For single or multicomponent flows, you will enable the cubic equation of state real gas models by selecting real-gas-soave-redlich-kwong, real-gas-peng-robinson, real-gas-aungier-redlich-kwong, or real-gas-redlich-kwong from the Density drop-down list in the Create/Edit Materials dialog box.

 Setup →   Materials → Create/Edit...

The required inputs for the cubic equation of state real gas models for single component flow and mixtures are described below.

Single Component Flow

Figure 8.41: The Cubic Equation of State Model for a Real-Gas Fluid

When any of the cubic equation of state models is enabled, enter the following material properties in the dialog box:

Mixtures

Figure 8.42: The Cubic Equation of State Model for a Real-Gas Mixture

When one of the cubic equation of state models is selected from the Density drop-down list, specify the following material properties for the mixture material in the dialog box:

You also need to enter the following material properties for each of the mixture components in the dialog box:

When you are modeling a real-gas mixture, the following methods are available for the mixture critical constants:

If the operating conditions in your model are in the subcritical regime, select Vapor or Liquid as the Real Gas State in the Operating Conditions dialog box (Figure 8.43: The Operating Conditions for a Real Gas State). Alternatively, you can use the define/operating-conditions/set-state text command. Note that the default is Vapor and therefore Vapor is assumed if no changes are made to the Real Gas State settings. If the operating conditions in your model are entirely in the supercritical regime, this setting will have no effect.

Figure 8.43: The Operating Conditions for a Real Gas State

In addition, you may specify the state for a specific fluid zone or a specific phase (if you are using multiphase modeling) by typing the following text command at the Ansys Fluent console prompt:

> define boundary-conditions modify-zones change-zone-state
Select a name/id from fluid zones list [(liquid-zone fluid-1)] liquid-zone
Set zone real-gas state:
  -1:use global setting
  0:liquid
  1:vapor
[-1]  0

The flow modeling of real-gas flow is more complex and challenging than simple ideal-gas flow. Therefore, the solution might converge at a slower rate with real-gas flow than when running ideal-gas flow. 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.

Special considerations apply if the operating regime in your model is fully or partly subcritical, where the physical state may be vapor, liquid, or a vapor/liquid mixture. In the cubic EOS real-gas model, the phase state is determined by the selection of the cubic root to calculate the molar-volume. This is illustrated in Figure 8.44: The PV Diagram for the Cubic Equation of State Real Gas Model, which shows a pressure versus molar volume (PV) diagram, with a subcritical isotherm ABFED at temperature T calculated from a cubic EOS. Curve C1CC2 represents the critical isotherm and curve ACD the saturation dome. Points A, F, and D represent the three roots of the EOS at the saturation pressure Ps, where A corresponds to the molar volume of saturated liquid, D to the molar volume of saturated vapor and point F does not have a physical significance. Points A1 and D1 correspond to the liquid and vapor molar volumes at pressure P1, which is lower than the saturation pressure. The liquid and vapor molar volumes at pressure P2, which is higher than the saturation pressure, are marked as A2 and D2 respectively. Points B and E, where the partial derivatives of pressure with respect to volume are 0, are called spinodal points. The loci of these points for all subcritical temperatures, the spinodal curve, is shown with the dashed curve BCE and sets the boundary beyond which the equation of state is no longer valid, because the local derivative of pressure with respect to volume becomes positive. State points inside the dome, up to the spinodal curve, are called “metastable” because normally they only exist temporarily in small local regions until phase change occurs.

Figure 8.44: The PV Diagram for the Cubic Equation of State Real Gas Model

It is important to realize that the current implementation of the cubic equations of state real gas model does not determine the saturation conditions and does not model the two-phase flow where liquid and vapor coexist. In the subcritical regime and for conditions where the cubic EOS has three roots, the phase state selection is controlled by your input of Vapor or Liquid for Real Gas State in the Operating Conditions dialog box (Figure 8.43: The Operating Conditions for a Real Gas State). The default setting is Vapor, which corresponds to the state points to the right of the vapor spinodal. With reference to Figure 8.44: The PV Diagram for the Cubic Equation of State Real Gas Model, for the operating point at temperature T and pressure P1, if the Real Gas State is set to Vapor, the molar volume at point D1, which corresponds to superheated vapor, will be selected in the calculation. On the other hand, if you have selected Liquid, the molar volume of point A1 will be taken, and the corresponding state will be metastable superheated liquid. Similarly for pressure P2, which is higher than the saturation pressure Ps, if you select Liquid for Real Gas State, the liquid molar volume at A2 will be computed, and if you select Vapor the molar volume will correspond to the subcooled vapor state point D2.

