Flux Entry Research Articles

Interface Wetting and Reaction Behavior between Mold Flux and Submerged Entry Nozzle under an External Electric Field

Herein, to provide theoretical foundation and experimental data for using electric field to control the service performance and improve the service life of submerged entry nozzle, the effect of the external electric field on the interfacial reaction and wetting behavior between the mold flux and the slag‐line materials is studied. The results show that the wettability of mold flux on the surface of the slag‐line materials is mainly affected by the dissolution and spreading under normal conditions. However, the electrochemical reaction and electric force generated by the applied electric field are also important factors for driving and changing the wettability between the mold flux and slag‐line materials. Under the positive electric field, the total duration of complete spreading and wetting is 227 s, which is 13 s shorter than that under normal conditions; the contact angle after complete wetting is about 17°, which is 1.5° lower than that under normal conditions. Under the negative electric field, the total duration of complete spreading and wetting is 260 s, which is 20 s longer than that under normal conditions; the contact angle after complete wetting is about 21°, which is 2.5° higher than that under normal conditions.

Combined Radiation and Convection in Developing Flow in a Parallel Plate Channel With Real Gas Behavior: The Case of Gas Cooling

Abstract The effect of real gas volumetric radiation on the thermal development in laminar parallel plate channel flow of H2O and/or CO2 in the case of gas cooling has been investigated numerically. The nongray radiation effects of the gas have been treated using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The results reveal that gas radiation results in significantly higher total heat transfer to the cooled channel wall, with an attendant more rapid drop in gas mean temperature. Gas radiation is seen to increase the local convective and total (radiative plus convective) Nusselt number for increasing radiating species mole fraction for both H2O and CO2 and for increasing gas inlet temperature. The influence of gas radiation on the thermal development is less pronounced for CO2 than for H2O. An apparent thermally fully developed condition may exist for this combined convection-radiation problem with real gases in the gas cooling scenario, and radiation has the effect of significantly extending the thermally developing region. Combined hydrodynamic and thermal development yields higher heat transfer than the thermally developing condition. Smaller channel wall spacing results in lower radiative heat transfer and the aforementioned radiation effects are diminished. Local convective and radiative flux and thermal entry length also increase with elevated gas total pressure.

HiPIMS deposition of superconducting Nb thin films onto Cu substrates

The use of energetic condensation techniques, such as HiPIMS, has the potential to significantly improve the superconducting performance of coated Cu cavities by producing bulk-like Nb thin films. However, in order to benefit from these techniques, a deeper understanding of the effects of, and the relationship between, the different deposition parameters, the physical properties of the deposited films and the superconducting performance of the films, is still required. This contribution details the effects of different HiPIMS deposition parameters on the morphological, crystallographic and superconducting properties of Nb thin films deposited onto electropolished Cu substrates. Higher duty cycles (8 – 20 %) than previous HiPIMS investigations were used in the majority of the coatings. The films displayed more bulk-like crystallographic properties, indicated here by the lattice parameter, with a “transition zone” apparent in the early stages of film growth. A reduction in the surface roughness as well as interfacial voids, compared to typical DC MS Nb films, was also observed. The superconducting performance of the films, specifically the first magnetic flux entry field, showed a marked reliance on the substrate bias and the film thickness. Conversely, the HiPIMS duty cycle appears to have a comparatively insignificant effect.

Vortex nucleation barriers and stable fractional vortices near boundaries in multicomponent superconductors

The magnetization process of a superconductor is determined by the potential barrier for vortex nucleation and escape. In multicomponent superconductors, fractional vortices with a winding in the phase of only one of the components can be stable topological solitons that carry a fraction of the flux quantum. While the formation of such objects in the bulk costs logarithmically or linearly divergent energy, these objects were shown to be stable near the samples' boundaries in the two-component London model. Therefore the conventional Bean-Livingston picture of magnetic flux entry does not apply to these superconductors, since the entry process can involve fractionalization of a vortex. In this paper, we address the nonlinear problem of determining the potential barrier for fluxoid penetration in a multicomponent superconductor, including the effects of various intercomponent couplings, by using the recently developed gauged string method. The method allows numerically exact (i.e., convergent) calculation of a sphaleron configuration in a gauge theory and thus the height of the nucleation barrier. We show how the fractionalized nucleation processes result in multiple sphalerons and intermediate states due to the complex shape of the energy landscape of multicomponent superconductors.

