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16.3.4 Defining Physical Properties for the Unburnt Mixture

The fluid material in your domain should be assigned the properties of the unburnt mixture, including the molecular heat transfer coefficient ( $\alpha$ in Equation  16.2-4), which is also referred to as the thermal diffusivity. $\alpha$ is defined as $k/\rho c_p$, and values at standard conditions can be found in combustion handbooks (e.g., [ 188]).

For both adiabatic and non-adiabatic combustion models, you will need to specify the Laminar Flame Speed ( $U_{l}$ in Equation  16.2-4) as a material property. If you want to include the flame stretch effect in your model, you will also need to specify the Critical Rate of Strain ( $g_{\rm cr}$ in Equation  16.2-15). As discussed in Section  16.2.2, $g_{\rm cr}$ is set to a very high value ( $1 \times 10^8 s^{-1}$) by default, so no flame stretching occurs. To include flame stretching effects, you will need to adjust the Critical Rate of Strain based on experimental data for the burner. Since the flame stretching and flame extinction can influence the turbulent flame speed (as discussed in Section  16.2.2), a realistic value for the Critical Rate of Strain is required for accurate predictions. Typical values for CH $_4$ lean premixed combustion range from 3000 to 8000 s $^{-1}$ [ 415]. Note that you can specify constant values or user-defined functions to define the Laminar Flame Speed and Critical Rate of Strain. See the separate UDF Manual. for details about user-defined functions.

For adiabatic models, you will also specify the Adiabatic Temperature of Burnt Products ( $T_{\rm ad}$ in Equation  16.2-21), which is the temperature of the burnt products under adiabatic conditions. This temperature will be used to determine the linear variation of temperature in an adiabatic premixed combustion calculation. You can specify a constant value or use a user-defined function.

For non-adiabatic models, you will instead specify the Heat of Combustion per unit mass of fuel and the Unburnt Fuel Mass Fraction ( $H_{\rm comb}$ and $Y_{\rm fuel}$ in Equation  16.2-23). FLUENT will use these values to compute the heat losses or gains due to combustion, and include these losses/gains in the energy equation that it uses to calculate temperature. The Heat of Combustion can be specified only as a constant value, but you can specify a constant value or use a user-defined function for the Unburnt Fuel Mass Fraction.

To specify the density for a premixed combustion model, choose premixed-combustion in the Density drop-down list and set the Density of Unburnt Reactants and Temperature of Unburnt Reactants ( $T_u$ and $\rho_u$ in Equation  16.2-24). For adiabatic premixed models, your input for Temperature of Unburnt Reactants ( $T_u$) will also be used in Equation  16.2-21 to calculate the temperature.

The other properties specified for the unburnt mixture are viscosity, specific heat, thermal conductivity, and any other properties related to other models that are being used in conjunction with the premixed combustion model.


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