Cryogenics最新文献

The work discusses on the behavior of dielectric properties of various commercially available insulators with respect to temperature (4.2 K to 300 K) and operating frequency range of 2.52 KHz to 500 KHz. A conventional parallel plate-based capacitor setup was designed and developed considering various conditions. The dielectric constant was found to be very dependent on the pre-breakdown partial discharges at low temperatures. At 4.2 K the discharges move far away from the electrodes and exert high electric stress on the sample under test, which results in the breakdown or the decrease in the dielectric strength. The relative permittivity (${ϵ}_r$) also decreased rapidly with the increase in frequency in most of the samples, this decrease is due to the reduction of space charge polarization effect. The correlation between the dielectric properties, operating frequencies and temperature have been studied in detailed.

The adsorption characteristics of the selected activated carbon have an important effect on the performance of the sub-Kelvin helium sorption cooler. In this paper, a test system for the adsorption amount of the carbons based on a G-M cooler was built, which has the advantages of being detachable and high gas seal performance. The adsorption isotherm of the three carbons (5.3 K-40 K, 0–0.6 MPa) for ⁴He were experimentally tested, and the data were fitted according to the Dubinin theory. In addition, the calculation methods of sorption cooler were summarized and a model of cooling power prediction was established. Based on the above theoretical model and experimental data, a carbon with high specific surface area and excellent adsorption properties was selected as filler, and a prototype of ⁴He sorption cooler was developed. The lowest temperature and holding time of the prototype are 923 mK and 8.47 h, respectively, in good agreement with the theoretical model. The model and the experimental method can provide reference for the design of sorption coolers at sub-Kelvin temperatures.

The Multistage LNG cryogenic submersible pump is one of the crucial power equipment in LNG transport process. However, the high hydraulic losses and low efficiency of this pump make it necessary to optimize the structure of its impeller in order to reduce energy losses. In this study, an optimization strategy based on particle swarm optimization (PSO) and least squares support vector regression (LSSVR) machine is proposed to optimize the structure of impeller. The optimization of the impeller structure with the PSO-LSSVR method increased the head and efficiency of the LNG cryogenic submersible pump by 0.57% and 2.72%, respectively. Comparing the calculation results of PSO-LSSVR method with the CFD results, we found that the relative error between them don’t exceed 3%, which verified the calculation accuracy of the method. Then, an enstrophy dissipation theory is introduced to quantitatively analyze the pump energy loss. Comparing the impellers before and after optimization revealed that the maximum reduction in impeller energy loss was 18%.. The distribution and mechanism of vortex generation inside the impeller were analyzed with the Q-criterion and the relative vortex transport equation. The relative vortex transport equation revealed that the horseshoe vortex structure at the leading edge of the optimized impeller blade was suppressed, and the relative vortex stretching term and the Coriolis force term, which dominate the evolution of the vortex structure, were reduced. This study will provide related reference for the optimal design of Multistage LNG cryogenic submersible pumps.

Rare-earth (Re)Ba₂Cu₃O_7−x (ReBCO) no-insulation (NI) coil is widely concerned due to its excellent electromagnetic and thermal properties. However, the presence of the turn-to-turn shunts in NI coils leads to that complexity of numerical simulation is increased. In this paper, a modified J model is proposed and the corresponding explicit–implicit hybrid algorithm is designed to calculate NI coils. The numerical results are in good agreement with the experimental data and the circuit model. The homogenization model is also proposed to simulate the large-scale NI coils in the background magnets. The modified J model has good accuracy and fast calculation speed, which can also be used to solve electromagnetic fields of insulation coils efficiently.

The paper presents experimental data on the recovery heat flux during the boiling of superfluid helium (He II) on a flat heater located inside a U-shaped channel. The lower part of the channel is filled with monodisperse porous backfill. The paper includes information on the experimental cell, the data processing and synchronization technique, and the results of experiments conducted at different heater immersion depths and liquid temperatures. The paper presents also a mathematical description of heat transfer processes in superfluid helium for confined conditions. The calculated values are compared with experimental data. The effect of the porous backfill permeability on the recovery heat flux, including the case of a free channel is discussed. It is demonstrated that the increase in the heat flux in the channel initiates oscillations of the phase interface.

