IMAGES OF THE 2-DIMENSIONAL FIELD AND TEMPERATURE-GRADIENTS TO QUANTIFY MIXING RATES WITHIN A NON-PREMIXED TURBULENT JET FLAME

The two-dimensional structure of the instantaneous temperature field a nd the temperature gradients were imaged in a turbulent jet flame usin g Planar Rayleigh Scattering. Two types of regions are observed: thin thermal layers in which temperature gradients are large and in which i ntense thermal mixing occurs, and broad homogeneous thermal zones in w hich temperature gradients are negligible. Many of the thermal gradien t layers appear to be created by vortex motions, since the thin layers are parallel and are rolled up into spiral-shaped patterns. Flame-vor tex interactions are shown which result in local flame extinction, and in some cases the vortices appear to penetrate through the viscous fl ame gases in the radial direction. A distinct cusp-shaped entrainment pattern is observed that is believed to result from counterrotating vo rtex pairs. The local thermal mixing rate is quantified by the thermal dissipation rate (($) over right arrow X(T)), which is deduced from t he measured temperature gradients. Profiles of ($) over right arrow ch i(T) are compared with a theoretical scaling relation. The thinnest th ermal layers were 0.6 mm thick; the sparcity of thin mixing layers, as compared with nonreacting flows at the same jet Reynolds number, is d ue to the large gas diffusivity associated with flames. The joint prob ability density function pdf(T, chi(T)) of a scalar (temperature) and its gradient was measured within the flame. The joint pdf displays a c lipped Gaussian dependence on T and a log-normal dependency on chi(T) at all locations except the flame-air boundary, which is dominated by intermittency.