TEMPORAL STRUCTURE OF THE BOUNDARY LAYER IN LOW REYNOLDS NUMBER, LOW PRESSURE TURBINE PROFILES

摘要

Viscous effects in the suction side of low pressure turbines account for about 1/2 of the total turbine losses. Modern design practices simultaneously include decreasing the profiles' Reynolds number and increasing their aerodynamic load, thereby compromising the suction side boundary layer flow. The objective of the present investigation is to experimentally elucidate the spatial and temporal structure of the suction side boundary layer in the low Reynolds number regime. Under steady state approaching flow conditions, the boundary layer undergoes laminar separation shortly after the external flow velocity peak is reached. The separation produces a low kinetic energy, uniform pressure fluid region. Between this and the external flow, a laminar shear layer develops. The laminar shear layer undergoes a sudden, non linear instability process when its distance to the suction side wall is compatible with the shear layer most unstable scale. This instability promotes the formation of large scale vortices that reattach the flow to the suction side wall. When the approaching flow includes moving wakes that simulate the previous turbine stage, the above description is modified. The laminar shear layer undergoes a global instability triggered by the passing wakes' perturbation field. Later on the viscous layer reattaches as the wakes' perturbation field accelerates the fluid. After each wake passage there is a transient, relaxation period characterized by the growth of the low energy, recirculating fluid region and the simultaneous lift of the shear layer away from the suction side wall. The measurements conducted allow the identification of flow regions, as well as temporal and spatial scales that account for the downstream evolution of the viscous layer integral parameters.