CFD Performance Prediction of Cryogenic High-Speed Bearings for Reusable Pump-Fed Liquid Rocket Engines

Reusable rockets are currently the most promising design solution for reducing space transportation costs by saving the efforts of maintenance and increasing the number of launches. The maintenance of turbomachinery requires large costs. Among these, the bearings of the turbopump are some of the most critical components. Several previous studies have suggested that journal bearings can replace ball bearings to extend their lifetime, and hybrid (combined hydrostatic and hydrodynamic) bearings are particularly considered one of the ideal solutions for rocket propulsion systems. In this research, we conducted a numerical analysis to contribute to the development of cryogenic hybrid bearings that enable stable and wide-ranging operations. We performed three-dimensional computational fluid dynamics (3D-CFD) simulations for the tested journal bearings, based on Reynolds-Averaged Navier-Stokes simulations with sub-models accounting for the effects of turbulence and cavitation. Static characteristics of high-speed hybrid bearings were investigated focusing on the geometry employing liquid methane of the University of Pisa. Through a series of simulations changing the shaft eccentricity conditions, we investigated the inlet and outlet pressure ratios and flow rates - important parameters representing the characteristics of journal bearings. Furthermore, internal flow in the narrow clearance inside the bearings, based on simulation results, clarified the mechanism of occurring cavitation and its effect on bearing performance. The difference in the pressure distribution and its fluid force with and without consideration of cavitation was also evaluated through the comparison of these results. We clarified the cavitation model had a large effect on the prediction accuracy. When not considering the cavitation model, we would underestimate the results in an underestimation of the shaft load and an overestimation of the flow rate. We discussed the relationship among cavitating areas, their void fraction, bearing flow characteristics, and dependence on the shape of test bearings and elucidated their contribution to the bearing characteristics.