Cryogenics最新文献

High temperature superconducting (HTS) bulks have strong flux pinning capabilities and are widely used in various fields. Their self-stabilizing characteristics also provide new ideas for ultra-high-speed rail transit. For HTS maglev systems, operational stability, curve negotiation and safety when subjected to external forces are very important. Due to the hysteresis effect of superconducting bulks, they do not always return to their initial positions after deviating from the levitated position in an alternative external magnetic field. In some cases, the levitation system can be destroyed. Studies have shown that preloading can enhance quasi-static levitation performance. Therefore, this paper conducts a detailed analysis of the quasi-static levitation and guidance forces of HTS bulks above a Halbach permanent magnet guideway (PMG) under conditions with and without preloading. Additionally, the dynamic responses of the HTS bulks under lateral or vertical pulsed excitations are studied, with a particular focus on the final equilibrium position offset after disturbance. The results indicate that preloading can suppress the attenuation of the levitation force, enhance the guidance performance, and raise stiffness in both lateral and vertical directions. It also effectively suppresses position deviation from disturbance and increases the maximum excitation force threshold for system instability. This study provides practical insights for HTS maglev applications.

The burgeoning demand for large-scale energy storage has catalyzed the advancement of Liquid Air Energy Storage (LAES) technology. However, the liquid-phase propane cold storage process in LAES systems encounters a significant challenge: the dissolution of nitrogen protection gas in propane poses a substantial risk to both operational safety and performance. Given the dearth of data on the diffusive solubility of nitrogen in propane under constant atmospheric pressure and low-temperature conditions, this study constructed a testbench and conducted experiments within a temperature range of 96–227 K at a pressure of 0.1 MPa. The molar diffusive solubilities at 227 K, 176 K, 139 K, and 96 K are 0.030 %, 0.177 %, 0.297 %, and 0.962 %, respectively. Besides, this study also fits a calculation equation for the diffusive solubility of nitrogen in propane, which applies to the propane cold storage process in LAES systems.

Venturi tube can be used to measure the flow rate of stable single-phase fluid, which plays an important role in chemical industry, energy, aerospace and other fields. Due to the complex physical properties of cryogenic fluids, it is of great significance to study the cavitation characteristics of cryogenic fluids for practical engineering. In this paper, the modified Zwart cavitation model is used to study the evolution characteristics of cryogenic cavitation in Venturi tube and its relationship with turbulent kinetic energy under different pressure ratios by using dimensionless number P_r instead of cavitation number. The P_r value affects the development of cavitation to a large extent. When P_r = 1.3, cavitation is in a stable development mode. When P_r = 2.3, the development mode of cavitation changes from steady state to dynamic state. The temporal and spatial correlation between cavitation and vortex structure is studied by Q-criterion, and the geometric similarity between cavitation cloud and vortex structure in the development process is analyzed. The entropy production caused by velocity gradient change, turbulent dissipation and wall shear stress is further analyzed by entropy diagnosis method. The results show that the change of P_r value plays a leading role in the distribution of entropy production, and the generation and collapse of cavitation in the evolution process also have a great influence on the distribution of entropy production.

Metal-organic deposition using trifluoroacetates (TFA-MOD) is known to yield uniform superconducting wires by a liquid growth mode. However, it has been difficult to prepare thick films because of drying stress during the calcining process. To avoid the drying stress, conventional crack-preventing chemicals such as H(CH₂)₈COOH are applied in conventional metal–organic deposition. However, large amounts of hydrogen atoms react with fluorine atoms during calcining process in TFA-MOD, and the consequent increased harmful carbon residue decreases superconductivity of the resulting films. To avoid the chemical reaction, new crack-preventing chemicals such as H(CF₂)₈COOH were applied to prepare single-coated thick films. A low ratio of hydrogen atoms decreases the chemical reaction and generates hydrogen fluorine gas, consequently suppressing the carbon residue. Above the calcining temperature, the crack-preventing chemical is decomposed into low-boiling-point chemicals such as CF₂CF₂ or CF₃CF₃. Consequently, single-coated thick film having low carbon residue and sufficient superconducting current per width was realized. For a long time, the authors have studied other possible candidate crack-preventing chemicals. Newly introduced fluorine ion measurement of decomposed materials during the calcining process revealed the nature of the crack-preventing chemicals. Based on the accumulated results, we have concluded that among over one million chemicals there are only two groups suitable for preparing single-coated thick superconducting films by TFA-MOD. One group is hydrogenated perfluoro-carboxylic acids such as H(CF₂)₈COOH and the other group is perfluoro di-carboxylic acids. With H(CF₂)₈COOH, using a single-coating process we were able to achieve a 560 nm-thick YBa₂Cu₃O_6.93 film having J_c of 4.70 MA/cm² (77 K,0T). Compared with a standard 150 nm-thick YBa₂Cu₃O_6.93 film having J_c of 7.70 MA/cm² (77 K,0T), the critical current per width is improved to about 227 %.

