Entrapped Air Pocket Behavior during Hydraulic Transient Events

The effect of entrapped air in the transient pressure wave propagation is analyzed using experimental data collected under controlled laboratory conditions. An air pocket is created at an intermediate section of a 15 m long copper pipe and subjected to a transient pressure variation generated by the fast closure of a valve located at the downstream end. Tests are carried out for several initial flow conditions and for six initial air pockets sizes, collecting the pressure-head signal both at the downstream end and at the air pocket location. The observed pressure drop values, maximum overpressures and pressure wave damping are compared with the no-air pocket test and the differences are analyzed and discussed. Pressure variations can be 30% higher than Joukowsky’s pressure pulse due to the reflected pressure waves associated with the entrapped air contraction and expansion. Results show that there is an air pocket volume for which the overpressures reach a maximum value, independently of the initial flow conditions.