Concentration profile of particles across a flame propagating through an iron particle cloud

The number density profile of particles across a flame propagating through an iron particle cloud has been examined experimentally. The iron particles were suspended in air and ignited by an electric spark. Measurements were performed using high-speed photomicrography combined with laser light scattering technique. It is shown that for relatively large (agglomerated) particles the number density of iron particles changes in the range of x smaller than 11.0 mm, where x is the distance from the leading edge of the combustion zone. The number density increases with the decrease of x in the range 0.6 ≤ x ≤ 11.0 mm, reaches a maximum at x ≈ 0.6 mm, and then decreases. The maximum value of the number density is about 2.6 times larger than that at the region far ahead of the flame (x >11.0 mm). This increase in the number density of particles must cause a change of the lower flammability limit. By assuming that the increase in the number density is caused by the velocity difference of particles from surrounding gas flow, the profile of the number density of particles has been estimated on the basis of measured velocities of particles. The estimated number density profile of particles agrees well with that of the measured profile. The increase in the number density of particles just ahead of the flame will appear not only in iron particle cloud but also in any two-phase combustion systems, such as combustible particle cloud, combustible spray and so on.