Fuel rich flames 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. 

Measurements of temperature and concentration in CO-H2-CH4 (or natural gas) flames were carried out. Rate profiles were developed for two excess air and two slightly fuel-rich flames as a function of temperature. Substitution of natural gas for methane does not bring about a marked change in the overall reactivity of these systems. Application of a modified theory analysis to these multiple-fuel flame mixtures allows one to satisfactorily correlate calculated values of the burning velocity with measured values. 

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... 

Concentration profiles of OH radicals in several fuel-rich flames are presented in the bottom right of Figure 29.8. All profiles rise steeply in the first few millimetres and reach a maximum at roughly the same position, at a height of 5 mm above the burner base. This maximum is followed by a more gradual decrease toward the exhaust of the burner. The authors found that both the general shape and the position of the maximum OH mole fraction of the CRDS measurements correlated well with other results obtained by LIF spectroscopy. 

A two-step global mechanism has been derived by de Soete [198] for the description of NO and N2 generation from fuel nitrogen. Parameters of the model were fitted to concentration profiles in flames. This model is widely used in post-processor packages of commercial CFD codes, although it works poorly under fuel-rich conditions. 

Fig. 9 shows concentration profiles of some selected species in a fuel rich, premixed methane flame. " Clearly, even the chemistry of combustion of a simple hydrocarbon fuel such as methane is considerably complex. The fuel molecule, before it ultimately produces CO2 and H2O, undergoes a series of intermediate... 

Fig. 9 (A) Major species concentration profiles and (B) Some of the polycyclic aromatic hydrocarbons (PAH) formed in a fuel-rich, premixed laminar methane flame the formation of a large number of intermediates and by-products are evident. Highly toxic benzo-fl-pyrene is the 3rd PAH from the bottom. (From Ref. " l)...

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