Pressure limits of flammability

It is often observed experimentally that if the relative concentrations of the reactants and the initial temperature of the mixture are kept fixed but the pressure p is decreased, then a limiting pressure is reached, below which flame propagation cannot be achieved. The flammable range of 0 usually narrows as the pressure is decreased, and below a critical pressure flame propagation does not occur for any value of 0. These observations are consistent with equation (24), in which the main pressure-dependent quantities are ml and L(Tf In terms of the overall order n (the pressure exponent) of the overall reaction rate, ml p . It will be seen in the following section that L(Tf) p , where m 0 for conductive or convective losses and 0 m 1 for radiative losses. Since n 1 for practically all flames, the right-hand side of equation (24) decreases more rapidly than the left-hand side as p decreases, and the limiting equality may therefore be expected to be surpassed at a sufficiently low pressure. 

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