[Techies]

Hi guys,
I'm working with Air at 25C in CFX
If I specify temperature above 25 C, what are air properties I need to modify ? Example for Pr=0.71
and what are the differences between Air at 25 C condition and Air Ideal Gas in setup ? I see Air at 25C use density and Air Ideal Gas use Modal Mass

[ghorrocks]

The "Air at 25C" material is an incompressible fluid with constant properties.

The "Air Ideal Gas" material is a compressible fluid model (using the ideal gas model) with constant properties.

If you want to change any of the properties (eg you want an incompressible fluid at a different temperature) then just add the new fluid as a material in CFX-Pre. The easiest way to do this is to duplicate the Air at 25C one.

[Techies]

Originally Posted by ghorrocks

The "Air at 25C" material is an incompressible fluid with constant properties.

The "Air Ideal Gas" material is a compressible fluid model (using the ideal gas model) with constant properties.

If you want to change any of the properties (eg you want an incompressible fluid at a different temperature) then just add the new fluid as a material in CFX-Pre. The easiest way to do this is to duplicate the Air at 25C one.

thanks, have a good day

[ghorrocks]

Oh, and I should add that the ideal gas material only uses the ideal gas model if you select "total energy" as the thermal model. If you use any other thermal model it will be an incompressible material.

[evcelica]

Originally Posted by ghorrocks

Oh, and I should add that the ideal gas material only uses the ideal gas model if you select "total energy" as the thermal model. If you use any other thermal model it will be an incompressible material.

Glenn, what do you mean by this exactly?
I thought when using thermal energy with ideal gas it is still compressible, it just did not include the full equation for enthalpy (adiabatic heating/cooling will not be included) but the density will still vary with pressure and temperature. Meaning Q = m * Cp * DT. So it would still be applicable for natural convection and simulations where enthalpy change due to pressure changes are not of concern.

[ghorrocks]

Yes, I think you are correct Erik. I understand the Thermal Energy option does act as you describe.

[Techies]

If i use rotating domain, cfx suggests Total Energy instead of Thermal Energy because Thermal E neglect rotational energy contributions.
So how are rotational energy contributions important under rotating condition ?
And what are the difference between inlet/outlet rotating and non-rotating ?

[evcelica]

I was not suggesting to use thermal energy model. Use total energy, it takes into account all fluid thermodynamic equations.

[Techies]

Originally Posted by evcelica

I was not suggesting to use thermal energy model. Use total energy, it takes into account all fluid thermodynamic equations.

Thanks. So what about inlet/outlet rotating and non-rotating ?

[ghorrocks]

The rotating frame of reference needs to be taken into account in the calculation of total temperature, enthalpy and pressure. There may be other differences as well. I don't know exactly why the total energy option is recommended over thermal energy for rotating cases.

[Opaque]

Thermal Energy eqn = Total Energy eqn - Mechanical Energy eqn

It only accounts for the energy transfer by advection, diffusion and viscous dissipation. It does not account for mechanical effects, i.e. shaft work

Thermal Energy should never be used when there is work input/output through the system boundaries