Analysis Of The Pressure Fluctuations And Vortical Flow Characteristics In A Francis Turbine Draft Tube With Air Admission

This paper introduced the flow instabilities in the operating of the hydro turbines.Based on the investigations of the pressure fluctuations in experiments of the model or prototype turbines,numerical simulations were done to make a deep investigation of the air admission effect on the pressure fluctuations and the vortex flow characteristics in the draft tube.In order to improve the simulation accuracy,the simulation method for multiphase turbulence flow was investigated.Considering the compressibility of air,the multiscale vortex and the influence of surface tension,a modified multiphase cavitation model based on the Level set method and turbulence model were adopted.The modified simulation method was verified by comparing the simulation results with the experimental data of the ventilated cavitation around a NACA0015 hydrofoil.Secondly,the internal flow and pressure fluctuations in a Francis turbine operating under partial load conditions were simulated based on the modified simulation method.It is indicated that with the decreasing of the cavitation number,the vortex rope turned to be a vapor rope from a water rope.Besides the pressure fluctuation induced by the rotating of the vortex rope,cavitation surge also induced a pressure fluctuation.Air injected into the draft tube can alleviated the pressure fluctuations partially with a small air injected flow rate,and totally with a large air flow rate.Then,the effect of the air admission on the vortical flow in the draft tube was investigated based on the simulation results.The results show that the changes of the vorticity is mainly occurs near the surface of the vortex rope,and composed by vortex stretching,dilatation and baroclinic torque term.Each term has a reverse action and makes the vortex rope rotating.Air admission can change the distribution of the vorticity and make it changes from a spiral rope to a cylindrical one.It is indicated that the injected air can increase the pressure level of the vortex rope core and change the distribution of the pressure gradient,and then make the types of vortex breakdown changes.Besides,air admission also changes the distribution of the specific energy from the crown to the band,and finally makes it turned to be a supercritical state which is similar to the distribution of full load condition.Lastly,the comparison between runner crown injection and spindle hole injection was make.It is indicated that with the same air flow rate,air injected from the runner crown is more effective than injected from the spindle hole.Also,based on the understanding of the connection between vortex flow and pressure fluctuation,a grooved runner cone was designed.Simulation results showed that with a reasonable designed grooved runner cone,the pressure fluctuations at part load conditions can be alleviated effectively.While at designed or full load conditions,the grooved runner cone did not bring an unfavorable influence.