Algorithms for the calculation of psychrometric properties from multi-fluid Helmholtz-energy-explicit models

Abstract

Psychrometric properties of humid air are widely used in the analysis and modeling of thermal systems. In this work, we present a method for obtaining these properties from the multi-fluid mixture formulation of the GERG mixture model. This mixture model was originally developed to model the thermodynamics of natural gas mixtures, and more recently has been extended to model thermodynamic properties relevant for carbon capture and storage. The primary advantage of this formulation is that the dry air composition is not fixed, and can be adjusted to suit the application, for instance for combustion flue gases, for air in submarines, for Martian atmospheres, etc. Furthermore, this multi-fluid framework employs the highest accuracy pure-fluid equations of state in the literature.We present algorithms that can be used to calculate the quantity of water in the saturated vapor in vapor-liquid and solid-vapor equilibria, and other properties that arise out of these equilibria calculations, such as relative humidity and humidity ratio. We also present algorithms for converting other common sets of independent variables (e.g., wet-bulb temperatures), to the natural variables of the mixture model.Comprehensive discussion of the developed algorithms is provided, as well as sample code in the C++ language.