## Chapter 12: Real Fluid Properties

This chapter describes:

- Setting up a Dry Real Gas Simulation
- Table Interpolation and Saturation Curve Clipping
- Equilibrium Phase Change Model
- Setting up an Equilibrium Phase Change Simulation
- Important Considerations
- Real Gas Property (RGP) File Contents
- Real Gas Property (RGP) File Format
- Parameters in the .rgp File Controlling the Real Gas Model

For many classes of problems, the thermodynamic properties for a gaseous pure substance are closely approximated by the ideal gas equation of state. The assumptions associated with an ideal gas are especially suited for compressible gas flows at low density, which in practical terms can be stated to be appropriate under the following conditions:

1. At very low pressures, ideal gas equations of state can be used, regardless of temperature, with good accuracy as long as no phase change occurs.

2. When the pressure is low relative to the critical pressure and the temperature is high relative to the critical temperature.

For flows where the pressure and temperature ranges vary, such that the density approaches a significant fraction of the critical density, molecular interactions start to become significant and the ideal gas equation of state is no longer appropriate. In this case, a real gas equation of state may be required to give a better representation of the fluid behavior. This includes regimes with flows of dense gases, or liquids at high pressures above the critical point, or, possibly flows with phase change.

Within CFX, you can use real gas equations of state that are appropriate for this type of regime. CFX has the capability to deal with flows of both superheated vapors and subcooled liquids, while at the same time using equilibrium or non-equilibrium models for phase change mass transfer rates.

This documentation describes the real fluid model capabilities within CFX. It describes material property set-up, model set-up and modeling considerations for performing calculations that do not involve phase change (vapors or liquids only) and also calculations that do involve phase change (condensing vapors or liquids with evaporation, cavitation or boiling).

The most general setup in CFX enables both equilibrium and non-equilibrium phase change models to be run, and there are specific, built-in models that apply to both kinds of phase change modeling. From a modeling standpoint, this document mainly focuses on how to set up and run the homogeneous equilibrium phase change model, however much of the advice is equally applicable to the non-equilibrium models as well.

Non-equilibrium phase change models are available when using Eulerian multiphase and Particle Transport. The physical properties of the two phases undergoing phase change are set up in a manner similar to what is described here for both of those approaches. For details, see:

- Thermal Phase Change Model

- Mass Transfer.

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