A computational study of the structure, stability, dynamics, and response of low stretch diffusion flame

Diffusional thermal instabilities occur because of the imbalance between the diffusion of heat and the diffusion of species into the reaction zone. While the phenomenon of diffusional thermal instability (DTI) has been recorded in detail in the high stretch extinction regime of diffusion flames, fundamental understanding of the DTI near the low stretch radiation induced extinction regime is lacking. Low stretch flames are relevant to the fire safety characteristics in many practical engineering fields. In the current study we investigate the phenomena of DTI for low stretch diffusion flames with radiative heat loss and map the one-dimensional and two-dimensional flame structure, stability and dynamics.; Low stretch nonpremixed combustion including radiative heat loss is modeled using a planar counterflow configuration (PCC) and an axisymmetric counterflow configuration (ACC) to highlight the effects of flow configuration on low stretch flame instabilities. It is found that the 1D low stretch diffusion flames in both ACC and PCC systems initially lose their stability to pulsations which lead to oscillatory extinction for stretch rates much higher than the 1D steady state extinction limit. The effects of the Lewis number and reaction rate on this 1D oscillatory instability is also presented along with a fast Fourier transform (FFT) analysis to map the frequency and amplitude characteristics of the oscillatory instability.; The stability of the 2D flame is mapped using different initial profiles. The 2D low-stretch diffusion flame in both ACC and PCC systems was found to lose its stability to wavy flames very close to the 1D neutral stability point subsequently leading to steady/unsteady stationary/traveling cellular flames for PCC system and unsteady cellular flames for ACC system. These multi-dimensional flame phenomena are shown to extend the dynamic extinction limits predicted by the 1D model. Comparisons between the ACC system and the PCC system also show the effects of flow configuration on multi-dimensional flame phenomenon for low-stretch radiative diffusion flames. A 2D bifurcation of the flame solution close to the 1D low-stretch radiation induced extinction limit is identified.