A 3D finite element mechanical-electric numerical model was constructed to analyze the mechanical-electric performance of TSSC cables under torsional and radial loads at 77 K. The results indicate that when subjected to torsional load, the tapes in the outermost region within the slot of TSSC cable initially exhibit a decreasing trend, gradually expanding towards the inner region. The magnitude of this decrease progressively diminishes in the direction from outer to inner. Conversely, under radial loading, the normalized critical current density of the tapes in the innermost region within the slot initially decreases and expands outward along the radius. The tapes in the middle region remain mostly unaltered, while the outer region follows a pattern of increasing from inside out and reaches its peak at layers 28 and 29. Moreover, under torsional loads, TSSC cables display a higher normalized critical current density in the counterclockwise direction compared to the clockwise direction. To enhance the critical current density performance of TSSC cables under torsional loads, it is suggested to appropriately reduce the pitch, decrease the width of superconducting tape, increase the number of stacked tapes, enlarge the width of the slot and the diameter of the diversion trench, as well as increase the inner diameter while decreasing the outer diameter of the helical core. The choice of material for helical cores is not significantly impactful. On the other hand, under radial loads, factors such as tape width, number of stacked tapes, slot width, diversion trench diameter, and inner and outer diameters of the helical core have similar effects. Additionally, increasing the pitch length or adding more slots can enhance the critical current density performance of TSSC cables. Selecting different materials for the helical core also contributes positively to this improvement.

To study the separation of helium isotopes by cryogenic adsorption, figuring out the adsorption characteristics of helium isotopes at low temperature is necessary. In this paper, a cryogenic adsorption measurement device has been established. It consists of a cryostat with a GM Cryocooler as cold source, a Setaram gas sorption instrument and a temperature controller. The helium-4 adsorption isotherms were studied under different temperatures and pressures (15–300 K, 0–1.5 MPa) by volumetric method. The results showed that a little helium was adsorbed by activated carbon at 300 K and 77 K. The amount of adsorbed helium increased with the temperature decreased from 20 K to 15 K. At pressures lower than 0.2 bar, the adsorption capacity increased rapidly as the pressure increased. Besides, the isosteric heats of adsorption were obtained, which was around 515 J/mol and changed little. Moreover, the cryogenic adsorption results were analyzed by Langmuir model and Toth model.

Cryogenic distillation is a significant method for producing pure oxygen and nitrogen gas. In the past, due to the high cost and difficulty of conducting cryogenic experiments, the structured packings used in cryogenic distillation are mainly developed and designed by referring to the testing results of room-temperature fluids. The real flow characteristics of cryogenic fluids on structured packings, such as flow pattern and wettability which significantly determines the distillation performance, have seldom been revealed. In this study, with the help of a three-dimensional CFD model and a visual experimental setup, the liquid nitrogen (LN₂) flow characteristics within the geometric unit of a 45° inclined packing element were revealed detailedly under different liquid loads and crimping angles. Comparison studies were also conducted to show the different film flow characteristics between water and LN₂. Additionally, optimization of the crimping angles based on 250Y structured packing was explored. Both simulation and experiment show that the flow pattern and wetting mechanism of the hydraulic test on water are different from those of LN₂ in the actual cryogenic distillation. The LN₂ has much better film flow performance than water due to its different physical properties. In addition, it is found that the crimping angle has a significant effect on the wetting area ratio. For LN₂, the crimping angle of 90° can basically result in a high enough wetting area ratio, while for water, the crimping angle as large as 120° is needed to achieve a high wetting area ratio. It shows that testing and designing the structured packing with water as the main working medium will bring apparent errors in real cryogenic distillation process.

The salt pill suspensions play a critical role in determining the performance of adiabatic demagnetization refrigerators (ADR). Kevlar suspensions are commonly used to hold the salt pill in place and to minimize the thermal leak. However, traditional solutions such as tensioning the Kevlar and fixing it on mechanical frame with the help of epoxy or running it around a pulley are quite challenging. This paper presents an innovative suspension using 3D printing technology and a polyether ether ketone (PEEK) multi-ring suspension has been designed and tested. Numerical analysis shows that a single piece of the suspension may contribute about 1.10 μW thermal leak between 4 K and 0.3 K. To test its performance, a two-stage ADR is built with Gadolinium Gallium Garnet (GGG) as the first stage and chromium potassium alum (CPA) salt pill as the second stage which is supported by the PEEK suspension. The GGG stage is mainly used to provide an initial temperature for CPA stage. Typically, starting from 1 K@4 T, a no-load lowest temperature of 66.1 mK is achieved at a demagnetization rate of 0.005 T/s in the experiments. The thermal leak of the PEEK suspension with and without 1 K heat sink has been analyzed and compared, with the former one showing a big reduction of the thermal leak. This paper confirms the feasibility of PEEK as an ADR suspension material and provides a potential 3D printing method for complex suspension designs.