In case the calculations fall inside the saturation dome, the solver will not limit the solution, but if conditions are predicted that extend beyond the vapor spinodal curve a warning will be issued:

temperature is below the spinodal point in 12 cells on zone 3.

Cubic equations of state real gas models are not available for two-phase flows but can be applied to conditions of supercritical pressure and subcritical temperature , where the fluid is in the liquid state, and the supercritical liquid co-exists with gas and supercritical fluid in the same simulation. In those cases, phase change may take place, without any of the flow conditions falling inside the saturation dome.

For multicomponent simulations, when Diffusion Energy Source is enabled in the Species Model dialog box, the species energy diffusion is by default suppressed in the supercritical liquid regime and across the gas-liquid boundary. The diffusion energy source can be included in the liquid regime using the define/models/species/liquid-energy-diffusion? text command.

If your simulation contains a liquid stream, the appropriate modeling approach in the various operating condition regimes is as follows:

The temperature gives an indication of the maximum droplet temperature for applicability of the DPM models for each droplet material. These temperature limits are listed in Table 8.2: Temperature Limits for Droplet Materials in Ansys Fluent Database prodb.scm for many of the droplet materials in Ansys Fluent's propdb.scm materials database. For supercritical pressures (), the DPM models can still be used provided the droplet temperatures remain below the limit .

For high pressure simulations the boiling point will be different from the normal boiling point, and for varying pressure applications the boiling point will vary with the droplet location in the domain.

When a cubic equation of state real gas model is enabled in a simulation that includes evaporating droplet particles, the boiling point is calculated from the vapor pressure data directly, as the temperature where the saturation vapor pressure equals the domain pressure. In addition, the latent heat of the evaporating or boiling droplet will vary with the droplet temperature.

The latent heat at temperature is given by

(8–104)

where is the normal boiling point and is the latent heat at the normal boiling point.

In the Create/Edit Materials dialog box you must enter the Normal Boiling Point (NBP) and the Latent Heat at NBP for the droplet-particle Material Type. These inputs will be used for calculating the Latent Heat at the reference temperature (see Equation 12–509) and the latent heat in the droplet energy balance during vaporization and boiling according to Equation 8–104.

Finally when a cubic equation of state real-gas model is enabled in a simulation with droplet models, the condition for switching from the vaporization to the boiling law will be

(8–105)

where is the saturation vapor pressure and P is the domain pressure. If while in the boiling law, the model will switch back to the vaporization law. Under supercritical pressure conditions (), the vaporization models are applicable at droplet temperatures below the critical point, and the switching condition from vaporization to boiling is never met because the vapor pressure curve is defined only up to the critical point.

For the multicomponent droplets in the supercritical pressure regime, failure of the saturation temperature calculation algorithm may indicate that the droplet has approached the critical temperature, provided that you have entered accurate vapor pressure data for all components. In such a case, you can use the following text command:

define/models/dpm/options/treat-multicomponent-saturation-temperature-failure?

Dump multicomponent particle mass if the saturation temperature calculation fails? [no] y

When this option is enabled, Ansys Fluent dumps the particle mass into the continuous phase if the saturation temperature calculation fails.

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 cubic equations of state real gas model include the following:

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

If you are modeling a real-gas mixture you can report the composition-dependent mixture critical properties

By default, the NIST functions are called directly each time the thermal properties are updated during an iteration. Alternatively, you can use NIST look-up tables in your simulation as described below. Computing the look-up table reduces the computation time because the NIST functions do not need to be called each time the thermal properties are updated.

To create a full NIST look-up table, first enable the NIST real gas model as described in Using the NIST Real Gas Models, and then select Create Lookup Table.

Figure 8.49: The NIST Fluid Data Dialog Box With Lookup Table Enabled

For both single and multi-species real gas models, you need to specify:

For single-species, you also need to specify the number of points for Saturation (Bubble/Dew Curve) properties.

For multi-species, you also need to specify the mixture composition on a mass or mole basis. You must ensure that the sum of all constituent fractions add up to 1 when specifying mass/molar fractions for the components. The maximum number of species is 20.

Figure 8.50: The NIST Fluid Data Dialog Box For Fixed Composition

If properties are needed at conditions outside the limits you have specified for the table, Fluent automatically calls the original NIST model functions to calculate the thermal properties. If more than 10% of the property evaluations require computation outside the look-up table, Fluent prints a message indicating the percentage of calculations using direct computation. This can be used as an indicator to adjust the temperature and pressure ranges for the table to minimize the computational cost.

