# Probable improper implementation of optional thermal layers in mixed multi-region BC

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 February 26, 2015, 07:48 Probable improper implementation of optional thermal layers in mixed multi-region BC #1 New Member   Volker Join Date: Aug 2014 Location: Germany Posts: 15 Rep Power: 10 Hi all, The feature I am concerned about is the option to insert additional thermal layers at a boundary. It is implemented in the turbulentTemperatureCoupledBaffleMixed BC which may be applied for heat transfer between different regions e.g. when the solver chtMultiRegionFoam is used. I have some doubts on the calculation of the overall (series) heat transfer coefficient (contactRes_) in version 2.3.1. I cite the corresponding lines in the file: turbulentTemperatureCoupledBaffleMixedFvPatchScala rField.C Code: ``` 113 if (dict.found("thicknessLayers")) 114 { 115 dict.lookup("thicknessLayers") >> thicknessLayers_; 116 dict.lookup("kappaLayers") >> kappaLayers_; 117 118 if (thicknessLayers_.size() > 0) 119 { 120 forAll (thicknessLayers_, iLayer) 121 { 122 const scalar l = thicknessLayers_[iLayer]; 123 if (l > 0.0) 124 { 125 contactRes_ += kappaLayers_[iLayer]/l; 126 } 127 } 128 } 129 }``` This calculation sums up thermal conductivities of the layers in the following way contactRes_ = kappa1/Layer1 + kappa2/Layer2 + …. This would mean something like additional, parallel heat paths. However from my understanding the layers were intended as series heat transfer and consequently the inverse values should be summed up: 1/contactRes_ = Layer1/kappa1 + Layer2/kappa2 + … Interestingly the second formulation may be found in the very similar BC file turbulentTemperatureRadCoupledMixedFvPatchScalarFi eld.C Code: ``` 119 if (dict.found("thicknessLayers")) 120 { 121 dict.lookup("thicknessLayers") >> thicknessLayers_; 122 dict.lookup("kappaLayers") >> kappaLayers_; 123 124 if (thicknessLayers_.size() > 0) 125 { 126 // total thermal transmittance by harmonic averaging 127 forAll (thicknessLayers_, iLayer) 128 { 129 const scalar l = thicknessLayers_[iLayer]; 130 if (l > 0.0) 131 { 132 contactRes_ += l/kappaLayers_[iLayer]; // inverse sum 133 } 134 } 135 contactRes_ = 1.0/contactRes_; // new total inverse 136 } 137 }``` In case you make use of thermal layers in the turbulentTemperatureCoupledBaffleMixed BC, better have a look. I certainly would prefer the second version for a layered problem. Regards, Volker

 September 2, 2021, 06:06 #2 New Member   Fab Join Date: Apr 2019 Posts: 24 Rep Power: 5 I'm bumping this old thread just to share my experiments with kappaLayers and coupled Bcs in multiregion solvers. I have strange issues that look a lot like what is described above. So I set up a test case where I have an interface solid/fluid where the solid heats the fluid. I have found that if you put kappaLayers only on the fluif side, instead of reducing the heat flux passing through the interface, you increase it ! So my guess is that it is a parallel flux kind of thing implementation that adds a thermal path if you put kappalayers only on one side of the interface... Or the sign of the heat flux matters... If you put kappalayers on both sides or on solid side (where the flux come from), the heat flux is effectively reduced. So my question is, do you need to put h/2 on both sides to have a thermal insulation layer of effective height h or just put h on both sides ? I think you should put the same height h on both sides and it will result in an effective height of h.

January 28, 2022, 15:45
#3
New Member

Balaji
Join Date: May 2013
Posts: 21
Rep Power: 11
Quote:
 Originally Posted by Rotidpor I'm bumping this old thread just to share my experiments with kappaLayers and coupled Bcs in multiregion solvers. I have strange issues that look a lot like what is described above. So I set up a test case where I have an interface solid/fluid where the solid heats the fluid. I have found that if you put kappaLayers only on the fluif side, instead of reducing the heat flux passing through the interface, you increase it ! So my guess is that it is a parallel flux kind of thing implementation that adds a thermal path if you put kappalayers only on one side of the interface... Or the sign of the heat flux matters... If you put kappalayers on both sides or on solid side (where the flux come from), the heat flux is effectively reduced. So my question is, do you need to put h/2 on both sides to have a thermal insulation layer of effective height h or just put h on both sides ? I think you should put the same height h on both sides and it will result in an effective height of h.
Hi,

Did you find the solution to this problem?

Does it adds up in parallel or in series, the efective thermal conductivites?

For example

// windshield 2 1 & 2 mm
thicknessLayers (0.002 0.001 0.002);
kappaLayers (0.8 0.2 0.8);
}

I have a windshield of three layers with 2, 1 and 2 mm respectively, i would like to use the effective thermal conductivities of 0.8, 0.2, 0.8 so that the heat transfer is reduced progressively.

Is the turbulentTemperatureRadCoupledMixedFvPatchScalarFi eld.C file replaced with proper calculation?

Thanks

 February 4, 2022, 05:11 #4 New Member   Fab Join Date: Apr 2019 Posts: 24 Rep Power: 5 Well, I must admit that I've been super lazy recently and my solution to this problem is to just create and mesh the layers. It is as bad as it sounds but at least it works without any trouble