Gas jet simulation
 

Hello everybody,
I'm simulating a jet of air full of water vapor in the atmosphere (cylindrical profile) (on Ansys fluent 12.1) so it's a gas-gas simulation; I'm using the spicies option without mixture or volume of fluid and i'm getting good contours of velocity but a constant contours of density in all the volume, which means that fluent doesn't mix the too fluids (gases) whereas it is supposed to decrease the density of the air full of water vapor until atmospherical density by the fact that the water vapor will be evaporated, can anyone please tell me the appropriate model to simulate that, thank you very much