Unsteady numerical investigation of the effect of wakes with eddy shedding in different axial turbine aerofoils

Abstract

In the past it was found experimentally and numerically that eddy shedding on trailing edges of turbine aerofoils has significant effect on the aerodynamic performance. Aerofoils of gas turbines usually have relatively thick trailing edges for reasons of mechanical integrity, and hence strong unsteady wakes are created. The current investigation considers trailing edges without cooling injection; hence the area of application is limited to the low- and medium-pressure stages of a gas turbine. In this work a numerical investigation is performed with the aim to weigh the loss production inside these unsteady wakes against the available kinetic energy of the unsteady fluid to formulate a so-called unsteady recovery factor. Furthermore, several design exercises are presented on a gas-turbine rotor profile, which is interacting with the unsteady wake of an upstream vane with eddy shedding. This shows that the stage performance is quite significantly influenced comparing frontloaded, midloaded, and aftloaded profile designs in contradiction to standard steady design approaches. It was found that a frontloaded rotor design under consideration of interaction with an unsteady wake can increase the stage performance of a two-dimensional midsection considerably, while under steady assumptions it has about the same or worse performance. This result shows the importance of unsteady methods in turbine design and also potential new ways for improved profile design.