Cavitation in cryogenic liquids is governed by a tighter coupling of pressure drop, phase change, and local heat transfer than cavitation in room-temperature liquids. Evaporation-induced cooling lowers the local saturation pressure, suppresses vapor growth, and shifts cavity dynamics from inertia-dominated behavior toward thermally limited regimes. This review synthesizes recent progress in governing physics, experiments and diagnostics, numerical modeling, and engineering control of cryogenic cavitation in liquid nitrogen, liquid oxygen, and liquid hydrogen. Experimental evidence from Venturi tubes, converging–diverging nozzles, orifices, valves, inducers, pumps, and single-bubble tests is compared using shared observables such as temperature depression, cavity morphology, dominant frequency, pressure pulsation, vapor fraction, and erosion aggressiveness. Representative results include condensation-front-controlled shedding in liquid‑nitrogen Venturi flow above a pressure ratio of about 2.23, transition of nozzle cavitation dynamics around 77–78 K, a nonlinear dependence of single-bubble breakup time on subcooling over 66–76.4 K, and a methane-pump cavitation cycle about five times longer at 0.146 MPa than at 0.3 MPa. Numerical approaches are assessed by equation-of-state (EOS) and real-fluid property treatment, equilibrium and non-equilibrium phase-change closure, turbulence and interface resolution, validation basis, uncertainty treatment, and applicability range, covering transport-based and homogeneous-relaxation models, compressible formulations, pure and hybrid large-eddy simulations, entropy-based diagnostics, and high-fidelity interface-resolved calculations. Engineering implications are further summarized for operating-state management, geometry-based suppression, staged pressure-drop redistribution, cavitation confinement in auxiliary components, and material-side damage tolerance. Remaining priorities include uncertainty-qualified cryogenic datasets, cross-fluid validation from liquid nitrogen to liquid oxygen and liquid hydrogen, transient cavitation during startup and shutdown, and non-equilibrium interfacial heat and mass transfer closure.
The central solenoid (CS) of a tokamak is engineered to withstand significant fluctuations in operating currents and rapid changes in magnetic fields. These capabilities are essential for initiating plasma breakdown and ensuring subsequent plasma shaping and control. A novel design has recently been introduced for the CS system of a next-generation experimental fusion device. This system consists of six stacked coils (CS3L, CS2L, CS1L, CS1U, CS2U, and CS3U), each divided into two submodules. The inner submodules, situated in the high-magnetic-field region, are made from YBCO high-temperature superconductors (HTS), while the outer submodules, located in the lower-field area, utilize Nb₃Sn. Among these, the HTS1U and HTS1L submodules are expected to endure the most demanding conditions, experiencing the highest magnetic fields as well as the greatest mechanical and thermal stresses. Each HTS module is constructed from five hex-pancakes, wound with a conductor based on the HTS CORC (Conductor on Round Core) strand concept.In the present study, quench simulations for the selected HTS conductor are performed using the THEA code by CryoSoft. The quench is initiated by a heat pulse, rectangular in both time and space. Two cases regarding the conductor's operating conditions prior to the current dump are considered: (i) operation at constant maximum current and a constant magnetic field profile (corresponding to the initial steady state), and (ii) operation under variable current and magnetic field profiles, following the standard operational scenario. It is assumed that following quench detection and a specific time delay, the operating current is dumped exponentially, with the magnetic field profile along the conductor decreasing proportionally to the current. Heat transfer between adjacent turns and pancakes, as well as heat loads from magnetization, coupling, and eddy current AC losses, are taken into account. The analysis aims to estimate the maximum hot-spot temperature during the quench.
This study investigates the magnetic levitation and guidance characteristics of Ag-doped DyBa2Cu3O7-δ superconducting bulks, synthesized by the single-direction melt growth (SDMG) technique. The levitation force, vertical stiffness and lateral guidance force are measured at varying cooling heights (CHs) of 5 mm, 25 mm and 50 mm, with a constant working height (WH) of 5 mm for lateral force measurements. DyBa2Cu3O7-δ samples are cooled using liquid nitrogen (77 K) and thermal stabilization is achieved before force data collection. Results show that levitation forces increase with CH, while the levitation force curves exhibit both attractive and repulsive behavior depending on the CH. The lateral guidance force is higher for the Ag2 sample than for the Ag1 sample and decreased as the temperature decreased. Vertical stiffness also increased with CH and decreased as the levitation gap decreased, confirming heightened guidence forces at smaller distances. The flux density distributions, including both lateral (Bx) and vertical (By) components, are found to significantly impact the levitation and guidance force performance. Specifically, configurations with concentrated Bx gradients improved levitation stability, while configurations with distributed By led to stronger guidance forces. These findings underscore the importance of optimizing the PMG design to balance Bx for levitation and By for guidance in superconducting systems.
