A LES investigation of off-design performance of a centrifugal pump with variable-geometry diffuser

The effect of the orientation of the diffuser blades on the performance and detailed flow-physics through a centrifugal pump is investigated at design and an off-design conditions, with the latter corresponding to 40% of the nominal flow-rate. A Large Eddy Simulation (LES) approach was adopted, validated for both load conditions in earlier studies. It is shown that an adjustment of the diffuser geometry at off-design produces a significant improvement of the pump efficiency, thanks to the lower incidence at the leading edge of the stator blades. For comparison, simulations were carried out also at design flow-rate with the same setting angles of the diffuser blades. At off-design separation on their suction side is substantially decreased, as well as back-flow phenomena at the impeller/diffuser interface. The flow through the stationary channels becomes more uniform, although separation is still experienced on their shroud side, caused by incorrect inflow from the impeller. Due to the smoother interaction between moving and stationary parts, turbulent kinetic energy undergoes a decrease of almost an order of magnitude. In contrast, results at nominal flow-rate show better performance with the original geometry. At both loads the impact of the setting angle of the diffuser blades on the flow through the impeller is actually limited to the pressure side of its blades, near their trailing edge. At the reduced flow-rate separation and back-flow phenomena at the shroud, rotor blades suction side and impeller inlet are still present and practically unaffected, being mainly caused by the pressure gradients through the impeller, rather than by impeller/diffuser interaction.