Fluid Property Functions in Polar and Parabolic Coordinates

This paper presents two methods for reallizing fluid property functions in Modelica simulation models. Each makes use of a coordinate transformation that aligns one coordinate with the saturation curve. This provides for a precise representation of the fluid property function at the saturation curve, and for connected domains of interest including the liquid, vapor, supercritical and two-phase regions. Both approaches make use of spline function approximation in the aligned coordinates, and are numerically efficient, well conditioned, and allow for efficient calculation of derivatives up to any desired order that are precise up to processor numerical tolerance.