Cryogenic Liquid Research Articles

Large amplitude vibration of annular and circular functionally graded composite plates under cooling thermal shocks

In this paper, large amplitude thermally induced vibration (TIV) of annular and circular plates is analyzed under cooling shocks. A novel comparison of the TIV response is presented under rapid cooling and heating shock loads. This is the first time that TIV study has been presented for any flexural structure under thermal cooling. Structures can experience cooling shock loads in many circumstances, for example when exposed to spills of cryogenic liquids. The functionally graded composite plates are made of 304 grade stainless steel (SUS304) and low-carbon steel (AISI1020) and are graded through the plate thickness to have a continuously varying spatial composition profile in thickness direction. Thermo-mechanical properties are assumed to be temperature dependent. The plates are modeled using first-order shear deformation theory with von Karman geometric nonlinearity. The Generalized Differential Quadrature Method (GDQM) and Newton–Raphson technique are employed to solve the governing equations. A Fourier heat equation is used to evaluate temperature distribution over time using GDQM and Crank–Nicolson approaches. The validation study is conducted based on the literature and FEM software ABAQUS. Detailed parametric study is presented focusing on the effect of thermal shock magnitude, rapidity of the thermal shock application, plate geometry, material distribution, geometrical nonlinearity and temperature dependency of material properties. Differences in the response due to these factors have been shown between cooling and heating thermal loads. Results show that importance of the inertia effect is substantially higher under sudden cooling than it is under sudden heating, for the same thermal load magnitude and rapidity of thermal shock. Moreover, it is shown that stresses induced by thermal cooling can potentially reach yield strength of the material with relatively large amplitudes of fluctuations. Therefore, to estimate realistically the structural behavior of thin/slender plates under rapid cooling, the inertia effect should be taken into the account and TIV analysis needs to be performed.

Cavitation in cryogenic fluids: A critical research review

Cavitation occurs as the fluid pressure is lower than the vapor pressure at a local thermodynamic state and may result in huge damage to the hydraulic machinery. Cavitation in cryogenic liquids is widely present in contemporary science, and the characteristics of cryogenic cavitation are quite different from those of water due to thermal effects and strong variations in fluid properties. The present paper reviews recent progress made toward performing experimental measurements and developing modeling strategies to thoroughly investigate cryogenic cavitation. The thermodynamic properties of cryogenic fluids are first analyzed, and different scaling laws for thermal effects estimation are then introduced. As far as cryogenic cavitation experimental research is concerned, the progress made in the cavitation visualization and cavity dynamics and the synchronous measurements of the multi-physical field are mainly introduced. As for the study on numerical simulation of cryogenic cavitation, the commonly used cavitation models and turbulence models are, respectively, classified and presented, and the modifications and improvements of the cavitation model and turbulence model for thermal effect modeling of cryogenic cavitation are examined. Then, several advances of critical issues in cryogenic fluid cavitation research are reviewed, including the influences of thermal effects, unsteady shedding mechanisms, cavitation–vortex interactions, and cavitation-induced vibration/noise. This review offers a clear vision of the state-of-the-art from both experimental and numerical modeling viewpoints, highlights the critical study developments and identifies the research gaps in the literature, and gives an outlook for further research on cryogenic cavitation.

Microscopic mechanisms behind nucleate boiling heat transfer enhancement on large-aspect-ratio concave nanostructured surfaces for two-phase thermal management

