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Nusselt calculation

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Old   November 6, 2019, 10:57
Default Nusselt calculation
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hi every body

i need to determine the local Nusselt number in vertical channel and then plot the nusselt versus length of the channel by openfoam but i dont know do it
how can i preform it?
(the wall of this channel under condition of constant heat flux and phase change occures)

any help, comment and reply would be appreciated

thanks in advance
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Old   November 10, 2019, 14:35
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any help, comment and reply would be very very appreciated
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Old   November 15, 2019, 09:16
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I'm not sure I got it right, but that could be a solution. If it's ok with you, if you calculate it at the end.

Choose your location where you want to determine the nusselt number. Add some lines in a sampleDict (you dont need the line data but the wall temperature is the first entry of the line). For example the nusselt number at location y=0.1 and 0.2.


Code:
type sets;
interpolationScheme cellPoint;
setFormat raw;
writeControl    runTime;
writeInterval    1;

fields
(
    TMean //or T 
);

TYPE lineUniform;


sets
(

    s00 
    {
        type            $TYPE; 
        axis            distance;
        nPoints        3000;
        start          (0.0 0.1 0.0);     
         end           (0.01 0.1 0.0);
    }

      s01
    {
        type            $TYPE; 
        axis            distance;
        nPoints        3000;
        start          (0.0 0.2 0.0);     
        end           (0.01 0.2 0.0);
    }
 )
With the wall temperature at a certain location T_w,i, the mean temperature of the flow T_m and the given heat flux at the wall q_w you can determine your local heat transfer coefficient alpha_i
alpha_i = q_w / (T_m - T_w,i)


And your local nusselt number

Nu_i = (alpha_i * D_h) / lambda


If you need a time evaluation you can also include the sampleDict in the controlDict and write the wall temperature at certain times. But still, this is a simple post processing solution. You need to calculate it afterwards, which could be time consuming if you need your local nusselt number at many time steps.
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Old   November 15, 2019, 10:10
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very very thanks for your kind reply

I use the swake for calculate the heat transfer coefficient in your idea this code can be corrected?

thanks in advance

code:
(q=30000)
h1
{
type swakExpression;
valueType surface;
surfaceName X1;
surface {
type plane;
basePoint (0 0.1 0);
normalVector (0 1 0);
interpolate true;
}
verbose true;
variables
(
"T_inf=sum(U.y*Sf().y*T)/sum(U.y*Sf().y);"
"Ts{patch'heater}=sum(T*area())/sum(area());"
);

expression "30000/(Ts-T_inf)";
accumulations (
average
);
//writeSurfaceOnConstruction true;
// autoWriteSurface true;
//surfaceFormat vtk;
outputControlMode timeStep;//outputTime;//
outputInterval 10;
//writeStartTime no;
}
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