Characterization of the local flame structure and the flame surface density for freely propagating premixed flames at various Lewis numbers

The local flume structure and the flame surface density for freely propagating premixed turbulent flames are measured by laser sheet tomography and image processing techniques. Different fuel/air mixtures (methane, propane. and hydrogen) have been successively spark-ignited at atmospheric pressure in order to evaluate the influence of thermodiffusive effects characterized by the Lewis number, at quasi-constant ratio u'/S-L(0). From the instantaneous flame front positions and the mono-dimensional mean progress variable distribution, spatial statistics including the turbulent flame brush, the mean orientation factors, and the components of the normal vector to the flame front have been characterized and compared with numerical and experimental results available in the Literature. The temporal evolution of the flame surface density was also determined by several approaches and analyzed in terms of maximum flame surface density evolution and location of maximum flame surface density. It clearly appears that the maximum flame surface density decreases as the flame propagates and remains independent of the Lewis number in the (C) space. The influence of the turbulence level u'/S-L(0) on the maximum flame surface density is also presented and compared with various results available in the literature. The peak location is a little less than the symmetrical value (C) = 0.5, and increases slightly as the flame propagates. An estimation of the Bray number, N-B. which characterizes the nature of the diffusion by turbulence (gradient or counter-gradient). is also proposed in order to characterize the turbulent flux of <C> (or Sigma).