Rob
Ansys Employee

You probably want both species and multiphase if the gas is a mix of air and vapour. 

Setting a constant mass fraction on the boundary is simple. RH is a function of pressure, temperature and volume fraction so you'd need to use an expression or UDF: neither will be simple. 

Re laminar/turbulent, that's for you to decide. Given the channel size do you think it will be turbulent? What about the head space?