NUMERICAL VALIDATION OF A NON-IDEAL GAS MODEL APPROACH FOR STEAM COMPRESSOR FLOW SIMULATION CLOSE TO THE SATURATION LINE

Kurzfassung

The use of high-temperature heat pumps (HTHP) can help replace fossil fuels with green electricity to decarbonize industrial production processes. An operation of a Rankine-based HTHP’s with water vapor as a working medium requires a special design of the turbo machinery, in the particular case of the compressor, used in them. In this study, a recently developed non-ideal gas model approach for low superheated steam for 3D-CFD steady state RANS simulations is deeper analyzed and validated. It is already noted that deviations in flow solutions and performance can occur when designing the centrifugal compressor with steam and a calorically perfect gas model is used in CFD simulations. The application of real gas models for a precise modeling of gas properties is already a well-known procedure in CFD-simulations but is not a common standard in turbo machinery design. The drawback of these real gas models, however, is a significantly more intensive computation time, stability issues and convergence problems. These models are only partially suitable for a 3D CFD-supported optimization process of a centrifugal compressor stage. For this purpose, the recently proposed non-ideal gas model with adapted NASA polynomials is used, which retains the advantages of a calorically perfect gas model in terms of computational speed, stability and convergence behavior. The adapted polynomials determine the change of state in temperature and pressure, assuming a polytropic process. For the validation of this gas model approach, comparisons with different real gas models using steam as working medium are performed and presented. A pressure and temperature dependency of the specific heat capacity and polynomial function is demonstrated, which is compared with the computed progression from the real gas models. 3D-CFD steady state RANS simulations of a centrifugal compressor stage with in-house and commercial CFD solvers specially developed for turbo machinery flow calculations are used. The influence of the different gas models on 0D compressor map parameters such as pressure ratio and efficiency are analyzed. The CFD solutions are examined and compared in detail with regard to the distributions of the thermodynamic gas properties. This is intended to provide as a basis for recognizing and understanding the changes in the flow field and their effects on the targeted vapor compression. The influence on the operating range (OR) of the centrifugal compressor stage is shown, which must ensure safe operation of the HTHP system. Especially when a multi-stage compression system is used, the OR must be well predicted. The use of the proposed adapted non-ideal gas model approach can improve an advantageous outcome in calculation speed, stability and convergence behavior of turbo machinery geometry optimization for centrifugal compressor stages.