Constant properties water with variable density as a function of temperature

JuPa · October 30, 2014, 18:10

Hi,

In the material properties for water is it possible to use constant properties for everything except density?

I want density to be a function of temperature and pressure.

I've read that the Redlich Kwong real gas model may be able to do this. However if I turn it on, is it as difficult to converge with as full IAPWS (where every property is a function of pressure and temperature)?

Thank you

AliMadayen · October 30, 2014, 20:30

Hi. Sorry but I can't get the problem right.
Isn't it possible by a simple expression? or there is something that makes it more complicated?

ghorrocks · October 31, 2014, 05:16

Yes, you can use constant properties except for density. What density equation of state do you want to apply?

JuPa · October 31, 2014, 05:47

I have an Eulerian-Eulerian free surface setup. I am modelling water liquid and water vapour, separated by a free surface.

I would like to apply variable density for the water liquid equation of state.

So far I am modelling variable density in the water vapour phase as an ideal gas assumption.

I've just stumbled across a fourth order polynomial expression of density as a function of temperature for the water liquid phase, which I may implement. This is the equation:

Density [kg/m^3] = (-2.98043*10^-8*T^4)+(4.93093*10^-5*T^3)+(-3.25853*10^-2*T^2)+(9.18937*T)+82.2757

Obviously to use the above expression you would need to call upon a dimensionless temperature (just divided T by 1 [K]) to give the correct units of density [kg/m^3].

Bar this approach, are there any others which would give me variable density for water liquid, but constant properties for everything else?

ghorrocks · November 1, 2014, 05:43

Rather than generate a dimensionless temperature use the correct units in your function. For instance:

Density = (-2.98043*10^-8 [kg m^-3 K^-4]*T^4)+(4.93093*10^-5[kg m^-3 K^-3]*T^3)+(-3.25853*10^-2[kg m^-3 K^-2]*T^2)+(9.18937[kg m^-3 K]*T)+82.2757 [kg/m^3]

If you wanted variable density but constant everything else and you know this equation is the density versus temperature function then I would just use this, rather than trying to use the more sophisticated models.

October 30, 2014, 18:10	Constant properties water with variable density as a function of temperature	#1
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Hi, In the material properties for water is it possible to use constant properties for everything except density? I want density to be a function of temperature and pressure. I've read that the Redlich Kwong real gas model may be able to do this. However if I turn it on, is it as difficult to converge with as full IAPWS (where every property is a function of pressure and temperature)? Thank you

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October 30, 2014, 20:30		#2
AliMadayen New Member Ali Madayen Join Date: Apr 2014 Posts: 22 Rep Power: 12	Hi. Sorry but I can't get the problem right. Isn't it possible by a simple expression? or there is something that makes it more complicated?

October 31, 2014, 05:16		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Yes, you can use constant properties except for density. What density equation of state do you want to apply?

October 31, 2014, 05:47		#4
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	I have an Eulerian-Eulerian free surface setup. I am modelling water liquid and water vapour, separated by a free surface. I would like to apply variable density for the water liquid equation of state. So far I am modelling variable density in the water vapour phase as an ideal gas assumption. I've just stumbled across a fourth order polynomial expression of density as a function of temperature for the water liquid phase, which I may implement. This is the equation: Density [kg/m^3] = (-2.9804310^-8T^4)+(4.9309310^-5T^3)+(-3.2585310^-2T^2)+(9.18937*T)+82.2757 Obviously to use the above expression you would need to call upon a dimensionless temperature (just divided T by 1 [K]) to give the correct units of density [kg/m^3]. Bar this approach, are there any others which would give me variable density for water liquid, but constant properties for everything else?

November 1, 2014, 05:43		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Rather than generate a dimensionless temperature use the correct units in your function. For instance: Density = (-2.9804310^-8 [kg m^-3 K^-4]T^4)+(4.9309310^-5[kg m^-3 K^-3]T^3)+(-3.2585310^-2[kg m^-3 K^-2]T^2)+(9.18937[kg m^-3 K]*T)+82.2757 [kg/m^3] If you wanted variable density but constant everything else and you know this equation is the density versus temperature function then I would just use this, rather than trying to use the more sophisticated models.