17.3.2 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 this equation in the separate Theory Guide), 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., [ 40]).

For both adiabatic and non-adiabatic combustion models, you will need to specify the Laminar Flame Speed ( $U_{l}$ in this equation in the separate Theory Guide) as a material property, in the Create/Edit Materials dialog box. 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 this equation in the separate Theory Guide). As discussed in this section in the separate Theory Guide , $g_{\rm cr}$ 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 this section in the separate Theory Guide), 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 $^{-1}$ [ 95]. 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 Burnt Temperature ( $T_{\rm ad}$ in this equation in the separate Theory Guide), 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 this equation in the separate Theory Guide). ANSYS 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 Adiabatic Unburnt Density and Adiabatic Unburnt Temperature ( $T_u$ and $\rho_u$ in this equation in the separate Theory Guide). For adiabatic premixed models, your input for Adiabatic Unburnt Temperature ( $T_u$) will also be used in this equation in the separate Theory Guide 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.