Mixed-flow turbines for automotive turbochargers: Steady and unsteady performance

Abstract

Turbochargers are finding increasing application to automotive diesel engines as cost effective means for improving their power output and efficiency, and reducing exhaust emissions; these requirements have led to the need for highly loaded turbocharger turbines. A mixed-flow turbine is capable of achieving its peak isen-tropic efficiency at reduced velocity ratios compared to a typical radial inflow turbine; it is therefore possible to improve the turbocharger/engine matching. These turbines differ from the commonly used radial turbines in that the flow approaches the rotor in the non-radial direction; in the extreme a mixed-flow turbine would become an axial machine. The steady and unsteady performances of a mixed-flow turbocharger turbine with a constant blade inlet angle have been investigated. The steady flow results indicated that the mixed-flow turbine obtains a peak efficiency (total-to-static) of 75 per cent at a velocity ratio of 0.61, compared with that of a typical radial-inflow turbine which peaks at a velocity ratio of 0.7. The performance and flow characteristics were found to deviate significantly from the equivalent steady state values commonly used in turbocharger turbine design.