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8.16 Real Gas Models

Some engineering problems involve fluids that do not behave as ideal gases. For example, at very high-pressure or very low-temperature conditions (e.g., the flow of a refrigerant through a compressor) the flow cannot typically be modeled accurately using the ideal-gas assumption. Therefore, the real gas model allows you to solve accurately for the fluid flow and heat transfer problems where the working fluid behavior deviate from the ideal-gas assumption.

The FLUENT density-based solvers provide two real gas options for solving this type of flows:

Both models allow the user to solve for either a single-species fluid flow or a multiple-species mixture fluid flow.



Real Gas Models Limitations


The following limitations exist for the real gas models:



Using the Real Gas Models


When you enable one of the real gas models (NIST or User Defined) and select a valid material, FLUENT's functionality remains the same as when you model fluid flow and heat transfer using an ideal gas, with the exception of the Materials panel (see below). The information displayed in the Materials panel is not used by the solver because control of all relevant property evaluations is relinquished to the selected real gas model.

The general procedure for using the real gas model is as follows:

1.   Enable one of the density-based solvers (whichever is appropriate for your case).

2.   Activate one of the real gas models.

3.   Set up and write your case file.

4.   Calculate a solution and perform postprocessing.

Enabling the Density-Based Solver

You can enable the density-based solver in the Solver panel. The real gas models can be used with either the implicit or the explicit formulation.

Define $\rightarrow$ Models $\rightarrow$ Solver...

figure   

The real gas models do not work with the pressure-based solvers.

Activating the Real Gas Models

When one of the density-based solvers is enabled, you will be able to activate one of the real gas models listed after typing the following text command at the FLUENT console prompt:

define $\rightarrow$ user-defined $\rightarrow$ real-gas

The list of available real gas models will be displayed:

  nist-real-gas-model
  nist-multispecies-real-gas-model
  user-defined-real-gas-model
  user-defined-multispecies-real-gas-model

The next sections provide additional information about activating and using these models.

Writing Your Case File

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 case of NIST the real gas model). Consequently, whenever you read your case file in a later session, FLUENT will load and report this information to the console during the read process.

Postprocessing

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 real gas models include the following:




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