Before turning on the non-premixed combustion model, you must enable turbulence calculations in the Viscous Model Dialog Box.

 Setup → Models → Viscous   Edit...

If your model is non-adiabatic, you should also enable heat transfer (and radiation, if required).

 Setup → Models → Energy   ON

 Setup → Models → Radiation   Edit...

Figure 8.7: Reacting Systems Requiring Non-Adiabatic Non-Premixed Model Approach in the Theory Guide illustrates the types of problems that must be treated as non-adiabatic.

Your first task is to define the type of reaction system and reaction model that you intend to use. This includes selection of the following options:

You can make these model selections using the Species Model dialog box (Figure 20.6: The Species Model Dialog Box (Chemistry Tab)).

 Setup → Models → Species   Edit...

For a single-mixture-fraction problem, you will perform the following steps:

Figure 20.5: Calculating the Chemistry Look-Up Table

The look-up table is the stored result of the integration of Equation 8–17 (or Equation 8–25) and Equation 8–19 (in the Theory Guide). The look-up table will be used in Ansys Fluent to determine mean species mass fractions, density, and temperature from the values of mean mixture fraction (), mixture fraction variance (), and possibly mean enthalpy () and mean scalar dissipation () as they are computed during the Ansys Fluent calculation of the reacting flow. See Look-Up Tables for Adiabatic Systems and Figure 8.8: Visual Representation of a Look-Up Table for the Scalar (Mean Value of Mass Fractions, Density, or Temperature) as a Function of Mean Mixture Fraction and Mixture Fraction Variance in Adiabatic Single-Mixture-Fraction Systems and Figure 8.10: Visual Representation of a Look-Up Table for the Scalar as a Function of Mean Mixture Fraction and Mixture Fraction Variance and Normalized Heat Loss/Gain in Non-Adiabatic Single-Mixture-Fraction Systems in the Theory Guide.

For a problem that includes a secondary stream (and, therefore, a second mixture fraction), you will perform the first two steps listed above for the single-mixture-fraction approach and then prepare a look-up table of instantaneous properties using Equation 8–13 or Equation 8–15 in the Theory Guide.

You should use the non-adiabatic modeling option if your problem definition in Ansys Fluent will include one or more of the following:

Note that the adiabatic model is a simpler model involving a two-dimensional look-up table in which scalars depend only on and (or on and ). If your model is defined as adiabatic, you will not need to solve the energy equation in Ansys Fluent and the system temperature will be determined directly from the mixture fraction and the fuel and oxidant inlet temperatures. The non-adiabatic case will be more complex and more time-consuming to compute, requiring the generation of three-dimensional look-up tables. However, the non-adiabatic model option allows you to include the types of reacting systems described above.

Select Adiabatic or Non-Adiabatic in the Chemistry tab of the Species Model dialog box.

The system Operating Pressure is used to calculate density using the ideal gas law. For non-adiabatic simulations, the Compressibility Effects under PDF Options can be enabled to account for cases where substantial pressure changes occur in time and/or space. In such cases it is assumed that the species mass fractions do not change with pressure, and the density is calculated as

(20–1)

where is the density at the specified Operating Pressure (), and is the local mean pressure in an Ansys Fluent cell.

When the Compressibility Effects option is enabled, the flow operating pressure (set in the Operating Conditions dialog box) can differ from the Non-Premixed model operating pressure. To distinguish this difference, the Operating Pressure name tag in the Species Model dialog box changes to Equilibrium Operating Pressure when the compressibility effects option is enabled.

Note that for the Operating Pressure or Equilibrium Operating Pressure, you should specify a value close to the absolute pressure in the system.

See Solution Strategies for Non-Premixed Modeling for details about enabling compressibility effects.

If you are modeling a system consisting of a single fuel and a single oxidizer stream, you do not need to enable a secondary stream in your PDF calculation. As discussed in Definition of the Mixture Fraction in the Theory Guide, a secondary stream should be enabled if your PDF reaction model will include any of the following:

To include a secondary stream in your model, turn on the Secondary Stream option under Stream Options in the Chemistry tab.

When the secondary stream is present, only instantaneous species mass fraction, temperature and mixture properties are stored inside the PDF table. The probability density function (PDF) calculates averaged species mass fraction and mixture properties at the run time. After a PDF table has been generated or read into Ansys Fluent, you can select the shape of the assumed PDF from the Probability Density Function drop-down list (PDF Options group box):

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. That is, you will define the elemental fraction not the individual species. When this option is disabled, you will define the chemical species that are present in each stream and the mass or mole fraction of each species, as described in Defining the Stream Compositions. The option for defining an empirical fuel stream is particularly useful for coal combustion simulations (see Modeling Coal Combustion Using the Non-Premixed Model) or for simulations involving other complex hydrocarbon mixtures.

To define a fuel or secondary stream empirically

You can define a rich limit on the mixture fraction when the equilibrium chemistry option is used. Input of the rich limit is accomplished by specifying a value of the Rich Flammability Limit for the appropriate Fuel Stream, Secondary Stream, or both. You will not be allowed to specify the Rich Flammability Limit if you have used the empirical definition option for fuel composition.

Ansys Fluent will compute the composition at the rich limit using equilibrium. For mixture fraction values above this limit, Ansys Fluent will suspend the equilibrium chemistry calculation and will compute the composition based on mixing, but not burning, of the fuel with the composition at the rich limit. 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 , , or exceeds the limit. This RFL model is often more accurate than the assumption of chemical equilibrium for rich mixtures, and also avoids complex equilibrium calculations, speeding up the preparation of the look-up tables. An RFL value of approximately twice the stoichiometric mixture fraction is appropriate.

