Primary zone

Combustion. The primary reaction carried out in the gas turbine combustion chamber is oxidation of a fuel to release its heat content at constant pressure. Atomized fuel mixed with enough air to form a close-to-stoichiometric mixture is continuously fed into a primary zone. There its heat of formation is released at flame temperatures deterruined by the pressure. The heat content of the fuel is therefore a primary measure of the attainable efficiency of the overall system in terms of fuel consumed per unit of work output. Table 6 fists the net heat content of a number of typical gas turbine fuels. Net rather than gross heat content is a more significant measure because heat of vaporization of the water formed in combustion cannot be recovered in aircraft exhaust. The most desirable gas turbine fuels for use in aircraft, after hydrogen, are hydrocarbons. Fuels that are liquid at normal atmospheric pressure and temperature are the most practical and widely used aircraft fuels kerosene, with a distillation range from 150 to 300 °C, is the best compromise to combine maximum mass —heat content with other desirable properties. For ground turbines, a wide variety of gaseous and heavy fuels are acceptable. 

The first commercial oil-fumace process was put into operation in 1943 by the Phillips Petroleum Co. in Borger, Texas. The oil-fumace blacks rapidly displaced all other types used for the reinforcement of mbber and today account for practically all carbon black production. In the oil-fumace process heavy aromatic residual oils are atomized into a primary combustion flame where the excess oxygen in the primary zone bums a portion of the residual oil to maintain flame temperatures, and the remaining oil is thermally decomposed into carbon and hydrogen. Yields in this process are in the range of 35 to 50% based on the total carbon input. A broad range of product quaHties can be produced. 

Larger tubular, or single-can, units usually have more than one nozzle. In many cases a ring of nozzles is placed in the primary zone area. The radial and circumferential distribution of the temperature to the turbine nozzles is not as even as in tubo-annular combustors. In some cases, high stresses are exerted on the turbine casing leading to casing cracks. 

For NO control only, steam is injected into the combustor directly to help reduce the primary zone temperature in the combustor. The amount of steam injected is in a ratio of 1 1 with the fuel. In this cycle, the steam is injected upstream of the combustor and can be as much as 5-8 percent by weight of the air flow. This cycle leads to an increase in output work and a shght increase in over l efficiency. Corrosion problems due to steam injection have been for the most part over-... 

The steam used in this process is generated by the turbine exhaust gas. Typically, water at 14.7 psia (1 Bar) and 80 °F (26.7 °C) enters the pump and regenerator, where it is brought up to 60 psia (4 Bar) above the compressor discharge and the same temperature as the compressor discharged air. The steam is injected after the compressor but far upstream of the burner to create a proper mixture which helps to reduce the primary zone temperature in the combustor and the NO output. The enthalpy of State 3 hi,) is the mixture enthalpy of air and steam. The following relationship describes the flow at that point ... 

The importanee of air veioeity in the primary zone is known. In the primary zone fuel-to-air ratios are about 60 1 the remaining air must be added somewhere. The seeondary, or dilution, air should only be added after the primary reaetion has reaehed eompletion. Dilution air should be added gradually so as not to queneh the reaetion. The addition of a flame tube as a basie eombustor eomponent aeeomplishes this, as shown in Figure 10-6. Flame tubes should be designed to produee a desirable outlet profile and to last a long time in the eombustor environment. Adequate life is assured by film eooling of the liner. 

With torroidal air flow, combustors will operate without visible smoke when properly developed for a primary-zone equivalence ratio below 1.5. Visible smoke is an air-pollution problem. 

Cross-sectionai area. The combustor cross section can be determined by dividing the volumetric flow at the combustor inlet by a reference velocity which has been selected as being appropriate for the particular turbine conditions on the basis of proven performance in a similar engine. Another basis for selecting a combustor cross section comes from correlations of thermal loading per unit cross section. Thermal loading is proportional to the primary zone air flow because fuel and air mixtures are near stoichiometric in all combustors. 

In practice it has been found that the diameter of holes in the primary zone should be no larger than 0.1 of the liner diameter. Tubular lines with about 10 rings of eight holes each give good efficiency. As discussed before, swirl vanes with holes yield better combustor performance. 

