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Gas Turbine Systems Theory 67
Impulse
In the impulse type turbine there is no net change in pressure
between rotor inlet and rotor exit. Therefore, the blades Relative
Discharge Velocity will be the same as its Relative Inlet Velocity. The
nozzle guide vanes are shaped to form passages, which increase the
velocity and reduce the pressure of the escaping gases.
Reaction
In the reaction turbine the nozzle guide vanes only alter the
direction of flow. The decrease in pressure and increase in velocity
of the gas is accomplished by the convergent shape of the passage
between the rotor blades.
The differences between the impulse and reaction turbine may
be depicted visually with the help of the velocity triangles (Figure
4-13). In the impulse turbine W 1
= W 2
. In the reaction turbine W 2
=
C 1
, and W 1
= C 2
.
There are two types of impulse turbines: a velocity compounded
impulse turbine and a pressure compounded impulse turbine. The velocity
compounded impulse turbine is often referred to as the Curtis
Turbine and the pressure compounded impulse turbine is referred to
as the Rateau Turbine.
Figure 4-14 summarizes, graphically, the pressure and velocity
through the various types of turbines. Note that in the impulse tur-
Figure 4-13. Comparison of impulse and reaction turbines.