| A theoretical and experimental investigation of a premixed turbulent flame is described. The objective was to measure certain flame surface properties and to develop and to assess a new statistical method to numerically simulate the instantaneous flame surface structure. In addition, measurements were made to assess the effects of preferential diffusion on high Reynolds number flames.;Measurement techniques that were used included Rayleigh-scattering to determine the mean and fluctuating reaction progress variable, unconditional and conditional laser velocimetry to characterize the turbulence properties within the flames, and laser light sheet imaging to characterize flame surface properties. Flame surface properties were numerically simulated using a two-dimensional flame propagation algorithm combined with a statistical time series simulation of unburned gas velocities along the flame surface. It was found that flame surface area (and thus the local-turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Preferential diffusion reduced the rate of increase of these properties for stable conditions (i.e., fuel-equivalence ratios greater than 1.8) and increased the rate of increase of these properties for unstable conditions. Simulated flame properties duplicated the measured trends but underestimated the effects of turbulence because the present simulation is two dimensional and significant three dimensional effects occur in the experiment. However, the three dimensional effects are understood, and future extension of the method to a three-dimensional simulation, to remove 2-D deficiencies, appears to be computationally feasible for high Reynolds number conditions of interest for practical applications. |
