Computational Fluid Dynamics Laboratory

Investigation of Extinction and Reignition Phenomena in Turbulent Non-premixed Flames using PDF Methods

Joint PDF model in homogenous turbulence

Mixing models for closure of the scalar joint probability density function (jPDF) in Reynolds-Averaged Navier-Stokes (RANS) or the scalar joint filtered density function (jFDF) in Large Eddy Simulation (LES) often employ a constant mixing frequency for all scalars and do not account for the effects of chemical reaction. This study compares the performance of several different mixing models with regard to their ability to predict three-stream mixing and nonpremixed flame extinction/reignition dynamics in homogeneous turbulence. The mixing models considered include Interaction by Exchange with the Mean (IEM), the Modified Curl (MC), the Euclidean Minimum Spanning Tree (EMST), and the method of Multiple Mapping Conditioning (MMC) (the latter only in the context of pure mixing). The jPDF predictions from the different mixing models are compared to results obtained from Linear Eddy Model (LEM) simulations.

Below is an animation of (clockwise from left) (1) particles in a box colored by temperature (2) joint PDF of reactants (3) joint PDF of reactant and product (4) Evolution of scalar means and (5) evolution of PDF conditioned on stoichiometric.



LES/FMDF of idealized non-premixed flames

The next part of the study investigates the effect of using the same three subgrid mixing models on the dynamics of flame extinction and reignition, using a hybrid LES and Filtered Mass Density Function (FMDF) approach. An idealized, piloted non-premixed jet flame with a one-step exothermic reaction F + rOx -- (1+r)P is studied with chemical kinetics parameters chosen to result in local extinction and reignition for the given Reynolds number. The relative performance of the mixing models is considered regarding extinction/reignition dynamics through examination of instantaneous flame images and scatter plots, as well as relevant statistical measures.

Below is an animation of the temperature and reaction rate contours in an idealized turbulent flame with the white circle indicating the region of localized extinction.


Modeling realistic flames using LES in FLUENT

Below is an animation of temperature contours of the Sandia flame D [1] using the partially premixed combustion model is FLUENT. flame images

[1] Barlow, R. S., and Frank, J.H., Proc. Combust. Inst., 27: 1087, (1998).

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