### 12.4.3. Using a General Set-up

You may also combine two materials together to create your own homogeneous binary mixture. To do this, first set up the two materials representing the two thermodynamic states.

It is very important that the two material definitions use consistent reference states for the enthalpy and entropy. The difference between the two material enthalpies must equal the latent heat at the selected reference temperature and pressure. To make this easier, CFX-Pre allows homogeneous binary mixtures to be created only from materials within the same material group.

When creating the Homogeneous Binary Mixture, select the two materials that make up the mixture in the Material1 and Material2 fields. The thermodynamic state of one fluid should be liquid and the thermodynamic state of the other should be gas.

Under Saturation Properties, set the Option to `General`. You must then set the saturation properties (vapor pressure as a function of temperature) for the mixture. This may be done in one of two ways.

#### 12.4.3.1. Constants or Expressions

If you choose to use constant saturation conditions, then, because they are dependent, you should set values for saturation pressure and temperature that are consistent. For example, for a mixture of saturated liquid and water vapor at 1 atm, the saturation temperature is 100 °C; for a mixture of saturated liquid and water vapor at 100 °C, the saturation pressure is 1 atm.

If you use CEL expressions or User Fortran for both pressure and temperature, you should ensure that your function or expression guarantees that for all on the saturation curve.

#### 12.4.3.2. Antoine Equation

You can choose to use this equation for the saturation pressure and supply the flow solver with the necessary model constants. These are tabulated in several thermodynamics texts, see, for example, [84]. Automatic is the only option allowed for temperature in this case because the flow solver calculates saturation temperature from the Antoine equation.

The Antoine Equation correlates vapor pressure as a function of temperature using three coefficients as follows:

| (12–8)

where is the Pressure Scale used to scale the units for the vapor pressure, is the Antoine Reference State Constant, is the Antoine Enthalpic Coefficient and is the Antoine Temperature Coefficient. Depending on the reference you use to obtain the constants, Equation 12–8 can also be written in different forms, so some care should be taken when entering coefficients from the literature. For example, the tables in [84] assume that the Antoine equation is written as follows:

| (12–9)

where is in bars and is 1.0 bar. So, if you are running the flow solver in the SI unit system, this requires that you add 5.0 to and multiply that result by the natural logarithm of 10.0, multiply by the natural logarithm of 10 and subtract 273.15 K from the tabulated values of . If you are running in a non-SI unit system this would require that you also convert the constants to return the right units of pressure.

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