Numerical Simulations And Flow Control Methods Of Separated Flow Within A Low Speed Axial Turbine

Turbine is one of the key components of air jet engine and ground gas turbine.The boundary layer is mainly laminar flow at very low inlet Reynolds number.The laminar boundary layer is easy to separate in the case of the reverse pressure gradient and transform from laminar to turbulence.Flow separation has strong influence on the aerodynamic performance and stability of turbine.So it is of great significance to study the separated flow and flow control methods in the turbine.Many flow control methods have been proposed for the linear cascade,but there is few research on those control methods in the practical application of annular turbine.To solve the above problem,this thesis has done the following studies:(1)The internal flow mechanism of low speed turbine under low Reynolds number has been studied.The flow at low Reynolds number of the low-speed axial-flow turbine is simulated with the laminar model,Shear Stress Transport(SST)model,transition SST model and the Large Eddy Simulation(LES)with WALE model.According to the comparison of the numerical simulation results and the experimental results,both the LES and transition SST model can accurately predict the internal flow of the turbine.In this thesis,the flow of low-speed turbine was analyzed in detail based on the results of large eddy simulation,which includes stator flow,stator-rotor interaction,rotor flow.The following conclusions can be reached:The large-scale separation flow exists on the surface of the stationary blade,and the separation flow flows radially inward;The loss of speed at the stationary blade root was significantly higher than that of the top,which is due to the Separation flow,reflux collective effect;The stator wake can make the velocity of the fluid near the wake significantly lower,but the effect of wake on the flow angle is small.The decisive factor for the flow angle is the separation flow on the blade surface;on the inlet of the rotor,the low-speed Micro-group moves in the radial direction while moving circumferentially,and the low-speed Micro-group moves in the circumferential direction.The effects of free stream turbulence intensity of the turbine flow is also studied.It is shown that the separation flow on the stationary blade is delayed with the increase of the turbulence intensity.(2)A variety of flow separation control methods have been tested in this thesis,including four passive flow control methods,C-type dimple,V-type dimple,grit strip and rectangle bosses,and the active flow control method,vortex generator jets(VGJs).When the turbulence intensity is 2%,three passive flow control method have no obvious effect on the flow separation.The flow separation can be postponed when the grit strip is located at leading edged and the maximum thickness of the blade.However,if the grit strip is placed before the separation it leads to premature separation.While turbulence intensity is 0.5%,all the passive control methods have no obvious effects.Only the vortex generator jets can be used to control the separation flow.When the jet blowing ratio is less than 2,the control effect is not good enough.When the blowing ratio is 2,the flow separation can be effectively controlled.For the separation control of low speed turbines at low Reynolds number,the reason for the different control modes have different control effects were analyzed.For passive flow control methods,the high velocity flow is brought into the boundary layer by the vortex generated by the control devices,which can postpone the flow separation.For active flow control methods,not only the energy from the high velocity flow,but also the energy from the jets are brought into the boundary layer.The turbulence kinetic energy of the boundary layer is increased by both these two ways.