Chapter 13. Introduction to Modeling Species Transport

Chapter13.Introduction to Modeling Species Transport

and Reacting Flows FLUENT provides several models for chemical species transport and chemical reactions.

This chapter provides an overview of the species transport and reaction models available in FLUENT.Details about the models are provided in Chapters14–18.Models for engine ignition and pollutant formation are described in Chapters19and20,respectively.

?Section13.1:Overview of Species and Reaction Modeling

?Section13.2:Approaches to Reaction Modeling

?Section13.3:Choosing a Reaction Model

13.1Overview of Species and Reaction Modeling

FLUENT can model species transport with or without chemical reactions.For information about modeling species transport without chemical reactions,see Section14.4:Species Transport Without Reactions.

Chemical reactions that can be modeled in FLUENT include the following:

?Gas phase reactions that may involve NO x and other pollutant formation,etc.

?Spark and autoignition(Chapter19:Engine Ignition Model)that involve volumetric

and chemical kinetics of reacting?ow.

?Surface reactions(e.g.,chemical vapor deposition)in which the reaction occurs at

a solid(wall)boundary.

?Particle surface reactions(e.g.,coal char combustion)in which the reaction occurs

at the surface of a discrete-phase particle.

Additional information on modeling of droplet/particle reactions is presented in Sec-tion23.3:Heat and Mass Transfer Calculations.

Introduction to Modeling Species Transport and Reacting Flows

13.2Approaches to Reaction Modeling

FLUENT provides?ve approaches to modeling gas phase reacting?ows:

?Generalized?nite-rate model

?Non-premixed combustion model

?Premixed combustion model

?Partially premixed combustion model

?Composition PDF Transport model

A brief overview of each model is provided in Sections13.2.1–13.2.5.

See Section13.3:Choosing a Reaction Model for guidelines on choosing a model.

13.2.1Generalized Finite-Rate Model

This approach is based on the solution of transport equations for species mass fractions.

The reaction rates that appear as source terms in the species transport equations are com-puted from Arrhenius rate expressions.You can use chemical kinetic mechanisms from the FLUENT database,create one yourself,or import a mechanism in Chemkin format.

For turbulent?ows,turbulence-chemistry interaction can be ignored using the Laminar Finite-Rate model,or modeled with the Eddy Dissipation[215]or EDC models[214].

The Generalized Finite-Rate Model is suitable for a wide range of applications including premixed,partially premixed,non-premixed turbulent combustion,and ignition delay in diesel engines(Section19.2:Autoignition Model).FLUENT provides several options to handle the sti?numerics of detailed kinetic mechanism.See Chapter14:Modeling Species Transport and Finite-Rate Chemistry for details.

13.2.2Non-Premixed Combustion Model

In this approach individual species transport equations are not solved.Instead,trans-port equations for one or two conserved scalars(the mixture fractions)are solved and individual component concentrations are derived from the predicted mixture fraction distribution.This approach has been speci?cally developed for the simulation of tur-bulent di?usion?ames and o?ers many bene?ts over the?nite-rate formulation.In the Non-Premixed Combustion Model,turbulence e?ects are accounted for with the help of an assumed shape Probability Density Function(PDF).Reaction mechanisms are not required;species and temperature can be modeled as in chemical equilibrium.Alterna-tively,the steady Laminar Flamelet model can include local?nite-rate kinetic e?ects due to straining by the turbulence.See Chapter15:Modeling Non-Premixed Combustion for details.

13.2Approaches to Reaction Modeling

13.2.3Premixed Combustion Model

This model can be applied to turbulent combustion systems that are of the purely pre-mixed type.In these problems perfectly mixed reactants and burned products are sepa-rated by a?ame front.The“reaction progress variable”is solved to predict the position of this front.The in?uence of turbulence is accounted for by means of a turbulent?ame speed.See Chapter16:Modeling Premixed Combustion for details.

13.2.4Partially Premixed Combustion Model

The partially premixed combustion model has been developed for turbulent reacting ?ows that have a combination of non-premixed and premixed combustion.The mix-ture fraction equations and the reaction progress variable are solved to determine the species concentrations and position of the?ame front,respectively.The knock model (Section19.2:Autoignition Model)demonstrates the use of the concepts discussed in Chapters16and17.

13.2.5Composition PDF Transport Combustion Model

The composition PDF transport model simulates realistic?nite-rate kinetic e?ects in turbulent?ames.Arbitrary chemical mechanisms can be imported into FLUENT,and kinetic e?ects such as non-equilibrium species and ignition/extinction can be captured. This model is applicable to premixed,non-premixed,and partially premixed?ames.Note that the model is computationally expensive.See Chapter18:The Composition PDF Transport Model for details.

Introduction to Modeling Species Transport and Reacting Flows

13.3Choosing a Reaction Model

The?rst step in solving any problem involving species transport and reacting?ow is to determine which model is appropriate.Consider the following guidelines:?For cases involving the mixing,transport,or reaction of chemical species,or re-

actions on the surface of a wall or particle(e.g.,chemical vapor deposition),use

the generalized?nite-rate model.See Chapter14:Modeling Species Transport

and Finite-Rate Chemistry for more information about the generalized?nite-rate

model.

?For reacting systems involving turbulent di?usion?ames that are near chemical

equilibrium where the fuel and oxidizer enter the domain in two or three distinct

streams,use the non-premixed combustion model.See Chapter15:Modeling Non-

Premixed Combustion for more information about non-premixed combustion.

?For cases with a single,perfectly premixed reactant stream,use the premixed com-

bustion model.See Chapter16:Modeling Premixed Combustion for more informa-

tion about premixed combustion.

?For cases involving premixed?ames with varying equivalence ratio in the domain,

use the partially premixed combustion model.See Chapter17:Modeling Partially

Premixed Combustion for more information about partially premixed combustion.

?For turbulent?ames where?nite-rate chemistry is important,use the Laminar

Flamelet model(see Chapter15:Modeling Non-Premixed Combustion),the Lam-

inar Finite-Rate or EDC model(see Chapter14:Modeling Species Transport and

Finite-Rate Chemistry),or the composition PDF Transport model(see Chap-

ter18:The Composition PDF Transport Model).