4. Directing the hot gases to the turbine
compressors with their respective turbines and
section
interconnecting shafts that form two independent
rotor systems.
The location of the combustion section is
The axial-flow type of engine has definite
directly between the compressor and the turbine
advantages. The advent of the split-spool axial
sections. The combustion chambers are arranged
compressor made these advantages even more
positive by offering greater starting flexibility and
coaxially with the compressor and turbines. The
improved high-altitude performance.
chambers must be in a through-flow position to
function efficiently.
The advantages of the axial-flow compressor
About one-fourth of the air entering the
are as follows:
combustion chamber area mixes with the fuel for
1. High peak efficiencies
combustion. This is primary air. The remaining
air (secondary air) serves as flame control.
2. Low frontal area for given airflow
Keeping the temperature of the heated gases down
3. Straight-through flow, allowing high ram
to a level at which the liners, turbine nozzles, or
efficiency
4. Increased pressure rise by increasing the
blades will not burn. These basic requirements
number of stages, with negligible losses
apply to all combustion sections.
Another general requirement of combustion
The disadvantages of the axial-flow com-
chambers is air pollution emission reduction.
Pollution emissions are particles of matter, such
pressor are as follows:
as smoke, carbon monoxide, partially burned
hydrocarbons, and nitric oxides. In general,
1. Good efficiencies are possible over narrow
e x h a u s t smoke becomes a problem when
rotational-speed range only
combustors operate at pressure greater than 10
2. Difficulty of manufacture and high cost
atmospheres and when the fuel-air ratio in the
3. High starting-power requirements
primary zone of the combustor is rich. For
Relative to compressors, the fan of a turbofan
example, in the idle range of operation, both
smoke particles and partially burned hydro-
engine should be mentioned now. The fan
accelerates a large mass of air rearward. It requires
carbons emit. During the combustion process,
relatively low drive power, and has a pressure ratio
emission levels of nitric oxide increase with
temperature increases to about 2,600F. At this
of 2 to 1 or less. It can be thought of as a
precompressor, as air enters the compressor inlet
temperature, these emission levels begin to taper
off. There is research being conducted to correct
at a pressure about 1.5-2.0 to 1 atmosphere.
problems, but many new factors may influence
the solution of the pollution problem. Present
approaches to reducing exhaust emissions include:
COMBUSTION SECTION
1. Cut visible smoke by improving primary
The combustion section provides the means
zone fuel-air mixing, but not sacrifice altitude
for and houses the combustion process. Its
relight capability.
function is to raise the temperature of the air
2. Reduce carbon monoxide and unburned
passing through the engine. This process releases
hydrocarbon emissions. Increase fuel atomization
energy contained in the air and fuel. The major
and optimizing the fuel-air ratio in the primary
part of this energy drives the compressor. The
remaining energy creates the reaction (or
combustion zone.
propulsion) and passes out the rear of the engine
3. Reduce nitric oxide emissions by minimiz-
ing the amount of time the fuel-air mixture spends
in the form of a high-velocity jet.
in the combustor (by using short cans) or lowering
The primary function of burning the fuel-air
t h e temperature in the primary zone of
mixture includes:
combustion.
Remember that these approaches are not
1. Providing the means for proper mixing of
solutions. Only ideas to give you some insight to
the fuel and air to assure good combustion
the problems and possible solutions.
2. Burning this mixture efficiently
All combustion chambers contain the same
3. Cooling the hot combustion products to a
basic elements: a casing, a perforated inner liner,
temperature that the turbine blades can withstand
a fuel injection system, some means for initial
under operating conditions
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