Chapter 12. Introduction to Modeling

Species Transport and Reacting Flows

FLUENT provides several models for chemical species transport andchemical reactions. This chapter provides an overview of the speciestransport and reaction models available in FLUENT. Details about themodels are provided in Chapters 13{16. Models for pollutant formationare described in Chapter 17.

Section 12.1: Overview of Species and Reaction Modeling Section 12.2: Approaches to Reaction Modeling Section 12.3: Choosing a Reaction Model

12.1 Overview of Species and Reaction Modeling

FLUENT can model species transport with or without chemical reactions.For information about modeling species transport without chemical re-actions, see Section 13.4.

Chemical reactions that can be modeled in FLUENT include the follow-ing:

Gas phase reactions that may involve NOx and other pollutantformation, etc.

Surface reactions (e.g., chemical vapor deposition) in which thereaction occurs at a solid (wall) boundary.

Particle surface reactions (e.g., coal char combustion) in which thereaction occurs at the surface of a discrete-phase particle.

Additional information on modeling of droplet/particle reactions is pre-sented in Section 19.3.

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Introduction to Modeling Species Transport and Reacting Flows

12.2 Approaches to Reaction Modeling

FLUENT provides four approaches to modeling reacting flows:

Generalized nite-rate model Non-premixed combustion model Premixed combustion model Partially premixed combustion model

A brief overview of each model is provided in Sections 12.2.1{12.2.4. SeeSection 12.3 for guidelines on choosing a model.

12.2.1 Generalized Finite-Rate Model

This approach is based on the solution of transport equations for speciesmass fractions, with the chemical reaction mechanism dened by you.The reaction rates that appear as source terms in the species trans-port equations are computed from Arrhenius rate expressions, from theeddy dissipation model of Magnussen and Hjertager [149], or from theEDC model [148]. Models of this type are suitable for a wide range ofapplications including premixed, partially premixed, and non-premixedcombustion. See Chapter 13 for details.

12.2.2 Non-Premixed Combustion Model

In this approach individual species transport equations are not solved.Instead, transport equations for one or two conserved scalars (the mix-ture fractions) are solved and individual component concentrations arederived from the predicted mixture fraction distribution. This approachhas been specically developed for the simulation of turbulent diusionflames and oers many benets over the nite-rate formulation. In theconserved scalar approach, turbulence eects are accounted for with thehelp of a probability density function or PDF. Reaction mechanisms arenot dened by you; instead the reacting system is treated using a flamesheet (mixed-is-burned) approach or chemical equilibrium calculations.See Chapter 14 for details.

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12.3 Choosing a Reaction Model

The laminar flamelet model is an extension of the non-premixed com-bustion model to account for aerodynamic strain-induced departure fromchemical equilibrium. See Section 14.4 for details.

12.2.3 Premixed Combustion Model

This model has been specically developed for combustion systems thatare of the purely premixed type. In these problems perfectly mixedreactants and burned products are separated by a \flame front". The\reaction progress variable" is solved to predict the position of this front.The influence of turbulence is accounted for by means of a turbulentflame speed. See Chapter 15 for details.

12.2.4 Partially Premixed Combustion Model

The partially premixed combustion model has been developed for sys-tems that have a combination of non-premixed and perfectly premixedcombustion. The mixture fraction equations and the reaction progressvariable are solved to determine the species concentrations and positionof the flame front, respectively. See Chapter 16 for details.

12.3 Choosing a Reaction Model

The rst step in solving any problem involving species transport andreacting flow is to determine which model is appropriate. Consider thefollowing guidelines:

For cases involving the mixing, transport, or reaction of chemicalspecies, or reactions on the surface of a wall or particle (e.g., chem-ical vapor deposition), use the generalized nite-rate model. SeeChapter 13 for more information about the generalized nite-ratemodel.

For reacting systems involving turbulent diusion flames that arenear chemical equilibrium where the fuel and oxidizer enter thedomain in two or three distinct streams, use the non-premixedcombustion model. See Chapter 14 for more information aboutnon-premixed combustion.

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Introduction to Modeling Species Transport and Reacting Flows

For cases with a single, perfectly premixed reactant stream, use thepremixed combustion model. See Chapter 15 for more informationabout premixed combustion.

For cases involving premixed flames with varying equivalence ratioin the domain, use the partially premixed combustion model. SeeChapter 16 for more information about partially premixed com-bustion.

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