# Conjugated heat transfer (CHT): Solid not cooling down

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 August 5, 2017, 16:01 Conjugated heat transfer (CHT): Solid not cooling down #1 Member   Join Date: Oct 2012 Posts: 32 Rep Power: 12 Hi all, I have a problem understanding the conjugated heat transfer mechanism present at my model. The geometry consists of a block of copper (Dimensions: 10 mm (width) x 10 mm (height) x 100 mm (length) with a circular cutout (diameter 5 mm). Air at a temperature of 200 K and an inlet velocity of 1 m/s flows through the circular cutout of the copper block. The copper block has an initial temperature of 300 K and has a convective heat transfer of 10 W/mēK to the ambient temeprature of 300 K. Due to symmetry, I only modelled 1/4 of the problem. I set up my simulation model following the official ANSYS Tutorial Heat Transfer from a Heating Coil and the advice given in this video tutorial. After the simulation, I evaluated the average temperature of the with this expression: volumeAve(Temperature)@Solid The results look pretty much how I would expect them. I evaluated the temperature at both the solid and the fluid symmetry wall. You can see how the cold air stream gets heated up by the solid with increasing pipe length. The image below shows the results from a steady state simulation: steadystate.png I simulated 3 different cases and evaluated the average temperature of the solid accoroding to the formular stated above: Steady state simulation: 279.0 K Transient simulation (duration: 3.600s = 1h / timestep: 10s): 269.2 K Transient simulation (duration: 18.000s = 5h / timestep: 10s): 269.2 K I always thought that the steady state simulation represents the final solution after a certian (unknown) amount of time, whereas a transient simulation also provides information about the history of the solution. However, the results above tell a different story, do you have any explanation for that? It would make sense to me if both transient cases would predict higher temperatures compared to the steady state case. Just for the record: All cases use the same BC and the same mesh. Many thanks in advance!

 August 6, 2017, 13:29 #3 Member   Join Date: Oct 2012 Posts: 32 Rep Power: 12 Dear Glenn, thanks a lot for the advice with the imbalance, that saved my day . I had to run the steady state simulation much longer to obtain convergence of the thermal energy. The (converged) temperature of the Solid is now 269.5 K, which is in accordance to the transient simulation. Once again, thank you very much for helping me!

 August 6, 2017, 18:54 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,372 Rep Power: 139 OK, you found a big error. But don't forget the rest of the comments in case there is a smaller error - make sure the other key sources of error are under control.

 Tags cht, cht problem, conjugated heat transfer