18 Multiparameter equations of state

Abstract

Accurate thermophysical properties of fluids are needed for the development of reliable mathematical models of energy systems. Although significant improvements are being made in predicting thermodynamic properties of pure fluids and mixtures using theory-based methods, there is a need for more accurate equations of state both for applications in engineering system design and analysis and to satisfy scientific data needs. Current practice in the development of computer programs, property tables, and charts involves the correlation of selected experimental data for a particular fluid or mixture using a model, which is accurate for calculating properties over a wide range of pressures and temperatures. A typical thermodynamic property formulation is based on an equation of state, which allows the correlation and computation of all thermodynamic properties of the fluid, including properties such as entropy that cannot be measured directly. The term “fundamental equation” is often used in the literature to refer to empirical descriptions of one of four fundamental relations: internal energy as a function of volume and entropy, enthalpy as a function of pressure and entropy, Gibbs energy as a function of pressure and temperature, and Helmholtz energy as a function of density and temperature. Modern equations of state for pure fluid properties are usually fundamental equations explicit in the Helmholtz energy as a function of density and temperature.