A dry-shipper is a dewar used to transport frozen biomedical samples at cryogenic temperature. The inside of the dewar is lined with a porous material that absorbs and prevents the spillage of liquid nitrogen during transportation. In these porous materials vapor might be trapped during filling of the dry-shipper leading to a lower transport and storage time. The conditions under which the vapor is formed and the relationship with the porous material properties is not well understood. We studied the impact of the pore size distribution on the vapor retention in the porous materials by comparing liquid nitrogen absorption in aluminosilicate material with relatively large pores (1-100 μm) and calciumsilicate with small pores (∼0.45 μm). Both samples were immersed into saturated liquid nitrogen and a comparison of the absorbed liquid volume fraction with the porosity showed the calciumsilicate sample was completely filled with liquid, whereas the aluminosilicate contained a vapor fraction of about twenty percent. As a further investigation, we studied the absorption characteristics in subcooled liquid nitrogen. In this case, both materials absorbed liquid equivalent to their respective void fraction indicating no vapor pockets in the material. From these results, we propose a design property window for potential new porous materials for use in the dry-shippers.

An experimental setup incorporating a tank with a non-insulated common bulkhead inside and a zero-boil-off system outside was designed and established to investigate the transient thermodynamic characteristics of bipropellant liquid oxygen and methane during liquefaction, filling, and zero-boil-off storage processes. The results demonstrate the advantage of the non-insulated common bulkhead tank in storing liquid oxygen and liquid methane. The non-insulated common bulkhead provided an additional heat transfer pathway, allowing the cold energy released by liquid oxygen to be used to improve the cooling rate of methane and stabilize its storage state during the liquefaction and filling processes. Furthermore, the influence of the operational modes of the zero-boil-off system on the storage states of the fluids was examined. The cooling power delivered by the zero-boil-off system was identified as the most critical factor affecting the storage state of the fluids in the common bulkhead tank. In the absence of cooling power, the temperature rising rates in the liquid oxygen and liquid methane compartments were 4.3 K/h and 4.4 K/h, respectively. Compared with the results during the helium circulator standby stage, the pressure difference between the two compartments increased by 62.73 % in the absence of cooling power. The findings indicate the necessity of maintaining adequate cooling power to achieve stable zero boil-off storage, providing guidelines for the design and operation of practical liquid oxygen-liquid methane common bulkhead storage tanks with zero boil-off requirements.

A diagnostic method for screw compressor faults using vibration signals was introduced and applied to a new type of helium oil-injected screw compressor in the 80L/h helium liquefier. The study revealed that the destruction of the oil film due to impurity particles could lead to rotor rubbing and poor meshing, compromising the reliability of these compressors. Based on frequency-domain analysis, the major vibration occurred at the meshing frequency of screw rotors and a series of its harmonics. The vibration of the characteristic frequency of bearing parts also indicated that the bearing had slight wear in the early stage. The vibration waveform’s time domain diagram exhibited clear clipping, and the axis trajectory shew disorder, which were common characteristic of rotor rubbing. When compressing helium with low molecular weight, the meshing clearance, tooth tip clearance, and end clearance of the rotor were significantly lower than those of traditional refrigerants or air media. Additionally, the thermal gap caused by thermoelastic deformation was smaller due to the large adiabatic index of helium. Therefore, rubbing faults between rotors and between rotors and casing were more likely to occur. The article recommended establishing a vibration signal monitoring system, optimizing the design of helium screw rotor profiles, and setting up reliability standards for screw compressor operation, such as limiting the vibration speed to ≤ 8 mm/s. Additionally, attention should be given to cleaning gas pipelines in cryogenic engineering and monitoring compressor vibration and noise signals during operation to prevent rotor damage due to particle impurities.