SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
In the paper the asymptotic structure of eigenfunctions of the absolutely continuous spectrum of the scattering problem is described. The case of three one-dimensional quantum particles interacting by repulsive pair potentials with a compact support is considered.
Material processing with laser offers a reliable tool in a large variety of micromachining technology, ranging from on-disc inscriptions to cell ablations through DNA combing and imprinting for fabrications of micro and nanofluidic devices. In these applications lasers are designed to operate in specific regimes characterized by their powers and wavelengths, so understanding characteristic properties of the distinct possible laser operation regimes turns out to be a relevant step toward optimization of their uses as well as improvement of the technology. In this work we examine the stability of a model of femtosecond laser intended for laser inscriptions in nonlinear transparent media, taking into consideration the laser-induced material damage and multiphoton ionization. The model approximates the laser dynamics by a complex Ginzburg–Landau equation with a Kerr plus a high-order nonlinear term accounting for K-photon ionization, coupled to a Drude equation for time evolution of the electron plasma density. Following the Benjamin–Feir instability theory it is shown that depending on characteristic parameters of the model, multiphoton ionization processes can stabilize continuous-wave or pulse regimes. These parameters include the group-delay dispersion, the linear and nonlinear gains, the phase shift per roundtrip and the plasma ionization rate.
Detection of radioxenon and radioargon produced by underground nuclear explosions is one of the primary methods by which the Comprehensive Nuclear-Test–Ban Treaty (CTBT) monitors for nuclear activities. However, transport of these noble gases to the surface via barometric pumping is a complex process relying on advective and diffusive processes in a fractured porous medium to bring detectable levels to the surface. To better understand this process, experimental measurements of noble gas and chemical surrogate diffusivity in relevant lithologies are necessary. However, measurement of noble gas diffusivity in tight or partially saturated porous media is challenging due to the transparent nature of noble gases, the lengthy diffusion times, and difficulty maintaining consistent water saturation. Here, the quasi-steady-state Ney–Armistead method is modified to accommodate continuous gas sampling via effusive flow to a mass spectrometer. An analytical solution accounting for the cumulative sampling losses and induced advective flow is then derived. Experimental results appear in good agreement with the proposed theory, suggesting the presence of retained groundwater reduces the effective diffusivity of the gas tracers by 10–1000 times. Finally, by using a mass spectrometer, the method described herein is applicable to a broad range of gas species and porous media.
In the current energy and environment scenario, it is imperative to develop energy efficient routes for chemical manufacturing that also pave the way for mitigation of greenhouse gas emissions. This work presents an efficient pathway for continuous syngas production via a chemical looping conversion of the two most potent greenhouse gases—CH 4 , and CO 2 . The well-known dry-reforming process of converting CH 4 , and CO 2 to syngas is energy-intensive and suffers from catalyst deactivation. The chemical looping approach, on the other hand, provides avenues for mitigating catalyst deactivation and enabling improved energy efficiency. The key to such process enhancements lies in the intricate structure–function relationships of the catalyst and its correlation to the process variables. We present the reduction and oxidation characteristics of 5 wt.% Ni/Ce 1-x Zr x O 2 -based catalysts (x = 0, 0.4, and 0.625). We demonstrate low temperature CH4 activation over Ni-promoted samples as opposed to pure Ce 1-x Zr x O 2 . Moreover, our results depict an optimum regeneration of these catalysts when oxidized by CO 2 , and H 2 O, which allows for chemical looping operation of steam reforming of methane as well. Process variables were tuned to optimize the CH 4 conversion (over 80%), and H 2 /CO ratio at 650 °C. The critical surface reactions—carbon accumulation and gasification, and thermocatalytic CO 2 splitting were investigated to elucidate the dynamic nature of the catalyst surface. In conclusion, the impact of this work lies in showcasing the opportunities to design chemical looping reactors for energy efficient syngas production from waste greenhouse gases.
