Flow Development in Radial Turbine Rotors

Abstract

This paper surveys the development of the primary and secondary flows in the rotors of radial-inflow turbines. Information previously scattered throughout the literature has been brought together, and it has been possible to create a coherent picture and a good understanding of the complex flow processes which occur. The secondary flow is generated by cross-passage forces due to the turning of the blades, and Coriolis forces. Near the leading edge these give rise to a strong vortex adjacent to the pressure surface, moving low momentum fluid from hub to tip. This feature helps to explain why best efficiency occurs typically at 20°–30° negative incidence. Attempts to correlate the optimum incidence angle using traditional slip factor expressions can give quite misleading results, but a new approach based on the blade loading shows considerable promise. Nearer the exit there is motion of fluid from hub to tip near the suction surface and a vortex in the suction surface-shroud corner, and this is linked to the highly non-uniform flow at exit. The latter effect makes the prediction and correlation of rotor deviation information very difficult, despite the development of a rational exit averaging procedure. The present deviation data are sparse and not easy to correlate.