A continuous combustion device usually has a primary combustion zone and a secondary combustion zone. Ideally, from a combustion or pollution aspect, or both, the primary combustion zone fuel/air ratio should be kept as close as possible to an optimum value which may be constant over the operating range of the combustion device. This does not normally happen. A gas turbine engine used as a propulsion unit on an aircraft, for example, will operate in varying operative conditions for different thrust settings. When an aircraft is on the ground, the thrust setting is relatively low to permit stopping or taxiing. When the aircraft initiates a take-off, the thrust is typically increased to its maximum setting until the aircraft reaches a cruising altitude and then is tapered back to an intermediate setting for a normal cruising flight. However, the fixed geometry of the conventional continuous combustion device provides a range of primary combustion zone fuel/air ratios which can go from over-rich to over-lean when the operative conditions vary.
It is well-known that the constituent emissions from a combustion device exhaust are formed by diverse processes depending on different, or even opposite, conditions, and therefore, problems are experienced when attempts are made to compensate for the variations in the operative conditions of the continuous combustion device. For example, the nitric oxide formation rate depends essentially on the temperature in the primary combustion zone and the availability of dissociated or free oxygen. A early or accelerated admission of cooling or dilution air to the primary zone can quench the reaction and restrict nitric oxide formation to low levels. This procedure may, however, increase hydrocarbons, smoke and carbon monoxide formation due to incomplete combustion.
In a conventional continuous combustion device used in a gas turbine engine at full load, carbon monoxide and hydrocarbons are practically non-existent, whereas nitric oxide emissions are at their peak. A continuous combustion device optimized for full load pollutant emissions would have a leaner than normal primary zone fuel/air ratio, and its yield in hydrocarbons and carbon monoxide would be higher, whereas nitric oxides would be considerably reduced, such a combustion device would not be practical for a normal application in a gas turbine engine where the fuel/air ratio is varied over a wide range, especially its stability would be poor and the emissions of hydrocarbons and carbon monoxide emissions would be very high when the engine is idling.
To maintain those objectionable or harmful exhaust emissions from a gas turbine engine combustor at an acceptable level, prior art combustion devices have provided means for varying the distribution of air flow within a combustor and means for providing automization, premixing and substantial vaporization to maintain the primary combustion zone fuel/air ratio within a narrow range when the operative conditions vary. One example of reducing harmful emissions in all modes of engine operations is described in U.S. Pat. No. 3,952,501, entitled GAS TURBINE CONTROL, naming John A. Saintsbury as inventor and issued Apr. 27, 1976. Saintsbury suggests a longitudinally adjustable baffle that is used to control the direction of air flow into the combustor to effect a substantially optimum proportionate distribution of combustion air throughout the combustor at all power levels. The fraction of primary zone airflow will be gradually reduced as the power is decreased, holding the fuel/air substantially to the predetermined optimum value. This procedure reduces the production of carbon monoxide and unburned hydrocarbons at low power because combustion takes place at a more favourable fuel/air ratio. The nitric oxide production is inherently low at reduced power because of the lower temperature of inlet air to the combustor. Moreover, more cooling air is diverted into the secondary zone, whereby the hot gases could be more efficiently cooled.
The nitric oxide produced in gas turbine engines is produced in the combustion process where the highest temperature in the cycle normally exists. Therefore, one way to limit the amount of nitric oxide produced is to limit the combustion temperature. Experience has shown that it is not enough to just limit the average temperature because when fuel is burned as drops of liquid or a diffusion gas flame, the combustion proceeds at near the stoichiometric value and the local temperature is very high, thus producing excessive nitric oxide. To produce the lowest possible nitric oxide, thoroughly premixing all of the fuel and combustion air in a mixing chamber separate from the combustion chamber itself is suggested in U.S. Pat. No. 5,477,671, entitled SINGLE STAGE PRE-MIXED CONSTANT FUEL/AIR RATIO COMBUSTOR and issued to Mowill on Dec. 26, 1995. Mowill describes in his patent, a compressed air valve and a fuel valve both under the control of a controller, to provide a preselected lean fuel/air ratio mixture for introduction to the combustion zone of an annular housing. Compressed air conduits are used to channel a portion of the total compressed air flow to a premixer and the remainder to a dilution zone of the combustor, and a fuel conduit is used to deliver all of the fuel to the premixer.
Another example is described in U.S. Pat. No. 3,905,192, entitled COMBUSTOR HAVING STAGED PRE-MIXING TUBES and issued to Pierce et al. on Sep. 16, 1975. Pierce et al. describe, in this patent, a gas turbine engine having an annular combustor with a plurality of staged premixing tubes extending from the forward end thereof. Each tube directs a flow to the combustor through two concentric flow passages. A moveable tube section is arranged to direct all of the air through both flow passages or just through one passage. Fuel is directed into the staged premixing tube for mixing with air generally flowing through the central flow passage. Swirler vanes are provided in each of the flow passages to provide for rotation of air passing therethrough. The air flow proportion through the two concentric flow passages can be varied by the moveable tube section and, therefore, the fuel/air premixing ratio is adjusted.
However, since the proportion of air entering through the outer flow passage into the primary zone decreases as the proportion of air entering through the central flow passage into the primary zone in a premixed condition increases, the total amount of air reaching the primary zone through the both flow passages cannot be significantly regulated and, in fact, finally affects the improvement of the combustion conditions in the primary combustion zone.