Equivalence Ratio Gradient Effects on Local Flame Front Topology and Heat Release Rate in Stratified, Iso-Octane/Air Turbulent V-Flames

Effects of partial premixing on the local behaviour of turbulent flames were investigated using a novel rectangular slot burner that could generate controlled, transverse variations in mixture strength along its exit plane. Turbulent, iso-octane/air V-flames were anchored in reactant mixtures of varying mean gradients in equivalence ratio (ϕ) and a unique windowing approach was devised in which 3-pentanone tracer planar laser induced fluorescence (PLIF) was used to determine an analysis region of interest (ROI) within the flame. Iso-contours of equivalence ratio were traced up to the mean position of the flame front to define the width of the ROI, which was specific to each mean ϕ gradient flame condition. This analysis methodology enabled fair comparison among gradient settings, and ensured that any observed variation in local flame behaviour could be attributed specifically to mean ϕ gradient effects, rather than simple mixture strength effects. The product of near-simultaneously acquired, spatially registered instantaneous OH and CH2O PLIF images was used as a measure of heat release rate (HRR) and to calculate flame topology data. Results within comparable ROIs were analyzed to elucidate gradient effects, providing several important insights. When comparing consistent ranges of ϕ within an ROI, the mean ϕ gradients considered had little influence on local flame surface density and curvature probability density functions in near-stoichiometric flame regions. Subtle differences in HRR and instantaneous flame thickness were observed among reference-premixed and gradient flame conditions for locally rich, near-stoichiometric, and lean flame regions. Results implied that back-support from the heated products altered the local HRR (and hence the reaction zone) along the stratified branches of the V-flame, and further suggested that different mechanisms of back-support may occur between locally rich and lean flame regions. Results consistently provided evidence of the potential interaction between neighbouring flame regions, despite being limited to relatively weak spatial gradients in equivalence ratio. The much steeper ϕ gradients observed in stratified charge engines and gas turbines are expected to influence flame behaviour more significantly.