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 TM 5-685/NAVFAC MO-912
connected by tubes to allow flame propagation dur-
are engine emplacement features that may vary at
ing ignition and operation.
different installations. Air entering the duct passes
through a filter assembly. The filters remove debris
3-16. Gas turbine fuel system.
and other material that would otherwise be drawn
System components. The system provides the engine
into the engine compressor and other operating ar-
with the proper amount of fuel to sustain operation.
eas causing damage. Usually the lowest part of the
System components include filters, a fuel manifold,
plenum is equipped with a drain for removal of
fuel tubes, and nozzles. Off-engine components in-
moisture.
clude the fuel control equipment and a supply sys-
b. Sequence of euents. Combustion causes an in-
tem.
crease in gas temperature proportionate to the
a. Fuel. Fuel (liquid and/or natural gas) enters
amount of fuel being injected, a moderate increase
the tubular fuel manifold ring via the supply sys-
in velocity, and a negligible decrease in pressure.
tem. The fuel tubes direct the fuel from the mani-
Approximately 25 percent of the compressor's total
fold to the fuel nozzles which are mounted in the
air flow is used for combustion at an air/fuel ratio of
fuel swirlers (see fig 3-22 and 3-23). Compressor
discharge air flows radially inward through the
air output is fed to the combustor and to cool com-
primary swirler in the combustion liner, which
bustor liners for cooling combustion gases before
rotates the air circumferentially and mixes it with
they enter the turbine.
the fuel. Air entering radially inward through
(1) The sequence of events during turbine en-
the secondary swirler is caused to rotate in the
gine start-up and operation is as follows:
opposite direction. As the two counter-rotating mix-
(a) Air is drawn into the compressor by ro-
tures join, the fuel mixes completely with the air.
tating the engine. Rotation is accomplished by the
This process promotes complete mixing of the fuel
engine starter. The engine is rotated to the speed at
and air and, therefore, more complete burning of
which it becomes self-sustaining.
the mixture resulting in less smoke emission and
(b) As the engine shaft is rotated and accel-
more uniform temperature distribution within the
erated by the starter, fuel is fed to the combustor.
combustor.
When the air pressure is high enough, the air/fuel
b. Ignition. Ignition is accomplished by one or
mixture is ignited by an electrical spark.
two igniter plugs. At ignition, the igniters are acti-
(c) The electrical spark is deactivated after
vated and fuel is injected into the swirlers. After
ignition occurs. Since the air/fuel mixture is con-
ignition, the igniters are deactivated (refer to para
tinuously fed to the combustor by the turbine and
3-15b( 1)).
compressor, and since there is a flame in the com-
bustor after ignition, engine operation is self-
3-17. Gas turbine cooling system.
sustaining.
a. Approximately 25 percent of the air entering a
(d) Rotation of th e engine by the starter is
combustor is mixed with fuel and burned. The re-
necessary after combustion takes place to help ac-
maining air is mixed with the products of combus-
celerate the engine to rated speed. Once the engine
tion to reduce the temperature of gases entering the
speed has increased to approximately 60 percent of
turbine to a safe operating level. Cooling is accom-
rated speed, the starter is deactivated.
plished by engine airflow.
(e) Gas turbine engines have dual-fuel capa-
b. Three forms of air cooling of the vanes and
bility since they may use either liquid or gaseous
blades are used, either separately or in combina-
fuel. Generating units with these engines are reli-
tions. The types of cooling are convection, impinge-
able and virtually free of vibration.
ment, and film (see fig 3-24).
(2) Types of combustors. Combustors for gas
(1) Convection. For convection cooling, air
turbine engines for generators are either cannular
flows inside the vanes or blades through serpentine
or annular-type with newer engines usually having
paths and exits through the blade tip or holes in the
an annular combustor. The annular-type engine is
trailing edge. This form of cooling is used in the
described in this manual. See figure 3-21 for de-
area of lower gas temperature (see fig 3-25).
tails. The annular combustor consists of a continu-
(2) Impingement. Impingement cooling is a
ous circular inner and outer casing or shell; the
form of convection cooling, accomplished by direct-
space between the casings is open. The cannular
ing cooling air against the inside surface of the
combustor consists of inner and outer combustion
airfoil through small internal high velocity air jets.
casings mounted coaxially around the engine
Cooling is concentrated at critical sections, such as
compressor/rotor shaft. A cluster of burner cans are
leading edges of vanes and blades (see fig 3-26).
located between the two casings. The cans are inter-
3-29
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