Hi

I am trying to model conduction in the duct walls, free convection on the surface and fluid flow inside the duct. How do I specify the conduction and convection on the duct. Can that be specified in boundary conditions?

This is conjugate heat transfer problem ->you need solid cells, probably you can estimate the natural convective heat flux and use a wall-flux bc

Hi, you have to activate the option buoyancy on. Then you enforce a wall boundary condition on the duct surface with conductivity option.