The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the susceptibility to the transverse crack can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate two types of double phase-transformation technologies. The distribution and morphology of ferrites under different cooling conditions were analyzed, and the effects of ferrite distribution and morphology on the double phase-transformation technologies were explored to obtain the suitable double phase-change technology for the continuous casting process. The results indicate that, under the thermal cycle TH0 [the specimens were cooled down to 913 K (640 °C) at a cooling rate of 5.0 K/s (5.0 °C/s)], the width of prior austenite grain boundaries was thick, and the dislocation density at grain boundaries was high. It had strong inhibition effect on crack propagation; under the thermal cycle TH1 [the specimens were cooled down to 1073 K (800 °C) at a cooling rate of 5.0 K/s (5.0 °C/s) and then to 913 K (640 °C) at a cooling rate of 1.0 K/s (1.0 °C/s)], the width of prior austenite grain boundary was thin, and the dislocation density at grain boundaries was low. It was beneficial to crack propagation. After the first phase change, the developed film-like ferrite along the austenite grain boundaries improved the nucleation conditions of new austenitic grains and removed the inhibition effect of the prior austenite grain boundaries on the austenite grain size.
The effect of the electroslag remelting-continuous rapid solidification (ESR-CRS) process on element segregation and carbide precipitation in GCr15 bearing steel was investigated. The results showed that a microstructure with fewer primary carbides and less segregation can be achieved via the ESR-CRS process. In the specimen subjected to ESR, the morphology of primary carbide changed from angular to lumplike. After the ESR-CRS process, the dimension of primary carbide and the mass fraction of Cr in primary carbide decreased. With the increase of cooling intensity during the ESR-CRS process, the microstructure of ESR ingot became more refined and uniform and the size of large primary carbides in ESR ingot gradually decreased. Suppressing the formation of grain boundary cementite and primary carbide during ESR of GCr15 steel is beneficial to inhibiting the presence of large secondary carbide formation after annealing. With the increase of cooling intensity, the mass fraction of carbides in ESR ingot decreased and the mass fraction of Cr in carbides increased, whereas the types of carbides did not change; all the M{sub 3}C, M{sub 3}C{sub 2}, and M{sub 7}C{sub 3} exist before and after ESR-CRS. Al{sub 2}O{sub 3} inclusions promoted the formation of Ti(N,C) by serving as preferred heterogeneous nucleation sites, whereas the formation of Ti(N,C) was suppressed through the refinement of Al{sub 2}O{sub 3} inclusions by increasing the cooling intensity.
Solidified shell bulging is supposed to be the main reason for slab center segregation, while the influence of thermal shrinkage rarely has been considered. In this article, a thermal shrinkage model coupled with the multiphase solidification model is developed to investigate the effect of the thermal shrinkage, solidification shrinkage, grain sedimentation, and thermal flow on solute transport in the continuous casting slab. In this model, the initial equiaxed grains contract freely with the temperature decrease, while the coherent equiaxed grains and columnar phase move directionally toward the slab surface. The results demonstrate that the center positive segregation accompanied by negative segregation in the periphery zone is mainly caused by thermal shrinkage. During the solidification process, liquid phase first transports toward the slab surface to compensate for thermal shrinkage, which is similar to the case considering solidification shrinkage, and then it moves opposite to the slab center near the solidification end. It is attributed to the sharp decrease of center temperature and the intensive contract of solid phase, which cause the enriched liquid to be squeezed out. With the effect of grain sedimentation and thermal flow, the negative segregation at the external arc side (zone A1) and the positive segregation near the columnar-to-equiaxed transition at the inner arc side (position B1) come into being. Besides, it is found that the grain sedimentation and thermal flow only influence solute transport before equiaxed grains impinge with each other, while the solidification and thermal shrinkage still affect solute redistribution in the later stage.
The separation of rare earth elements (REEs) using solvent extraction adding complexing agents appears to be an alternative to saponification of the extractant. We evaluated the effect of lactic acid concentration on didymium (praseodymium and neodymium) and lanthanum extraction with 2-ethylhexyl phosphonic acid mono-2-ethyl hexyl ester [HEH(EHP)] as extractant. First, we investigated in batch experiments the separation of lanthanum (La) and didymium (Pr and Nd) using McCabe–Thiele diagrams to estimate the number of extraction stages when the feed solution was or was not conditioned with lactic acid. Additionally, we conducted continuous liquid–liquid extraction experiments and evaluated the influence of lactic acid concentration on the REE extraction and separation. The tests showed that the extraction percentage of REEs and the separation factor Pr/La increased when the lactic acid concentration increased, but the didymium purity decreased. Lanthanum, praseodymium, and neodymium extraction rate were 23.0, 89.7, and 99.2 pct, respectively, with 1:1 aqueous/organic volume flow rate and feed solution doped with 0.52 mol L{sup −1} lactic acid. The highest didymium purity reached was 92.0 pct with 0.26 mol L{sup −1} lactic acid concentration.
