Page 53 - The Jet Engine

Combustion chambers

33. In the fuel rich regions of the primary zone, the
hydrocarbons are converted into carbon monoxide
and smoke, Fresh dilution air can be used to oxidize
the carbon monoxide and smoke into non-toxic
carbon dioxide within the dilution zone. Unburnt
hydrocarbons can also be reduced in this zone by
continuing the combustion process to ensure
complete combustion.
34. Oxides of nitrogen are formed under the same
conditions as those required for the suppression of
the other pollutants, Therefore it is desirable to cool
the flame as quickly as possible and to reduce the
time available for combustion. This conflict of
conditions requires a compromise to be made, but
continuing improvements in combustor design and
performance has led to a substantially 'cleaner'
Fig. 4-10 Combustion efficiency and air/fuel combustion process.
ratio.

Combustion stability
28. Combustion stability means smooth burning
and the ability of the flame to remain alight over a
wide operating range.

29. For any particular type of combustion chamber
there is both a rich and weak limit to the air/fuel ratio,
beyond which the flame is extinguished. An
extinction is most likely to occur in flight during a
glide or dive with the engine idling, when there is a
high airflow and only a small fuel flow, i.e. a very
weak mixture strength.

30. The range of air/fuel ratio between the rich and
weak limits is reduced with an increase of air velocity,
and if the air mass flow is increased beyond a certain
value, flame extinction occurs. A typical stability loop
is illustrated in fig. 4-11. The operating range defined
by the stability loop must obviously cover the air/fuel Fig. 4-11 Combustion stability limits.
ratios and mass flow of the combustion chamber.

31. The ignition process has weak and rich limits
similar to those shown for stability in fig. 4-11. The MATERIALS
ignition loop, however, lies within the stability loop
since it is more difficult to establish combustion under 35. The containing walls and internal parts of the
'cold' conditions than to maintain normal burning. combustion chamber must be capable of resisting
the very high gas temperature in the primary zone. In
Emissions practice, this is achieved by using the best heat-
32. The unwanted pollutants which are found in the resisting materials available, the use of high heat
exhaust gases are created within the combustion resistant coatings and by cooling the inner wall of the
chamber. There are four main pollutants which are flame tube as an insulation from the flame.
legislatively controlled; unburnt hydrocarbons
(unburnt fuel), smoke (carbon particles), carbon 36. The combustion chamber must also withstand
monoxide and oxides of nitrogen. The principal corrosion due to the products of the combustion,
conditions which affect the formation of pollutants are creep failure due to temperature gradients and
pressure, temperature and time. fatigue due to vibrational stresses.

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