Modeling of Gas Foil Bearings for Supercritical CO2 Turbomachinery

by Thomas Conboy & Steve Wright
Sandia National Laboratories

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Sandia National Laboratories has been a leader in hardware development of the S-CO2 Brayton cycle, and is in possession of unique facilities for testing turbomachinery in supercritical fluid flow environments. To achieve conditions necessary for higher efficiency electrical conversion, operation of the Sandia split-flow Brayton loop will soon progress into higher temperature, larger compression ratio operation regimes, requiring greater rotation frequencies and thrust loads. During this next phase of testing, accurate thrust bearing load capacity estimates will be needed to prevent costly failures. Also, windage and heat transfer mechanisms at the thrust bearing lubrication layer will need to be better understood to reduce parasitic losses and to minimize bearing surface temperature. To address these concerns, the Sandia Compression Loop has been modified into a thrust bearing test rig. The data generated in these upcoming tests will be ideal for the basis of benchmarking a new computational model.

For the present study, a simplified, theoretical thrust bearing model has been created by evaluating the Reynolds equation along the thrust disk surface. This has allowed for an analysis of geometric and environmental parameters influencing bearing design for maximum thrust load support. Work is in progress to add more features such as temperature dependence, complex foil and support structural models, turbulent film theory, and coupling to the code Simulink for dynamic simulation of heat transfer. This will provide a useful tool to Sandia researchers during future test runs, and to turbomachinery designers seeking to optimize thrust bearing geometry and cooling mechanisms.