Page 46 - The Jet Engine

Combustion chambers

Fig. 4-1 An early combustion chamber.

COMBUSTION PROCESS various devices for metering the airflow distribution
along the chamber.
4. Air from the engine compressor enters the
combustion chamber at a velocity up to 500 feet per 6. Approximately 20 per cent of the air mass flow is
second, but because at this velocity the air speed is taken in by the snout or entry section (fig. 4-2).
far too high for combustion, the first thing that the Immediately downstream of the snout are swirl vanes
chamber must do is to diffuse it, i.e. decelerate it and and a perforated flare, through which air passes into
raise its static pressure. Since the speed of burning the primary combustion zone. The swirling air
kerosine at normal mixture ratios is only a few feet induces a flow upstream of the centre of the flame
per second, any fuel lit even in the diffused air tube and promotes the desired recirculation. The air
stream, which now has a velocity of about 80 feet per not picked up by the snout flows into the annular
second, would be blown away. A region of low axial space between the flame tube and the air casing.
velocity has therefore to be created in the chamber,
so that the flame will remain alight throughout the 7. Through the wall of the flame tube body, adjacent
range of engine operating conditions. to the combustion zone, are a selected number of
secondary holes through which a further 20 per cent
5. In normal operation, the overall air/fuel ratio of a of the main flow of air passes into the primary zone.
combustion chamber can vary between 45:1 and The air from the swirl vanes and that from the
130:1, However, kerosine will only burn efficiently at, secondary air holes interacts and creates a region of
or close to, a ratio of 15:1, so the fuel must be burned low velocity recirculation. This takes the form of a
with only part of the air entering the chamber, in what toroidal vortex, similar to a smoke ring, which has the
is called a primary combustion zone. This is achieved effect of stabilizing and anchoring the flame (fig, 4-3).
by means of a flame tube (combustion liner) that has The recirculating gases hasten the burning of freshly

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