For the Secondary Stream, the rich flammability limit controls the equilibrium calculation for the secondary mixture fraction. If your secondary stream is not a fuel, you should use an RFL value of 1. 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 exceeds the limit. (Note that it is the secondary mixture fraction and not the partial fraction that is used here.)

Select Adiabatic or Non-Adiabatic in the Chemistry tab of the Species Model dialog box. See the discussion in Choosing Adiabatic or Non-Adiabatic Options about the two options.

The system Operating Pressure is used to calculate density using the ideal gas law. When the Compressibility Effects option is enabled, the name Operating Pressure is changed to Equilibrium Operating Pressure since the non-premixed combustion model operating pressure can differ from the flow operating pressure. Specifying the Operating Pressure for the System provides more information about this value.

You can use the steady or unsteady diffusion flamelet model for reactions in liquid systems. To do so, enable Liquid Micro-Mixing under PDF Options. The Liquid Micro-Mixing option is discussed in detail in Liquid Reactions in the Theory Guide.

If you are generating a flamelet file yourself, you will need to read in the chemical kinetic mechanism and thermodynamic data. The mechanism and thermodynamic data must be in CHEMKIN format  [79].

To read in a CHEMKIN mechanism, select the Create Flamelet option in the Chemistry tab of the Species Model dialog box and click the Import CHEMKIN Mechanism... button. Using the Import CHEMKIN Format Mechanism dialog box, import the gas-phase files into Ansys Fluent as described in Importing a Volumetric Kinetic Mechanism in CHEMKIN Format. Note that for flamelet generation, only a gas-phase kinetic mechanism and a thermodynamic database are required.

Note that the import is limited to mechanisms with 700 or fewer species.

To import an existing flamelet file

After you have completed this step, you can skip ahead to the Table tab of the Species Model dialog box (see Calculating the Look-Up Tables).

The unsteady diffusion flamelet model can only be enabled from a valid steady-state, steady diffusion flamelet solution. When enabled, the unsteady diffusion flamelet model will change this case to unsteady and postprocess marker probability equations on the frozen flow field. You should hence ensure that the starting steady-state, steady diffusion flamelet solution is fully converged.

When the Unsteady Diffusion Flamelet is enabled in the Chemistry tab, the Import Flamelet File for Restart... button appears in the dialog box, allowing you to run the simulation from a previously saved case, data and unsteady flamelet file.

The Unsteady Diffusion Flamelet Model requires four user inputs in the Flamelet tab:

When these inputs have been set, clicking the Initialize Unsteady Flamelet Probability button initializes the marker probability equation for each flamelet, automatically enabling the Unsteady solver, while disabling all equations except the Unsteady Flamelet Probability equation in the Solution Controls task page. This initialization in the Flamelet tab also sets the Time Step Size in the Run Calculation task page.

The diesel unsteady laminar flamelet model can only be enabled when conditions for compression-ignition are met:

The basic steps for setting the diesel unsteady laminar flamelet models are as follows.

In order to simulate multiple cycles in internal combustion engines, flamelets should be reset at the end of every cycle. In addition, the burned trapped gases must be modeled, which can be done in one of the two ways. The first and recommended approach is to use the Diesel Unsteady Flamelet Reset option. The second approach is to use the inert (EGR) model and the Inert EGR Reset option. You can access the Diesel Unsteady Flamelet Reset and Inert EGR Reset options via the Dynamic Mesh Events dialog box. There, you need to set the crank angle at which this event occurs (usually shortly before the inlet valves open) and the participating fluid zones (usually only the combustion chamber and not the intake and exhaust port zones).

When the Diesel Unsteady Flamelet Reset event is executed, all flamelets are deleted and a new flamelet is introduced with a state set to the probability-weighted average condition of all flamelets present before reset. The other new flamelets are introduced during a new cycle in a similar fashion to that described in Using the Diesel Unsteady Laminar Flamelet Model.

When the inert EGR reset event is executed with the diesel unsteady flamelet model, the burnt gas in the selected Inert EGR Reset zones is converted to inert, all flamelets are deleted, and a new unburnt flamelet is introduced into the domain.

In combustion, a large number of intermediate and product species may be produced from a small number of initial boundary species. In Ansys Fluent you must specify only the species composition of your boundary species in the fuel, oxidizer, and (if appropriate) secondary streams. Ansys Fluent will calculate all intermediate and product species automatically. The following suggestions may be helpful in the definition of the system chemistry:

If you want to include a new species in your reacting system that is not available in the chemical database, you can add it to the database file, thermo.db. The format for thermo.db is detailed in [79]. If you choose to modify the standard database file, you should create copies of the original file.

As mentioned in Defining the Problem Type, you can define the composition of a fuel stream (that is, the standard fuel or a secondary fuel) empirically. For an empirically-defined stream, you will need to enter the atomic mass or mole fractions in addition to the inputs for lower caloric (heating) value of the fuel and the mean specific heat of the fuel that were described previously.

The heat of formation of an empirically defined stream is calculated from the heating value and the atomic composition. The fuel inlet temperature and fuel specific heat are used to calculate the sensible enthalpy. The molecular weight is used for the computation of the unburnt stream density. Note that the unburnt density is only required if the stream enters via an inlet boundary, or if you are using the partially-premixed model.

