
The next step in the nonpremixed combustion modeling process in 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 Section 25.14 for details about solution initialization.
Solve Initialize Initialize...
Next, begin calculations in the usual manner.
Solve Iterate...
During the calculation process, FLUENT reports residuals for the mixture fraction and its variance in the fmean and fvar columns of the residual report:
iter cont xvel yvel k epsilon fmean fvar 28 1.57e3 4.92e4 4.80e4 2.68e2 2.59e3 9.09e1 1.17e+0 29 1.42e3 4.43e4 4.23e4 2.48e2 2.30e3 8.89e1 1.15e+0 30 1.28e3 3.98e4 3.75e4 2.29e2 2.04e3 8.88e1 1.14e+0 
(For twomixturefraction calculations, columns for psec and pvar will also appear.)
UnderRelaxation Factors for PDF Equations
The transport equations for the mean mixture fraction and mixture fraction variance are quite stable and high, underrelaxation can be used when solving them. By default, an underrelaxation factor of 1 is used for the mean mixture fraction (and secondary partial fraction) and 0.9 for the mixture fraction variance (and secondary partial fraction variance). If the residuals for these equations are increasing, you should consider decreasing these underrelaxation factors, as discussed in Section 25.9.2.
Density UnderRelaxation
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. FLUENT allows you to underrelax the change in density to alleviate this difficulty. The default value for density underrelaxation is 1, but if you encounter convergence trouble you may wish to reduce this to a value between 0.5 and 1 (in the Solution Controls panel).
Tuning the PDF Parameters for TwoMixtureFraction Calculations
For cases that include a secondary stream, the PDF integrations are performed inside FLUENT.
The parameters for these integrations are defined in the Species Model panel (Figure 15.16.2).
Define Models Species...
The parameters are as follows:
For simulations involving nonadiabatic multiple strained flamelets, looking up the fourdimensional PDF tables can be CPUintensive if a large number of species exist in the flamelet files. In such cases, the Number of Flow Iterations Per Property Update controls the updating of the mean molecular weight, which involves looking up the PDF tables for the species mass fractions.
For the Eulerian unsteady laminar flamelet model, a marker probability equation is solved in an unsteady mode. Residuals for uflaprob will be displayed.