Fluent’s inert model can only can be used with non-adiabatic non-premixed or partially premixed combustion model. That is why it is greyed out for the adiabatic case. This option was enabled for the adiabatic case in fluent releases prior to R17.0, but this was a defect. The inert stream affects the solution due to its influence on enthalpy, specific heat, and density of the mixture. Inert calculations therefore require the enthalpy information, H_pdf, that is available in the non-adiabatic PDF tables: H(cell enthalpy) = (1-y_inert)*H_pdf+y_inert*H_inert; For non-adiabatic case H (left hand side) is known and H_pdf is also known from mean mixture fraction, we calculate the temperature, then calculate H_inert and go back to the above equation iteratively. For the adiabatic case, the cell enthalpy is function of mean mixture fraction only. The above equation can still be attempted with H(cell) calculated from Fmean, but H_pdf could only be calculated using a modified_fmean that is a function of y_ inert. This would indirectly make the above equation iterative over y_inert, which is not desirable as y_inert is to be solved for.