When the premixed combustion model is used, FLUENT calculates density using the ideal gas law. For the adiabatic model, pressure variations are neglected and the mean molecular weight is assumed to be constant. The burnt gas density is then calculated from the following relation:
where the subscript refers to the unburnt cold mixture, and the subscript refers to the burnt hot mixture. The required inputs are the unburnt density ( ), the unburnt temperature ( ), and the burnt adiabatic flame temperature ( ).
For the non-adiabatic model, you can choose to either include or exclude pressure variations in the ideal gas equation of state. If you choose to ignore pressure fluctuations, FLUENT calculates the density from
where is computed from the energy transport equation, Equation 16.2-22. The required inputs are the unburnt density ( ) and the unburnt temperature ( ). Note that, from the incompressible ideal gas equation, the expression may be considered to be the effective molecular weight of the gas, where is the gas constant and is the operating pressure.
If you want to include pressure fluctuations for a compressible gas, you will need to input the effective molecular weight of the gas. The density will be calculated from the ideal gas equation of state.