fluid_mechanics

690 Chapter 12 ■ Turbomachines
usually of the axial-flow type. Often they are multistage turbomachines, although single-stage compressible
turbines are also produced. They may be either an impulse type or a reaction type. With
compressible flow turbines, the ratio of static enthalpy or temperature drop across the rotor to this
drop across the stage, rather than the ratio of static pressure differences, is used to determine reaction.
Strict impulse 1zero pressure drop2 turbines have slightly negative reaction; the static enthalpy
or temperature actually increases across the rotor. Zero-reaction turbines involve no change
of static enthalpy or temperature across the rotor but do involve a slight pressure drop.
A two-stage, axial-flow impulse turbine is shown in Fig. 12.38a. The gas accelerates through
the supply nozzles, has some of its energy removed by the first-stage rotor blades, accelerates again
through the second-stage nozzle row, and has additional energy removed by the second-stage rotor
blades. As shown in Fig. 12.38b, the static pressure remains constant across the rotor rows.
Across the second-stage nozzle row, the static pressure decreases, absolute velocity increases, and
the stagnation enthalpy 1temperature2 is constant. Flow across the second rotor is similar to flow
across the first rotor. Since the working fluid is a gas, the significant decrease in static pressure
across the turbine results in a significant decrease in density—the flow is compressible. Hence,
more detailed analysis of this flow must incorporate various compressible flow concepts developed
in Chapter 11. Interesting phenomena such as shock waves and choking due to sonic conditions at
the “throat” of the flow passage between blades can occur because of compressibility effects. The
interested reader is encouraged to consult the various references available 1e.g., Refs. 2, 3, 202 for
fascinating applications of compressible flow principles in turbines.
The rotor and nozzle blades in a three-stage, axial-flow reaction turbine are shown in Fig.
12.39a. The axial variations of pressure and velocity are shown in Fig. 12.39c. Both the stationary
and rotor blade 1passages2 act as flow-accelerating nozzles. That is, the static pressure and enthalpy
1temperature2 decrease in the direction of flow for both the fixed and the rotating blade rows.
This distinguishes the reaction turbine from the impulse turbine 1see Fig. 12.38b2. Energy is removed
from the fluid by the rotors only 1the stagnation enthalpy or temperature is constant across
the adiabatic flow stators2.
Rotor blades
(Rotor)
Moving
blades
Nozzles
Moving
blades
(Rotor)
Fixed
blades
(Stator)
Nozzles
(Stator)
(a)
(a)
Stagnation
enthalpy or
temperature
Static
enthalpy or
temperature
(b)
Static
pressure
Static
pressure
Absolute
velocity
Absolute
velocity
(b)
F I G U R E 12.38 Enthalpy, velocity,
and pressure distribution in two-stage impulse
turbine.
(c)
F I G U R E 12.39 Enthalpy,
pressure, and velocity distribution in a
three-stage reaction turbine.