Images of the two-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 and the temperature gradients were imaged in a turbulent jet flame using Planar Rayleigh Scattering. Two types of regions are observed: thin thermal layers in which temperature gradients are large and in which intense thermal mixing occurs, and broad homogeneous thermal zones in which temperature gradients are negligible. Many of the thermal gradient layers appear to be created by vortex motions, since the thin layers are parallel and are rolled up into spiral-shaped patterns. Flame-vortex interactions are shown which result in local flame extinction, and in some cases the vortices appear to penetrate through the viscous flame gases in the radial direction. A distinct cusp-shaped entrainment pattern is observed that is believed to result from counterrotating vortex pairs. The local thermal mixing rate is quantified by the thermal dissipation rate (χ_T), which is deduced from the measured temperature gradients. Profiles of χ_T are compared with a theoretical scaling relation. The thinnest thermal layers were 0.6 mm thick; the sparcity of thin mixing layers, as compared with nonreacting flows at the same jet Reynolds number, is due to the large gas diffusivity associated with flames. The joint probability density function pdf (T, χ_T) of a scalar (temperature) and its gradient was measured within the flame. The joint pdf displays a clipped Gaussian dependence on T and a log-normal dependency on χ_T at all locations except the flame-air boundary, which is dominated by intermittency.