燃烧模型

1）非预混燃烧模型
非预混方法仅能用于当反应流动系统满足几个要求时。第一、流动是湍流。第二、反应系统包括一个燃料流、一个氧化剂流，并且随意包括一个次要流（另外一个燃料或氧化剂，或者一个非反应流）。最后、化学动力学必须迅速以使流动接近化学平衡。
2) 非预混燃烧模型的限制
 The chemical system must be of the diffusion type with discrete fuel and oxidizer inlets (spray combustion and pulverized fuel flames may also fall into this category).
 The Lewis number must be unity. (This implies that the diffusion coefficients for all species and enthalpy are equal, a good approximation in turbulent flow).
 When a single mixture fraction is used, the following conditions must be met:
1) Only one type of fuel is involved. The fuel may be made up of a burnt mixture of reacting species (e.g., 90% CH4 and 10% CO) and you may include multiple fuel inlets. The multiple fuel inlets must have the same composition, however. Two or more fuel inlets with different fuel composition are not allowed (e.g., one inlet of CH4 and one inlet of CO). Similarly, in spray combustion systems or in systems involving reacting particles, only one oil-gas is permitted.
2) Only one type of oxidizer is involved. The oxidizer may consist of a mixture of species (e.g., 21% O2 and 79% N2) and you may have multiple oxidizer inlets. The multiple oxidizer inlets must, however, have the same composition. Two or more oxidizer inlets with different composition are not allowed (e.g., one inlet of air and a second inlet of pure oxygen).
 When two mixture fractions are used, three streams can be involved in the system. Valid systems are as follows:
1) Two fuel streams with different compositions and one oxidizer stream. Each fuel stream may be made up of a mixture of reacting species (e.g., 90% CH4 and 10% CO). You may include multiple inlets of each fuel stream, but each fuel inlet must have one of the two de ned compositions (e.g., one inlet of CH4 and one inlet of CO).
2) Mixed fuel systems including gas-liquid, gas-coal, or liquid-coal fuel mixtures with a single oxidizer. In systems with a gas-coal or liquid-coal fuel mixture, the coal volatiles and char are treated as a single composite fuel stream.
3) Coal combustion in which volatile and char oil-gases are tracked separately.
4) Two oxidizer streams with di erent compositions and one fuel stream. Each oxidizer stream may consist of a mixture of species (e.g. 21% O2 and 79% N2). You may have multiple inlets of each oxidizer stream, but each oxidizer
inlet must have one of the two defined compositions (e.g., one inlet of air and a second inlet of pure oxygen).
5) A fuel stream, an oxidizer stream, and a non-reacting secondary stream.
 The flow must be turbulent.

You should use the non-adiabatic modeling option if your problem definition in FLUENT
will include one or more of the following:
 radiation or wall heat transfer
 multiple fuel inlets at different temperatures
 multiple oxidant inlets at different temperatures
 liquid fuel, coal particles, and/or heat transfer to inert particles

如果PDF反应模型中将包括下列情形之一，则要启动次要流：
 两种不相似的气体燃料流：在这些模拟中，燃料流定义为燃料中的一种，次要流定义为第二种燃料。
 不相似气体和液体燃料的混合燃料系统：在这些模拟中，燃料流定义为气体燃料，次要流定义为液体燃料（或者，反之亦然）。
 不相似气体燃料和煤燃料的混合燃料系统：在这些模拟中，燃料流必须定义为煤，次要流必须定义为气体燃料。见14.3.5节，用非预混燃烧
模型模拟煤燃烧。
 煤和液体燃料的混合燃料系统：在这些模拟中，燃料流必须定义为煤，次要流必须定义为液体燃料。见14.3.5，用非预混燃烧模型模拟煤燃烧。
 煤燃烧：使用次要流可以更精确地模拟煤燃烧。燃料流必须定义为焦炭，次要流必须定义为煤的挥发分。见14.3.5，用非预混燃烧模型模拟
煤燃烧。
 单一燃料和两个不相似的氧化剂流：在这些模拟中，燃料流定义为燃料，氧化剂流定义为氧化剂中的一种，次要流定义为第二种氧化剂。

