated air bubble (air pocket) immersed in a liquid flow. For this purpose, an air pocket of large aspect ratio is immobilized in a main liquid duct flow. This particular geometry differs from that of previous works since the air pocket is not influenced by the presence of other bubbles (slug/plug flow). The characteristic zones of the air–water interface are identified experimentally and discussed as a function of the Reynolds number up to 26,000. Two-dimensional and unsteady numerical calculations without mass transfer across the air–water interface are carried out in order to study the behaviour of the air pocket under the same conditions as in the experiment. The features of the results are that the shape of the air pocket and its front are quite well reconstructed by the numerical calculations. However, the comparison of the air pocket length evolution with the experimental data shows a slightly different behaviour attributed to the mass transfer across the air–water interface. The numerical study provides a further insight into the wavy behaviour of the interface that is responsible for the vortices in the air pocket.