Best practice when using thin surfaces
 

Hello,

Let's imagine I want to model flow through two concentric cylinders (see attached), where the flow through pipe D1 is 20 Deg C, and flow through pipe D2 is 100 Deg C.

If I want want to know how much heat is transferred from D2 to D1 I can do this using thin surfaces in CFX.

The question is when drawing the geometry for CFX, should the diameter of D1 be its:

1. Internal diameter (which excludes the pipe wall thickness)
or
2. External diameter (which includes the pipe wall thickness)

Thank you

If the assumption of a thin surface is valid then it should not make much difference.

I would start by putting the surface at the mid-plane. If one fluid domain was particularly sensitive to the precise diameter you might make that one the correct diatameter. Of course this puts an error on the other domain. You will have to decide whether this is important or not.