CFD Online Discussion Forums - Janaf coefficient for gaz mixture + nMoles ?

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- OpenFOAM (https://www.cfd-online.com/Forums/openfoam/)

- - Janaf coefficient for gaz mixture + nMoles ? (https://www.cfd-online.com/Forums/openfoam/175917-janaf-coefficient-gaz-mixture-nmoles.html)

Janaf coefficient for gaz mixture + nMoles ?

Hi all,

I would like to change the gas in the solver XiFoam, (running methane instead of propane). The thing is, I don't know how to calculate the Janaf coefficient for gas mixture. Does anybody know ?
Another question more specific about this solver I think, WHY is there nMoles = 1 for products, whereas it's set to 24.8095 for reactants (in the combustionProperties file). Where does this 24.8095 comes from ?

Thanks a lot, hopping one of you hold the answer,
Adrien

Ok, about the gas mixture,I've found my answer, using the Dalton's law in order to determine the janaf coeff of a gas mixture.

https://en.wikipedia.org/wiki/Dalton%27s_law

Leading to : Cp = http://direns.mines-paristech.fr/Sit...gaz-ideaux.png xi . Cpi where xi is the mole fraction.

Unfortunately, I can't handle why there's nmoles = 24.8095 for reactants and nmoles = 1 for products...(According to me, they all should be at 1...) If someone have the answer, let me know.

Have a nice ay,
Adrien

Hi adrieno,

To change the cp coefficients you can use the adiabaticFlameT utility.

cd $FOAM_UTILITIES/thermophysical/adiabaticFlameT
adiabaticFlameT controlDict

you ll get values for the fuel specified in controlDict.

However I have the same problem about 'nmoles' could not find an answer.
If you find an answer please share.

Hi Uyan,

I don't understand how to use the controlDict you've told me about :

Code:

/*--------------------------------*- C++ -*----------------------------------*\

| =========                 |                                                 |

| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |

|  \\    /   O peration     | Version:  2.2.2                                 |

|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |

|    \\/     M anipulation  |                                                 |

\*---------------------------------------------------------------------------*/

FoamFile

{

    version     2.0;

    format      ascii;

    note        "settings for calculating the adiabatic flame temperature";

    class       dictionary;

    object      controlDict;

}

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

P       1e5;

T0      300.0;

H2

{

    fuel H2;

    n    0;

    m    2;

}

CH4

{

    fuel CH4___ANHARMONIC;

    n    1;

    m    4;

}

ETHYLENE

{

    fuel C2H4;

    n    2;

    m    4;

}

PROPANE

{

    fuel C3H8;

    n    3;

    m    8;

}

OCTANE

{

    fuel C8H18(L)_isooctane;

    n    8;

    m    18;

}

N-HEPTANE

{

    fuel C7H16_n-heptane;

    n    7;

    m    16;

}

$H2;

// $CH4;

// $PROPANE;

// ************************************************************************* //

Could you be more specific please, sorry for my ignorance.

Another question : Do you know how to adapt the coeff in the combustionPorperties file ? for another gas like methane in my case (I just handle the laminar speed with Su but I'm a bit lost with all the other coefficients)

Thanks a lot,
Adrien

Hi Adrian,

Type these two commands on an OpenFOAM terminal,

Code:

cd $FOAM_UTILITIES/thermophysical/adiabaticFlameT 

adiabaticFlameT controlDict

this will print out cp coefficients you need for the fuel specified in the controlDict. In the controlDict you have shown here it is H2.

Well for other values in combustionProperties dictionary you ll have to look for values from literature and try them out.

Thank you very much for your time Uyan,

Does it gives the Cp value for a stoechiometric mixture ? (This is what I'm looking for).
And just to know, is it possible to have it for another stoechiometric value ?

For the other coefficients, this is what I was trying to do... but it's hard to find especially as the Weller combustion model doesn't seem very common. Even the laminar flame speed seems to differ from a revue to another ! Aaaa combustion is such a complex science for neophyte !

Thank you again Uyan.

Hi Adrian,

For you and others who wondered why the "nMoles" is not equal to 1 in the reactants side but 24.8. I finally found the answer how to calculate. So I post it here for anyone who is stuck there.

In the XiFoam tutorial file (moriyoshiHomogeneous), thermophysicalProperties dictionary contains properties for reactants and products as following.

Code:

reactants

{

    specie

    {

        nMoles          24.8095;

        molWeight       29.4649;

    }



   .....

Code:

products

{

    specie

    {

        nMoles          1;

        molWeight       28.3233;

    }

This case is for a stoichoemetric mixture of Propane and air. So Propane and Air mixture is considered as reactants.

Reactants ---------------> Products

$C_3H_8 + 5\bigg( O_2 + 3.76 N2 \bigg) -----> 3CO_2 + 4H_2O + 18.8 N2$

For reactants
$nMoles = 1 + 5 + 18.8 = 24.8 \sim 24.8095.$

$molWeight = \frac{44 + 5\times (32 + (3.76 \times 28) ) }{1 + 5\times(1+3.76)} \sim 29.4$

For products it is considered as one species, so nMoles = 1

Thank you Uyan, that's what I've done finally in my simulation, but that was just a guess finding the way the weller combustion model was working...