[ashghan]

Hello,

I am actually trying to implement a boundary condition with heat transfer through the external wall. The equation is

Tw=(Tair-(alphain+(l/d)+alphaout)(Tair-Tinf))/alphain
 
 where Tw = wall temperature
 
 Tair = air temperature (calculated from the solver)
 alphain = heat transfer coefficient on the inside of the wall( calculated from the solver)
 l= thermal conductivity;
 d= virtual wall thickness;
 alphaout=heat transfer coefficient on the outside of the wall
 Tinf= outside temperatue

Has anybody got any idea on how to implement this BC using groovy? with alphain and Tair generating from the solver. Till now I have tried something like this:

wand2
    {
        type groovyBC;
        value uniform 293.15;
       fractionExpression "0";
       variables "alphaa=20;Tinf=300;d=0.015;l=23;Tw=(Tair-(alphain+(l/d)+alphaout)(Tair-Tinf))/alphain";
                    timelines (
                    );
}

which does'nt work and gives me an error like this

--> FOAM FATAL ERROR: 
 Parser Error for driver PatchValueExpressionDriver at "1.30" :"syntax error, unexpected '('"
"(Tair-(alphaa+(l/d)+alphaout)(Tair-Tinf))/alphaa"
                               ^
-------------------------------|

Context of the error:


- From dictionary: /home/ashwin/Desktop/wallbc/0/T.boundaryField.wand2
  Evaluating expression "(Tair-(alphaa+(l/d)+alphaout)(Tair-Tinf))/alphaa"


    From function parsingValue
    in file lnInclude/CommonValueExpressionDriverI.H at line 1081.

FOAM exiting

I am using buoyantSimpleFoam Solver. I have no idea on how to generate the Tair and heat transfer coefficient from the solver.

I hope somebody here has an idea about it.


Regards
Ashwin

[Tobi]

Hi Ashwin,

hmmm your problem seems "easy".

First - the solver calculates Alpha (in the air Domain) and you will get the temperature of the wall.

You want to extend the wall with an thickness of "l" to simulate a pipe or sth like that and the heat Transfer from the pipe to the outer fluid (i think air).

So you will do a fluid - solid - fluid Simulation.

a) you can do it with chtMultiRegion
b) you can build your own BC - as you tried

In your case (b) you made some mistakes.

1. you do not calculate the heat Transfer from fluid to solid surface - that do the solver. Therefor you do not Need alphain.

2. Tair is not a OpenFOAM variable. I think you have to use "T" or "T()" instead.

3. I did not get your Point with Tw, Tair, Tinf. What do you want to achive? In my opinion you Need:

- conductivity
- Tinf
- Thickness


Any questions?
Regards Tobi

[ashghan]

Hello Tobias,

First of All, Thank you for the reply.

Yeah I need conductivity, Thickness, The outside wall temperature = Tinf and the heat transfer coefficient on the outside = alphaout as a user input to calculate the Tw= wall temperature on the inside.

I am applying this to a Normal wall(with a thickness d) in a room, to calculate the inside wall temperature.


Regards
Ashwin

[akidess]

I think you're trying to apply a mixedBC / Robin boundary condition: http://www.cfd-online.com/Forums/ope...acianfoam.html

[ashghan]

Originally Posted by akidess

I think you're trying to apply a mixedBC / Robin boundary condition: http://www.cfd-online.com/Forums/ope...acianfoam.html

Hello,

I am trying to do something similar to the external wall heat flux.

Regards
Ashwin

[Tobi]

Hi Ashwin,

ähm ... hmmm i am a bit confused

Just to clear my mind. You want to bring heat energy into your system ?
Well alphaout you have to define yourself

[ashghan]

Hello Tobias,

Yes I am bringing the heat Energy from outside the system and yeah I give alphaout, conductivity, thickness and outside temperature as a user input.

The alphain and Tair are derived from the solver. So when I use a Value expression like

Tw=(T-(alpha+(l/d)+alphaout)(T-Tinf))/alpha

It does'nt work. So how do I calculate the Tw?

http://www.flickr.com/photos/115947529@N08/12211738076/

[Tobi]

Originally Posted by ashghan

Hello Tobias,

Yes I am bringing the heat Energy from outside the system and yeah I give alphaout, conductivity, thickness and outside temperature as a user input.

