IGNITION, EXTINCTION, AND FLAMMABILITY LIMITS

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. 

Adoption of the energy-based viewpoint may be motivated by the fact that energetic aspects practically always are relevant to the phenomena to be studied. Seldom ever can energy conservation be neglected in addressing 

This ignition criterion is immanently reasonable. It can also be shown to be roughly equivalent to a number of other reasonable physical statements—for example, that the rate of liberation of heat by chemical reactions inside the slab must approximately balance the rate of heat loss from the slab by thermal conduction. The rule also clearly represents an over-simplified approximation discrepancies exceeding a factor of 2 or 3 will not be surprising. A formula expressing the rule is 

The approximate correlation between quenching distance and minimum ignition energy expressed by equation (3) is (roughly) substantiated experimentally [4]. 

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