Cryogenics最新文献

Operando magnetization remains a critical issue for practical applications of single-grain bulk high temperature superconductors (HTSs) as strong trapped field magnets (TFMs). It has been previously proved that the magnetization efficiency can be improved by exploiting flux jumps during the pulsed field magnetization (PFM). In this study, we proposed a novel PFM sequence combining a transverse magnetization followed by a vertical magnetization namely the crossed fields magnetization. It was found the final trapped field distribution greatly depended on the direction of the transverse field exhibiting chirality features. Numerical simulation revealed the redistribution of the induced current during the vertical PFM indicating more intense flux motion will be involved. As a result, the external magnetic field required to magnetize the bulk was reduced by about 10 % at 30 K and the magnetization efficiency was improved.

Density is a critical parameter in the liquefied natural gas (LNG) industry, which necessitates the study of measurement methods. Standard methods for calculating LNG density are employed in downstream LNG custody transfer, but it still faces challenges in density measurement in the upstream, such as in LNG plants and LNG refueling station. In this paper, a gravimetric density measurement method was established for LNG plants, which is SI-traceable as it relies on volume and mass. A home-made sampling device was developed and employed to obtain the LNG volume and mass. The principle involves trapping LNG in a known-volume quantitative tube within the sampling device. The LNG is then transferred to a vacuum sampling cylinder and weighed using a mass compactor. This approach not only provides representative samples but also enables compositional analysis while performing density calculations according to ISO 6578. Feasibility and practicality tests of this novel method were conducted in an LNG plant. The test results indicate that the reproducibility is 0.6 %(RSD), and the relative expanded uncertainty is 2.0 % (k = 2). Compared to the revised Klosek and McKinley method (RKM), it shows a relative difference of −0.3 % to 1.5 %, demonstrating the applicability to LNG plants.

Accurate modeling of cryogenic boiling heat transfer is vital for the development of extended-duration space missions. Such missions may require the transfer of cryogenic propellants from in-space storage depots or the cooling of nuclear reactors. Purdue University in collaboration with NASA has assembled a database of cryogenic flow boiling data points from steady-state heated-tube experiments dating back to 1959, which has been used to develop new flow boiling correlations specifically for cryogens. Computational models of several of these experiments have been constructed in the Generalized Fluid System Simulation Program (GFSSP), a network flow code developed at NASA’s Marshall Space Flight Center. The new Purdue-developed universal correlations cover the full boiling curve: onset of nucleate boiling, nucleate boiling, critical heat flux, and film boiling. These correlations have been coded into GFSSP user subroutines. The fluids modeled in this study are liquid hydrogen and liquid helium. Predictions of local wall temperature and pressure drop are presented and compared to the test data.

The prevalence of technologies that operate at cryogenic temperatures requires that the cooling systems used to maintain these systems be carefully designed. This research focuses on understanding the thermal performance of the heat path between the heat source (the technology being cooled) and the cooling source (the cryocooler). Specifically, this work characterizes through measurement the thermal properties of common heat path materials, with a focus on bulk thermal conductivity and thermal contact resistance of pressed contacts. A framework for using these measurements to optimize a bolted contact is proposed and demonstrated.

Over the past few years, liquefied natural gas (LNG) evaporated gas re-liquefaction technology has rapidly developed. This technology is particularly useful in trans-oceanic LNG trade, which has grown to be a hot sector in the world’s energy trade. And there are several limitations on board. Reducing energy consumption, improving efficiency, and lowering costs are crucial factors in the BOG re-liquefaction process. This review provides a comprehensive review of the recent advancements in the design and optimization of BOG re-liquefaction processes. It includes a detailed analysis and comparison of existing BOG re-liquefaction technologies, and specifically discusses two types of re-liquefaction processes: dynamic and steady-state. Furthermore, this review examines different perspectives and directions for improving these two re-liquefaction technologies. Finally, the challenges faced by the BOG re-liquefaction process for LNG carriers are presented, along with future directions for addressing these challenges.

This study presents the measurement of the thermal expansion of aluminum alloy 6061 at cryogenic temperatures using Helium as the cooling medium. Three distinct tests were conducted to evaluate thermal expansion: two with gradual and natural heating of the material, and a third with temperature stabilization at key points. Measurements were carried out using Fiber Bragg Grating Sensors (FBGS), which provided precise and reliable data on the material's thermal behavior. The obtained results were compared with reference curves from the National Institute of Standards and Technology (NIST), showing good agreement and validation of the employed methods. This research highlights the effectiveness of using FBGS in measuring thermal expansion under cryogenic conditions and the importance of heating procedures in obtaining accurate data.

