Diffusion flames species distribution

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. 

Unlike premixed flames, which have a very narrow reaction zone, diffusion flames have a wider region over which the composition changes and chemical reactions can take place. Obviously, these changes are principally due to some interdiffusion of reactants and products. Hottel and Hawthorne [5] were the first to make detailed measurements of species distributions in a concentric laminar H2-air diffusion flame. Fig. 6.5 shows the type of results they obtained for a radial distribution at a height corresponding to a cross-section of the overventilated flame depicted in Fig. 6.2. Smyth et al. [2] made very detailed and accurate measurements of temperature and species variation across a Wolfhard-Parker burner in which methane was the fuel. Their results are shown in Figs. 6.6 and 6.7. 

Case 3 Both temperature and species variation - In this case, additional information is required. This could be obtained from another diagnostic or a mathematical model. Smith (10) used an extensive mathematical model of a laminar hydrogen diffusion flame to predict the species distribution throughout the flame having this, the temperature could be inferred from the Rayleigh scattering intensity. 

Palmer and his co-workers have observed more complex C2 emission from diffusion flames of alkali metals (Na or K) in haloforms and carbon tetra-halides [160-165], The Swan bands show a much broader v distribution than those emitted from other systems, and although the v = 6 level is sometimes excited preferentially, it is clear that more than one process excites the A 3I1B state in the diffusion flame reactions. Detailed interpretation is difficult and is hampered by a lack of precise thermochemical data for some of the species involved. Tewanson, Naegeli, and Palmer [165] have suggested that three quite distinct mechanisms may cause excitation (i) the association of C... 

Direct aspiration of the sample into the hydrogen diffusion flame is always associated with certain problems. The low temperature of the flame may lead to formation of solid particles, which reduce the production of emitting species and decrease sensitivity. The solvent and other components of the analyte solution may alter the temperature and disturb the radical distribution and concentration within the flame body. The distribution of the sample vapors all over the flame may be responsible for generation of more than one emitting species, such as with tin(II) bromide. Production of excited species will also be affected by the difference of temperature at various points of the flame. Moreover, the emitting species will spread over a wide region and the intensity per unit area of flame facing the detector will be low. Finally, the residence time of the analyte within the flame is short and cannot be increased since it is mainly controlled by the flow rate of the support gas. 

Figures 4.6—4.8 are the results for the stoichiometric propane-air flame. Figure 4.6 reports the variance of the major species, temperature, and heat release Figure 4.7 reports the major stable propane fragment distribution due to the proceeding reactions and Figure 4.8 shows the radical and formaldehyde distributions—all as a function of a spatial distance through the flame wave. As stated, the total wave thickness is chosen from the point at which one of the reactant mole fractions begins to decay to the point at which the heat release rate begins to taper off sharply. Since the point of initial reactant decay corresponds closely to the initial perceptive rise in temperature, the initial thermoneutral period is quite short. The heat release rate curve would ordinarily drop to zero sharply except that the recombination of the radicals in the burned gas zone contribute some energy. The choice of the position that separates the preheat zone and the reaction zone has been made to account for the slight exothermicity of the fuel attack reactions by radicals which have diffused into...

Diffusion in solids does not ensure the experimentally observed velocity of combustion wave propagation in the systems which are traditionally considered as gasless and burned in the mode of solid flames (gasless solid-state combustion). The phenomenology of indirect interactions, the thermochemistp and dynamics of the gas-phase carriers formation, as well as their participation in the reactants transport are studied in the systems Mo-B and Ta-C. The distributions of the main species in the gas phase of the combustion wave are measured in situ with the use of a dynamic mass-spectrometry (DMS) technique which allows for high temporal and spatial resolution. The detailed chemical pathways of the processes were established. It was shown that the actual mechanism of combustion in the systems under study is neither solid state nor gasless and the reactions are fiilly accomplished in a narrow front. 

---