In general, you will not need to modify the constants used in the equations presented in Section 16.2. The default values are suitable for a wide range of premixed flames. If, however, you want to make some changes to the model constants, you will find them under Model Constants in the Species Model panel.
You can set the Turbulence Length Scale Constant ( in Equation 16.2-6), the Turbulent Flame Speed Constant ( in Equation 16.2-4), the Stretch Factor Coefficient ( in Equation 16.2-14) and the Turbulent Schmidt Number (Sc in Equation 16.2-1).
For a non-adiabatic premixed combustion model, note that the value you specify for the Turbulent Schmidt Number will also be used as the Prandtl number for energy. (The Energy Prandtl Number will therefore not appear in the Viscous Model panel for non-adiabatic premixed combustion models.) These parameters control the level of diffusion for the progress variable and for energy. Since the progress variable is closely related to energy (because the flame progress results in heat release), it is important that the transport equations use the same level of diffusion.