Exhaust gases dispersion simulation in open atmosphere
 

Hi guys,
I am doing a project which involves the simulation of exhaust gases in an open atmosphere. I have done the modelling and meshing using GAMBIT and now I am trying to solve the problem using FLUENT. The thing is I haven't performed dispersions or mixing of two fluids in Fluent before so, I have a couple of questions to ask.

The grid is 3d and a huge one (building complex) so I am using Fluent on Cluster. In the problem exhaust gases (natural gas engines) are coming out from circular duct and their conditions are given (temperature, mass flow rate). Atmospheric conditions are also given (wind speed, direction).
BC of my domain is velocity inlet for wind inlet, pressure outlet for outlet, wall for building and ground.
Now my questions are:

1. What should be best boundary conditions for the exhaust duct outlet in fluent for the given problem? (I used Mass flow inlet)

2. How to define/specify different fluids at the same time in grid i.e. Air & exhaust gases?

3. What would be the best model for this problem? (I am thinking for K-Epsilon Standard)

In my results I would like to have flue gas dispersion, velocity profile and temperature profile. So what would be simplified strategy to solve this problem. :)

If you have further questions, post below
I am a beginner in CFD and any assistance you can provide would be greatly appreciated.
I am looking forward for your help. . . :)