• Aspect-ratio effects of nano-cavities on nanoscale boiling are quantitatively illustrated. • The incipient nucleation time and temperature are significantly enhanced for deeper cavities. • An optimal cavity depth for maximal nucleate boiling enhancement is obtained. • Thermal energy accumulation efficiency of liquid at the initial nucleation site are elaborated. • Microscopic mechanisms are elucidated thoroughly by molecular dynamics simulations. Enhancing boiling heat transfer and elaborating underlying mechanisms are of great significance to assist in improving the efficiency of energy systems and solving thermal management bottlenecks in advanced electronics. Numerous experimental studies have proved nanostructured surfaces enable boiling heat transfer to be enhanced strikingly. Nevertheless, it is a great challenge to perform more in-depth research due to the limitation of the observation accuracy of conventional experimental approaches. To optimize designs of nanostructured boiling surfaces and fully reveal the microscopic mechanisms, the aspect-ratio effects of rectangular nano-cavities on nucleate boiling heat transfer are investigated systematically by molecular dynamics simulations in this study. The nano-cavities have the same width of 3 nm and different depths of 2 nm to 8 nm, corresponding to aspect ratios from 2:3 to 8:3. The results illustrate that, compared with the plain surface, concave nanostructured surfaces can cause a significant heat accumulation effect, resulting in remarkable reinforcements in absorbed heat flux and bubble nucleation. The nucleate boiling heat transfer enhancements originating from large-aspect-ratio nano-cavities are fully verified by the notable reductions in the incipient nucleation time and boiling initiation temperature. Attractively, for cavities with the same width, an optimal depth for maximal nucleate boiling enhancement can be obtained, which is 7 nm in the present study. In this case, the incipient nucleation time is about eight times shorter than that for the plain substrate and the decrease in boiling initiation temperature is up to 53 K. Moreover, a novel and straightforward insight into the microscopic enhanced mechanism has been proposed. It is elucidated thoroughly from the perspective of the thermal energy accumulation of liquid at the initial nucleation site by taking into account the heat absorbed from the solid wall and the heat transferred to the adjacent cryogenic liquid comprehensively. These analysis and simulation results are in great agreement. This study provides significant guidance for state-of-the-art two-phase thermal management applications.

Thermal destratification of cryogenic liquid storage tanks by continuous bubbling of gases

This paper focuses on thermal destratification and pressurisation inside thermally stratified storage tanks by continuous gas bubbling. The primary purpose of doing these studies is to better understand the effect of bubble dynamics on thermal destratification and quantify the extent of destratification. The volume of fluid and interface compression method of OpenFOAM CFD code is utilised for the present analysis. Different values of inlet gas velocities (Vg), orifice diameters (do), and arrangement of the orifices in triangular and square fashion with different pitches (p/do) are considered. In addition, the effect of gravitational forces (g/ge) on thermal destratification is also reported. For all these cases, the effectiveness of thermal destratification is quantified in terms of a newly defined parameter, the destratification index (Id). For Vg = 1 m/s, the Id value is maximum compared to lower Vg values. It is seen that when the gas velocity increased from 0.3 m/s to 1.0 m/s, the average effectiveness in thermal destratification (Idavg) and pressure at the ullage increased by 44.38% and by 64.81%, respectively. The Idavg and pressure at ullage increased by 96.29% and 14.91%, respectively, when the g/ge ratio changed from 0.3 to 3. Compared to the triangular arrangement with p/do = 10, the calculated Idavg increased by 30.67% when gas inlets were arranged with a square pitch of 10. For p/do = 4, 6 and 8, the increments in Idavg are of the order of 12.86%, 19.43% and 21.92%, respectively, for gas inlets arranged in a square fashion as compared to the triangular arrangement. It is found that continuous bubbling with gas inlets arranged in square pitch p/do = 10 gives higher effectiveness in thermal destratification. Thus, by these studies, one can develop a thermal destratification mechanism with continuous bubbling for optimum performance. Also, these studies give an overall idea of sparger design for getting the correct gas flow rate for thermal destratification within the cryogenic liquid storage tanks.

An Extensive Review of Liquid Hydrogen in Transportation with Focus on the Maritime Sector

The European Green Deal aims to transform the EU into a modern, resource-efficient, and competitive economy. The REPowerEU plan launched in May 2022 as part of the Green Deal reveals the willingness of several countries to become energy independent and tackle the climate crisis. Therefore, the decarbonization of different sectors such as maritime shipping is crucial and may be achieved through sustainable energy. Hydrogen is potentially clean and renewable and might be chosen as fuel to power ships and boats. Hydrogen technologies (e.g., fuel cells for propulsion) have already been implemented on board ships in the last 20 years, mainly during demonstration projects. Pressurized tanks filled with gaseous hydrogen were installed on most of these vessels. However, this type of storage would require enormous volumes for large long-range ships with high energy demands. One of the best options is to store this fuel in the cryogenic liquid phase. This paper initially introduces the hydrogen color codes and the carbon footprints of the different production techniques to effectively estimate the environmental impact when employing hydrogen technologies in any application. Afterward, a review of the implementation of liquid hydrogen (LH2) in the transportation sector including aerospace and aviation industries, automotive, and railways is provided. Then, the focus is placed on the maritime sector. The aim is to highlight the challenges for the adoption of LH2 technologies on board ships. Different aspects were investigated in this study, from LH2 bunkering, onboard utilization, regulations, codes and standards, and safety. Finally, this study offers a broad overview of the bottlenecks that might hamper the adoption of LH2 technologies in the maritime sector and discusses potential solutions.

