# What exactly is h in externalWallHeatFluxTemperature BC?

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 January 16, 2015, 09:24 What exactly is h in externalWallHeatFluxTemperature BC? #1 Member   Petr Furmanek Join Date: Jan 2012 Location: Prague, Czech Republic Posts: 60 Rep Power: 5 Hi all! Maybe it is a stupid question, but after looking into the code I'm still a bit confused about true meaning of h in the externalWallHeatFluxTemperature. If I assume the following boundary problem internal fluid -> wall -> environment there are two heat transfer coefficients coming into play - alpha_1 between the internal fluid and the wall and alpha_2 between the wall and the environment. So which one of these is h?

 January 17, 2015, 05:39 #2 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,115 Rep Power: 19 Hi, If you take a look at the sources, here's the way heat flux is calculated: Code: case fixedHeatTransferCoeff: { scalar totalSolidRes = 0.0; if (thicknessLayers_.size() > 0) { forAll (thicknessLayers_, iLayer) { const scalar l = thicknessLayers_[iLayer]; if (kappaLayers_[iLayer] > 0.0) { totalSolidRes += l/kappaLayers_[iLayer]; } } } q = (Ta_ - Tp)*(1.0/h_ + totalSolidRes); break; } Ta_ is ambient temperature, Tp is temperature on the patch. Now, assuming you neglected additional thermal resistance, h can be calculated as 1/h = 1/h_{flud->wall} + 1/h_{wall} + 1/h_{wall->external}

January 17, 2015, 16:51
#3
Member

Petr Furmanek
Join Date: Jan 2012
Location: Prague, Czech Republic
Posts: 60
Rep Power: 5
Hi Alexey,

thanks for the reply. I'm using OF v 2.3.x with a little bit different code

Code:
if (thicknessLayers_.size() > 0)
{
forAll (thicknessLayers_, iLayer)
{
const scalar l = thicknessLayers_[iLayer];
if (kappaLayers_[iLayer] > 0.0)
{
totalSolidRes += l/kappaLayers_[iLayer];
}
}
}
hp = 1.0/(1.0/h_ + totalSolidRes);
and then

Code:
else if (mode_ == fixedHeatTransferCoeff)
{
Qr /= Tp;
refValue() =  hp*Ta_/(hp - Qr);
valueFraction() =
(hp - Qr)/((hp - Qr) + kappa(Tp)*patch().deltaCoeffs());
}
where Qr is radiative heat flux. You're right that from theory the overall hp value is
Quote:
 1/hp = 1/h_{fluid->wall} + 1/h_{wall} + 1/h_{wall->external}
I wasn't sure if there is something neglected in the code or not... So if it isn't then apparently

1/h_{wall} = totalSolidRes

so I presume

1/h_ = 1/h_{fluid->wall} + 1/h_{wall->external}

I suppose I have to estimate the 1/h_{fluid->wall} for example from

as mentioned in https://en.wikipedia.org/wiki/Heat_transfer_coefficient

and then do a similar thing for 1/h_{wall->external} to get the h_ for input of the boundary condition...

 January 18, 2015, 05:05 #4 Member   Petr Furmanek Join Date: Jan 2012 Location: Prague, Czech Republic Posts: 60 Rep Power: 5 After looking into code of mixedFvPatchScalarField and fluidThermo I think I got it. The h_ is the outer heat transfer coefficient - the 1/h_{wall->external}. Heat transfer from internal fluid -> wall is solved using the kappa() field, inside the wall with the use of totalSolidRes which leaves the h_{wall->external} to be set by the user. ksara likes this.

 January 18, 2015, 09:35 #5 Senior Member   Alex Join Date: Oct 2013 Posts: 276 Rep Power: 12 Exactly Petr, h is the external heat transfer coefficient, the value of the solid resistance is calculated if you provide values of the kappa layers and thickness layers, otherwise it's neglected. Regards, Alex __________________ I'm newbie in OpenFOAM's world and not an English-speaking, so if I make any mistake a correction will be welcome!

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