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-   -   Strange nheptane density (http://www.cfd-online.com/Forums/openfoam-solving/58547-strange-nheptane-density.html)

 tsencic September 8, 2008 07:48

I investigated some fluid prop

I investigated some fluid properties and I found a strange low density for heptane used in OpenFOAM.
In src/thermophisicalModels/liquids/C7H16 is said that density is described by NSRDSfunc5 with coefficients (61.38396836, 0.26211, 540.2, 0.28141).
In src/thermophysicalModels/thermophisicalFunctions/NSRDSfunctions/NSRDSfunc5.h is given the function:
return a/pow(b,1+pow(1-T/c,d))
When I print this on a graph, I obtain a density that is lower than 360kg/m3, while I think it should be over 700kg/m3.
Beside other sources, the NIST pages on the web http://webbook.nist.gov/chemistry/fluid/
conferm that it should be around 700kg/m3.
Is it a mistake or I am missing something?

 niklas September 8, 2008 10:31

I've checked the data and it l

I've checked the data and it looks ok to me.
at what temperature and pressure are you looking?

approximate values...
rho(T = 300) = 679
rho(T = 400) = 585
rho(T = 500) = 446
rho(T = 540) = 270

the data is using the NSRDS functions and coefficients (reference is in the source-files)

and is only valid at around athmospheric pressures and up to critical temperature (around 540 K), but I'd say that accuracy around critical conditions is probably poor.

NIST also reveals that at p=1bar liquid does not exist above ~370K (which is ~normal boiling point)

at 100 bar NIST yields
rho(300K) ~ 687
rho(500K) ~ 509
rho(540K) ~ 463

so not too shabby by NSRDS neglecting the pressure influence Id say

 tsencic September 9, 2008 03:12

Sorry, my mistake! n-heptane i

Sorry, my mistake! n-heptane is OK.
It is n-heksadecane C16H34 that has strange low density:
coeffs: rho_(61.94656776, 0.25442, 720.6, 0.3238),
func: return a_/pow(b_, 1 + pow(1 - T/c_, d_));
aproximate values
rho(T = 300) = 355
rho(T = 400) = 332
rho(T = 500) = 305
rho(T = 540) = 293

I do not have the refference, but I would expect that C16H34 has bigger density.

 niklas September 9, 2008 07:07

Yup, something is weird here..

Yup, something is weird here...

I ran hexadecane in supertrapp and got this:
p = 100 bar.
rho(T=300) = 790.2
rho(T=400) = 708.4
rho(T=500) = 637.6

for n-heptane:
rho(T=300) = 686.3
rho(T=400) = 601.6
rho(T=500) = 504.0

so supertrapp predicts reasonable values for n-heptane, but that is not a guarantee that it will do so for hexadecane, however, its supposed to be pretty accurate for hydrocarbons.

so I then ran hexadecane in openfoam and got this.
rho(T=300) = 768.8
rho(T=400) = 697.9
rho(T=500) = 618.9

so Im guessing the mistake is on your side.

this was my setup in thermophysicalProperties.

liquidComponents
(
C16H34
);

liquidProperties
{
C16H34 C16H34 defaultCoeffs;
}

 borise September 16, 2008 17:57

Hi! I have a short question

Hi!

I have a short question to those of you who solve the equation of state for a number of liquids.

The NIST webpage includes calculations for greenhouse gases like C02 and CH4. Are these equations included in Openfoam or in some other software (together with the sourcecode)?

thanks!
Boris

 niklas September 17, 2008 02:08

Short answer is - nope. Even

Even shorter is - no haha.

A while back i implemented Peng Robinson EOS and sent it to Henry, but it has not been included yet
and I think its because he really wants to re-write the entire thermo treatment before making it into an even bigger beast.
For ideal gases it's ok, but its hard to extend it to handle mixtures with complex mixing rules.

 borise September 17, 2008 10:40

Thank you, Niklas, for the sho

Thank you, Niklas, for the short answer. I am also interested in EOS calculators outside of Openfoam. As long as these are open source, it shouldn't be too difficult to access them from Openfoam.

I am asking, because the project freesteam.sourceforge.net was mentioned in another post, which calculates the IAPWS water and steam properties.

In fact the IAPWS formulation appears to be based on the methane formulation, and so I am wondering whether the following two references have been the target of a project like freesteam, which would be interesting for atmospheric modelling:

Span, R., Wagner, W. A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa. J. Phys. Chem. Ref. Data 25 (1996) 1509-1596

Setzmann, U., Wagner, W. A new equation of state and tables of the thermodynamic properties for methane covering the range from the melting line to 625 K at pressures up to 1000 MPa. J. Phys. Chem. Ref. Data 20 (1991) 1061-1155

Boris

 haghajani June 29, 2009 06:52

Peng-Robinson EOS

Dear All,

I want to implement Peng-Robinson EOS, and make use of it in carbon dioxide high pressure release.

I think it is not included yet as Niklas mentioned; I coludn't find it, at least.

Would you please let me have you advise on the how to derive it suitable for OF.

As I had a look into current perfect gas Eos, it seems the psi(rho/P) parameter is more important than rho. Because, later rho is been calculate by, rho = p_*psi