Calculation Technique of Some Thermodynamic Functions using the Results of Joint Thermal and Acoustic Measurements

Abstract

An original method of calculating isochoric and isobaric heat capacity based on the fundamental relationships of thermodynamics is presented. The theoretical development of the methodology for calculating isochoric and isobaric heat capacity can be used to obtain high metrological data on the thermophysical properties of new low-boiling working substances of power plants operating on the Rankine organic cycle and to replenish existing databases. The technique allows the use of heterogeneous experimental results of acoustic and thermal measurements to calculate the basic thermodynamic functions without information about the values of ideal gas functions, and also to evaluate the consistency of experimental data. Formulas are derived for calculating isobaric and isochoric heat capacities, which are based on reliable equations describing experimental data on the speed of sound and density depending on state parameters. The method was tested using both the characteristics of the reference substances (H2O and CO2), and own new experimental data on the thermal and acoustic properties of octafluorocyclobutane (c-C4F8) in the liquid phase for temperatures of 320 and 340 K and pressures from 4 to 10 MPa. The isochoric heat capacity was calculated from measurements of the density of octafluorocyclobutane and the speed of sound in it. According to the latest series of density measurements in a limited region of the states of the liquid phase of octafluorocyclobutane obtained by the constant volume piezometer method, the isochoric heat capacity was calculated using the equation proposed in this work. The uncertainty of the experimental data used for the test calculation of the density did not exceed 0.25%, and the speed of sound did not exceed 0.05%. The standard deviation in the description of the dependence of thermal quantities was 0.23%, and acoustic—0.15%.