Low Specific Speed Pumps with Atypical Impellers

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Low specific speed pumps are usually characterized by relatively low hydraulic efficiency. This fact is usually attributed to so called local eddy, which develops within the blade channel and causes flow blockage. Modified impellers with very thick trailing edges and recirculation channels were designed to improve pump performance. Unsteady CFD simulations and experimental measurements were carried out for original and modified impellers. Contribution will discuss influence of the new design on stability of the characteristic curve, energy dissipation and pressure pulsations within the spiral case. Dissipation function is evaluated in different parts of the spiral case and correlation between the hydraulic loss and filling of the spiral case by liquid is drawn for the particular impeller designs. The impeller with channels (i.e. with thick trailing edges) reaches at low flow rates higher hydraulic efficiency in comparison with conventional impeller with blades. Recirculation channels decrease the value of achievable hydraulic efficiency in whole range but its value at lower flow rates is still higher than for conventional impeller. However, the shift of maximum efficiency values toward higher flow rates occurs. The stability of specific energy characteristic curve is slightly worse for impeller with channels as well as for impeller with recirculation channels. This is manifested near the shut-off point and therefore it is very difficult to prove it by CFD simulation. This situation can be even worse for incorrect configuration of recirculation channels when flatter specific energy characteristic curve is achieved. Presence of swirling flow within recirculation channels also influences pressure pulsations in the spiral case.

low specific speed, spiral case, pressure pulsations, energy dissipation, characteristic curve stability

KLAS, R.; POCHYLÝ, F.

9. 9. 2013

EPFL Laboratory for Hydraulic Machines

Lausanne, Swiss

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