Shaohang Yan

The violent collapse of a vapor bubble within a confined, high-shear environment commonly occurs in hydrodynamic bearing and apical leaf space, which presents a complex interplay of inertial, viscous, and thermal effects. The special thermodynamic properties of cryogenic fluids like liquid hydrogen (LH2) induce highly sensitive cavitation. The collapse dynamics of a single LH2 bubble within a shearing micro-clearance is numerically investigated by the thermodynamic-modified cavitation model. The transient, multi-scale physics are deconstructed using proper orthogonal decomposition, dynamic mode decomposition, and fast Fourier transform. The increase in shearing rate leads to a significant attenuation of the peak pressure impact generated during the violent collapse phase. This pressure reduction is quantitatively linked to the interplay of inertial, viscous, and surface tension forces, as characterized by the Reynolds and Weber numbers. The frequencies and time–frequency features of pressure and mass transfer oscillations are also shown to be strongly modulated by the shearing rate and confinement height. The analysis results reveal that high shearing rates dissipate oscillatory energy more effectively throughout the collapse–rebound period, thereby mitigating the final localized pressure pulse. This study provides a foundation for understanding and potentially controlling cryogenic cavitation dynamics in high-performance fluid systems.

The liquid-lubricated spiral-grooved thrust bearing is widely applied in rotational devices with high speed. In the bearing, hydrodynamic effect induced cavitation and axial clearance undulation interact with each other. The load capacity, friction torque, stiffness, and other bearing parameters oscillate with the periodic undulation of clearance. The oscillation of bearing performance induces instability of high rotational speed devices, which can lead to serious mechanical failure. In this investigation, the coupling effect of cavitation and sinusoidal oscillating clearance on high-speed spiral-grooved thrust bearing performance is numerically researched. The influence of vibration period, clearance amplitude, and equilibrium clearance height on bearing performance is acquired by comparison. The load capacity, friction torque, and other parameters are dealt with by fast Fourier transform, wavelet transform, and Hilbert-Huang transform. The transient, frequency, and time-frequency characteristics of bearing performances are analyzed and compared. According to the analytical results of the load capacity, the influence extent of these three vibration factors on the bearing performance is evaluated. The structure of herringbone spiral grooved thrust bearing should be optimized to acquire better load and cavitation performances under vibration conditions.

In high-speed motors, there is a huge amount of heat generation from core and winding losses, which may result in thermal failures or motor performance deterioration. In the prevention of heat accumulation, efficient cooling technology is critical for smooth and reliable motor movement. This paper summarizes the diverse application of high-speed motor and thermal requirements, such as in electrical devices, turbo-machinery, and high-precision machine tools. Three paths of case convection—cooling, internal ventilation cooling and spindle core cooling—are analyzed. Methods for configuring thermal resistance and improving cooling efficiency are summarized. Among them, coolant flow characteristics and flow channel shapes, gas supply ventilation systems, and methods to reduce air resistance, as well as axial cooling and integrated heat pipe structures, are extensively investigated. Finally, the development prospects of high-speed motor cooling are also forecasted. At present, the primary research directions are to reduce the heat generated by the heat source, utilize the latent heat of the coolant, optimize the cooling flow path of the shell, design an axial air-cooling circulation system, and enhance the heat dissipation of the spindle.

The hydrogen hydrostatic thrust bearing (HHTB) is a key component of hydrogen liquefaction that impacts turbo-expander characteristics. To analyze the feasibility of using the HHTB in this application, characteristics of HHTBs were calculated using a CFD model. To upgrade the performance of the HHTB, the impacts of bearing structure and operating parameters on static performance were investigated. Dynamic characteristics of the HHTB were studied using the dynamic grid method. It was found that the load capacity of the HHTB is less than that of helium-lubricated bearings but higher than that of air- and methane-lubricated bearings. The turbulent kinetic energy of hydrogen is higher than that of other gases. Load capacity can be enhanced through boosting supplied pressure, expanding the diameter of supply orifices, reducing gas film clearance, increasing the orifices quantity and setting a circumferential groove. A reduction in disturbance amplitude slightly increased the bearing’s dynamic stiffness. The dynamic stability of the HHTB was improved by a small film clearance in response to disturbance.

In cryogenic fluid storage and delivery, the rapid contraction and rebound of bubbles are prone to occur during bubble collapse due to the pressure saltation. With the contraction and rebound of bubbles, the pressure and temperature in the bubbles fluctuate greatly, which affects the service life of fluid machinery. During bubble contraction and rebound, there is an accompanied complex heat and mass transfer process. According to the thermal properties of cryogenic fluids, a single-bubble collapse model is proposed considering the temperature variations inside the bubble. In order to study the variation in temperature and pressure during bubble collapse in cryogenic fluids, the contraction and rebound of a single bubble in liquid hydrogen are investigated numerically under various operating pressures and supercooling degrees. The numerical results of the model indicate that there are periodic contraction and rebound of the bubble when the pressure rises suddenly. Furthermore, the periods and attenuation rates of bubbles in different media are studied and compared. For the most concerned pressure and temperature characteristics, the relationship between the peak pressure, the attenuation rate of the temperature and the dimensionless number is proposed.

The liquefaction of hydrogen is considered to be a crucial process in the large-scale utilization of hydrogen energy. In hydrogen liquefaction, hydrogen turbo-expander is a key refrigerating machine for high liquefaction efficiency. Performance of the turbo-expander is directly affected by the hydrogen gas bearings. To obtain a deep understanding of the performance characteristics of hydrogen gas bearings, the static and dynamic characteristics of herringbone grooved journal bearings under hydrogen and other lubricating gases were numerically calculated and compared. The bearing load capacity and critical mass of hydrogen gas bearings were slightly lower than those of helium-, air- and nitrogen-lubricated bearings. To improve the performance of the hydrogen gas bearings used in high-speed turbo-machinery, the influence of working conditions was analyzed. It is found that the load capacity of hydrogen gas bearings can be improved by increasing the ambient pressure, reducing the gas film clearance, and raising the bearing eccentricity ratio. Meanwhile, the critical mass increases, and the bearing dynamic stability is enhanced.

Frost usually grows on the cooling surface of heat transfer equipment, which often operates under different working pressures. In such conditions, one of the fundamental influencing factors of frosting, the diffusion coefficient of water vapor around the cooling surface, is different from that in the normal atmosphere pressure. In order to investigate the pressure effect on frosting, a test rig for frosting visualization is designed. The role of working pressure around the cooling surface on the frost crystal morphologies and frost layer characteristics is evaluated under different air parameters and temperatures of cooling surface. The vertical growth and lateral distribution of frost crystals are strongly related to working pressure around the cooling surface. More frost crystal morphologies and faster growth rate of frost layer are presented at a lower pressure. The frost thickness, frost accumulation and density reduce with higher pressure. The dimensionless correlation of frost thickness is proposed considering working pressure around the cooling surface, humid air parameters, and temperature of cooling surface.