Post-flame Zone Research Articles

Error estimates for Rayleigh scattering density and temperature measurements in premixed flames

Rayleigh scattering has become an accepted technique for the determination of total number density during the combustion process. The interpretation of the ratio of total Rayleigh scattering signal as a ratio of densities or temperatures is hampered by the changing composition through a flame, since the average Rayleigh scattering cross-section depends on the gas composition. Typical correction factors as a function of degree of reaction, fuel and equivalence ratio were calculated. The fuels considered were H2, CH4, C2H4, C2H6 and C3H8. Factors as low as 0.7 and 0.56 were found for the heaviest hydrocarbon fuel at large equivalence ratio for interpreting the Rayleigh scattering intensity as gas density and inverse temperature, respectively. This is primarily due to the presence of CO and H2 as intermediates. As CO and H2 are subsequently oxidized to CO2 and H2O, these factors approach 1.0. Conversely, the worst case, when using H2 as a fuel, occurs in the post flame zone. However, the correction factors for H2 are near 1.0 and the errors involved will, in general, remain within the expected experimental accuracy of a typical Rayleigh scattering system. Linear correlations of correction factors with equivalene ratio and with the product of equivalence ratio and fuel molecular weight were found and presented. The interpretation of Rayleigh scattering as temperature was found to have larger errors than the interpretation as density. Corrections for changes in gas composition were applied to Rayleigh scattering temperature measurements in the post flame region of CH4 and C3H8 flames with equivalence ratios of 0.75 and 1.0. The corrected temperatures were in excellent agreement with thermocouple measurements.

An Experimentally Verified NOx Prediction Algorithm Incorporating the Effects of Steam Injection

A Perfectly Stirred Reactor model, kinetic rate data, and equilibrium adiabatic flame temperatures have been incorporated into a closed form NOx prediction algorithm. The algorithm accounts for combustor inlet parameters and steam injection. Combining this model with the operating map for the gas turbine allows prediction of NOx with variation in parameters such as inlet guide vane angle, ambient temperature, and load. If the actual values of the model input variables are measured, real time prediction of NOx emissions may be generated using a microcomputer. This signal may then be used as an input to NOx abatement systems such as Selective Catalytic Reduction. The semitheoretical technique is based upon the extended Zeldovich chain reaction kinetics for the production of NOx in the post-flame zone. The temperature and concentration of major product species in the post-flame zone are taken to be those appropriate to equilibrium. Steam injection and inlet effects enter the model through their influence on the abiabatic equilibrium flame temperature. Mixing effects are accounted for empirically.

Higher hydrocarbon combustion: 2. Fuel-rich [formula omitted] mechanism

A method is presented for using a measured hydrogen atom profile as a probe of reaction details, and for refining the reaction mechanism by imposing the constraint that the hydrogen atom profile be correctly calculated. A reaction mechanism is derived for the fuel-rich, atmospheric pressure combustion of an acetylene/methane mixture at 2000K in the postflame zone of a rich (φ = 1.66) ethylene/air flame. The resulting chain-branched mechanism has the characteristic of, on net, forming free radicals in the hydrocarbon oxidative reaction sequence. The implications of this result are discussed in the context of radical scavenging by hydrocarbons and the system approach to equilibrium.

Nitric oxide formation in an ammonia-doped methaneoxygen low pressure flame

Mass spectrometry and absorption spectroscopy have been used to measure the concentration profiles of most of the species appearing in a slightly rich H 4 O 4 low pressure flame doped with NH 3. The mole flux profiles of these species and the net reaction rate profiles of the reactions producing NO or N 2 from NH 3 have been derived. The data lead to the following conclusions: 1. a. The reactions ▪ and ▪ are not in equilibrium if the JANAF value of ΔH f(NH) is used in the calculations. If the value of Kaskan and Nadler [1] is used, the last reactions appear to be equilibrated. 2. b. The HCNCN system is in partial pseudoequilibrium near the flame front. 3. c. The measured NO yield is in good agreement with De Soete's [2] empirical correlation. 4. d. In the flame zone NH 3 is transformed to HCN or N 2 by the reaction sequence ▪ but in the post flame zone, HCN and CN decay to NO or N 2 by ▪ The reactions involving N atoms are the most important paths, in good agreement with results published by Morley [3] and Haynes [4].

C 1 and C 2 chemistry in rich mixture, ethylene/air flames

Decay profiles of methane, acetylene, and hydrogen atoms are modeled in the post flame zone of four fuel-rich, atmospheric presure ethylene/air flat flames, at fuel equivalence ratios of 1.71 to 1.83 and adiabatic flame temperatures of 1992 to 2126 K. Chain branching is required in the fuel-rich hydrocarbon combustion mechanism for the simulation of hydrogen atom measurements. The mechanism of hydrocarbon suppression of free radical overshoot is discussed.

