Fluid flow due to temperature differences

Hello,
This is not my strong side so I would appreciate some recommendations.
I want to calculate the cooling oil flow in a cylindrical geometry in which there are a number of heat producing elements. The oil should change its specific weight due to the heat transfer and this should result in a free flow.
I would probably need more than one physical model, i.e. heat transfer and buoyant flow.
In Comsol I found some possibilities, such as single phase flow and multi phase flow.
As I said, this is not my strong side.
Are there any experts who can lead me to the right area?
Thank you
Kamil