The usage of splitter blades to enhance the performances of low specific speed pumps is common practice. Based on experimental and numerical studies, the influence of the addition of one and two splitter blades is investigated on a very low specific speed pump to assess their impact not only on the performance characteristics but also on the losses in all pump domains. First, the main characteristic curves are discussed and it is shown that the usage of splitter blades enhances the head of the pump while not impairing its efficiency. Secondly, a detailed analysis of the losses in the pump reveals that splitter blades improve the flow in all parts of the pumps, but the volute. The flow at the impeller outlet shows that splitter blades largely benefit the slip factor and discharges a more blade-congruent flow in the volute. However, higher absolute velocity at the outlet of the impeller with splitter blades increases friction at the volute wall, as confirmed by the average wall shear stress in the different tested cases.
Low specific speed pumps find applications in a broad range of domains, but suffer from a low efficiency and a risk of head instability close to shut-off. The numerical computations on these pumps performed in the last years have shown discrepancies with experimental results. Recent studies suggest that the use of wall-functions underpredicts the losses of these pumps, especially at overload. The reason has been attributed to a detachment zone downstream the volute tongue, not well captured with the wall-function approach. This paper focuses on the influence of the volute casing on a pump with a specific speed of 8.9 on two issues. First, the influence of the wall modelling approach relatively to the low-Reynolds number method on the performance prediction is discussed. The results are, as expected, an underprediction of the losses when the wall-function approach is used. With a larger volute, the difference between the two wall modeling approaches is smaller. Secondly, the influence of the volute throat area enlargement on the pump performances is discussed. Both the head and efficiency are improved at the design point and at overload with an increased volute throat area. However the part-load head decreases and the head flattens. The study of the flow at part-load, in the region at the outlet of the impeller reveals that with a larger volute, larger flow exchanges are present, contributing to additional mixing losses and head loss.
The very low specific speed pump (nq = 8.9) operated in turbine mode was analyzed. The experimental and numerical studies were carried out in order to show effect of different blade layout on pump-as-turbine (PaT) performance. In total three different impellers were analyzed. One impeller consisting of four main blades and two impellers consisting of four main blades and different arrangement of splitter blades. Either single splitter blade or two splitter blades are placed between each of main blades. While measurements pointed out the main PaT performance, the simulations enable to analyze internal flow fields and point out the mechanisms of performance variation using different impellers. The main aim of this study is to clarify usability of very low specific speed pump for energy recovery in terms of pump as turbine operation or for storage capability in terms of pump-turbine.
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