Early, yet still often-cited, mathematical models for electromagnetic stirring (EMS) in continuous casting are re-examined and found to contain a surprising anomaly: the solutions obtained were not unique. Analysis for the case of a round billet under rotary EMS shows how to avoid this behavior, whilst still making use of the experimental data that motivated the original models. The relevance of this result for current-day modeling of EMS is highlighted, particularly in the context of modulated EMS.
In continuous casting of steel, argon gas is often injected to prevent clogging of the nozzle, but the bubbles affect the flow pattern, and may become entrapped to form defects in the final product. Further, an electromagnetic field is frequently applied to induce a braking effect on the flow field and modify the inclusion transport. In this study, a previously validated GPU-based in-house code CUFLOW is used to investigate the effect of electromagnetic braking on turbulent flow, bubble transport, and capture. Well-resolved large eddy simulations are combined with two-way coupled Lagrangian computations of the bubbles. The drag coefficient on the bubbles is modified to account for the effects of the magnetic field. The distribution of the argon bubbles, capture, and escape rates, are presented and compared with and without the magnetic field. The bubble capture patterns are also compared with results of a previous RANS model as well as with plant measurements.
Consistent and uniform lubrication of the solidifying shell, especially in the meniscus, is crucial for the smooth continuous casting operation and production of strands free of surface defects. Thus, the current study established a coupled model to study the inflow behavior of liquid slag to the mold-strand channel, taking the solidification of steel and slag and the periodic oscillation of mold into account. The difficulties and solutions for the simulation were described in detail. The predicted profiles of the slag rim and initial shell were in good agreement with the reports. The main results indicated that liquid slag could be squeezed out and back into the slag pool in a negative strip period while a large amount of liquid slag could infiltrate into the mold-strand channel. Thus, the amount of slag consumed in the negative strip period was relatively small compared with that in the positive strip period. The predicted variation of slag consumption during mold oscillation was periodic, and the average value was 0.274 kg/m{sup 2}, which agreed well with the slag consumption in industrial practice. The current model can predict and optimize the oscillation parameters aiming at stable lubrication conditions.
To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr{sub 2}O{sub 3} but decrease with the addition of B{sub 2}O{sub 3}. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr{sub 2}O{sub 3}, and then further decreases to 25.3 pct with the addition of B{sub 2}O{sub 3}; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B{sub 2}O{sub 3}-1.10 pct MnO-2.10 pct Cr{sub 2}O{sub 3} shows the best properties for the continuous casting of Cr-contained steels.
In the current study, steel samples were etched and hooks on several successive layers of longitudinal and transverse slab sections were obtained to investigate the three-dimensional (3-D) distribution of hooks in low-carbon continuous casting slabs. The data from industrial trials and numerical calculation were used to explain the variation in the distribution and morphology of hooks. Around the slab perimeter, the mean depth of hooks at the center and edge of the wide face and the edge of the narrow face was larger than that of other locations. The 3-D hook demonstrated a varied characteristic on different layers, as the observed surface changed in the longitudinal direction. By several layers of the transverse section being polished off, hooks in the transverse direction below the slab surface were obtained, in which the truncated hook and the joint hook between the wide face and narrow face were observed at the slab corner. The same oscillation mark could show as hook type and depression type, depending on the occurrence of the overflow between the solidified meniscus and the copper plate. Industrial trials and the numerical calculation illustrated that the transient variation of hooks in the 3-D space was related to the level fluctuation of molten steel in the mold and temperature fluctuation with casting time and the fluid flow and temperature near the meniscus. Finally, the influence area and volume of hooks on the capture of inclusions and bubbles were discussed.
