The Mechanism Of The Influence Of High-Load Low-Pressure Turbine Blade Vibration On The Separation And Transition

High load design of low-pressure turbine blades is an effective way to improve the thrust-to-weight ratio of aero engines.The high-load low-pressure turbine will inevitably cause a sharp increase in the adverse pressure gradient of the stator passage.At high altitude cruise,the Reynolds number of the low-pressure turbine is very low.The lowspeed fluid,such as the boundary layer on the suction surface of the blade,is insufficient to resist the adverse pressure gradient,and separation is extremely likely to occur.Additionally,due to the lower inlet temperature of the low-pressure turbine and the limited driving force provided by the elementary stage,the blades are generally longer and thinner.It is very easy for the blades to vibrate during operation.Blade vibration will affect the flow phenomena of the low-pressure turbine,especially the high-load low-pressure turbine blades with boundary layer separation.Therefore,it is of great importance to deeply understand the influence of blade vibration on the internal flow of high-load low-pressure turbines.This paper adopts steady and unsteady numerical simulations,uses the SST(shear stress transport)turbulence model coupled with the γ-Reθ transition model,takes the PACK-B low-pressure turbine blade as the research object,and studies the influence mechanism of high-load low-pressure turbine blade vibration on separation and transition.Firstly,the flow characteristics of blades under different Reynolds numbers without vibration were studied,and the influence of Reynolds number on the velocity field and boundary layer parameters of the suction surface was analyzed in detail.Furthermore,the influence of different vibration modes(translation and torsion),frequencies(0-175 Hz),and amplitudes(0-1.5%chord length)of the blade on the boundary layer characteristics of the suction surface under low Reynolds number(Re=25,000)conditions was explored.The influence mechanism of blade vibration on boundary layer separation and transition was summarized.The main conclusions are as follows:When the blade is without vibration,a closed separation bubble appears at the trailing edge of the blade.Increasing the Reynolds number promotes the transition in advance,which significantly reduces the size of the separation bubble.With the increase of Reynolds number,the total pressure loss of the blade decreases gradually.When the blade vibrates,the relative motion between the blade vibration and the fluid,and the fluid inertia cause the separated flow and the main flow to mix in advance.The boundary layer transition is advanced,effectively inhibiting the development of the separation bubble,weakening the internal reflux mixing of the separation bubble,reducing the boundary layer thickness,and greatly reducing flow loss.Increasing the vibration amplitude or frequency of the blade will weaken the turbulence pulsation level in the separation and transition process,thereby reducing the additional shear stress caused by turbulence pulsation and the resulting viscous dissipation,greatly reducing the total pressure loss,and improving the aerodynamic performance of the blade.When the vibration frequency of the blade exceeds 150 Hz,the separation bubble profile changes,and secondary separation bubbles are also produced,which will increase the blade profile loss.Secondary separation bubbles should be avoided as much as possible.Continuing to increase the vibration frequency beyond 150 Hz will increase the total pressure loss.In addition,changing the torsional vibration component has little effect on the aerodynamic performance of the blade.