When an empirically-defined fuel or secondary stream is specified in the Chemistry tab (equilibrium chemistry model only) of the Species Model dialog box, you must specify the atom fractions of C, H, O, N, and S in that stream instead of the species mass or mole fractions. To avoid confusion, the species fraction inputs for an empirically-defined stream will be grayed out in the table within the Boundary tab, leaving only the fields for atom fractions (that is, c, h, o, n, and s).

Liquid fuel combustion can be modeled with the discrete phase and non-premixed models. In Ansys Fluent, the fuel vapor, which is produced by evaporation of the liquid fuel, is defined as the fuel stream. (See Defining the Stream Compositions.) The liquid fuel that evaporates within the domain appears as a source of the mean fuel mixture fraction.

Within Ansys Fluent, you define the liquid fuel discrete-phase model in the usual way. The gas phase (oxidizer) flow inlet is modeled using an inlet mixture fraction of zero and the fuel droplets are introduced as discrete phase injections (see Setting Initial Conditions for the Discrete Phase). The property inputs for the liquid fuel droplets are unaltered by the non-premixed model (see Setting Material Properties for the Discrete Phase). Note that when you are requested to input the gas phase species destination for the evaporating liquid, you should specify the species that make up the evaporating stream.

If the fuel stream was defined as a mixture of components, you should select the largest of these components as the “evaporating species”. Ansys 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.

If your model involves coal combustion, the fuel and secondary stream compositions can be input in one of several ways. You can use a single mixture fraction (fuel stream) to represent the coal, defining the fuel composition as a mixture of volatiles and char (solid carbon). Alternatively, you can use two mixture fractions (fuel and secondary streams), defining the volatiles and char separately. In two-mixture-fraction models for coal combustion, the fuel stream represents the char and the secondary stream represents volatiles. This section describes the modeling options and special input procedures for coal combustion models using the non-premixed approach.

There are three options for coal combustion:

 

20.1.4.5.5. The Coal Calculator

The Coal Calculator dialog box automates the calculations described above for setting up a coal case from the proximate and ultimate analyses.

Figure 20.12: The Coal Calculator Dialog Box

The inputs to the Coal Calculator dialog box are:

1. Coal Proximate Analysis, which is the mass fraction of Volatile, Fixed Carbon, Ash and Moisture in the coal.

2. Coal Ultimate Analysis, which is the mass fraction of atomic C, H, O, N and optionally S, in the Dry-Ash-Free (DAF) coal.

3. The option to use a Secondary Stream. If enabled, the two mixture fraction model will be set with the primary stream representing char as , and an empirical secondary stream representing the volatiles.

4. The Coal Particle Material Name. A DPM Combusting Particle Material will be created with this name. The default name is coal-particle.

5. The Coal As-Received HCV (Higher Calorific Value).

6. The High Temperature Volatile Yield. Proximate analyses are generally done with slower heating rates and lower temperatures than would occur in a real flame. Therefore, enhanced devolatization at higher temperatures can cause the volatile yield to exceed the proximate analysis fraction. To model this, the actual volatile fraction used is calculated as that specified in the Proximate Analysis input multiplied by the High Temperature Volatile Yield. The actual Fixed Carbon fraction is then calculated as one minus the sum of the actual Volatile, Ash, and Moisture fractions.

7. Fraction of N in Char (DAF). This input is used in calculating the split of atomic nitrogen for the Fuel NOx model.

   When the OK button is clicked, Ansys Fluent makes the following changes:

   1. The empirical fuel atomic compositions in the Boundary tab are set, and the Non-Adiabatic model is enabled as required for DPM. The Empirical Fuel Lower Calorific Value is calculated as follows. First the DAF LCV of the coal is computed as,

      (20–4)

      where and are the proximate moisture and ash fractions, is the ultimate fraction, and are the molecular weight of water and atomic hydrogen, respectively, and is the latent heat of water.

      If you are using the Secondary Stream option, is calculated from using,

      (20–5)

      where and are the proximate fixed carbon and volatile fractions, respectively.

   2. A combusting particle material is created with Volatile Component Fraction and Combustible Fraction calculated from the ultimate and proximate analyses. The Discrete Phase Model (DPM) is enabled.

   3. For the Fuel NOx model, the char N conversion is set to NO, and the Fuel NOx  Volatile and Char mass fractions are set according to the ultimate and proximate compositions. Note that even though some of the Fuel NOx parameters are changed, the Fuel NOx model itself is not enabled.

After the Coal Calculator has set up the relevant models, you must build the PDF Table by clicking Calculate PDF Table in the Table tab. You will also need to create injections if you have not done this yet. After converging your coal combustion case, you may want to enable the NOx model for postprocessing nitrogen-oxide pollutants.

Because Ansys Fluent calculates all intermediate and product species automatically during the equilibrium calculation, certain species will be included that are generally not in chemical equilibrium. Principal among these are the NOx species. Specifically, the NOx reaction rates are slow and should not be treated using an equilibrium assumption. Instead, the NOx concentration is predicted most accurately using the Ansys Fluent NOx postprocessor, where finite-rate kinetics are included (see NOx Formation). The NOx species can be safely excluded from the equilibrium calculation since they are present at low concentrations and have little impact on the density, temperature, and other species.

