Air bubble breakup in shear water flows generated by a plug conduit: An experimental investigation

In the context of a sudden contraction plug conduit, the near-wall area experiences a significant shearing effect of water flow, however, the extent to which this shearing effect occurs in bubble-water flow and the related variation mechanisms of air bubble size and number remain unclear. This study employs a model test method to investigate the diffusion process of bubble-water flow in a sudden contraction plug conduit. The size and number of bubbles, as well as their distribution along the shearing section under varying initial air volume conditions, are studied in detail using a high-speed image acquisition system. The experimental findings reveal a self-similar relationship between the number and size of bubbles and their cross-sectional distribution over time. The bubble number and size vary in three stages, i.e., quasi-suspension, shearing, and shearing completion stages. The direction perpendicular to the conduit exhibits peak values in bubble number distribution over the three stages, with peak value location varying with the near-wall area. As time progresses, the peak value increases, and a larger initial air volume corresponds to a smaller distance of the peak value location from the wall. The size of air bubbles near the wall is consistent with the minimum diameter of air bubbles in shear flow and is hardly affected by the initial air volume. These results aid in comprehending the change law of two-phase water and air flow under a strong shearing effect in the plug conduit, and provide useful insights for hydraulic design in fluid engineering.