The energy equation can be simplified in the special case that all species diffusivities are the same and equal to thermal conductivity divided by specific heat capacity,

Equation 133.

This equation Equation 133 holds when the Lewis number is unity for all components: . For turbulent flow, assuming for all components is usually just as good as the common practice of using the fluid viscosity for the default component diffusivity (unity Schmidt number, ). For , the energy equation Equation 132 simplifies exactly to the following:

Equation 134.

This equation Equation 134 has the advantage that only a single diffusion term needs to be assembled, rather than one for each component plus one for heat conduction. This can significantly reduce numerical cost, in particular when the fluid consists of a large number of components.