Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d ₁₀ < 15 μm and particle concentrations from ≈10¹⁰ to 10¹¹ m^?3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d ₁₀ = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d ₁₀ = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.