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Power and Enthalpy in fvOptions file

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Old   April 25, 2019, 05:22
Default Power and Enthalpy in fvOptions file
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Raza Javed
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Hello everyone,

I am trying to generate a heat source using Power of 10W using fvOptions file. So for this, I have given the value of 10 to "h" in fvOptions like below:

    type            scalarSemiImplicitSource;
    active          true;
        selectionMode   all; // all, cellSet, cellZone, points
       // cellZone        heatSrc;
        //cellSet         c1;
        volumeMode      absolute; // specific;
            h     (10 0);

I have read in this forum that If I need a power of say 10W, then I just need to put that value as "h (10 0)" in fvOptions and the volumeMode should be "absolute".

The temperature of my heat source is changing when I check it in paraview but I don't understand that how this power(W) is changing my temperature? What is the mathematical relationship between them? Also some people also say that this 'h' is Enthalpy, so I am confused that if this is enthalpy then how can I put my power directly in "h" ?

Also I went through the solver log file to see how solver reads the fvOtption file. And it shows the following:

    Adding fvOptions

Creating finite volume options from "constant/fvOptions"

Selecting finite volume options model type scalarSemiImplicitSource
    Source: heatSource
    - selecting all cells
    - selected 10599 cell(s) with volume 123000

What does this mean?
-selected 10599 cell(s) with volume 123000

This value remains the same even if I change the value of "h" in my fvOptions file.

I shall be very thankful if someone can clear my doubts.

Thank you
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Old   April 26, 2019, 10:03
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> -selected 10599 cell(s) with volume 123000

This simply means, that your fvOptions is applied to 10599 cells which have a combined volume of 123000 m.

Lets assume you have the following equation:

\nabla\cdot \left(\alpha \nabla h \right) = 0

This is the equation for heat conduction inside a solid. It is an equation for enthalpy, but can also be formulated with temperature directly. You can add an explicit source term S to this equation:

\nabla\cdot \left(\alpha \nabla h \right) = S

Now this source term describes internal heat generation. Again, please check a fundamentals book in heat transfer for that.

scalarSemiImplicitSource has an explicit and implicit part.
h     (10 0);
This means that to the equation for h and explicit part of 10 should be added and an implicit part of 0. This changes our equation to
\nabla\cdot \left(\alpha \nabla h \right) = 10

I hope this helps
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fvoptions, heat sources, openfoam, power, temperature

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