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17.3.2 Modifying the Unburnt Mixture Property Polynomials

After building the PDF table 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 Equation  17.2-3). The laminar flame speed is automatically calculated as a piecewise-linear polynomial function of $\overline{f}$.

However, as outlined in Section  17.2, 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 [ 126]. 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, 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 Premix tab of the Species Model panel (Figure  17.3.1) before you save the PDF file.

Figure 17.3.1: The Species Model Panel ( Premix Tab)
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For each polynomial function of $\overline{f}$ under Partially Premixed Mixture Properties ( Unburnt Density, Unburnt Temperature, Unburnt Specific Heat, and Unburnt Thermal Diffusivity), you can specify values for Coefficient 1, Coefficient 2, Coefficient 3, and Coefficient 4 (the polynomial coefficients in Equation  17.2-3) in the appropriate Quadratic of Mixture Fraction panel (Figure  17.3.2). To open this panel, click the appropriate Edit... button in the Premix tab.

Figure 17.3.2: The Quadratic of Mixture Fraction Panel
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You can also specify the piecewise-linear Mixture Fraction and its corresponding laminar Flame Speed for 10 different points in the Piecewise Linear panel (Figure  17.3.3). 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 panel, click the Edit... button next to Laminar Flame Speed in the Premix tab.

Figure 17.3.3: The Piecewise Linear Panel
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Note also that if you choose to use a user-defined function for the laminar flame speed in the Materials panel, the piecewise-linear fit becomes irrelevant.


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