An optocoupler is widely used in electronic devices to protect sensitive circuits from voltage surges. While its characteristics are well understood at room temperature, its behavior at cryogenic temperatures remains unexplored, especially for quantum computing applications. This paper aims to characterize and model the 4N25 optocoupler from 300 K to 30 K. Electrical measurements of the input light-emitting diode (LED) and the output phototransistor were first performed in isolation and then repeated under coupled operation. Detailed characterization was first performed at room temperature and then repeated while cooling the device. Parameters associated with each measurement were extracted using MATLAB® and subsequently implemented and simulated in LTspice. Lastly, the current transfer ratio (CTR) of the optocoupler and the relationships between each parameter and temperature were investigated. The findings reveal that both the input LED current under fixed biasing and the output phototransistor current gain decrease significantly at cryogenic temperatures. Notably, the maximum CTR at each temperature depends on the diode biasing current. This study demonstrates that the 4N25 optocoupler remains functional down to 30 K, and that the maximum differential CTR can be achieved under appropriate biasing conditions. These behaviors were characterized and incorporated into a robust LTspice modeling framework.
Polylactide – based biocomposites reinforced with natural lignocellulosic fillers are promising materials for sustainable additive manufacturing; however, their mechanical performance is often limited by insufficient polymer-filler interfacial compatibility. Although various chemical modification methods have been proposed, environmentally friendly physical approaches remain insufficiently investigated. The aim of this study was to evaluate the influence of combined cryogenic mechanical activation of polylactide and modified fire flax on the structural characteristics and mechanical properties of composites produced by fused filament fabrication. Composite powders containing 10, 20, and 30 wt% fire flax were prepared by cryogenic co-milling and characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, particle-size analysis, and tensile testing. Cryogenic mechanical activation promoted improved powder homogenization and redistribution of intermolecular interactions, as confirmed by Fourier-transform infrared spectroscopy analysis. Compared with composites prepared without cryogenic activation, tensile strength increased by 11.1%, 9.5%, and 14.8% for composites containing 10, 20, and 30 wt% fire flax, respectively. The developed composites were successfully processed into filaments and fabricated by fused filament fabrication. The obtained results demonstrate that combined cryogenic mechanical activation represents an effective and environmentally friendly processing strategy for improving the structural homogeneity and mechanical performance of biodegradable polylactide/fire flax composites intended for additive manufacturing.
The swirl nozzle is regarded as a highly efficient and environmentally friendly device for nitrogen liquefaction. The objective of this work is to investigate nitrogen flow characteristics in the swirl nozzle and to analyze the influence of non-equilibrium condensation and the swirl device on the liquefaction characteristics of nitrogen. This work innovatively designs the supersonic nozzle with the swirl device and carries out numerical simulations on nitrogen swirl condensing flow employing gas-liquid control equations, internally consistent classical nucleation theory, and Gyarmathy droplet growth model. The findings demonstrate that the swirl nozzle could effectively liquefy nitrogen under swirl flow conditions. When employing the isentropic model without taking condensation into consideration, the computed results misestimate the temperature drop characteristics of nitrogen, with the peak deviation of 48.70%. In contrast to the non-swirl flow, the presence of swirl considerably increases the peak droplet growth rate while slightly lowering the liquid mass fraction at the outlet. Thus, it is essential to keep balance between swirl intensity and liquefaction efficiency when designing the swirl nozzle for realizing its optimal operation performance. With the reduction of inlet temperature from 163 K to 161 K, the liquid mass fraction at the exit grows from 16.36% to 18.73%. This indicates dropping the inlet temperature can promote the condensation process of nitrogen inside the swirl nozzle.
