Flame instability effects on the smallest wrinkling scale and burning velocity of high-pressure turbulent premixed flames

To explore the mechanism determining the smallest scale of flame wrinkles and turbulent burning velocity in a high-pressure environment, OH planar laser-induced fluorescence (PLIF) images of turbulent and non-turbulent premixed flames stabilized in a high-pressure chamber were analyzed for CH₄/air and C3H8/air mixtures. Fractal analysis was performed to investigate the characteristics of the scale and complexity of the flame wrinkles. It was found that fractal dimension increased with increasing u'/S_L for the whole pressure range in the experiments. The increase in the dimension was rapid at higher pressure. The fractal inner cutoff decreased with u'/S_L and pressure. However, at high pressure, the variation of the fractal inner cutoff with u'/S_L was very small, showing that the inner cutoff is almost constant over the wide range of u'/S_L. Comparison between the inner cutoff and various characteristic scales of turbulent flames was made. It was proved that, at high pressure, a significant correlation exists between the inner cutoff and the characteristic scale of flame instability, that is, Darrieus-Landau instability combined with diffusive thermal effects, where the characteristic instability scale is defined based on the wavenumber at the maximum growth rate of flame disturbances. Flame instability of the high-pressure flame without flow turbulence also was observed. The nominal burning velocity enlarged by the flame-area increase due to the flame instability and its variation with pressure was measured using a mean angle method for OH-PLIF images at pressure up to 3.0 MPa, and the pressure exponent was found to be 0.4. Based on these results, a concept to explain the pressure effects that appeared in the general correlation of the turbulent burning velocity obtained by Kobayashi et al. was proposed. That is, flame instability which produces small-scale wrinkles is significant in a high-pressure environment and overlaps with flame-area increase due to the turbulence, causing larger S_T/S_L.