Friction heating in heat transfer
Posted: Thu Aug 05, 2021 8:44 pm
Hello Yvan,
Please could I talk with you about the friction heating (kinetic energy effect) in rotating flows with heat transfer.
The case is a rotating cavity with two adiabatic discs, as shown in the figure below. There is expected to be a temperature rise on the discs from low to high radii, because the adiabatic discs will be heated by the rotating flow due to viscous friction. The temperature rise should be (Ωb)^2/(2*Cp)-(Ωa)^2/(2*Cp). In my case of Reφ=2*10^7, the temperature difference between r=b and r=a should be ~237K.
I ran the case in the incompressible module, selected the thermal scalar to Temperature, did wall-resolved RANS without wall function. In the results, the temperature on the disc was uniform, showing no frictional heating term was included in the energy equation. Then I switched the thermal scalar from Temperature to Enthalpy. The results are the same. So I suppose in the incompressible module, "friction term" is not included in the energy equation. In high Reφ cases, this may give wrong results, since the frictional heating will be significant and should not be neglected.
Then I switched the module from incompressible to compressible, ran wall-resolved RANS without wall function. Luckily, the temperature rise was correct on the adiabatic discs. So I suppose the energy equation in compressible module is fully formed, including friction term.
So my questions are:
1. How could I get the friction heating included in the incompressible module? Do I need to go into the source code and revise the energy equation (maybe add a source term)?
2. I want to use wall-modelled solutions to save computing time. But in the compressible module, when I switched the wall-resolved RANS to wall-modelled RANS (using 1-scale log law), the temperature rise on the adiabatic discs again went wrong. So I suppose in the compressible module, if I want to do wall-modelled solutions, I need to do some revisions to the thermal wall function. My supervisor told me to modify the definition of T+
Please could you suggest to me how could I achieve this? Should I go to code_saturne-master/src/base/cs_wall_function.h to modify it?
Sorry for the long post. Thank you very much for your very important help!
Best regards,
Ruonan
Please could I talk with you about the friction heating (kinetic energy effect) in rotating flows with heat transfer.
The case is a rotating cavity with two adiabatic discs, as shown in the figure below. There is expected to be a temperature rise on the discs from low to high radii, because the adiabatic discs will be heated by the rotating flow due to viscous friction. The temperature rise should be (Ωb)^2/(2*Cp)-(Ωa)^2/(2*Cp). In my case of Reφ=2*10^7, the temperature difference between r=b and r=a should be ~237K.
I ran the case in the incompressible module, selected the thermal scalar to Temperature, did wall-resolved RANS without wall function. In the results, the temperature on the disc was uniform, showing no frictional heating term was included in the energy equation. Then I switched the thermal scalar from Temperature to Enthalpy. The results are the same. So I suppose in the incompressible module, "friction term" is not included in the energy equation. In high Reφ cases, this may give wrong results, since the frictional heating will be significant and should not be neglected.
Then I switched the module from incompressible to compressible, ran wall-resolved RANS without wall function. Luckily, the temperature rise was correct on the adiabatic discs. So I suppose the energy equation in compressible module is fully formed, including friction term.
So my questions are:
1. How could I get the friction heating included in the incompressible module? Do I need to go into the source code and revise the energy equation (maybe add a source term)?
2. I want to use wall-modelled solutions to save computing time. But in the compressible module, when I switched the wall-resolved RANS to wall-modelled RANS (using 1-scale log law), the temperature rise on the adiabatic discs again went wrong. So I suppose in the compressible module, if I want to do wall-modelled solutions, I need to do some revisions to the thermal wall function. My supervisor told me to modify the definition of T+
Please could you suggest to me how could I achieve this? Should I go to code_saturne-master/src/base/cs_wall_function.h to modify it?
Sorry for the long post. Thank you very much for your very important help!
Best regards,
Ruonan