The Helmholtz free energy of pure fluid substances and fluid mixtures

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.The Helmholtz free energy is essential to the modelling of multiphase flow with the diffuse interface formalism. In models where it is a good approximation to let the temperature be constant and uniform, a simplified calculation of the Helmholtz free energy is performed, that leaves a number of functions of temperature unspecified (one per chemical component, in case of mixtures). If the model is to include the energy balance equation, i.e., account for flow with variable temperature, then the Helmholtz free energy must include these temperature dependent terms. This paper is an exposition of the determination of such terms, where three types of molecules are considered: (i) single conformation, (ii) multiple conformation, and (iii) polar. A specific example for each case is included: methane, n-pentane, and water. The Helmholtz free energy of fluid mixtures is also considered.