When you enable one of the NIST real gas models (pure fluid or multiple-species mixture) through the legacy TUI, the following limitations apply in addition to those listed in Limitations 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- or multi-species NIST real gas model, and then you select the fluid material from the REFPROP database.

By default, the NIST functions are called directly each time the thermal properties are updated during iteration. Alternatively, you can use the NIST look-up tables in your simulation as described below. Computing the look-up table offers two advantages:

To create a full NIST look-up table, first enable the NIST real gas model as described in Activating the NIST Real Gas Model, and then follow these steps:

Capabilities and Limitations on Using Multi-Species Full NIST look-up Tables

To create a saturation data look-up table for binary species mixtures (mixtures of two species only), first enable the NIST real gas model as described in Activating the NIST Real Gas Model and then follow these steps:

NIST Binary Saturation Files

NIST binary saturation files store saturation data lookup table for bubble and dew points for a binary mixture with mass fractions between 0 and 1. They can be used for mass and heat transfer modeling involving phase changes.

The binary saturation file structure is as follows. The file contains several data sets for bubble/dew point curves for a series of mass fractions. Each data set consists of the following three parts:

The example below shows the saturation dew point data for the first two curves, both with 12 pressure-temperature pairs. The first data set is for 0 and the second is for a 0.083333 mass fraction of the first species. The pressure-temperature critical condition is (4.872200e+06, 3.053220e+02) for the first curve and (5.547305e+06, 2.947561e+02) for the second curve.

 0.000000 
12 
1.142108e+00 9.036800e+01 
7.314020e+01 1.099093e+02 
1.202771e+03 1.294505e+02 
8.833626e+03 1.489918e+02 
3.890130e+04 1.685331e+02 
1.219542e+05 1.880744e+02 
3.025174e+05 2.076156e+02 
6.354397e+05 2.271569e+02 
1.182515e+06 2.466982e+02 
2.011733e+06 2.662395e+02 
3.201942e+06 2.857807e+02 
4.872200e+06 3.053220e+02 

 0.083333 
12 
1.509489e+05 1.891238e+02 
1.257388e+06 2.428513e+02 
2.252097e+06 2.631615e+02 
3.090473e+06 2.752623e+02 
3.775539e+06 2.831498e+02 
4.318958e+06 2.883822e+02 
4.735586e+06 2.917723e+02 
5.042345e+06 2.938262e+02 
5.258091e+06 2.949048e+02 
5.402834e+06 2.952901e+02 
5.495209e+06 2.951982e+02 
5.547305e+06 2.947561e+02 

..... 
.....

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). This information can be reported to the console by listing the material properties (using the command /define/materials/list-properties <material-name>) and viewed in the Create/Edit Materials dialog box by selecting Edit… for density.

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:

If the build is successful, then the compiled library will be placed in the appropriate architecture folder (for example, win64/2d). By default the library name is libudf.so (libudf.dll on Windows).

More information on compiled UDFs and building libraries using the Ansys Fluent graphical user interface can be found in the Fluent Customization Manual.

The UDRGM library can be compiled in the text command interface as follows:

Ansys Fluent will then start compiling the UDRGM C code and put it in the appropriate architecture folder.

Example:

> define/user-defined/compiled-functions
  load OR compile ? [load]  compile

  Compiled UDF library name: ["libudf"] my_lib

   Make sure that UDF source files are in the folder
   that contains your case and data files. If you have
   an existing libudf folder, please remove this
   folder to ensure that latest files are used.
  Continue?[yes] RETURN

  Give C-Source file names:
  First file name: [""] my_c_file.c RETURN

  Next file name: [""] RETURN

  Give header file names:
  First file name: [""] my_header_file.h RETURN

Load the UDRGM library:

/**********************************************************************/
/* User Defined Real Gas Model :                                      */
/* For Ideal Gas Equation of State                                    */
/*                                                                    */
/**********************************************************************/

#include "udf.h"
#include "stdio.h"
#include "ctype.h"
#include "stdarg.h"

#define MW 28.966   /* molec. wt. for single gas (Kg/Kmol) */
#define RGAS (UNIVERSAL_GAS_CONSTANT/MW)

static int (*usersMessage)(const char *, ...);
static void (*usersError)(const char *, ...);


