Direct comparison of turbulent burning velocity and flame surface properties in turbulent premixed flames

Direct comparison of the turbulent burning velocity (obtained from flame speeds) to the flame perimeter ratio has been made in turbulent premixed flames propagating freely downward for propane/air mixtures at various equivalence ratios, with u′/S_L of ranging from 1.4 to 5.3. The turbulent flame speed ranged from 2.6 to about 7 times the laminar flame speed at high turbulence intensities, while the flame perimeter ratio ranges from 1.4 to 3.3. In the current freely propagating flames, the global flame curvature can lead to an enhancement of the flame speed by a factor of up to 3.5. This global flame curvature is attributable to the wall heat loss in the current burner configuration, and flame brush thickness has been used as a measure of the global flame curvature. For flames involving coupling of the globally curved flame geometry with flow divergence or any flow non-uniformity, correcting for this geometrical effect requires a careful consideration of the flame topology and flow field. The difference between the observed flame speed and the 2-D flame perimeter ratio, after correcting for the global flame curvature effect, is attributed to the fact that the flame wrinkles in three-dimensions are associated with a larger flame surface area than that determined from the flame perimeter ratio data. This also points to a need to better understand the 3-D geometrical effects including the global flame curvature and the local flame wrinkle structure in turbulent premixed flames. The observed turbulent flame speed data for the most part follow the flame speed models of Bray and Damkohler, wherein the flame surface area increase is modeled as a function of turbulence and thermochemical properties. The above results, taken together, indicate that the fundamental assumption that the turbulent flame speed depends primarily on the increased flame surface area is valid. This concept can be used to estimate the turbulent flame speed within reasonable accuracy provided that the 3-D flame effects associated with the global flame curvature and local flame wrinkle structure are considered.