Ansys Employee

Difficult to say: partly because I can't see the model but also I'm not allowed to give detailed advice due to US export law. 

Heat transfer from the fluid to solid is handled in the solver & wall functions: avoiding an interface there is a good idea but not a requirement.  Also read up on y+ and wall functions (turbulence sections of the manual). 

Heat through the solid is a function of the material conductivity and surface area: we can use thin walls with a thickness, shell conduction or solid zones.