Thermal Structure and Burning Velocity of Flames in Non-volatile Fuel Suspensions

TL;DR

This study models flame propagation in non-volatile solid-fuel suspensions, revealing complex interactions between diffusion and kinetics that influence burning velocity and stability, challenging previous assumptions and highlighting the importance of coupled parameter determination.

Contribution

It introduces a self-consistent numerical model that predicts flame behavior without imposing external parameters, showing the interplay of diffusion and kinetics affects flame dynamics and stability.

Findings

Both diffusive and kinetic regimes can occur within the same flame.

Particle size influences burning velocity in non-intuitive ways.

New flame instability involving oscillating kinetic and diffusive regimes was identified.

Abstract

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.