The Influence of Spatial Discreteness on the Thermo-Diffusive Instability of Flame Propagation with Infinite Lewis Number

TL;DR

This paper investigates how spatial discreteness affects flame propagation and thermo-diffusive instability in systems with infinite Lewis number, revealing complex oscillatory behaviors and mode interactions through numerical analysis.

Contribution

It introduces a numerical model analyzing the impact of spatially discrete heat sources on flame stability and instability regimes, highlighting the excitation of higher instability modes.

Findings

Spatial discreteness can induce higher modes of thermo-diffusive instability.

Long-wavelength oscillations are observed near the stability boundary.

Discreteness can lead to chaotic flame behaviors.

Abstract

The dynamics of flame propagation in systems with infinite Lewis number and spatially discretized sources of heat release is examined, which is applicable to the combustion of suspensions of fuel particles in air. The system is analyzed numerically using a one-dimensional heat equation with a source term for the reaction progress variable, which is specified to have zero diffusivity, and the model reveals a spectrum of flame-propagation regimes. For the case of a switch-type reaction rate and homogeneous media (continuous regime), the flame propagates steadily at a velocity in agreement with analytical solutions. As the sources are spatially concentrated into {\delta}-function-like sources, propagation approaches the discrete regime with a fixed period between ignition of the sources, for which an analytic solution is also available for validation. When the source term is governed by an Arrhenius rate and the activation energy is increased beyond the stability boundary, the flame begins to exhibit a long-wavelength (4-5 times the thermal flame thickness) oscillation characteristic of the thermo-diffusive instability, in good agreement with prior stability analysis. When spatial discreteness is introduced, a competition is observed between the long-period oscillations of the thermo-diffusive instability and the pulsations associated with the rapid heat release of the concentrated sources. Interestingly, the presence of spatial discreteness is able to excite higher modes (period doubling and chaotic solutions) of the thermo-diffusive instability, suggesting that the introduction of discreteness may have an influence qualitatively similar to that of increasing activation energy. Relevance of the model parameters to experimental systems is then discussed.