Background on modeling fluid transport properties
In the context of thermofluid systems, fluid properties such as pressure, density, enthalpy, heat capacity, compressibility or speed of sound are calculated from an equation of state. Conversely, the transport properties such as viscosity, thermal conductivity, and surface tension are calculated from dedicated correlations. Of these, both viscosity and thermal conductivity are split into (shown below). The dilute (or zero density) contribution can be treated by kinetic gas theory, which is usually modeled by simple polynomials. However, the critical enhancement of viscosity can only be seen in a very small region and in the published literature it is neglected for all fluids except CO2 and water. For thermal conductivity, the critical enhancement is significant in a wider region and is usually modeled based on the model by Olchowy and Sengers. This model is dependent on the equation of state and the viscosity correlation; thus, it can only be employed if reliable models exist for these properties. As dilute and critical contribution rely on theory, little experimental data is required for the fitting.
If only very little experimental data is available, the background contribution can instead be modeled using extended corresponding states (ECS) theory. The basic idea of ECS is that: one fluid, R1234yf, at one state behaves like a reference fluid, R134a, at a different state (the so-called conformal state). The algorithms for finding the conformal state are quite complex and involve non-linear iterations that are hard to converge in some regions.
The advantage of the ECS model is that little experimental data is required to develop it, resulting in early publications using ECS for modeling the transport properties. However, the disadvantages of the ECS model are that it often can be used for a limited range and the algorithms are an additional source for potential numerical difficulties.