Can anyone suggest code (commer/home brewn) that perform well for problem where large buoyancy forces are important concern. Require field modeling, not zone. Application of room fires etc such as in building fires etc. Gracias in advance for suggestions.

Try looking at CFDRC web site - www.cfdrc.com

I have used STAR*CD (a commercial code) for buoyant flows, particularly fire problems for years. You may want to look at adapco's web site www.adapco.com (the North American distributor of STAR*CD) or Computational Dynamics web site www.cd.co.uk (the developer).