Study On Vortex Simulation And Tail - Edge Modification Of Turbine Fixed Vane

In the current hydropower development, hydropower units are developing towards the direction of high head, large flow, large-scale. The capacity of single unit becomes larger. Hydraulic vibration problems of large capacity unit which caused by Karman vortex are more easily aroused. It is well known that the structural vibration may be induced by vortex shedding at the trailing edge of vanes. When the frequency of vortex shedding coincides with one of the natural frequencies of vanes, resonance occurs, causing vibration and potentially initiating cracks in the vane geometry if the amplitude of the excitation force is sufficient. Vibration will cause unstable operation of unit. Therefore, the simulation and analysis of Karman vortex in the hydraulic turbine, which has important significance for the elimination of resonance problem caused by the Karman vortex.In this investigations, Francis turbine’s tandem cascade was established, and three conditions of flow field were simulated by the means of FLUENT. The Karman vortex shedding at the trailing edge of stay vane was analyzed on the basis of numerical solution. In addition, In order to put forward reasonable repair scheme, the trailing edge was modified for different shapes and their impact on flow field of downstream were analyzed.The numerical results show that, when the vortex moves downwards, the vorticity value is gradually reduced, and the vorticity decay rate is not uniform. The more close to the trailing edge, the faster vorticity value decay. Secondly, in the calculation of the Karman vortex shedding frequency, the relative velocity of separation point should be determined to be outside the boundary layer velocity, instead of relative velocity in the boundary layer. Thirdly, the smaller the oblique angle of trailing edge is, more easily eliminate vortex. It will result in the pressure of guide vane’s pressure side is relatively large if the oblique angle is the bigger. In order to prevent vibration and according to the results of comparative analysis, the trailing edge was modified to a 45°angle, both ends of the oblique surface were designed as transition with circular chamfer.