In general, it has been found that much visible smoke is formed in small, local fuel-rich regions. The general approach to eliminating smoke is to develop leaner primary zones with an equivalence ratio between 0.9 and 1.5. Another supplementary way to eliminate smoke is to supply relatively small quantities of air to those exact, local, over-rich zones. 

Use of a rieh primary zone in whieh little NO formed, followed by rapid dilution in the seeondary zone. 

Use of a very lean primary zone to minimize peak flame temperature by dilution. 

In a typieal eombustor as shown in Figure 10-19, the flow entering the primary zone is limited to about 10%. The rest of the flow is used for mixing the eombusted air and eooling the eombustor ean. The Maximum temperature is reaehed in the primary or stoiehiometrie zone of about 4040 °F (2230 °C) and after the mixing of the eombustion proeess with the eooling air the temperature drops down to a low of 2500 °F (1370 °C). 

The gas turbine eombustors have seen eonsiderable ehange in their design as most new turbines have progressed to Dry Low Emission NOx Combustors from the wet eombustors, whieh were injeeted by steam in the primary zone of the eombustor. The DLE approaeh is to burn most (at least 75%) of the fuel at eool, fuel-lean eonditions to avoid any signifieant produetion of NOx. The prineipal features of sueh a eombustion system is the premixing of... 

H20 and free radicals such as. Above the primary zone is the outer cone or secondary reaction zone. In this region, cooling occurs as a result of mixing with the surrounding atmosphere. This may lead in turn to the entrainment of impurities, such as sodium compounds which will increase... 

It is interesting to review a general pattern for oxidation of hydrocarbons in flames, as suggested very early by Fristrom and Westenberg [29], They suggested two essential thermal zones the primary zone, in which the initial hydrocarbons are attacked and reduced to products (CO, H2, H20) and radicals (H, O, OH), and the secondary zone, in which CO and H2 are completely oxidized. The intermediates are said to form in the primary zone. Initially, then,... 

Considering the wake of a flame holder as a stirred reactor may be inconsistent with experimental data. It has been shown [66] that as blowoff is approached, the temperature of the recirculating gases remains essentially constant furthermore, their composition is practically all products. Both of these observations are contrary to what is expected from stirred reactor theory. Conceivably, the primary zone of a gas turbine combustor might approach a state that could be considered completely stirred. Nevertheless, as will be shown, all three theories give essentially the same correlation. 

Stirred reactor theory was initially applied to stabilization in gas turbine combustor cans in which the primary zone was treated as a completely stirred region. This theory has sometimes been extended to bluff-body stabilization, even though aspects of the theory appear inconsistent with experimental measurements made in the wake of a flame holder. Nevertheless, it would appear that stirred reactor theory gives the same functional dependence as the other correlations developed. In the previous section, it was found from stirred reactor considerations that... 

The primary zone of a gas turbine combustor is modeled as a perfectly... 

However, even when the overall equivalence ratio in the primary zone is very... 

The emissions were greatly influenced by the primary zone equivalence ratios. The residence times at the high-temperature region decreased with decreased fuel-air ratio, resulting in a drastic increase in CO and UHC emissions. CO and UHC emissions also increased for very lean mixtures because of lower combustion temperatures. For the baseline cases NOj, emissions were found to increase with increased fuel-air ratio whereas, with porous inserts installed, varying fuel-air ratio generally had little influence on the NOj, concentrations. 

Primary zone size is important with regard to efficiency and limits also. Within practical limits, a larger primary zone cross-sectional area will provide the best performance 138). Possible reasons arc lower velocities, less wall impingement by fuel, larger zone of low velocity, and less wall quenching of chemical reactions. The best axial distribution of open area of a combustor will depend on required operating conditions, the pressure loss characteristics, and the shape of the air entry ports. It will also depend on fuel-injection and fuel-volatility characteristics, as these factors will affect the amount of vapor fuel present at any location. If proper burning environment is to be obtained, these factors must be matched, and compromises in performance must be expected. 

Total combustor volume and primary zone volume should be as large as possible within the limits imposed by other engine components and pressure-loss characteristics. 

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