Finite element analysis to enhance the efficiency of the secondary roof of double skin roofs (DSRs)

The significant side effect of rapid urbanization in developing cities is urban heat island (UHI), resulting in elevated ambient air temperatures. This causes a further increase in the indoor air temperature of a building bringing thermal discomfort to the inhabitants. It is a well-established fact that the significant electric consumption in buildings creates thermal comfort conditions for the inhabitants. Cool roofs are one of the promising passive technologies to curb the entry of heat flux into the buildings through ceilings, and a double skin roof (DSR) is a cool roof technique in which an air channel inhibits the heat flux entry. This paper presents an opinion to increase the efficiency of the secondary roof of a DSR by providing perforations and fins (extended surfaces) to it by way of a simulation using Ansys software, a finite element analysis software, and incorporating constant peak ambient boundary conditions on the roof for five hours to obtain the temperature distribution. The simulated results show that such a secondary roof design is 16.43 % more efficient in decreasing the temperature at its lower surface than the conventional one.

Improvement of the first flux entry field by laser post-treatment of the thin Nb film on Cu

In superconducting radiofrequency accelerating cavities for particle accelerators, the Nb/Cu structure is one of the alternatives to bulk Nb but R&D is required to achieve a reproducible performance that competes with bulk Nb. The DC entry field H en and critical temperature were studied in the thin Nb films deposited by magnetron sputtering on Cu substrate. The Nb surfaces were further treated by Nd:YAG laser at four energy doses (D1 = 350, D2 = 233, D3 = 175 and D4 = 140 J cm−2). The superconducting properties and other characteristics obtained by atomic force microscope, scanning electron microscope and x-ray diffraction were compared before and after laser treatment. The laser treatment increased the field of first flux entry by up to 65% compared to a non-irradiated sample. The laser irradiation also led to reduction of surface roughness, improvement of the surface morphology, reduction in crystallite size and increase of lattice parameter.

The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth

The annual flux of extraterrestrial material on Earth is largely dominated by sub-millimetre particles. The mass distribution and absolute value of this cosmic dust flux at the Earth's surface is however still uncertain due to the difficulty in monitoring both the collection efficiency and the exposure parameter (i.e. the area-time product in m2.yr). In this paper, we present results from micrometeorite collections originating from the vicinity of the CONCORDIA Station located at Dome C (Antarctica), where we performed several independent melts of large volumes of ultra-clean snow. The regular precipitation rate and the exceptional cleanliness of the snow from central Antarctica allow a unique control on both the exposure parameter and the collection efficiency. A total of 1280 unmelted micrometeorites (uMMs) and 808 cosmic spherules (CSs) with diameters ranging from 30 to 350 μm were identified. Within that size range, we measured mass fluxes of 3.0 μg.m−2.yr−1 for uMMs and 5.6 μg.m−2.yr−1 for CSs. Extrapolated to the global flux of particles in the 12-700 μm diameter range, the mass flux of dust at Earth's surface is 5,200±12001500 tons.yr−1 (1,600±500 and 3,600±7001000 tons.yr−1 of uMMs and CSs, respectively). We indicate the statistical uncertainties expected for collections with exposure parameters in the range of 0.1 up to 105 m2.yr. In addition, we estimated the flux of altered and unaltered carbon carried by heated and un-heated particles at Earth's surface. The mass distributions of CSs and uMMs larger than 100 μm are fairly well reproduced by the CABMOD-ZoDy model that includes melting and evaporation during atmospheric entry of the interplanetary dust flux. These numerical simulations suggest that most of the uMMs and CSs originate from Jupiter family comets and a minor part from the main asteroid belt. The total dust mass input before atmospheric entry is estimated at 15,000 tons.yr−1. The existing discrepancy between the flux data and the model for uMMs below 100 μm suggests that small fragile uMMs may evade present day collections, and/or that the amount of small interplanetary particles at 1 AU may be smaller than expected.

Superconducting NbN thin films for use in superconducting radio frequency cavities

In recent years, renewed interest has been given to alternatives to bulk Nb for use in superconducting radio frequency (SRF) cavities. Much of this research has involved the use of thin films of Nb or other alternative high T c materials. Given the recent insights into the potential of single layer high materials and the prospective performance improvements due to the use of multilayer (ML) superconductor–insulator–superconductor (SIS) film structures, NbN stands as a potential candidate for use in future accelerating cavities. Much of the research completed thus far has focused on the deposition of NbN onto single crystal substrates such as Si and MgO, or onto bulk Nb. In this study, the deposition of high T c NbN thin films onto copper substrates, in the form of both single layers and as part of an ML SIS structure, using DC magnetron sputtering (DC MS) was explored. The effects of the deposition parameters on the film microstructure and superconducting properties of the NbN films as well as the challenges involved in depositing ML SIS films onto copper substrates are reported on. A maximum T c = 16.1 K has been achieved for single layer NbN films, as determined by AC susceptometry measurements. Initial results for the SIS film structures, deposited onto copper in the form of Nb/AlN/NbN, have also shown an enhancement in the first magnetic flux entry field value above that achieved by a single Nb layer. This enhancement has been found to be highly dependent on the quality of the SIS film coatings.

