The Radial Turbine, for Low Specific Speeds and Low Velocity Factors

The main factors which influence radial-turbine performance are discussed. After a short description of the losses in radial turbines, the concept of the diffusion parameter is introduced. It is shown how the turbine geometry influences performance and, in particular, the reaction. The influence of a variation of the reaction on performance of radial turbines with high and low specific speed is discussed. Three-dimensional effects appear especially in the exducer region of the turbine. They may contribute to backflow and to a distorted distribution of the meridional velocity. The basic approach to improving turbines in the range of low-velocity factors and low specific speed is indicated. It appears that an attempt to reach impulse conditions in radial turbines is difficult, and only a rather small efficiency improvement can be expected. The approach to be taken in adapting a turbine design to low-specific-speed operation is pointed out, and the deterioration of the blade loading pattern due to such a reduction of the specific speed is shown.