Two-Phase Modelica Media example

Question

I am trying to develop a simulation in OpenModelica of a flow that has a single substance that will be liquid or vapor. The Modelica.Media.Water models do have two phases, but are extremely complicated, and would be very hard to reproduce for a completely different substance. I would like to find a simple example of a two phase medium that I can work from. There is a partial package TemplateMedium and a partial package PartialTwoPhaseMedium, but I don't see any examples of how to write a completely new Medium that can be in either of two phase.
If anyone can provide a simple example, or just a list of the minimum set of properties and equations that are required that would be extremely helpful as a starting point.

To address some of the question in comments: I am just getting started on this model, so I am trying to understand the details of how the Media model is constructed, and what my specifics are included in the model versus what has to be added for each new media. I working with propylene, so there is good data available. This is one of the media that is included in CoolProp, so being able to use ExternalMedia and CoolProp would be very useful, but I believe that these are not yet working with OpenModelica, from a number of comments and bug reports.

Answer 1

Generally, your medium model can be written in Modelica or you can reuse an existing external library. Writing good medium models is a lot of work, so reusing existing libraries is usually a good idea. This is the approach taken by ExternalMedia (open source) or TILMedia (commercial). If you are interested in an open-source workflow, ExternalMedia in combination with Coolprop is a reasonable decision. All three projects OpenModelica, ExternalMedia and CoolProp accept contributions from the community, so maybe you should help improving these instead of writing your own library. There is a lot of work going on already, I am unsure of the current status. Writing qualified bug reports (including steps to reproduce the problem) is also a very welcome way to contribute.

For some applications, it might be good to have the Medium model directly in Modelica. This is the approach taken by Modelica.Media (obviously), HelmholtzMedia and the commercial media libraries from XRG or Modelon (not 100% sure about that). There are some more implementations, but these are neither open source nor commercial, only information are e.g. conference papers. The examples you can look at include the R134a medium from the MSL or the code from the HelmholtzMedia library. Also, looking at the ExternalMedia implementation might help.
For fluids that cannot change phase, there are some good examples in the Annex60 library.

As you have a pure substance that can change phase, your new medium should extend from PartialTwoPhaseMedium. PartialTwoPhaseMedium is partial, defining only what functions are there, but (mostly) not the algorithms of the functions. You will have to write an algorithm for each and every function that is available in the interface and does not have an algorithm in order to be fully compatible. For a start, you should implement at least one of the setState funtions, e.g. the setState_ph function. Then later, implement at least one setSat function and the BaseProperties.

If you implement your own medium, you also have the choice of how to model it: Using the full multiparameter Helmholtz energy equation of state, a simpler equation of state like Peng-Robinson or other cubic EoS, some polynomials or splines, table-based methods like TTSE or SBTL and probably many more options.