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Combustion efficiency
Combustion stability Emissions
Materials 43
INTRODUCTION
1. The combustion chamber (fig. 4-1) has the
difficult task of burning large quantities of fuel,
supplied through the fuel spray nozzles (Part 10),
with extensive volumes of air, supplied by the
compressor (Part 3), and releasing the heat in such
a manner that the air is expanded and accelerated to
give a smooth stream of uniformly heated gas at all
conditions required by the turbine (Part 5). This task
must be accomplished with the minimum loss in
pressure and with the maximum heat release for the
limited space available.
2. The amount of fuel added to the air will depend
upon the temperature rise required. However, the
maximum temperature is limited to within the range
of 850 to 1700 deg. C. by the materials from which
the turbine blades and nozzles are made. The air has
already been heated to between 200 and 550 deg. C.
by the work done during compression, giving a
temperature rise requirement of 650 to 1150 deg. C.
from the combustion process. Since the gas
temperature required at the turbine varies with
engine thrust, and in the case of the turbo-propeller
engine upon the power required, the combustion
chamber must also be capable of maintaining stable
and efficient combustion over a wide range of engine
operating conditions.
3. Efficient combustion has become increasingly
important because of the rapid rise in commercial
aircraft traffic and the consequent increase in
atmospheric pollution, which is seen by the general
public as exhaust smoke.
35
COMBUSTION PROCESS
4. Air from the engine compressor enters the
combustion chamber at a velocity up to 500 feet per
second, but because at this velocity the air speed is
far too high for combustion, the first thing that the
chamber must do is to diffuse it, i.e. decelerate it and
raise its static pressure. Since the speed of burning
kerosine at normal mixture ratios is only a few feet
per second, any fuel lit even in the diffused air
stream, which now has a velocity of about 80 feet per
second, would be blown away. A region of low axial
velocity has therefore to be created in the chamber,
so that the flame will remain alight throughout the
range of engine operating conditions.
5. In normal operation, the overall air/fuel ratio of a
combustion chamber can vary between 45:1 and
130:1, However, kerosine will only burn efficiently at,
or close to, a ratio of 15:1, so the fuel must be burned
with only part of the air entering the chamber, in what
is called a primary combustion zone. This is achieved
by means of a flame tube (combustion liner) that has
various devices for metering the airflow distribution
along the chamber.
6. Approximately 20 per cent of the air mass flow is
taken in by the snout or entry section (fig. 4-2).
Immediately downstream of the snout are swirl vanes
and a perforated flare, through which air passes into
the primary combustion zone. The swirling air
induces a flow upstream of the centre of the flame
tube and promotes the desired recirculation. The air
not picked up by the snout flows into the annular
space between the flame tube and the air casing.
7. Through the wall of the flame tube body, adjacent
to the combustion zone, are a selected number of
secondary holes through which a further 20 per cent
of the main flow of air passes into the primary zone.
The air from the swirl vanes and that from the
secondary air holes interacts and creates a region of
low velocity recirculation. This takes the form of a
toroidal vortex, similar to a smoke ring, which has the
effect of stabilizing and anchoring the flame (fig, 4-3).
The recirculating gases hasten the burning of freshly
Combustion chambers
36
Fig. 4-1 An early combustion chamber.
injected fuel droplets by rapidly bringing them to
ignition temperature.
8. It is arranged that the conical fuel spray from the
nozzle intersects the recirculation vortex at its centre.
This action, together with the general turbulence in
the primary zone, greatly assists in breaking up the
fuel and mixing it with the incoming air.
9. The temperature of the gases released by
combustion is about 1,800 to 2,000 deg. C., which is
far too hot for entry to the nozzle guide vanes of the
turbine. The air not used for combustion, which
amounts to about 60 per cent of the total airflow, is
therefore introduced progressively into the flame
tube. Approximately a third of this is used to lower the
gas temperature in the dilution zone before it enters
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