Effect of Inlet Section of Circular Section Spiral Case on Performance of Ultra-Low Specific Speed Diagonal Flow Turbine

Variation of section area at volute inlet is a key factor affecting turbine performance. By changing the inlet section of the turbine volute, the inlet angle of the guide vane is changed, so the guide vane parameters will also be changed, which will affect the turbine performance. In order to study the influence of different volute inlet section areas on the overall hydraulic performance of ultra-low specific speed diagonal flow turbine, this paper redesigns the volute inlet section area of an ultra-low specific speed diagonal flow turbine. On the premise of applying the same runner, seven different volute inlet circular sections are designed. The numerical simulation of the full flow channel is carried out for each scheme, and the influence of inlet circular section area of different volutes on hydraulic performance of the ultra-low specific speed diagonal flow turbine is analyzed and compared. The results show that with the increase of flow rate, the calculated head and runner loss of the turbine increase, and the hydraulic efficiency of the turbine first increases and then decreases. When the turbine runs under small flow conditions (less than rated flow), the hydraulic efficiency of the turbine increases with the decrease of the diameter of the inlet circular section of the volute. When the turbine operates under high flow condition (greater than rated flow), the hydraulic efficiency of the turbine decreases with the decrease of the diameter of the inlet circular section of the volute. Through comparative analysis of seven schemes, the optimum radius of the inlet circular section of volute for the ultra-low specific speed diagonal flow turbine is 127mm and the corresponding inlet angle of guide vane is 32°. At this time, the efficiency of the turbine is 81.59% and the calculated head is 4.821m. The flow pattern inside the turbine is more uniform without whirlpools. It provides a reference for future design of similar type turbines and selection of suitable volute inlet circular section area.