Ab initio theory of the impact of grain boundaries and substitutional defects on superconducting Nb3Sn

Grain boundaries play a critical role in superconducting applications of Nb3Sn: in dc applications, grain boundaries preserve the material’s inherently high critical current density by pinning flux, while in ac applications grain boundaries can provide weak points for flux entry leading to significant dissipation. We present the first ab initio study to investigate the physics of different grain boundary types in Nb3Sn and their impact on superconductivity using density-functional theory. We identify an energetically favorable selection of high-angle tilt and twist grain boundaries of distinct orientations. We find that clean grain boundaries free of point defects reduce the Fermi-level density of states by a factor of two, an effect that decays back to the bulk electronic structure ∼1–1.5 nm from the boundary. We further calculate the binding free-energies of tin substitutional defects to multiple boundaries, finding a strong electronic interaction that extends to a distance comparable to that of the reduction of density of states. Associated with this interaction, we discover a universal trend in defect electronic entropies near a boundary. We then probe the effects of defect segregation on grain boundary electronic structure and calculate the impact of substitutional impurities on the Fermi-level density of states in the vicinity of a grain boundary. We find that titanium and tantalum defects have little impact regardless of placement, whereas tin, copper, and niobium defects each have a significant impact but only on sites away from the boundary core. Finally, we introduce a model for a local superconducting transition temperature and consider how grain boundary composition affects as a function of distance from the boundary plane. The methodology established in this manuscript can be applied to other A15 superconductors in addition to Nb3Sn.

Effect of hydrophilic silica and dual coagulation bath on structural and mechanical properties of PVDF membrane for membrane distillation

The water scarcity threatens environmental health and human development. Membrane distillation (MD) is one of the most applicable processes for purifying water using a hydrophobic membrane. In this study, the synergetic effect of SiO2 nanoparticles as well as employing the dual coagulation bath on physical and mechanical properties of Polyvinylidene Fluoride (PVDF) flat-sheet membranes produced by dry-wet phase inversion (DIPS) technique has been investigated. The results of microstructural analysis using Scanning Electron Microscope (SEM) demonstrated that by adding nanoparticles while the pore size decreased noticeably, the percentage of porosity significantly increased. Also, it has been revealed that by utilizing isopropanol as the first coagulation bath the finger-like macro-voids became smaller in size, and the share of sponge-like structures rose remarkably. The membrane performance was tested by Vacuum Membrane Distillation (VMD) for measuring the flux and Liquid Entry Pressure (LEPw) laboratory setup. It can be seen that by increasing the content of SiO2 nanoparticles to 6 wt.% while the LEPw approximately halved, the flux soared to about 10000 g/m2h. Moreover, mechanical testing showed that although the tensile strength of nanocomposite samples fabricated in isopropanol dual coagulation bath was improved by up to 66%, their ductility slightly declined. Furthermore, the hydrophobicity of each membrane was examined via contact angle measurements. Finally, it was found that all membranes completely rejected the NaCl in rejection test. Graphical abstract.

Geometrical surface pinning in the nonlinear AC susceptibility of HTS tapes

The analytic expression for the nonlinear magnetic susceptibility of a thin disk derived from Bean’s model with a uniform J C for predicts a peak-peak ratio of 7.1 (5.4) for the real (imaginary) components of the third harmonic χ 3 susceptibility. Our measurements show a peak-peak ratio closer to 1.1 in the real component and a noise limited lower estimate of ≈ 7 in the imaginary component. The anomalous third harmonic can be explained by the presence of a geometrical surface barrier for flux entry and exit which we have included as a small region of enhanced current density, J C,G, close to the surface of the disk. The geometrical surface current density is predicted to be approximately equal to J C,G ≈ 2B C1/μ 0 d. In 2G HTS tapes with d ≈ 1μm and B C1 ≈ 5mT this is J C,G ≈ 8GAm−2. We have measured both J C,B and J C,G using the nonlinear susceptibility of a Superpower tape without artificial pinning in fields orthogonal to the tape up to 35T. We find a geometrical surface current of J C,G ≈ 10GAm−2 and an average critical current density J C which is in good agreement with transport measurements performed on material from the same reel of tape.

