Entrapped air blocking the flow in pipeline systems is a common cause of increased pumping costs. At present, air is generally removed via valves or pipeline excavation and drilling. This becomes inefficient in large networks where the precise location of the air is unknown. Fluid transients are a potential tool for detecting and locating air in pipelines. The effect of a stationary air pocket part of the way along the pipe, which occupies the main flow path and acts as a blockage without causing a hydraulic jump or column separation, has not previously been studied experimentally. This paper presents experimental results for a transient pulse interacting with an in-line air pocket for a range of pocket sizes and system pressures. In accordance with the impedance theory, the reflective power of the air increases with pocket size. Other notable characteristics of the interaction include frequency-dependent transmissivity, an out-of-phase reflection, and a substantial reflection under zero base flow. These effects set air pockets apart from solid blockages, allowing a transient detection methodology to differentiate between the two cases, although they have similar effects at steady-state.
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