air outlets
gas inlet
Figure 1-... A typical plate and fin type re.enerator for an industrial .as tur.ine.
Combustors
All gas turbine combustors perform the same function， they increase the temperature of the high-pressure gas. The gas turbine combustor uses very little of its air (10%) in the combustion process. The rest of the air is used for cooling and mixing. New combustors are also circulating steam for cooling purpose. The air from the compressor must be diffused before it enters the combustor. The velocity of the air leaving the compressor is about 400-600 ftjsec (122-183 mjsec) and the velocity in the combustor must be main-tained below 50 ftjsec (15.2mjsec). Even at these low velocities care must be taken to avoid the flame to be carried on downstream.
The combustor is a direct-fired air heater in which fuel is burned almost stoichiometrically with one-third or less of the compressor discharge air. Combustion products are then mixed with the remaining air to arrive at a suitable turbine inlet temperature. Despite the many design differences incombustors， all gas turbine combustion chambers have three features: (1) a recirculationzone， (2) a burning zone (with a recirculationzone， whichextends to the dilution region)， and (3) a dilution zone， as seen in Figure 1-23. The air entering a combustor is divided so that the flow is distributed between threemajor regions (1) PrimaryZone， (2) Dilution Zone and
(3) Annular space between the liner and casing.
The combustion in a combustor takes place in the Primary Zone. Com-bustion of natural gas is a chemical reaction that occurs between carbon， orhydrogen， and oxygen. Heat is given off as the reaction takes place. The products of combustion are carbon dioxide and water. The reaction isStoichiometric， which means that the proportions of the reactants are such that there are exactly enough oxidizer molecules to bring about a complete reaction to stable molecular forms in the products. The air enters thecombustor in a straight throughflow， or reverse flow.孔ost aero-engineshave straight through flow type combustors.孔ost of the large frame type units have reverse flow. The function of the recirculation zone is to evap-
Recirculation Zone
Burning Zone Dilution Zone
Figure 1-... A typical com.ustor can with strai.ht throu.h flow.
orate， partlyburn， and prepare the fuel for rapid combustion within theremainder of the burning zone.Ideally， at the end of the burningzone， all fuel should be burnt so that the function of the dilution zone is solely to mix the hot gas with the dilution air. The mixture leaving the chamber should have a temperature and velocity distribution acceptable to the guide vanes and turbine.Generally， the addition of dilution air is so abrupt that ifcombustion is not complete at the end of the burning zone， chilling occurswhich prevents completion.However， there is evidence with some chambersthat if the burning zone is runover-rich， some combustion does occur within the dilution region. Figure 1-24 shows the distribution of the air in the various regions of the combustor. The Theoretical or Reference Velocity is the flow of combustor-inlet air through an area equal to the maximum cross section of the combustor casing. The flow velocity is 25 fps (7.6 mps) in a reverse-flow combustor; and between 80 fps (24.4 mps) and 135 fps
(41.1 mps) in a straight-through flow turbojet combustor.
Combustor inlet temperature depends on engine pressure ratio， load andenginetype， and whether or not the turbine is regenerative or nonregen-erative especially at the low-pressure ratios. The new industrial turbinepressure ratio"s are between17:1， and35:1， which means that the combustor inlet temperatures range from 850 0F (454 0C) to 1200 0F (649 0C). The newaircraft engines have pressure ratios， which are in excess of 40:1.
Combustor performance is measured by efficiency， the pressure decreaseencountered in the combustor， and the evenness of the outlet temperature profile. Combustion efficiency is a measure of combustion completeness.Combustion completeness affects fuel consumption directly， since the heat-ing value of any unburned fuel is not used to increase the turbine inlet
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本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 1(18)
