Design of a Highly Loaded Mixed Flow Turbine

Abstract

The high boost pressures and fuel–air ratios required for the next generation of turbocharged diesel engines imply an increased turbine expansion ratio without an increase in the speed of rotation. This leads to a requirement for high peak efficiency at lower values of blade speed/isentropic expansion velocity U/C than are normal today. The objective of this project was to achieve this with a mixed flow rotor with a positive inlet blade angle. Two rotors were manufactured and tested: one a ‘constant blade angle’ design and the other a ‘constant incidence’ design. In practice both achieved a peak efficiency at a low U/C value, but the constant blade angle design, at 0.84 total to static efficiency, was significantly more efficient than the constant incidence design at 0.77. These efficiencies are highly competitive, compared to current radial turbine design. It is suggested that the reasons for this difference are a lack of understanding of the incidence and its effects on a mixed flow rotor, and a region of diffusion in the shroud-trailing edge corner of the suction surface, apparently worse for the constant incidence design.