In order to simulate water-vapour diffusion through air (zero velocity), I created a multicomponent mixture of water-vapour and air. Also, the air part uses the constraint equation, and the water the transport equation. I specified the kinematic diffusivity.

However, when I try to run the simulation, I've found no mass transport (diffusion) for the water. In other words, the water vapour source is not mixing with the rest of the domain. Thus, I would like to ask if I need to turn on another parameter? Any suggestions?

The model is a square room, where the sides and the ceiling are walls. My floor is the type of inlet. The domain is set up with a certain water-vapour mass fraction (very small) while the floor is prescribed to a higher water vapour concentration than the rest of the domain.

is your flow model transient or stationary ? is your flow model turbulent or laminar ?

it should be transient and turbulent...