反應模擬的方法
FLUENT 提供了四種模擬反應的方法：
通用有限速度模型
非預混和燃燒模型
預混和燃燒模型
部分預混和燃燒模型

反應模型的選擇
解決包括組分輸運和反應流動的任何問題，首先都要確定什么模型合適。模型選取的大致方針如下：
通用有限速度模型主要用于：化學組分混合、輸運和反應的問題；壁面或者粒子表面反應的問題（如化學蒸氣沉積）。
非預混和燃燒模型主要用于：包括湍流擴散火焰的反應系統(tǒng)，這個系統(tǒng)接近化學平衡，其中的氧化物和燃料以兩個或者三個流道分別流入所要計算的區(qū)域。
預混和燃燒模型主要用：于單一、完全預混和反應物流動。
部分預混和燃燒模型主要用于：區(qū)域內(nèi)具有變化等值比率的預混和火焰的情況。
  For cases involving premixed flames with varying equivalence ratio in the domain, use the partially premixed combustion model.
  For turbulent flames where finite-rate chemistry is important, use the Laminar Flamelet model , the Laminar Finite-Rate or EDC model , or the composition PDF Transport model .

Generalized 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 computed 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 flows, turbulence-chemistry interaction can be ignored using the Laminar Finite-Rate model, or modeled with the Eddy Dissipation or EDC models.

Non-Premixed Combustion Model
In this approach individual species transport equations are not solved. Instead, transport 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 turbulent diffusion flames and offers many benenits over the finite-rate formulation.

Premixed Combustion Model
This model can be applied to turbulent combustion systems that are of the purely premixed type.

Partially Premixed Combustion Model
The partially premixed combustion model has been developed for turbulent reacting flows that have a combination of non-premixed and premixed combustion.

Modeling Species Transport and Finite-Rate Chemistry
1) Modeling Species Transport:通過求解mass transport equations 獲得各個組分的分布。
2）Finite-Rate Chemistry：
  Laminar nite-rate model: The effect of turbulent fluctuations are ignored, and reaction rates are determined by Arrhenius expressions.
  Eddy-dissipation model: Reaction rates are assumed to be controlled by the turbulence, so expensive Arrhenius chemical kinetic calculations can be avoided. The model is computationally cheap, but, for realistic results, only one or two step
heat-release mechanisms should be used.
  Eddy-dissipation-concept (EDC) model: Detailed Arrhenius chemical kinetics can be incorporated in turbulent flames. Note that detailed chemical kinetic calculations are computationally expensive.

species transport中的湍流與化學反應相互作用模型 1)laminar finite-rate,The laminar finite-rate model computes the chemical source terms using Arrhenius expressions, and ignores the effects of turbulent fluctuations. 2)eddy-dissipation Most fuels are fast burning, and the overall rate of reaction is controlled by turbulent mixing.This is usually acceptable for non-premixed flames, 3) finite-rate/eddydissipation model  in premixed flames, the reactants will burn as soon as they enter the computational domain, upstream of the flame stabilizer. To remedy this, FLUENT provides the finite-rate/eddydissipation model. 4) EDC model The eddy-dissipation-concept (EDC) model is an extension of the eddy-dissipation model to include detailed chemical mechanisms in turbulent flows.In the eddy-dissipation model, every reaction has the same, turbulent rate, and therefore the model should be used only for one-step (reactant--->product), or two-step (reactant---->intermediate, intermediate--->product) global reactions. The model cannot predict kinetically controlled species such as radicals. To incorporate multi-step chemical kinetic mechanisms in turbulent flows, use the EDC model

define material steps：species. It is important that you define the mixture properties before setting any properties for the constituent species, since the species property inputs may depend on the methods you use to define the properties of the mixture. The recommended sequence for property inputs is as follows:
1. Define the mixture species, and reaction(s), and define physical properties for the mixture.
2. Define physical properties for the species in the mixture. Remember to click on the Change/Create button after defining the properties for each species.
the most abundant species should be stay the last in the species list. You will change the order of the species list if you add or remove any species. When this occurs, all boundary conditions, solver parameters, and solution data for species will be reset to the default values. Thus, you should take care to redefine species boundary conditions and solution parameters for the newly defined problem. In addition, you should recognize that patched species concentrations or concentrations stored in any file that was based on the original species ordering will be incompatible with the newly defined problem.

 

Further, to reduce the computational expense of the chemistry calculations, you can increase the number of Flow Iterations
per Chemistry Update.

EDC模型中 volume fraction const 和time scale const的確定：一般默認值是最優(yōu)化的，沒必要修改它們

求解技巧：
1）Two-Step Solution Procedure (Cold Flow Simulation)：Temporarily disable reaction calculations by turning off Volumetric in the Species Model panel and Turn off calculation of the product species in the Solution Controls panel. Then Calculate an initial (cold-flow) solution. Finally solve the full problem
2) One of the main reasons a combustion calculation can have difficulty converging is that large changes in temperature cause large changes in density, which can, in turn, cause instabilities in the flow solution. When you use the segregated solver, FLUENT allows you to under-relax the change in density to alleviate this difficulty. The default value for density under-relaxation is 1, but if you encounter convergence trouble you may wish to reduce this to a value between 0.5 and 1

 

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