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Enthalpy of air?????

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Old   February 6, 2020, 04:51
Question Enthalpy of air?????
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Brett
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Morning all,

This is gonna sound dumb, but thats what forums are for I guess.

How do I calculate the enthalpy of air? or more precisely what is going on with different calculators.

For example if you look at:

https://www.irc.wisc.edu/properties/

and you use coolProp:

http://www.coolprop.org/fluid_proper...al-description

You will get different values for the enthalpy of "Air".

There is also a discrepancy when I look at my results in Ansys CFX post.

Does anyone have any thoughts on this?

Brett
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Old   February 6, 2020, 05:39
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Kasper
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I usually use the NASA polynomials when working with enthalpy of single gasses or gas mixtures. I just assume a gas composition of 21% O2 and 79% N2 when calculating the enthalpy of air.
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Old   February 6, 2020, 06:03
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Brett
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Thanks Kasper,

I imagine that's what CFX does too.

For the purposes of compression I can use the ideal gas laws but it would be nice to be able to draw a p-h diagram still.
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Old   February 6, 2020, 10:21
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I don't know the differences between these two software because I can't run the coolprop. But...

Basically you are asking for an equation of state. Unfortunately, everyone uses their own. There is no enforceable standard for the EOS for the enthalpy of air that makes everyone happy when a lot of people still use the ideal gas law.

To make matters worse, we don't all agree on what is the definition of dry-air. A lot of academic-minded people use 21% O2 and 79% N2 as air. Refprop uses 78.12% N2, 20.96% O2, and 0.92% Argon!

CFX and most CFD software most often calculates enthalpy by integrating the specific heat (either constant, or stepwise, or polynomial). Because most CFD deals with aero / gas dynamics, a very popular approach is to use NASA polynomials. Which approach CFX is actually using is a user input btw. None of these approaches account for changes due to pressure.

The nice thing about the two calculators you provide is that they are based on the work of Lemmon, whose work is also the basis of NIST's REFPROP calculator. What's remarkable about this body of work is that they use the Helmholtz equation of state, which is a sledgehammer in the world of EOS's and vastly superior in accuracy compared to the ideal gas law or any practical variant of the real gas laws and vastly superior still to the very popular NASA polynomials.

At the end of the day, you probably will always have the wrong enthalpy. So just use any approach that you like and live with it.
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