Page 284 - Fluid Mechanics and Thermodynamics of Turbomachinery

Radial Flow Gas Turbines 265

FIG. 8.12. Specific speed function for a 90 deg inward flow radial turbine (adapted from
Wood 1963).

FIG. 8.13. Specific speed-efficiency characteristics for various turbines (adapted from
Wood 1963).

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limits for the exhaust energy factor in gas turbine practice are 0.04 <.c 3 /c 0 / <
0.30, the lower value being apparently a flow stability limit.
The numerical value of specific speed provides a general index of flow capacity
relative to work output. Low values of  s are associated with relatively small
flow passage area and high values with relatively large flow passage areas. Specific
speed has also been widely used as a general indication of achievable efficiency.
Figure 8.13 presents a broad correlation of maximum efficiencies for hydraulic and
compressible fluid turbines as functions of specific speed. These efficiencies apply