Unified low-parametrical equation of state for engineering calculations of thermodynamic properties of substances

Abstract

Abstract.The new simple semi empirical equation of state for description of P --T data of "normal" substances was specified. New equation of state has 10 individual adjustable coefficients and it describes thermal properties of gas, liquid and fluid in the main with the accuracy within the error of experimental data, except of critical region. The caloric properties and the speed of sound of argon, nitrogen and carbon dioxide were calculated with the help of known thermodynamic equations and in general divergences between calculated and tabular caloric data do not exceed the experimental error. New equation can be used for engineering calculations at the deficit of experimental data, especially on the caloric properties of substances.It is believed that with development of computer engineering all (or most) problems related to description of the thermodynamic surface of single-component substances can be solved with the aid of formal multivariable equations containing many tens or even hundreds of empirical coefficients. It is difficult to accept this idea, since these equations have a number of well-known fundamental disadvantages.In this connection the problem of developing relatively simple physically based unified lowparametrical equations of state still seems quite relevant.In literature there are many (more than 200) low-parametrical equations of state for the singlecomponent substances. The most detailed analysis of these equations is presented in [1].In previous publications [2][3][4][5][6][7] we have derived the semi-empirical equations of state, which slightly yield in accuracy of experimental data description to multivariable equations, but they have some physical basis. At that, they take into account both the known and just determined conditions and restrictions, which should be met by the "true" equation of state (see [2][3][4][5][6][7]). They include the following:1. The preferred structure of the equation of state was determined [4]. It is shown that the compressibility factor of a single-component substance should contain a term that is a function of density only. 2. The equations for the second virial coefficient of substances with polar and nonpolar molecules were derived in [5] with the help of the modified stepwise potential of intermolecular interaction suggested by us.a Corresponding