A model is proposed for the ignition of quiescent multidroplet fuel mists which assumes that chemical reaction rates are infinitely fast, and that the sole criterion for successful ignition is the generation, by the spark, of an adequate concentration of fuel vapour in the ignition zone. From analysis of the relevant heat transfer and evaporation processes involved, expressions are derived for the prediction of quenching distance and minimum ignition energy. Support for the model is demonstrated by a close level of agreement between theoretical predictions of minimum ignition energy and the corresponding experimental values obtained using a specially designed ignition apparatus in which ignition energies are measured for several different fuels, over wide ranges of pressure, mixture composition and mean drop size. The results show that both quenching distance and minimum ignition energy are strongly dependent on droplet size, and are also dependent, but to a lesser extent, on air density, equivalence ratio and fuel volatility. An expression is derived to indicate the range of drop sizes over which the proposed model is valid.
A fundamental study of quenching distance measured by the flanged electrode method (Selection of ignition energy and diameter of flanged disk)