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Numerical and experimental study on keyhole and melt flow dynamics during laser welding of aluminium alloys under subatmospheric pressures
摘要： Porosity defects was highly related to the keyhole and melt flow dynamic during laser welding process. In this paper, a novel 3D numerical model was developed to describe the keyhole dynamic and melt flow behaviors during laser welding of 5A06 aluminium alloy under subatmospheric pressures. The effect of ambient pressure on laser welding process was taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation. A moving hybrid heat source model was employed to describe the laser energy distribution under subatmospheric pressures. Numerical results indicated that a wider and deeper keyhole with less humps was produced under subatmospheric pressure comparing with that of atmospheric pressure. The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures. The melt flow velocity on the keyhole wall was larger under a lower pressure. A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and improved the stability of molten pool. Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity. Bigger keyhole opening size, larger melt flow velocity, thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys. Based on the simulation results, the plasma distribution, weld formation and porosity defects had been demonstrated. The compared results showed that the simulation results exhibited good agreements with the experimental ones.
关键词： Porosity defects,Keyhole stability,Numerical simulation,Subatmospheric pressure,Melt flow dynamic,Laser welding
更新于2025-11-28 14:24:20
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In-situ Measurements and Thermo-mechanical Simulation of Ti-6Al-4V Laser Solid Forming Processes
摘要： Residual stresses and distortions are two technical obstacles for popularizing the Additive Manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in-situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the Laser Solid Forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed.
关键词： Numerical simulation,Laser Solid Forming (LSF),Thermo-mechanical analysis,Additive manufacturing (AM),In-situ measurements of residual stresses
更新于2025-11-28 14:24:20
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Performance improvement of baffle-type solar air collector based on first chamber narrowing
摘要： This paper proposes a novel idea to optimize thermal performance of baffle-type solar air collector by narrowing the first chamber through rearranging the baffles in the collector. The collector with five chambers divided by four baffles was investigated numerically and experimentally, and the mechanism of performance improvement was revealed by flow and heat transfer analysis. The simulation results show that the width of first chamber has significant influence on thermal efficiency, while has little influence on pressure drop. The maximum thermal-efficiency growth rate can be achieved when the width of the first chamber is 200 mm with the total chamber size of 2000 mm × 1000 mm × 120 mm, and the value is up to 16.90% compared with the model with evenly distributed baffles during the Reynolds number ranges from 1.8–5.5 × 10^3. A test rig was developed at the ratio of 1:0.5 to the numerical model. Four collector models were studied under three working conditions, and effectiveness of this method is verified. The results show that the first-chamber narrowing method has relatively stable optimization effect with a thermal-efficiency growth rate ranges from 9.73% to 16.10% in the experiment. It means that this method is not sensitive to scale change and has certain adaptability.
关键词： Experimental verification,Numerical simulation,Performance optimization,First chamber narrowing,Solar air collector
更新于2025-09-23 15:23:52
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Influence of Current Density on Orientation-Controllable Growth and Characteristics of Electrochemically Deposited Au Films
摘要： This paper is concerned with the stability analysis of time varying delayed stochastic Hopfield neural networks in numerical simulation. To achieve our expected conclusions, we will reform the classical contractive mapping principle in functional analysis, with some modifications, to adapt to our conditions and both the continuous and the discrete delayed models. Under the reasonable conditions, it is shown that, the Euler–Maruyama numerical scheme is mean square exponentially stable of exact solution dependent of step size. Further more, it is also shown that the backward Euler–Maruyama numerical scheme can share the mean square exponential stability of the exact solution independent of step size under the same conditions.
关键词： Numerical simulation,Time delay,Stochastic differential equation,Hopfield neural network,Stability
更新于2025-09-23 15:23:52
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Evaluation of Radiation Characteristics of Xenon Short Arc Lamp by Numerical Simulation; 数値解析を用いたキセノンショートアークランプの放射特性評価;
摘要： In this study, an arc numerical simulation model of xenon short arc lamp was developed by modifying numerical simulation model of free burning arc. The influence of the difference in the filling gas pressure on the energy consumed by the lamp voltage, current and gas radiation was investigated. It became clear that the radiation power of the xenon arc lamp strongly depends on the filling gas pressure. Furthermore, when the lamp is turned on with constant power, the radiation efficiency is determined by the balance between the radiation power density and the high temperature arc volume, and as a result, it is clear that the maximum value exists of the radiation power fraction.
