Burning limits and the role of small scale turbulence in high Karlovitz number premixed flames

Abstract

Premixed flames exhibit a progressive enhancement in burning rates with turbulence, with the ultimate limits governed by the onset of local extinctions. Piloted premixed burners have enabled access to turbulence far exceeding the theoretically predicted extinction limits, with the measured flame speeds still improving. This study explores the role of small scale turbulence on the burning rates of premixed flames using a piloted high Karlovitz number burner. Premixed CH$_4$-air mixtures with equivalence ratios ($ϕ$) from 0.7--1.0, stabilized using a $ϕ$-matched pilot flame, were subjected to increasing rms fluctuating velocities, $u'$, up to 36.0 m/s. The experimental setup enables a progressive increase in turbulent kinetic energy (TKE) at smaller scales, with the Kolmogorov scales reaching approximately 4 $μ$m under the highest turbulence condition. Global consumption flame speeds, $S_\mathrm{T,GC}$, derived from the binarized Planar Laser-Induced Fluorescence (PLIF) imaging of OH radicals, scale with turbulent diffusivity, $\mathcal{D}_\mathrm{T}$. Further increases in $u'$, however, lead to a maximum and an eventual decline in flame speeds. The measured flame speeds exhibit a common scaling with the measured upstream $u'$. Assessments of the flame structure using CH-PLIF images further reveal progressive straining and extinction events with increasing $u'$ despite flames maintaining their flamelet-like appearance, suggesting that the observed limits derive from the increasing extinction propensity across the flame front. Finally, these findings are incorporated into a scaling relation accounting for the role of straining and extinctions in the observed flame-speed trends.