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.
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