| 1. |
- Bai, Haitong, 1986-
(author)
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A Study of the Swirling Flow Pattern when Using TurboSwirl in the Casting Process
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The use of a swirling flow can provide a more uniform velocity distribution and a calmer filling condition according to previous studies of both ingot and continuous casting processes of steel. However, the existing swirling flow generation methods developed in last decades all have some limitations. Recently, a new swirling flow generator, the TurboSwirl device, was proposed. In this work, the convergent nozzle was studied with different angles. The maximum wall shear stress can be reduced by changing the convergent angle between 40º and 60º to obtain a higher swirl intensity. Also, a lower maximum axial velocity can be obtained with a smaller convergent angle. Furthermore, the maximum axial velocity and wall shear stress can also be affected by moving the location of the vertical runner. A water model experiment was carried out to verify the simulation results of the effect of the convergent angle on the swirling flow pattern. The shape of the air-core vortex in the water model experiment could only be accurately simulated by using the Reynolds Stress Model (RSM). The simulation results were also validated by the measured radial velocity in the vertical runner by the ultrasonic velocity profiler (UVP). The TurboSwirl was reversed and connected to a traditional SEN to generate the swirling flow. The periodic characteristic of the swirling flow and asymmetry flow pattern were observed in both the simulated and measured results. The detached eddy simulation (DES) turbulence model was used to catch the time-dependent flow pattern and the predicted results agree well with measured axial and tangential velocities. This new design of the SEN with the reverse TurboSwirl could provide an almost equivalent strength of the swirling flow generated by an electromagnetic swirling flow generator. It can also reduce the downward axial velocities in the center of the SEN outlet and obtain a calmer meniscus and internal flow in the mold.
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| 2. |
- Liu, Yu, 1990-
(author)
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Thermodynamics and Kinetics in Metallurgical Processes - with a Special Focus on Bubble Dynamics
- 2020
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Doctoral thesis (other academic/artistic)abstract
- Gas stirring is commonly used in the steelmaking processes to reinforce chemical reactions, kinetic transfer, and inclusion removal, etc. This dissertation concentrates on multiphase flows with gas bubbling to study fluid dynamics and thermodynamics in metallurgical processes. A study of bubble behavior has been carried out using a multiscale approach as follows: Prototype scale (macro) → Plume scale → Single bubble scale → Reaction scale (micro).Initially, previous works on physical modeling and mathematical modeling in relation to the gas bubbling in the ladle have been reviewed. From that, several aspects that can be improved were found:For physical modeling, such as mixing and homogenization in ladles, the general empirical rules have not been analyzed sufficiently;The mathematical models focusing on inclusion behaviors at the steel-slag interface need to be improved;The phenomena governing the transfer of elements, vacuum degassing, and the combination of fluid dynamics and thermodynamics, such as in desulfurization, need to be investigated further.The kinetics transfer with regards to temperature and element homogenization is one of the most extensive research fields in steel metallurgy. For the analysis on prototype scale, the optimal plug configuration has been studied for a 50t ladle. For stirring using bottom-blowing, a separation angle between dual plugs of 160 degree is mostly recommended, and the optimal dual-plug radial position is around 0.65R. Moreover, the influence of the tracer’s natural convection on its homogenization pattern cannot be neglected, especially for ‘soft bubbling’ conditions using low gas flow rates.Subsequently, in studies of the statistical behavior of gas bubbling in the plume, mathematical modeling using an Euler-Euler approach and an Euler-Lagrange approach have been compared. With respect to the bubble coalescence and breakup, the Euler-Lagrange approach is more accurate in predicting the flow pattern for gas injection using a porous plug. With regards to the effect of plug design on the statistical behavior of gas bubbling, gas injection using a slot plug promotes kinetic reactions close to the open eye due to the concentrated plume structure, and gas bubbling using a porous plug promotes a good inclusion removal because of the increased amount of bubbles.Focusing on single bubble behavior, under the same flow rate, as the top gauge pressure is reduced, the bubble diameter increases and the bubble generation frequency decreases. During the bubble ascent, a large bubble gradually reaches stable conditions by means of shedding several small bubbles. In a steel-argon system, under a flow rate in the range of 5.0(mL‧min-1)STP to 2000(mL‧min-1)STP, the bubble diameter is in the range of 6.0mm to 20.0mm. Under laminar conditions, the maximum bubble width is 65mm when the surrounding pressure is 0.2bar, and the steady bubble width is around 58mm under a pressure of 2.0bar.Finally, a coupling method, named Multi-zone Reaction Model, has been developed to predict the conditions in the EAF refining process. Using a combined injection of O2 and argon, and the same injected mass of O2, the decarburization rate increases due to an efficient kinetic mass transfer of carbon in the molten steel. Furthermore, using CO2 to replace argon, as the ratio of the CO2 content in the injection increases, the maximum hot spot temperature, the increment rate of average temperature, and the decarburization rate decrease dramatically.The research step from multiphase fluid dynamics to its coupling with high temperature thermodynamics is a large advancement in this study. Moreover, the research process using open source software to replace the commercial software is also an important technical route. This can help the transparent development of future modules for reacting flow in metallurgical processes.
