Fluid thermophysical properties modelling in an opensource platform: CoolFOAM

The evaluation of fluid properties is one important step in Computational Fluid Dynamics (CFD) simulations, especially when the ideal gas approximation is not reasonably accurate. Waste heat recovery units and refrigeration systems are typical examples of applications that require an accurate representation of real gases. Recently, researchers and industries are focusing on the development of refrigerant fluids capable to meet both technical and environmental requirements. The thermophysical models employed in the numerical analyses have to evaluate these fluid properties with sufficient accuracy to be a reliable support for designers. On the other hand, the computational effort overhead related to the usage of such models is not negligible and should be kept as lower as possible. CoolFOAM is a wrapper of the CoolProp library of thermophysical properties for OpenFOAM, one of the most used CFD open-source software. The CoolFOAM library allows OpenFOAM to obtain the thermophysical properties by means of direct calls to CoolProp. In this work, the CoolFOAM wrapper is upgraded with the online generation of lookup tables for a faster evaluation of the thermophysical properties of various fluids. The authors have analyzed the properties of R1234yf to evaluate the precision and the computational load of the wrapper developed, comparing it with OpenFOAM's standard models. Furthermore, a full three-dimensional CFD simulation of a scroll expander has been performed both with ideal and real gas properties, in order to evaluate the differences between the two modelling options. The results have shown a mass flow rate difference of 25 % and an efficiency difference of 30 % between the two models, maintaining a mass imbalance of 0.1 %. The outcome of this work represents a step towards the fast, accurate and reliable numerical simulation of ORC and vapour compression cycle components and systems.