Investigation into the impact of span-wise flow distribution on the performance of a mixed flow turbine

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. A turbocharger increases charge air density using a turbine to extract waste energy from the exhaust gas to drive a compressor.
Most turbocharger applications employ a radial inflow turbine. However, to ensure radial stacking of the blade fibers and avoid excessive blade stresses, the inlet blade angle must remain at zero degrees. Alternately, mixed flow turbines can offer non-zero blade angles while maintaining radial stacking of the blade fibers. The additional freedom to manipulate the blade leading edge and varying tip speed allow for varying leading edge incidence in the span-wise direction. Furthermore, the flow development in the volute does not necessarily lead to uniform inlet conditions.
The current paper investigates the performance of a mixed flow rotor passage under a range of span-wise flow distributions including that produced by a turbine volute. Initial unsteady pulsating simulations were conducted and the volute exit flows extracted. These distributions were then applied as boundary conditions to a single passage model. All simulations were carried out at a constant MFP and average leading edge relative flow angle.
It was observed that the different inlet flow distributions resulted in marked difference in passage flow characteristics. A 2.17% variation was observed between cases in the radial passage. A tilted passage was also included providing an increased axial flow component at the inlet. This passage was found to result in greater swallowing capacity when compared to that of the radial passage.