Equivalence ratio gradient effects on flame front topology in a stratified iso-octane/air turbulent V-flame

Abstract

The effect of partial premixing on the local topology of globally stoichiometric flames was considered using a novel burner that permits controlled transverse variation of equivalence ratio along a continuous stratified flame front. Five iso-octane/air V-flames with different levels of stratification were studied in experiments designed to quantify differences in flame surface density and curvature between a reference homogeneous case and four different gradient cases. The mean equivalence ratio along the stratified flame front was characterized for each flame condition using 3-pentanone planar laser induced fluorescence (PLIF) conditioned to the reactants. PLIF of OH and CH2O were subsequently used to obtain progress variable, flame surface density, and curvature measurements for locally stoichiometric flamelets subjected to varying equivalence ratio gradients. To ensure fair comparison among conditions and to eliminate potential effects due to variable mixture strengths, a unique variable interrogation window approach was used to fix the range of equivalence ratios considered in the analysis of flames under different gradients. Results showed a significant increase in flame wrinkling with increased gradients in equivalence ratio, as well as increased variability in the local intensities of OH PLIF images. The effect of increased flame surface density was more modest, balanced in part by an increase in flame brush thickness and ultimately by a decrease in flame length. Observed variations in curvature distribution were not nearly as prominent as those reported in the literature for globally lean methane/air V-flames, which suggests that although gradients in mixture strength may alter the overall structure and instantaneous behavior of globally stoichiometric combustion systems, their effect on the topology of locally stoichiometric flames may be limited.