Improvement of Cavitation Surge in a Double Suction Centrifugal Pump by Use of CFD

This paper discussed about suppressions of cavitation surge in a double suction centrifugal pump. In order to suppress the cavitation surge, CFD simulation was carried out. Cavitation surge was observed near the best efficiency point, and it was difficulty to operate the pump stably. The specific speed of the tested pump was about 81 [m³/min, min⁻¹, m] or 533 [ft., USGPM, min.⁻¹]. In general, the main cause of the cavitation surge is inlet reverse flows at the impeller inlet. In order to prevent the inlet reverse flow, two kinds of modification at the impeller inlet were applied. One was the reduction of impeller inlet area by using a suction ring, and the other was the reduction of impeller inlet diameter. To reduce the computational time, in CFD model, a half of the double suction centrifugal pump was modeled. CFD simulations were carried out using ANSYS CFX with the Rayleigh Preset cavitation model. It was confirmed that the head fluctuation caused by the cavitation phenomena was predicted qualitatively by use of unsteady CFD simulation in the original pump impeller. The head fluctuation was about the 16% of the time averaged head and the very low frequency was confirmed by the FFT analysis. In addition, the relationship between head characteristics and cavitation behavior was observed clearly. The objective of the suction ring was to eliminate the head fluctuations caused by the cavitation. It was concluded that the suction ring was very effective to prevent the cavitation surge. On the other hand, the decrease of impeller inlet diameter was effective to reduce the head fluctuations, which became half of that for the original pump impeller. As a result, it was suggested that the complete suppression of the cavitation surge by the reduction of impeller inlet diameter was difficult in this case. It was concluded that unsteady CFD simulations with cavitation model is very effective for clarification of the impeller inlet modification on the cavitation surge.