  Flame propagation through a non-volatile solid-fuel suspension is studied
using a simplified, time-dependent numerical model that considers the influence
of both diffusional and kinetic rates on the particle combustion process. It is
assumed that particles react via a single-step, first-order Arrhenius surface
reaction with an oxidizer delivered to the particle surface through gas
diffusion. Unlike the majority of models previously developed for flames in
suspensions, no external parameters are imposed, such as particle ignition
temperature, combustion time, or the assumption of either kinetic- or
diffusion-limited particle combustion regimes. Instead, it is demonstrated that
these parameters are characteristic values of the flame propagation problem
that must be solved together with the burning velocity, and that the a priori
imposition of these parameters from single-particle combustion data may result
in erroneous predictions. It is also shown that both diffusive and kinetic
reaction regimes can alternate within the same flame and that their interaction
may result in non-trivial flame behavior. In fuel-lean mixtures, it is
demonstrated that this interaction leads to certain particle size ranges where
burning velocity increases with increasing particle size, opposite to the
expected trend. For even leaner mixtures, the interplay between kinetic and
diffusive reaction rates leads to the appearance of a new type of flame
instability where kinetic and diffusive regimes alternate in time, resulting in
a pulsating regime of flame propagation.
