Thermodynamics of pure liquid water: Sound speed measurements to 700 MPa down to the freezing point, and an equation of state to 2300 MPa from 240 to 500 K

摘要

Accurate thermodynamic properties for aqueous solutions under an increasing range of pressures, temperatures, and compositions are needed to address a variety of technical and scientific challenges. This study provides measurements for improving the high-pressure and low-temperature representation of liquid water. Sound speeds of pure liquid water are reported between 0.1 and 700 MPa, from 353 K down to the melting curves of the ice phases. The new sound speed measurements have a relative standard uncertainty of 100-170 ppm (from 0.1 to 700 MPa), with standard uncertainties of 20 mK for the temperature measurements and 0.02-0.14 MPa (from 0.1 to 700 MPa) for the pressure measurements. Using additional published measurements, a new equation of state is derived extending from 240 to 500 K and from 0.1 to 2300 MPa, covering much of the subcritical domain of water up to the ice VI-ice VII transition. Analyses of measurements and construction of the equation of state are accomplished with a flexible computational thermodynamic framework based on local basis functions in the form of tensor B-splines. Relative to IAPWS-95 (the most comprehensive representation available), improvements in the accuracies of density, sound speed, and specific heat are expected above 100 MPa, particularly near the solid-fluid phase boundaries. Published under license by AIP Publishing.