Study On Unsteady Flow Mechanism Of The Integrated Aggressive Inter-Turbine Duct In Low Reynolds Number Condition

Aggressive inter-turbine duct(AITD)which has ultra-high bypass ratio and ultra-short axial length is widely applied in the modern high perforence turbofan engine to meet the demand of lighter and more efficient aircraft engines.However,within such duct,the curvature change of the flow channel is greater and the reverse pressure gradient in stream-wise direction is enhanced.There are swirling flow,wake,shock and tip clearance leakage flow of upstream high pressure turbine(HPT),and some engines even have structs in this transition section.In addition,the turbine's Reynolds number in high-altitude cruise state is greatly reduced.Resulted from the above,flow fields of the AITD and its downstream low pressure turbine(LPT)are on the edge of separation,which will cause high aerodynamic losses.The integrated AITD is to integrate them with the wide-chord guide vane which replaced original LPT nozzle in an AITD and becomes a promising technology to restrain the additional ITD's three-dimensional separation by improving the flow field structure.However,how to improve the AITD's performance is still a difficult problem.Since there are many studies on suppressing the blade suction surface's separation by inherent unsteady wake's“calmed region”effect,the anuthor utilizes periodic wake to suppress the boundary layer separation of the LPT nozzle's suction surface in the integrated AITD by computatioinal simulation methold.Focusing on the unsteady flow mechanism of the integrated AITD,following three aspects of the subject are presented in this dissertation:(1)Internal flow mechanisms of the integrated AITD,especially the separation and transition mechanisms of LPT nozzle's suction surface boundary layer are discussed.The LPT nozzle accounts for the largest proportion of the integrated AITD's loss in steady and unsteady high-altitude cruise design state.The transport property of the periodic wake and the“negative jet” effect are elaborated.Suppression and transition flow mechanisms of the separation-bubble on the LPT nozzle's suction surface are studied,and the“calmed region”effect is found in the wake-induced transition process.(2)Incoming flow condition's impact on the three dimensional unsteady flow performance of the integrated AITD,especially the separation and transition mechanism of LPT nozzle's suction surface boundary layer are discussed.Research shows that the flow performace of the integrated AITD is improved with the increase of the Reynolds number and the influence of freestream turbulence intensity(T_u)on integrated AITD is not significant.The impact of Tu on the integrated AITD's flow property depends on the balance of the positive and negative effect because the influence of the Tu on the integrated AITD is dual.(3)The influence of wake features on the three dimensional unsteady flow performance and the optimization of unsteady wake feature of the integrated AITD are carried.For the two wake features studied in this dissertation,the wake intensity has great influence on the integrated AITD's unsteady flow property,especially the LPT nozzle's suction surface boundary layer.Both wake intensity and wake sweep frequency have dual effects on the unsteady performance of the integrated AITD.Finally,wake feature is optimized to maximize the aerodynamic performance of the integrated AITD.