High-fidelity flame-front wrinkling measurements derived from fractal analysis of turbulent premixed flames with large Reynolds numbers

Abstract

This work assesses the level of wrinkling that premixed flames exhibit when subjected to turbulence levels in excess of ten times that of previous experimental studies of this nature. This assessment was facilitated by performing fractal analysis of isocontours extracted from simultaneously acquired planar Rayleigh scattering (PRS) and planar laser-induced fluorescence (PLIF) images of OH. Images were collected from 21 separate flames with turbulent Karlovitz (KaT,P) and Reynolds numbers (ReT,0) as large as 210 and 99,000, respectively. Values of the inner-cutoff scale (εi) and fractal dimension (FD) were extracted from two separate isocontours associated with each imaging technique. Results indicate that values of εi normalized by the laminar flame thickness (δL,P) follow the scaling: εi/δL,P∼KaT,P−0.22, which holds for results from prior studies but deviates from those derived from theoretical principles. Cases with main-flame equivalence ratios (ϕm) substantially less than that of their pilot flame (ϕp) did not follow the aforementioned scaling; however, adjusting δL,P and SL values such that they corresponded to the average of ϕm and ϕp was observed to reconcile this discrepancy. Yet, cases with ϕm ∼ ϕp did not deviate much from the scaling above. The influence of isocontour choice and targeted scalar on values of εi and FD was assessed and found to be much more significant for the former than the latter. Additionally, εi and FD exhibit a modest increase with axial distance from the flame base, corroborating prior results.