Your model design and boundary conditions really depend on what exactly you want to find out. Have a look at relevant papers which might be modelling something similiar to what you would like to achieve. Check what equationsand setups they are using and if you can apply them to your problem. Some suggestions:
Erinc Dobrucali, Selma Ergin: An investigation of exhaust smoke dispersion for a generic frigate by numerical analysis and experiment
Oliver Mărunţălu, Gheorghe Lăzăroiu, Elena Elisabeta Manea, Dana Andreya Bondrea, Lăcrămioara Diana Robescu: Numerical Simulation of the Air Pollutants Dispersion Emitted by CHP Using ANSYS CFX
Teixeira, Jose C. F.;Teixeira, Senhorinha F. C. F.;Cunha, Pedro;Silva, Angela M.: THERMAL DRIVEN DISPERSION OF SMOKE IN A PARKING SPACE