If the diffusive transport of energy or species is dominating, we have to take the diffusion across inlets into account. This is not included per default in ANSYS CFX. You have to add the expert parameter diffusion at inlets = t. in ANSYS CFX Pre : Insert ->Solver ->Expert Parameter->Discretization->Diffusion Scheme The Peclet number is a measure for the importance of diffusion relative to convection. It is the ratio of the rate of advection of a physical quantity by the flow to the rate of diffusion of the same quantity driven by an appropriate gradient. For values smaller equal 1 you have to consider the diffusion. For energy the Peclet Number is defined as: Pe = Tr*Pr= Charcteristic_Length * Velocity * Density * cp / lambda For species you could substitute the Pr Number by the Schmidt Number.