Combustion behavior of iron particles suspended in air

The combustion zone propagating through an iron particle cloud and the comb ustion behavior of individual iron particles have been examined by using hi gh-speed photomicrographs. Propagation of the combustion zone of 4-5 mm in width was observed as the movement of a luminous zone which consists of bur ning iron particles. In the region just behind the leading edge, burning pa rticles of various diameters are examined. As the distance from the leading edge becomes larger, smaller particles are fading away, and then only larg e particles are observed to remain luminous in the region where the distanc e is larger than 2 mm. Each iron particle burns at the combustion zone with out gas phase flame. The burn-out time (the duration of light emission) is proportional to the diameter of iron particle when the particle diameter is not so large. It agrees well with the result of a simple analysis. As the particle diameter becomes larger, the burn-out time becomes much larger tha n that predicted by the simple analysis.