How does CFX treat an opening when heat transfer is modelled?
 

For the transfer of heat to occur, there has to be a driving force. I.e. a temperature difference.

It is my understanding that the opening temperature is only used when the fluid re-enters the domain.

In that case how does CFX determine the driving force at the opening? What is delta T at the opening? What temperature difference does CFX use to determine how much heat leaves at the opening? And since it's an opening, a heat transfer coefficient cannot be used. What is the formulation?

Is the opening equivalent to implementing a wacking great heat transfer coefficient in the code at the opening with some mass sinks?

No. At an opening the temperature at the boundary is dependant on the flow direction:

If the flow is out of the domain then the boundary temperature is convected from the interior nodes.

If the flow is into the domain then the boundary temperature is set as the defined temperature.

Thank you. :)