Effects of flow and chemistry on OH levels in premixed flame-vortex interactions

Abstract

The transient response of OH during laminar flame-vortex interactions is studied experimentally forpremixed CH4-air flames. Quantitative measurements of OH laser-induced fluorescence are performed for this purpose. The effect of flow geometry is investigated by comparing a line-vortex/V-flame interaction and a toroidal-vortex/counterflow-flame interaction. At an equivalence ratio of =1.2, a N2-diluted V-flame exhibits an abrupt increase in peak OH signal. This increase is not observed in the axisymmetric configuration for comparable flame chemistry and vortex size. The two-dimensional flame surface contour, as traced by the peak OH signal, is also topologically matched for the two geometries. Under richer conditions, an increase in OH is observed during toroidal vortex-counterflow-flame interactions. The distribution and temporal evolution of excess OH along the vortex contour are quite different for the two geometries. Results suggest a strong coupling between chemistry and flame stretch. Distributions of stretch along the vortex contour differ for the two geometries. The effect of chemistry is investigated for lean and rich conditions. An undiluted rich flame (=1.4) is compared with a diluted rich flame (=1.3, 10% N2 dilution) with similar adiabatic flame temperature and laminar flame speed. Under the richest conditions (=1.4), an OH increase is observed for undiluted flames, indicating that the mechanism for increasing OH during a flow transient is present even without high levels of N2 dilution. An undiluted lean flame (=0.6) is compared with a diluted rich flame (=1.2, 25% N2 dilution) with similar laminar flame speed. For the lean flame, the counterflow flame-vortex interaction exhibits a decrease in the peak OH signal, which is consistent with previous studies of both V-flames and freely propagating axisymmetric flames.