Experimental investigation of the inner structure of premixed turbulent methane/air flames in the thin reaction zones regime

The thermal flame front thickness of premixed turbulent methane/air flames stabilized on a Bunsen-type burner was investigated experimentally. The instantaneous velocity and temperature fields were measured using the particle image velocimetry and Rayleigh scattering techniques, respectively. The Karlovitz number was varied from 1.2 to 20.7, indicating that the measured data are located within the thin reaction zones regime. The normalized preheat zone and reaction zone thicknesses decreased with increasing non-dimensional turbulence intensity in ultra-lean premixed turbulent flames under a constant equivalence ratio of 0.6, whereas they increased with increasing equivalence ratios from 0.6 to 1.0 under a constant bulk flow velocity. These normalized thicknesses were higher than unity for equivalence ratios ranging from 0.8 to 1.0, indicating that the internal structure of the flame fronts deviates from the thin laminar flamelet assumption. However, the mean widening for progress variable contours of premixed turbulent flames in comparison with corresponding values of unstrained premixed laminar flames was insignificant, implying that the influence of turbulent eddies on the flame front broadening is negligible. The normalized preheat zone and reaction zone thicknesses showed no overall trend with increasing non-dimensional longitudinal integral length scale. The normalized preheat zone and reaction zone thicknesses decreased by increasing the Karlovitz number, suggesting that increasing the total stretch rate is the controlling mechanism in the reduction of flame front thickness for the experimental conditions studied in this work. The probability density functions of the progress variable in front of the preheat layers were found to be insensitive to an increase in the non-dimensional turbulence intensity.