increase in volume through three stages. (If this
compressor and partially because area C is a lower
did not happen rapidly, back pressure from area
pressure area. The end of area C is the turbine
nozzle section. Here you will find a decrease in
D would cause area C to become choked.) The
gases in the combustor would back up into the
pressure and an increase in velocity. The high-
compressor. There they would disrupt airflow and
velocity, high-temperature, low-pressure (LP)
gases are directed through the inlet nozzle to the
cause a condition known as surge, or compressor
first stage of the turbine rotor (area D). The high-
stall. This condition can destroy an engine in a
matter of seconds. Surge will be explained later
velocity, high-temperature gases cause the rotor
in our discussion of axial-flow compressors.
to rotate by transferring velocity energy and
The gases from the last turbine stage enter
thermal energy to the turbine blades. Area D is
a divergent area. Between each rotating turbine
the exhaust duct where they are sent to the
stage is a static stage or nozzle. The nozzles
atmosphere. The leading portion of the exhaust
duct is part of a divergent area. Further divergence
perform the same function as the stators in the
reduces the pressure and increases the volume of
compressor.
the warm gases and aids in lowering the velocity.
A turbine nozzle is a stator ring with a series
The exhaust gases enter the atmosphere at or
of vanes. The vanes direct the combustion gases
uniformly and at the proper angle to the turbine
slightly above atmospheric pressure. This depends
on the length and size of the exhaust duct.
blades. The passages between the vanes are
Refer to figure 1-6 and compare the graph and
designed as diverging nozzles. Each succeeding
the actual operation of the cycle. Air enters the
stage imparts velocity to the gases as they
intake at constant pressure (point A). It is
pass through the nozzle. Each nozzle converts
compressed as it passes through the compressor
heat and pressure energy into velocity energy by
(line A-B in fig. 1-6 and area A in fig. 1-8).
controlling the expansion of the gas.
Between the end of area B and the beginning of
Each stage of the turbine is larger than the
area C in figure 1-8, combustion occurs and
preceding one. The drop in pressure is quite rapid;
volume increases (fig. 1-6, line B-C). As the gases
consequently, each stage must be larger to use the
energy of a lower pressure, lower temperature,
pass through area D (fig. 1-8), the gases expand
with a drop in pressure and an increase in volume
and larger volume of gas.
(fig. 1-6, line C-D). The gases are discharged to
Atmospheric air is raised in pressure and
the atmosphere through the exhaust duct at
velocity and lowered in volume in area A by the
constant pressure (fig. 1-6, line D-A and fig. 1-8,
compressor. Each stage can only compress air
exhaust). At this point, you should have the basic
about 1.2 times. In the turbine rotor (area D), the
knowledge of how a simple gas turbine works.
gases give up thermal and pressure energy and
Figure 1-9.--GTE pressure-temperature-volume relationship.
1-8
