Plot of Cp (of Air) genrated from Cantera mismatches with the NASA Cp (of Air) plot

[Gibin Varghese]

Hi all, 

First of all, I am not a highly skilled Cantera user.  Please find below my doubts regarding the Cantera.

1) I was actually trying to use Cantera as a thermographic plot generator (for plotting Cp, Cv, Molecular weight etc) and I was successful in generating the Cp and Cv plot for air using cantera. Unfortunately, the plot doesn't match with the literature (I referred to the NASA paper).  I have attached the Resulting Cantera plot and its deviation from the NASA literature plot and also the matlab program which I have written for the cantera plot generation. I have also written some programs for other gases and compared with the existing literature plots, sadly was also not matching for all temperature ranges. 

2) How good is it to use this Cantera software for the prediction of Cp, Cv values of  different gases?

3) Is there any good sources where I can get the different Thermodynamic reactions and its corresponding thermodynamic  values (required by cantera) for different gases. I am in need of in particular of gases like Air and SF6.

It would be great, if someone can help me regarding the above.

Many many thanks in advance.

With Best Regards,

Gibin Varghese

[wal...@rxnsys.com]

Gibin,

You don't post where your original data comes from - the NASA set. I used the data from here: https://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html as a comparison. The plots all fall on top of each other (I plotted Cp as solid lines and the ratio of specific heats as dash-dot). I do not see the divergence behavior your red line shows.

I included my Matlab file (compare_SpHeat.m), the digitized data from Engineering Toolbox (Air.xlsx), and a comparison plot (Cp_Comparison.tif).

To answer your questions:

2) How good is it to use this Cantera software for the prediction of Cp, Cv values of  different gases?

The short answer is that Cantera is as good of a predictor of variables as your underlying data file. When you use the different objects - Air() vs. GRI30() - you are using different reaction mechanisms. I am going to skip over a lot and I would recommend you look at CHEMKIN documentation (should be available even though the software is not). It will explain a lot of the details.

Anyway, each reaction mechanism consists of at least species data and reaction data. Sometimes, there are transport properties, but that only matters if you're looking at changes in viscosity and some other things, not for Cp/Cv, molecular weight, etc. The Air() object does not have transport properties loaded but the GRI30() mechanism does.

The species data relies on polynomials that are used to generate curve fits (you can find more information here: http://combustion.berkeley.edu/gri-mech/data/nasa_plnm.html). These polynomials were largely developed at NASA by Bonnie McBride and Sanford Gordon and are colloquially referred to as "the NASA polynomials". For air, especially non-reacting, the NASA polynomial thermodynamic data is considered very good and I would trust it implicitly.

3) Is there any good sources where I can get the different Thermodynamic reactions and its corresponding thermodynamic  values (required by cantera) for different gases. I am in need of in particular of gases like Air and SF6.

Generally, there are the well-known reaction mechanisms (some listed here: https://www.cerfacs.fr/cantera/index.php) that are available to everyone. More exotic ones are usually kept at research labs or facilities and may or may not be available to the public.

I hope this helps, good luck!

Sibylle

[Gibin Varghese]

Hi  Sibylle,

Thanks a lot for the valuable information you shared. It helped me quite a lot.

I just need one more information. Do you know from where I can find a yaml file or a cti file for SF6 gas at a temperature range of 300-3000K?

Many many thanks in advance.

With Best Regards,

Gibin Varghese