Extinction flammability limits

Flammability Limits Ignition of a Flammable Mixture and Limit Flame Extinction... 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

The creation of a steady flame hole was previously carried out by Fiou et al. [36]. In their experiments, a steady-annular premixed edge flame was formed by diluting the inner mixture below the flammability limit, for both methane/air and propane/air mixtures. They found that a stable flame hole was established when the outer mixture composition was near stoichiometry. Their focus, however, was on the premixed flame interaction, rather than on the edge-flame formation, extinction, or propagation. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

Clearly, minimizing pollutant is easy to obtain by simply injecting less fuel in a chamber. The problem however is that below a certain equivalence ratio (i.e. below a certain amount of kg of fuel per kg of air), combustion simply stops [379 306 340]. The existence of this flammability limit makes optimization delicate because bringing the combustor close to extinction is dangerous (for aircrafts and helicopters for example, that is definitely something which must be avoided for obvious reasons). 

Flammability limits are limits of composition or pressure beyond which a fuel-oxidizer mixture cannot be made to burn. They are of practical interest especially in connection with safety considerations because mixtures outside the limits of flammability can be handled without concern about ignition. For this reason, extensive tabulations of limits of flammability have been prepared [1], [2]. Meanings of these tabulations and their relationships to ignition and extinction phenomena will be considered here in Section 8.2. 

Rates of chemical reactions always have a bearing on ignition, extinction, and flammability limits. There are many situations in which analyses of these phenomena reasonably may employ one-step, Arrhenius approximations to the rates. This fact enables common theories to be developed on the basis of energy considerations, which serve to correlate a number of different observed characteristics of ignition, quenching, and flammability limits. We shall focus our attention here on results explained by energy-conservation requirements and heat losses. In so doing, we exclude the consideration of special effects associated with finer details of chemical kinetics, such as radical diffusion or surface reactions. 

FIGURE 8.3. Illustration of the dependence of the adiabatic flame temperature and of the Arrhenius factor on the equivalence ratio, exhibiting extinction condition for defining flammability limits. 

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