THE PREDICTION OF NOx FORMATION FOR THE COMBUSTION OF NITROGEN-DOPED DROPLETS OF HEXANE INSIDE A REFRACTORY TUBE

Abstract The detailed kinetic model developed by Tang and Churchill for the prediction of the combustion of premixed-gaseous or evaporated-liquid hydrocarbon fuels in a refractory plug flow burner has been extended to account for nitrogen compounds in the fuel. In the modified model the nitrogen compounds are postulated to decompose thermally to HCN in the flame front prior to oxidation. The concentrations of NO2 computed from this model for 0.795% wt. nitrogen in hexane are found to be in excellent agreement with measured values for fuel-lean mixtures doped with diethylamine, isobutylamine, pyridine and piperidine, and in fair agreement for fuel-rich mixtures up to an equivalence ratio of 1.3. The formation of NO2 from fuel-nitrogen is found to be essentially complete after 1 ms in the postflame zone. All nitrogen-containing species except N2 and NO are computed to be in negligible concentration for fuel-lean mixtures. Significant concentrations of HCN, NH3 and HN2 are predicted for the combustion of ver...

The formation of thermal and fuel Nox for radiantly stabilized combustion

Previous studies of combustion in a refractory tube with premixed propane and air or vaporized hexane droplets in air have demonstrated the existence of multiple stationary states; thin, stable and non-oscillatory flames; and a low production of thermal NO x (5–40 ppm). These studies were all limited to an equivalence ratio of approximately 0.80. In the current work a complete range of equivalence ratios was investigated for hexane droplets, vaporized completely prior to combustion. The concentration of thermal NO x was found to peak at about 100 ppm for the stoichiometric ratio, to fall off rapidly as the equivalence ratio increased above unity and to fall off even more rapidly to less than 5 ppm as the equivalence ratio decreased to 0.6. The decrease for <1 is attributable to the lower flame temperatures but that for >1 is presumably due to the absence of free oxygen as well. Experiments were also carried out for hexane doped with 0.795% wt. nitrogen in the form of diethylamine, isobutylamine, pyridine and piperidine. The formation of NO x from these materials was found to be independent of the residence time in the post-flame zone. This result implies complete conversion within the flame front in accordance with prior conjectures. The conversion of fuel-nitrogen to NO x was essentially 100% for 0.6<<1, fell off rapidly for >1 and precipitously for <0.6. Thermal NO x was found to be additive to fuel NO x . This behavior was quantitatively similar for all four nitrogen compounds and is in close agreement with prior results of other investigators for other nitrogen compounds and flat laminar flames. Increasing the concentration of nitrogen in the fuel resulted in a decreased conversion to NO x , particularly for >1, again in agreement with the observations of prior investigators for flat laminar flames.

A THEORETICAL MODEL FOR COMBUSTION REACTIONS INSIDE A REFRACTORY TUBE

Abstract A kinetic and thermal model has been developed for the evaporation and combustion of a single chain of uniformly sized droplets of hexane in a turbulent stream of air inside a refractory tube. A single global rate expression is utilized for the oxidation of hexane to CO and H20, but 44 reversible, free-radical mechanisms thereafter. Adiabatic flow is postulated for the post-flame zone and plug flow throughout. An integro-differential energy balance for the wall is avoided by the use of an experimental wall temperature profile for each set of conditions. The model is restricted to external conditions such that evaporation is completed prior to ignition. Calculated concentrations of NO are in good agreement with measured values for fuel-lean mixtures over a wide range of post-flame residence times. Calculated concentrations of CO are up to twice as high as experimental values but demonstrate the same dependence on residence time and equivalence ratio, suggesting that some of the individual rate con...

Temperature measurements of the hydroxyl radical and molecular nitrogen in premixed, laminar flames by laser techniques

Temperature profiles in an atmospheric pressure, premixed, laminar, methane-air flame were measured andsystematically compared by two different nonperturbing, laser light scattering methods. The rotationaltemperature of the lowest vibrational state of the electronic ground state of OH was measured by laser-induced fluorescence. The vibrational temperature of the electronic ground state of N(2) was also measured by laser Raman scattering. The first quantitative comparison of these two temperatures with a spatial resolution of less than 100 microm throughout both the reaction zone and the postflame zone is presented. The analysis of the data indicates that the N(2) vibrational temperature is in equilibrium with the OH rotational temperature at all points.

