Research On The Mechanism Of Boundary Layer Flow In Low Pressure Turbine Blade

The low pressure turbine(LPT) plays an important role in aircraft engine, whose performance heavily depends on the efficiency of LPT. With the development of aircraft engine,further request has been put forward on LPT.Thus the generation of high lift and high efficiency blade profiles is the main trend in the development of aircraft engine. Considering the low Reynolds number environment in which LPT operates during aircraft cruise and as the increase of blade loading, the blade surface is prone to show the phenomenon of boundary layer separation, transition and reattachment, which will lead to decreased efficiency and deteriorate performance. Therefore the detailed analysis of the mechanism of boundary layer on LPT blades surface is significant to develop high performance aircraft engine.First, the influence of the turbulent intensity, incident angle and Reynolds number of the steady incoming flow on the transition process of T106 A blade boudary layer are researched detailedly, and the turbulent intensity is considered as the most important parameter which palys an important influence in the transition process. When turbulent intensity exceed 4%, natural transition or bypass transition will happen in boundary layer,and the phenomenon of boundary layer separation will disappear. When turbulent intensity is below 3%, separation transition will happen in boundary layer,and the phenomenon of boundary layer separation will became very significant. The influence of incident angle of incoming flow on boundary layer is mainly shown at the front of leading edge of bade, while the Reynolds number will significantly influence the boundary layer at the rear part of the suction side. When the Reynolds number is below 26000, an open bubble will disappear. As a result, it decreases the efficiency of blade to a great extent.Under the condition of unsteady flow with periodic incoming wakes, the transport process of wakes in blade passage are researched, and the shape change and reorientation process of the wake are also analyzed. Through analyzing the streamline, vorticity and velocity changes in the boundary layer with time, it can be considered that the separation bubble at the rear part of the suction side will periodically produce and disappear with the wakes migrating downstream. Through drawing the space-time diagram of displacement thickness, momentum thickness and shape factor, the interaction between boundary layer and wakes is researched and the calmed region after wake is discovered, which play an important role in suppressing the separation of the boundary layer. Through analyzing the distribution and variation of dissipation function in blade passage, the source and cause of energy loss are revealed. The loss mainly comes from the area near the leading edge of blade and the region between the separation zone and main flow, as well as the blade wake zone because of high velocity gradient in these area. While velocity gradient is small in separation bubble,the separation zone is just the accumulation area of low-energy fluid rather than the source of energy loss. The mechanism of wake suppressing the separation of boundary layer in different Reynolds number conditions is also researched. The higher Reynolds number are, the more significant increase of the blade performance the wake improves. When the Reynolds number equals 26000, the timeaveraged efficiency of blade will increase 1.1%, compared to no incoming wakes, thus widely improving the stability of blade. The mechanism of wake suppressing the separation of boundary layer provides an important reference for designing high lift blade profiles.Finally, the boundary layer development and transition mechanism was investigated using vortex generator in steady flow. Three different vortex generators with different amplitudes, different wave numbers and different location are respectively designed to investigate the effect which the vortex generators play on the control of the boundary layer at the rear part of suction surface. Research indicated that vortex generators can accelerate the transition process.High-energy fluid in the main flow was driven to nearwall zone by the rotation effect of vortexs which were produced by vortex generators, thus increasing the fluid momentum inside the boundary layer and greatly suppressing the separation of the boundary layer. Based on the three geometric constructions of vortex generators, the change of amplitude is the most important to the control effect of boundary layer. Under the high Reyonlds number conditions, the efficiency of blade will decrease because of increasing energy loss produced by vortex generators. Under the low Reyonlds number conditions vortex generators can decrease the size of separation bubble, thus greatly increasing the efficiency of blade.