Effects of Axial Gap on the Vane-Rotor Interaction in a Low Aspect Ratio Turbine Stage

An extensive experimental analysis on the subject of unsteady flows in a low aspect-ratio turbine stage was performed at the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano. Three different vanerotor axial gaps were considered ranging from 16 to 50% of the stator axial chord. Steady flow measurements at different axial planes in the stator-rotor gap were carried out to provide a complete description of the threedimensional flowfield entering the rotor for different axial gaps. The blade row interaction and its dependence on the axial gap were evaluated by means of phase-resolved aerodynamic measurements downstream of the rotor. Unsteady numerical simulations were also performed to support the interpretation of the experiments. Results show a strong dependence of the time-averaged and phase-resolved flowfield on the stator-rotor spacing. The blade row interaction is mainly driven by the vortex-blade interaction in the hub region, and by the rotor incidence unsteadiness produced by the stator flow structures. Different interaction phenomena take place for the different axial gaps, depending on the magnitude of the stator vortices and on the superposition between the stator wake and potential field. On the contrary, the tip region is dominated by the rotor aerodynamics, which are found to be almost steady in the relative frame. Spanwise efficiency profiles evidence the different effects related to the single blade row as well as to the wake and vortex interactions on the performance. The maximum overall efficiency and the minimum unsteadiness are achieved for a vane-rotor axial gap equal to one third of the stator axial chord, or five sixths of the stator throat.