Effects of Valve Closing Law and Unsteady Friction Model on Damping Pressure Waves in a Network

Water-hammer is a transient condition which may occur in a network as a result of rapid or slow valve closures, pump failures, changes in turbine loading, etc. It creates high and low pressure waves which travel along the system and decay as a result of wall shear stress. Comparison o experimental and theoretical results revealed the failure of steady or quasi-steady models in correctly predicting the daming process of the pressure waves. In fact, the velocity profiles have greater gradients under unsteady conditions which results in higher shear stresses compared to the steady condition. In this paper, the transient condition in a network (valve-pipe-tank system) is investigated by implementing one of the unsteady friction models (Brunone model) into the method of characteristics (MOC). Results show that using the unsteady friction model damps the pressure waves more rapidly, the absence of which may result in disagreement between theoretical and experimental values. In addition, this work shows that pressure rise due to the water hammer phenomenon cannot be correctly determined without effecting the unsteady friction factor. The valve closure law affects pressure rise prediction.