In modern civil aeroengines the fan is driven by the low pressure turbine. In order to match the low rotational speed of the fan, the low pressure turbine consists of large diameter components and several stages, up to six or seven. An individual stage of a low pressure turbine consists of a row of stationary blades (stators) followed by a row of rotating blades (rotors). This arrangement necessitates radial clearance gaps above the tips of the rotor blades and below the hub end of the stator blades. Therefore, a fraction of the gas does not pass through the blade rows, but instead leaks through the tip and hub cavities into the downstream blade row. The leakage flow over shrouded turbine rotor blades is driven by the pressure drop across the blade row. The leakage flow experiences very little change in tangential velocity as it moves through the rotor shroud cavity, hence, when the leakage flow re-enters the mainstream downstream of the rotor trailing edge, it has higher tangential velocity than that of the mainstream flow.

The aim of this research was to improve the understanding of the effects of the rotor shroud leakage flow on the aerodynamics of the downstream stator. Based on the improved understanding, a new design for the stator geometry was produced which has a performance that is more robust to increases in the rotor shroud leakage mass flow rate, so that the efficiency of the turbine is better maintained over the in-service lifetime of the aeroengine.

The experimental measurements for the present study have been undertaken on the Peregrine Turbine Facility in the Whittle Laboratory. Measured contours of streamwise vorticity at the exit of the downstream stator identified two positive streamwise vortex structures. A computational study revealed that one vortex was caused by a large separation at the leading edge of the stator. It is demonstrated that this vortex can be eliminated by the appropriate redesign of the stator. The second vortex is caused by the roll up of the inlet streamwise vorticity sheet, which results from the radial gradient of tangential velocity between the rotor shroud leakage flow and the mainstream flow at the stator inlet. This vortex is a fundamental consequence of the rotor shroud leakage flow and cannot be eliminated within the stator passage.