Empirical correlations for the third virial coefficients of nonpolar, polar and quantum fluids in a wide temperature range

The third virial coefficient considers the real-fluid imperfection caused by the molecular-triplet interaction. Based on the corresponding state principle and proper classification of fluids, a new third-virial-coefficient model was proposed in this work for nonpolar, polar, and quantum fluids over a wide temperature range. The correlations were formed as a sum of the simple spherical term and the deviation term caused by non-sphericity and polarity, or as a sum of the simple spherical term and the quantum term caused by quantum effects. With easily accessible characteristic parameters, the correlations were determined using ab initio values and reliable experimental data. The new model met the three physically-based mathematical conditions proposed in this work, and could provide reliable predictions at extremely high temperatures. To validate the accuracy of the new third-virial-coefficient model, the ab initio values and experimental data were compared, all of which were in good agreement with this work. In addition, the truncated virial equations of state, composed based on this study and our previous study for the generalized second-virial-coefficient model, were compared to the multiparameter equations of state. The three-term truncated virial equations of state have a wider valid P-T region than the two-term equations. This shows that the new model can provide accurate descriptions and physically reasonable predictions for simple, nonpolar, non-associated polar, associated polar, and quantum fluids over a wide temperature range. Also, the three-term truncated virial equation of state performs well in the low-to-moderate density region, making it potentially useful for reliable predictions at high temperatures where experiments are difficult to conduct. © 2022