The effect of strain rate on the temperature field structure in a turbulent non-premixed flame using planar Rayleigh scattering.

Abstract

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 counter-rotating vortex pairs. The local thermal mixing rate is quantified by the thermal dissipation rate which is deduced from the measured temperature gradients. Profiles of the mean thermal dissipation rate were compared to a theoretical scaling relation of Peters. The thinnest thermal layers were 0.6 mm thick. The joint probability density function (PDF) of a scalar (temperature) and its gradient was measured within the flame. The joint PDF displays a clipped Gaussian dependence on temperature and a log-normal dependence on thermal dissipation rate at all locations except the mean flame-air boundary, which is dominated by intermittency. The two-dimensional structure of the instantaneous mixture fraction field and the scalar dissipation rate field for a propane jet have been imaged at the flame liftoff position using planar Rayleigh scattering of turbulent non-reacting propane jets. Thin high gradient regions were observed and the flammable zone thickness was less than 115 $\mu$m. Large scale, vortex-like structures appear to cause a thinning of the mixing layer downstream of the vortex structure and a broadening of the gradient upstream of the structure. This observation is consistent with flame-vortex models, which predict a similar thinning and broadening of the flammable zone due to extensive and compressive strain, respectively.