Modelica.Media.UsersGuide

Library Modelica.Media is a free Modelica package providing a standardized interface to fluid media models and specific media models based on this interface. A fluid medium model defines algebraic equations for the intensive thermodynamic variables used in the mass and energy balance of component models. Optionally, additional medium properties can be computed such as dynamic viscosity or thermal conductivity. Medium models are defined for single and multiple substance fluids with one and multiple phases.

A large part of the library provides specific medium models that can be directly utilized. This library can be used in all types of Modelica fluid libraries that may have different connectors and design philosophies. It is particularily utilized in the Modelica_Fluid library (the Modelica_Fluid library is currently under development to provide 1D thermo-fluid flow components for single and multiple substance flow with one and multiple phases). The Modelica.Media library has the following main features:

• Balance equations and media model equations are decoupled. This means that the used medium model does usually not have an influence on how the balance equations are formulated. For example, the same balance equations are used for media that use pressure and temperature, or pressure and specific enthalpy as independent variables, as well as for incompressible and compressible media models. A Modelica tool will have enough information to generate as efficient code as a traditional (coupled) definition. This feature is described in more detail in section Static State Selection.
• Optional variables, such as dynamic viscosity, are only computed if needed in the corresponding component.
• The independent variables of a medium model do not influence the definition of a fluid connector port. Especially, the media models are implemented in such a way that a connector may have the minimum number of independent medium variables in a connector and still get the same efficiency as if all medium variables are passed by the connector from one component to the next one (the latter approach has the restriction that a fluid port can only connect two components and not more). Note, the Modelica_Fluid library uses the first approach, i.e., having a set of independent medium variables in a connector.
• The medium models are implemented with regards to efficient dynamic simulation. For example, two phase medium models trigger state events at phase boundaries (because the medium variables are not differentiable at this point).

This User's Guide has the following main parts:

• Medium usage describes how to use a medium model from this library in a component model.
• Medium definition describes how a new fluid medium model has to be implemented.
• ReleaseNotes summarizes the changes of the library releases.
• Future provides information about the upcoming version 3.1 of Modelica.Media and some changes to this version to enable forward compatibility.
• Contact provides information about the authors of the library as well as acknowledgements.

Package Content

Name	Description
MediumUsage	Medium usage
MediumDefinition	Medium definition
ReleaseNotes	Release notes
Future	Future
Contact	Contact

Modelica.Media.UsersGuide.ReleaseNotes

Version included in Modelica 3.0

See top-level release notes for MSL.

Version 1.0, 2005-03-01

Many improvements in the library, e.g., providing mixtures of the ideal gases, table based media, test suite for all media, improved and updated User's Guide.

Version 0.9, 2004-10-18

• Changed the redeclaration/extends within packages from the experimental feature to the language keywords introduced in Modelica 2.1.
• Re-introduced package "Water.SaltWater" in order to test substance mixtures (this medium model does not describe real mixing of water and salt).
• Started to improve the documentation in Modelica.Media.UsersGuide.MediumDefinition.BasicStructure

Version 0.792, 2003-10-28

This is the first version made available for the public for the Modelica'2003 conference (for evaluation).

Modelica.Media.UsersGuide.Future

Planned changes for version 3.1

Several changes are planned for version 3.1 of Modelica.Media. In order to have an easy transition, the current verison is moved to the ObsoleteModelica-package and current users can continue to use ot for many years. The goal for the new version is to cover many more fluids, be easier to use for users and less for implementors that want to include their own models. A beta-version of the new media libray will be available in the first quarter of 2009.

The main user-visible changes are:

• No distinction between pure fluids and mixtures on the interface level: pure fluids are mixtures with 1 component
• Compressibility will in the future be a flag that can be set for those media where it makes sense. It will thus be possible to treat an ideal gas as incompressible, which is adequate for certain applications, and also use the same medium model for thermal-only applications, and thermal-hydraulic ones.
• All functions will have derivatives to enable analytic Jacobians.
• Mole-fraction based media for process applications

For further information on the upcoming version, please contact the Author

Forward compatibility

In order to simplify transition to the upcoming version of Modelica.Media for Modelica 3.1, a new enumeration has been introduced: IndependentVariables with the entries ph, pT, pTX, phX and dTX. Users are advised to use this enumeration to determine the independent variables of a medium. If they are going to use the function based interface of the current Modelica.Media (and not use the BaseProperties from Modelica.Media), they should not have any trouble using the next version without any need for changes in their code.

Modelica.Media.UsersGuide.Contact

Main author and maintainer:

Hubertus Tummescheit
Modelon AB
Ideon Science Park
SE-22730 Lund, Sweden
email: Hubertus.Tummescheit@Modelon.se

Acknowledgements:

The development of this library has been a collaborative effort and many have contributed:

• The essential parts of the media models have been implemented in the ThermoFluid library by Hubertus Tummescheit with help from Jonas Eborn and Falko Jens Wagner. These media models have been converted to the Modelica.Media interface definition and have been improved by Hubertus Tummescheit.
• The effort for the development of the Modelica.Media library has been organized by Martin Otter who also contributed to the design, implemented part of the generic models, contributed to the User's Guide and provided the generic test suite Modelica.Media.Examples.Tests.
• The basic idea for the medium model interface based on packages is from Michael Tiller who also contributed to the design.
• The first design of the medium model interface is from Hilding Elmqvist. The design and the implementation has been further improved at the Modelica design meetings in
  Dearborn, Nov. 20-22, 2002
  Dearborn, Sept. 2-4, 2003
  Lund Jan. 28-30, 2004
  Munich, May 26-28, 2004
  Lund, Aug. 30-31, 2004
  Dearborn, Nov. 15-17, 2004
  Cremona Jan. 31 - Feb. 2, 2005.
• Hans Olsson, Sven Erik Mattsson and Hilding Elmqvist developed symbolic transformation algorithms and implemented them in Dymola to improve the efficiency considerably (e.g., to avoid non-linear systems of equations).
• Katrin Pröß implemented the moist air model
• Rüdiger Franke performed the first realistic tests of the Modelica.Media and Modelica_Fluid libraries and gave valuable feedback.
• Francesco Casella has been the most relentless bug-hunter and tester of the water and ideal gas properties. He also contributed to the User's Guide.
• John Batteh, Daniel Bouskela, Jonas Eborn, Andreas Idebrant, Charles Newman, Gerhart Schmitz, and the users of the ThermoFluid library provided many useful comments and feedback.