To force the exclusion of a species from the equilibrium calculation, click the Control tab in the Species Model dialog box (Figure 20.13: The Species Model Dialog Box (Control Tab)).

Figure 20.13: The Species Model Dialog Box (Control Tab)

Under Species Excluded From Equilibrium, enter the chemical formula for the desired species in the Add/Remove Species field. Next, click Add to add the species to the Species list or Remove to remove an existing species from the Species list.

If there are species that you want to include in your PDF table that would be ignored by Ansys Fluent due to their low concentration (for example, CH, CH₂, CH₃ for the NO_x calculation), you can force Ansys Fluent to include them using the text interface:

define → models → species → non-premixed-combustion

When the console window prompts you with Force Equilibrium Species to Include..., specify the appropriate species by entering the chemical formula(s) in double quotes (for example, "ch", "ch2").

Note that you will have to first set up the inputs for the fuel and oxidizer before you are given the option to include the species.

When the steady diffusion flamelet model is selected, and you have created or imported a flamelet, you can adjust the controls for the flamelet solution in the Control tab of the Species Model dialog box (Figure 20.14: The Species Model Dialog Box (Control Tab) for the Steady Diffusion Flamelet Model).

Figure 20.14: The Species Model Dialog Box (Control Tab) for the Steady Diffusion Flamelet Model

The Initial Fourier Number sets the first time step size for the solution of the flamelet equations (Equation 8–47 and Equation 8–48 in the Theory Guide). This first time step size is calculated as the explicit stability-limited diffusion time step size multiplied by this value. If the solution diverges before the first time step is complete, the value should be lowered.

The Fourier Number Multiplier increases the time step size at subsequent times. Every time step after the first is multiplied by this value. If the solution diverges after the first time step, this value should be reduced.

During the numerical integration of the flamelet equations, the local error is controlled to be less than

(20–6)

where represents the species mass fractions and temperature at point in the 1D flamelet. is the value of the Relative Error Tolerance and is the value of the Absolute Error Tolerance, both of which you can specify.

Because steady flamelets are obtained by time-stepping, they are considered converged only when the maximum absolute change in species fraction or temperature at any discrete mixture-fraction point is less than the specified Flamelet Convergence Tolerance. Between time steps, the flamelet species fractions and temperature will sometimes oscillate, which causes absolute changes that are always greater than the flamelet convergence tolerance. In such cases, Ansys Fluent will stop the flamelet calculation after the total elapsed time has exceeded the Maximum Integration Time.

When modeling gas-phase combustion using the Eulerian unsteady laminar flamelet model, the flamelet fields are initialized to a burning, steady-flamelet solution in order to model ignition. However, assuming steady-flamelet profiles for slow-forming species is inaccurate. A better approximation is to identify the slow species and to set them to zero, which is done in the Control tab. By default, Ansys Fluent selects some NO_x species (NO, NO₂, N₂O, N, NH, NH₂, NH₃, NNH, HCN, HNO, CN, H2CN, HCNN, HCNO, HOCN, HNCO, HCO), as well as liquid water H₂<l> and solid carbon C<s> to be zeroed. See Figure 20.15: Method to Zero Out the Slow Chemistry Species.

Figure 20.15: Method to Zero Out the Slow Chemistry Species

In the Flamelet tab of the Species Model dialog box (Figure 20.16: The Species Model Dialog Box (Flamelet Tab)), you will enter values for parameters of the flamelet(s).

Figure 20.16: The Species Model Dialog Box (Flamelet Tab)

The Flamelet Parameters are as follows:

Click Calculate Flamelets to begin the diffusion flamelet calculation. Sample output for a flamelet calculation is shown below.

 Generating flamelet 1 at scalar dissipation  0.01 /s
 
Time (s)   Temp (K)   Residual
1.679e-05   2233.7   3.779e+00
5.038e-05   2233.0   7.734e-02
1.175e-04   2231.5   1.648e-01
2.519e-04   2228.6   3.652e-01
5.206e-04   2223.6   8.295e-01
1.058e-03   2215.7   2.100e+00
2.133e-03   2205.5   3.540e+00
4.282e-03   2197.0   4.607e+00
8.581e-03   2193.6   6.639e+00
1.718e-02   2193.1   4.905e+00
3.437e-02   2193.4   5.792e+00
6.877e-02   2194.3   4.659e+00
1.375e-01   2195.3   3.922e+00
2.751e-01   2192.2   3.181e+00
5.502e-01   2188.6   2.549e+00
1.100e+00   2184.8   1.639e+00
2.201e+00   2182.9   4.604e+00
4.402e+00   2186.8   1.307e+00
8.804e+00   2189.6   4.420e-01
1.761e+01   2190.0   8.581e-02
3.522e+01   2190.0   1.199e-02
7.043e+01   2190.0   1.735e-03
1.409e+02   2190.0   4.217e-04
2.817e+02   190.0    6.892e-05
5.635e+02   2190.0   6.777e-06

Flamelet successfully generated 

In the Flamelet tab of the Species Model dialog box (Figure 20.17: The Flamelet Tab for the Unsteady Diffusion Flamelet Model), you will enter values for parameters of the flamelet.

Figure 20.17: The Flamelet Tab for the Unsteady Diffusion Flamelet Model

The Unsteady Flamelet Parameters are as follows:

Click Initialize Unsteady Flamelet Probability to initialize the unsteady flamelet and its probability marker equation. Ansys Fluent is now ready for postprocessing the 1D unsteady flamelet and the 2D/3D unsteady marker probability equation.