关键词： Numerical Simulation,Xenon Arc Lamp,Radiation,Xenon,Energy Balance
更新于2025-09-23 15:23:52
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The influence of Marangoni effect on the growth quality of multi-crystalline silicon during the vacuum directional solidification process
摘要： A multi-field coupling model of heat-flow-thermal stress was established to investigate the influence of Marangoni effect on the growth quality of multi-crystalline silicon (mc-Si) during the vacuum directional solidification (VDS) process. The simulation results showed that the Marangoni effect has a significant effect on the distribution of temperature and thermal stress as well during the VDS process due to the increased velocity of melts. The enhanced flow incurred more homogeneous temperature distribution of silicon melts and reduced radial temperature gradient, which then leads to the flatter solid/liquid (s/l) interface. However, as temperature gradient of the crystal increased, thermal stress of the silicon ingot was strengthened as a result. The growth quality of crystal was more desirable when adopting a pulling-down rate of 10 μm/s, in which case the (111) surface was advantageous throughout the entire crystal growth process by XRD detection. Finally, the reliability of the numerical simulation result was verified by the experiment.
关键词： Multi-crystalline,Melt flow,Vacuum directional solidification,Numerical simulation,Marangoni effect
更新于2025-09-23 15:22:29
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Comparison between different strategies for the realization of flashing-light effects – Pneumatic mixing and flashing illumination
摘要： A major limitation for the phototrophic cultivation of microalgae in photobioreactors is the low culture density which results in expensive harvesting and downstream processes. It is often mentioned that intensive mixing of the culture can improve the efficiency of light utilization by realizing flashing-light effects which enhance the growth of cell cultures. Alternatively, flashing light sources could provide a way to realize this effect without the need of supplying additional energy for fast mixing. While the effects of mixing have been investigated at various reactor scales, experiments with flashing illumination have been mostly conducted in small geometries. In addition, few studies have been conducted for photobioreactors with larger light paths, being characteristic for the production scale. By means of numerical simulations, we evaluate pneumatic mixing and flashing illumination with regard to their ability to realize flashing-light effects in a 5 cm diameter bubble column filled with a suspension of Chlamydomonas reinhardtii. To the best of our knowledge, a numerical comparison between the effects of flashing illumination and pneumatic mixing on the culture growth has not been reported in literature. A thorough comparison of the two methods requires a robust numerical tool which integrates the computation of the fluid flow and the light field, as well as the growth kinetics of algal cells. In the present work, we compute the three-dimensional flow field in a bubble column photobioreactor tracking also micrometer particles in order to simulate the movement of algae. The spectral light field is computed by solving the three-dimensional Radiation Transfer Equation (RTE) at different wavelengths and biomass concentrations. The coupling of the flow and light fields enables the computation of the spatio-temporal light exposure of individual algae cells, which is used to estimate the respective dynamic photosynthesis reaction rates under different operating conditions of the bubble column reactor. We found, numerically, that the contribution of pneumatic mixing alone is negligible in comparison to flashing-light effects, for the investigated operating conditions. In contrast, illumination with flashing LED leads to an increase of the growth rate up to a factor of 2.5 at flashing frequencies higher than 50 Hz in a PBR with industrially relevant operating conditions. Thereby, a proper selection of the duty cycle is not only needed to prevent photoinhibition but also to maximize the effects of flashing light at the same time. According to these results, the utilization of flashing LED sources can provide a mostly unused way to improve photobioreactor productivity in the future.
关键词： Mixing,Photobioreactor,Flashing light,Numerical simulation,Bubble column
更新于2025-09-23 15:22:29
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[IEEE 2018 IEEE 38th International Conference on Electronics and Nanotechnology (ELNANO) - Kiev (2018.4.24-2018.4.26)] 2018 IEEE 38th International Conference on Electronics and Nanotechnology (ELNANO) - Simulated Phantom Projections for Reconstruction Quality Control in Digital Tomosynthesis
摘要： IMASIM software was used to generate sets of X-ray projections for quality control in digital tomosynthesis. Corresponding projections of a polymethylmethacrylate(PMMA) phantom with the same size were taken using x-ray tomosynthesis imaging equipment. A specially designed fiduciary marker bead phantom was used to measure the relative positions of imaging system elements. A good correspondence was found between real and simulated projections and reconstructed slices. The ability to create simulated projections with predefined imperfections gives valuable benefits to analysis of overall tomosynthesis performance.
