Improved Performance of a Radial Turbine Through the Implementation of Back Swept Blading

Abstract

This report details the numerical investigation of the performance characteristics and internal flow fields of an 86 mm radial turbine for a turbocharger application. A new blade was subsequently designed for the 86 mm rotor which departed from the conventional radial inlet blade angle to incorporate a 25° inlet blade angle. A comparative analysis between the two geometries is presented. Results show that the 25° back swept blade offers significant increases in efficiency while operating at lower than optimum velocity ratios (U/C). This enhanced efficiency at off-design conditions would significantly improve turbocharger performance where the turbine typically experiences lower than optimum velocity ratios while accelerating during engine transients. A commercial CFD code was used to construct single passage steady state numerical models. The numerical predictions show off-design performance gains of 2% can be achieved, while maintaining design point efficiency. Primary and secondary flow patterns are examined at various planes within the turbine blade passage and reasons for the increase in performance are discussed. A finite element analysis has been conducted to assess the stress implications of introducing a non-radial angle at turbine rotor inlet. A modal analysis was also carried out in order to identify the natural frequencies of the turbine geometry, thus calculating the critical speeds corresponding to the induction of the excitational frequencies from the stator vanes. Although the new blade design has resulted in stress increases in some regions, the numerical study has shown that it is feasible from both an aerodynamic and structural point of view to increase the performance characteristic of a radial turbine through the implementation of back swept blading.