与其他模型的比较
In many reacting systems, the combustion is not in chemical equilibrium. FLUENT offers several approaches to model chemical non-equilibrium, including the finite-rate , EDC , and PDF transport models, where detailed kinetic mechanisms can be incorporated. There are two approaches in the non-premixed combustion model to simulate chemical non-equilibrium. One is the Rich Flammability Limit (RFL) option, where rich regions are modeled as a mixed but unburnt mixture of Modeling Non-Premixed Combustion pure fuel and a leaner equilibrium burnt mixture. The other approach is the Laminar Flamelet model, where chemical non-equilibrium due to turbulent flame stretching is included.

非平衡的处理：
When the chemical time-scale is comparable to the fluid convection time-scale, the species can be considered to be in global chemical non-equilibrium. Such cases include NOx formation and low-temperature CO oxidation.

Rich Flammability Limit (RFL) Option:A value of 1.0 for the rich limit implies that equilibrium calculations will be performed over the full range of mixture fraction. When you use a rich limit that is less than 1.0, equilibrium calculations are suspended whenever f, ffuel, or fsec exceeds the limit. For the Secondary Stream, the rich flammability limit controls the equilibrium calculation for the secondary mixture fraction. For fuel streams, an RFL value of approximately twice the stoichiometric mixture fraction is appropriate. If your secondary stream is not a fuel, you should use and RFL value of 1. For a secondary fuel stream, you can consider modifying the value to use the RFL model. A value of 1.0 for the rich limit implies that equilibrium calculations will be performed over the full range of mixture fraction. When you input a rich limit that is less than 1.0, equilibrium calculations are suspended whenever fsec exceeds the limit. (Note that it is the secondary mixture fraction fsec and not the partial fraction psec that is used here.)

燃料进口温度的确定：
Fuel is the temperature of the fuel inlet in your model. In adiabatic simulations, this input (together with the oxidizer inlet temperature) determines the inlet stream temperatures that will be used by FLUENT. In non-adiabatic systems, this input should match the inlet thermal boundary condition that you will use in FLUENT (although you will enter this boundary  ondition again in the FLUENT session). If your FLUENT model will use liquid fuel or coal combustion, define the inlet fuel temperature as the temperature at which vaporization/ devolatilization begins . For such non-adiabatic systems, the inlet temperature will be used only to adjust the look-up table grid (e.g., the discrete enthalpy values for which the look-up table is computed). Note that if you have more than one fuel inlet, and these inlets are not at the same temperature, you must define your system as non-adiabatic. In this case, you should enter the fuel inlet temperature as the value at the dominant fuel inlet.

boundary 中species的输入：The empirical fuel option provides an alternative method for defining the composition of the fuel or secondary stream when the individual species components of the fuel are unknown.
1）燃料中含S元素的处理
If you wish to include the sulfur that may be present in a hydrocarbon fuel, note that this may hinder the convergence of the equilibrium solver, especially if the concentration of sulfur is small. It is therefore recommended that you include sulfur in the calculation only if it is present in considerable quantities.
2）In addition to gaseous species, solid and liquid species can be included in the chemistry calculations. They are often indicated by an "<l>" or an "<s>" in parentheses after the species name. its density is calculated by FLUENT's chemical property database file propdb.scm. if you are using the thermodynamic database file thermo.db that is also supplied with FLUENT. If you are using a custom thermodynamic database file and want to include a condensed species in the equilibrium system that does not exist in propdb.scm, a density of 1000 kg/m3 will be assumed. The condensed species density can be changed in the Materials panel after the PDF table has been calculated. If you modify the condensed species density in this manner, you will then need to recalculate the PDF table.
3)If the fuel stream was defined as a mixture of components, you should select the largest of these components as the evaporating species". FLUENT will ensure that the mass evaporated from the liquid droplet enters the gas phase as a source of the fuel mixture that you defined. The evaporating species you select here is used only to compute the diffusion controlled driving force in the evaporation rate.

