Pump efficiency drops largely with a decrease in specific speed N_s in a low N_s range. In order to reveal the mechanism of large loss and to find a method of improving impeller performances in the range of N_s<100 [rpm, m~3/min ], seven kinds of centrifugal impellers were tested in a axisymmetrie parallel-walled diffuser channel. Three impellers have the same outlet angle of β~2=22.5 ° with a different N_s from 80 to 200, and the other four have radial vanes (β_2=90°) ranging from N_s =50 to 80, three of which are closed type and one is full open type.The results showed that the conventional pump design is not suitable for a very low , N_s range and that the discharge at the optimum efficiency point (BEP) is far from the designed discharge. The Q3D calculation of impeller flow revealed that this discrepancy between the BEP and the dedigned is mainly due to large separation of flow in an impeller channel. The results also revealed that an efficiency of radial impellers is much higher than that of 22.5°-impellers of the same N_s, in the very low N_s range, though they have unstable performance characteristics. A full open-type impeller has no unstable characteristics and has relatively high efficiency in the very low N_s range.
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