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Concept
Chemical reaction model
Résumé
Chemical reaction models transform physical knowledge into a mathematical formulation that can be utilized in computational simulation of practical problems in chemical engineering. Computer simulation provides the flexibility to study chemical processes under a wide range of conditions. Modeling of a chemical reaction involves solving conservation equations describing convection, diffusion, and reaction source for each component species.
Ri is the net rate of production of species i by chemical reaction and Si is the rate of creation by addition from the dispersed phase and the user defined source. Ji is the diffusion flux of species i, which arises due to concentration gradients and differs in both laminar and turbulent flows. In turbulent flows, computational fluid dynamics also considers the effects of turbulent diffusivity. The net source of chemical species i due to reaction, Ri which appeared as the source term in the species transport equation is computed as the sum of the reaction sources over the NR reactions among the species.
These reaction rates R can be calculated by following models:
Laminar finite rate model
Eddy dissipation model
Eddy dissipation concept
The laminar finite rate model computes the chemical source terms using the Arrhenius expressions and ignores turbulence fluctuations. This model provides with the exact solution for laminar flames but gives inaccurate solution for turbulent flames, in which turbulence highly affects the chemistry reaction rates, due to highly non-linear Arrhenius chemical kinetics. However this model may be accurate for combustion with small turbulence fluctuations, for example supersonic flames.
The eddy dissipation model or the Magnussen model, based on the work of Magnussen and Hjertager, is a turbulent-chemistry reaction model. Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing. In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly.
Source officielle
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