Mixed flow turbines offer additional design freedom compared with conventional radial turbines. This is useful in automotive turbocharger applications to reduce rotor inertia, which can be very beneficial for the transient response of a highly-boosted downsized passenger car powertrain. The aim of this work was to understand the aerodynamic and mechanical impacts of varying the chord length, particularly for turbocharger applications where off-design performance and transient response are pivotal. The baseline mixed flow rotor for this study had a blade cone angle of 30° and an inlet blade angle of 30°. Two variations were produced; one with the TE extended in the downstream direction across the entire blade span, the second with the chord extended at the hub corner only. When the blade was extended at both hub and shroud, the inertia and stress levels increased significantly and the blade eigenfrequencies reduced. There was significant improvement in peak efficiency, however this was mainly due to better exhaust diffuser performance and therefore would not be realised in most turbocharger installations. The blade that was extended at only the hub corner incurred very little additional inertia, and the centrifugal stresses and blade eigenfrequencies were improved. Consequently, it was possible to reduce the blade thickness at the TE in order to achieve a more aerodynamically optimised design. In this case, the mechanical performance was acceptable and there were efficiency improvements of up to 1.1% pts at off-design conditions, with no reduction in peak efficiency or maximum mass flow rate.

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