[Engineers_Firelli]

Hello OpenFOAM users,


I'm new on the forum. I'm trying to simulate a hot gas flow through ducts in OpenFOAM. I need to setup a boundary condition that combines:
* a heat transfer coefficient + outer temperature (representing the convection between the external wall and the ambient)
* an emissivity at the outer wall + outer temperature (representing the radiation from the externall wall to the ambient)


(* perhaps it would be nice also to include the conduction in the wall, but this may be calculated analytically and included in the heat transfer coefficient mentioned above)


I have struggled a lot with externalWallHeatFluxTemperature. I am simulating a simple case with a cylindrical tube (1 inlet, 1 outlet, 1 wall). The inlet is hot fluid (specified uniform velocity), and at the outlet there is a defined pressure (1 bar).



I am using externalWallHeatFluxTemperature for the wall.

walls
{
type externalWallHeatFluxTemperature;
mode coefficient;

//kappaMethod fluidThermo;
Ta constant 298.15;
h uniform 0.01;

emisssivity 0.95;
//thicknessLayers List<scalar> 1(0.1);
//kappaLayers List<scalar> 1(5.0);
//qr none;
//relaxation 0.3;
//qrRelaxation 0.3;
value $internalField;
}




I tried many different combinations of the parameters, but it seems that setting "emissivity" has no effect. Even, if I check the results file at some timestep different from "0", the emissivity does not even show up !


walls
{
type externalTemperature;
h uniform 0.01;
Ta
{
type constant;
value 298.15;
}
refValue uniform 298.15;
refGradient uniform 0;
valueFraction uniform 4.18458e-06;
value nonuniform List<scalar>
}


I looked into the code for externalWallHeatFluxTemperature for mode coefficient and I saw the coefficient there is defined as I would expect: h_rad = (Tp^4 - Ta^4)/(Tp-Ta) is added to the h defined by the user.



However I cannot make it function to compute radiation as well. It only uses the h that I provide.




I also tried to include a radiation model in my simulation (P1) (although I actually don't want to simulate the radiation inside the fluid domain, JUST to calculate the radiation from the external wall of the duct to the ambient), but it didn't change this behaviour.


I checked different related posts on the forum, but none seemed to give a good solution to this.



Has anyone had success in implementing convection + radiation (+conduction) using externalWallHeatFluxTemperature ? Did it work as you had expected ? Was the radiation contribution to the heat loss from the wall computed correctly ?



I am using OpenFOAM 11, the solver is fluid (and I noticed the same behaviour with CHTMultiRegion - now called foamMultiRun).




Thanks in advance.
Theo

[Engineers_Firelli]

I have tested the same case with OpenFOAM 2306 and 2406, and it worked exactly as expected. Even setting h = 0.000001 but with emissivity 0.99, a large amount of heat was released from the flow of hot gas, so the gas cooled down significantly.


simpleCase_externalWall_Radiation.zip

Screenshot_tube_longitudinal_section.jpg

In OpenFOAM 11 (solver "fluid"), the emissivity did not make any difference and in the same case as mentioned above, the gas stayed hot throughout the entire duct !


wall
{
type externalWallHeatFluxTemperature;
mode coefficient;
Ta constant 290.0;
h uniform 1e-06;
emissivity 0.99;
kappaMethod fluidThermo;
value $internalField;
}




Any idea where this difference is coming from ?