Superconducting properties and surface roughness of thin Nb samples fabricated for SRF applications

Using a thin Nb layer on Copper substrate has several advantages compared with the bulk Nb in construction of Superconducting Radiofrequency accelerating cavities (SRF) for particle accelerators. We were evaluating the properties of two series of Nb layers deposited on Cu substrate, mainly by determining the start of magnetic flux penetration into the sample – the first magnetic flux entry field B en, proportional to the B c1. The values of B en are compared with the surface roughness and surface morphology of the Nb layers, which have a strong influence on B en. The surfaces of the samples were also treated by Nd:YAG laser depending on laser irradiation dose. The results of B en and surface roughness before and after laser polishing are compared.

Cooperative response of magnetism and superconductivity in the magnetic superconductor RbEuFe4As4

A friendly coexistence of superconductivity and magnetism is quite an unusual physical phenomenon. However, the authors find that in the new high-T${}_{c}$ magnetic superconductor RbEuFe${}_{4}$As${}_{4}$ the magnetic and superconducting subsystems show a remarkable cooperative response. In the external magnetic field, the magnetic Eu sublattice acts as an internal pump of the magnetic flux, while the superconducting component works as a valve controlling the flux entry through the pinning of flux-carrying Abrikosov vortices and establishes the self-organized critical state.

Metabolic reorganization in winter: Regulation of pyruvate dehydrogenase (PDH) during long-term freezing and anoxia

Wood frogs, Rana sylvatica, can undergo prolonged periods of whole body freezing during winter, locking as much as 65–70% of total body water into extracellular ice and imposing both anoxia and dehydration on their cells. Metabolic rate depression (MRD) is an adaptation used by R. sylvatica to survive these environmental stresses, where a finite amount of ATP generated through anaerobic metabolism is directed towards maintaining pro-survival functions, while most ATP-expensive cellular processes are temporarily reduced in function. Pyruvate dehydrogenase (PDH) is a vital metabolic enzyme that links anaerobic glycolysis to the aerobic TCA cycle and is an important regulatory site in MRD. PDH enzymatic activity is regulated via reversible protein phosphorylation in response to energetic demands of cells. This study explored the posttranslational regulation of PDH at three serine sites (S232, S293, S300) on the catalytic E1α subunit along with protein expression of four pyruvate dehydrogenase kinases (PDHK1-4) in response to 24 h Freezing, 8 h Thaw, 24 h Anoxia, and 4 h Recovery in the liver and skeletal muscle of R. sylvatica using Luminex multiplex technology and western immunoblotting. Overall, inhibitory regulation of PDH was evident during 24 h Freezing and 24 h Anoxia, which could indicate a notable reduction in glycoytic flux and carbon entry into the tricarboxylic acid cycle as part of MRD. Furthermore, the expression of PDHK1-4 and phosphorylation of PDH at S232, S293, and S300 were highly tissue and stress-specific, indicative of how different tissues respond differently to stress within the same organism.

PVDF/h-BN hybrid membranes and their application in desalination through AGMD

A new procedure to produce poly(vinylidene fluoride)/boron nitride hybrid membrane is presented for application in membrane distillation (MD) process. The influence of hexagonal boron nitride (h-BN) incorporation on the performance of the polymeric membranes is studied through the present investigation. For this aim, h-BN nanopowders were successfully synthesized using the simple chemical vapor deposition (CVD) route and subsequent solvent treatments. The resulting h-BN nanosheets were blended with poly(vinylidene fluoride) (PVDF) solution. Then, the prepared composite solution was subjected to phase inversion process to obtain PVDF/h-BN hybrid membranes. Various examinations such as scanning electron microscopy (SEM), wettability, permeation flux, mechanical strength and liquid entry pressure (LEP) measurements are performed to evaluate the prepared membrane. Moreover, Air gap membrane distillation (AGMD) experiments were carried out to investigate the salt rejection performance and the durability of membranes. The results show that our hybrid PVDF/h-BN membrane presents higher water permeation flux (~18 kg/m2 h) compared to pristine PVDF membrane. In addition, the experimental data confirms that the prepared nanocomposite membrane is hydrophobic (water contact angle: ~103 degree),has a porous skin layer (>85%), as well competitive fouling resistance and operational durability. Furthermore, the total salt rejection efficiency was obtained for PVDF/h-BN membrane. The results prove that the novel PVDF/h-BN membrane can be easily synthesized and applied in MD process for salt rejection purposes.

