Experimental Study of Velocity-Scalar Filtered Joint Density Function for Improving Large-Eddy Simulation of Turbulent Combustion

摘要

The filtered mass density function (FMDF) of mixture fraction and other filtered variables used in large-eddy simulation of turbulent combustion were studied using measurement data obtained in turbulent partially premixed methanelair (Sandia) flames. For subgrid-scale (SGS) scalar variance small compared to its mean, the FMDF was not far from Gaussian, and the SGS scalar was well mixed. For large SGS variance, the FMDF became bimodal, and the conditionally filtered scalar dissipation was bell-shaped, indicating the existence of a diffusion (dissipation) layer structure, which was similar to the mixture fraction profile in the counter-flow model for laminar flamelets. The conditionally filtered temperature near the stoichiometric mixture fraction decreased progressively with increasing SGS scalar variance. Local extinction events appeared to occur mostly when the SGS scalar variance was large, suggesting the possibility of flamelet extinction. The results suggested that the mixing regimes and the associated mixture fraction structure could potentially have strong influences on the combustion regime and extinction/reignition in turbulent non-premixed flames.