Liquid fuel combustion can be modeled with the discrete phase and non-premixed models. In FLUENT, the fuel vapor, which is produced by evaporation of the liquid fuel, is defined as the fuel stream. (See Section 15.9.) The liquid fuel that evaporates within the domain appears as a source of the mean fuel mixture fraction.
Within FLUENT, you define the liquid fuel discrete-phase model in the usual way. The gas phase (oxidizer) flow inlet is modeled using an inlet mixture fraction of zero and the fuel droplets are introduced as discrete phase injections (see Section 22.12). The property inputs for the liquid fuel droplets are unaltered by the non-premixed model (see Section 22.14). Note that when you are requested to input the gas phase species destination for the evaporating liquid, you should input the species that comprises the evaporating stream.
If the fuel stream was defined as a mixture of components, you should select the largest of these components as the "evaporating species''. FLUENT will ensure that the mass evaporated from the liquid droplet enters the gas phase as a source of the fuel mixture that you defined. The evaporating species you select here is used only to compute the diffusion controlled driving force in the evaporation rate.