PRESSURE FLUCTUATIONS REDUCTION IN CENTRIFUGAL PUMPS: INFLUENCE OF IMPELLER GEOMETRY AND RADIAL GAP

A numerical investigation was carried out in a volute type centrifugal pump to study the influence of splitter blades and radial gap and decrease the pressure fluctuations levels at the blade passage frequency for the same operating conditions. In this kind of turbomachinery, the strong interaction between the flow leaving the impeller and entering the volute casing generate pulsating pressures which propagate toward the inlet and the discharge of the pump. They are mainly associated to the blade passage frequency presenting high amplitudes near the tongue and at impeller discharge. Three configurations were considered in the numerical tests, the first hydraulic is the reference geometry to be optimized and the other two are the versions modified for decrease pressure fluctuations. The geometrical changes concern: adding splitter blades to the original impeller and increasing the radial gap between the splitter impeller and the volute tongue. The simulations for all configurations were performed at the same flow rate. It was necessary to decrease the rotational speed of the optimized configurations to reach the same operating point. The pressure signals were collected by virtual sensors placed at several locations of the pump mainly at the impeller-volute interface plus at inlet and at outlet ducts. Then they were treated and processed obtaining the frequency spectrum by means of the Fast Fourier Transform, so the numerical results were compared and discussed. A commercial CFD code has been used to predict the pulsating pressures and the Unsteady Reynolds Averaged Navier-Stokes (URANS) approach has been applied to solve the unsteady, incompressible and turbulent flow. The k-omega SST model was used to take into account the turbulence effects and the standard wall functions were applied. The numerical procedure employed for the unsteady simulations was determined after an investigation of the influence of boundary conditions on the pressure signals specially at the outlet duct, showing that the condition of outflow does not have influence on the numerical results.