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15.6.3 Overview of the Problem Setup Procedure

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

1.   Choose the chemical description of the system: equilibrium, steady flamelet, unsteady flamelet, or diesel unsteady flamelet (Figure  15.6.1).

2.   Indicate whether the problem is adiabatic or non-adiabatic.

Figure 15.6.1: Defining Equilibrium Chemistry
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3.   (steady laminar flamelet model only) Import a flamelet file or appropriate CHEMKIN mechanism file if generating flamelets (Figure  15.6.2).

4.   (unsteady laminar flamelet model only) Import a flamelet file for restart (Figure  15.8.1).

Figure 15.6.2: Defining Steady Laminar Flamelet Chemistry
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5.   Define the chemical boundary species to be considered for the streams in the reacting system model. Note that this step is not relevant in the case of flamelet import (Figure  15.6.3).

Figure 15.6.3: Defining Chemical Boundary Species
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6.   (steady and unsteady laminar flamelet model only) If you are generating flamelets, compute the flamelet state relationships of species mass fractions, density, and temperature as a function of mixture fraction and scalar dissipation (Figure  15.6.4). For unsteady laminar flamelets, you will initialize the unsteady flamelet probability (Figure  15.11.2).

Figure 15.6.4: Calculating Steady Flamelets
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7.   Compute the final chemistry look-up table, containing mean values of species fractions, density, and temperature as a function of mean mixture fraction, mixture fraction variance, and possibly enthalpy and scalar dissipation. The contents of this look-up table will reflect your preceding inputs describing the turbulent reacting system (Figure  15.6.5).

Figure 15.6.5: Calculating the Chemistry Look-Up Table
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The look-up table is the stored result of the integration of Equations  15.2-16 (or 15.2-24) and 15.2-18. The look-up table will be used in FLUENT to determine mean species mass fractions, density, and temperature from the values of mean mixture fraction ( $\overline{f}$), mixture fraction variance ( $\overline{f^{'2}}$), and possibly mean enthalpy ( $\overline{H}$) and mean scalar dissipation ( $\overline{\chi}$) as they are computed during the FLUENT calculation of the reacting flow. See Section  15.2.4 and Figures  15.2.8 and 15.2.10.

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  15.2-12 or 15.2-14.


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