Toward the Simulation of Flashing Cryogenic Liquids by a Fully Compressible Volume of Fluid Solver

We present a fully compressible single-fluid volume of fluid (VOF) solver with phase change for high-speed flows, where the atomization of the liquid can occur either by the aerodynamics or by the effect of the local pressure. The VOF approximation among a non-miscible phase (non-condensable gas) and a mixture of two fluids (liquid and vapor) represents the liquid core of the jet and its atomization. A barotropic model is used in combination with the equation of state (EoS) to link the mixture density to pressure and temperature. The solver is written with the aim to simulate high-pressure injection in gas–liquid systems, where the pressure of the liquid is great enough to cause significant compression of the surrounding gas. Being designed in an C++ object-oriented fashion, the solver is able to support any kind of EoS; the aim is to apply it to the simulation of the injection of liquid propellant in rocket engines. The present work includes the base development; a verification assessment of the code is provided by the solution of a set of numerical experiments to prove the boundedness, convergence and accuracy of the method. Experimental measurements of a cavitating microscopic in-nozzle flow, available in the literature, are finally used for a first validation with phase change.

Combined cooling, heating, power and oxygen for hospital buildings employing photovoltaic power and liquefied methane

To enhance the energy and gas supply security and reduce the greenhouse gas emission simultaneously, this paper presents a new cryogen-based co-production concept of combined cooling, heating, power and oxygen (CCHPO) for hospital buildings. By integrating with local photovoltaic power generation, two cryogenic liquids of liquefied methane and liquefied oxygen are used to store and produce multiple energies and medical gas. Detailed system modeling and performance analysis are carried out regarding the actual energy and gas consumption data from hospital buildings. The results obtained show the proposed CCHPO solution can be expected to fulfill the simultaneous requirements of energy conservation during normal operation and sustainable energy and gas supply during emergency operations.

Self-heating of cryogenic high electron-mobility transistor amplifiers and the limits of microwave noise performance

The fundamental limits of the microwave noise performance of high electron-mobility transistors (HEMTs) are of scientific and practical interest for applications in radio astronomy and quantum computing. Self-heating at cryogenic temperatures has been reported to be a limiting mechanism for the noise, but cryogenic cooling strategies to mitigate it, for instance, using liquid cryogens, have not been evaluated. Here, we report microwave noise measurements of a packaged two-stage amplifier with GaAs metamorphic HEMTs immersed in normal and superfluid 4He baths and in vacuum from 1.6 to 80 K. We find that these liquid cryogens are unable to mitigate the thermal noise associated with self-heating. Considering this finding, we examine the implications for the lower bounds of cryogenic noise performance in HEMTs. Our analysis supports the general design principle for cryogenic HEMTs of maximizing gain at the lowest possible power.

Экспериментальные исследования проливов криогенных жидкостей (топлив)

