Influence Analysis of Runner Inlet Diameter of Hydraulic Turbine in Turbine Mode with Ultra-Low Specific Speed

The hydraulic turbine in turbine mode (HTTM) with an ultra-low specific speed (HTTM-ULSS) has the advantages of a simplified structure, high efficiency, and good stability and has great application value in the industry. However, the influence of the runner inlet diameter (D1) on the performance of HTTM-ULSS has not yet been fully studied. Therefore, the three-dimensional models of Francis runners were established in the ultra-low specific speed range by examining D1 = 0.49 m, 0.5 m, and 0.51 m, and the two-stage hydraulic turbine models were constructed with flow passage components. Then, internal flow and energy characteristics were calculated using Fluent 16.0 software. Further, the influence of D1 on HTTM performance was studied by comparing numerical simulation results. The results show that the water head of the HTTM-ULSS can reach 540.87 m when D1 = 0.51 m, showing its powerful ability to recover the pressure energy in high-pressure water. Moreover, the head and efficiency are closely related to D1; when D1 increases, the circulation at the runner inlet increases, resulting in an enhancement in the ability to recover the water head and decreases in efficiency and in the operating range of the high-efficiency zone; with D1 increasing, the flow pattern inside the runner becomes better, but the high-pressure area of the blade increases. When selecting the D1, attention should not only be paid to the ability to recover the water head but also to the pressure of the runner blades and the internal water flow pattern.