Here, instead of using static characteristics of gas journal bearing such as the damping coefficients and the misalignment angle for stability analysis, a linear method is presented for studying the whirl stability of a rigid rotor supported on both cylindrical bearing and externally pressurized bearing. First, by using linearization method, the gas film forces can be represented as 8-coefficents, and they are calculated through perturbation method, the static performance is validated by published literature. Then, based on the Routh-Hurwitz criterion, the stability threshold is determined and graphically presented regarding several non-dimensional system parameters, which reflects the effects of both the eccentricity and supply pressure on the system’s linear stability. This effective and computationally cheap method of stability analysis is helpful for the prior design of gas bearing.

High-speed turbo-expander is the key component of large-scale cryogenic liquefaction system and the gas thrust bearing is widely applied in turbo-expander due to the introducing exceptionally low contamination to the gas flow and low viscosity. However, the load capacity of gas bearing is low and it is often difficult to obtain the load capacity because of nonlinear problem. The paper establishes a mathematic prediction model to obtain the load capacity of gas thrust bearing directly and quickly. The model is universal for hydrogen, helium, nitrogen and air working fluids. A new dimensionless variable (Φ t) is proposed and the function between the Φ t and load capacity is found. Meanwhile, an experimental study is carried out to validate the mathematic prediction model. The results from the prediction model show good agreement with the experimental data. Based on the model, the load capacity for various working fluids is the same as long as the Φ t is the same, which can be used to conduct experimental research on the replacement of potentially flammable such as hydrogen by inert gas.

With the continuous development of low-temperature turbo-expanders, the rotation speed of the rotor is getting faster. The diameter of the impeller has become one of the factors that limit the rotation speed. In this article, a new idea to improve braking capability and reduce the diameter of the brake impeller is proposed. The brake impeller of a small low-temperature air turbo-expander test bench is used as a prototype, influence of the combination of circumferential bending and sweeping at the trailing edge of the brake impeller on the braking ability is studied. It is found that the braking ability is significantly improved. To simplify the description, the combination of a forward bending of 10° and a sweeping of 20° is called combination 1. For the current brake impeller, combination 1 is the most suitable. Compared with the original impeller, the braking power of combination 1 has increased by 99.6 W, an increase of 10.4 %. Forward bending and forward sweep substantially increase the wrap angle of the brake impeller, thereby increasing the air outlet speed and improving the braking ability, but at the expense of the flow stability of the brake impeller.

The high-speed hydrogen turbo-expander, serves as a key component of the large-scale hydrogen liquefaction system, receives a lot of attention. Being an essential part of the turbo-expander, gas bearings are widely used for its pollution-free and low viscosity advantages. However, studies on characteristics of hydrogen bearing, especially on the static and dynamic characteristics of externally pressurized hydrogen journal bearing, have not been conducted by many. Here, the finite difference method (FDM) was employed to solve the Reynolds equation thus obtaining the pressure distribution and the static load capacity. The partial derivative method (PDM) was used to calculate the dynamic stiffness coefficients and damping coefficients of hydrogen bearing. The critical mass was used to analyze the linear instability of the hydrogen bearing-rotor system. Relationship between various design parameters, including eccentricities, orifice diameter, clearance, supply pressure and rotor speeds, and static and dynamic behaviours of hydrogen bearing has been established. The results indicate that load capacity increase and volume flow rate decrease is viable through parameter optimization. The stability of the hydrogen bearing-rotor system is evaluated quantitatively.