Numerical Simulation of the Flow in a Centrifugal Pump With a Vaned Diffuser

This work presents a numerical investigation of the water flow in the first stage of a two-stage centrifugal pump with a vaned diffuser. The geometry consists of a pipe intake, a 8-blade impeller, a 12-blade diffuser and an outlet extension chamber. The numerical modeling comprises a transient rotorstator interface connection between the impeller and the other static domains, and it was implemented in the commercial code ANSYS-CFX. The numerical runs were carried out for four impeller speeds and a wide range of volumetric flow rates. The standard k-ε turbulence model was used. An experimental loop is also used to measure pump head values to validate the numerical approach. Comparison of numerical head values with the experimental data showed a good agreement. Similarity relations used for the numerical head values for different impeller speeds also shows good agreement within the range studied. A transient analysis of the pressure values at the impeller-diffuser interface showed that, using a steady state frozen rotor approximation as the initial condition for the transient calculation, generally no more than half of a complete impeller revolution is needed for the pump to achieve temporal periodicity. This numerical procedure saves significantly the computational time. Moreover, the numerical results confirm that, once the periodic regime is achieved, an azimuthal periodicity at each 90° interval is also achieved, just as expected from the 8-to-12 blades ratio between impeller and diffuser. Comparison of the numerical efficiencies of the single-stage pump with the experimental counterpart showed significant discrepancies. These must be related to the geometric simplifications of the numerical model and volumetric pressure losses of the real pump not included in the numerical model. Consequently, the Best Efficiency Point of the single-stage pump was found to be different from the two-stage assembly, and the flow field analysis apparently confirms this feature.