# Feasibility Analysis of Adopting the Hydrogen Hydrostatic Thrust Bearing
## 1 Introduction  
Hydrogen is non-toxic and non-polluting, and has a low greenhouse effect, which is considered green energy [1; 2; 3; 4]. Hydrogen energy is mainly utilized in the following applications: hydrogen internal combustion engines, fuel cells, nuclear fusion materials and solid oxide batteries. More and more attention has been paid to related technologies [5; 6; 7; 8]. Currently, hydrogen technology is primarily focused on producing and storing hydrogen. The most economical method of storing and transporting hydrogen over long distances is currently in the form of liquid hydrogen, which requires a hydrogen liquefaction process [9; 10; 11; 12]. In large hydrogen liquefaction systems, high-speed hydrogen turbo-machinery plays a critical role. The main equipment that has a cooling effect during the hydrogen liquefaction process is the hydrogen turbo-expander. Turbo-expander characteristics have a significant impact on the liquefaction rate and its economic efficiency [13; 14; 15; 16]. As a result of its low viscosity and non-contamination, the gas bearing has been extensively adopted in turbo-expanders [17; 18; 19]. Based on the different directions of support force, gas bearings can be classified as journal bearings and thrust bearings. The application of gas bearings in a turbine system is presented in Figure 1.  
Based on the mechanism of bearing force generation, hydrodynamic bearings and hydrostatic bearings are the two main forms of application of gas bearings [20; 21; 22]. For hydrostatic bearings, bearing load capacity is achieved by means of an external gas supply. Generally speaking, the hydrostatic bearing has better reliability, and its bearing capacity can be artificially adjusted to fit operating requirements [23; 24; 25; 26]. In hydrogen turbo-expanders, due to their low density, large flow rate and enthalpy difference, the axial thrust bearing capacity may not be sufficient, which may lead to mechanical failure. Therefore, research regarding thrust bearing is particularly important. [PERSON] et al. carried out an extensive analysis of the impacts of small orifices on hydrostatic thrust bearing [27]. [PERSON] et al. verified the above research results. In addition, they conducted experimental and numerical analyses regarding the relationship between surface roughness and bearing performance. It was found that bearings with a gas orifice had better stiffness and damping coefficients compared to composite throttling [28]. [PERSON] et al. numerically evaluated the impact of orifice structure on bearing performance under different operational conditions using a computational fluid dynamics (CFD) model. Their results showed that bearing performance was influenced by the form of the inner cavity. As film clearance decreased, some parameters, such as pressure drop, gas vortex and turbulence intensity, decreased. The bearing load capacity decreased slightly as the spindle speed increased [29]. [PERSON] et al. conducted experiments on and simulations of mass flow characteristics in hydrostatic bearings. It was found that characteristics of the nozzle strongly influenced gas mass flow characteristics at the orifice [30]. [PERSON] et al. studied bearing characteristics using a throttle hole based on a three-dimensional model [31]. [PERSON] et al. manufactured a bearing compensated by a porous material restricttor to enhance bearing performance; composite restricttors for optimal bearing parameters were provided [32]. [PERSON] et al. put forward a bearing with a flexible equalizing pressure groove according to the principle of fluid-structure coupling. Its stiffness was improved by 59%, which was validated by numerical calculation [33]. [PERSON] et al. analyzed the impacts of speed and film clearance on pressure distribution and spindle static stiffness based on CFD models. It was found that pressure was significantly influenced by the film's thickness.