The alphain and Tair are derived from the solver. So when I use a Value expression like

Tw=(T-(alpha+(l/d)+alphaout)(T-Tinf))/alpha

It does'nt work. So how do I calculate the Tw?

http://www.flickr.com/photos/115947529@N08/12211738076/

Hi you have a logical mistake.
You do not need Tw ... Tw is calculated from the solver!

Regards
Tobi

[ashghan]

Originally Posted by Tobi

Hi you have a logical mistake.
You do not need Tw ... Tw is calculated from the solver!

Regards
Tobi

Hi,

Now I am using this piece of code

wand2
    
        {
                 type groovyBC;
                 value uniform 293.15;
                 variables "alphaout=20;Tinf=300;d=0.05;l=0.05;";
                 Value Expression "(T-(alpha+(l/d)+alphaout)*(T-Tinf))/alpha";
                 timelines       ();
               
                 }

and I get the following error

#0  Foam::error::printStack(Foam::Ostream&) at ??:?
#1  Foam::sigFpe::sigHandler(int) at ??:?
#2   in "/lib/x86_64-linux-gnu/libc.so.6"
#3  Foam::heRhoThermo<Foam::rhoThermo, Foam::pureMixture<Foam::constTransport<Foam::species::thermo<Foam::hConstThermo<Foam::incompressiblePerfectGas<Foam::specie> >, Foam::sensibleEnthalpy> > > >::calculate() at ??:?
#4  Foam::heRhoThermo<Foam::rhoThermo, Foam::pureMixture<Foam::constTransport<Foam::species::thermo<Foam::hConstThermo<Foam::incompressiblePerfectGas<Foam::specie> >, Foam::sensibleEnthalpy> > > >::correct() at ??:?
#5  
 at ??:?
#6  __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"
#7  
 at ??:?
Floating point exception (core dumped)

Any ideas on this?

Regards
Ashwin

[olivierG]

hello,

Why you don't use the "externalWallHeatFluxTemperature" BC ?

regards,
olivier

[ashghan]

Originally Posted by olivierG

hello,

Why you don't use the "externalWallHeatFluxTemperature" BC ?

regards,
olivier

Hello,

I tried to use that, but I am not getting good heat balance. The HeatFlux's seem to be bad.

Regards
Ashwin

[olivierG]

hello,

If something is wrong with "externealWallHeatFluxTemperature", you may report as a bug. This will help all OF user, including you.

If you use use groovyBC, you may follow the advice from Akidess:

type groovyBC;
value uniform 293.15;
variables "alphaout=20;Tinf=300;d=0.05;l=0.05;alphawall=l/d;K=DT*rho*Cp;htot=alphaout+alphawall;";
valueExpression "Tinf";
fractionExpression "1.0/(1.0 + K/(mag(delta())*htot))";

NB: you need to adapt the conductivity "DT" name, density "rho" and "Cp" name.

regards,
olivier

[ashghan]

Hello Oliver,

I have a question from your code.

Why do we need DT and K ? Since l is the thermal conductivity provided.

Regards
Ashwin

[olivierG]

Hello,

Check and read the link from Akidess for more info.
DT, rho , Cp are for the bulk properties.

regards,
olivier

[ashghan]

Hello Tobias, Akidess and Oliver,

I have another question regarding external wall heat flux temperature. Can I input Q in [W/m³] instead of [W/m²] making it as a volumetric source term?

Regards
Ashwin

[ashghan]

Hello,

My Expression is working when I use h instead of alpha. Is it correct? I took it as h w.r.t to convective heat transfer coefficient notation. Please leme know if I am wrong.

valueExpression "(T-(h+(l/d)+alphaout)*(T-Tinf))/h";

If I use alpha I get an error saying

Parser Error for driver PatchValueExpressionDriver at "1.5-9" :"field alpha not existing or of wrong type"

Regards
Ashwin

[esujby]

Hello,

I am currently trying to model a solar volumetric receiver, i have conducted optical ray tracing analysis and have value for the heat flux at the aperture of the receiver. i have the data for the heat flux in 3 formats, grid form, matrix form, and table form. the heat flux represents a spot focus area (3cm radius) with energy density values in the formats mentioned. if you can shed some light on this issue it would be really helpful.

thanks