Experimental studies were carried out, the analysis of the processes of spillage, spreading and evaporation of the cryogenic liquids (fuels) on a solid surface were carried out. It is shown that the intensity of evaporation of the cryogenic liquids during spills is influenced by many factors: the type of surface on which the spill occurred (concrete, steel, soil, sand, etc.), its penetrating ability, temperature, thickness of the receiving layer, humidity, atmospheric mobility, and, etc. The evaporation rate is determined by the total heat flow coming to the liquid from the external sources. In this case, the heat flow coming from the underlying surface is dominant. Studies of the evaporation of cryogenic liquids during spills on various surfaces showed that the evaporation process has a non-stationary character. This is especially evident at the initial stage of evaporation when its intensity decreases 5֪–6 times in the first 1.5 minutes. After 6–8 minutes, the process goes into a quasi-stationary mode, and the evaporation rate decreases slowly. In case of spills on the solid surfaces, the evaporation process is satisfactorily described (depending on the nature of the surface) by the calculated ratios obtained. Seepage of liquid deep into the sand increases the intensity of evaporation by 2–3 times. Presence of moisture in the ground, on the one hand, reduces the seepage of liquid due to its freezing, and on the other hand, additional heat is released during freezing to vaporize the liquid. Presence of moisture in the ground can increase the evaporation rate by up to 50 % at a non-stationary evaporation site. Based on two scenarios of cryogenic liquids spilling (instantaneous or prolonged expiration) onto a limited or unlimited surface, physicomathematical models of their spreading during leakage were developed. These models, in turn, are implemented as methods for assessing the hazardous factors of fires under various scenarios of their occurrence and explosions on the territory of the facility and in the residential area near the facility.

Development of a controlled-atmosphere, rapid-cooling cryogenic chamber for tribological and mechanical testing.

Mechanical testing of seals, bearing materials, and mechanisms in cryogenic environments is a rapidly growing field of research, as it promises improvements in equipment performance and reliability for applications such as space exploration, liquid hydrocarbon storage, and superconducting devices. Cooling of test equipment is usually performed within a well-insulated test chamber, via direct or indirect evaporation of liquid cryogen. State-of-the-art equipment is frequently insufficient for rigorous testing, being expensive and cumbersome, cooling slowly, struggling to replicate relevant environmental conditions, and/or failing to reach the temperature of the cryogen. Herein, we employ a rapid prototyping approach using polymer 3D printing to iteratively refine cryogen-based cooling of a tribometer. The final design greatly exceeds the minimum temperature of state-of-the-art equipment, cooling a chamber to liquid nitrogen temperatures (-196 °C) while maintaining dry test conditions. When modified for use on a cryogenic tensile tester, the design cools to -150 °C in 149s, significantly improving upon state-of-the-art performance. By utilizing this 3D-printed equipment, we find that components produced via Fused Deposition Modeling with unmodified, commodity polylactic acid have favorable mechanical properties in a cryogenic environment: tensile strength of 110 MPa, elongation at break of 10%, and specific wear of 5.6 × 10-5mm3/Nm against stainless steel. By leveraging 3D printing for rapid manufacture of production-quality parts, highly refined cooling chamber designs have been experimentally developed for both a tribometer and a load frame in rapid succession, enabling significant improvements in cryogenic test capabilities.

Mechanistic insight into impact of clay on compressive elasticity of poly(N-alkyl acrylamide)/Laponite-based nanocomposite cryo-beads

Relationship between formation pathway and Hertzian elasticity of poly(N-isopropylacrylamide-co‑sodium acrylate)/Laponite® XLG, P(NIPA-NaA)/LAP, based nanocomposite cryo-beads was evaluated. Millimeter-sized nanocomposite cryo-beads were prepared by combination of dropwise freezing into cryogenic liquid (DFLC) method with sedimentation cryogelation method. pH-sensitive 3D-templete was formed by conducting free-radical crosslinking cryopolymerization of NIPA and NaA in the presence of synthetic hectorite Laponite® XLG as inorganic crosslinker and N,N′-methylenebis(acrylamide) as chemical crosslinker. The effects of network template, ionic comonomer, inorganic clay incorporation, pH- and ionic strength of swelling medium on the swelling and elasticity were evaluated as a function of bead size. The deformability of swollen cryo-beads was investigated using Hertzian elasticity. The crosslink density and Hertz modulus of nanocomposite beads vary with the droplet size and the polymer spheres with low crosslink density tend to absorb more water. In DFLC method, the desired sphere size and swelling can be adjusted by selecting the needle size used for dripping. The swelling degree increased significantly as the size of nanocomposite cryo-beads increased. At a fixed swelling ratio, the cylindrical nanocomposite cryogels exhibited greater elastic moduli and crosslink density than that of spherical gels. Correlation of elastic modulus of cryo-beads with swelling and cross-linked polymer concentration predicted a higher scaling exponent than the classical estimate for swelling in good solvents. It was investigated how the presence of 30 mol% pH-sensitive anionic comonomer NaA in the structure of all nanocomposite gels affects their pH-dependent swelling behavior. At pH < 5.7, the gels exhibited a lower swelling ratio, whereas at pH > 6.6 they tended to swell and followed second-order swelling kinetics by Fickian diffusion. When pH of swelling medium was alternately changed, all gels exhibited a pH-switch and swelling rate was two times higher than shrinkage rate. By investigating the effect of KCl, NaCl and K2CO3 on the swelling of cryobeads, it was determined how the salt cation changes the electrostatic repulsion between network charges. Nanocomposite cryo-beads showed fast pH-responsiveness and immediately reabsorbed the water they lost due to pH change, therefore, they can be designed as drug release carriers by pH-switching modulation.