DEFINE_ON_DEMAND(I_do_nothing)
{
  /* This is a dummy function to allow us to use */
  /* the Compiled UDFs utility      */
}


void IDEAL_error(int err, char *f, char *msg)
{
  if (err)
    usersError("IDEAL_error (%d) from function: %s\n%s\n", err, f, msg);
}

void IDEAL_Setup(Domain *domain, cxboolean vapor_phase, char *filename,
                 int (*messagefunc)(const char *format, ...),
                 void (*errorfunc)(const char *format, ...))
{
  /* Use this function for any initialization or model setups*/
  usersMessage = messagefunc;
  usersError  = errorfunc;
  usersMessage("\nLoading Real-Ideal Library: %s\n", filename);
}

double IDEAL_density(cell_t cell, Thread *thread,
                     cxboolean vapor_phase, double Temp, double press, double yi[])
{
  double r = press / (RGAS * Temp); /* Density at Temp & press */
  return r;      /* (Kg/m^3) */
}

double IDEAL_specific_heat(cell_t cell, Thread *thread,
                           double Temp, double density, double P, double yi[])
{
  double cp = 1006.43;
  return cp;     /* (J/Kg/K) */
}

double IDEAL_enthalpy(cell_t cell, Thread *thread,
                      double Temp, double density, double P, double yi[])
{
  double h = Temp * IDEAL_specific_heat(cell, thread, Temp, density, P, yi);
  return h;      /* (J/Kg) */
}

#define TDatum 288.15
#define PDatum 1.01325e5

double IDEAL_entropy(cell_t cell, Thread *thread,
                     double Temp, double density, double P, double yi[])
{
  double s = IDEAL_specific_heat(cell, thread, Temp, density, P, yi) * log(fabs(Temp / TDatum)) +
             RGAS * log(fabs(PDatum / P));
  return s;      /* (J/Kg/K) */
}


double IDEAL_mw(double yi[])
{
  return MW;     /* (Kg/Kmol) */
}

double IDEAL_speed_of_sound(cell_t cell, Thread *thread,
                            double Temp, double density, double P, double yi[])
{
  double cp = IDEAL_specific_heat(cell, thread, Temp, density, P, yi);
  return sqrt(Temp * cp * RGAS / (cp - RGAS)); /* m/s */
}

double IDEAL_viscosity(cell_t cell, Thread *thread,
                       double Temp, double density, double P, double yi[])
{
  double mu = 1.7894e-05;
  return mu;     /* (Kg/m/s) */
}

double IDEAL_thermal_conductivity(cell_t cell, Thread *thread,
                                  double Temp, double density, double P,
                                  double yi[])
{
  double ktc = 0.0242;
  return ktc;      /* W/m/K */
}
double IDEAL_rho_t(cell_t cell, Thread *thread,
                   double Temp, double density, double P, double yi[])
{
  /* derivative of rho wrt. Temp at constant p */
  double rho_t = -density / Temp;
  return rho_t;     /* (Kg/m^3/K) */
}

double IDEAL_rho_p(cell_t cell, Thread *thread,
                   double Temp, double density, double P, double yi[])
{
  /* derivative of rho wrt. pressure at constant T */
  double rho_p = 1.0 / (RGAS * Temp);
  return rho_p;    /* (Kg/m^3/Pa) */
}

double IDEAL_enthalpy_t(cell_t cell, Thread *thread,
                        double Temp, double density, double P, double yi[])
{
  /* derivative of enthalpy wrt. Temp at constant p */
  return IDEAL_specific_heat(cell, thread, Temp, density, P, yi);
}

double IDEAL_enthalpy_p(cell_t cell, Thread *thread,
                        double Temp, double density, double P, double yi[])
{
  /* derivative of enthalpy wrt. pressure at constant T  */
  /* general form dh/dp|T = (1/rho)*[ 1 + (T/rho)*drho/dT|p] */
  /* but for ideal gas dh/dp = 0        */
  return 0.0 ;
}

UDF_EXPORT RGAS_Functions RealGasFunctionList =
{
  IDEAL_Setup,                   /* initialize */
  IDEAL_density,                 /* density */
  IDEAL_enthalpy,                /* enthalpy */
  IDEAL_entropy,                 /* entropy */
  IDEAL_specific_heat,           /* specific_heat */
  IDEAL_mw,                      /* molecular_weight */
  IDEAL_speed_of_sound,          /* speed_of_sound */
  IDEAL_viscosity,               /* viscosity */
  IDEAL_thermal_conductivity,    /* thermal_conductivity */
  IDEAL_rho_t,                   /* drho/dT |const p */
  IDEAL_rho_p,                   /* drho/dp |const T */
  IDEAL_enthalpy_t,              /* dh/dT |const p  */
  IDEAL_enthalpy_p               /* dh/dp |const T  */
};
/**************************************************************/