Further studies on the ignition and flame quenching of quiescent dust clouds

Abstract

The ignition and quenching of dust clouds of carbon, graphite, coal, aluminium and magnesium have been investigated. New experimental data on quenching distance and minimum ignition energy of graphite and coal dust in air and in oxygen-nitrogen mixtures are reported. These results show the influence of particle size, volatile matter, dust concen­tration and oxygen enrichment on the ignition and quenching processes. To explain these and earlier experimental observations, a new formula for the quenching distance has been derived. This expression incorporates ‘chemical reaction’ and ‘radiation heat loss’ terms in addition to a ‘diffusion’ term, which is retained from its earlier version (Ballal 1980). The new formula shows a significantly better prediction of the old and the new experimental results. The importance of chemical reaction and radiation heat loss in the study of ignition and quenching of dust clouds is discussed. Also, a strong similarity between the ignition and quenching characteristics of dust clouds, fuel mists and gaseous fuel-air mixtures is elucidated.