Numerical Analysis Of Vortex Bands And Pressure Fluctuations In The Draft Tube Of Francis Turbines

The instability of turbine units is a serious and common problem.One of the main causes of turbine vibration is the pressure fluctuation induced by vortex band in the draft tube.So,it's very important to study the relations between the draft tube vortex band and the pressure pulsation.At present,the methods used in studying the draft tube vortex band and pressure pulsation are numerical simulation and experimental research.Considering the economic costs and the limitation of the test conditions,the numerical simulation method becomes the mainstream method in studying the pressure pulsations in the draft tube.The UG software was used to establish the 3D model of a hydraulic turbine,and the ICEM CFD was used to generate meshes.The SST turbulence model in CFX was selected to calculate the steady and unsteady flow conditions in the whole flow passage of the turbine,to study the flow characteristics of vortex bands in the draft tube and the stability of the internal flow field.Main research contents and achievements are as follows:(1)At the optimum unit speed,four typical calculation conditions were selected,which are two small flow conditions,the optimal condition and the large flow condition.The steady state calculation results show that the spiral vortex band appears at small flow conditions,the smaller the opening is,the larger the rotating radius of the vortex band will be,which means the worsening of eccentric vortex;the vortex band becomes slender cone at the optimal condition;and the vortex band becomes columnar vortex at large flow condition.The unsteady state calculation results show that the two symmetrical monitoring points have phase difference at small flow conditions.The rotating frequency of the vortex band is 0.333 times of the rotating frequency.The pressure fluctuation is mainly distributed in the inlet cone and elbow of the draft tube,and the amplitude of pressure fluctuation in the inlet section is larger than that in the elbow section.The two symmetrical monitoring points don't have phase difference at the optimum flow condition,and the rotating frequency of the vortex band is 1 time of the rotating frequency.The pressure fluctuation is mainly distributed in the straight section near the elbow of the draft tube.At large flow condition,the rotating frequency of the vortex band is 0.22 times the rotating frequency.The pressure fluctuation moves from the cone section to the elbow section.This indicates that the operation at small flow conditions is unstable,and the vibration is serious.(2)At the high unit speed,a small flow condition was selected for comparison.The steady state calculation results show that the eccentricity of the vortex band in the straight section of the draft tube is very small at the high unit speed.The spiral vortex band is mainly distributed at the elbow section.The radius of the vortex core is larger than the radius of the vortex core at low unit speed.The unsteady state calculation results show that the rotating frequency of the vortex band is 0.44 times of the rotating frequency at the high unit speed.Comparing with the low unit speed,the rotating frequency is increased.The pressure fluctuation is mainly distributed in the elbow section,and the pressure fluctuation has low frequency.The research results can provide guidance for the optimum design of hydraulic turbines.