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8.16.1 The NIST Real Gas Models



Overview and Limitations of the NIST Real Gas Models


The NIST real gas models are available in the density-based solvers. They use the National Institute of Standards and Technology (NIST) Thermodynamic and Transport Properties of Refrigerants and Refrigerant Mixtures Database Version 7.0 (REFPROP v7.0) to evaluate thermodynamic and transport properties of approximately 39 pure fluids or a mixture of these fluids.

The REFPROP v7.0 database is a shared library that is dynamically loaded into the solver when you activate one of the NIST real gas models in a FLUENT session. Once the NIST real gas model is activated, control of relevant property evaluations is relinquished to the REFPROP database, and any information for a fluid that is displayed in the Materials panel is ignored by the solver. However, all postprocessing functions will properly report and display the current thermodynamic and transport properties of the real gas.

The limitation on the real gas models listed in the previous section apply to NIST Real gas model in addition to the following:



The REFPROP v7.0 Database


The NIST real gas model uses 39 pure fluids from the REFPROP v7.0 database. The pure-fluid refrigerants and hydrocarbons that are supported by REFPROP v7.0 and used in the NIST real gas model are listed in Table  8.16.1.

The REFPROP v7.0 database employs the most accurate pure-fluid equations of state that are currently available from NIST. These equations are based on three models:

For fluid consists of multispecies-mixture the thermodynamic properties are computed by employing mixing-rules applied to the Helmholtz energy of the mixture components.

For details about these thermodynamic models, refer to Appendix B in the web-based REFPROP v7.0 User's Guide that is accessible from the NIST web site at:
www.nist.gov/srd/nist23.htm


Table 8.16.1: Hydrocarbons and Refrigerants Supported by REFPROP v7.0
           
r23 r32 r41 r125 r134a  
r143a r152a r227ea r236ea r236fa  
r245ca r245fa r22 r123 r124  
r141b r142b r11 r12 r13  
r113 r114 r115 r116 r14  
r218 rc318 ammonia carbon dioxide propane  
isobutane propylene nitrogen oxygen argon  
methane ethane butane water    



Using the NIST Real Gas Models


When you enable one of the NIST real gas models (single-specie fluid or multiple-species mixture) and select a valid material, FLUENT's functionality remains the same as when you model fluid flow and heat transfer using an ideal gas, with the exception of the Materials panel (see below). The information displayed in the Materials panel is not used by the solver because control of all relevant property evaluations is relinquished to the REFPROP database.

Activating the NIST Real Gas Model

When the density-based solver is used, you will be able to activate one of the NIST real gas models. Activating one of the NIST real gas models is a two-step process. First you enable either the single-species NIST real gas model or the multi-species NIST real gas model, and then you select the fluid material from the REFPROP database.

1.   Enabling the appropriate NIST real gas model:

If you are solving for a single-specie flow then you should enable the single-specie NIST real gas model by typing the following text command at the FLUENT console prompt:

> define/user-defined/real-gas/nist-real-gas-model

use NIST real gas? [no]  yes

On the other hand, if you are solving for multi-specie mixture then you should enable the multi-species NIST real gas model by typing the following text command at the FLUENT console prompt:

> define/user-defined/real-gas/nist-multispecies-real-gas-model

use multispecies NIST real gas? [no]  yes

The list of available pure-fluid materials you can select from will be displayed:

ammonia.fld   nitrogen.fld  r116.fld   r13.fld    r227ea.fld  r32.fld
argon.fld     oxygen.fld    r11.fld    r141b.fld  r22.fld     r41.fld
butane.fld    propane.fld   r123.fld   r142b.fld  r236ea.fld  rc318.fld
co2.fld       propylen.fld  r124.fld   r143a.fld  r236fa.fld  water.fld
ethane.fld    r113.fld      r125.fld   r14.fld    r23.fld
isobutan.fld  r114.fld      r12.fld    r152a.fld  r245ca.fld
methane.fld   r115.fld      r134a.fld  r218.fld   r245fa.fld

2.   Select material from the REFPROP database list:

If the single-specie real gas model is selected, then you need to enter the name of one fluid material when prompted:

select real-gas data file [""] "r125.fld"

figure   

You must enter the complete name of the material (including the .fld suffix) contained within quotes ( " ").

