Experimental investigation of three-dimensional flame-front structure in premixed turbulent combustion: II. Lean hydrogen/air Bunsen flames

Abstract

Comprehensive laser measurements of three-dimensional flame-front structures for turbulent lean hydrogen/air premixed Bunsen flames are reported in this continuation paper. The local scalar front appears lamella-like for both the reaction progress variable and the OH mole fraction. This lamella-like feature cannot be predicted by the commonly accepted combustion-regime diagrams. The flame residence time relevant to the turbulent flames investigated here may be much smaller than the unstretched laminar value used for constructing these regime diagrams. Superadiabaticy and flame-front bulges convex toward the reactants are clearly observed, representing effects of the less-than-unity Lewis number of the mixture. The average size of flame bulges is found to increase with the turbulence integral length scale. Moderate positive correlations exist between the in-plane two-dimensional curvature and the three-dimensional progress-variable gradient. OH mole fraction is also correlated with the progress-variable gradient. Local flame orientation in three-dimensional space is close to an isotropic distribution, which is attributed to flame-surface wrinkling being strongly nonpassive. More backward-facing flame fronts are formed nearer the unburnt than the burnt side of the turbulent flame brush, suggesting the importance of large-scale flame-front bulges in turbulent flame propagation. Higher progress-variable dissipation rates are measured than for the unstretched laminar flame. By comparison with previously published data for turbulent hydrocarbon/air premixed flames, the present dissipation-rate measurements suggest that chemical reactions do not play an important role in the destruction or generation of progress-variable fluctuations. Discrepancies are observed even in the qualitative trends of some statistics of the progress-variable dissipation rate when compared with DNS data modeled with detailed chemistry.