pdf求解参数：
Number of Mean Mixture Fraction Points：Increasing the number of points will yield a more accurate PDF shape, but the calculation will take longer.
Number of Mixture Fraction Variance Points:Lower resolution is acceptable because the variation along the f2' axis is, in general, slower than the variation along the f axis of the look-up tables.
Number of Secondary Mixture Fraction Points：A larger number of points will give a more accurate shape for the PDF, but with a longer calculation time.
Maximum Number of Species：The maximum number of species that can be included is 100
Number of Mean Enthalpy Points：The number of points required will depend on the chemical system that you are considering, with more points required in high heat release systems
Minimum Temperature：The minimum temperature should be set 10{20 K below the minimum system temperature.
Include Equilibrium Flamelet：This option is available when generating or importing multiple flamelets, as well as when a single flamelet is considered. This option is not available with the equilibrium chemistry model.

媒燃烧的注意点：
(Using two mixture fractions to model coal combustion is more accurate than using one mixture fraction as the volatile and char streams are modeled separately. However, the two-mixture-fraction model incurs significant
additional computational expense since the multi-dimensional PDF integrations are performed at run-time.)

When coal is used with another (gaseous or liquid) fuel of different composition, you must model the coal with one mixture fraction and use a second mixture fraction to represent the second (gaseous or liquid) fuel. The stream associated with the
coal composition is defined as detailed below for single-mixture-fraction models.

Moisture in the coal can be considered by adding it in the fuel composition as liquid water, H2O(l). The moisture can also be defined as water vapor, H2O, provided that the corresponding latent heat is included in the discrete phase material inputs
in FLUENT. If the liquid water is used as a boundary species, it should be removed from the list of excluded species (see Section 15.4.5: Forcing the Exclusion and Inclusion of Equilibrium Species).

In the Set Injection Properties panel, you will specify for the Oxidizing Species one of the components of the oxidizer stream. This species concentration field will be used to calculate the diffusion-controlled driving force in the char burnout law

Forcing the  Inclusion of Equilibrium Species:In the case that there are species that you want to have available for postprocessing that would be ignored by FLUENT due to their low concentration (e.g., CH, CH2, CH3 for the NOx calculation), you can force FLUENT to include them using the text interface. input the ommond in the consol interface : define-->models--->species--->non-premixed-combustion

求解参数设置：
Input the Mean Mixture Fraction and Mixture Fraction Variance：In general, the inlet value of the mean fractions will be 1.0 or 0.0 at flow inlets: the mean fuel mixture fraction will be 1.0 at fuel stream inlets and 0.0 at oxidizer or secondary
stream inlets; the mean secondary mixture fraction will be 1.0 at secondary stream inlets and 0.0 at fuel or oxidizer inlets. The fuel or secondary mixture fraction will lie between 0.0 and 1.0 only if you are modeling fuel gas recycle. The fuel or secondary mixture fraction variance can usually be taken as zero at inlet boundaries.

turn off diffusion at inlet:By default, FLUENT includes the diffusion flux of mixture fraction at inlets. To turn off inlet diffusion, use either the define/models/species/inlet-diffusion text command, or the Species Model panel.

The transport properties in a non-premixed combustion problem can be defined as functions of temperature, if desired, but not as functions of composition. In practice, since turbulence effects will dominate, it will be of little benenit to include even the temperature dependence of these transport properties.

