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enigma November 11, 2009 20:08

Chemical Equilibrium Composition of Air and other Species
Hello Everyone,

I would like to set up an excel file from which I can read out the ratio of specific heats (gamma) of air atelevated temperatures (up to about 5000K).

The ratio of specific heats is defined as (where cp is the specific heat capacity of air at constant pressure):
gamma = cp / (cp-R)

where R is the specific gas constant of air. It can be expressed by the universal gas constant and the molecular mass of the substance:
R = Runiversal / Mair

The specific heat capacity is a function of temperature and can be taken from McBride, "NASA Glenn Coefficients for Calculating Thermodynamic Properties of individual Species", 2002 (∨=false&qs=Ntt%3D NASA%2BGlenn%2BCoefficients%2Bfor%2BCalculating%2B Thermodynamic%2BProperties%2Bof%2Bindividual%2BSpe cies%26Ntk%3Dall%26Ntx%3Dmode%2Bmatchall%26Ns%3DHa rvestDate%257c1%26N%3D0)

I implemented the data in the excel file and calculated gamma versus temperature. Here the plot for various species:

Where I got stuck:
Comparing the plot to previously published data, I realised that there is a significant difference especially for higher temperatures. In the plot, I assumed that the specific gas constant is constant over the temperature domain. This is not the case as the gas (here air) starts to dissociate at about 2000K changing the specific gas constant. The specific gas constant of associated (N2, O2) air is smaller than for dissociated air (N, O).

The question now is: how can I calculate the specific gas constant of air at elevated temperatures where dissociation happens? Are there any simple expressions? Here a plot of the equilibrium composition of Air, taken from Anderson's textbook on hypersonics.

I hope that I could depict the issue clearly and that there is someone in here who is proficient on the topic.

Thank you very much,

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