Epoxy resin (EP) plays a crucial role in safeguarding superconducting magnets. One of the major concerns related to its usage is its inherent susceptibility to cracking under cryogenic temperatures and the strong electromagnetic forces experienced during the operation of superconducting magnets. In this study, we utilize molecular dynamics (MD)simulation and cryogenic experiments to conduct a comprehensive investigation aimed at gaining a profound understanding of the cryogenic toughening mechanism in hyperbranched polymers-toughened (HBPs) EPs. Five different crosslinking models of EP composites were established by MD simulations. The performance parameters obtained from the MD simulation calculations are highly consistent with the experimental results, which included the glass transition temperature, coefficient of thermal expansion, mechanical properties, free volume and atomic mean square displacement. Moreover, the relationship between structural changes and properties of the MD models was investigated. This research method provides a new avenue of exploration for superconducting magnet encapsulation resin materials.

Stacked structures of high-temperature superconducting (HTS) tapes are usually employed to enhance current-carrying capacity in cable or magnet applications, but their multilayer characteristics may result in local hot spots due to uneven cooling. Besides, the inconsistencies along the length of tapes introduced during manufacturing process and the significant transverse Lorentz forces experienced in magnet operations can lead to weak parts, bringing uncertainties to thermal stability. Finite-element methods are widely used to predict thermal propagations, but 3D models encounter challenges such as distorted mesh elements and convergence issues, particularly when combining electromagnetic and heat transfer modules for long-distance wires. In this work, we have developed a Dimensional Coupling Method (DCM) to assess the thermal impact of weak parts in stacked wires applying coupled 1D and 2D models. The 2D model analyzes the electromagnetic and heat characteristics of stacked surfaces, and provides an initial heat source for the 1D model, which evaluates thermal propagation longitudinally. Simulation results of the 1D module are then transferred back to update the 2D outcomes. Models of distinct dimensions are coupled sequentially in physical steps but simultaneously in the time domain. Our approach is verified by 3D model benchmarks and offers a computational cost reduction of approximately 60 % compared to the benchmarks, making it more suitable for applications with large I_op/I_c ratios. Specially, multi-layer stacked wires under low ratios cases are also been analyzed. What’s more, two influencing factors of heat propagation, the weak-part length and position, are also investigated.

Liquid hydrogen plays an important role in the large scale storage and long-distance transportation. The development of hydrogen turbo-expander is the key to increase the efficiency of hydrogen liquefaction, reducing the cost of liquid hydrogen production, storage and transportation. In this paper, a numerical model of hydrogen nozzle is established and validated against experimental data. The performance of four traditional nozzle profiles in hydrogen turbo-expanders is simulated and analyzed. The poor uniformity of outlet flow angle is generally found in these nozzle profiles, leading to nozzle passage and impeller incidence losses in turbo-expanders. A genetic algorithm based method is proposed to optimize the nozzle profiles. The optimization objectives involve the nozzle efficiency, the uniformity of the nozzle outlet angle and the uniformity of the outlet Mach number. The deviations in the outlet angle and Mach number of the optimized nozzle are reduced by 50.26 % and 14.03 %, respectively, while the nozzle efficiency reaches 98.64 %. The matching characteristics of the optimized nozzle with the impeller are obtained via simulation of a hydrogen turbo-expander, and the results indicate the expansion efficiency can be increased by 1.53 %.

In order to carry out vibration reduction work on a cryogen-free dilution refrigerator system pre-cooled by a GM cryocooler, this paper first introduces the method of vibration measurement, particularly presenting the results and verification of displacement measurement obtained through acceleration integration. Subsequently, based on the construction of our dilution refrigerator, the paper reports experimental evidence of the direct impact of vibration on temperature and provides an effective vibration reduction scheme for the system. The study finds that vibrations transmitted through the path of “low-temperature part of GM- copper braid − support structure − various cold plates” have the most significant impact, hence making good internal soft connections is key. The influence of vibrations from the room-temperature part of GM can be mitigated to some extent by suspending the cold head, either by a separate frame or using the dilution refrigerator’s own frame. The use of a structure composed of air springs and honeycomb rubber in the suspension system has proven to be effective. Our final experimental results have demonstrated that with adequate vibration damping, the impact of vibrations from the GM on temperature can be essentially eliminated, allowing the dilution refrigerator to achieve a temperature level close to that when pre-cooled by a PT. The research in this paper has certain reference value for the construction and vibration mitigation of ultra-low temperature refrigeration systems.