If a process is not adiabatically, the fluid is absorbing or emitting heat through the wall of a geometry.
This can be simulated with a conjucated heat transfer sovler. The solver needs two regions. One for the fluid, the second one for the wall.
Inside the region of the fluid, heat transfer happens because of conduction and convection. For the wall its only conduction. The influence of the wall-material and the fluid condition are both considered.
heat conduction through a flange
The picture below shows the coubled simulation of the flow inside a reactor head with the heat transfer through the flange. On the outside of the reactor the heat is transfered to the air.