Modification of the Redlich-Kwong-Aungier Equation of State to Determine the Degree of Dryness in the CO2 Two-phase Region

Abstract

The degree of dryness is the most important parameter that determines the state of a real gas and the thermodynamic properties of the working fluid in a two-phase region. This article presents a modified Redlich-Kwong-Aungier equation of state to determine the degree of dryness in the two-phase region of a real gas. Selected as the working fluid under study was CO2. The results were validated using the Span-Wanger equation presented in the mini-REFPROP program, the equation being closest to the experimental data in the CO2 two-phase region. For the proposed method, the initial data are temperature and density, critical properties of the working fluid, its eccentricity coefficient, and molar mass. In the process of its solution, determined are the pressure, which for a two-phase region becomes the pressure of saturated vapor, the volumes of the gas and liquid phases of a two-phase region, the densities of the gas and liquid phases, and the degree of dryness. The saturated vapor pressure was found using the Lee-Kesler and Pitzer method, the results being in good agreement with the experimental data. The volume of the gas phase of a two-phase region is determined by the modified Redlich-Kwong-Aungier equation of state. The paper proposes a correlation equation for the scale correction used in the Redlich-Kwongda-Aungier equation of state for the gas phase of a two-phase region. The volume of the liquid phase was found by the Yamada-Gann method. The volumes of both phases were validated against the basic data, and are in good agreement. The results obtained for the degree of dryness also showed good agreement with the basic values, which ensures the applicability of the proposed method in the entire two-phase region, limited by the temperature range from 220 to 300 K. The results also open up the possibility to develop the method in the triple point region (216.59K-220 K) and in the near-critical region (300 K-304.13 K), as well as to determine, with greater accuracy, the basic CO2 thermodynamic parameters in the two-phase region, such as enthalpy, entropy, viscosity, compressibility coefficient, specific heat capacity and thermal conductivity coefficient for the gas and liquid phases. Due to the simplicity of the form of the equation of state and a small number of empirical coefficients, the obtained technique can be used for practical problems of computational fluid dynamics without spending a lot of computation time.

Modification of scaled equation of state to determine the pressure in the CO2 critical region

“…As the crossover function between the regular and the scaling parts is used the classical function of the extinction probability of temperature and density fluctua tions from [7]. In paper [11] was presented the method to determine the main thermodynamics properties in the two phase region of the carbon dioxide. This method is based on Aungiermodified the RedlichKwong equation of state, which can predict fluid properties in the twophase region with suf ficient accuracy.…”

“…This method is based on Aungiermodified the RedlichKwong equation of state, which can predict fluid properties in the twophase region with suf ficient accuracy. Methodology from [11] was decided to applied and modified for using in the near critical region of CO 2 .…”

“…where ω -the compressibility factor for a working fluid. Scaled and shift parameters were added to the regular part of the crossover equation of state by analogy with methodology from paper [11] for achieving more accurate results for pressure. As temperature range from 300 K to 305 K contains several regions of working fluid, twophases and supercritical, scaled parameters will be individual for both regions.…”

Effect detection of using a modified Redlich-Kwong-Aungier equation of state on the calculation of carbon dioxide flow in a centrifugal compressor

“…The study [12] presents a mathematical model for determining the basic thermodynamic properties in the twophase region of carbon dioxide. This mathematical model is based on the modified Redlich-Kwong-Aungier equation of state [13], which can predict the properties of the liquid phase in the two-phase region accurately enough. Equations from [14] were taken as a basis and modified for use in the pure liquid and transcritical regions of CO 2 .…”

“…For modeling the CO 2 condensation cycle, the Redlich-Kwong-Aungier equation of state was selected, which provides high accuracy in calculations in the supercritical and gaseous regions. The theoretical approach to its modification is described in [13]. In the critical point region, where phase transition occurs, significant fluctuations in thermodynamic parameters such as pressure, temperature, and density are observed.…”