The Calculation of the Compressibility, Thermal Expansion Coefficient and the Determination the Evaporation Curve for Some Important Refrigerants

The paper presents the mathematical model of computating the compessibility, thermal expansion coefficient, evaporation curve, velocity of sound and other thermophysical properties by means of statistical thermodynamics for some imortant refrigerants. The paper features all important forms of motion of molecules and atoms [translation, rotation, internal rotation, vibration], the influence of excitation of electrons and nuclei into higher energy levels as well as the effect of the intermolecular potential energy. Utilized as the intermolecular potential was the Lennard Jones potential. To computate the thermodynamic values of the state in the realm of real fluids we used grand canonical distribution. For the realm of real fluids Johnson-Zollweg-Gubbins model based on the modified BWR and great number of Monte Carlo simulations was applied. For the calculation in twophase region we applied the method of equilibrium conditions between two phases. The mathematical model enables the calculation in both suband supercritical region. By means of the mathematical model described above we were able to compute thermodynamical properties in one and two-phase region and to draw phase diagrams for some technically significant substances. The results of the analysis are compared with data on experiments showing a relatively good agreement. INTRODUCTION In refrigeration processes in liquid-gas region are of vital importance. In order to design devices for this field of activity, it is necessary to be familiar with the thermodynamic properties of state in a one-phase and two-phase environment. In most cases the thermodynamic tables and diagrams and different empirical functions obtained from measurements are used. Calculation of thermodynamic properties of state is possible by the claasical or statistical thermodynamics. Classical thermodynamics has no insight into the microstructure, but it allows the calculation of thermodynamic function of state with assistance of macroscopic observation of phenomenons. Unlike the classical thermodynamics, however, the statistical thermodynamics does enable the computation of the thermodynamic functions of the state by studying molecular structure of the matter. COMPUTATION OF THERMODYNAMIC PROPERTIES OF STATE To calculate thermodynamic functions of state we applied the canonical partitition (Lucas, 1992). Utilazing the semi-classical formulation for the purpose of the canonical ensemble for the N indistiguishable molecules can be expressed as follows (Lucas, 1992): Z==~J .. Jexi _..!!_)·dr1dr2 •• drNdp1dp2 .• dpN "(I) N!h l kT where f stands for the number of degrees of freedom of individual molecule, H designates the Hamiltonian molecule system. By means of the canonical ensemble for the system of N molecules can be like this: Z == ZoZtranszvibzrotzirzeiZnuczconf (2)