A numerical analysis is performed to assess the capability of common simulation methods, in particular Ansys CFX, to predict the performance and NPSH curve of a centrifugal pump at very low specific speed for both, design and off-design conditions.
In all cases, we use an entire numerical model containing the impeller, the volute casing, the side chambers as well as suction pipe and pressure pipe. A three-dimensional setup is used, testing the following numerical models: steady, i.e. frozen rotor model, unsteady model accounting for the impeller movement and the relative impeller-volute position, single-phase flow as well as cavitating flow conditions.
The global performance of the pump is assessed in terms of pressure head, power consumption and pump efficiency for single-phase flow. Furthermore, the drop of the pump head and Net Positive Suction Head (NPSH) characteristics are analyzed for cavitating flow conditions. Numerical results are validated against experimental data.
Regarding non-cavitating flow conditions, the trend of the characteristic curves is well predicted, while absolute performance values differ from measured data significantly. The results of steady and unsteady calculations deviate from each other by less than 2%. Concerning cavitating flow, unsteady simulations have to be performed in particular for overload conditions, in order to obtain convergence of the solver. The trend of the measured NPSH curve is well captured with default cavitation model parameters. For nominal and overload, the predicted NPSH curve underestimates the measured one significantly.