An Experimental Investigation on Air Pocket Entrapments Caused by Shear-Flow Instabilities in Rapid-Filling Pipes

Abstract

Rapid filling conditions in closed conduits, such as the case of sewer filling during intense rain events, can lead to air pocket formation. Such formation creates many operational issues in urban water systems, including the possibility for surging, manhole cover displacement and/or geysering. These problems linked to air pocket formation can cause disruptions in urban areas and results in significant repair costs. To understand flow characteristics under such conditions, an experimental research on air pocket entrapment mechanism based on shear flow instabilities (SFI) was developed. A fully filled horizontal water pipe was opened at the downstream end to create a cavity flow and air intrusion of varying thickness. After some time, a second valve was maneuvered near the upstream end triggering inflow admission and flow pressurization. The inflow front pushed air in high velocity over the free surface, and in some cases, air pocket entrapment was observed. Existing threshold criteria for SFI have been assessed with the experimental observations of pocket formation. Also, velocity for pressurized front and air intrusion, and pressure values during the experimental runs have been measured. This study sheds more light on the issues and associated risk of air pocket entrapment in existing water systems.