Testing helium turbo-expanders is often cost-prohibitive and technically demanding due to the extreme operating environments required. To address this, we propose a cross-fluid similarity framework that employs air as a viable surrogate for helium. Using the first-stage impeller of a radial-inflow turbo-expander as the benchmark, we derive similarity criteria rooted in compressible flow physics and the congruence of inlet velocity triangles. This framework adopts the tip Mach number (Mau) and flow coefficient (φ) as the foundational scaling parameters, supported by an iterative method to determine equivalent operating points for air. Comparative numerical analyses reveal that when Mau and φ are well matched, the internal aerodynamic and thermodynamic characteristics—including temperature, pressure, and Mach number fields—show remarkable fidelity to the original helium conditions. Key flow phenomena, such as vortex evolution and irreversible entropy production, remain spatially and qualitatively consistent across both fluids. These findings provide mechanistic validation for the use of air in preliminary turbo-expander testing, offering a cost-effective and reliable pathway for the design optimization and performance characterization of cryogenic helium systems.
The 4He Joule-Thomson cryocooler is a critical component of space cryogenic systems, as it is not only able to provide a stable precooling environment for sub-Kelvin refrigerators but also can directly provide cooling power for certain optical devices. To meet the cooling demand of multi-temperature in space detection missions, this paper presents the development and testing of a 4He Joule-Thomson cryocooler precooled by a two-stage pulse tube cryocooler. The hybrid cryocooler is capable of simultaneously providing cooling capacities at 80 K, 20 K, and 4 K. A no-load temperature of 3.77 K is achieved with a total input power of 462.4 W. Experimental results demonstrate that with a total input power of 632.1 W, the three-stage cooling capacities are 4.06 W at 69.83 K, 204.69 mW at 17.54 K, and 51.8 mW at 4.08 K, respectively.
To address the problems of high energy consumption of hydrogen liquefaction process and insufficient utilization of cold energy of liquefied natural gas, a novel pressurized liquefied natural gas precooling hydrogen liquefaction system is proposed. The system was subjected to parameter optimization, exergy analysis and economic analysis, and compared with the atmospheric pressure liquefied natural gas precooling hydrogen liquefaction system. The results show: In the optimal operation, the specific energy consumption, coefficient of performance, figure of merit, and exergy efficiency of the system were 6.68 kWh/kg LH2, 0.1847, 0.6156, and 46.04%, respectively; the system exergy loss are mainly distributed in the heat exchange equipment and the turbine; the ortho-Para hydrogen conversion process was accompanied with an exothermic reaction, resulting in a small increase in the reactor outlet hydrogen temperature; compared to atmospheric liquefied natural gas precooling system, pressurized liquefied natural gas precooling system performed better and consumed less liquefied natural gas. The research indicates that pressurized liquefied natural gas precooling strategy has significant potential to reduce hydrogen liquefaction and storage and transportation costs and to improve the liquefied natural gas cold energy combined utilization efficiency
Electrification has been proposed as a route to decarbonizing air travel. Conventional electric motors are too heavy to achieve the power densities required for aviation so superconducting motors for aircraft are being developed. Superconductors however, work at cryogenic temperatures, which indispensably require a reliable and efficient cooling mechanism. The Robinson Research Institute is developing a 3 MW superconducting motor using an HTS (High Temperature Superconductor) rotor operating at 50 K. The motor is intended to run at a shaft speed of 4500–6000 rpm to directly drive a ducted fan. A key challenge is the removal of heat from a cryogenic spinning rotor to a stationary refrigerator so it can be rejected at ambient temperatures. Previous motors have utilized externally pumped or thermo-syphoned cryogens to cool the rotor, which require complex rotary seals and a heavy external cold box for the refrigerator. The aircraft application demands a compact and low weight solution. In a synchronous superconducting motor, a popular configuration is to have the rotor with direct current (DC) field coils and the stator with alternating current (AC) coils. This configuration significantly reduces cooling load on the rotor as DC superconducting coils have few losses. Most of the heat to be lifted to ambient from the rotor comes from conduction down the shaft(s), especially if the DC coils are energised with a flux pump. However, the rotor is rotating, so the heat needs to be transferred across an interface to the non-rotating (stationary) world. This work proposes using a novel concept using a stationary cryocooler's cold heat exchanger as part of a pump to circulate gaseous helium inside the rotor and to use the rotor's spinning action to perform the pumping. In this way the rotating interface for heat transfer becomes the pump. A rotating gas seal is still necessary, but this can be at near ambient temperature and pressure. Computational fluid dynamics (CFD) analysis has indicated that the system will work with acceptable losses and requires less than 20 W of cooling to keep a motor's rotor at 50 K. This paper presents the CFD modelling, results of the proof-of-concept experiments that validated the CFD model, and will present further improvements to the concept, demonstrating a feasible cooling method and its application to a superconducting rotor.
