Influence of clearance flow on efficiency prediction of Francis turbines

The disk friction loss of a Francis runner is crucial to the prediction of turbine efficiency, and the leakage flow through the gap near the runner band plays an important role to the flow structure of the draft tube, influencing the performance of a turbine. Therefore, investigation of runner leakage flow is very important for predicting accurately the turbine performance, especially turbine efficiency. In this paper, three-dimensional turbulent flow of a model Francis turbine has been simulated by using the CFD code ANSYS-CFX, with consideration of the runner leakage flow. Both steady and unsteady simulations have been conducted for different operating points of the turbine, and the turbulence was simulated with shear stress transportation (SST) turbulence model together with automatic near wall treatment. The disk friction loss of the runner was examined quantitatively, and CFD results were compared with those from the model test. The results show that the turbine efficiency predicted by CFD simulations with consideration of the runner disk friction loss is in very good agreement with the result from the model test for the operation points near the optimal point, and the one by CFD without the runner disk friction loss overestimates the turbine efficiency. For the operating points far away from the optimal point, the turbine efficiency predicted by CFD with consideration of runner disk loss is lower than the value from model test, due to flow separations or backflows existing in the flow field. The reduction in efficiency due to the runner disk friction loss becomes bigger with the increase of unit speed. The reduction in efficiency caused by the runner band surface is much higher than by the crown surface. In addition, the disk friction loss caused by the runner inner surface is basically equivalent to the one by the outer surface. Therefore, for a complex geometry, the outer disk friction loss can be estimated roughly by the inner one of the runner for simplification. Furthermore, at the same head, the leakage mass flow through the gap near the runner band is nearly constant, depending only on the pressure difference between the inlet and outlet of the gap. Moreover, the leakage flow through the gap near the runner band decreases the meridional velocity near the hub at the runner outlet at the optimal point, causing flow separation near the pier of the draft tube and producing extra hydraulic loss for the turbine. Based on the CFD results obtained from unsteady flow simulations for the optimal point, it is also found that the distributions of the inner and outer disk frictional loss in one period are influenced by the runner blade passing frequency, consisting of 13 peaks and valleys occurring at the same time. However, the phenomenon of rotor-stator interaction induced by the runner rotation has very limited influence on the disk friction loss. The relative peak to peak fluctuation is only 0.15% in one turbine period, denoting that the disk friction loss is nearly independent of the relative position between the runner and guide vanes. In addition, the same phenomenon has been found on the mass flow of the leakage near the runner band, with the relative peak to peak fluctuation of 0.03%. This research can provide useful reference for the prediction of disk friction loss of Francis turbine. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved.