Preferential Diffusion Effects on the Surface Structure of Turbulent Premixed Hydrogen/Air Flames

Abstract

An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. 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. Finally, preferential diffusion reduced/increased the rate of increase of these properties for stable/unstable conditions. Taken together, these observations imply that models and correlations of turbulent premixed flame properties for turbulent jet flames must account for effects of flame development, e.g., distance from the flameholder or the point of ignition, and preferential diffusion, in order to adequately characterize flame properties.