Abstract: In order to improve the efficiency of pump-as-turbine (PAT), a specific speed 193 centrifugal pump with noncircular section spiral volute is chosen as a research object and one kind of special impeller is designed for the turbine's working condition. The relational expression between trapezoidal geometry characters and radius of equivalent circle is derived based on the area equal principle. The velocity moment at impeller inlet is acquired on the basis of constant velocity moment theory. And the relational expression between blade inlet angle and rated flow rate is derived. Considering the relatively large inlet width of blade, the blade axial projection draw is divided into two parts. Three outlet angles at the intersection of the three meridional streamline and blade trailing edge are calculated, respectively. The new special turbine impeller is modeled by using ANSYS BladeGen and NX software. The impeller is manufactured and the performance test is carried out on an open PAT test rig. To understand how the performance is improved, a verified computational fluid dynamics technique is adopted in the turbine mode performance prediction of PAT with the two impellers. The comparison of new impeller and the original PAT experimental results shows that the highest efficiency is raised from 71.9% to 77.3%. The best efficiency is increased by 7.5%. Compared with original PAT performance curves, the efficiency curve of PAT with new impeller is flat, and the high efficiency operating range of the special impeller is wider than that of the original impeller. The new impeller can adapt to large variation range of flow rate. The efficiency of PAT with new impeller is above 72% between 75-130 m³/h operating ranges. Numerical calculation results show that the new special impeller blade inlet angle can match well with the flow of the spiral volute outlet. The energy transformation is more uniform from the blade inlet to the blade outlet in new impeller. Hydraulic loss analysis shows that the hydraulic loss within the new special impeller is significantly less than that of the original impeller at a high flow rate. The experimental best efficiency point is consistent with the given design flow rate. Thus, it is verified that the impeller design theory and method proposed in this paper is reasonable. Research results have important meaningful to enrich the PAT impeller design theory and method.

Key words: blade angle, impeller, numerical simulation, pump, pump as turbine, trapezoidal cross-section volute