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| 3. |
- Xu, Yonggui
(author)
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A Study of Bubble and Inclusion Behaviors in a Liquid Steel Bath
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The bubble formation processes in a water model experiment were measured by a high-speed camera. Nozzle diameters of 0.5 mm, 1 mm and 2 mm were investigated under both wetting and non-wetting conditions. The bubble sizes and formation frequencies as well as the bubbling regimes were identified for each nozzle size and for different wettabilities. The results show that the upper limits of the bubbling regime were 7.35 L/h, 12.05 L/h and 15.22 L/h under wetting conditions for the 0.5 mm, 1 mm and 2 mm nozzle diameters, respectively. Meanwhile, the limits were 12.66 L/h, 13.64 L/h and 15.33 L/h for the non-wetting conditions. In addition to experiments, numerical modeling was performed. The Volume-of-Fluid (VOF) method was used to track the interface between the gas and liquid. The simulation results were compared to experimental observations from an air-water system. The comparisons show a satisfactory good agreement between the two methods (maximum difference is 0.029 s during a bubble formation period). Simulations from the argon-steel system show that the effect of the nozzle size on the bubble formation is insignificant for the current studied metallurgical conditions. The upper limits of the bubbling regime were approximate 60 L/h and 80 L/h for a 2 mm nozzle for wetting and non-wetting conditions, respectively. In addition, a poor wettability leads to a bigger bubble size and a lower frequency compared to a good wettability, for the same gas flow rate. The fundamental aspects of rising argon bubbles in molten metal flow were investigated by numerical simulations. The results show that 3~10 mm bubbles rise in a spiral way with strong instabilities which cause them to change their instantaneous shapes. In addition, 10~20 mm bubbles rise rectilinearly and their shapes are kept almost steady. All these bubbles’ terminal velocities are around 0.3 m/s, which are in accordance with literature data. The simulation results of bubble bursting at the liquid surface show that when the surface tension is 1.4 N/m, the critical bubble size is 9.3 mm. Also, the ejection is found to increase with an increased surface tension value, unless a critical bubble size is reached. The single bubble passage through the liquid-liquid interface was numerically simulated. The calculation results show that the passing patterns at the steel-slag interface are oscillation-pass, oscillation-breakup, oscillations-pass, pass and pass-breakup for the 3, 5, 7, 10 and 15 (20) mm bubbles, respectively. For a 5 mm bubble, it was found that an increase of the interfacial tension from 0.04 to 0.8 N/m results in a delayed bubble passage time. The results also show that the bubble experiences an oscillations-breakup process if the interfacial tension value is up to 1.15 N/m. However, a higher interfacial value (1.8 N/m) can make the bubble pass again but with a longer passage time. The inclusion removal mechanism due to a bubble wake flow was studied using a water model and using a three dimensional numerical model. The individual particle motion was tracked by the discrete phase model (DPM). The average bubble velocity and particle velocity from simulation are of 2.5% and 28.9% differences compared to the water model experimental results. The predictions in an argon-steel-inclusion system show that the removal rate per bubble is increased with an increased bubble size. However, the inclusion removal rate per unit bubble volume can be improved by decreasing the bubble size. Also, the particle rising zone was found to be 1.625 and 5 times of the bubble size in width and height, respectively. It is also shown that the bigger inclusions are more easily removed compared to the smaller ones. The Euler-Euler two-phase simulation model was used to investigate gas stirring in a ladle. In addition, water model experiments were carried out to validate the predicted flow field by using the UVP velocity measurement method. The simulation results show a maximum 0.04 m/s difference in axial velocity compared to the experimental observations. The flow patterns under different gas flow rates were obtained. Simulations for 120 s without the stochastic turbulent motions show that more than 30% of the 400 μm light inclusions can be removed. Also, that the 1~100 μm light inclusions have their highest removal percent for a 3 m3·h-1 gas flow rate. The size (1~100 μm) of heavy inclusion shows little effect on the inclusion removal percent. The 400 μm heavy inclusions are harder to remove compared to the smaller inclusions when the gas flow rate is or higher than 2 m3·h-1. Also, the inclusion transport behavior with the stochastic turbulent motions shows that both light and heavy inclusions are removed by up to 94% with a 1 m3·h-1 gas flow rate after 40 s. When the gas flow rate is 3 m3·h-1, the inclusion removal percent can be improved from 77% to 90% after 20 s and it reaches to almost 99% after 40 s. A further increase of the gas flow rate reduces the inclusion removal percent slightly.
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| 4. |
- Yin, Jun, 1991-
(author)
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A study of mold flux entrapment and gas entrainment in an ingot casting process
- 2021
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Doctoral thesis (other academic/artistic)abstract
- The focus of this work is to study the gas entrainment and mold flux entrapments during the ingot filling process based on physical and numerical modelling.The formation of the free surface was investigated in the uphill teeming method, which illustrates a dynamic change at a quasi-steady state. The influence of several turbulent models on the result was studied. The Reynolds stress turbulence model predictions show a good agreement to the experimental result compared to predictions using k-ԑ based turbulence models. It was found that the Weber number at the free surface is smaller than 12.3, when the inlet velocity is 0.5 m·s-1. This indicates a minor chance for mold flux entrapments, based on previously reported results. In order to reach a calm free surface using an even high inlet velocity, the side teeming process is proposed. A rotational flow field was found to be generated in the side teeming process, due to the horizontal teeming of the molten steel. A vortex cannot be found in the side teeming process because of the weak strength of the swirling flow. However, surface disturbances can be seen close to the wall of the mold, but they are small and vanish at lower teeming velocities.An optimization of the filling angles in the side teeming process was studied to reduce and eliminate the surface disturbances at the edge of the mold. The result showed that the 90 degrees filling angle results in a calm free surface without surface disturbances. Therefore, this design is recommended to use in the ingot casting process.The trumpet was also studied in this work to better understand the gas entrainment phenomenon. Water model experiments were carried out to measure the gas entrainment rate during a quasi-steady. Numerical simulations were performed and the results showed a good agreement to the experimental results. The formation of a big bubble was observed at the bend, which is due to the generation of a low-pressure region. Finally, an angled runner design was constructed to ease the gas entrainment rate. The results showed that the 30-degree angled runner can result in less entrained gas in the horizontal runner and lower hump height compared to the traditional design.
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