Fixed Temperature vs. fixed heat flux | buoyantCavity case

Zena27 · February 22, 2023, 05:52

Hi foamers,

I am working with buoyantSimpleFoam on the buoyant cavity case. I worked out the tutorial with cold and hot walls with temp diff = 19.6 Celsius. Currently,

I want to replicate the same case but with fixedGradient BC, i.e., heatFlux in and heat flux out instead of hot and cold walls. Any idea about how should be the value of the gradient to impose?

I tried calculating the average gradient on the hot and cold walls from buoyantCavity case, but it wasn't representative at all.

Thank you!

Regards,
Zena

Hr_kules · February 22, 2023, 08:23

Hi, i recently started using the same solver and tried the fixedGradient as well as the externalWallheatfluxTemperature BC. The later has the advantage that you choose between flux and power.
Both work fine.
If you use the fixedGradient you can get the desired heat flux [w/m^2] by dividing the desired heat flux by the thermal conductivity of the material. For example i wanted to have a heat flux of 1000 w/m^2 so i divided 1000 by 0,607 the thermal conductivity of water at initial temperature and got 1647,44. That value was the fixed gradient.

Hope this helps

Zena27 · February 22, 2023, 09:15

Hello,
I got what you say but I still have a question.

How to replicate the fixedTemperature case entirely with heatflux / fixedGradient BCs ?

In other words how to choose the heat flux in/out value?

I tried getting the temperature gradient on the hot and cold walls using OpenFOAM (grad) functionObject, and then took the weighted average of the gradient on each wall using paraview. When I used this gradient as BC in the fixedGradient case, it didn't give the same result.

Thank you

Hr_kules · February 22, 2023, 09:18

Have you tried wallHeatFlux functionobject? it usually just prints out the heat flux, you can "measure" it and then use it again in a externalWallHeatFluxTemperature BC and check if is the same.

Zena27 · February 22, 2023, 10:16

Hi,
I have just tried the walHeatFlux functionObject along with the externalWallHeatFluxTemperature.

The results of the fixedGradient case and the externalWallFlux case matched but they didn't match with the original buoyantCavity case of fixedTemperature BC.

Is this originally a reasonable comparison?

February 22, 2023, 05:52	Fixed Temperature vs. fixed heat flux \| buoyantCavity case	#1
Zena27 Member Zeinab Join Date: Feb 2023 Posts: 32 Rep Power: 3	Hi foamers, I am working with buoyantSimpleFoam on the buoyant cavity case. I worked out the tutorial with cold and hot walls with temp diff = 19.6 Celsius. Currently, I want to replicate the same case but with fixedGradient BC, i.e., heatFlux in and heat flux out instead of hot and cold walls. Any idea about how should be the value of the gradient to impose? I tried calculating the average gradient on the hot and cold walls from buoyantCavity case, but it wasn't representative at all. Thank you! Regards, Zena

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February 22, 2023, 08:23		#2
Hr_kules Senior Member Nico Join Date: Jan 2022 Location: Germany Posts: 122 Rep Power: 6	Hi, i recently started using the same solver and tried the fixedGradient as well as the externalWallheatfluxTemperature BC. The later has the advantage that you choose between flux and power. Both work fine. If you use the fixedGradient you can get the desired heat flux [w/m^2] by dividing the desired heat flux by the thermal conductivity of the material. For example i wanted to have a heat flux of 1000 w/m^2 so i divided 1000 by 0,607 the thermal conductivity of water at initial temperature and got 1647,44. That value was the fixed gradient. Hope this helps

February 22, 2023, 09:15		#3
Zena27 Member Zeinab Join Date: Feb 2023 Posts: 32 Rep Power: 3	Hello, I got what you say but I still have a question. How to replicate the fixedTemperature case entirely with heatflux / fixedGradient BCs ? In other words how to choose the heat flux in/out value? I tried getting the temperature gradient on the hot and cold walls using OpenFOAM (grad) functionObject, and then took the weighted average of the gradient on each wall using paraview. When I used this gradient as BC in the fixedGradient case, it didn't give the same result. Thank you

February 22, 2023, 09:18		#4
Hr_kules Senior Member Nico Join Date: Jan 2022 Location: Germany Posts: 122 Rep Power: 6	Have you tried wallHeatFlux functionobject? it usually just prints out the heat flux, you can "measure" it and then use it again in a externalWallHeatFluxTemperature BC and check if is the same.

February 22, 2023, 10:16		#5
Zena27 Member Zeinab Join Date: Feb 2023 Posts: 32 Rep Power: 3	Hi, I have just tried the walHeatFlux functionObject along with the externalWallHeatFluxTemperature. The results of the fixedGradient case and the externalWallFlux case matched but they didn't match with the original buoyantCavity case of fixedTemperature BC. Is this originally a reasonable comparison?