OFF-DESIGN PERFORMANCE OF A HIGHLY LOADED LP TURBINE CASCADE UNDER STEADY AND UNSTEADY INCOMING FLOW CONDITIONS

The off-design performance of a highly loaded LP turbine cascade has been experimentally investigated, at the Aerodynamics and Turbomachinery Laboratory of Genova University, under steady and unsteady incoming flow conditions. Tests have been performed for different Reynolds numbers (Re = 70000 and Re= 300000), in order to cover the typical LP turbine working range. The incidence angle has been varied between i = -9 degrees and +9 degrees, in order to test off-design conditions characterizing the engine. For the unsteady case upstream wake periodic perturbations have been generated by means of a tangential wheel of radial rods. The cascade and the moving bars system have been located over a common bearing in order to make them rigidly rotating. This solution allows a proper comparison of the cascade robustness at the incidence angle variation under steady and unsteady incoming flows, since all the other operating parameters have been kept the same. In order to survey the variation of the unsteady boundary conditions characterizing the off-design operation of the downstream cascade, time-mean and time-resolved wake structures have been analyzed in detail. For what concerns the cascade performance, profile aerodynamic loadings and total pressure loss coefficients at the cascade exit have been surveyed for the different incidence angles under both steady and unsteady inflows. Different total pressure loss sensitivity at the incidence angle variation has been observed for the steady and the unsteady inflow conditions. Hot-wire anemometer has been employed to obtain the time-mean pressure and suction side boundary layer velocity profiles at the blade trailing edge for the different conditions. The integral parameters at the cascade exit plane help to justify the different loss trend vs incidence angle found for the steady and the unsteady cases, explaining the different sensibility of the blade profile when this operates under realistic unsteady inflow condition.