Computational fluid dynamics based transient investigation of the penstock in a hydropower plant

Abstract Water hammer is a hydraulic transient phenomenon that occurs at hydropower plants (HPP) when the discharge is rapidly decreased or closed, which can damage the HPP components, mainly penstock. Sudden Pressure rise and collapse of water columns or cavity forms due to the low pressure may cause severe damage. Understanding transient flow behavior is necessary to prevent harmful pressure fluctuations and ensure power plant safety. The present 3D computational fluid dynamics (CFD) study is carried out to predict pressure fluctuations, the occurrence of any cavitation, and water column separation at critical locations in the water conductor system from the surge tank to the turbine inlet due to low pressure caused by the closing of the hydraulic turbine guide vane valve at downstream of the penstock. The hydraulic domain of the penstock, connecting the head race tunnel to the turbine inlet, is created as per the HPP UHL-III actual water conductor system elevations, bent bifurcations, and geometric sizes. Transient CFD simulations for different load rejection conditions were performed in the Ansys CFX by modifying the Unsteady Reynolds Average Navier’s Stoke equation (URANS), incorporating the elastic effect of penstock material, and considering the variable density of the water, taking the compressibility. Transient pressure in the penstock was recorded at various locations. Further, minimum pressure inside the flow domain that falls in the no-column separation zone was also obtained.