The Influence Of Upstream Wakes On The Boundary Layer In Turbine Blade

Modern high-lift blade designs incorporated into the low-pressure turbine(LPT)of high bypass ratio turbofan engines is used to reduce the weight of the turbine by increasing blade load and decreasing the number of turbine blades,but typically exhibit a separation bubble on the suction surface of the airfoil.Especially,LPT usually work in a low Reynolds number environment,which will lead to serious loss and reduce the efficiency of LPT.By used of upstream wake generated by the relative motion of the rotor and stator in LPT can controlled the development of the downstream blade boundary layer.Therefore,it is necessary to study the interaction between the upstream wakes and the boundary layer for design high lift LPT blade.The separation bubble and the turbulence loss are governed by the transition process in the separated shear layer.However,the wake-induced transition,the natural transition,and the instability induced by the Klebanoff streaks complicate the transition process.In this paper,the boundary layer on the suction surface of a high-lift LPT was investigated with upstream wakes at three condition.This paper base on PAKB airfoil and investigated under numerical simulation and experiment.The numerical simulation was performed with the CFX software using large eddy simulations(LES),and the experiment was performed on a linear cascade.This paper is focus on the aerodynamic issues of high-lift blade design in LPT,following two aspects of the subject are addressed in this dissertation:(1)In this paper,the wake is divided into the wake center and the wake tail,the unsteady formation process of the streaks and the wall shear stress caused by the wake are analyzed.It was found that after entering the blade passage,the wake center does not contact the leading edge of blade but causes the wall shear stress of the front part on the suction surface to increase.However,it is not possible to form strong Klebanoff streaks at the leading edge of the blade by shear sheltering effect.Only the wake tail can form Klebanoff streaks when it contacts the blade.The wake-amplified streaks under the forcing of the wake tail can arrive at the separation bubble and the transition zone.(2)In this paper,the interactions between the wakes and the boundary on the leading edge of the blade at 0° incidence and +10° incidence are mainly discussed.+10° incidence gives rise to an adverse pressure gradient(APG)over the forward suction surface.The adverse pressure gradient enhances the shear sheltering effect and,as a result,reduces penetration and amplify the streaks.However,strong Klebanoff streaks are amplified by the APG at +10° incidence and experience a weak viscous decay due to the moderately favorable pressure gradient(FPG).Between the wake passing events,weak Klebanoff streaks induced by FSTI are also amplified by the APG at +10° incidence but barely amplified at 0° incidence.Klebanoff streaks appear almost in the whole wake passing cycle,resulting in an earlier transition onset and a longer transition process at +10° incidence.