Densified HKUST-1 Monoliths as a Route to High Volumetric and Gravimetric Hydrogen Storage Capacity.

We are currently witnessing the dawn of hydrogen (H2) economy, where H2 will soon become a primary fuel for heating, transportation, and long-distance and long-term energy storage. Among diverse possibilities, H2 can be stored as a pressurized gas, a cryogenic liquid, or a solid fuel via adsorption onto porous materials. Metal–organic frameworks (MOFs) have emerged as adsorbent materials with the highest theoretical H2 storage densities on both a volumetric and gravimetric basis. However, a critical bottleneck for the use of H2 as a transportation fuel has been the lack of densification methods capable of shaping MOFs into practical formulations while maintaining their adsorptive performance. Here, we report a high-throughput screening and deep analysis of a database of MOFs to find optimal materials, followed by the synthesis, characterization, and performance evaluation of an optimal monolithic MOF (monoMOF) for H2 storage. After densification, this monoMOF stores 46 g L–1 H2 at 50 bar and 77 K and delivers 41 and 42 g L–1 H2 at operating pressures of 25 and 50 bar, respectively, when deployed in a combined temperature–pressure (25–50 bar/77 K → 5 bar/160 K) swing gas delivery system. This performance represents up to an 80% reduction in the operating pressure requirements for delivering H2 gas when compared with benchmark materials and an 83% reduction compared to compressed H2 gas. Our findings represent a substantial step forward in the application of high-density materials for volumetric H2 storage applications.

Comparative study of three insulation materials installed on type C independent tank for offshore LNG transportation

The traditional choice of insulation material for offshore LNG transportation with type C independent tank was polyurethane foam (PUF). However, aiming to further cut down the insulation cost and spare additional arrangement space, the requirement of searching for the alternative materials still exists. Polyisocyanurate board (PIRB) and silica aerogel blanket (SAB) are chosen as candidates in this study since these two materials have been proved to be available in other fields. Despites their advantages, the seams and joints due to the specific installation techniques for PIRB and SAB could be the causes of potential defects when transporting cryogenic liquid. In this paper, the comparative study of three materials associated with their specific installation techniques was conducted to discuss if PIRB and SAB are available for offshore LNG transportation. Therein, the insulation performances were investigated through the evaporation experiment of LN2 with a 1.5 m3 type C experimental tank. The equivalent conductive model was adopted in the numerical analysis with finite element method (FEM) to determine the temperature field of insulation and the absorbed heat from the LN2 evaporation. It is proved that three types of insulation materials are well behaved and available for transporting cryogenic liquid. In respect of the thermal resistance, the ranking of three types of insulation materials is: SAB, SPF and PIRB. Higher thermal resistance of insulation leads to thinner thickness and additional arrangement space in the cargo hold, and more ship construction cost can be cut down due to the possible scaling down of ship dimensions. However, from the perspective of insulation material cost, the cost reduction due to the insulation thickness decrease is not significant as the tank continually scales up, and it is more practical to apply SPF or PIRB on a full-scale type C independent tank.

