Investigation On Separation And Transition Control Of Ultra-high-lift Low Pressure Turbine Blade Under Steady Flow And Unsteady Flow

The low-pressure (LP) turbine blades are being designed for increasing higher values of aerodynamics loading is an efficiency way for lower weight and higher thrust-to-weight ratio of modern aircraft engines. Further increase of the blade loading will result in an even higher adverse pressure gradient, especially the LP turbines operate at low Reynolds numbers as the aircraft operates to high altitude cruise. Consequently the boundary layer of flow is prone to separation, making profile’s performance deteriorate, leading to increased losses and declining LP turbine efficiency. The inherently periodic wakes in aircraft engine from the previous blade row can suppress the separation bubble, however the bubble will reestablish between the wakes. Furthermore, further increasing the blade loading (Zweifel number above1.4) will result a larger separation bubble on the blade suction surface, so the periodic incoming wake may not be strong enough to suppress the separation bubble. Therefore, further understanding the separation and transition mechanism and find the effective control strategies will be one of the most critical technologies for high performance LP turbine design. Focusing on the boundary layer performance of ultra-high-lift LP turbine airfoils and the flow passive control based on surface roughness techniques in steady and unsteady flow, following two aspects of the subject are addressed in this thesis:(1) The passive flow control mechanisms of ultra-high-lift low pressure turbine blade in steady flow. Numerical study was conducted to improve the performance of a forward-loaded ultra-high-lift (Zweifel number1.4) low pressure turbine blade using surface roughness, dimple and surface trips. The boundary layer development and transition mechanism was investigated using these passive flow control methods in steady flow. A parametric study of these control methods on the effect of the profile losses in different Reynolds numbers is conduct. The profile loss is further reduced when the dimples’deep vs width ratio is optimized, the results indicate that the profile losses can lower about33%.(2) The combined effects of upstream unsteady wakes and surface passive flow control methods of boundary layer development on the ultra-high-lift low-pressure turbine blade. The unsteady effects of wakes are simulated by equidistant parallel cylindrical bars traversed across the inlet flow. At first, the development of separation bubble and wake induced transition are analysis on smooth wall in a wake passing period. The different passive flow control methods are investigated to reduce the profile losses in unsteady flow, which indicates the dimple can reduce the profile losses about9.5%. Then the detial flow mechanism of the optimized dimple is analysis in unsteady flow, the result indicate that the dimple influence the reverse vortex of the pulse velocity. The influence of Reynolds numbers and FSTI are investigated in a wake passing period using optimized dimple.