Effusive molecular energy flux ($F_E=2k_{B}Tn\overline{v}/4$)

An equilibrium Maxwell-Boltzmann gas has number flux $n\overline{v}/4$ onto a plane. Molecules crossing the plane are weighted by their positive normal velocity, so their mean translational kinetic energy is $2k_{B}T$, rather than the bulk value $3k_{B}T/2$. The translational energy flux is therefore $F_E=n\overline{v}{k}_{B}T/2$. Internal degrees of freedom contribute their mean energy per crossing molecule when independent of translational velocity.