I use the values in the janaf file and make as you tell. (coeff = coeff N2 * 0.75 + coeff02 * 0.25)
Just a remark, I think that there is AIR in the janaf file so you haven't to calculate the coefficients

but why they're a little different.there is not air for old version of janaf that OF uses.

yes.your question caused my mistake to be corrected.
then to store for future users the correct coefficients for air in JANAF old version(7-coefficients that OpenFOAM uses) are:
did you find direct coefficients for 7-coefficient format for air in JANAF?

do you know the keyword for puting Cp's in each time step to see in paraView?(like the command Mach for mach number

No I don't know any keyword to do that. I don't use the janaf coefficients because the temperatures of my application are out of the range of temperatures possible with janaf.

is it possible to calculate the janaf coeffs for temperture below 30 K for air?

Its a very low temperature.I think maybe the gas not can be considered perfect in this low temperature.
when we use Cp(T) and Cv(T) the gas is assumed be perfect gas.are you pressures very low too?
you maybe need to use real gas equations of state.

Source for data
 

1.) Concerning reliable data, you can also ask goolge for "NIST <gas needed> thermodynamic properties". Then you quite quickly should find some data for different gases, among others air.
From these you should be able to calculate the coefficients with octave or matlab. I am still digging in the code what would be the right way of including the coefficients (they have to be modified with some factor)...

2.) Air below 30K will be quite challenging, aside from that being non-physical most probably. According to some NIST-information, air seems to become solid around 60 K. Thus you will not only be in the liquid phase but in a solid phase...

Hi all,
the link for JANAF table in here:http://openfoamwiki.net/index.php/Janaf seems have been broken.does anybody know another link for old version like that?
thanks.

Hi immortality,
Do you manage to use the Janaf Coefficient ?
I would like to use them for my simulation, but I'm wondering about the units used in OpenFOAM in oder to adapt the coefficient I've found.
Do you know the units used in OpenFOAM ?
Because that's really a mess... between J, cal or mol,kg... pfff I'm a bit lost from on source to another. To be sure of what I do, I need to know what OpenFoam is working with.

Air JANAF values
 

Hey, I know it's been a while since this thread was opened, but I did the same things you guys did but obtained much different values.
The links for the JANAF tables no longer work in openfoam.wiki so I used the values in this table for O2 (21%) and N2 (79%): http://combustion.berkeley.edu/gri_m...0/thermo30.dat

For O2:
high: 3.28253784E+00 1.48308754E-03-7.57966669E-07 2.09470555E-10-2.16717794E-14 -1.08845772E+03 5.45323129E+00
low: 3.78245636E+00-2.99673416E-03 9.84730201E-06 -9.68129509E-09 3.24372837E-12-1.06394356E+03 3.65767573E+00

For N2:
high: 0.02926640E+02 0.14879768E-02-0.05684760E-05 0.10097038E-09-0.06753351E-13 -0.09227977E+04 0.05980528E+02
low: 0.03298677E+02 0.14082404E-02-0.03963222E-04
0.05641515E-07-0.02444854E-10-0.10208999E+04 0.03950372E+02

In matlab I simply did:
Air_high = (0.79*N2_high) + (0.21*O2_high);
Air_low = (0.79*N2_low) + (0.21*O2_low);

I get for air:
highcpcoeffs: 3.00137854640000 0.00148695005540000 -6.08269040490000e-07 1.23755416750000e-10 -9.88622096400000e-15 -957.586304200000 5.86979569090000

lowcpcoeffs: 3.40027066560000 0.000483195742400000 -1.06301195790000e-06 2.42372488110000e-09 -1.25025170230000e-12 -1029.93906860000 3.88890578330000

Any ideas why our answers are different?

Thanks!

Hi,
did you validate your results?
I used the results from previous post (the one you are reffering to) and my values for enthalpy of air (calculated by OF) were way off.

Hey,

Thanks for your response. I've been doing simulations with those value, and so far I haven't gotten any issues... but doesn't mean I'm right! Try my values and see what you get? Do you agree with my procedure? Please let me know what outcome you get.

Thanks!

I tried your values and I got specific enthalpy 173 817.2412 for temperature 469.1499949. That is not correct since the h/T should be equal to cp which is roughly 1020 for this temperature.
Or am I missing something? How did you validate that you are using correct values?

I can not comment on the procedure you used since I am only getting familiar with janaf. But as far as I can tell, your procedure looks logical.

Hello. I think I have some experience with OLD! JANAF and openFOAM.

To calculate all a0 - a6 and b0 - b6 coefficient we need to use some additional tools like Cantera.

With Cantera installed we can use cpPolynomials.py:
http://chomikuj.pl/Sheaker/Studia/Me...,5632310992.py (I hope You can download it for free without registration) to calculate coefficient of Cp(T) of defined gas mixture for specific temperature ranges. Remember that Cantera outputs Cp(T) = [J/molK] while openFoam needs Cp(T)/R = [-].

To calculate b5 and b6 coefficient make sure Your H(T=273,15K) is equal to Standard Enthalpy of Formation and Your S(T=273,15K) is equal to Standard Entropy. Values of Standard Enthalpy of Formation and Standard Entropy comes from Cantera basic output.

To calculate a5 and a6 just make Your functions tangent near T_common.

I hope it put some light on JANAF coefficient.

Interesting. I didn't about the validation process as I thought what I had done was logical and would work...

Would you mind checking my procedure above and letting me know where my process is wrong? Wonder why the method I used wouldn't work?

Thanks in advance!

I believe Your procedure is right. Remember that Your case is valid in rage 300 - 5000 K while immortality's case if valid in rage 200 - 6000 K. Check attachment.

I think that difference at higher temperature are cause by different data in Cantera and Chemkin II.

I found my solution very similar to openFoam predefine coefficients for air as oxidant, octane/propane as fuel and stoichiometric burn products of those fuels.

Have a nice day.
Oskar

er10, would you try to evaluate enthalpy in your case, divide by temperature and get back to me with values of cp?

How:
add this at the end of your controlDict:

functions { writeFields { type writeObjects; libs ( "libutilityFunctionObjects.so" ); objects ( h ); } }

Hi Oskar,

So my method uses values from Chemkin II. Your plot seems to show that CP values in the lower range are different but not by much... would it be wrong to use the 300-5000 K case for test cases where I am seeing temperature above 293K?

Many thanks