[Jar0ld]

When can fluent show negative energy in volumes?
In case we have 3 materials concrete, steel and air. All materials initializated as 273.15 K temperature.

[MKuhn]

As the internal energy, like the enthalpy, of a system cannot be measured directly you only deal with the diffrence between different states. Internal energy is a thermodynamic potential, so in order to measure the enthalpy of a system, you must refer to a defined reference point; therefore what we measure is the change in internal energy, ΔU. So start your calculation and see how the values will change.

[Jar0ld]

In my case at 0 timepoint I have positive temperature and then it will become negative. After Initialization I plot Mass-average total energy of air and concrete, both are negative. By patch I give them +25°C temperature, Mass-average total energy of concrete become 0 j/kg, but air energy become -99966 j/kg. Air Total energy become 0 j/kg at about 416 K.

[MKuhn]

Be careful with patching values in selected cells. After that you will not have a valid solution in most cases. After patching you have to run the solver until convergence to calculated all temperature depending (or other variables wich are dependend of the patched property) properties properly.

[Jar0ld]

Dear Kuhn finally what you advice me, how to solve problem with ploting energy in material?

[MKuhn]

See attached pictures


--Moritz

[Jar0ld]

Thanks. I will try )

[Jar0ld]

I attach Data Total Energy, Temperature and Mass of air and building part at 2000 and 10000 seconds

10000 sec.PNG

2000 sec.PNG

[Jar0ld]

Sorry but fluent take reference(where energy =0) temperature for air as ≈416 K and for concrete 298.15. How can I change it?

[MKuhn]

You dont have to change it. Absolute values of enthalpy or energy are quite useless, as you work only with the difference of these values between two states. It is also ok, if these values are negative.
Check user or theory guide of fluent if you can change the reference state. But in my opinion it is not necessary.