Gas Turbine Handbook : Principles and Practices

68 Gas Turbine Handbook: Principles and Practices
Figure 4-14. Impulse and reaction turbine pressure & velocity profiles.
bine no pressure drop or expansion occurs across the moving rows.
While in the reaction turbine the fixed nozzles perform the same
function as in the impulse turbine.
Turbines may be either single- or multiple-stage. When the turbine
has more than one stage, stationary vanes are located upstream
of each rotor wheel. Therefore, each set of stationary vanes forms a
nozzle vane assembly for the turbine wheel that follows. The rotor
wheels may or may not operate independently of each other, depending
upon the type of engine and the power requirements of the turbine.
Some gas turbines, based on their design objectives, incorporate
two compressors and two turbines—with a shaft-in-a-shaft design.
In this case the first (high pressure) turbine drives the last (high
pressure) compressor, and the last (low pressure) turbine drives the
first (low pressure) compressor. This is referred to as a split-shaft
machine.
The turbine wheel, or turbine blade and disc assembly, consists
of the turbine blades and the turbine disc. The blades are attached