Influence of Blade Number on the Performance of Hydraulic Turbines in the Transition Stage

To analyze the effect of blade number on the performance of hydraulic turbines during the transient stage in which the flow rate is not constant, six hydraulic turbines with different blade numbers are considered. The instantaneous hydraulic performance of the turbine and the pressure pulsation acting on the impeller are investigated numerically by using the ANSYS CFX software. The ensuing results are compared with the outcomes of experimental tests. It is shown that the fluctuation range of the pressure coefficient increases with time, but the corresponding range for the transient hydraulic efficiency decreases gradually when the flow velocity transits to larger values. During the transition to small flow velocity, the fluctuation range of the pressure coefficient gradually decreases as time passes, but the corresponding fluctuation range of its transient hydraulic efficiency gradually becomes larger. The fluctuation range in the Z9 case is small during the transition. The main frequency of transient hydraulic efficiency pulsation is equal to the blade frequency. At the main frequency, Z7 has the largest amplitude of the hydraulic efficiency pulsation, Z10 has the smallest amplitude, and the difference between Z7 and Z9 is limited. As the number of blades grows, the pressure pulsation during the transition process gradually decreases, but the pressure pulsation of Z10 at the volute tongue is larger. In the steady state, Z9 has the highest efficiency and in the transient stage, the pressure coefficient fluctuation range is small. Accordingly, for the hydraulic turbine Z9, the performance is optimal.