Design and Analysis of a Radial Turbine with Back Swept Blading

Abstract

This report documents the detailed design and numerical analysis of an 86 mm radial turbine with a 25° back swept inlet blade angle. The original blade geometry, which was from an 86 mm radial turbine for a turbocharger, contained purely radial blades, as conventional design dictates. The original blade design was subsequently modified to include back swept blading at inlet to increase performance at lower than optimum velocity ratios. Results from the numerical analysis are presented and compared. It is shown that the 25° back swept blade offers significant increases in efficiency while operating at lower than optimum velocity ratios (U/C). Improvements in efficiency at off-design conditions would notably improve turbocharger performance where the turbine typically experiences lower than optimum velocity ratios while accelerating during engine transients. Automotive turbochargers, particularly those of city bound vehicles, would tend to spend a significant portion of their time operating at lower than optimum velocity ratios. Thus, an increase in turbine efficiency within this operating range would result in more torque available for turbocharger acceleration, increasing boost air pressure during engine transients, benefiting engine response and emissions. A CFD study of the 86 mm baseline radial rotor and the 86 mm 25° back swept rotor was conducted. Numerical predictions show off-design performance gains of 2% can be achieved, while maintaining design point efficiency. A finite element analysis has been carried out to assess the changes in blade stress levels as a result of introducing a non-radial angle at turbine rotor inlet. A modal analysis was also performed in order to identify the natural frequencies of the turbine geometry, thus calculating the critical speeds corresponding to the induction of excitational frequencies from the stator vanes. While the new blade design experiences increased stress levels within some regions, the numerical study has shown that it is feasible from both an aerodynamic and structural point of view to increase the performance of a radial turbine through the addition of back swept blading. Several rotors have been manufactured at Queen’s University of Belfast and are currently being tested to obtain experimental performance measurements for radial turbines with a range of inlet blade angles in order to verify the numerical predictions detailed herein.