The existence of two flame speeds

The dependence of the calculated flame speed on the initial reactant concentrations arises primarily through the influence of the concentrations on the adiabatic flame temperature T f. If the reactant concentration is 

An accurate application of equation (24) to the calculation of flammability limits entails accounting for variations other than that of El ROT a f Relevant parameters are exhibited explicitly in equation (24). That limits usually are somewhat wider for flames propagating upward in tubes than for flames propagating downward [1], [2] may be attributed to a larger value of L(Tf) for downward propagation. The evaluation of L Tf) will be considered in Section 8.2.5, where questions will be addressed relating to the physics of losses and also to the accuracy with which equation (24) follows from equation (21). It has already been indicated in Section 5.3.7 

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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