Influence on cavitation characteristics of large axial-flow pumping unit by uniformity of conduit inflow

Regarding to the effect on cavitation characteristics of large axial-flow pumping unit caused by the elbow-inlet conduit, establishment of accurate prediction methods is necessary. In this paper, based on the velocity triangle theory, the relationship of net positive suction head (NPSH) and uniformity of inlet velocity was firstly deduced. And then computational fluid dynamics (CFD) technology was used to simulate external characteristic curve of large axial-flow pumps and the results of experiment and simulation were compared. In conclusion, the research shows that under the optimal conditions, the head error is 3.58% while the efficiency error is 3.31%, which indicates the correctness of numerical simulation. What's more, properties and cavitation characteristics of pumping unit were predicted, and uniformity of inlet velocity got bad due to the influence of elbow-inlet flow, which lead to 7.83% reduction of head and 12.04% increment of NPSH. By comparing uniformity of inlet velocity, it can be find that at impeller inlet section, the inlet flow dose not enter into inside impeller vertically and uniformly, but at an angle with axial streamline, and weight average angle of inflow velocity (θ) become maximum when cavitation occurred. Actually, putting θ into formula, the NPSH error of both axial flow pump model and pumping unit are reduced to less than 5%, which meets the requirements of engineering application.