On elongated air pockets in downward sloping pipes.

The behaviour of elongated air pockets in downward sloping pipes is investigated. The observed flow regimes in co-current air-water flow in downward sloping pipes are described. The effects of pipe diameter and surface tension on the air pocket motion are quantified and two criteria for the observed flow regime transitions are derived. One criterion stems from energy considerations and marks the transition from the presence of one elongated air pocket to multiple air pockets. The second criterion predicts the required water velocity to start the motion of elongated air pockets in the downward direction. Both criteria were validated with experimental data for a range of downward sloping pipe angles, lengths and diameters. [ABSTRACT FROM AUTHOR]