Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

or
(4.22b)
after using eqn. (4.21).
If b3 = b 2, the reaction is zero; if b 3 = a 2 the reaction is 50%. These two special cases
are discussed below in more detail.
Zero reaction stage
Axial-flow Turbines: Two-dimensional Theory 103
From the definition of reaction, when R = 0, eqn. (4.18) indicates that h2 = h 3 and
eqn. (4.22a) that b2 = b3. The Mollier diagram and velocity triangles corresponding to
these conditions are sketched in Figure 4.5. Now as h02rel = h 03rel and h 2 = h 3 for R = 0
it must follow, therefore, that w2 = w3. It will be observed from Figure 4.5 that, because
of irreversibility, there is a pressure drop through the rotor row. The zero reaction stage
is not the same thing as an impulse stage; in the latter case there is, by definition, no
pressure drop through the rotor. The Mollier diagram for an impulse stage is shown in
Figure 4.6 where it is seen that the enthalpy increases through the rotor. The implica-
FIG. 4.5. Velocity diagram and Mollier diagram for a zero reaction turbine stage.
FIG. 4.6. Mollier diagram for an impulse turbine stage.