Numerical and experimental study on dynamics of unsteady pipe flow involving backflow prevention assemblies

When control valves at the end of pipeline close simultaneously, two pressure waves are generated at each end and the waves propagate toward to the other end. The pressure waves continue to move back and forth along pipelines until they are damped out to next steady states. This study provides information on the experimental data and the numerical simulation of a rapid hydraulic transient event called water hammer. The energy loss term due to friction in the present model consists of quasi-steady contribution and unsteady contribution. For the present model, an equivalent friction coefficient is used to replace the quasi-steady friction coefficient, inclusive unsteady friction loss and minor energy loss factors. The unsteady component has been related to the combination of the instant flow acceleration and instant flow convective acceleration. The numerical results of the present model are compared with the experimental records. The computer results by the present model which is based on the unsteady friction 1D model was successful to follow general trends of water hammer phenomena, corresponding with sudden changes in flow. This study later extends to the dynamic characteristics of backflow prevention assemblies under a rapid transient condition. When numerous hydraulic devices are installed on water distribution systems, the dynamic characteristics of such hydraulic devices have a significant influence on the intensity of the associated water hammer waves. A backflow prevention assembly plays important roles as not only a safety hydraulic device but also an energy dissipater. A numerical program for rapid transient pipe flow interconnected with a backflow prevention assembly has been developed using the present coupling model and numerically solved by the method of characteristics.