Density measurements in subcooled water at pressures up to 400 MPa

Despite many applications in industry and science, water is still unknown and continuously under study, since it is a complex fluid with several anomalies and especially at high pressures and low temperatures, and outside the range of stability in the so called “metastable states”. The thermodynamic states of liquid water below the normal freezing point (supercooled water) need to be accurately studied because of the lack of accurate experimental data of the hermodynamic properties. Experimental measurements, especially of density, are also necessary because they form the basis to implement a fundamental equation of state, from which all thermodynamic properties can be calculated. The purpose of this thesis is to measure the density of cooled and supercooled water with high accuracy, i.e. both in stable and metastable states, in the temperature range of (243 and 283) K, and at high pressures, from (140 to 400) MPa, by means of a pseudo-isochoric method. The experimental apparatus mainly consists of a high pressure vessel, specifically designed for this experiment, of known volume as a function of temperature and pressure. The density of subcooled water is basically obtained by measuring, at constant mass, the equilibrium pressure by changing the temperature avoiding crystallization. Furthermore, since the paramount goal of performing accurate experimental measurements is the development of an equation of state, a complementary activity, consisting of the implementation of a fundamental equation of state in terms of the Helmholtz energy, is carried out. Finally, an overview of how an equation of state is developed, and the practical example of fundamental equations implementation for two alkanes (n-hexadecane and n-docosane) are presented.