Thanks for your reply. In the case of humid air I do believe humid air is a mixture of liquid water and air, so it does seem like species would be the right case.

I would like to learn the case of condensation vapour as well. Would I use multiphase in that case, with phase 1 being air and phase 2 being water-vapour, phase 3 being water-liquid and activating evaporation-condensation effect? 

Also, is there a difference between steam and water vapor in Fluent? Also using species and mixture species I have noticed that among the selected species available there is only h2o. Is there a difference between h2o and water-vapor/water-liquid?

Thank you.