Studies on Transitional Behavior of Separated Boundary Layer on the Suction Surface of an LP Turbine Airfoil for Aeroengines

摘要

In modern high bypass turbofan engines, low-pressure turbine (LPT) stages are required to provide very huge power output to drive large fan for propulsion and additional booster stages very efficiently. Due to the relatively low-speed rotation, the aerodynamic loading of the LPT stages is usually quite high and inevitably the blade count in the LPT stage tends to be very large for maintaining the stage efficiency as high as possible. As a result, LPT section is one of the heaviest parts of the engine, which could amount to about one-third of the engine's total weight. The current design trend of aeroengines is therefore to decrease the number of blades in LPT stages in order to achieve a drastic reduction of engine weight, manufacturing and maintenance costs and total sfc (specific fuel consumption) of aircraft. However, the reduction of the blade number surely induces an increase of the aerodynamic loading on each blade, resulting in the appearance of large separation or separation bubble on the blade suction surface due to the strong adverse pressure gradient, particularly under low Reynolds number conditions. Since this separated flow around the blade causes a significant loss in engine efficiency, there have been a number of relevant studies on separated boundary layer on high-lift LPT blades. Mayle classified the boundary layer transition on LPT blade into three modes in his pioneering paper, describing that separated-flow transition mode could be the most important one for LPT. Nevertheless, it is still necessary to investigate the separated boundary layer because of relatively few studies dealing with its transitional behavior in detail under realistic flow conditions such as Reynolds number and freestream turbulence. Since boundary layer transition and separation depend strongly on these two factors and their interaction, it is quite obvious that understanding of the separated boundary layer subjected to such flow disturbance and development of an accurate method to predict its transition is crucial for lighter and more efficient aeroengines.