Modeling soot formation in turbulent methane–air jet diffusion flames

A filtered-laminar-flame PDF sub-grid scale closure for LES of premixed turbulent flames. Part I: Formalism and application to a bluff-body burner with differential diffusion

The influence of the turbulence–chemistry interaction (TCI) for n-heptane sprays under diesel engine conditions has been investigated by means of computational fluid dynamics (CFD) simulations. The conditional moment closure approach, which has been previously validated thoroughly for such flows, and the homogeneous reactor (i.e. no turbulent combustion model) approach have been compared, in view of the recent resurgence of the latter approaches for diesel engine CFD. Experimental data available from a constant-volume combustion chamber have been used for model validation purposes for a broad range of conditions including variations in ambient oxygen (8‑21% by vol.), ambient temperature (900 and 1000 K) and ambient density (14.8 and 30 kg/m3). The results from both numerical approaches have been compared to the experimental values of ignition delay (ID), flame lift-off length (LOL), and soot volume fraction distributions. TCI was found to have a weak influence on ignition delay for the conditions simulate...

Influence of turbulence–chemistry interaction for n-heptane spray combustion under diesel engine conditions with emphasis on soot formation and oxidation

A finite volume Conditional Moment Closure (CMC) formulation has been developed as an LES sub-grid combustion model. This allows unstructured meshes to be used for both LES and CMC grids making the method more applicable to complex geometry. The method has been applied to an oxy-fuel jet flame. This flame offers new challenges to combustion modelling due to a high CO 2 content in the oxidiser stream and significant H 2 content in the fuel stream. The density ratio of the two streams is of the order 5 and the viscosity of the two streams will also differ. All the flames simulated showed localised extinction in the region around 3–5 jet diameters downstream of the nozzle, which is in very good agreement with the experiment. Trends for conditional and unconditional statistics with changing levels of H 2 in the fuel are correctly captured by the LES–CMC method, although different levels of agreement are observed for different species and temperature and possible reasons for this are discussed. The degree of extinction is also correctly predicted to increase as the H 2 content of the jet is reduced, showing the ability of the CMC method to predict complex turbulence-chemistry interaction phenomenon in the presence of changing fuel composition.

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Numerical Simulation of Oxy-Fuel Jet Flames Using Unstructured LES-CMC

Conditional moment closure prediction of soot formation in turbulent, nonpremixed ethylene flames

The modelling of differential diffusion in nonpremixed reacting turbulent flows is considered. The present approach is based on the conditional moment closure (CMC) method. As in the nonreacting case the terms involving the deviations from the conditional averages, the eyterms, cannot be neglected and need modelling. The development of new closures for the eyterms and their validation is aided by DNS studies. Non-unity Schmidt number effects of reacting scalars in homogeneous iso-tropic decaying turbulence are investigated. Chemical kinetics are approximated by irreversible one-step temperature independent and temperature dependent reactions. It is found that the ey-terms and their modelling in reacting flows strongly resemble ey-terms and -models in nonreacting flows (Kronenburg and Bilger, 1997). However, a new definition of a suitable differential diffusion variable is necessary for accurate enclosure in reacting flows. The dependence of the ey-modelling on reaction rate and temperature effects is anal...

A. Kronenbljrg

Papers

Modelling Differential Diffusion in Nonpremixed Reacting Turbulent Flow: Model Development