This work defines density factor as the ratio of before ignition density to after ignition density of the ignition mixture. This work provides an estimation method for explosive limits of various fuels under room temperature and pressure by showing that for a large universe of fuels, constant adiabatic flame temperature and density factor are appropriate approximations at the lower explosive limit while only a constant density factor might be an appropriate approximation at the upper explosive limit. Thus the assumption of a constant adiabatic flame temperature can be used in calculating lower explosive limit while the assumption of a constant density factor can be used in approximating upper explosive limit.

SUMMARY Flammability limits are important safety parameters of each flammable material. Thermal theory of flame propagation suggests existence of a threshold temperature above which a flame propagates. Density theory of flame propagation suggests existence of a threshold density factor above which a flame propagates; density factor is defined as the ratio of before ignition to after ignition density. Based on a general version of thermal theory of flame propagation, constancy of values of adiabatic flame temperature at each flammability limit can be used to predict flammability limits. Similarly, based on a general version of the density theory of flame propagation, constancy of values of density factor at each flammability limit can be used to predict flammability limits. This work reports and compares predictions of general versions of both thermal theory and density theory of flame propagation for lower flammability limits and for upper flammability limits. This work shows that thermal theory and density theory both perform well at predicting the lower flammability limit while density theory performs far better than thermal theory at predicting the upper explosive limit.