Evaluating the use of leaned stator vanes to produce a non-uniform flow distribution across the inlet span of a mixed flow turbine rotor

An off-design operating condition of significant importance for automotive turbocharging applications is at low turbine velocity ratios, since it is relevant to engine transient response and efficient energy extraction from pressure pulses in the unsteady exhaust flow. In order to improve off-design performance and reduce rotor inertia, mixed flow turbine (MFT) rotors are increasingly being used to replace conventional radial flow turbines. However, the reduction in leading edge radius from shroud to hub in a MFT, coupled with the increasing tangential velocity of the flow due to conservation of angular momentum, results in increasingly positive incidence towards the hub side of the leading edge. This region produces separation from the suction surface and generates significant loss within the rotor passage. This study aimed to determine if the losses in a MFT could be reduced by the use of leaned stator vanes, which deliberately created a significant spanwise variation of flow angle between hub and shroud at rotor inlet. Turbine performance with a series of leaned vanes was compared against that of a straight vane using a validated CFD model. By reducing incidence at the hub, it was found that increasing vane lean improved turbine performance at all operating points considered. An increase of 3.2%pts in stage total-to-static efficiency was achieved at a key off-design operating point, which compared favourably with an experimentally measured 2.6%pt improvement at the same operating condition. Results from unsteady CFD models further confirmed this level of improvement.