Two phase flow and cavitation in centrifugal pump: a theoretical and experimental investigation

Existing designs of centrifugal pumps can only handle very modest amounts of gas before they deprime. A normal centrifugal pump cannot usually handle any liquid having more than 7-9 percent gas content (by volume). Even this small percentage creates drop in head on the order of 20 percent as compared with 100 percent liquid. Furthermore the cavitation phenomenon considered is one of the most important problems in designing and operating a centrifugal pump. Especially when the uprating of pumps and driving at higher speeds to reduce size, its importance becomes greater. The physical mechanism of both two phase flow and cavitation remains to a great extend unexplained. In this investigation an attempt has been made to study these phenomena. A control volume method was employed to derive a mathematical model for a centrifugal pump under two phase flow conditions. The model combines the ideal (Eulers) two phase head of a centrifugal pump with additional losses due to compressibility and condensation of the gas phase. Systematic tests were carried out on a centrifugal pump of conventional design in order to established the causes of two phase head degradation and support the derived mathematical model. The excellence of the agreement obtained between the mathematical model and the experimental results gives confidence that it will be possible in the near future to predict the two phase head-capacity curve for any centrifugal pump. High speed video observations identified the basic mechanism leading to flow breakdown when pumping two phase mixtures and two phase flow and cavitation.