16.8.1 Input of Mixture Fraction Boundary Conditions

When the non-premixed combustion model is used, flow boundary conditions at inlets and exits (i.e., 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, $\overline{f}$, and the mixture fraction variance, $\overline{f^{'2}}$, 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 (e.g., Figure  16.8.1).

Boundary Conditions

Figure 16.8.1: The Velocity Inlet Dialog Box Showing Mixture Fraction Boundary Conditions

Click the Species tab and input 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 this figure and discussed in this section in the separate Theory Guide. The fuel or secondary mixture fraction variance can usually be taken as zero at inlet boundaries.