Sound velocity and isentropic exponents for gases with different acentric factors by using the Redlich-Kwong-Soave equation of state

Abstract

In previous publications, three real gas isentropic exponentsk pv ,k Tv ,k pT have been introduced, which when used instead of the classical isentropic exponentk=c p /c v in the ideal gas isentropic change equations, the latter may be applied with great accuracy to real gases. The first of the exponents may also be employed for calculating the real gas sound velocity as α=(k pv pv)1/2. The exact values of the sound velocity α and of the three isentropic exponentsk pv ,k Tv ,k pT of a real gas may be calculated only if the exact thermal and caloric equations of state of this real gas are available. For most real gases, however, such equations are not available. In the present work, correlations are presented for calculating quantities α,k pv ,k Tv ,k pT for all real gases, in terms of the acentric factor which characterises their deviation from the ideal gas. The correlations, which are based on the Redlich-Kwong-Soave generalized equation of state, provide the above quantities in a generalized reduced form in terms of the reduced temperature, of the reduced volume and of the compressibility factor, i.e. independently of any gas properties except for the acentric factor. Therefore, it becomes possible to plot the generalized sound velocity and isentropic exponents on the generalized compressibility chart (Nelson-Obert diagram) for various values of the acentric factor. Comparisons with exact values in the case of real air, calculated by use of exact thermal and caloric equations, show that the accuracy of the correlations developed is satisfactory.