The over-/undershoots in the gradient of the mass fraction are due to use of cell averaged values for computing the gradients in the boundary cells (Least-Squares or Green-Gauss cell-based gradient reconstruction schemes) and should not occur with a node-based scheme. In the default Least-Squares gradient reconstruction formulation, the cell center values are used for computing the cell gradients adjacent to the inlet/outlet boundaries. In order to resolve this issue, you need to change the default Least-Squares-gradient correction be setting the value of recon/cell-lsq/cortype to 2: (rpsetvar ‘ recon/cell-lsq/cortype 2) More information regarding the Least-Squares gradient correction options: There are three gradient correction options available in ANSYS Fluent for correcting the Least-Squares gradients at the boundaries which are controlled by recon/cell-lsq/cortype. One can set the value of this rpvar to 0,1(default), or 2. Here 0 means that the solver will zero out the gradients at the boundary face, 1 means that solver will zero out the normal gradient at the boundary faces, and 2 means that the boundary values are extrapolated implicitly at the boundaries faces. In general, the default value is recommended for most of the applications as it was tested vastly and was found to be fairly accurate and robust, but the setting can be changed when the obtained solutions are not satisfactory.