Time scales of the scalar field in turbulent premixed conical flames

Boukhalfa A., GÖKALP İ.

Symposium (International) on Combustion, cilt.22, sa.1, ss.755-761, 1989 (Scopus)

Özet

The temporal characteristics of five conical methane-air turbulent premixed flames are investigated by one-point laser induced Rayleigh scattering technique. They correspond to combinations of three exit Reynolds numbers and three equivalence ratios and are representative of the wrinkled flame regime, the corrugated flame regime and the thickened flame regime of premixed turbulent combustion. The density pdfs are explored to assess the structural differences between these flames. It is found that the constant levels of the intermediate burning states in the density pdfs decreases with increasing large eddy Damköhler number and reach a plateau value for the highest Damköhler number flames. These levels increase with u′/SL. The scalar field time statistics of these flames are investigated through the density spectra, flamelet crossing frequencies and the integral time scale of the density field. This investigation shows that the flame closest to the wrinkled flame model has a singular behaviour, whilst the four others behave very similarly. For a constant exit Reynolds number, temporal statistics of the scalar field do not depend on the equivalence ratio, while, for a constant equivalence ratio, they strongly depend on the cold flow characteristics. In particular, the flamelet crossing frequencies increase with the mean exit velocity, whilst the integral time scale of the scalar field decreases. In order to provide some modelling ideas, the relationships between the mean flamelet passage time and the integral time scale of the cold flow, on the one hand, and the most energetic and the most dissipative frequency ranges of the density spectra and the maximum value of the flamelet crossing frequency, on the other hand, are explored. © 1997 Combustion Institute.