Saturday, January 7, 2012

Combustion stability

Fig. 4-11 Combustion stability limits.
28. Combustion stability means smooth burning and the ability of the flame to remain alight over awide 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 ahigh airflow and only a small fuel flow, i.e. a veryweak 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 loopis illustrated in fig. 4-11
. The operating range definedby the stability loop must obviously cover the air/fuelratios 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 hydrocarbon (unburnt fuel), smoke (carbon particles), carbonmonoxide and oxides of nitrogen. The principalconditions which affect the formation of pollutants arepressure, 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 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’ combustion process.
35. The containing walls and internal parts of the combustion chamber must be capable of resisting the very high gas temperature in the primary zone. In practice, this is achieved by using the best heatresisting materials available, the use of high heat resistant coatings and by cooling the inner wall of the flame tube as an insulation from the flame.
36. The combustion chamber must also withstand corrosion due to the products of the combustion, creep failure due to temperature gradients and fatigue due to vibrational stresses.

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