Defining Fluid Properties in Altair Flow Simulator
Altair EmployeeFluid Property Sources
The FS solver has “built-in” tables and equations for several fluids.
Gas: Air (ideal gas) and mixtures of Air and Steam
Liquids: Water, Jet-A Fuel, 3 types of Oil, Propylene and Ethylene Glycol
NASA CEA can be used as a source of fluid properties as well as modeling combustion. It has a long list of gases and some liquids to choose from. The choices are mostly fuels and oxidizers. NASA CEA does use ideal gas assumptions so should only be used at moderate temperatures and pressures. If a model includes a combustion element, NASA CEA must be used for the fuel properties.
Coolprop is an open access program and has been integrated into FS. It has many options for gases and liquids including refrigerants like R134a. It is the best source for supercritical and multi-phase properties. Note the multi-phase in FS is still under development and should be used with caution. If the pressures and temperatures require real gas properties, use Coolprop or User defined for properties.
User defined properties is also an option if the other property sources do not fit your needs. Properties can be input as constants, temperature dependent tables, or pressure and temperature dependent tables. FS uses linear interpolation with the tables. Only include enough points to accurately represent the property curve. More table points than needed can lead to longer run times. The option to specify enough properties to model a real gas was added in version 2024.
Defining Fluid Property Sources
The material manager is used to setup properties from NASA CEA, Coolprop, or “user defined” before they can be applied to a fluid chamber. The “built-in” properties (Ex: Air (ideal gas), Water) can be applied to a fluid chamber without doing anything in the material manager.
Using Fluid Properties in the Model
The fluids are assigned to boundary chambers, accumulators, and tanks. Fluids are propagated downstream from the upstream boundary chamber, accumulator, or tank so internal chambers and elements will automatically have the proper fluid. It is a good idea to set the proper fluid for downstream boundary chambers in case the boundary becomes an upstream boundary during the solver iterations. The Materials options listed for a chamber depend on the type of element attached.
Multiple Fluids and Fluid Mixtures
A Flow Simulator model can contain more than 1 fluid. These fluids can remain in separate circuits (unmixed) or be combined in a fluid mixture.
Some fluid mixing is possible but not all combinations are supported. Some common mixing options are shown below.
Avoid mixing fluids from NASA CEA with Coolprop. NASA CEA can process many gas mixtures. For instance, a custom mixture of components for Natural Gas is possible using NASA CEA. Coolprop has trouble with some fluid mixtures. FS has an option to use built-in mixing laws if Coolprop has trouble with the mixture.
Checking Fluid Properties
Flow Simulator can output fluid properties at each model chamber to a file with “.prop” extension. Details of the NASA CEA run can also be written to a file for combustion elements. The *.res file also lists fluid properties for some of the elements (ex: density at each station of an advanced tube element).
