Strain Rate and Pressure Effects on Multi-branched Counterflow Flames

TL;DR

This paper investigates how strain rate and pressure influence multi-branched counterflow flames, revealing conditions for flame coexistence, flame structure variations, and the role of endothermic reactions in complex combustion scenarios.

Contribution

It introduces a computationally efficient simulation approach for multi-branched flames with realistic conditions, avoiding symmetry assumptions and exploring critical parameters affecting flame behavior.

Findings

Multiple flames coexist at low strain and high pressure.

Higher strain rate or lower pressure reduces the number of flames.

Negative heat release regions indicate endothermic reactions.

Abstract

This study presents methane-air counterflow simulations, in computationally efficient similar form, allowing combustible mixtures to flow from one or both directions in order to learn more about multi-branched propagating flame structures (e.g., a triple flame). These structures with both premixed and non-premixed flames are commonly seen in more practical combustion analyses. A range of realistic mass mixture fractions and asymmetric chemical rate laws are examined while avoiding the commonly forced unreal symmetric behavior with one-step second-order kinetics. Moreover, a survey of critical parameters is performed varying pressure and normal strain rate to define the flame structure and detect different characters. Three flames can co-exist if the strain rate is low enough and the pressure is high enough. However, at higher strain rate and/or lower pressure, only one or two flames might be obtained. Negative regions of heat release rate are observed and linked to potential endothermic reactions. With a rich premixed mixture at low strain rates and pressures, high exothermic reactions producing CO$_2$ and H$_2$O, and consuming CO and H$_2$ causes a heat-release-rate peak. Unexpected character of the lean and rich premixed flames is observed, leading to the conclusion that these flames are diffusion-controlled.