By Graham Goldin and Genong Li, Fluent Inc.

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FLUENT 6.1 is unique in its vast offerings for simulating reacting flow. Many models are available for gas, solid, and liquid fuels, for both gas-phase and surface reactions.

GaAs growth on a rotating substrate and parasitic deposition on reactor walls

For gas-phase combustion modeling, rapid solutions can be obtained using the fast chemistry assumptions in the eddy dissipation (or Magnussen), equilibrium mixture fraction, or premixed models. These models are the work-horses of current combustion simulations and are widely employed. The eddy dissipation model assumes that reactions occur infinitely fast and that the reaction rate is limited by the turbulent mixing rate. The equilibrium mixture fraction model tracks the progress of a mixture fraction and its variance rather than multiple species, and makes use of a PDF function for the turbulence-chemistry interaction. It can include intermediate species and radicals in a reaction system as long as the reactions are fast and those species can be assumed to be in equilibrium. The flamelet model extends the equilibrium mixture fraction model to include finite rate effects due to high flow strain rate.

The recent gas-phase combustion models in FLUENT incorporate full finite-rate chemistry into flame simulations. FLUENT can import detailed kinetic mechanisms in CHEMKIN format, and the use of ISAT (see the articles on pages 6 and 7) now makes the solution of multi-dimensional chemistry simulations affordable. The eddy dissipation concept (EDC) model and the PDF transport model, imple- PDF transport particles colored by temperature for a turbulent jet diffusion flame GaAs growth on a rotating substrate and parasitic deposition on reactor walls mented through a collaboration with Professor Stephen Pope of Cornell, are available to account for turbulence-chemistry interaction. Both models allow for arbitrary complex reaction mechanisms.

Coal combustion begins with the devolatilization of fuel from the coal particle, and much effort has been spent on accurately capturing this process, since it ultimately governs the flame temperature and hence, pollutant formation. The recently introduced chemical percolation devolatilization (CPD) model uses coal structure data based on nuclear magnetic resonance measurements to characterize devolatilization, and predictions of off-gases have agreed well with data. Liquid fuel combustion begins with an accurate description of the spray and its breakup into droplets. Evaporation follows, and subsequently, combustion. Models for all of these processes are available in FLUENT and compatible with the deforming mesh model, so that in-cylinder combustion can be readily simulated.

Surface reaction modeling, used by engineers in the semiconductor industry and for applications such as gas reformers and catalytic converters, involves reactions between species in the gas phase and on a surface. For a deposition process, gaseous species are adsorbed at the surface, where reactions take place. These reactions leave behind deposited surface species and cause the release of other species back into the gas phase. For etching processes, there may be no deposited surface species, only reactions that produce species that are released into the gas. To correctly model both deposition and etching, FLUENT allows for three types of species, which can be either reactants or products: gas species, site species (adsorbed at the surface), and bulk species (left behind on the surface following the reaction).

PDF transport particles colored by temperature for a turbulent jet diffusion flame

Many other reaction models are also available. Those for the prediction of pollutant formation, such as NOx and soot, are widely used and customizable, if desired. Chemical reactions in packed bed reactors can be simulated using either the porous media model or the fixed-bed Eulerian granular multiphase model, with the added option of specifying different reactions in different zones. In addition to the many built-in capabilities, reactions that depend on micromixing or population balance theory can be simulated through user-defined functions.

In summary, reaction modeling is a mature capability in FLUENT that covers a wide range of applications, and the articles on the next several pages present just a sampling of these. More examples can be found on our website, www.fluent.com, or by calling your local Fluent office or distributor.

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