If the multiple-species real gas model is selected, then you need to enter the number of species in the mixture:

Number of species  [] 3

followed by the name of each fluid selected from the list shown above:

select real-gas data file [""] "nitrogen.fld"

select real-gas data file [""] "co2.fld"

select real-gas data file [""] "r22.fld"

Upon selection of a valid material (e.g., r125.fld), FLUENT will load data for that material from a library of pure fluids supported by the REFPROP database, and report that it is opening the shared library ( librealgas.so) where the compiled REFPROP database source code is located.

/usr/local/Fluent.Inc/fluent6.2/realgas/lib/r125.fld

Opening "/usr/local/Fluent.Inc/fluent6.2/realgas/
ultra/librealgas.so"...
Setting material "air" to a real-gas...

Matl name: "R125"
         : "pentafluoroethane  !full name"
         : "354-33-6"
Mol Wt   : 120.021

Critical properties:
 Temperature : 339.173 (K)
 Pressure    : 3.6177e+06 (Pa)
 Density     : 4.779 (mol/L) 573.582 (kg/m^3)

Equation Of State (EOS) used:
Helmholtz Free Energy (FEQ)
EOS:"FEQ  Helmholtz equation of state for R-125 of Lemmon and Jacobsen (2002)."

EOS Range of applicability
 Min Temperature: 172.52 (K)
 Max Temperature: 500 (K)
 Max Density    : 1691.1 (kg/m^3)
 Max Pressure   : 6e+07 (Pa)

Thermal conductivity Range of applicability
 Min Temperature: 172.52 (K)
 Max Temperature: 500 (K)
 Max Density    : 1691.1 (kg/m^3)
 Max Pressure   : 6e+07 (Pa)

Viscosity Range of applicability
 Min Temperature: 172.52 (K)
 Max Temperature: 500 (K)
 Max Density    : 1692.3 (kg/m^3)
 Max Pressure   : 6e+07 (Pa)

figure   

Once the real gas model is activated, any information for a fluid that is displayed in the Materials panel is ignored by FLUENT.

figure   

For mixture flows, not all combinations of species mixtures are allowed. This could be due to lack of data for one or more binary pairs. In such situation an error message generated by NIST will be returned and displayed on the FLUENT console, and no real gas material is allowed to be created. In some combinations the mixing data will be estimated, a warning message will be displayed on the FLUENT console and the mixture material allowed to be created.



Solution Strategies and Considerations for NIST Real Gas Model Simulation


The flow modeling of NIST real-gas flow is much more complex and challenging than simple ideal-gas flow. Therefore, you should expect the solution to converge at much slower rate with real-gas flow than when running ideal-gas flow. Also due to the complexity of the equations used in property evaluations, converging a solution with real-gas model is in general done at much lower CFL (Courant-Friedrichs-Lewy) value. It is recommended that you first attempt to converge your solution using first-order discretization then switch to second-order discretizations and re-iterate to convergence.

It is important to realize that the real-gas properties in NIST are defined within a limited/bounded range. It is important that the flow conditions you are prescribing fall within the range of the database. It is possible that you specify flow at a state that is physically valid but otherwise not defined in the database. In this situation the solution will diverge or immediately generate an error message on the FLUENT console as soon as the state crosses the limit of the database. In some instances, the actual converged state is just within the bounded defined database but only transitory outside the range. In this situation the divergence can be avoided by lowering the CFL value so a less aggressive convergence rate is adapted.

Finally, if you attempt to initialize the flow from an inlet flow conditions and an error message is generated from one of the property routines, then this is a good indicator that the flow conditions you have specified is not defined within the range of the database.


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