18.2.2 Modifying the Unburnt Mixture Property Polynomials

After building the PDF table, ANSYS FLUENT automatically calculates the temperature, density, heat capacity, and thermal diffusivity of the unburnt mixture as polynomial functions of the mean mixture fraction, $\overline{f}$ (see this equation in the separate Theory Guide). The laminar flame speed is automatically calculated as a piecewise-linear polynomial function of $\overline{f}$.

However, as outlined in this section in the separate Theory Guide , the laminar flame speed depends on details of the chemical kinetics and molecular transport properties, and is not calculated directly. Instead, curve fits are made to flame speeds determined from detailed simulations [ 27]. These fits are limited to a range of fuels (H $_2$, CH $_4$, C $_2$H $_2$, C $_2$H $_4$, C $_2$H $_6$, and C $_3$H $_8$), air as the oxidizer, equivalence ratios of the lean limit through unity, unburnt temperatures from 298 K to 800 K, and pressures from 1 bar to 40 bars. If your parameters fall outside this range, ANSYS FLUENT will warn you when it computes the look-up table. In this case, you will need to modify the piecewise-linear points in the Premixed tab of the Species Model dialog box (Figure  18.2.1) before you save the PDF file.

Figure 18.2.1: The Species Model Dialog Box ( Premixed Tab)

For each polynomial function of $\overline{f}$ under Partially Premixed Mixture Properties ( Adiabatic Unburnt Density, Adiabatic Unburnt Temperature, Unburnt Cp, and Unburnt Thermal Diffusivity), you can specify values for Coefficient 1, Coefficient 2, Coefficient 3, and Coefficient 4 (the polynomial coefficients in this equation in the separate Theory Guide) in the appropriate Quadratic of Mixture Fraction dialog box (Figure  18.2.2). To open this dialog box, click the appropriate Edit... button in the Premixed tab.

Figure 18.2.2: The Quadratic of Mixture Fraction Dialog Box

You can also specify the piecewise-linear Mixture Fraction and its corresponding Laminar Flame Speed for 10 different points in the Piecewise Linear dialog box (Figure  18.2.3). The first set of values is the lower limit and the last set of values is the upper limit. Outside of either limit, the laminar flame speed is constant and equal to that limit. To open this dialog box, click the Edit... button next to Laminar Flame Speed in the Premixed tab.

Figure 18.2.3: The Piecewise Linear Dialog Box

Note also that if you choose to use a user-defined function for the laminar flame speed in the Create/Edit Materials dialog box, the piecewise-linear fit becomes irrelevant.