Investigation of Real Gas Effects on Centrifugal Compressor Analytical Methods for Supercritical CO2 Power Cycles

As supercritical carbon dioxide (sCO2) power cycles have shown potential to be the next generation power cycle, an immense amount of research has gone into developing this system. One of the setbacks facing development and optimization of this cycle is the unknown in current design and analysis methods ability to accurately model turbomachinery working with sCO2. Due to the desired inlet conditions to the compressor close proximity to the critical point, accurate design and analysis of this power cycle component is one of the main concerns. The present study provides aerodynamic analysis of a centrifugal compressor impeller blade with sCO2 as the working fluid through a comparative study between three dimensional (3D) computational fluid dynamics (CFD) and a one dimensional (1D) mean line analyses. The main centrifugal compressor in reference to a 100 MW sCO2 closed loop Recuperated Recompression Brayton cycle is investigated. Through the use of conventional loss correlations for centrifugal compressors found in the literature, and geometrical parameters developed through a past mean line design, losses were calculated for the specified compressor impeller. The aerodynamic performance is then predicted through the 1D analysis. Furthermore, the boundary conditions for the CFD analysis were derived through the mean line analysis of the centrifugal compressor to carry out the 3D study of the sCO2 impeller blade. As the Span and Wagner equation of state has been proven to be the most accurate when working in the vicinity of the critical point, this real gas equation of state was implemented in both analyses. Consequently, a better understanding was developed on best practices for modeling a real gas sCO2 centrifugal compressor along with the limitations that currently exist when utilizing commercial CFD solvers. Furthermore, the resulting performance and aerodynamic behavior from the 1D analysis were compared with the predicted conclusions from the CFD analysis. Past literature studies on sCO2 compressor analysis methodology have been focused on small scale power cycles. This work served as the first comparison of 1D and 3D analysis methodology for large scale sCO2 centrifugal compressors. The lack of commercial CFD codes able to model phase change within sCO2 turbomachinery and the possible breach of flow properties into the saturation region at the leading edge of the impeller blade creates a limit to the operating conditions that can be simulated within these analysis tools. Further, the rapid expansion rate within this region has been predicted to cause non-equilibrium condensation leading the fluid to a metastable vapor state. Due to the complexity of two phase models, a proposed methodology to model sCO2 compressors as single phase is to represent metastable properties through the extrapolation of equilibrium properties onto the liquid domain up until the spinodal limit. This equation of state definition with metastable properties was used to model a 3D converging-diverging nozzle due to the similar flow dynamics occurring when compared to a compressor blade channel. The equation of state was implemented through a temperature and pressure dependent property table amended with metastable properties using the NIST REFPROP Database. Modeling was performed for inlet conditions with varied closeness to the fluid\u27s critical point. Investigation on the accuracy of utilizing this table to define sCO2 properties with respect to its resolution was executed. Through this, it was determined that the resulting interpolation error was highly influenced on the closeness to the critical point. Additionally, the effect on the capable modeling operating region when utilizing the metastable real gas property table through single phase modeling was examined