Measurement of the instantaneous detailed flame structure in turbulent premixed combustion

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Simultaneous two-dimensional UV-Rayleigh thermometry and 2D laser-induced fluorescence of OH radicals were used to analyze highly turbulent, premixed natural gas/air flames with up to 130-kW heat release, where the turbulence conditions are approximately homogeneous in the measurement area. The turbulent Reynolds number has been varied between 60 and 2500, and the turbulent Karlovitz numbers ( Ka ), between 0.03 and 14. The simultaneously recorded single-shot images resolve the instantaneous structure of turbulent premixed flames to a spatial resolution of about 0.1 mm. For flames with Ka <1, the local flame-front structure (temperature and OH concentration profiles normal to the flame front) is similar to calculations Δ of laminar premixed flames. For flames with 1< Ka <14, the mean thermal flame-front thickness is significantly decreased compared with laminar unstrained calculations of flames with the same stoichiometry. This is contrary to the expectation of thickened flames (Klimov-Williams criterion). The mean flame thickness based on the OH concentration ascend is found to be independent of Ka . Obvious strongly localized deviations from a laminar-like flamelet structure were found for flames with Ka >5. In a few regions, temperatures above 1400 K, but no OH radicals, were observed, indicating regions with local extinction. A comparison of the size of small-scale eddies with the laminar eldovich flame-front thickness and the thermal thickness indicates that these eddies do have much smaller influence than has been expected by the Klimov-Williams criterion.