Material Properties by expressions

testaccount · March 28, 2019, 08:58

Hey folks,

i want to simulate the flow of liquid oxygen. In a preliminary simulation i used a constant property liquid and everything worked fine.

Now i want to account for changes in fluid properties. I defined expressions for the density, heat capacity, dynamic viscosity and the thermal conductivity as functions of p and T (pressure and Temperature). I checked the units and the values and they are correct. If i don't setup a table CFX-Pre starts complaining. I tried to start a simulation with and without a table but either way the solver stops immediately.

Is there a major mistake in my setup?
Do i need a table?
I heard about problems when defining density and the heat capacity at the same time but i couldn't find any related threads.

Thanks in advance

Opaque · March 28, 2019, 09:27

The CFX-Solver nearly always creates a table from the fundamentals properties given by expressions.

For example, how would compute Enthalpy from a Cp expression? How would you compute Entropy from expressions for density and Cp?

Thermodynamic properties are not independent, they MUST satisfy the Maxwell relationships; otherwise, the energy balance equations will not work properly.

See ANSYS CFX documentation for a light description of the minimum requirements.

March 28, 2019, 08:58	Material Properties by expressions	#1
testaccount New Member Join Date: Mar 2019 Posts: 1 Rep Power: 0	Hey folks, i want to simulate the flow of liquid oxygen. In a preliminary simulation i used a constant property liquid and everything worked fine. Now i want to account for changes in fluid properties. I defined expressions for the density, heat capacity, dynamic viscosity and the thermal conductivity as functions of p and T (pressure and Temperature). I checked the units and the values and they are correct. If i don't setup a table CFX-Pre starts complaining. I tried to start a simulation with and without a table but either way the solver stops immediately. Is there a major mistake in my setup? Do i need a table? I heard about problems when defining density and the heat capacity at the same time but i couldn't find any related threads. Thanks in advance

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March 28, 2019, 09:27		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	The CFX-Solver nearly always creates a table from the fundamentals properties given by expressions. For example, how would compute Enthalpy from a Cp expression? How would you compute Entropy from expressions for density and Cp? Thermodynamic properties are not independent, they MUST satisfy the Maxwell relationships; otherwise, the energy balance equations will not work properly. See ANSYS CFX documentation for a light description of the minimum requirements.