Falling Film Flow and Heat Transfer of Cryogenic Liquid Oxygen on Different Structural Surfaces

The accurate prediction of the falling film characteristics of cryogenic liquids is necessary to ensure good evaporation performance, due to their special physical properties. In this study, the film flow and heat transfer characteristics on four different structures were investigated, and the performance of the cryogenic liquid oxygen was compared with other fluids with higher temperatures, which demonstrates the influence of structures and liquid mediums. The VOF model was used to capture the film surface in the simulation model. The results show that for the four structures, liquids with higher kinematic viscosity tend to have greater film thickness, and the sensible heat transfer coefficients are inversely related to the nominal thermal resistance of falling film flow. Both on the smooth plate and the corrugated plate, the film wettability depends on the kinematic viscosity, rather than the dynamic viscosity, and the effect of kinematic viscosity is greater than that of surface tension. Both the local heat transfer coefficient and its fluctuation amplitude decrease gradually along the flow direction on the triangular corrugated plate, and the vortices are easier to produce at the wall troughs when the film viscosity is higher. At the bottom of the horizontal tube, the increases in local film thickness of the liquid oxygen are less than those of the water and the seawater. More liquid tends to accumulate at the bottom of the round tube, while it easily detaches from the film surface of the elliptical tube. For the horizontal tubes, the local heat transfer coefficients decrease rapidly when θ = 0–5°, and increase sharply at θ = 175–180°.

Experimental and numerical investigation of change in boil-off gas and thermodynamic characteristics according to filling ratio in a C-type cryogenic liquid fuel tank

This paper covers the experimental and numerical investigation of the changes in boil-off gas and thermodynamic characteristics due to filling ratio (FR) in a cryogenic liquid fuel tank. An experiment on a vertically oriented C-type fuel tank with an insulation system was carried out using LN2 to allow validation of the simulation results. The data for temperature inside the tank and mass flow rate of boil-off gas were acquired with respect to the different FRs. It was observed that the amount of BOG tends to increase in the form of a quadratic function based on the trend line, as FR increases. The simulation seemed to well express the complex physical phenomenon associated with heat transfer, heat flow, and vaporization occurring in the entire system of the tank. Especially the flow visualization of the simulated physical quantities helped to understand physical phenomena more easily, such as a mechanism of BOG caused by vaporization, which are not easy to approach in experiments. On the other hand, the BOR according to the FR showed a very slight difference of less than 3% on average compared to the experiment, which made it possible to confirm the validity of the multi-physics simulation technique.

Carbon emissions, techno-economic and machinability assessments to achieve sustainability in drilling Ti6Al4V ELI for medical industry applications

In the last decade, cryogenic machining has made a significant impact as a sustainable machining technique, demonstrating machinability improvements in terms of tool life, surface quality of machine parts, low environmental effects and reduced cost of overall machining operation. A near-to-dry machining technique, Electrostatic Minimum Quantity Lubrication (EMQL) has also emerged as a sustainable machining technique, which is an environmentally-friendly alternative to the flood machining process. Although these coolant/lubrication strategies have been studied extensively, there is no direct comparison of both these machining techniques in the aspects of ecology, economy and machining performances. In this novel study, cryogenic liquid carbon dioxide (LCO2) and EMQL techniques are used for high-speed drilling of Ti6Al4V ELI (extra low interstitial) grade, and the sustainability and machinability assessment of these techniques are evaluated which can help medical implant manufacturing industries to implement these techniques as an economic and sustainable replacement of the conventional cooling/lubrication techniques. Ti6Al4V ELI is widely used in the medical industry for Hip and Knee joint endoprosthesis, bone screws, collar bone and dental implants. The sustainability aspects were evaluated in terms of carbon emission and cost analysis of the processes. It was found that the EMQL cutting condition exhibited 27.69% higher carbon emission and 67.60% higher cost of machining operation compared to cryogenic LCO2 cutting condition. Moreover, the tool life experienced when machining using cryogenic LCO2 cutting condition was at least 4 times higher than tool life found when using the EMQL cutting condition. The surface roughness of the workpiece material was higher (56.40%) when using the EMQL coolant. These results were attributed to adhesion wear and chip clogging phenomena which were found to be the main factors for reduced tool life and higher surface roughness in EMQL cutting condition. Therefore, from this study, it can be concluded that cryogenic LCO2 coolant performs significantly better compared to EMQL coolant, providing improved machinability of the titanium workpiece and superior sustainability of the cutting operation.

