When you choose to solve conservation equations for species, Icepak predicts the local mass fraction of each species, YI, through the solution of a convection-diffusion equation for the ith species. This conservation equation takes the following general form:
 | (40–7) |
where SI is the rate of creation by addition from user-defined sources. An equation of this form will be solved for N –1 species where N is the total number of fluid phase species present in the system.
Mass Diffusion in Laminar Flows
is
the diffusion flux of species i, which arises due to concentration gradients. Icepak uses the
dilute approximation, under which the diffusion ux can be written
as
 | (40–8) |
Here Di,m is the diffusion coefficient for species i in the mixture.
Mass Diffusion in Turbulent Flows
In turbulent flows, Icepak computes the mass diffusion in the following form:
 | (40–9) |
where Sct is the turbulent Schmidt number, 
(with a default setting of 0.7).
Treatment of Species Transport in the Energy Equation
For many multicomponent mixing flows, the transport of enthalpy due to species diffusion
 | (40–10) |
can have a significant effect on the enthalpy field and should not be neglected. In particular, when the Lewis number
 | (40–11) |
is far from unity, this term cannot be neglected. Icepak will include this term by default.