Large Eddy Simulation Of Unsteady Flow Field Evolvement In The Low-Pressure Turbine Blade

Since the beginning of the new century, the development of aviation industry has put forward higher request on aircraft engine. The pivotal issue in low pressure turbine(LPT) design is the generation of high lift blade profiles that contribute to reduce the count and weight of blades per row, in order to maintain high efficiency and low specific fuel consumption as far as possible. The key challenge to accomplish these goals is the low Reynolds number environment during aircraft cruise. When the blade loading increases, the phenomenon of boundary layer separation, transition and reattachment may take place on the blade surface, which lead to decreased efficiency and deteriorated performance. Therefore, detailed analysis of the unsteady flow field in LPT blade, especially the evolution mechanism of the boundary layer and coherent structures, is significant to develop high performance aircraft engine.Based on the dynamic Smagorinsky model, a large eddy simulation program of compressible Navier-Stokes equations was developed to carry out the research, which is suitable for the cases where there are dynamic and static domain simultaneously. The calculation program was verified by the cases of flow around a circular cylinder, flow around side-by-side circular cylinders and flow over a flat plate under adverse pressure gradient. The results showed the development process of cylinder wakes and the transition mechanism of laminar boundary layer under a strong adverse pressure gradient. Then the evolution characteristics of flow field in a LPT blade under the experimental condition are studied. Through analyzing the distribution and variation of dissipation function, the source and cause of energy loss were revealed. On this basis, comparison analysis under different incidence angle and Reynolds number were performed. The evolution process of boundary layer and large scale coherent structures near the pressure side were emphatically described at the cases of negative incidences. The influence of Reynolds number on the rear part of suction side was also investigated. Finally, flow fields around the LPT blades with periodic incoming wakes were studied. According to the obtained results under different reduced frequencies, the action mechanism of incoming wakes on the separating boundary layer as well as the impact on aerodynamic loss were analyzed.The computational geometry considered here is that of T106 high-lift LPT blade. The flow field characteristics are researched from both time-averaged and instantaneous results. The static pressure coefficient peaks near 62% of the axial chord. Due to the adverse pressure gradient and viscous effect, the laminar boundary layer separates from the suction surface. The shear layer goes through the process of spanwise vortex rolling up,(43) vortexes growing and large scale vortexes breakdown. However, the transition does not finish in the blade passage but completes in the blade wake zone. Based on the distribution of dissipation function, the loss comes from the area near the whole pressure surface and the front part of suction surface, the region between the separation zone and main flow, as well as the blade wake zone. The separation zone is just the accumulation area of low-energy fluid where the velocity gradient is small. It’s not the source of energy loss.The influence of the incidence angle on the flow field is mainly shown at the front of the suction side and the whole pressure side. With the incidence angle changing from positive to negative, the separation bubble near the leading edge disappears and the blade loading decreases gradually. When the incidence angle equals-5 degree, boundary layer separation starts to appear on the pressure side. At the case of incidence angle equaling-10 degree, the length of time-averaged separation bubble grows to 39% of the axial chord. From instantaneous results of-10 degree angle of incidence, the spanwise vortexes roll up near the leading edge on the pressure side and gradually evolve into streamvise vortexes. High-energy fluid in the main flow was driven to near-wall zone by the rotation effect of streamwise vortexes, which increases the fluid momentum inside the boundary layer. Then the boundary layer reattaches on the middle part of the pressure surface. The streamwise vortexes are stretched by the strong acceleration of the flow until they run out of the passage.The effect of Reynolds number on the flow field is embodied at the rear part of the suction side. As the Reynolds number grows, the length of the separation bubble in the leading edge increases and the initiating position of spanwise vortex moves upstream. Yet the separation point at the rear part of the suction side migrates downstream. Spanwise vortex which rolls up from the shear layer disappears when the Reynolds number equals 2.0?105. Instead, many small scale structures come into being and the transition process completes before the boundary layer shed s from the trailing edge. In high Reynolds number environment, the high dissipation region is closer to the blade surface near the rear part of the suction side. The width of the blade wake gets narrower and the loss at the exit plane becomes more less.The periodic incoming wakes increase the turbulence intensity at the inlet plane. At the same time, the fluid impinges the blade with smaller positive incidence. These two factors result in the separation bubble near the leading edge fading away. As the reduced frequency rises, the peak points of Reynolds stress and turbulence kinetic energy at the rear part of the suction side moves closely to the suction surface. The separation is also suppressed and the space-time diagram of wall shear stress shows “isolated island” phenomenon. Once the incoming wakes arrive at the suction side, the separation zone shrinks immensely or disappears entirely. At the intermittent stage of wakes, the Kelvin-Helmholtz(K-H) instability plays the leading role in the transition process. From the perspective of energy loss, it is not said that the higher reduced frequency is better for the LPT performance. When the reduced frequency is too high, the dissipation in the incoming wakes is relatively large. Although it reduces the loss at the rear part of suction side and the zone of blade wake zone, it leads to the main flow region becoming one of the principal sources of the loss. Hence, there is an optimum reduced frequency for the best aerodynamic performance.