Reduced properties
Encyclopedia
In thermodynamics
, the reduced properties of a fluid are a set of state variables normalized by the fluid's state properties at its critical point
. These dimensionless thermodynamic coordinates, taken together with a substance's compressibility factor
, provide the basis for the simplest form of the theorem of corresponding states
.
Reduced properties are also used to define the Peng-Robinson equation of state, a model designed to provide reasonable accuracy near the critical point. They are also used to critical exponents, which describe the behaviour of physical quantities near continuous phase transitions.
divided by its critical pressure 
:
where the actual temperature and critical temperature are expressed in absolute temperature scales (either Kelvin
or Rankine). Both the reduced temperature and the reduced pressure are often used in thermodynamical formulas like the Peng-Robinson equation of state.
at the substances critical pressure and temperature:
This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
, the reduced properties of a fluid are a set of state variables normalized by the fluid's state properties at its critical point
Critical point
Critical point may refer to:*Critical point *Critical point *Critical point *Construction point of a ski jumping hill-See also:*Brillouin zone*Percolation thresholds...
. These dimensionless thermodynamic coordinates, taken together with a substance's compressibility factor
Compressibility factor
The compressibility factor , also known as the compression factor, is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behavior. In general, deviation from ideal behavior becomes more significant the closer a gas is to a phase change, the lower the...
, provide the basis for the simplest form of the theorem of corresponding states
Theorem of corresponding states
According to van der Waals, the theorem of corresponding states indicates that all fluids, when compared at the same reduced temperature and reduced pressure, have approximately the same compressibility factor and all deviate from ideal gas behavior to about the same degree.Material constants that...
.
Reduced properties are also used to define the Peng-Robinson equation of state, a model designed to provide reasonable accuracy near the critical point. They are also used to critical exponents, which describe the behaviour of physical quantities near continuous phase transitions.
Reduced pressure
The reduced pressure is defined as its actual pressure divided by its critical pressure :Reduced temperature
The reduced temperature of a fluid is its actual temperature, divided by its critical temperature:where the actual temperature and critical temperature are expressed in absolute temperature scales (either Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
or Rankine). Both the reduced temperature and the reduced pressure are often used in thermodynamical formulas like the Peng-Robinson equation of state.
Reduced specific volume
The reduced specific volume (or "pseudo-reduced specific volume") of a fluid is computed from the ideal gas lawIdeal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...
at the substances critical pressure and temperature:
This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.