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Erich March 24, 1999 19:16

Isobaric compressibility of Air
What is the isobaric compressibility (beta) of air at STP for use in convection analysis within the momentum equation for buoyancy effects? My units are English and its units should be Temperature^-1.

Where the momentum source term is dens*g*beta(T-Tref).


Erich F.

Heinz Wilkening March 25, 1999 08:24

Re: Isobaric compressibility of Air

For a ideal gas (p*v=R*T) like air

beta = 1 / T_ref,

as beta just is the proportional factor beween density and referenes density times temperatuer change for isobaric conditions.

rho = rho_ref * beta * ( T - T_ref )

Temperature for T_ref must be absulute value such as Kelvin or in your case Rankine.

Ciao Heinz

Erich March 25, 1999 11:25

Re: Isobaric compressibility of Air

Heinz Wilkening March 26, 1999 06:17

Re: Isobaric compressibility of Air

sorry I put a mistake into my answer. The density change is:

Delta rho = rho_ref * beta * ( T - T_ref )

This is, what is needed for buoyancy effect in an incompressible NS-simulation of natural convection.

Sorry Heinz

Erich March 26, 1999 10:25

Re: Isobaric compressibility of Air
Hello Heinz,

From your idea yesterday, I dug up my old heat transfer book and found the correct equation. I am currently trying to model the flow in a building with buoyancy effects and am not too successful, yet. My experience is in turbomachinery so this is new to me.

Any advice from all would be appreciated. My inlet flow to the building is heated and instead of stratifying up near the ceiling, it short circuits directly to the outlet. According to test data the flow does not exhibit this behavior. I am using CFX Tascflow and have buoyancy, etc turned on for the run.

Thanks again, Erich

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