A comparison between dynamic and scalar timescales in lean premixed turbulent flames

A comparison between the dynamic and scalar timescales in turbulent premixed flames is presented. Methane-air turbulent Bunsen-type flames are used in this study. They are all lean flames and basically in the flamelet regime. Laser Doppler velocimetry (LDV), Mie scattering and Rayleigh scattering techniques allowed the determination of dynamic (both conditional and nonconditional) and scalar (density) timescales. For both dynamic and scalar temporal scales, we used the autocorrelation function of the LDV signal and the Rayleigh scattering signal, respectively. The main results of the paper indicate that there are significant differences between the conditional (in the reactants) dynamic timescales and those evaluated from nonconditional velocity measurements. The nonconditional velocity timescales are much closer to the scalar timescales, which indicates that they are strongly influenced by the flame front dynamics. Therefore, to evaluate the turbulent premixed flame regimes, only turbulence parameters based on conditional measurement are meaningful, as the nonconditional ones incorporate part of the flame response to the turbulence structure in the reactants. Comparisons between scalar and dynamic timescales, as well as between integral and dissipation timescales, are made. They can be useful in turbulent premixed combustion models for the mean reaction term. In general, the results presented here indicate a complex effect of the u′/S_L, ratio, which seems to depend on the way this parameter is varied.