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Enthaply as calculated by CEA/JANAF Polynomials

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Old   December 16, 2014, 10:37
Default Enthaply as calculated by CEA/JANAF Polynomials
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Hello all,

First time posting here, but this problem has been giving me a headache for the last week.

I am using the CEA data base (http://www.grc.nasa.gov/WWW/CEAWeb/) and have written a programme to extract the coefficients for searched species which is all fine. My values for Cp as a function of temperature are coming out fine, however I am struggling with the calculated enthalpy.

Cp/R = a1*T^-2 + a2*T^-1 + a3 + a4*T + a5*T^2 + a6 T^3 + a7T^4

and when I input temperature I get a reasonable value for Cp (calorifically perfect air has a Cp of ~1004 J/kg K this gives 1004.81 at 300K and it varies with temperature) thats great, however:

H/RT = -a1*T^-2 + a2*lnT/T + a3 + a4*T/2 + a5*T^2/3 + a6T^3/4 + a7*T^4/5 + b1/T

gives values for enthalpy which I am struggling to understand. (again using calorifically perfect gas for air as a sanity check, H=Cp*T, therefore at 300K H~=1004*300~=301200 J/kg K)

The above polynomial however gives a value of -2475 J/gk K at 300K.
For Thermally Perfect Gas:
H = integral (T-ref to T) Cp(s) ds + H-ref
H = integral (298.15 to T) Cp(s) ds + H(298.15)
where H(298.15) is the heat of formation (which for air is -4333 J/kg K)

Unfortunately all the documention I can find is quite vague and just says you get enthalpy from the polynomial, however I would like to actually understand the fundamentals if anyone is able to explain it better.

Cheers.
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