The flamelet tables may be written to a file for import into later sessions of Ansys Fluent. You may want to do this, for example, to change the number of discretization points in the PDF table, or to plot the flamelet profiles in Ansys Fluent. The flamelet tables should be saved before you create the PDF table:

 File → Write → Flamelet...

For the flamelet model, you can display or write flamelet curves. Click the Display Flamelets... or Display Unsteady Flamelet... button. If you have a single flamelet, as for the unsteady diffusion flamelet model, you can access the Flamelet 2D Curves dialog box (Figure 20.18: The Flamelet 2D curves Dialog Box).

Figure 20.18: The Flamelet 2D curves Dialog Box

For the steady diffusion flamelet model with more than one flamelet, you can display 2D plots and 3D surfaces showing the variation of species fraction or temperature with the mean mixture fraction or scalar dissipation using the Flamelet 3D Surfaces dialog box (for example, Figure 20.19: The Flamelet 3D Surfaces Dialog Box).

To access this dialog box, click the Display Flamelets... button in the Flamelet tab of the Species Model dialog box, as shown in Figure 20.16: The Species Model Dialog Box (Flamelet Tab).

Figure 20.19: The Flamelet 3D Surfaces Dialog Box

To display the flamelet tables graphically, use the following procedure:

Figure 20.20: Example 2D Plot of Flamelet Data and Figure 20.21: Example 3D Plot of Flamelet Data show examples of a 2D curve plot and 3D surface plot of a flamelet table.

Figure 20.20: Example 2D Plot of Flamelet Data

Figure 20.21: Example 3D Plot of Flamelet Data

The default algorithm for the two-mixture-fraction model is to perform PDF integrations of the equilibrium state relationships at run time. Since these are multi-dimensional integrals, the default two-mixture-fraction model can be computationally demanding.

Alternatively, you may want to pre-compute these integrations and create 4D look-up tables for adiabatic simulations, or 5D tables for non-adiabatic simulations. Such high-dimensional tables are computationally expensive to build, and may require large memory and disk storage, but can offer substantial improvement in run time speed.

The option to create a fully-tabulated two-mixture-fraction table is available in cases with the two-mixture-fraction model enabled, via the text command:

define/models/species/full-tabulation?

If you are modeling pollutant formation, note that the full tabulation option is not compatible with the mixture-fraction option for PDF Mode in the Turbulence Interaction Mode tab settings. See Setting Turbulence Parameters for details on Turbulence Interaction options for pollutant modeling.

Complex chemistry and non-adiabatic effects may make the equilibrium calculation more time-consuming and difficult. In some instances the equilibrium calculation may even fail. You may be able to eliminate any difficulties that you encounter by trying the calculation as an adiabatic system. Adiabatic system calculations are generally quite straightforward and can provide valuable insight into the optimal inputs to the non-adiabatic calculation.

Additional stability issues may arise for solid or heavy liquid fuels that have been defined using the empirical fuel approach. You may find that, for rich fuel mixtures, the equilibrium calculation produces very low temperatures and eventually fails. This indicates that strong endothermic reactions are taking place and the mixture is not able to sustain them. In this situation, you may need to raise the heating value of the fuel until Ansys Fluent produces acceptable results. Provided that your fuel will be treated as a liquid or solid (coal) fuel, you can maintain the desired heating value in your Ansys Fluent simulation. This is accomplished by defining the difference between the desired and the adjusted heating values as latent heat (in the case of combusting solid fuel) or heat of pyrolysis (in the case of liquid fuel).

The look-up tables may be stored in a file that you can read back into later sessions of Ansys Fluent. The look-up tables should be saved before you exit from the current Ansys Fluent session.

 File → Write → PDF...

By default, the file will be saved as formatted (ASCII, or text). To save a binary (unformatted) file, turn on the Write Binary Files option in the Select File dialog box.

It is important for you to view your temperature and species tables to ensure that they are adequately but not excessively resolved. Inadequate resolution will lead to inaccuracies, and excessive resolution will lead to unnecessarily slow calculation times.

After a PDF table has been generated or read into Ansys Fluent, you can display 2D plots and 3D surfaces showing the variation of species mole fraction, density, or temperature with the mean mixture fraction, mixture fraction variance, or enthalpy using the PDF Table dialog box (for example, Figure 20.24: The PDF Table Dialog Box (Non-Adiabatic Case With Flamelets)). The PDF Table dialog box can be accessed in one of two ways:

Figure 20.24: The PDF Table Dialog Box (Non-Adiabatic Case With Flamelets)

To display the look-up tables graphically, use the following procedure:

Figure 20.25: Mean Species Mole Fraction Derived From an Equilibrium Chemistry Calculation and Figure 20.26: Mean Temperature Derived From an Equilibrium Chemistry Calculation show examples of 2D plots derived for a very simple hydrocarbon system.

Figure 20.25: Mean Species Mole Fraction Derived From an Equilibrium Chemistry Calculation

Figure 20.26: Mean Temperature Derived From an Equilibrium Chemistry Calculation

Figure 20.27: 3D Plot of Look-Up Table for Temperature Generated for a Simple Hydrocarbon System shows an example of a 3D surface derived for the same system.

Figure 20.27: 3D Plot of Look-Up Table for Temperature Generated for a Simple Hydrocarbon System

A sample diffusion flamelet file in the standard format is provided below. Note that not all species are listed in this file.

