Multiscale analysis of turbulence-flame interaction based on measurements in premixed flames

Abstract

Multi-scale analysis of turbulence-flame interaction is performed using experimental data sets from three methane- and propane-fired premixed, turbulent V-flames, at an approach flow turbulent Reynolds number of 450 and a ratio of r.m.s. fluctuating velocity from the mean to laminar flame speed of between 2.1 and 3.0, straddling the border between corrugated flamelets and thin reaction zone in the Borghi-Peters diagram. The measurements were made in the plane of a single laser sheet using stereo particle image velocimetry SPIV and planar laser-induced fluorescence to measure three orthogonal components of velocity and flame OH. Methods to approximate the remaining, unmeasured, out of plane derivatives are described. The instantaneous SPIV images were bandpass filtered at user-specified characteristic length scales L_ω and L_s (for vorticity and strain rate, respectively) resulting in instantaneous bandpass-filtered velocity fields, u̲_b^L_ω and u̲_b^L_s, which were further analysed to give the bandpass filtered vorticity field, ω̲^L_ω=∇×u̲_b^L_ω, the strain-rate field, e_ij^L_s, and the tangential strain rate field a_T^L_s. This work quantifies two aspects of turbulence-flame interaction. The first aspect is that of the flame interaction of eddies of size L_s on the turbulence, as found by the statistics of the alignment of vorticity with strain rate. We find that vortical eddies with scale about L_ω=2δ_th (where δ_th is the flame thickness) are stretched by L_s structures which are larger than about 2 L_ω, with this factor broadly true also for vortical eddies of scales L_ω=4δ_th and L_ω=6δ_th. Within the limitations of the data set, these findings are consistent with those in the literature on reacting and non-reacting flows, suggesting that the premixed flame has had little influence on the vortex stretching mechanism. The second aspect of turbulence-flame interaction examined is that of flame surface-averaged tangential strain rate imparted by eddies. Eddies with length scales L_s smaller than 3 or 4δ_th are likely to have the strongest individual contribution but eddies of this length scale and smaller may contribute only about 1/5th of the total tangential strain rate. This is to be compared with the value of 10% that has been reported in the literature based on analysis of DNS predictions of premixed flames at turbulent Reynolds numbers up to 110. Eddies with length scale L_s larger than about 20δ_th contribute a negligible amount to the total tangential strain rate. We have found no evidence that the Lewis number up to about 1.8 has an observable effect, but this may reflect the limitations of the current experiment. In the context of large eddy simulations (LES) of premixed combustion, these results are preliminary experimental evidence supporting the suggestion that resolving turbulence scales down to a few multiples of δ_th might be adequate to capture much of the flame straining caused by turbulence.