In some cases, you may wish to include only the convective transport of mixture fraction through the inlets of your domain. You can do this by disabling inlet mixture-fraction diffusion. By default, FLUENT includes the diffusion flux of mixture fraction at inlets. To turn off inlet diffusion, use either the define/models/species/inlet-diffusion? text command, or the Species Model panel
Define Models Species...
and disable the Inlet Diffusion option.