gas(same density with air) dispersion in the air ?
 

Dear all:
I want to simulate one gas (has the same density with air) disperses in the air ,which model should I use ?
In this forum, somebody said there are two methods, one is using Combustion model without reaction, another is solve another scalar transport equation.
So I think using RAS model and solving another scalar transport equation is better, because I am newer , How could I do this ? If I should write my own solver ?
Thank you very much.