Conceptual design and performance evaluation of a novel cryogenic integrated process for extraction of neon and production of liquid hydrogen

Novel integration of a hydrogen liquefaction process and cryogenic rectification of air is presented. The thermodynamics analysis including the 2nd Law and sensitivity analysis regarding this process are conducted. The proposed cryogenic process can produce 111.3 kg/s of nitrogen and 42 kg/s of oxygen with argon. In addition, this process produces more than 132 × 103 tons of liquid hydrogen and 751 tons of crude neon per year. Moreover, as the number of the used compressor is very fewer than of the conventional process, the required power consumption of the proposed cryogenic plant is almost low. Based on the exergy analysis, using expanders, providing the requested refrigeration condition by solar heat exchangers and proper streams, and integrating two processes with the similar operating condition, the exergetic efficiency of the cycle is more than 70 %. The sensitivity analysis of operating parameters such as pressure, temperature, reflux ratio, and the number of stages of column presents their the importance of them on the recovered neon, oxygen, and nitrogen. Moreover, the production of cryogenic liquids is more sensitive to operating pressure and temperature than the reflux ratio and the number of stages.

Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-body, Actively Shielded, 3 T MRI Magnets Wound Using MgB2 Strands for Liquid Cryogen Free Operation.

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB2 strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB2 wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

Optimum coating thickness for the fastest chilldown of Teflon-coated stainless steel flat plate in liquid nitrogen pool

In this study, a novel model was developed to accurately determine the optimum thickness of a low-thermal-conductivity coating layer applied on a stainless steel flat plate resulting in the fastest chilldown of the plate in a cryogenic liquid pool. A low-thermal-conductivity layer coated on a metal surface is known to significantly reduce the cryogenic chilldown time of the metal by facilitating the early transition from film to nucleate boiling. However, there is still a lack of models to estimate the optimum coating thickness. An attempt has been made to address this issue. To do so, the authors first investigated the transient pool boiling of liquid nitrogen on Teflon-coated stainless-steel plates with various coating thicknesses ranging from 0 to 300 μm. Both the pool mass and plate temperature over time were measured and used to determine the heat flux into the liquid pool. A coating layer that results in a lower thermal resistance during film boiling would lead to a higher thermal resistance in nucleate boiling. As the thickness of the coating layer increases, the thermal resistance in film boiling decreases and the transition from film to nucleate boiling occurs earlier. The early transition of the boiling regimes was successfully explained by the assumption of intermittent liquid-solid contact during the film-boiling regime. The coating thickness in the range of 100–150 μm was observed to be the optimum thickness that resulted in both the smallest thermal resistance and earliest transition of boiling regimes. Subsequently, a new model to estimate the optimum coating thickness was derived based on experimental observations and the assumption of intermittent liquid-solid contact. The findings of this study are expected to be beneficial in enhancing the chilldown efficiency of cryogenic systems.

Numerical Study of Effect of Pressurant Gas Species on Thermal Behavior in Cryogenic Tank

This study numerically reveals effect of pressurant gas species on thermal behavior in a cryogenic propellant tank for space propulsion systems. The simulation was conducted for a ground experiment using liquid nitrogen in which gaseous helium and the same species as the liquid, gaseous nitrogen, were used as prepressurants. The tank was sealed after prepressurization to observe self-pressurization (pressure rise with evaporation) and thermal stratification (high-temperature liquid near the surface). A higher evaporation rate and a thicker thermal layer were observed for helium prepressurization in the experiment. To clarify the underlying physics, a simulation considering phase change and conjugate heat transfer was developed. The simulated pressure and temperature closely matched the experimental results. Conjugate heat transfer played an important role; that is, the heat flow through the ullage part of the tank skin into the cryogenic liquid was a dominant heat transfer process, causing evaporation to occur mainly at the contact point of the wall and the gas–liquid interface. Afterward, nitrogen vapor rose along the tank wall due to buoyancy under nitrogen prepressurization. However, buoyancy in the ullage is lower under helium prepressurization since light helium stayed in the upper part of the ullage, and a radial vapor flow is produced from the contact point, leading to a higher heat flux from the tank wall to the liquid nitrogen. This mechanism shows that evaporation rate and heat flow into the liquid are higher for helium prepressurization than for nitrogen prepressurization.