Centrifugal pumps are characterized by the performance curves, showing the dependency between its main operating parameters for a range of external conditions. One of the most important curves is the head curve, which shows the dependency between the volume flow rate and the pressure head, induced by the pump. Stable operating conditions occur when the pump curve has a negative slope for the point of intersection with the system resistance curve (dH/dQ < 0). Most of the pump curves have a “stable” negative slope of the head curve for the most part of the possible operating range. Nevertheless, centrifugal pumps of low specific speed display a tendency to generate an unstable pump performance curve, especially if designed with a high head coefficient at the best efficiency point . These curves are characterized by lowering the pressure head near the shut-off area (drooping curves). Operating range of such pumps could be significantly limited in the case of high static head component for the system resistance curve (firefighting applications) or parallel operation.
The flow pattern for the part load conditions differs from the one by the optimum conditions. There are two recirculation zones, occurring at the inlet and at the outlet of the impeller . This paper concentrates on the investigation of the pressureside recirculation and its influence on the curve slope. The pressure-side recirculation may have various position and intensity in the case of a radial impeller with an axis-parallel trailing edge [3, 4]. There are a few measures that influence positively the stability of the performance curve [3, 5].
The tests contain an experimental setup with comparison of experimental data to numerical simulations. Subject of the experimental investigations is a single volute radial centrifugal pump with a specific speed nq = 35 min−1. The casing was modified in order to provide optical access to the impeller outlet area. Radial velocity components are measured by using the Particle Image Velocimetry (hereafter: PIV) methods in order to define the backflow zones for the part load regimes. Measured operating points are then compared to numerical simulations carried out by Computational Fluid Dynamics (hereafter: CFD). The flow pattern gained by CFD allows analyzing the phenomena of the pressure side recirculation in detail, also in areas, where the access with measuring instruments is limited.
The investigation is performed on two modifications of the pump impeller — applying the slots opening in the back shroud  and diagonal trimming of the impeller , as well as for the original (unmodified) case. The combination of the described methods gives a clear vision on the 3D recirculating structures for the cases of stable and unstable behavior of the curve.