Schnelle Berechnung realer Gaseigenschaften im Programmsystem AC² (SBRG-AC²)

Abstract

The aim of the project was to develop fluid-property-calculation algorithms and corresponding software for calculating the real thermophysical properties of gases and gas mixtures, and to implement them in the AC² program system. Furthermore, the property library libSBTL_95 for water/steam, developed in the predecessor project SBRS-AC², was to be implemented in COCOSYS. At first, a calculation algorithm for real gas mixtures was developed, in which the influence of the real mixture is calculated using a virial equation for gas mixtures. The properties of the real individual gases are calculated using the Spline-Based Table Look-up method (SBTL). In terms of thermodynamic consistency and range of validity, this approach meets the requirements of ATHLET and COCOSYS. The FluidMixtures property library was developed for the gas mixtures used in AC2, which in turn uses the SBTL property libraries for pure substances. Comparative calculations with REFPROP (NIST, USA) show that the new property calculations for gases and gas mixtures are significantly more accurate than the model used previously, in which non-condensable gases are modeled as ideal gases and the influence of mixing is neglected. The new calculation method is suitable for numerical process simulations and reproduces the fluid properties with high accuracy and minimal computing time. The SBTL method developed at the HSZG has been an international standard for the fast and accurate calculation of the properties of water and steam since 2015. A plugin was created for the newly developed FluidMixtures property library, enabling its use in AC2 (ATHLET and COCOSYS). In the course of the work, it became apparent that the existing implementation of the conservation laws and solution variables in ATHLET is not applicable to generic mixture models. This made it necessary to derive generic equations for the conservation laws with generic solution variables. The corresponding partial differential equations were implemented in ATHLET and the solver for the generic solution variables was modified. In addition, the fluid property calculations for water and steam in COCOSYS (thermohydraulic module) were refactored and verified. The models and their implementation in AC2 were tested using simple validation calculations.