Measurement of flame surface density for turbulent premixed flames using PLIF and DNS

Results for flame surface density (FSD) in premixed turbulent flame kernels have been obtained from OH planar laser induced fluorescence (PLIF) and direct numerical simulations (DNS), and have been compared for similar values of global Lewis number and normalised turbulence intensity. Stoichiometric methane-air and lean hydrogen-air mixtures were studied, and the same post-processing techniques were employed for both experimental and DNS data in order to evaluate FSD statistics from spatial gradients of the reaction progress variable. Full 3D FSD statistics were obtained from the DNS data sets. Also, FSD statistics were obtained from two-dimensional cross-sections extracted from the DNS data sets which were found to be in qualitative agreement with the FSD statistics of PLIF data. The location of maximum FSD within the flame was found to be close to the middle of the flame brush for both methane-air and hydrogen-air flames, and was found to be slightly skewed about the middle of the flame brush for some methane-air flames. The PLIF data for both fuels showed a decrease in the maximum FSD with increasing turbulence intensity. This effect was not observed in the three-dimensional DNS analysis for methane-air flames, but was found to be consistent with both two-dimensional and three-dimensional analysis of the DNS data for hydrogen-air flames. The findings have been compared with the results of other experimental and DNS work reported in the literature and mechanisms have been suggested to explain the observed behaviour. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.