Premixed Flame Font Structure in Intense Turbulence
Six methane/air flames of equivalence ratio 0.7, Ka=1-17 stabilized on a intense turbulence low-swirl burner of were investigated by Rayleigh laser sheet measurements of the gas density within the preheat zone (approximately 400-1000 K). Lean premixed turbulent flames with Karlovitz numbers Ka, greater than unity are becoming of increasing practical importance but only limited detailed experimental data are available. This regime is often called the distributed reaction zone and a new approach to analyzing flame front broadening was developed to address issues of flame three dimensionality. Instantaneous progress variable, c, contour sets constitute the basic data and a statistical analysis of three aspects of this data are presented: (1) c contour spacing (2) flame front curvature, and (3) flame surface density. A detailed investigation revealed little change of these parameters with turbulence, even at the highest Ka numbers: the contour spacings were similar to those derived from laminar flame calculations and neither the flame front curvature nor the flame surface density varied significantly with c. Geometric effects of turbulence on flame front structure still predominate at these turbulence levels. The primary effect of increasing turbulence is increasingly to convolute the flame front and so increase the flame surface density, reduce the scalar length scales, and increase the burning rate. This finding is also reflected in the significant broadening of the flame front curvature distributions with increasing u′/SL. The data presented here indicate that the diagrams often used to delimit the various regimes of premixed turbulent combustion and hence guide modeling are based on parameters which need to be carefully investigated to determine their appropriateness. The results are consistent with an interpretation of combustion/turbulence interactions which sees the reaction zone thickness as the significant scalar length scale.