Centrifugal pumps of low specific speed display an inherent tendency to generate an unstable pump performance curve [1]. These curves are characterized by a head dropping at low flow rates that limits the operational range. Hence, for example centrifugal pumps with such performance curves are not suitable for a usage in firefighting applications or parallel operation.

However, there are a few actions that influence positively the stability of the performance curve [1, 2]. One is adding slots at the rear shroud, e.g. on the pressure side or the suction side of the blade. Slots at the pressure side of the blade stabilize the characteristic curve by increasing the head, while suction-side slots stabilize the characteristic curve by dropping it down [2].

The part load flow pattern of a centrifugal pump includes two recirculation zones. The first is located at the inlet of the impeller and caused by the blade suction geometry. The second recirculation zone forms at the outlet of the impeller. It is known that the recirculation zone at the pressure side of a radial impeller has various positions, sizes and structures depending on initial conditions [3]. This paper deals with the assumption that influencing the pressure side recirculation zone leads to a stable pump performance curve. Therefore the structure of the recirculation zone at the impeller outlet is being investigated and analyzed whereas geometrical changes on a centrifugal pump impeller are performed.

The tests contain an experimental setup and compare the results to numerical simulations. Subject of the experimental investigations is a centrifugal pump with a specific speed of 33 min−1, a flow rate of 650 m3/h and head of 47 m for the Nominal Point. Measurements are performed for analyzing the time resolved pressure fluctuations and visualizing the flow structures in the volute casing by using pressure transducers and particle image velocimetry (PIV). These data show the changing pressure and velocity field and enable an analyzing of the part load recirculation.

Furthermore, the measured operational points and the time resolved pressure data are compared to numerical simulations that are 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.

Within the present study different geometrical parameters are subsequently changed on the original impeller design. This concerns, for example, the earlier named slots in the rear shroud both on suction and pressure side of the blade.

Results show an influence of these subsequent design methods on the performance curve as well as on the efficiency of the centrifugal pump. Additionally, the time resolved pressure data are used for a validation of the CFD simulations and both results show a significant influence of the flow structure at the impeller outlet on the performance curve. Therefore, it can be shown that the recirculation zone of the impeller is affected by these actions.

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