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pipe and thus maintain the wall temperature at a safe
value.
169
4. The area of the afterburning jet pipe is larger than
a normal jet pipe would be for the same engine, to
obtain a reduced velocity gas stream. To provide for
operation under all conditions, an afterburning jet
pipe is fitted with either a two-position or a variablearea
propelling nozzle (fig. 16-2). The nozzle is
closed during non-afterburning operation, but when
afterburning is selected the gas temperature
increases and the nozzle opens to give an exit area
suitable for the resultant increase in the volume of
the gas stream. This prevents any increase in
pressure occurring in the jet pipe which would affect
the functioning of the engine and enables afterburning
to be used over a wide range of engine speeds.
5. The thrust of an afterburning engine, without
afterburning in operation, is slightly less than that of
a similar engine not fitted with afterburning
equipment; this is due to the added restrictions in the
jet pipe. The overall weight of the power plant is also
increased because of the heavier jet pipe and afterburning
equipment.
6. Afterburning is achieved on low by-pass engines
by mixing the by-pass and turbine streams before the
afterburner fuel injection and stabilizer system is
reached so that the combustion takes place in the
mixed exhaust stream. An alternative method is to
inject the fuel and stabilize the flame in the individual
by-pass and turbine streams, burning the available
gases up to a common exit temperature at the final
nozzle. In this method, the fuel injection is scheduled
separately to the individual streams and it is normal
to provide some form of interconnection between the
flame stabilizers in the hot and cold streams to assist
the combustion processes in the cold by-pass air.
OPERATION OF AFTERBURNING
7. The gas stream from the engine turbine enters
the jet pipe at a velocity of 750 to 1,200 feet per
second, but as this velocity is far too high for a stable
flame to be maintained, the flow is diffused before it
enters the afterburner combustion zone, i.e. the flow
velocity is reduced and the pressure is increased.
However, as 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 would be blown
away. A form of flame stabilizer (vapour gutter) is,
therefore, located downstream of the fuel burners to
provide a region in which turbulent eddies are formed
to assist combustion and where the local gas velocity
is further reduced to a figure at which flame stabilization
occurs whilst combustion is in operation.
170
Fig. 16-1 Principle of afterburning
171
Fig. 16-2 Examples of afterburning jet pipes and propelling nozzles.
8. An atomized fuel spray is fed into the jet pipe
through a number of burners, which are so arranged
as to distribute the fuel evenly over the flame area.
Combustion is then initiated by a catalytic igniter,
which creates a flame as a result of the chemical
reaction of the fuel/air mixture being sprayed on to a
platinum-based element, by an igniter plug adjacent
to the burner, or by a hot streak of flame that
originates in the engine combustion chamber (fig.
16-3): this latter method is known as ’hot-shot’
ignition. Once combustion is initiated, the gas
temperature increases and the expanding gases
accelerate through the enlarged area propelling
nozzle to provide the additional thrust.
9. In view of the high temperature of the gases
entering the jet pipe from the turbine, it might be
assumed that the mixture would ignite spontaneously.
This is not so, for although cool flames form at
172
Fig. 16-3 Methods of afterburning ignition.
temperatures up to 700 deg. C., combustion will not
take place below 800 deg. C. If however, the
conditions were such that spontaneous ignition could
be effected at sea level, it is unlikely that it could be
effected at altitude where the atmospheric pressure
is low. The spark or flame that initiates combustion
must be of such intensity that a light-up can be
obtained at considerable altitudes.
10. For smooth functioning of the system, a stable
flame that will burn steadily over a wide range of
mixture strengths and gas flows is required. The
mixture must also be easy to ignite under all
conditions of flight and combustion must be
maintained with the minimum loss of pressure.
CONSTRUCTION
Burners
11. The burner system consists of several circular
concentric fuel manifolds supported by struts inside
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