Best (easiest) way to implement a specific heat flux from a Wall to a Fluid region

FoamerLuc · February 22, 2021, 11:02

Dear Foamers,

I have been trying to find the best way to set-up my case for a while now:

What I am trying to simulate is the heat transfer from a standard heating coil to a turbulent flow of water. The water has a fixed flow rate and the temperature difference between the inlet and the outlet is used to calculate the absolute heat flux.

In the real-life model, I have a specific electric Power (~7 kW), which defines the absolute heat flux.

In the first Simulation, I used buoyantPimpleFoam and a fixedValue boundary condicition for the temperature. This generates a heatflux from the coil to the fluid (water) but it is far from realistic, because the surface temperature varies in real life and is difficult to calculate. What i observed with this model, is that the outlet-temperature always coverges to the temperature set on the coil-surface (probably because the heat flux is very high due to the fixed temperature value).

Side note: I also tried to use the incompressible buoyantBoussinesqPimpleFoam but i had quite a few issues with this solver.

A more realistic approach would be to set a constant heat flux from the coil to the water (basically a turbulentHeatFluxTemperature BC). I found similar cases using this BC but they are mostly run on older OF versions. (This BC doesn't seem to be available in newer OF versions, only the atmospheric BC "atmTurbulentHeatFluxtemperature")

An other approach would be a multiRegion-cHT Solver (cHTMultiRegionFoam, in which the Coil is defined as a seperate region.

What I would like to know, is if there are any other approches to define a constant heat flux from a wall to a fluid region. Maybe somebody has an idea or has dealt with a similar problem and has a solution that works?

Thanks in advance for your help!

Jean-Luc

tomf · February 23, 2021, 05:25

Hi,

You have not specified the version you use, but I think that the externalWallHeatFlux would work for your case.

The coil could be considered an external wall to your case.

Cheers,
Tom

FoamerLuc · February 23, 2021, 07:03

Hi Tom!

first of all thanks for your reply.

- I am using OpenFOAM v2012.

- I actually thought about that BC before. If I'm not mistaken, "externalWallHeatFlux" is a BC that provides a heat flux from the Fluid region to a Wall in order to simulate radiation and convection losses to the "outside".

If I treat the coil as an external Wall and use this BC (which makes sense in my opinion), I would need to ensure that the heat flux is directed into the fluid region and not the other way around.

Is that possible with this BC?

Thanks again for your help!

Cheers
Jean-Luc

tomf · February 24, 2021, 06:07

Hi,

It acts in both ways, depending on the sign of the heatflux. In general a positive heatflux would indicate the heating of the fluid by the wall and a negative heatflux would indicatie heating of the wall by the fluid if I remember correctly. But this would be revealed rather quickly in the simulation.

You can always test on a small domain, or maybe review the source code to check it.

Regards,
Tom

February 22, 2021, 11:02	Best (easiest) way to implement a specific heat flux from a Wall to a Fluid region	#1
FoamerLuc New Member Jean-Luc Join Date: Feb 2021 Posts: 2 Rep Power: 0	Dear Foamers, I have been trying to find the best way to set-up my case for a while now: What I am trying to simulate is the heat transfer from a standard heating coil to a turbulent flow of water. The water has a fixed flow rate and the temperature difference between the inlet and the outlet is used to calculate the absolute heat flux. In the real-life model, I have a specific electric Power (~7 kW), which defines the absolute heat flux. In the first Simulation, I used buoyantPimpleFoam and a fixedValue boundary condicition for the temperature. This generates a heatflux from the coil to the fluid (water) but it is far from realistic, because the surface temperature varies in real life and is difficult to calculate. What i observed with this model, is that the outlet-temperature always coverges to the temperature set on the coil-surface (probably because the heat flux is very high due to the fixed temperature value). Side note: I also tried to use the incompressible buoyantBoussinesqPimpleFoam but i had quite a few issues with this solver. A more realistic approach would be to set a constant heat flux from the coil to the water (basically a turbulentHeatFluxTemperature BC). I found similar cases using this BC but they are mostly run on older OF versions. (This BC doesn't seem to be available in newer OF versions, only the atmospheric BC "atmTurbulentHeatFluxtemperature") An other approach would be a multiRegion-cHT Solver (cHTMultiRegionFoam, in which the Coil is defined as a seperate region. What I would like to know, is if there are any other approches to define a constant heat flux from a wall to a fluid region. Maybe somebody has an idea or has dealt with a similar problem and has a solution that works? Thanks in advance for your help! Jean-Luc

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February 23, 2021, 05:25		#2
tomf Senior Member Tom Fahner Join Date: Mar 2009 Location: Breda, Netherlands Posts: 647 Rep Power: 32	Hi, You have not specified the version you use, but I think that the externalWallHeatFlux would work for your case. The coil could be considered an external wall to your case. Cheers, Tom

February 23, 2021, 07:03		#3
FoamerLuc New Member Jean-Luc Join Date: Feb 2021 Posts: 2 Rep Power: 0	Hi Tom! first of all thanks for your reply. - I am using OpenFOAM v2012. - I actually thought about that BC before. If I'm not mistaken, "externalWallHeatFlux" is a BC that provides a heat flux from the Fluid region to a Wall in order to simulate radiation and convection losses to the "outside". If I treat the coil as an external Wall and use this BC (which makes sense in my opinion), I would need to ensure that the heat flux is directed into the fluid region and not the other way around. Is that possible with this BC? Thanks again for your help! Cheers Jean-Luc

February 24, 2021, 06:07		#4
tomf Senior Member Tom Fahner Join Date: Mar 2009 Location: Breda, Netherlands Posts: 647 Rep Power: 32	Hi, It acts in both ways, depending on the sign of the heatflux. In general a positive heatflux would indicate the heating of the fluid by the wall and a negative heatflux would indicatie heating of the wall by the fluid if I remember correctly. But this would be revealed rather quickly in the simulation. You can always test on a small domain, or maybe review the source code to check it. Regards, Tom