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Isobaric compressibility of Air

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Old   March 24, 1999, 19:16
Default Isobaric compressibility of Air
  #1
Erich
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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).

Thanks,

Erich F.
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Old   March 25, 1999, 08:24
Default Re: Isobaric compressibility of Air
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Heinz Wilkening
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Hi,

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
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Old   March 25, 1999, 11:25
Default Re: Isobaric compressibility of Air
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Erich
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Thanks...
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Old   March 26, 1999, 06:17
Default Re: Isobaric compressibility of Air
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Heinz Wilkening
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Hi,

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
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Old   March 26, 1999, 10:25
Default Re: Isobaric compressibility of Air
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Erich
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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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