Influence of tip shroud cavities on low-pressure turbine main flow at design and off-design conditions

Abstract

A lot of studies on turbomachinery main flow optimisation have been performed in order to reach actual efficiency level of modern gas turbines. To go further in the study of aerodynamic losses sources, a better understanding on technological effects is required. Tip shroud cavities in low pressure turbine is an example. Indeed, the by-pass flow causes additional pressure losses. In addition, interactions between main flow and cavity flows, as well as the re-entering flow, cause mixing losses and modifications of flow angle. This paper investigates the contribution of tip shroud cavities in a low pressure turbine stage on flow structures using (Unsteady) Reynolds Averaged Navier-Stokes simulations. The ability of a steady simulation to predict the overall performance and flow physic of this kind of flow is well documented in the literature but time-resolved simulations are needed to deepen the analysis. This is an objective of this paper. Firstly, computations are compared and validated with experimental data from low speed turbine test rig. Then, an analysis of flow structures is made in the upstream region of the rotor, close to the shroud. The boundary layer, growing on the shroud walls, as well as stator wakes are sucked into the inlet cavity of the by-pass. It leads to a modification of the location where stator wakes are chopped by the rotor and then change the loading on the blades. Secondly, the previous mechanism could be affected by interactions between inlet cavity and main flow. In the present configuration reintroduction of inlet cavity fluid inside rotor channel is relatively small due to weak azimuthal pressure gradient. That is why an off-design simulation was run in order to increase these interactions and evaluate their impact on rotor flow.