Pure Component Liquid, Solid and Gas Properties
Properties
In terms of choices for pure component liquid, solid and gas properties ANSYS CFX contains the most flexible CFD solver on the market. Many different built-in property models are available with an extensive library of predefined materials. If a particular material or property model is not available is easy to simply create your own and use Tables (in a specially formatted text file), CFX Expression Language or User Fortran to create custom property models.
Ideal Gas Equations of State
The main choice of property model in a CFD application is the equation of state. For typical CFD applications it is usually sufficient to assume that a fluid has a constant density or obeys the ideal gas law. ANSYS CFX provides a library of over 100 materials that use either constant density or the ideal gas law.
Ideal gases can be modeled using constant specific heat capacity, however extra fidelity is provided by two built-in polynomial approximations for specific heat capacity:
Real Gas Equations of State
In some application areas, where flow conditions approach the saturation dome shown in the attached figure, possibly with phase change, the ideal gas law is not appropriate. To accurately predict properties in this regime ANSYS CFX provides a modified version of the Redlich Kwong equation of state that gives exact behavior at the critical point. Enthalpy, entropy, specific heat capacity, and all other thermodynamic properties are computed such that they are exactly consistent with this equation of state.
The Redlich Kwong library in ANSYS CFX also allows accurate prediction of flows with phase change, extending property calculations to allow flow conditions encompassing both superheated and saturated flow regimes. This allows flows with wetness to be modeled as in the sample calculation shown in the figure. The property models can be used with both the single phase equilibrium phase change model as well as the non-equilibrium thermal phase change model. When using the equilibrium phase change model, gas properties are predicted in the superheat region, mixture properties are calculated inside the dome using the lever rule, and pure liquid properties vary along the liquid saturation line.
The Real Fluid thermodynamics includes phase change throughout the vapor dome. This sample calculation shows wetness within a steam turbine.
A extensive library of materials which use the Redlich Kwong equation of state is supplied, including steam, typical hydrocarbons, refrigerants and various other common gases. If a material is not supplied then a flexible GUI is provided in ANSYS CFX pre-processing for setting up your own materials.
If the built-in Redlich Kwong library is not applicable then it is easy to setup your own real gas models. CFX can read tables of superheat region and saturation properties, as a function of temperature and pressure, from a specially formatted Real Gas Properties (RGP) text file. Several of these RGP files are also supplied with CFX for steam and some common gases.
Transport Properties
A variety of built-in options for specifying dynamic viscosity and thermal conductivity are available. These currently include Sutherland’s law for temperature dependent viscosity and thermal conductivity as well as additional kinetic theory based models that are applicable to both ideal and real gases.