Field of first magnetic flux entry and pinning strength of superconductors for rf application measured with muon spin rotation

The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry $H_{entry}$ and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation ($\mu$SR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400{\deg}C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure $H_{entry}$ of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.

A Three-Layers Plane Wall Exposed to Oscillating Temperatures with Different Amplitudes and Frequencies

A linear model of three layers plane wall exposed to oscillating temperatures with different amplitudes and frequencies was built by using a physical superposition. A physical superposition of two states was performed, one state is a wall which one surface is exposed to oscillating temperature and the other surface is exposed to zero relative temperature and a second state is a wall which one surface is exposed to relative zero temperature while the other surface is exposed to oscillating temperature with different amplitudes and frequencies. Temperature distributions were introduced for different amplitudes, frequencies and thermal conductivities. It was shown that increasing the frequency value decreases the temperature penetration length, high frequency value leads to extremum temperature values changes on the surface while low frequency value allows gradually temperature changes during the time period. Temperature distribution lines where there are at the same time heat flux entry and heat flux exit were not received for the same constraint frequencies.

A model of two cylindrical plane wall layers exposed to oscillating temperatures with different amplitudes and frequencies

A linear model of two cylindrical plane wall layers exposed to oscillating temperatures and frequencies was built using a physical superposition of two states. In the first state, the inner surface of a wall was exposed to oscillating temperature and the outer surface was exposed to a zero relative temperature. In the second state, the inner surface was exposed to a zero relative temperature while the outer surface was exposed to an oscillating temperature with different amplitude and frequency. Temperature distributions were derived for different amplitudes, frequencies, and thermal conductivities. Results show that increasing the frequency decreased the depth of temperature penetration. A high frequency led to extremum temperature values on the surface, while a low frequency allowed gradual temperature changes during the time period. Temperature distribution lines showing simultaneous heat flux entry and exit were not observed.

Superheating in coated niobium

Using muon spin rotation it is shown that the field of first flux penetration in Nb is enhanced by about 30% if coated with an overlayer of Nb3Sn or MgB2. This is consistent with an increase from the lower critical magnetic field up to the superheating field of the Nb substrate. In the experiments presented here coatings of Nb3Sn and MgB2 with a thickness between 50 and 2000 nm have been tested. does not depend on material or thickness. This suggests that the energy barrier at the boundary between the two materials prevents flux entry up to of the substrate. A mechanism consistent with these findings is that the proximity effect recovers the stability of the energy barrier for flux penetration, which is suppressed by defects for uncoated samples. Additionally, a low temperature baked Nb sample has been tested. Here a 6% increase of was found, also pushing beyond .

Development of a control model to regulate temperature in a solar receiver

Solar irradiation intermittency is a big challenge in operating the solar thermochemical processes due to its direct effect on flux entry into the cavity reactor/receiver. A control system driven by heat transfer model developed for a receiver can compensate impact of fluctuation in solar radiation. This study presents an optimal dynamics and control strategy assessing the performance of a novel method based on aperture size adjustment in comparison to feedstock flow adjustment. The controller is designed based on a mathematical model representing dynamic behavior of the system. The model tackles transient behavior of the system by coupling unsteady energy balance equation solver with Monte Carlo (MC) ray tracing method. Simulations per numerical model provide thorough comparison of the effectiveness of temperature control via aperture size adjustment versus gas flowrate adjustment in open loop and closed loop circuits. Open loop simulations feature step response method where the results confirm that change in aperture size influences the system dynamics more effective than the change in gas flowrate. Closed loop simulations feature two separate controllers designed to adjust aperture size and gas flowrate. Performance of these controllers are compared in tracking desired setpoints and compensating the effect of fluctuation in solar irradiance as the process disturbance. Results of setpoint tracking simulations show that average settling time of the system controlled by aperture size is 30% lower than the system controlled by gas flow rate which exhibits that control performance by aperture size adjustment is superior. As for disturbance rejection, results obtained by using aperture size based controller reveals that the overshoot of the system is about three times lower than the system controlled by gas flowrate. Efficiency analysis also confirmed that adjusting the aperture size or gas flow rate resulted to the same absorption efficiency in the solar receiver. These findings justify the impact of aperture size as a better parameter for temperature control of a solar receiver.