Investigations On Flow Separation Mechanism And Control Methods Of A Low Speed Axial Flow Turbine

Aero engines operating at high altitudes and micro turbines usually work at very low inlet Reynolds numbers.There may exist flow separation and transition within the turbine,which has severe impact on the aerodynamic characteristics of the turbine.The flow inside an axial flow turbine is complicated three-dimensional unsteady flow.Numerical simulations become an important method to study this type of flow.Three-dimensional numerical model of a low speed axial turbine is established.The influence of Reynolds number on the internal flow of the turbine is studied.Secondly,considering the flow separation on the surface of stator,the effects of different flow separation control methods on the internal flow are studied.Finally,the influence of blade geometry optimization on the flow separation of the turbine is studied.The results show that with the increase of the Reynolds number,the flow separation of the stator suction surface is delayed.The boundary layer thickness at the exit of the stator is increased.The total pressure loss and velocity loss at the downstream of the stator are decreased.The loss area caused by the secondary flow at the downstream of the rotor exit is reduced.The total pressure losses caused by the hub passage vortex,the tip passage vortex and the tip leakage vortex are increased.The tip leakage vortex is largest source of velocity loss.At a low inlet Reynolds number,the flow separation of the suction surface is delayed when the boss is installed at the leading edge or the maximum blade thickness point.The boss has no effect on flow separation control when it is installed just before the separation point.The rough belt has obvious effect on flow separation whenever it is installed at three positions.The C-shaped groove and the V-shaped groove have no obvious etTect on flow control.At a high inlet Reynolds number,the effects of the boss and the rough belt are similar to that at the low inelt Reynolds number..The effect of C-shaped groove is same to the previous two methods.While,the effect of the V-groove is weak when it is installed at the leading edge position.Moreover,it has no effect when it is installed just before the separation point.Optimization of the inclination angle of the blade reduces the radial flow of the fluid at the top of the blade,and increases the fluid velocity at the downstream outlet of the stator,which reduces the total pressure loss of the whole turbine.However,the flow separation area at the tip of the blade increases.