[Rob Strong]

Hi,

names Rob. We currently work on a heat transfer problem between a solid and fluid domain. Simulation works fine but I'am not happy with the absolute values. So i broke the problem down to the heat transfer in solid Bodys and noticed that Ansys Thermal and CFX give me different results.

In Thermal simulations I add 50W on one side of a block, 4 sides isolated and I measure 50W output on the opposite site.
The same case in CFX I only get 41W of output. The side walls exchange 0W with the surrounding. I tried different simulation conditions and I always seem to loose energy.

Does anyone of you have an idea were the missing energy went?

What I tried:
- Fixed Temperatures instead of heat flux
- Different boundary conditions for side walls (symmetry, heat exchange, ...)
- Changed the dimensions


My Setup:
- Ansys 2019
- Thermal and CFX steady state

[ghorrocks]

Your simulation is not adequately converged. The default convergence settings do not work very well for heat transfer simulations as residuals are not an ideal thing to measure convergence. For heat transfer simulations you should add imbalances as a convergence criteria (in CFX-Pre, solver settings tab).

Secondly, how tight a residual or imbalance tolerance should you use? You should do a sensitivity study to see the error generated by various convergence criteria. Choose a convergence criteria which gives you an accuracy you are happy to accept.

Finally, you will find double precision numerics will help.

[Rob Strong]

Hi ghorrocks,

thank you for your imput! I will try to change the convergence criteria and run a few test.

cheers Rob