Mixing of liquid nitrogen with gaseous nitrogen in a conduit
 

I am looking forward to perform an analysis wherein, i want a regulated temperature of gaseous nitrogen in the range of 120 to 130 K. I am planning to mix gaseous nitrogen with liquid nitrogen in a pipe.
Due to evaporation of liquid nitrogen, gaseous nitrogen will cool down.
Given some pressure inlet BC, which model is suitable for such analysis, Multispecies transport model (non-reacting) or Multiphase model (VOF or mixture).

The conditions are near the critical point, so you first need a real EOS model. Moreover with a density based solver you have to play with large spurious velocity and pressure oscillations, when you pass the two phase region or the widdom line. Generally such simulations are performed with a HEM model, because surface tension is neglectable and the phase interface is quite diffusive.

I am no Fluent expert, so i can't suggest you a solver, but the simulations are quite challenging, especially when you have to deal with more than one component.

So good luck!