Numerical Evaluation of Alford Forces Acting on an Axial Expander for Supercritical CO2 Application

Nowadays, supercritical carbon dioxide (S-CO2) cycles are of great interest in the scientific research especially considering the energy transition that is occurring. The S-CO2 high density and relatively low viscosity make it an interesting fluid for power generation. For large heat sources, large flowrates of fluid can be obtained. Therefore, the development of axial flow expanders can allow large power generations. In the presence of rotor eccentricities, the aerodynamic loading of free-standing blades is not constant tangentially and will promote the lateral vibration of the rotor. The dynamic phenomenon that arises is known as Thomas-Alford force. The Thomas-Alford force determines an increase of the vibration level of the machine and a higher risk of instabilities. In this paper, a preliminary investigation of a S-CO2 axial expander stage is performed. Different correlations proposed in the literature are adopted to estimate the magnitude of the Thomas-Alford force. A mono-dimensional code and a simplified computational fluid dynamics (CFD) model are adopted to obtain the parameters of the stage considered. In this preliminary investigation, only free-standing blades are considered. The results obtained show a good agreement between 1D and CFD inputs required by the different correlation used. Despite this, the cross coupled stiffness calculated are widely dependent on the correlation used; then, this study can be considered as the starting point for more detailed investigations validating the correlations behavior in this environment through an unsteady CFD and/or a proper test campaign.