Flow Characteristics Analysis Of Inter-blade Vortices For Runners In A Low-Specific-Speed Francis Turbine

Hydropower units often carry the tasks of peak and frequency regulation of the power grid.The frequent changes in the output of the units cause the turbines to inevitably run under conditions that deviate from optimal conditions.When the Francis turbine runs at a point deviating from the optimal working condition,the inflow between the water flow inlet and the blade cannot meet the zero angle of attack,resulting in the formation of a blade vortex in the flow channel after the water flow separation.The appearance of the blade channel vortex will cause uneven force on the runner blade and induce blade pressure pulsation,which will cause abnormal noise and vibration of the unit and seriously affect the safe,efficient and stable operation of the hydroelectric generating unit.In this paper,the internal flow state of the low specific speed Francis turbine under partial operating conditions is measured by the PIV test.The experimental data is analyzed and processed using the proper orthogonal decomposition principle,combined with CFD software for numerical simulation of the internal flow of the turbine.The main contents of this study are as follows:(1)Transparency of the flow components of the model turbine,a PIV test system is built according to the structure of the turbine,and the flow field structure of the plane cross section at the height of 1/2 blade at the entrance of the turbine runner is measured.The absolute velocity streamlines of A1-404 and A2-404 conditions are relat ively uniform,and there is no obvious change in the flow field structure at different times;In the absolute velocity flow field of A1-606 working condition and A2-606 working condition,vortices of different scales appear in the flow field,and the flow s tate is not stable(2)The inlet velocity triangle is used to decompose the absolute velocity flow field,and the velocity and streamline structure of the decomposed relative velocity flow field are analyzed.The relative velocity streamline of A1-404 working condition is close to the arc line,which is easy to form a vortex;The water flow in A2-404 working condition flows in near zero angle of attack without obvious vortex structure.A large-scale vortex structure appears in the relative velocity flow field between A1-606 and A2-606 conditions,and the flow state is turbulent.(3)Based on the principle of proper orthogonal decomposition,the dimensionality reduction analysis is performed on the relative velocity flow field,and the main flow mode structure in the original input flow field data is extracted.(4)Establish a three-dimensional model and structured mesh of the volute,distributor,runner and draft tube of the model turbine.The three-dimensional steady turbulent flow numerical simulation calculation of the entire flow channel is performed on the above calculation domain,and the blade surface pressure contour,the velocity of the flow surface at different blade heights and the streamline structure are analyzed.The results show that the distribution position of the inter-blade vortex is related to the angle of attack of the inlet flow.