Thermophysical Functions

The first argument of all built-in thermophysical property functions is the name of the substance. This argument is a string that may be provided as a string constant (enclosing quote marks are optional) or a string variable that contains the name of the fluid. The fluid may be any of the built-in fluids provided with EES, any of ideal gas fluids provided with the NASA ideal gas data base, or any of the fluids in the Solutions or Incompressible substances libraries. EES also allows User-Supplied Property Data.

Arguments are separated with the list separator character, which is a comma for the U.S. numerical format and a semicolon for the European numerical format.

It may appear that some substances in the built-in property data base, e.g., N2 and Nitrogen, CO2 and CarbonDioxide, H2O and Steam (or Water), are duplicated, but this is not true. Whenever a chemical symbol notation (e.g., N2, CO2, CH4) is used, the substance is modeled as an ideal gas and the enthalpy and entropy values are based on JANAF table references. The JANAF table reference for enthalpy is based on the elements having a specific molar enthalpy value of 0 at 298 K (537 R). The entropy of these substances is based on the Third Law of Thermodynamics. Whenever the substance name is spelled out (e.g., Steam (or Water), Nitrogen, R134a, CarbonDioxide, Methane, etc.) the substance is modeled as a real fluid with subcooled, saturated, and superheated phases. In this case, it is possible to use the $Reference directive to shift the reference state so that the substances use the same reference states as the corresponding ideal gases. Exceptions to this rule occur for Air and AirH2O, both of which are modeled as ideal gases. AirH2O is the notation for air-water vapor mixtures, i.e., psychometrics. Additional information concerning the methods, reference states, and ranges of applicability for the thermophysical properties are provided in the online help for each substance.

All arguments in thermophysical property functions, aside from the substance name, are identified by a single case-insensitive letter followed by an equal sign. Arguments must be separated with commas (semi-colons for the EU format) and may be in any order, provided that the substance name is first. The value or algebraic expression representing the value of the argument follows the equal sign. The letters that are recognized in function arguments and their meaning are:

B= wet-bulb temperature (only for substance AIRH2O)

C=mass concentrationof the solute in % (only for Solution)

D=dew-point temperature (only for substance AIRH2O)

H=specific enthalpy

P=pressure

Q=quality (only for NH3H2O. Use X for quality for real fluids)

R=relative humidity (only for substance AIRH2O)

S=specific entropy

T=temperature

U=specific internal energy

V=specific volume (=1/density)

W=humidity ratio (only for substance AIRH2O)

X=quality (for real fluids, mass fraction for NH3H2O, otherwise not applicable)

Many of the thermodynamic functions can take alternate sets of arguments. For example, the ENTHALPY function for steam can be accessed with temperature and pressure as arguments; alternatively, the same function could be accessed with entropy and quality as arguments. In general, any valid set of arguments can be supplied for thermodynamic functions.

EES does not require the function argument to have a known value. For example:

h1 = ENTHALPY(Steam,T=T1,P=P1)

will return the value of h1 corresponding to known temperature and pressure, T1 and P1. If, however, the value of h1 is known, but T1 is unknown, the same equation will return the appropriate value of the temperature. Alternatively, the temperature could be found by:

T1 = TEMPERATURE(Steam,h=h1,P=P1)

The latter method is preferable in that the iterative calculations implemented for steam are less likely to have convergence difficulty.

Note that there are four built-in property Procedures that return multiple properties in one EES statement.

*IDEALGASTHERMOPROPS returns all of the thermodynamic properties for an ideal gas given any two

*IDEALGASMIXTUREPROPS returns thermodynamic and transport properties for mixtures of ideal gases.

*REALTHERMOPROPS returns all of the thermodynamic properties for a pure real fluid, given any two.

*PSYCHPROPS returns psychrometric properties given temperature, pressure, and one of (humidity ratio, relative humidity, wetbulb or dewpoint)