Flame concentration profiles

Fuel-Rich Flames. Concentration Profiles. Typical mole fraction curves for a substoichiometric flame are shown in Figure 2-a for Flame C. Mole fraction profiles in Flames C and D were nearly identical for each corresponding species in the substoichiometric flames in the presence of methane or natural gas. 

For the analysis of the chemical structure of flames, laser methods will typically provide temperature measurement and concentration profiles of some readily detectable radicals. The following two examples compare selected LIF and CRDS results. Figure 2.1 presents the temperature profile in a fuel-rich (C/O = 0.6) propene-oxygen-argon flame at 50 mbar [42]. For the LIF measurements, 1% NO was added. OH-LIF thermometry would also be possible, but regarding the rather low OH concentrations in fuel-rich flames, especially at low temperatures, this approach does not capture the temperature rise in the flame front [43]. The sensitivity of the CRDS technique, however, is superior, and the OH mole fraction is sufficient to follow the entire temperature profile. Both measurements are in excellent agreement. For all flames studied here, the temperature profile has been measured by LIF and/or CRDS. 

Other measurements such as gas species and soot all have importance in fire plumes but will not be discussed here. As we have seen for simple diffusion flames, the mixture fraction plays a role in generalizing these spatial distributions. Thus, if the mixture fraction is determined for the flow field, the prospect of establishing the primary species concentration profiles is possible. 

Prompt NO mechanisms In dealing with the presentation of prompt NO mechanisms, much can be learned by considering the historical development of the concept of prompt NO. With the development of the Zeldovich mechanism, many investigators followed the concept that in premixed flame systems, NO would form only in the post-flame or burned gas zone. Thus, it was thought possible to experimentally determine thermal NO formation rates and, from these rates, to find the rate constant of Eq. (8.49) by measurement of the NO concentration profiles in the post-flame zone. Such measurements can be performed readily on flat flame burners. Of course, in order to make these determinations, it is necessary to know the O atom concentrations. Since hydrocarbon-air flames were always considered, the nitrogen concentration was always in large excess. As discussed in the preceding subsection, the O atom concentration was taken as the equilibrium concentration at the flame temperature and all other reactions were assumed very fast compared to the Zeldovich mechanism. 

It has been shown recently [25] that concentrations of NOj, tend to reduce with increase in the amplitude of discrete-frequency oscillations. The mechanisms remain uncertain, but may be associated with the imposition of a near-sine wave on a skewed Gaussian distribution with consequent reduction in the residence time at the adiabatic flame temperature. Profiles of NO, concentrations in the exit plane of the burner are shown in Fig. 19.6 as a function of the amplitude of oscillations with active control used to regulate the amplitude of pressure oscillations. At an overall equivalence ratio of 0.7, the reduction in the antinodal RMS pressure fluctuation by 12 dB, from around 4 kPa to 1 kPa by the oscillation of fuel in the pilot stream, led to an increase of around 5% in the spatial mean value of NO, compared with a difference of the order of 20% with control by the oscillation of the pressure field in the experiments of [25]. The smaller net increase in NO, emissions in the present flow may be attributed to an increase in NOj due to the reduction in pressure fluctuations that is partly offset by a decrease in NOj, due to the oscillation of fuel on either side of stoichiometry at the centre of the duct. 

Blevins and Gore [14, 15] found that low-stretch-rate partially premixed flames involve multiple peaks in the profiles of intermediate hydrocarbon species. In particular, the CH species existing between the premixed and the diffusion flame part of the partially premixed flames were observed to react with NO and create an intermediate NO consumption zone. DuPont et al. [16] for low-stretch-rate flames also found the double peaks of intermediate hydrocarbon species and the NO consumption zone. However, Tanoff et al. [17] used the CH peak to characterize the location of the rich premixed flame and the OH peak to characterize the location of the diffusion flame. The NO concentration profiles showed that the peak NO mole fractions first increased and then decreased with increasing levels of partial premixing. However, the emission index of NO was not reported. 

The steady-state approximation is often used for the atomic and free radical intermediates occurring in combustion processes. The validity of this approximation has been examined in connection with the theoretical calculation of laminar flame velocities (3, 20, 21) in premixed gaseous systems. The steady-state approximation is occasionally useful for obtaining first-order estimates for flame-propagation velocities but should probably not be used in estimating concentration profiles for reaction intermediates. Some additional observations on the steady-state approximation are contained in Appendix I. 

Figure 1. Vertical concentration profiles for DMS, chlorophyll a and DMSP in the western basin of the Cariaco Trench off Venezuela (10° 39 N, 65°30 W). DMS was determined by sparging and gas chromatography with a flame photometric detector. Particulate DMSP was determined bv base treatment of material collected on 0.22 pm filters and analysis of the DMS released free DMSP was determined as DMS released upon base treatment of sparged water samples obtained after initial DMS analysis. Chlorophyll a data from W. Cooper and R. Zika (personal communication).

The hydroxyl concentration profile for a stoichiometric CH -air flame is presented in Figure 8. Here the maximum mole fraction observed and the predicted mole fraction are equal to better than 10% accuracy. The abscissas of the theoretical and the experimental results were matched by setting the theoretically predicted temperature equal to the measured hydroxyl rotational temperature. At all positions in the flame the hydroxyl 2j[(v,=o) state exhibited a Boltzmann distribution of rotational states. This rotational temperature is equal to the N2 vibrational temperature to within the +100 K precision of the laser induced fluorescence and laser Raman scattering experiments. An example of this comparison is given in Figure 9. 

