OH fluorescence images of the quenching of a premixed flame during an interaction with a vortex

Local quenching of a premixed laminar flame has been imaged using OH planar laser induced fluorescence as the flame interacts with the well-defined strain field of a single laminar vortex. This flame-vortex interaction represents a fundamental "building block" of premixed turbulent combustion. Since the OH concentrations reach a maximum in the flame-front which is 10 times larger than the value in the burnt gas, the measured [OH] profiles indicate the instantaneous shape of the reaction zone. Local quenching is confirmed by the disappearance of the reaction zone. Four different vortex strengths were studied; the weakest vortex causes flame wrinkling only; stronger vortices create pockets of unburned gas. Local quenching occurs at the leading edge of the vortex when the Karlovitz number (based upon the vortex diameter and rotational velocity) exceeds 0.17, which is significantly less than the value 1 predicted previously. Quenching is believed to result from a combination of flame stretch and radiative heat loss in the burnt gas. Effects of flame curvature are quantified; the highest OH fluorescence intensities occur when the flamefront is convex towards the reactants, as expected for these flames for which Lewis number <1. These experiments help to quantify regimes for which the local flamelet approach is valid in the modeling of turbulent premixed flames.