 HEADER
 STOICH_SCADIS   1.000000E-02
 NUMOFSPECIES    60
 GRIDPOINTS      50
 STOICH_Z        6.618652E-02
 PRESSURE        1.013250E+05
 BODY 
 Z
  0.000000000E+00 8.273315430E-03 1.240997314E-02 1.447830200E-02 1.551246643E-02 
  1.602954865E-02 1.654663086E-02 2.068328857E-02 2.481994629E-02 3.309326172E-02 
  4.136657715E-02 4.963989258E-02 5.377655029E-02 5.791320801E-02 6.204986572E-02 
  6.618652344E-02 6.912528540E-02 7.206404735E-02 7.500280931E-02 7.794157127E-02 
  7.867626176E-02 7.941095225E-02 8.088033323E-02 8.381909519E-02 8.675785714E-02 
  8.969661910E-02 9.263538106E-02 9.557414302E-02 1.014516669E-01 1.073291909E-01 
  1.132067148E-01 1.190842387E-01 1.249617626E-01 1.367168104E-01 1.425943343E-01 
  1.484718583E-01 1.543493822E-01 1.572881441E-01 1.602269061E-01 1.672799348E-01 
  1.743329635E-01 1.884390209E-01 2.166511357E-01 2.448632505E-01 2.730753653E-01 
  3.407844408E-01 4.084935163E-01 5.709952975E-01 7.659974350E-01 1.000000000E+00 
  TEMPERATURE
  3.000000000E+02 6.268546798E+02 7.802882604E+02 8.544333704E+02 8.908936814E+02 
  9.089771589E+02 9.269654154E+02 1.067550740E+03 1.203418117E+03 1.462664100E+03 
  1.707202552E+03 1.934669997E+03 2.037622483E+03 2.128475386E+03 2.200397351E+03 
  2.244572249E+03 2.254177637E+03 2.245072250E+03 2.220638695E+03 2.186735136E+03 
  2.177535664E+03 2.168109526E+03 2.148737393E+03 2.108856496E+03 2.068758176E+03 
  2.029154219E+03 1.990673114E+03 1.953782923E+03 1.885112633E+03 1.821857119E+03 
  1.761797286E+03 1.703540208E+03 1.647197627E+03 1.543457773E+03 1.496624533E+03 
  1.452642813E+03 1.410968313E+03 1.390849359E+03 1.371237774E+03 1.326602075E+03 
  1.286669965E+03 1.222176660E+03 1.134221753E+03 1.064763074E+03 1.004219101E+03 
  8.845385948E+02 7.871510772E+02 6.152847269E+02 4.605312740E+02 3.000000000E+02 
  massfraction-h
  0.000000000E+00 8.470246609E-15 1.689876963E-13 7.823505733E-13 1.604836416E-12 
  2.259465253E-12 3.137901769E-12 2.297692554E-11 1.299314819E-10 2.009221085E-09 
  3.364446760E-08 4.504321512E-07 1.488600091E-06 4.041919731E-06 9.259044043E-06 
  1.764854561E-05 2.446982906E-05 2.948355957E-05 3.067005444E-05 2.858096141E-05 
  2.763204904E-05 2.660609671E-05 2.447858861E-05 2.024662524E-05 1.571691024E-05 
  1.207106883E-05 9.769042791E-06 8.144795604E-06 5.683704776E-06 3.862438419E-06 
  2.529575950E-06 1.577642294E-06 9.493198194E-07 3.597224020E-07 2.397461608E-07 
  1.672683885E-07 1.109095223E-07 8.352078253E-08 5.847760877E-08 2.024498078E-08 
  6.804126487E-09 1.241975019E-09 1.664800458E-10 3.824650210E-11 1.058872219E-11 
  1.103611172E-12 2.070333150E-13 5.521048377E-15 6.116361056E-17 0.000000000E+00 
  massfraction-o2
  2.330000000E-01 2.037872969E-01 1.891809523E-01 1.818777426E-01 1.782261116E-01 
  1.764002861E-01 1.745744526E-01 1.599674402E-01 1.453586904E-01 1.161277458E-01 
  8.685446531E-02 5.764270922E-02 4.337649028E-02 2.986903537E-02 1.792882492E-02 
  8.656537748E-03 4.260825775E-03 1.718417261E-03 5.787962652E-04 1.809733781E-04 
  1.319197735E-04 9.626119601E-05 5.195145104E-05 1.624530279E-05 5.000207346E-06 
  1.625791857E-06 5.935574928E-07 2.314963476E-07 5.242565474E-08 1.388575920E-08 
  4.396692299E-09 1.701489188E-09 7.617025651E-10 3.172983452E-10 2.146818725E-10 
  1.500353242E-10 1.052693603E-10 8.680804793E-11 7.032480088E-11 3.785587351E-11 
  1.627110024E-11 1.631110236E-12 2.316226342E-14 1.904385354E-16 8.159316969E-19 
  2.851864505E-21 7.569901388E-23 1.016058744E-25 1.304158252E-26 0.000000000E+00 
  massfraction-oh
  .  .   .    .    .
  .  .   .    .    .
  .  .   .    .    .

Ansys Fluent will check whether all species in the flamelet data file exist in the thermodynamic properties databases thermo.db. If any of the species in the flamelet file do not exist, Ansys Fluent will issue an error message and halt the flamelet import. If this occurs, you can either add the missing species to the database, or remove the species from the flamelet file.

