PartialLinearFluidGeneric pure liquid model with constant cp, compressibility and thermal expansion coefficients 
A selection of variables that uniquely defines the thermodynamic state 

Base properties of medium 

Set the thermodynamic state record from p and T (X not needed) 

Set the thermodynamic state record from p and h (X not needed) 

Set the thermodynamic state record from p and s (X not needed) 

Set the thermodynamic state record from d and T (X not needed) 

Return thermodynamic state so that it smoothly approximates: if x > 0 then state_a else state_b 

Return the pressure from the thermodynamic state 

Return the temperature from the thermodynamic state 

Return the density from the thermodynamic state 

Return the specific enthalpy from the thermodynamic state 

Return the specific entropy from the thermodynamic state 

Return the specific internal energy from the thermodynamic state 

Return specific Gibbs energy from the thermodynamic state 

Return specific Helmholtz energy from the thermodynamic state 

Return velocity of sound from the thermodynamic state 

Return isentropic exponent from the thermodynamic state 

Return isentropic enthalpy 

Return specific heat capacity at constant volume 

Return specific heat capacity at constant volume from the thermodynamic state 

Return the isothermal compressibility kappa 

Return the isobaric expansion coefficient 

Return density derivative w.r.t. pressure at const specific enthalpy 

Return density derivative w.r.t. specific enthalpy at constant pressure 

Return density derivative w.r.t. pressure at const temperature 

Return density derivative w.r.t. temperature at constant pressure 

Returns the partial derivative of density with respect to mass fractions at constant pressure and temperature 

Return molar mass 

Return temperature from pressure and specific enthalpy 

Return temperature from pressure and specific entropy 
ThermoStates 
Value: Modelica.Media.Interfaces.Choices.IndependentVariables.pTX Type: IndependentVariables Description: Enumeration type for independent variables 

mediumName 
Value: "unusablePartialMedium" Type: String Description: Name of the medium 
substanceNames 
Value: {mediumName} Type: String[:] Description: Names of the mixture substances. Set substanceNames={mediumName} if only one substance. 
extraPropertiesNames 
Value: fill("", 0) Type: String[:] Description: Names of the additional (extra) transported properties. Set extraPropertiesNames=fill("",0) if unused 
singleState 
Value: false Type: Boolean Description: = true, if u and d are not a function of pressure 
reducedX 
Value: true Type: Boolean Description: = true if medium contains the equation sum(X) = 1.0; set reducedX=true if only one substance (see docu for details) 
fixedX 
Value: true Type: Boolean Description: = true if medium contains the equation X = reference_X 
reference_p 
Value: 101325 Type: AbsolutePressure (Pa) Description: Reference pressure of Medium: default 1 atmosphere 
reference_T 
Value: 298.15 Type: Temperature (K) Description: Reference temperature of Medium: default 25 deg Celsius 
reference_X 
Value: fill(1 / nX, nX) Type: MassFraction[nX] (kg/kg) Description: Default mass fractions of medium 
p_default 
Value: 101325 Type: AbsolutePressure (Pa) Description: Default value for pressure of medium (for initialization) 
T_default 
Value: Modelica.SIunits.Conversions.from_degC(20) Type: Temperature (K) Description: Default value for temperature of medium (for initialization) 
h_default 
Value: specificEnthalpy_pTX(p_default, T_default, X_default) Type: SpecificEnthalpy (J/kg) Description: Default value for specific enthalpy of medium (for initialization) 
X_default 
Value: reference_X Type: MassFraction[nX] (kg/kg) Description: Default value for mass fractions of medium (for initialization) 
C_default 
Value: fill(0, nC) Type: ExtraProperty[nC] Description: Default value for trace substances of medium (for initialization) 
nS 
Value: size(substanceNames, 1) Type: Integer Description: Number of substances 
nX 
Value: nS Type: Integer Description: Number of mass fractions 
nXi 
Value: if fixedX then 0 else if reducedX then nS  1 else nS Type: Integer Description: Number of structurally independent mass fractions (see docu for details) 
nC 
Value: size(extraPropertiesNames, 1) Type: Integer Description: Number of extra (outside of standard massbalance) transported properties 
C_nominal 
Value: 1.0e6 * ones(nC) Type: Real[nC] Description: Default for the nominal values for the extra properties 
cp_const 
Value: Type: SpecificHeatCapacity (J/(kg·K)) Description: Specific heat capacity at constant pressure 
beta_const 
Value: Type: IsobaricExpansionCoefficient (¹/K) Description: Thermal expansion coefficient at constant pressure 
kappa_const 
Value: Type: IsothermalCompressibility (¹/Pa) Description: Isothermal compressibility 
MM_const 
Value: Type: MolarMass (kg/mol) Description: Molar mass 
reference_d 
Value: Type: Density (kg/m³) Description: Density in reference conditions 
reference_h 
Value: Type: SpecificEnthalpy (J/kg) Description: Specific enthalpy in reference conditions 
reference_s 
Value: Type: SpecificEntropy (J/(kg·K)) Description: Specific entropy in reference conditions 
constantJacobian 
Value: Type: Boolean Description: If true, entries in thermodynamic Jacobian are constant, taken at reference conditions 
This information is part of the Modelica Standard Library maintained by the Modelica Association.
This linear compressibility fluid model is based on the assumptions that:
That means that the density is a linear function in temperature and in pressure. In order to define the complete model, a number of constant reference values are needed which are computed at the reference values of the states pressure p and temperature T. The model can be interpreted as a linearization of a full nonlinear fluid model (but it is not linear in all thermodynamic coordinates). Reference values are needed for
Apart from that, a user needs to define the molar mass, MM_const. Note that it is possible to define a fluid by computing the reference values from a full nonlinear fluid model by computing the package constants using the standard functions defined in a fluid package (see example in liquids package).
In order to avoid numerical inversion of the temperature in the T_ph and T_ps functions, the density is always taken to be the reference density in the computation of h, s, u and cv. For liquids (and this model is intended only for liquids) the relative error of doing so is 1e3 to 1e4 at most. The model would be more "correct" based on the other assumptions, if occurrences of reference_d in the computations of h,s,u and cv would be replaced by a call to density(state). That would require a numerical solution for T_ps, while T_ph can be solved symbolically from a quadratic function. Errors from this approximation are small because liquid density varies little.
One of the main reasons to use a simple, linear fluid model is to achieve high performance in simulations. There are a number of possible compromises and possibilities to improve performance. Some of them can be influenced by a flag. The following rules where used in this model:
Modelica.Media.CompressibleLiquids Cold water model with linear compressibility 

Modelica.Media.CompressibleLiquids.Common Base class for liquid, linear compressibility water models 