Correlations for estimating flammability limits of pure fuels and fuel-inert mixtures

Flammability limits are one of the most important characteristics to evaluate fire hazards associated with fuels. Currently, numerous correlations, based on either chemistry (stoichiometric oxygen number) or energy (heat of combustion), have been proposed to estimate flammability limits. In this study, a thermal model is proposed to incorporate the dual role (quenching and heating) of each component in a mixture. The energy term plays a vital role in Excess Heating Potential (EHP) and Quenching/Heating Ratio (QHR) and hence an association between flammability limits and energy terms can be established. In addition, oxygen calorimetry bridges chemistry formula with energy release of a certain fuel, thus all existing correlations are unified under the principle of energy conservation. Since this methodology is universal, it can be applied to estimate the flammability limits of any mixtures or guide future correlations with improved precision. Advantages and limitations for this methodology are also discussed.

► Out of a thermal theory, various correlations for flammability limits are proposed.
► New correlations incorporate the contributions of fuel and diluent in a consistent framework.
► New correlations are compatible with existing correlations, showing the universality of the thermal theory.
► Various fuel combinations are used to find the overall performance of each correlation.
► Guidance to make better correlations for a special fuel combination is proposed.