Effect of trailing-edge boundary conditions on acoustic feedback loops in high-pressure turbines

Abstract

The occurrence and sensitivity of acoustic feedback loops was investigated for a linear high-pressure turbine (HPT) vane cascade at realistic operating conditions, i.e. isentropic exit conditions of Re = 570, 000 and M = 0.92. Forced compressible Navier–Stokes simulations were conducted to study the temporal response of various base flows to an initial small-amplitude pulse. One of the objectives was to assess the impact the quality of the base flow has on the instability behaviour, i.e. whether base flows obtained from computationally more affordable approaches such as Reynolds-averaged Navier–Stokes (RANS) produce similar results as those obtained from highly resolved large-eddy simulations (LES). Using base flows obtained from both LES and RANS of a high-pressure turbine vane with no-slip trailing-edge boundary conditions, the temporal pulse response revealed the presence of an unstable acoustic feedback loop, i.e. with amplitude increasing quickly over time. It was also found that the base flows were globally unstable with respect to three-dimensional instabilities, regardless of the perturbation location. However, the stability analysis performed on the RANS generated base flow exhibited much increased growth rates due to the recirculation region downstream of the blade trailing edge featuring a higher reverse velocity.In order to understand the sensitivity of the acoustic feedback loop to modifications of the trailing-edge boundary conditions, additional stability analyses were conducted on base flows obtained from simulations of the same HPT vane configuration with trailing-edge ejection at two different rates. For the base flows with added trailing-edge ejection, the pulse responses did not exhibit pressure waves originating from the trailing edge impinging on the suction side of the adjacent blade. More importantly, although an acoustic feedback loop was also present in these cases, the amplitude decayed over time, thus the flow was stable with regards to two- and three-dimensional disturbances. The results suggest that it is primarily the reduction in reverse flow velocity, or even the full removal of the trailing-edge recirculation region when applying trailing-edge ejection, that is responsible for the suppression of the acoustic feedback loop and the growth of three-dimensional instabilities.