Mechanism Research Of Unsteady Flow In High-Lift Low Pressure Turbine Under Dynamic Roughness

From takeoff to cruise,the operating Reynolds number for the highly-loaded low-pressure turbine(LPT)in an aircraft gas turbine engine decreased significantly.At low Reynolds numbers,the boundary layer on the LPT blades are largely laminar,making them susceptible to flow separation near the aft portion of the blade suction surface,with associated loss increase and performance drop.Many active flow control techniques,which can be shut off when not required for flow control,have been developed to prevent the separation in an attempt to reduce the total pressure losses at different Reynolds numbers.Dynamic roughness,firstly proposed by Huebsch in recent years,is treated as a new active flow control method,which suppressed separation over the entire leading edge of the airfoil by unsteady surface roughness.It has the advantages of low energy consumption,blade structure integrity and separation control potential etc.In this paper,this method is introduced to the flow control of a highly-loaded PAKB LPT cascade at low Reynolds numbers in order to reduce the flow losses..Firstly,the effects of static roughness location on the LPT cascade performance were investigated for Reynolds numbers ranging from 25 k to 100 k.The sine roughness elements were placed at three different positions.L1 was placed upstream of the suction velocity peak location.L2 was placed between the velocity peak location and the separation onset location while L3 was placed downstream of the separation onset location.The steady numerical results showed that combined action of the turbulent wet area and the separation bubble size is the key to alter cascade flow losses,which was influenced by the bump.Static bump of L1 can slightly reduce the total pressure coefficient for Reynolds number of 25 k,while L2 and L3 increase the flow losses for other Reynolds numbers.And then dynamic roughness translated from the static roughness at the frequency of 180 Hz and the amplitude of 1mm was studied with unsteady numerical simulations.To analyze time-averaged performance of unsteady condition,this paper put forward the power loss coefficient representing the level of mechanical energy loss at the loss of entropy.It was found that the dynamic bump had comprehensive better control effects then the static one at the same positions.The cascade loss was reduced by46.9%,40.1% and 34.0% at L1,L2 and L3 compared to the smooth cascade.Loss sources at high Reynolds numbers mainly consist of separation bubbles,free shedding vortex and the turbulent wet under dynamic roughness.When it came to Reynolds number of 25 k,the suction surface was covered by laminar vortex,which formed the “rolling bearing” effect.The laminar vortex was able to eliminate the separation bubble and reduce the turbulent loss greatly.Finally,a parametric study about amplitude and frequency of the dynamic roughness placed at L1 was conducted at Reynolds number of 25 k.When the amplitude is constant,with the increase of the vibration frequency,the number of shedding vortices on the surface of the suction surface increases and the spacing decreases,which is helpful to break the original laminated laminar flow separation bubble and to delay the transition point.When the critical frequency is reached,suction turns to full laminar flow state.When the vibration frequency is constant,the disturbance of the pressure wave increases with the increase of the vibration amplitude,and the breaking effect of the large-scale separation bubble of the smooth cascade becomes stronger,and after reaching the critical amplitude,the separation bubble at the trailing edge of the convex hull becomes Vortex,the original smooth vane under the large-scale separation of the bubble completely disappeared.