Modeling of the Shear Viscosity of Sulfur Hexafluoride : Free-Volume Model, Thermodynamic Scaling and Molecular Dynamics Representation
Résumé
Sulfur hexafluoride is a very important compound in many fields of technology. It is a relatively inert compound that is very stable over a wide range of temperatures and pressures. From the modeling point of view, the sulfur hexafluoride molecule is quite large and relatively spherical. For these reasons, from a fundamental point of view sulfur hexafluoride can be considered as a model fluid which could provide a stringent test on the applicability of models aiming at representing its thermophysical properties. Viscosity at very low density states (i.e. in the "diluted-gas" conditions) having been carefully studied and modeled by Quiñones-Cisneros et al1, we focus in this work to the viscosity behavior for the denser states. Thus, the purpose of this work is to check how recent approaches are able to represent the sulfur hexafluoride viscosity over a large range of thermodynamic conditions with a limited number of adjustable parameters. With this intention, three recent approaches with important physical background are considered: a free-volume model2, a thermodynamic scaling model3,4 and a correlation5 representing the dynamic viscosity using the equivalent Lennard-Jones 12-6 (LJ) fluid. The experimental database employed for adjustment (1562 data points) considers a large temperature (225.18 to 473.15 K) and pressure intervals (0.0264 to 51.21 MPa). The absolute average deviation is 3.8% for the Lennard-Jones model (1 parameter; see Fig.1 close to the critical temperature), 1.7% for the free volume model (3 parameters) and 1.5 % for the thermodynamic scaling model (6 parameters). Thus, it is shown that when physically based approaches are employed, a limited number of parameters are sufficient to represent accurately the shear viscosity of sulfur hexafluoride. Furthermore it has been confirmed, using the thermodynamic scaling approach, that the repulsive steepness of the sulfur hexafluoride interaction potential is higher than usually found for non polar spherical molecule.