I am modelling fluid flow past a heat exchanger (HEX).  The HEX has been modelled as a series of solid copper pipes with a temperature differential across them (this is a suitable approximation as the flow rate through the HEX is controlled to achieve the required temperature range).  This extracts heat from an external fluid.

The issue I'm having is that when looking at the cut plot for the fluid temperature, fluid temperatures are visible inside the HEX tubes.  I would assume that this isn't possible as (a)the tubes are an impermeable solid and (b)the temperature in the tubes has been defined hence should not be influenced by interactions in the simulation.

What's more, in some simulations this does not occur. The project set-up does not change, only the geometry and power output of the heat source is altered.

I've attached screen captures of the model working as expected, with no fluid temperatures in the tubes, and the faulty model, with fluid temperatures showing in the tubes.

Any suggestions as to why this is happening would be gratefully received!

Thanks in advance.