The measurements of temperature and species concentrations profiles in premixed, laminar flames play a key role in the development of detailed models of hydrocarbon combustion. Systematic comparisons are given here between a recent laminar methane-air flame model and laser measurements of temperature and species concentrations. These results are obtained by both laser Raman spectroscopy and laser fluorescence. These laser probes provide nonintrusive measurements of combustion species for combustion processes that require high spatial resolution. The measurements reported here demonstrate that the comparison between a model and the measured concentrations of CH, O2,... 

Concentration Profiles. The relative fluorescence intensity profiles for OH, S2, SH, SO, and SO2 were converted to absolute number densities according to the method already outlined. Resulting concentration profiles for a rich, sulfur bearing flame are exhibited in Figure 17. H-atom densities were calculated from the measured OH concentrations and H2 and H2O equilibrium values for each flame according to Equation 6. Similar balanced radical reactions were used to calculate H2S and S concentrations 6). Although sulfur was added as H2S to this hydrogen rich flame, the dominant sulfur product at early times in the post flame gas is S02 ... 

If the activation energy is fixed at 76.25 kcals, then the resulting pre-exponential factor for the flame data is calculated to be 1.76 x 10, almost identical with Ref. 1. The scatter in the flame data for the narrow temperature range covered has an average deviation of 28% from the value recommended by Ref. 1. While the cause(s) of the scatter are not known, the largest uncertainties are attributable to errors in temperature, 0 atom concentration and small errors in the concentration profiles which are magnified by taking numerical derivatives. 

Temperature measurements (corrected for radiation) were made with silica-coated Pt-Pt/10% Rh thermocouples, about 4 mils in diameter. The temperature and species concentration profile as a function of distance through the flame provided the basic data for the kinetic analyses. 

Validation of the Global Rates Expressions. In order to validate the global rate expressions employed in the model, temperature and concentration profiles determined by probing the flames on a flat flame burner were studied. Attention was concentrated on Flames B and C. The experimental profiles were smoothed, and the stable species net reaction rates were determined using the laminar flat-flame equation described in detail by Fristrom and Westenberg (3) and summarized in Reference (8). 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

There are a number of possible approaches to the calculation of influences of finite-rate chemistry on diffusion flames. Known rates of elementary reaction steps may be employed in the full set of conservation equations, with solutions sought by numerical integration (for example, [171]). Complexities of diffusion-flame problems cause this approach to be difficult to pursue and motivate searches for simplifications of the chemical kinetics [172]. Numerical integrations that have been performed mainly employ one-step (first in [107]) or two-step [173] approximations to the kinetics. Appropriate one-step approximations are realistic for limited purposes over restricted ranges of conditions. However, there are important aspects of flame structure (for example, soot-concentration profiles) that cannot be described by one-step, overall, kinetic schemes, and one of the major currently outstanding diffusion-flame problems is to develop better simplified kinetic models for hydrocarbon diffusion flames that are capable of predicting results such as observed correlations [172] for concentration profiles of nonequilibrium species. 

These experimental measurements on flat flame burners revealed that when the NO concentration profiles are extrapolated to the flame-front position, the NO concentration goes not to zero, but to some finite value. Such results were most frequently observed with fuel-rich flames. Fenimore [9] argued that reactions other than the Zeldovich mechanism were playing a role in the flame and that some NO was being formed in the flame region. He called this NO, prompt NO. He noted that prompt NO was not found in nonhydrocarbon CO-air and H2-air flames, which were analyzed experimentally in the same manner as the hydrocarbon flames. The reaction scheme he suggested to explain the NO found in the flame zone involved a hydrocarbon species and atmospheric nitrogen. The... 

Another important feature of this analysis was that for fixed values of fei 7, and for the imposed condition of satisfactory prediction of measured burning velocities, the H atom concentration profiles in specific flames were not appreciably affected by the particular combination selected from the adjustable parameters concerned with reactions (viii), and (xviii)— (xxii), i.e. the rate coefficients g and ftj 9, and the ratios ftga/fegi ( a fesa)/ 2 0> 21/ 20 and 2 2/ 20- This implies that, despite somewhat incomplete characterization at this stage, the flame and the computational approach may be used to study the reactions of its radical species with trace additives. Such an analysis with D2O, D2 and CO2 as the trace additives, has been used by Dixon-Lewis [172] to obtain information about the rate coefficients fe j d a. 1 2 3 >... 

The hypothesis that the HO2 formed in reaction (iv) is rapidly removed (thus preventing its re-dissociation) has recently been examined for flame systems by Dixon-Lewis et al. [182]. On the assumption of equilibration of the fast, bimolecular, electron spin conserving reactions (i), (ii) and (iii), it is possible to compute concentration profiles for all the chemical species in the recombination region of a wide variety of flame systems. The calculation requires knowledge of the rate coefficients 4, Sa and ki 7— 225 which control the rate of electron spin removal (recombination). The rate of recombination via HO2 is calculated as the difference... 

From studies of the concentration profiles through dichlorodifluoro-methane/fluorine flames at low pressures, Homann and MacLean have proposed a chain mechanism involving fluorine and chlorine atoms as chain carriers [151(b)]. 

Levy and Merryman examined the structure of flat H2S-O2 flames with a mass spectrometer and micro-collection/analysis procedure. Concentration profiles for SO, SO2 and SO3 were measured, the [SO] being obtained very indirectly. The O atom concentration was computed by two alternate procedures, assuming that one of the following equilibria was established ... 

Important information on the NH3-O2 flame was reported by Maclean and Wagner . They used a mass spectrometer to measure the concentration profiles of NO, H2O, N2, NH3, H2, and N2O. Absorption spectroscopy gave the profiles of OH and NH. Emission of NH2, NO, NH, and OH was also monitored. The end products of the reaction, in order of importance, were H2O, N2, H2, and NO. The detailed kinetics could not be unravelled but the authors felt that reaction (6) was supported by their data. 

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