Numerical simulation and validation of effects of blade inlet angle on performance of pump-as-turbine

Abstract: Small and micro hydropower systems represent an attractive solution for generating electricity at low cost and with low environmental impact. The pump-as-turbine (PAT) approach is promising in this application due to its low purchase and maintenance cost. Due to that the conventional backward-curve centrifugal impellers do not effectively match the turbine's running, the performance of the PAT is usually undesirable. Therefore, to improve significantly the performance of PAT, the method for determining the blade inlet angle that plays an important role in the energy conversion was investigated deeply, and one kind of special impeller with forward-curved blades was designed for the turbine working condition in this paper. Firstly, based on the conservation theorem of angular momentum, the relationship expression between the blade inlet angle and the design flow rate was deduced. Moreover, in order to validate the relationship expression and investigate the effects of the blade inlet angle on the performance of PAT, 4 special impellers with different blade angles were designed by using ANSYS BladeGen and NX software. The validity of numerical simulation was first confirmed through the comparison between numerical and experimental results. The 4 impellers with different blade angles were numerically investigated by use of a verified computational fluid dynamics (CFD) technique. The performance within PATs was investigated through analyzing the external characteristic curves obtained by CFD. The results show that the flow rates of 4 PATs at BEP (best efficiency point) obtained by CFD are about 85, 90, 100 and 110 m3/h while the flow rates by theoretical calculation are about 83.6, 90, 99.2 and 108.6 m3/h, respectively, as the blade inlet angle is 60°, 72°, 90° and 105°. Based on the results, the flow rate of numerical BEP is very close to that of theoretical BEP, and the flow rate of BEP increases with the enlargement of the blade inlet angles. And, the theoretical relationship expression between the blade inlet angle and the design flow rate presents the effective prediction of the turbine model operations of centrifugal pumps with special impellers. The maximum efficiency of PAT decreases with the increase of the blade inlet angle. And the drop rate of the maximum efficiency increases with the increase of the blade inlet angle. The results indicate that the calculating method of the blade inlet angle is reasonable. Smaller angle is matched with relatively lower rated flow rate while bigger angle with higher rated flow rate. The performance of PAT is better and the high efficiency range is wider when the blade inlet angle is designed in a reasonable range. Additionally, the energy loss within the impeller reaches the minimum if suitable blade inlet angle is selected. So considering the efficiency and the high efficiency range of PAT, the value of blade inlet angle is recommended in a reasonable range between 60° and 90° when the spiral volute of this high specific speed pump is used as turbine flume. This paper is very instructive to the design of the special impeller used in the PAT.