This study establishes a volume-averaged model for transport phenomena to study V-shaped segregation in continuous casting blooms with different superheating and cooling rates by coupling fluid flow, macro-scale heat transfer, and solute transport. The results demonstrate that the modified Won–Thomas model considering back diffusion in the solid phase corresponds well with the measured carbon segregation. V-shaped segregation decreases with decreasing superheating and increases slightly with decreases in the cooling rate. The results of original position statistical distribution analysis correspond well with the simulation results. This phenomenon can be exploited to improve the quality of bloom.
The drawback of steel fluid flow phenomena for a continuous casting mold under an electromagnetic stirrer with a traveling magnetic field is the resulting severe meniscus fluctuations due to the high-velocity and excessive downward flow in the casting direction. This can result in slab surface defects due to mold flux or inclusion entrainment. To inhibit these defects in the as-cast steel products, the implementation of braking forces using static magnetic fields in combination with a traveling magnetic field was studied in the current study. The numerical results of the fluid flow in the mold under a combined traveling and static magnetic field operation show not only 50 pct lower downward flow speed, but also more enhanced rotational flow to improve washing effects. The numerical results were validated in commercial-scale operations. The results suggest the optimized range of the ratio between the traveling and static magnetic flux densities is 65 to 75 pct to ensure steel quality improvements; this range maintains a balance between the fluid forces from the jet flow and the magnetic forces in the mold.
Integrated casting and rolling in a series production line is well established for the non-ferrous metals through the use of twin-belt casting technology. However, this twin-belt casting with high mass flow is not yet commercialized in the continuous production of steel alloys. A novel steel casting for the in-line rolling (named LUUP method) is presented and one main focus of this work is to design a casting machine: twin-belt caster. Influence of mass flow rate and dimensional ratio on the solidification, compensation, and total process length is addressed. A gap-dependent belt and dam block side heat transfer is modeled. The admissible casting speed due to the limitations of caster length and shell strength is addressed. The total process length only depends on the mass flow rate. A 2D traveling slice numerical model is presented with realistic boundary conditions for the entire process starting from the meniscus to roll mill entry.
Scale-adaptive simulation (SAS) of the transient gas–liquid two-phase flow in a laboratory-scale continuous-casting mold is presented. The main objective is to investigate the applicability of the scale-adaptive unsteady Reynolds-averaged Navier–Stokes turbulent model (URANS SAS) for predicting the transient multiscale turbulent structures in a two-phase flow. Good quantitative agreements with the experimental data and the large eddy simulation (LES) results are obtained both for the time-averaged velocity field and for the transient turbulent characteristics. The introduction of the von Karman length-scale into the turbulence-scale equation allows the SAS model to dynamically adjust to the resolved turbulent structures. The LES-like pulsating behavior of the air gas and the large-scale liquid eddy magnitudes in the unsteady regions of flow field are captured by the SAS model. The classical − 5/3 law of power spectrum density (PSD) of the axial velocity is kept properly for the single-phase turbulent flow. For two-phase flow, the decay of PSD is too steep at the high-frequency region; the predicted PSD obtained with SAS is damped stronger than that estimated by LES. The SAS model offers an attractive alternative to the existing LES approach or to the other hybrid RANS/LES models for strongly unsteady flows.
This study focuses on an experimental investigation of the fluid flow in round bloom continuous casting using a 1:3 model of the industrial casting process. A swirling flow nozzle, represented by the specific design of the RHI Magnesita GYRONOZZLE, is used to produce a swirling motion in the cylindrical mold. The test section is integrated into the Mini-LIMMCAST facility at HZDR, which is operated at room temperature using the ternary alloy GaInSn. Systematic measurements of horizontal and vertical velocity profiles are performed by means of the Ultrasound Doppler Velocimetry. The second part of the study focuses on the interaction between the flow driven by the GYRONOZZLE and concurrent electromagnetic stirring in the mold (M-EMS) by applying rotating magnetic fields (RMFs) at different magnetic flux densities. The effect of the GYRONOZZLE on the flow pattern inside the mold is examined with and without superimposed RMFs and compared to those of a standard single-port nozzle. The measurements reveal that the GYRONOZZLE induces a swirling flow in the whole mold. It is further shown that the influence of a simultaneously applied RMF is mainly restricted to the lower part of the mold since the transport of angular momentum to the top is suppressed by the jets pouring out from the GYRONOZZLE.