RE: [cantera-users] Constant pressure specific heat calculation

[Zehe, Michael J. (GRC-RTM0)]

Hello Kevin et al:

 

                Have you tried the online CEA at http://cearun.grc.nasa.gov ?

 

Michael J. Zehe

NASA Glenn R.C.

 

From: canter...@googlegroups.com [mailto:canter...@googlegroups.com] On Behalf Of Kevin Holst
Sent: Tuesday, January 08, 2013 1:30 PM
To: canter...@googlegroups.com
Subject: [cantera-users] Constant pressure specific heat calculation

 

Group,

 

I've been checking out Cantera for a little while now, and I've run into a question that I think I know the answer to, but I just wanted to be sure. First some background, NASA's CEA code is considered the gold standard, in my line of work, for equilibrium thermo calculations. Unfortunately it's slow and cumbersome to use in computer programs. Cantera has been much easier to use for basic equilibrium calculations, and can be easily worked into existing code. So in an effort to get a warm, fuzzy feeling about using Cantera rather than CEA, I decided to compare the two for the pressure/temperature range of interest. When using the same set of NASA9 polynomial curve fits, everything compares VERY well between CEA and Cantera, except Cp (and presumably Cv, although that is not explicitly given as an output in CEA).

 

Does Cantera only calculate the 'frozen' Cp (as defined in the CEA Analysis Report http://www.grc.nasa.gov/WWW/CEAWeb/RP-1311.pdf in section 2.5), leaving out the contributions due to the reactions? If so, was there a specific rationale for doing that, and is there a function built in that I don't know about that calculates the equilibrium Cp?

 

I suspect that the answer to the first question is "no," based on a comment, reproduced below, in 'IdealMolalSoln.cpp' (thanks for commenting the code so well by the way!)

 

/*
 * Molar heat capacity at constant pressure: Units: J/kmol/K.
 *  * \f[
 * \bar{c}_p(T, P, X_k) = \sum_k X_k \bar{c}_{p,k}(T)
 * \f]
 *
 * Units: J/kmol/K
 */

 

 

-Kevin Holst

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[Ray Speth]

Kevin,

Cantera computes the heat capacity assuming that the composition is fixed. This definition is useful because it works even when the mixture is not at equilibrium, not to mention being a much simpler calculation. There is not, to my knowledge a built-in function for calculating the quantity you're looking for, but I think it would be easy to do numerically by computing the enthalpy of equilibrium states at T and T+dT and getting dh/dT from that.

Regards,

Ray

[Mary Spellman]

Kevin:

[Mary Spellman]

[Mary Spellman]

Kevin,

I am trying to get the NASA thermodynamic coefficients file to use with CEA, but all I can find is the file 'gri30' which allows me to set up an object for a whole group of compounds all at once, and do a 'tp' problem with the group.  But I wish to make an object that will allow me to set up and work with temperatures (or pressures) for individual compounds in other types of CEA runs like uv or hp or rocket problems.  Can you tell me how to do it?  Thank you in advance.

-MS-