Numerical Investigation on Stiffness and Damping of Gas Foil Journal Bearings Supporting High-speed Rotor of Cryogenic LH2 Turbopumps

Abstract

The turbopumps are used in used in a cryogenic engine of space launch vehicle for higher thrust and longer burn duration. These turbopumps are considered as most critical components in a cryogenic engine and consists of components such as turbine, impellers, inducer, shaft, thrust balancer, radial bearings, seals, housing, etc. In the cryogenic, CE-20 engine of GSLV-MkIII, a turbopump was used for pumping LH2 with an operating speed of 38,000 rpm. The radial load of nearly 2000 N of the LH2 is taken care of using angular contact ball bearings. These ball bearings are subjected to extreme adverse temperature and unbalance load, which leads to small life. Further, the working capacity of ball bearings is limited to the DN index of the rotor. Gas foil bearings has been proved to have long life and high-speed capability, with exceptional reliability, over a wide range of temperatures and various process fluids. However, high radial stiffness and damping is often a challenge in the development of foil bearings. Current research target to predict the load carrying capacity, radial stiffness and damping of bump type foil bearing which can be used in a test case LH2 turbopump rotor with a 50.8 mm diameter and rotating at 38,000 rpm. A numerical model is developed by utilizing the finite difference method to evaluate the load carrying capacity and dynamic coefficients based on perturbation approach. The load carrying capacity was found to be close to the radial load developed in a 30 kg rotor with G2.5 balancing grade. Further, a detailed study is performed on the impact and sensitivity of various bearing parameters such as length-to-diameter ratio, eccentricity ratio, bearing number, whirl ratio, and bearing compliance on the stiffness and damping of gas foil bearing. The results reveal that the stiffness increases with the bearing number and decreases with increased bearing compliance whereas the damping shows an opposite nature. Further, the stiffness coefficients tend to increase and the damping coefficients tend to decrease corresponding to increase in speed. The authors believe the detailed investigation on the stiffness and damping will be quite useful to the bearing designers and engineers around the world.