Introduction

For more than three decades now, radial-inflow turbines have been established as a viable alternative to its axial-flow counterpart, specifically in power-system applications. Despite its relatively-primitive means of fabrication, radial turbines are capable of extracting a large per-stage shaft work in situations with low mass-flow rates. This turbine category also offers little sensitivity to tip clearances, in contrast to axial-flow turbines. Nevertheless, the turbine large envelope, bulkiness and heavy weight (Fig. 10.1), virtually prohibits its use in propulsion devices.

Figure 10.1: Isometric and meridional views of a typical radial-inflow turbine stage.

Components of Energy Transfer

Figure 10.2 shows the velocity diagrams at the rotor inlet and exit stations within a typical radial-turbine stage. As derived in Chapter 4, the combined Euler/energy-transfer equation can be expressed, for the specific shaft work ( w _s), as

where the subscripts 1 and 2 m refer to the rotor inlet and mean-radius exit stations, respectively. The velocity components in expression (10.1) are all shown in Figure 10.2.

Figure 10.2: Component identification and velocity triangles.

For all three terms in expression 10.1 to contribute positively to the shaft-work production, the following velocity-component relationships must be satisfied:

This implies a streamwise decline in radius across the rotor. It also underscores the use of the phrase inflow in referring to this turbine category.

This implies an accelerating (nozzle-like) blade-to-blade passage, as shown in Fig. 10.2.

This condition calls for a large stator-exit velocity (Fig. 10.2) or, equivalently, a large stator-exit swirl angle ( ?