Effects of Blade Count on Three-Bladerow Interaction in a Transonic Turbine

This paper presents some numerical parametric studies of the multi-row interaction mechanisms for a one and half stage (NGV-rotor-stator) transonic turbine. Firstly both steady and unsteady flows under the nominal operating condition for this turbine have been validated against the experimental data available. The sub-harmonic interaction induced by the two fundamental passing frequencies from the upstream and downstream vanes has been identified in the rotor row. But more significant is an aperiodic unsteady flow pattern characterized by variable amplitudes and inter-blade phase angles in the downstream stator row. Although the time-averaged blade forces only vary by about 5%, the maximum unsteady force can be changed by factor of three among stator blades. The parametric studies have revealed a strong dependence of the aperiodic flow behavior on blade count ratio between the NGV and the stator. The spatial mode of the unsteadiness amplitude variation is shown to correspond exactly to the spatial wavelength due to the NGV-stator interference. The longer the spatial NGV-stator interference wavelength, the larger the aperiodic unsteady loading variation. Given that the spatial mode amplifies the unsteady loading aperiodically on the stator, the present results suggest that the choice of stator-stator relative blade counts may be used to limit the maximum unsteady force on the downstream stator.