Investigation of subgrid-scale mixing of mixture fraction and temperature in turbulent partially premixed flames

The filtered mass density function (FMDF) of mixture fraction and temperature 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 mean of the FMDF conditional on the subgrid-scale (SGS) scalar variance at it given location are found to vary from unimodal to bimodal, cot-responding to quasi-equilibrium distributed reaction zones and laminar flamelets (including extinguished flamelets), respectively. The conditionally filtered mixture fraction dissipation for small SOS variances has a relatively weak dependence on the mixture fraction, and is not sensitive to temperature for extinguished samples. For large SOS variance the large dissipation is concentrated in the cliffs and increases with decreasing temperature. The conditionally filtered temperature dissipation for small SOS variances is the highest for intermediate temperature. For large SGS variance the dependence is more complex and the pilot gas appears to be playing an important role. The different SOS mixture fraction structures for small and large SOS variances, as reflected by the unimodal and bimodal FMDF, have a strong impact on the small-scale mixing and turbulence-chemistry interaction, as reflected by the results for the conditionally filtered temperature dissipation. The results have implications for understanding and modeling multiple reactive scalar SOS mixing. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.