 

PDF方程的欠松弛因子 （Under-Relaxation Factors for PDF Equations）
平均混合分数和混合分数变化变量的输运方程相当稳定，当求解它们时可用高度欠松弛。通过默认，平均混合分数（和次要部分分数）的欠松弛因子为1，混合分数变化变量（或次要部分分数变化变量）为0.9。如果这些方程的残差增加，应考虑降低这些欠松弛因子，如22.9节所讨论。
密度欠松弛 （Density Under-Relaxation）
燃烧计算可能收敛困难的主要原因之一是温度大变化引起密度大变化，而密度大变化能引起流动解的不稳定。FLUENT允许欠松弛密度中的变化以减轻困难。密度欠松弛的默认值为1，但是如果遇到麻烦，就会希望将该值减少到0.5和1之间（在“Solution Controls”面板）。

 

预混燃烧模型：

在使用预混燃烧模型时有以下限制：
 必须使用非耦合求解器。预混燃烧模型在两种耦合求解器中都不能得到。
 预混燃烧模型只对湍流、亚音速模型有效。这一类型的火焰成为爆燃。在爆炸中，可燃混合物被冲击波后面的热量点燃，这一类型的燃烧可以使用非耦合和耦合求解器用有限速率模型模拟。有关限速率模型。
 预混燃烧模型不能和污染物（如碳烟和NOx）模型一起使用。但完全预混系统可以用部分预混模型模拟。
 不能用预混燃烧模型模拟反应的离散相粒子。只有惰性粒子可以使用预混燃烧模型。

预混燃烧模型的输入：

修改模型的常熟：你可以设置湍流长度尺度常数(Turbulence Length Scale Constant， 方程15.2-6 中的CD)，湍流火焰速度常数(Turbulence Flame Speed Constant ，方程15.2-4 中的A)，－－－（文献[276]中推荐A 的缺省值为0.52，对于大多数预混火焰都是适合的。缺
省的D C 值为0.37，对于大多数预混火焰也是适合的。）拉伸因子系数(Stretch Factor Coefficient ，方程15.2-11 中的μstr)－－－（文献[276]推荐的μstr的缺省值为0.26（在无反应流动中测得），对于大多数预混合火焰都适用）。和湍流施密特数（Turbulent Schmidt Number，方程15.2-1 中的Sct）－－－（For a non-adiabatic premixed combustion model, note that the value you specify for the Turbulent Schmidt Number will also be used as the Prandtl number for energy. These parameters control the level of diffusion
for the progress variable and for energy. Since the progress variable is closely related to energy , it is important that the transport equations use the same level of diffusion.）

Setting Boundary Conditions for the Progress Variable
For premixed combustion models, you will need to set an additional boundary condition at flow inlets and exits: the progress variable, c. Valid inputs for the Progress Variable are as follows:
 c = 0: unburnt mixture
 c = 1: burnt mixture

初始化进程变量：
通常，将进程变量c 处处初始化设置为1(燃烧后)，并允许未燃混合物( 0 => c )从入口进入燃烧域将火焰吹回稳定器，已经足够。另一种更好的初始化方法是在火焰保持器的上游插入一个初始值0(未燃)，在下游区域插入一个值1（已燃）（已经在求解初始化Solution Initialization 面板中初始化了流动场）----patch。

 

部分预混燃烧的模拟

模型的输入
There are two ways to specify a premixed inlet boundary condition:
(a) If you defined the fuel composition in the Boundary tab to be the premixed inlet species, then you should set f = 1 and c = 0 in the boundary conditions panel.
(b) If you set the fuel composition to pure fuel in the Boundary tab, you will need to set the correct equivalence ratio (0 < f < 1) and c = 0 at your premixed inlet boundary condition.
For example, if the premixed inlet of methane and air is at an equivalence ratio of 0.3, you can
(a) specify the mass fraction of the fuel composition of YCH4 = 0.017, YO2 = 0.236,
and YN2 = 0.747 in the Boundary tab and f = 1 and c = 0 in the boundary conditions panel.
(b) specify the mass fraction of the fuel composition of YCH4 = 1.0 in the Boundary tab and f = 0.017 and c = 0 in the boundary conditions panel.
Method (a) is preferred since it will have more points in the flame zone thanmethod (b).

 

 

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