Calculation of thermodynamic properties of SF6 including the critical region. Combined thermal equation of state with a small number of parameters

Abstract

A new unified combined equation of the state of liquid, gas, and fluid with a small number of adjustable constants is considered. Using this equation, tables of sulfur hexafluoride SF6 thermodynamic properties are compiled and proposed as standard reference data. The equation of state has the form of an explicit dependence of p = p(ρ, T) and does the following: transfers the thermal and caloric properties of SF6 in the regular component of the thermodynamic surface with the accuracy of the experiment; includes critical exponents β, α, and γ, taken in agreement with the three-dimensional Ising model; and corresponds to the relations of thermodynamics and the scale theory for properties in the critical region. The adjustable constants are obtained by approximating the p,ρ,T-data of SF6 using a combined equation of state in the ranges of 224 ≤ T ≤ 623 K, 0.1 ≤ p ≤ 30 MPa, and 8 ≤ ρ ≤ 1842 kg/m3. It includes a new regular component for approximating p,ρ,T-data in the liquid and gas regions of states outside the critical region, four of the 13 constants that are determined by the conditions at the critical and triple points, the singular component that is a scale equation (six constants) in explicit form for the critical region, and a new crossover function for combining these equations. The total number of system-dependent constants is 21, including parameters of the critical point. The root-mean-square error in the description of p,ρ,T-data of SF6 by pressure is 1.05% in the entire range of gas and liquid states; the average absolute deviation is 0.51%.