Analysis on the fluctuation in the lean burn of methane-aIr Mixtures

This paper deals with the flame shape and fluctuation in the lean burn of methane-air mixtures. Cellular flames are formed on a flat burner at low equivalence ratios, and the cell size becomes larger as the equivaence ratio becomes lower. We measure the fluctuation of brightness of premixed flames, and obtain the RMS and power spectrum density of the brightness fluctuation. With a decrease in the equivalence ratio, the RMS of the brightness fluctuation increases. This is because the diffusive-thermal effect has a destabilizing influence at low equivalence ratios. The power spectrum density has sharp peaks, whose frequency corresponds to the oscillation frequency of premixed flames, and the 1/f spectrum appears in low frequency range. Moreover, we perform the time series analysis on the brightness fluctuation. We obtain the attractor, correlation dimension, Lyapunov exponent to investigate the characteristics of the fluctuation of premixed flames. The characteristics of fluctuation depend strongly on the equivalence ratio, i.e., on the intrinsic instability of premixed flames. The results suggest that the fluctuation analysis is applicable to the diagnostics of flame instability.