A Numerical and Experimental Investigation of the Impact of Mixed Flow Turbine Inlet Cone Angle

Mixed flow turbines offer potential benefits for turbocharged engines when considering off-design performance and engine transient behaviour. Although the performance and use of mixed flow turbines is described in the literature, little is published on the combined impact of the cone angle and the inlet blade angle, which are the defining features of such turbines.
Numerical simulations were completed using a CFD model that was validated against experimental measurements for a baseline geometry. The mechanical impact of the design changes was also analysed. Based on the results of the numerical study, two rotors of different blade angle and cone angle were selected and manufactured. These rotors were tested using the QUB low temperature turbine test rig, which allowed for accurate and wide range mapping of the turbine performance to low values of velocity ratio. The performance results from these additional rotors were used to further validate the numerical findings.
The numerical model was used to understand the underlying physical reasons for the measured performance differences through detailed consideration of the flow field at rotor inlet, and to document how the loss mechanisms and secondary flow structures developed with varying rotor inlet geometry. It was observed that large inlet blade cone angles resulted in strong separation and flow blockage near the hub at off-design conditions, which greatly reduced efficiency. However, the significant rotor inertia benefits achieved with the large blade cone angles were shown to compensate for the efficiency penalties and could be expected to deliver improved transient performance in downsized automotive engine applications.