You should not remove a species from the flamelet data file unless its species concentration is very small (10^-3 or less) throughout the flamelet profile. If you remove a low-concentration species, you will not have the species concentrations available for viewing in the Ansys Fluent calculation, but the accuracy of the Ansys Fluent calculation will otherwise be unaffected.

If a species with relatively large concentration is missing from the Ansys Fluent thermodynamic databases, you will have to add it. Removing a high-concentration species from the flamelet file is not recommended.

You will need to set appropriate boundary conditions at flow inlets and exits for the inert tracer mass fraction, . The tracer species mass fraction must be between zero and one, with the value of one meaning that all of the material entering the domain comes from the inert stream. The values for flow boundaries are set in the Inert Stream field of the inlet boundary condition dialog boxes, under the Species tab.

The main assumption of the inert model is that the composition of the inert stream does not change with combustion. For some dilutants, this is a very reasonable assumption, however, it is not valid for rich combustion where there is fuel in the exhaust stream. For cases where there is fuel or oxidizer left in the exhaust gas, the accuracy of your results will depend upon taking the fuel or oxidizer species into account when setting initial conditions.

 

20.1.9.2.2. Inert Composition

For combustion calculations that burn a hydrocarbon fuel, Ansys Fluent provides a straightforward way of setting the initial composition of the inert stream. The inert composition can be set by assuming a ratio of hydrogen to carbon in the following overall oxidation reaction (from Heywood [63]):

(20–8)

which can be rewritten in terms of the ratio of hydrogen to carbon atoms () in the fuel as

(20–9)

If Equation 20–9 is solved for the mass fractions of CO₂, H₂O and N₂, the following relations are obtained:

(20–10)

where

Setting the H/C ratio assumes that the burned gas resulted from the complete, stoichiometric combustion of that hydrocarbon fuel, and the only products of the combustion are CO₂, H₂O and N₂. An arbitrary composition for the inert stream can also be specified in the interface.

Ansys Fluent allows burned gases to be converted to an inert gas. This has been designed with in-cylinder combustion in mind to aid the simulation of multiple cycles of such engines using the partially-premixed combustion model with exhaust gas recirculation (EGR). At the end of a combustion stroke, the premixed progress variable of the burnt gases is unity. When fresh charge, with progress variable of zero, is mixed with the burnt trapped gases, combustion will be initiated unless these trapped gases are converted to inert.

This facility is accessed via the Dynamic Mesh Events dialog box, as described in Resetting Inert EGR. There, you will need to set the crank angle at which the event occurs (usually shortly before the inlet valves open) and at the fluid zones (usually the combustion chamber) where the burnt gases are converted to inert.

When Ansys Fluent performs an inert reset, the inert composition is calculated as the stoichiometric composition. For lean mixtures, the stoichiometric inert is mixed with oxidizer, and for rich mixtures the stoichiometric inert is mixed with fuel, so that overall stoichiometry is conserved. This entails:

When the non-premixed combustion model is used, flow boundary conditions at inlets and exits (that is, velocity or pressure, turbulence intensity) are defined in the usual way. Species mass fractions at inlets are not required. Instead, you define values for the mean mixture fraction, , and the mixture fraction variance, , at inlet boundaries. (For problems that include a secondary stream, you will define boundary conditions for the mean secondary partial fraction and its variance as well as the mean fuel mixture fraction and its variance.) These inputs provide boundary conditions for the conservation equations you will solve for these quantities. The inlet values are supplied in the boundary conditions task page, under the available tabs, for the selected inlet boundary (for example, Figure 20.30: The Velocity Inlet Dialog Box Showing Mixture Fraction Boundary Conditions).

 Setup →   Boundary Conditions

Figure 20.30: The Velocity Inlet Dialog Box Showing Mixture Fraction Boundary Conditions

Click the Species tab and specify the Mean Mixture Fraction and Mixture Fraction Variance (and the Secondary Mean Mixture Fraction and Secondary Mixture Fraction Variance, if you are using two mixture fractions). 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 flue gas recycle, as illustrated in Figure 8.15: Using the Non-Premixed Model with Flue Gas Recycle and discussed in Definition of the Mixture Fraction in the Theory Guide. The fuel or secondary mixture fraction variance can usually be taken as zero at inlet boundaries.

In some cases, you may want to include the diffusive transport of mixture fraction through the inlets of your domain. You can do this by enabling inlet mixture-fraction diffusion. By default, Ansys Fluent excludes the diffusion flux of mixture fraction at inlets. To enable inlet diffusion, use either the

define/models/ species/inlet-diffusion?

text command, or the Species Model dialog box (Figure 20.1: Defining Equilibrium Chemistry)

 Setup → Models → Species   Edit...

and enable the Inlet Diffusion option.

If your model is non-adiabatic, you should specify the Temperature at the flow inlets. Recall that the inlet temperatures were requested during the table construction in the Chemistry tab of the Species Model dialog box, and were used in the construction of the look-up tables. The inlet temperatures for each fuel, oxidizer, and secondary inlet in your non-adiabatic model should be defined, in addition, as boundary conditions in Ansys Fluent. It is acceptable for the inlet temperature boundary conditions to differ slightly from those you input for the look-up table calculations. If the inlet temperatures differ significantly from those in the Chemistry tab, however, your look-up tables may provide inaccurate interpolation. This is because the discrete points in the look-up tables were clustered around a finite range of the temperatures defined in the Chemistry tab, and your input in the Ansys Fluent inlet boundary condition may fall outside this range.

