[ghorrocks]

1. Is it OK? That depends on what you are trying to achieve. If close enough is good enough then it is fine. If you are looking for precise answers then no. Yes, it will alter the flow physics but whether it will do so enough to be significant depends on what you trying to get out of this model.

2. Is the outer structure the pipe? Which you intend to do a CHT simulation with? In this case I think it unlikely this is required for a LES simulation and yes, I would consider removing it.

3. The thermal time scales in the solid region are likely to be far longer than the time scales of the thermal fluctuations in the fluid. So if you are interested in the short time scales then the fluid has all the information you need.