Ignition and flame quenching of quiescent dust clouds of solid fuels

Experiments have been carried out to study the influence of particle size, dust concentration, pressure, mass transfer number and oxygen/nitrogen ratio on quenching distance and minimum ignition energy of dust clouds of solid fuels. The solids chosen were aluminium, magnesium, titanium and carbon. Ignition was accomplished by using sparks whose energy and duration could be varied independently. A separate series of tests was also conducted to ascertain optimum spark duration. The results of these tests show that particle size has a strong influence on the minimum ignition energy, the quenching distance and the optimum spark duration. To a lesser extent pressure, dust concentration, mass transfer number and oxygen/nitrogen ratio also affect ignition and quenching of dust clouds. A detailed comparison of results for the dust clouds and the liquid mists studied previously shows a strong similarity between the two. It appears that both solid and liquid fuels may be treated as members of a single family. Thus it is found that the formulae for quenching distance and minimum ignition energy derived previously for liquid fuel mists also satisfactorily predict the experimental results obtained for dust clouds of metals and carbon. Further, a typical calculation shows that for aluminium, for example, the critical particle Sauter mean diameter below which the formulae become invalid is around 6 $\mu$ m.