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the turbine and the remainder is used for cooling the
walls of the flame tube. This is achieved by a film of
cooling air flowing along the inside surface of the
flame tube wall, insulating it from the hot combustion
gases (fig. 4-4). A recent development allows cooling
air to enter a network of passages within the flame
tube wall before exiting to form an insulating film of
air, this can reduce the required wall cooling airflow
by up to 50 per cent. Combustion should be
completed before the dilution air enters the flame
tube, otherwise the incoming air will cool the flame
and incomplete combustion will result.
10. An electric spark from an igniter plug (Part 11)
initiates combustion and the flame is then selfsustained.
Combustion chambers
37
Fig. 4-2 Apportioning the airflow.
Fig. 4-3 Flame stabilizing and general airflow pattern.
11. The design of a combustion chamber and the
method of adding the fuel may vary considerably, but
the airflow distribution used to effect and maintain
combustion is always very similar to that described.
FUEL SUPPLY
12. Fuel is supplied to the airstream by one of two
distinct methods. The most common is the injection
of a fine atomized spray into the recirculating
airstream through spray nozzles (Part 10). The
second method is based on the pre-vaporization of
the fuel before it enters the combustion zone.
13. In the vaporizing method (fig.4-5) the fuel is
sprayed from feed tubes into vaporizing tubes which
are positioned inside the flame tube. These tubes
turn the fuel through 180 degrees and, as they are
heated by combustion, the fuel vaporizes before
passing into the flame tube. The primary airflow
passes down the vaporizing tubes with the fuel and
also through holes in the flame tube entry section
which provide ’fans’ of air to sweep the flame
rearwards. Cooling and dilution air is metered into
the flame tube in a manner similar to the atomizer
flame tube.
TYPES OF COMBUSTION CHAMBER
14. There are three main types of combustion
chamber in use for gas turbine engines. These are
the multiple chamber, the tubo-annular chamber and
the annular chamber.
Multiple combustion chamber
15. This type of combustion chamber is used on
centrifugal compressor engines and the earlier types
of axial flow compressor engines. It is a direct
development of the early type of Whittle combustion
chamber. The major difference is that the Whittle
chamber had a reverse flow as illustrated in fig. 4-6
but, as this created a considerable pressure loss, the
straight-through multiple chamber was developed by
Joseph Lucas Limited.
16. The chambers are disposed around the engine
(fig. 4-7) and compressor delivery air is directed by
ducts to pass into the individual chambers. Each
Combustion chambers
38
Fig. 4-4 Flame tube cooling methods.
chamber has an inner flame tube around which there
is an air casing. The air passes through the flame
tube snout and also between the tube and the outer
casing as already described in para. 6.
17. The separate flame tubes are all interconnected.
This allows each tube to operate at the same
pressure and also allows combustion to propagate
around the flame tubes during engine starting.
Combustion chambers
39
Fig. 4-5 A vaporizer combustion chamber.
Fig. 4-6 An early Whittle combustion chamber.
Tubo-annular combustion chamber
18. The tubo-annular combustion chamber bridges
the evolutionary gap between the multiple and
annular types. A number of flame tubes are fitted
inside a common air casing (fig. 4-8). The airflow is
similar to that already described. This arrangement
combines the ease of overhaul and testing of the
multiple system with the compactness of the annular
system.
Annular combustion chamber
19. This type of combustion chamber consists of a
single flame tube, completely annular in form, which
is contained in an inner and outer casing (fig. 4-9).
The airflow through the flame tube is similar to that
already described, the chamber being open at the
front to the compressor and at the rear to the turbine
nozzles.
20. The main advantage of the annular chamber is
that, for the same power output, the length of the
chamber is only 75 per cent of that of a tubo-annular
system of the same diameter, resulting in considerable
saving of weight and production cost. Another
advantage is the elimination of combustion
propagation problems from chamber to chamber.
21. In comparison with a tubo-annular combustion
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