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Old   September 20, 2019, 06:06
Default Setting real gas properties
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Abdullah Arslan
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Dear all,

I am stuck to a problem from last 4 months and my results follow the trends in paper but with 60-70% error.

I am doing methane steam reforming (MSR) using CEL expressions. Inlet conditions use bot velocity and pressure but we can use one at a time. To counter this I was using CEL expression to put pressure in partial form. MSR mixture has methane, CO,CO2,H2,H2O and N2.

Arr*((max(102 [kPa],pabs*my.CH4.molf))^0.47)*((max (102 [kPa],pabs*my.H2O.molf))^-0.01).

But post result pressure contour were like E-1 Pa max value though pabs is 1 bar.

The paper clearly states that it is ideal mixture. But yesterday, when i chose non ideal/real gas the option of mixture pressure appears.

However, by random values of dP/dV and dP/dT (maxwell's partial equations) I cannot start simulation.
I would appreciate:
1. A good link to study this maxwell's partial equations
2. What will be the guess value to start the simulation.
Attached Images
File Type: png BC and initial conditions.png (15.9 KB, 17 views)
File Type: png Non ideal mixture.png (47.5 KB, 17 views)
File Type: jpg Pressure contuour.jpg (32.7 KB, 18 views)
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Old   September 20, 2019, 07:55
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May I ask what you meant by

"However, by random values of dP/dV and dP/dT (maxwell's partial equations) I cannot start simulation."

Random values?

It would be great if you take a look in a thermodynamics textbook about Maxwell relationships, and understand the intricate connection between all the properties. It is definitely not random; therefore, those values must be specific to the material and equation of state you are working with. Anything else will be thermodynamically inconsistent.
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Old   September 20, 2019, 08:03
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Quote:
Originally Posted by Opaque View Post
May I ask what you meant by

"However, by random values of dP/dV and dP/dT (maxwell's partial equations) I cannot start simulation."

Random values?

It would be great if you take a look in a thermodynamics textbook about Maxwell relationships, and understand the intricate connection between all the properties. It is definitely not random; therefore, those values must be specific to the material and equation of state you are working with. Anything else will be thermodynamically inconsistent.
Just any value to start simulation like 0.01 or 2 or -2. I wanted to do that to see by setting 1 bar there does pressure contour changes (right or wrong). May be i am putting values which crash the simulation bcz these values make a non-physical system.

I am going through a lecture now. I can post its youtube link if its not against forum rules.

thank you
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Old   September 20, 2019, 11:30
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"I am going through a lecture now."

Please do. Set yourself an example, by writing the ideal gas equation and the corresponding relationships, i.e. write the derivatives being asked for in the software. Evaluate them and get a feeling for the order of magnitudes implied by that equation of state.

For the particular material used, try to visualize the thermodynamic surface and see what those slopes mean and how they vary for that specific material. It matters on which part of the thermodynamic space (P,v,T) you are in.

Handling real gas equations requires quite a bit of expertise and thermodynamics knowledge, and mixtures of real gases are another level above that. If you are not satisfied/convinced by the solution using ideal gas, or ideal mixture, you must know in advance what your goal is when jumping to real gases.
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Old   September 24, 2019, 09:30
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Quote:
Originally Posted by Opaque View Post
"I am going through a lecture now."

Please do. Set yourself an example, by writing the ideal gas equation and the corresponding relationships, i.e. write the derivatives being asked for in the software. Evaluate them and get a feeling for the order of magnitudes implied by that equation of state.

For the particular material used, try to visualize the thermodynamic surface and see what those slopes mean and how they vary for that specific material. It matters on which part of the thermodynamic space (P,v,T) you are in.

Handling real gas equations requires quite a bit of expertise and thermodynamics knowledge, and mixtures of real gases are another level above that. If you are not satisfied/convinced by the solution using ideal gas, or ideal mixture, you must know in advance what your goal is when jumping to real gases.
Thank you for the reply. Actually, the mixture is ideal but compressible. The density of mixture is calculated at P 1 atm and T 793K. As earlier I mentioned I wanted to add both speed and P as inlet condition but cannot, so I went for 'Real gas' option as it has Pressure settings. Anyway, in 'Materials' we can add value of density (indirectly pressure) by selecting 'value' and i tried that too. But no change in results.

Additional question: How can we select compressible/incomprehensible option in CFX? My density varies in domain but where Temperature varies (793-632K). So is my flow compressible?

Regards

Last edited by Goenitz; September 24, 2019 at 09:41. Reason: additional question
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Old   September 24, 2019, 13:06
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You said

"As earlier I mentioned I wanted to add both speed and P as inlet condition but cannot, "

Unless your flow is supersonic at the inlet, you cannot nor do want to set both variables. The setup becomes ill-posed.

The material selection determines if the model is compressible or incompressible. It is not an explicit user option, and it should not be. Say you select a variable density material, and able to force the equations into incompressible, the solution will be WRONG if it converges (unlikely)
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