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

OH-PLIF images of turbulent and non-turbulent premixed flames stabilized in a high-pressure chamber were analyzed for CH₄-air and C₃H₈-air mixtures to investigate the mechanism to determine the smallest scale of flame wrinkles and turbulent burning velocity in a high-pressure environment. Fractal analysis was carried out to study the characteristic of the scale and complexity of the flame wrinkles. 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. At high pressure, a significant correlation existed between the inner cutoff and the characteristic scale of flame instability, i.e., Darrieus-Landau instability combined with diffusive thermal effects. Flame instability of the high-pressure flame without flow turbulence was also observed. The nominal burning velocity enlarged by the flame-area increased due to the flame instability and its variation with pressure was measured using a mean angle method for OH-PLIF images at up to 3 MPa, and the pressure exponent was 0.4. 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 based on these results. Flame instability, which produces small scale wrinkles was significant in a high-pressure environment and overlapped with flame-area increase due to the turbulence, causing larger S_T/S_L. Original is an abstract.