Wall thermal boundary conditions should also be defined for non-adiabatic non-premixed combustion calculations. You can use any of the standard conditions available in Ansys Fluent, including specified wall temperature, heat flux, external heat transfer coefficient, or external radiation. If radiation is to be included within the domain, the wall emissivity should be defined as well. See Thermal Boundary Conditions at Walls for details about thermal boundary conditions at walls.

For a single-mixture-fraction system, when you have completed the calculation of the PDF look-up tables, you are ready to begin your reacting flow simulation. In Ansys Fluent, you will solve the flow field and predict the spatial distribution of and (and if the system is non-adiabatic or if the system is based on laminar flamelets). Ansys Fluent will obtain the corresponding values of temperature and individual chemical species mass fractions from the look-up tables.

When a secondary stream is included, Ansys Fluent will solve transport equations for the mean secondary partial fraction () and its variance in addition to the mean fuel mixture fraction and its variance. Ansys Fluent will then look up the instantaneous values for temperature, density, and individual chemical species in the look-up tables, compute the PDFs for the fuel and secondary streams, and calculate the mean values for temperature, density, and species.

Note that in order to avoid both inaccuracies and unnecessarily slow calculation times, it is important for you to view your temperature and species tables in Ansys Fluent to ensure that they are adequately but not excessively resolved.

You can read a previously defined Ansys Fluent case file as a starting point for your non-premixed combustion modeling. If this case file contains inputs that are incompatible with the current non-premixed combustion model, Ansys Fluent will alert you when the non-premixed model is turned on and it will turn off those incompatible models. For example, if the case file includes species that differ from those included in the PDF file created by Ansys Fluent, these species will be disabled. If the case file contains property descriptions that conflict with the property data in the chemical database, these property inputs will be ignored.

In the Species Model dialog box, select Non-Premixed Combustion under the Model heading. When you click OK in the Species Model dialog box, The Select File Dialog Box will immediately appear, prompting you for the name of the PDF file containing the look-up tables created in a previous Ansys Fluent session. (The PDF file is the file you saved using the File/Write/PDF... ribbon tab item after computing the look-up tables.) Ansys Fluent will indicate that it has successfully read the specified PDF file:

 Reading "/home/mydirectory/adiabatic.pdf"...
 read 5 species (binary c, adiabatic fluent)
 pdf file successfully read.
 Done.

After you read in the PDF file, Ansys Fluent will inform you that some material properties have changed. You can accept this information; you will be updating properties later on.

You can read in an altered PDF file at any time by using the File/Read/PDF... ribbon tab item.

If you are modeling a non-adiabatic system and you want to include the effects of compressibility, re-open the Species Model dialog box and turn on Compressibility Effects under PDF Options. This option tells Ansys Fluent to update the density according to Equation 20–1. When the non-premixed combustion model is active, you can enable compressibility effects only in the Species Model dialog box. For other models, you will specify compressible flow (ideal-gas, boussinesq, and so on) in the Create/Edit Materials dialog box. See Specifying the Operating Pressure for the System and Tuning the PDF Parameters for Two-Mixture-Fraction Calculations for more information about compressibility effects.

The next step in the non-premixed combustion modeling process in Ansys Fluent is the solution of the mixture fraction and flow equations. First, initialize the flow. By default, the mixture fraction and its variance have initial values of zero, which is the recommended value; you should generally not set nonzero initial values for these variables. See Initializing the Solution for details about solution initialization.

 Solution →   Initialization

Next, begin calculations in the usual manner.

 Solution →   Run Calculation

During the calculation process, Ansys Fluent reports residuals for the mixture fraction and its variance in the fmean and fvar columns of the residual report:

iter       cont       x-vel       y-vel          k     epsilon       fmean       fvar
  28    1.57e-3     4.92e-4     4.80e-4    2.68e-2     2.59e-3     9.09e-1    1.17e+0
  29    1.42e-3     4.43e-4     4.23e-4    2.48e-2     2.30e-3     8.89e-1    1.15e+0
  30    1.28e-3     3.98e-4     3.75e-4    2.29e-2     2.04e-3     8.88e-1    1.14e+0  

(For two-mixture-fraction calculations, columns for psec and pvar will also appear.)

 

20.1.12.4.3. Tuning the PDF Parameters for Two-Mixture-Fraction Calculations

For cases that include a secondary stream, the PDF integrations are performed inside Ansys Fluent.

The parameters for these integrations are defined in the Species Model Dialog Box (Figure 20.31: The Species Model Dialog Box for a Two-Mixture-Fraction Calculation).

 Setup → Models → Species   Edit...

Figure 20.31: The Species Model Dialog Box for a Two-Mixture-Fraction Calculation

The parameters are as follows:

Compressibility Effects

(non-adiabatic systems only) tells Ansys Fluent to update the density, temperature, species mass fraction, and enthalpy from the PDF tables to account for the varying pressure of the system.

Ansys Fluent provides several additional reporting options for post-processing calculations with the inert model. You can generate graphical plots or alphanumeric reports of the same items that are available with the non-premixed or partially premixed models, and in addition the following variables are available:

where the mass fraction of the ^th inert species, , is calculated as

where is the inert tracer and is the mass fraction of the ^th inert species defined in the Inert dialog box.

Note these variables appear in the Inert... category.