ANALYSIS OF UNSTEADY LOSS SENSITIVITY TO INCIDENCE ANGLE VARIATION IN LOW PRESSURE TURBINE

In the present work, Large Eddy Simulations (LES) of a low pressure turbine cascade have been carried out to understand the different sensitivity to incidence angle variation observed under steady and unsteady inflow conditions. Indeed, experimental evidence shows poor sensitivity to incidence angle variation for the steady case in a range of -9° ≤ i ≤ +9°, while losses grow rapidly in the unsteady flow environment. It will be shown that this is due to wake migration into the core flow region, producing an additional source of losses for the unsteady flow case. Both steady and unsteady simulations have been carried out at nominal and two positive and negative incidence angles. The LES data have been inspected to identify the physical reasons behind the different loss trends, after being validated by means of blade loading and loss coefficient distributions. To this end, Proper Orthogonal Decomposition (POD) has been applied to each dataset. POD modes clearly identify the different structures responsible for loss production in the steady and the unsteady operation of the cascade. The structures carried by upstream wakes have been isolated and identified both in the boundary layer and in the core flow. The process of bowing, tilting and reorientation of the wake filaments and related structures is shown to be the dominant mechanism responsible for the strong loss sensitivity to the incidence angle variation observed under unsteady inflow. Otherwise, structures responsible for the transition of the blade boundary layer do not exhibit significant variation for both the steady and the unsteady flow cases, thus further strengthening that the sensitivity to incidence variation is mainly due to wake migration into the core flow region. This loss source is not classically considered in literature correlations and actual design processes.