Rob
Ansys Employee
There should be a coupled wall between the fluid and solid regions, leave that alone as it's taking heat from the fluid and passing to the solid.
Mixed external bc is fine, but unless you are using Mechanical the system coupling option isn't doing anything. For an exhaust I'd probably just use a convective boundary, and given the metal is a fairly uniform thickness I'd just use a wall boundary with a thickness and shell conduction rather than try and mesh the metal bit.