Research On Flow Separation Mechanism And Control Method Of Axial Flow Turbines Under Low Operating Conditions

Modern gas turbines usually work under non-design conditions,especially when the internal flow of the turbine is prone to flow separation when operating under low conditions,which seriously deteriorates the performance and efficiency of the turbine.At present,there are few and scattered studies on the flow field of turbines under low operating conditions,and there is a lack of understanding of the mechanism of the separation flow phenomenon.Therefore,it is of great theoretical significance and engineering application value for the aerodynamic design of high-efficiency and wide-condition turbines to carry out research on the mechanism of separation flow in low operating conditions of axial flow turbines and further develop control methods for separation flow.Firstly,the separation problem caused by shock waves and their associated phenomena when transonic turbine stages work under low operating conditions is studied.The difference of internal flow field in turbines under low operating conditions and design conditions is compared by unsteady numerical calculation results,and shock wave structure and its influence on boundary layer,blade load and cascade loss are analyzed in depth.The results show that,compared with the design condition,the effect of the tail wave in the stator blade on the flow direction migration is weakened in the low operating conditions,and the shock wave moves forward on the application point of the adjacent blade,resulting in the increase of static pressure at the application point,obvious adverse pressure gradient,boundary layer thickening and flow separation.For the rotor blade,a stronger inverse pressure gradient appears near the leading edge of the suction surface under low operating conditions,resulting in serious flow separation and increased loss.Moreover,in low operating conditions,isentropic efficiency of turbine stage is about 3.6% lower than that in design condition.Subsequently,taking a two-stage axial flow turbine as the research object,aiming at the problem of separation induced by Angle of attack changes caused by flow condition changes in low operating conditions,the influence of Angle of attack changes on flow field structure and aerodynamic performance of axial flow turbines was studied by unsteady numerical calculation method.The results show that,compared with the design condition,the flow inlet Angle of the second stage stator blade increases,and the direct impact of the airflow on the blade suction surface at the position of 20% to 30% axial chord length increases the inverse pressure gradient,and the boundary layer thickens and flow separation occurs.The flow inlet Angle of the first and second rotor blades decreases,and the leading edge saddle point deviates seriously and intensifies separation.In low operating condition,isentropic efficiency of turbine stage is reduced by about 14% compared with design condition.Finally,combining the above mechanism with passive control means,a modified idea was put forward."The boundary layer is conditioned by promoting its early transformation and increasing the level of turbulent kinetic energy in the boundary layer,thus improving the pressure gradient ability." A columnar groove structure was installed near the separation area on the suction surface side of the blade.Cylindrical grooves with width/blade chord length of3.6% and depth/maximum blade thickness of 8.6% are set at 25% axial chord length of the second stage stator blade suction surface of the axial flow turbine.The isentropic efficiency of the turbine under design condition is reduced by 0.45% compared with the prototype,but that under low operating condition is increased by 2.09% compared with the prototype.The split flow mechanism and the proposed passive flow control scheme obtained in this paper can provide relevant references for the flow separation control of the axial flow turbine of gas turbines under low operating conditions.