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Individual Gas Constant Units (lbm & lbf)

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mrpi

mrpi

Mechanical
Jun 22, 2008
80
US
I was evaluating supersonic nozzle performance when I had a question about the units for the Gas Constant "R".

The Imperial units are:

[ft-lbf]/[lbm-°R]

What gave me a bit of trouble when balancing the choked mass flow equation is the relationship bewteen lbf and lbm.

(I typed the equation out in MS equation editor, but can't figure out how to put it here...)

Do you take these (lbm& lbf) as equivalent at STP conditions?

Thanks,
=stan

Beat to fit, paint to match.
 
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What are you doing with it? When you use it to calculate gas density (rho=(P)/(R(gas)*T) then the lbf cancels the lbf in the pressure and you end up with mass/volume.

For other uses you have to be really careful. I just calculated R for air in MathCad and got 53.355 (lbf*ft)/(lbm*R). If I just delete the lbf/lbm I get 1716.7 (ft^2/s^2*R). That looks like lbf/lbm is 1/g.

David
 
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I'm using the mass flow to calculate the resultant force.

I want lbm/s and in the numerator is a lbf (from a psi number) that I'm not sure how to get into lbf.

Can I post a jpg directly into this text area? I'll post the equation I'm using.

Beat to fit, paint to match.
 
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mdotequation.jpg
 
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Where gamma (and the one "g" I forgot in the numerator...) represent the specific heat ratio (air = 1.4).

Beat to fit, paint to match.
 
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mrpi:

This is not an answer to your specific question, but you might find it useful to look at the tabulation of a number of R values and their units to be found in this article on the online Citizendium:


Milton Beychok
(Visit me at www.air-dispersion.com)
.
 
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lbf/lb = g, since weight is mass * g.

In Mathcad, psi ~ lbf/in^2

TTFN

FAQ731-376
 
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