关键词： X-ray applications,digital X-ray tomosynthesis,quality control,biomedical imaging,numerical simulation
更新于2025-09-23 15:22:29
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Effect of crucible and crystal rotations on the solute distribution in large size sapphire crystals during Czochralski growth
摘要： In this study, the ?ow, temperature and solute concentration ?elds in the melt during the CZ growth process are numerically investigated. The results show that the magnitude and distribution of the solute concentration in the melt is strongly affected by the convective ?ow and thermal distribution. The maximum solute concentration always occurs at the crucible sidewall where the maximum temperature in the melt is found and the solute concentration at the crystal-melt interface increases from the triple point to the centerline. Heat transport from the side crucible wall towards the crystal-melt interface is enhanced by the crucible rotation. The level of the solute concentration inside the melt is reduced due to the lowering of the maximum temperature at the crucible wall. As a consequence, the distribution of the solute concentration along the crystal-melt interface becomes smaller and more uniform as the crucible rotation rate increases. However, after the crucible rotation rate becomes large enough, the maximum solute concentration and the solute concentration along the crystal-melt interface start to increase. Heat transport inside the melt is also affected by the crystal rotation. The centrifugal force induced by the crystal rotation generates a vortex below the crystal-melt interface. This vortex gets larger and stronger as the crystal rotation rate increases. In the smaller crystal rotation rate regime, this vortex is very small, suppressing the solute concentration at the crystal-melt interface. Therefore, the solute concentration along the crystal-melt interface becomes less when the crystal rotation rate is higher, although there is an increase in the maximum solute concentration in the melt due to the higher maximum temperature. In the higher crystal rotation rate regime, there is a reduction in the convexity of the crystal-melt interface due to enhancement of heat transport from the bottom wall of the crucible by vortex motion under the crystal-melt interface. Therefore, there is a switch to an increase in the transport of solute impurities into the crystal-melt interface. Hence, the solute concentration along the crystal-melt interface increases as the crystal rotation rate increases. However, with a further increase in the crystal rotation rate, as the shape of the crystal-melt interface changes becoming concave towards the melt, the solute concentration along the crystal-melt interface decreases because the maximum temperature is signi?cantly reduced. In this study, both counter- and iso-rotations are considered. The results of a comparison of the cases of iso- and counter-crystal rotation show that the lowest and most uniform solute distribution along the crystal-melt interface is achieved when there is no crystal rotation and the crucible rotation rate is ?xed at 1 rpm. In other words, the lowest and most uniform solute concentration can be achieved with the only crucible rotation.
关键词： Solute concentration,Czochralski method,Sapphire,Numerical simulation,Crucible and crystal rotations
更新于2025-09-23 15:21:21
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Design guidelines for high sensitivity ZnO nanowire gas sensors with Schottky contact
摘要： Zinc oxide nanowire (ZnO NW) gas sensor with single Schottky contact is capable of sensitive detection of gas molecules. In this study, we investigate the effect of design factors such as nanowire defect density, diameter, and length on the gas sensitivity using 3-D numerical simulation. The sensor with lower defect density or smaller NW diameter exhibits improved gas sensitivity, while length does not have an impact when not considering the external environment such as background gases and binding probability. Lower defect density causes low electron density within the NW in air environment, and the change in electron density due to gas adsorption is intensified, thus improving gas sensitivity. As the NW diameter decreases, the change in the electrical conductivity due to gas molecules is greatly increased due to an increase in the ratio of the depletion area to the entire NW area. In contrast, the nanowire length does not impact the gas sensitivity because the change in the electron density is independent of the length. These results are helpful to understand the sensing mechanism and provide design guidelines to maximize the sensitivity.
关键词： Zinc oxide,Gas sensitivity,Nanowire,Numerical simulation,Gas sensor
更新于2025-09-23 15:21:21