モデル燃焼器による低汚染燃焼法の実験的研究

An experimental study was made to investigate the mixing and NOx formation in a swirlstabilized, diffusion-type flame in a model combustor operating at lean fuel/air ratios. The city gas was injected from a fuel injector into an air flow passed through a swirler. The temperature, stable species and NOx concentration profiles in the combustor were measured over a range of air flow velocities through the swirler from 5 to 20m/s, fuel injection velocities from 36.4 to 145.4m/s, overall equivalence ratios Φ from 0.2 to 0.8 and nominal fuel injection angles from 60° to 180°. The effects of these parameters on NOx concentration in the flame zone and the emission level of NOx from the combustor were discussed. The effects of mixing rates were also estimated by examination of the local equivalence ratio profiles across the flame zone.In general, the flame clearly showed the characteristics of a fuel jet flame near the fuel injector. The temperature and the concentration profiles across the main flame zone and the recirculation zone change considerably with change in Φ. In the case of comparatively low value of Φ, it was shown that the high mixing rate had the effect of lowering the local equivalence ratio in the flame zone and the combustion gas temperature early in the post-flame zone. Then, the emission levels of NOx can be reduced by achieving high mixing rate due to the increase of air flow velocity and the variation of the fuel injection angle, at the expense of slight increase of the emission levels of CO.The minimum emission level of NOx obtained is 0.23g NO2/kg fuel at Φ of 0.2. Although the emission levels of NOx from the present combustor are still higher than those from lean premixed system, they were found to be much lower than those from conventional diffusion flame combustor.

Production of nitrogen compounds from molecular nitrogen in fuel-rich hydrocarbon-air flames

Measurements of HCN, NH 3 and NO concentrations in the burnt gases of a number of fuel-rich hydrocarbon-air flames have been made. Only HCN and/or NO leave the primary reaction zone of all flames studied, while NH 3 is formed later in the burnt gases. The behaviour of these species in the post-flame zone is qualitatively similar to that observed in flames seeded with fuel-nitrogen. The generation of nitrogen compounds from molecular nitrogen is obviously due not only to the Zeldovich mechanism but also to some as yet unidentified hydrocarbon-N 2 reaction. The characteristics of this reaction are that it begins to dominate in flames richer than φ = 1.2, and that the total amount of combined nitrogen generated increases only slowly as the flame is made richer, even though the distribution of this nitrogen amongst HCN, NH 3 and NO varies markedly.

NO x formation in premixed turbulent flames

NOx formation has been studied in lean, premixed, turbulent flames stabilized by perforated plates. The burner pressure was atmospheric, but other parameters were representative of gas turbine operating conditions. Prevaporized Jet A fuel was injected into preheated unvitiated air and thoroughly mixed prior to combustion. Mixture inlet temperature was 750°K and total residence times were approximately 4 msec. NOx, CO, CO2 and hydrocarbon levels were measured along the jet and recirculation zone axes. Results show a very rapid increase in NOx in the reaction zone which cannot be accounted for by O-atom radical overshoot. Post-flame zone NOx formation rates agree with the predictions of equilibrium theory. The relative contribution of “prompt NOx” to the final NOx levels increases with decreasing equivalence ratio. The rate of NOx formation in the reaction zone is found to be higher than in laminar flames. Turbulent transport processes are believed to be the dominant effect. NOx formation via nitrous oxide intermediate does not appear to be important under the present experimental conditions. Large amplitude temperature oscillations observed in turbulent premixed flames were found not to affect NOx formation rates.

Predictions of laminar flame speeds in boron-oxygen-nitrogen dust clouds

A detailed model of boron-oxygen-nitrogen dust-cloud flames including consideration of the details of boron particle ignition and the effects of oxygen depletion has been developed and used for prediction of flame speeds as functions of numerous parameters. Reasonably good agreement between measured flame speeds for the only two data points available on laminar boron dust cloud combustion and those predicted by this model has been obtained, although uncertainty concerning details of the experimental parameters results in this agreement being somewhat inconclusive. In addition, a simplified closed-form flame speed expression has been developed and the effects on predicted flame speeds of the various assumptions used in its development have been examined. The models have been used to study the effects of initial temperature, pressure, initial oxygen mole fraction, weight fraction particles, initial particle size, initial thickness of the oxide coating on the particles, radiation feedback from the postflame zone, and Nusselt Number. Mechanisms leading to the predicted dependencies are discussed.

Kinetics of nitric oxide formation in premixed laminar flames

The formation of nitric oxide in premixed methane/air in atmospheric-pressure flat flameswas studied at equivalence ratios ranging from 0.89 to 1.15. The peak rates of nitric oxide formation in the flame zone were found to be 1 to 2 orders of magnitude greater than the values in the post-flame zone, and to be consistent with the presence in the flame of oxygen radicals greatly in excess of equilibrium. A fair approximation of the measured rates is provided by the Zeldovich reactions when the free-radical concentrations are derived from the concentration of stable species by use of partial-equilibrium assumptions for the fast-flame reactions. A simplified method of predicting the rates of nitric oxide formation and carbon monoxide burnout in the flame zone provides partial explanation for the results obtained in this study.