8.16.2 The NIST Real Gas Models

Overview and Limitations of the NIST Real Gas Models

The NIST real gas models 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 an ANSYS 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 Create/Edit Materials dialog box 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 following limitations exist for the NIST real gas model: :

• Access to the Create/Edit Materials dialog box from the navigation pane or Define menu is restricted. Therefore, if solid properties have to be set and modified, then it should be done in the Create/Edit Materials dialog box before activating the real gas model.

• The NIST real gas model assumes that the fluid you will be using in your ANSYS FLUENT computation is superheated vapor, supercritical fluid, or liquid. Note that subcritical flow conditions, where vapor coexists with liquid in two-phase flow, are not supported. In addition, all fluid zones must contain the real gas; you cannot include a real gas and another fluid in the same problem.

• Pressure-inlet, mass flow-inlet, and pressure-outlet are the only inflow and outflow boundaries available for use with the real gas models.

• Non-reflecting boundary conditions should not be used with the real gas models.

• The mixture flow does not permit chemical reactions with the NIST real gas model.

• The real gas models cannot be used with any of the multiphase models. The model is compatible with the Lagrangian Dispersed Phase Models only for the massless and inert particle types. Please note that particle material properties have to be set in the Create/Edit Materials dialog box before activating the real gas model.

• You cannot modify material properties in the REFPROP database libraries, or add custom materials to the NIST real gas model.

The REFPROP v7.0 Database

The NIST real gas model supports 83 pure fluids from the REFPROP database. These include 39 materials that were originally included in REFPROP v7.0 plus the extra materials that were made available in the NIST web site later. 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 corresponding property file name appears in parentheses, where it does not coincide with the fluid name).

Please note that the database does not include transport property models for the following species: acetone, benzene, c4f10, c5fl2, cos, cyclohexane, cyclopropane, deuterium, fluorine, neopentane, nf3, propyne, r21, sf6, so2. As a result the NIST real gas model with those species can only be used for modeling inviscid flow.

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

• modified Benedict-Webb-Rubin (MBWR) equation of state

• Helmholtz-energy equation of state

• extended corresponding states (ECS)

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

Table 8.16.1: Hydrocarbons and Refrigerants Supported by REFPROP v7.0

1butene	acetone	ammonia	argon	benzene	butane
dodecane	cis-butene	c4f10	c5fl2	co	co2
(c12.fld)	(c2butene.fld)
cos	cyclohexane	cyclopropane	deuterium	heavy water	decane
	(cyclohex.fld)	(cyclopro.fld)	(d2.fld)	(d2o.fld)
dimethylether	ethane	ethanol	ethylene	fluorine	h2s
(dme.fld)
helium	heptane	hexane	hydrogen	ibutene	ihexane
ipentane	isobutene	krypton	methane	methanol	n2o
neon	neopentane	nf3	nitrogen	nonane	octane
	(neopentn.fld)
oxygen	parahydrogen	pentane	propane	propylene	propyne
	(parahyd.fld)			(propylen.fld)
r11	r113	r114	r115	r116	r12
r123	r124	r125	r13	r134a	r14
r141b	r142b	r143a	r152a	r21	r218
r22	r227ea	r23	r236ea	r236fa	r245ca
r245fa	r32	r365mfc	r41	rc318	sf6
so2	trans-butene	toluene	water	xenon
	(t2butene.fld)

Using the NIST Real Gas Models

When you enable one of the NIST real gas models (single-species fluid or multiple-species mixture) and select a valid material, ANSYS FLUENT's functionality remains the same as when you model fluid flow and heat transfer using an ideal gas, with the exception of the Create/Edit Materials dialog box (see below). The information displayed in the Create/Edit Materials dialog box 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

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-species flow then you should enable the single-species NIST real gas model by typing the following text command at the ANSYS FLUENT console prompt:

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

use NIST real gas? [no]  yes

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

> define/user-defined/real-gas-models/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:

1butene.fld   acetone.fld   ammonia.fld   argon.fld     benzene.fld
butene.fld    c12.fld       c2butene.fld  c4fl0.fld     c5fl2.fld  
co.fld        co2.fld       cos.fld       cyclohex.fld  cyclopro.fld
d2.fld        d2o.fld       decane.fld    dme.fld       ethane.fld   
ethanol.fld   ethylene.fld  fluorine.fld  h2s.fld       helium.fld
heptane.fld   hexane.fld    hydrogen.fld  ibutene.fld   ihexane.fld  
ipentane.fld  isobutan.fld  krypton.fld   methane.fld   methanol.fld
n2o.fld       neon.fld      neopentn.fld  nf3.fld       nitrogen.fld
nonane.fld    octane.fld    oxygen.fld    parahyd.fld   pentane.fld
propane.fld   propylen.fld  propyne.fld   r11.fld       r113.fld    
r114.fld      r115.fld      r116.fld      r12.fld       r123.fld
r124.fld      r125.fld      r13.fld       r134a.fld     r14.fld
r141b.fld     r142b.fld     r143a.fld     r152a.fld     r218.fld
r21.fld       r22.fld       r227ea.fld    r23.fld       r236ea.fld
r236fa.fld    r245ca.fld    r245fa.fld    r32.fld       r365mfc.fld
r41.fld       rc318.fld     sf6.fld       so2.fld       t2butene.fld
toluene.fld   water.fld     xenon.fld

2.   Select material from the REFPROP database list:

If the single-species 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"

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), ANSYS 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)

3.   If you would like to model flow in the liquid phase, then this needs to be specified in the set-phase command. Note that the default phase is vapor, so if you do not go through this step, vapor is assumed. In addition, if the flow conditions do not permit liquid to exist, a vapor calculation will be performed instead.

> define/user-defined/real-gas-models/set-phase

Select vapor phase (else liquid)?[yes]

Once the real gas model is activated, any information for a fluid that is displayed in the Create/Edit Materials dialog box is ignored by ANSYS FLUENT.

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 situations an error message generated by NIST will be returned and displayed on the ANSYS 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 ANSYS 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 the real-gas model is in general done at much lower Courant values when you are using the density-based solver, or at much lower under-relaxation values if you are using the pressure-based solver. 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 ANSYS 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 Courant value or under-relaxation factors 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.

Writing Your Case File

When you save your completed real gas model to a case file, the linkage to the shared library containing real gas properties will be saved to the case file (along with property data for the material you selected in the NIST real gas model). Consequently, whenever you read your case file in a later session, ANSYS FLUENT will load and report this information to the console during the read process.

Postprocessing

All postprocessing functions properly report and display the current thermodynamic and transport properties of the selected real gas model. The thermodynamic and transport properties controlled by the NIST real gas model include the following:

• density

• enthalpy

• entropy

• molecular weight

• molecular viscosity

• sound speed

• specific heat

• thermal conductivity In addition to the properties listed above, you can also report

• compressibility factor

• any quantities that are derived from the properties listed above (e.g., total quantities, ratio of specific heats)