Experimental study of scalar filtered mass density function in turbulent partially premixed flames

The mixture fraction filtered mass density function (FMDF) used in large eddy simulation (LES) of turbulent combustion is studied experimentally using line images obtained in turbulent partially premixed methane flames (Sandia flames D and E). Cross-stream filtering is employed to obtain the FMDF and other filtered variables. The means of the FMDF conditional on the subgrid-scale (SGS) scalar variance at a given location are found to vary from close to Gaussian to bimodal, indicating well-mixed and non-premixed SGS mixing regimes, respectively. The bimodal SGS scalar has a structure (ramp-cliff) similar to the counter-flow model for laminar flamelets. Therefore, while the burden on mixing models to predict the well-mixed SGS scalar is expected to lessen with decreasing filter scale, the burden to predict the bimodal one is not. These SGS scalar structures can result in fluctuations of the SGS flame structure between distributed reaction zones and laminar flamelets, but for reasons different from the scalar dissipation rate fluctuations associated with the turbulence cascade. Furthermore, the bimodal SGS scalar contributes a significant amount of the scalar dissipation in the reaction zones, highlighting its importance and the need for mixing models to predict the bimodal FMDFs. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.