Factors affecting flame propagation through dust clouds

The Malard-Le Chatelier picture of the combustionwave is amended by adding a term for radiant heat transfer. On introducing the rate of oxygen diffusion toward individual particles an expression for the burning velocity in dust clouds is obtained, representing its dependence on thermal conductivity, burning and ignition temperatures, radiation characteristics of the dust cloud, dust concentration and particle size.

This serves as working hypothesis in conductingexperiments on the effect of those factors upon stationary dust flames and flames traveling through quiescent dust clouds.

Results obtained with atomized aluminum are:o

1. 1.

   On the lean side of the stoichiometric ratiothe burning velocity increases with increasing concentration.

2. 2.

   The burning velocity increases with decreasingparticle size in the range studied.

3. 3.

   The failure to produce stationary dust flames with burner tubes smaller than 1/2-inch diameter indicates an effect of “radiation quenching” close to the rim in high temperature dust flames.

4. 4.

   A striking increase in the flame velocity ofclosed end ignited dust flames is attributed to turbulence.

The experimental results are in qualitative agreementwith theoretical expectations.