When a medium model is used in a balance volume, differential equations for the independent medium variables are present and therefore initial conditions have to be provided. The following possibilities exist:

Modelica has currently no language element to define steady state initialization. In the Modelica simulation environment Dymola, the option

Advanced.DefaultSteadyStateInitialization =true

can be set before translation. Then, missing initial conditions are provided by automatically setting appropriate state derivatives to zero.

Explicit start values can be defined with the "start" and
"fixed" attributes. The number of independent variables nx
need to be known which can be deduced from the medium
constants (nx = nXi + **if** singleState **then** 1 **else** 2).
Then, start values or initial equations can be defined
for nx variables (= p, T, d, u, h, Xi) from Medium.BaseProperties,
e.g., in the form:

replaceablepackage Medium = Medium.Interfaces.PartialMedium; Medium.BaseProperties medium1 (p(start=1e5, fixed=notMedium.singleState), T(start=300, fixed=true)); Medium.BaseProperties medium2;initial equationif notMedium.singleStatethenmedium2.p = 1e5;end if; medium2.T = 300;equation

If initial conditions are not provided for the independent medium variables, non-linear systems of equations may occur to compute the initial values of the independent medium variables from the provided initial conditions.

If non-linear systems of equations occur during initialization, e.g., in case of steady state initialization, guess values for the iteration variables of the non-linear system of equations have to be provided via the "start" attribute (and fixed=false). Unfortunately, it is usually not known in advance which variables are selected as iteration variables of a non-linear system of equations. One of the following possibilities exist:

- Do not supply start values and hope that the medium specific types have meaningful start values, such as in "Medium.AbsolutePressure"
- Supply start values on all variables of the BaseProperties model, i.e., on p, T, d, u, h, Xi.
- Determine the iteration variables of the non-linear systems of
equations and provide start values for these variables.
In the Modelica simulation environment Dymola, the iteration
variables can be determined by setting the command
`Advanced.OutputModelicaCode =`

and by inspection of the file "dsmodel.mof" that is generated when this option is set (search for "nonlinear").**true**

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