Feasibility Study of Foil Journal Bearing for LH2 Turbopump Used in Cryogenic Engine

Abstract

The turbopump feed system is used in a cryogenic engine for higher thrust and longer burn duration. The turbopump is one of the critical components in a cryogenic engine and consists of components such as inducer, turbine, impeller, shaft, radial bearings, seals, thrust balancer, housing, etc. In the CE-20 engine of GSLV-MkIII, two separate turbopumps were used for handling LH2 and LOX with an operating speed of 38,000 rpm and 15,000 rpm respectively. The thrust load of each rotor is balanced using a thrust balancer, whereas the radial load 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 and high wear and tear. Further, the working capacity of ball bearings is limited to the DN (Diameter-RPM) index of the rotor. Therefore, an alternate bearing system with a higher working capacity is essential for developing a cryogenic engine with higher performance. The necessity of high working capacity and higher reliability of the turbopump bearings motivated the authors to predict the feasibility of an LH2 lubricated foil journal bearing operating at 1.9 X106 DN speed (mm-rpm). The foil bearings studied is a bump-type foil bearing consisting of very thin top and bump foils (120 m) attached to the bearing base. The wedging action between the smooth top foil and the bearing base is responsible for the development of aerodynamic pressure, which helps to support the radial load of the rotor. This paper presents a simulation method to analyze the aerodynamic behavior of the bump-type foil bearings. The LH2 film is modeled using Reynold’s equation. The structural and Reynold’s equations are coupled together to determine the film thickness, pressure distribution, load-carrying capacity, torque, power, flowrates. attitude angles etc. The predicted load carrying capacity for a test case rotor with a 50.8 mm diameter and rotating speed of 38,000 rpm is nearly 375N, which is quite higher than the unbalance radial load developed in a 30 kg rotor with G2.5 balancing grade. From the initial investigation of static bearing parameters, the authors believe that bump type LH2 lubricated bearing will be quite suitable for cryogenic turbopump to improve its reliability and rotor bearing performances