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system, the wall area of a comparable annular
chamber is much less; consequently the amount of
cooling air required to prevent the burning of the
flame tube wall is less, by approximately 15 per cent,
This reduction in cooling air raises the combustion
efficiency (para. 27) to virtually eliminate unburnt
fuel, and oxidizes the carbon monoxide to non-toxic
carbon dioxide, thus reducing air pollution.
22. The introduction of the air spray type fuel spray
nozzle (Part 10) to this type of combustion chamber
Combustion chambers
40
Fig. 4-7 Multiple combustion chambers.
also greatly improves the preparation of fuel for
combustion by aerating the over-rich pockets of fuel
vapours close to the spray nozzle; this results in a
large reduction in initial carbon formation.
COMBUSTION CHAMBER PERFORMANCE
23. A combustion chamber must be capable of
allowing fuel to burn efficiently over a wide range of
operating conditions without incurring a large
pressure loss. In addition, if flame extinction occurs,
then it must be possible to relight. In performing
these functions, the flame tube and spray nozzle
atomizer components must be mechanically reliable.
24. The gas turbine engine operates on a constant
pressure cycle, therefore any loss of pressure during
the process of combustion must be kept to a
minimum. In providing adequate turbulence and
mixing, a total pressure loss varying from about 3 to
8 per cent of the air pressure at entry to the chamber
is incurred.
Combustion intensity
25. The heat released by a combustion chamber or
any other heat generating unit is dependent on the
volume of the combustion area. Thus, to obtain the
required high power output, a comparatively small
Combustion chambers
41
Fig. 4-8 Tubo-annular combustion chamber.
and compact gas turbine combustion chamber must
release heat at exceptionally high rates.
26. For example, at take-off conditions a Rolls-
Royce RB211-524 engine will consume 20,635 lb. of
fuel per hour. The fuel has a calorific value of approximately
18,550 British thermal units per lb., therefore
the combustion chamber releases nearly 106,300
British thermal units per second. Expressed in
another way this is an expenditure of potential heat
at a rate equivalent to approximately 150,000 horsepower.
Combustion efficiency
27. The combustion efficiency of most gas turbine
engines at sea-level take-off conditions is almost 100
per cent, reducing to 98 per cent at altitude cruise
conditions, as shown in fig. 4-10.
Combustion chambers
42
Fig. 4-9 Annular combustion chamber.
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
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
ignition loop, however, lies within the stability loop
since it is more difficult to establish combustion under
’cold’ conditions than to maintain normal burning.
Emissions
32. The unwanted pollutants which are found in the
exhaust gases are created within the combustion
chamber. There are four main pollutants which are
legislatively controlled; unburnt hydrocarbons
(unburnt fuel), smoke (carbon particles), carbon
monoxide and oxides of nitrogen. The principal
conditions which affect the formation of pollutants are
pressure, temperature and time.
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
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