In order to optimize steel flow and maximize the contact time of the inclusions with the slag layer inside the tundish, a proper flow-control arrangement must be designed, considering the shape, the dimensions of the prototype, and the plant operating conditions of the tundish. Physical and mathematical modeling has been used in this study, in a complementary fashion, to evaluate the influence of turbulence-inhibiting devices on the velocity fields, tracer dispersion, small-and large-particle trajectories, flow-pattern characteristics, and grade changes in a large-volume tundish. From the water model and mathematical simulation results, a flow-control system with the best performance was identified; this system must contribute to improving the productivity and cleanliness of the continuous-cast steel.
A water-physical model of a funnel-type thin slab mold fed by a two-port submerged entry nozzle (SEN) was employed to characterize the flow of liquid steel using dye tracer, particle image velocimetry, and video recording experiments. The overflow fluid flow pattern was the typical double-roll flow. A cyclic, low-frequency (%0.01 s À1 ), and energetic flow distortion of a short-lived (8 to 12 seconds) inducing high meniscus oscillation was identified. Its intensity grew with high casting speeds (5 and 7 m/min) and with a shallow SEN immersion position (200 mm from the meniscus level to the SEN tip). This distortion originated from the apparent existence of vortex flows located below the two discharging jets, which are formed by shear stresses in their ends that act on the surrounding fluid. These vortexes exert momentum transfer upward through a cascade mechanism from the lowest part of the discharging jets until reaching the region close to the SEN tip. This cascade momentum transfer widens the separation of both jets, enhancing the fluid velocities of the upper rolls, which promotes a high-amplitude standing wave. It is inferred, based on the experimental results, that this flow distortion originated from an instantaneous unbalance of the turbulent kinetic energy in the discharging jets. The negative production of kinetic energy is ascribed as the source of this unbalance, which is compensated by a higher contribution of the turbulent kinetic energy through mean convection and turbulent transport mechanisms manifested through higher velocities. After the restoration of the energy balance, the system yields a stable meniscus to repeat the low-frequency cycle.
A new design of a ladle shroud, obtained through water modeling, that controls turbulence of the entry jet in continuous casting tundishes is proposed. Particle Image Velocimetry (PIV) measurements indicate that this design decreases the impact velocity on the tundish bottom to close to 1/3 of that provided by a conventional ladle shroud. This achievement is due to a swirling jet that promotes a recirculatory flow in the horizontal planes of the tundish. The swirling effects help to dissipate the turbulence energy of the jet before it impacts the tundish bottom making possible decreases of fluid velocities that impact the back and front walls of the tundish. Turbulence models like k -e, k -w and RSM were applied to simulate the experimental PIV measurements of velocities in the fluid flow. Only the RSM model yielded results that agree remarkably well with the experimental determinations. These results make possible to avoid the employment of flow control devices such as dams, weirs, turbulence inhibitors and the like in tundishes.KEY WORDS: tundish; ladle shroud; turbulence dissipation; PIV; mathematical models. and a density of 1 030 kg/m 3 was fed through a syringe in the upper part of the ladle shroud. By following the tracks of these particles, using a Particle Image Velocimetry (PIV) equipment, velocity fields of water were recorded. Particle tracks were recorded by a Coupled Charged Device (CCD) equipped with a lens of depth field. All signals are processed in a PC; using the cross correlation technique and Fast Fourier Transforms they are converted into fluid velocities. [23][24][25] Figure 2 shows a scheme of the experimental setup. Two ladle shrouds were investigated the first one is of a conventional (LS) design and its dimensions are shown in Fig. 3(a) the second is a swirling ladle shroud (SLS) which consists of central pipe with three intermediate chambers that work as a brake of fluid velocity and an upper blade whose function is to start a swirling motion of the fluid all through the shroud length up to its tip. The tip of the shroud has bell shape to reinforce the braking effect on the fluid which flows into the tundish. Water flow rate was 5.83ϫ10Ϫ4 m 3 /s corresponding to 3.8 tons of liquid steel/min in the current tundish, according to the Froude criterion. This flow was maintained constant for this work. Theory of Turbulence ModelsThe most commonly used turbulence model is that devised by Jones and Launder 28) and known as k-e, it has many advantages; its concept is simple, is implemented in very commercial codes and it has demonstrated capability to simulate correctly many industrial processes like combustion, 27) fluid flow in tundishes 15,16) and multiphase flows 28) among many other applications. Nevertheless, it fails to provide reliable results of swirling flows and highly strained angular velocities of rotating flows.30) Since we are dealing here with a complex swirling flow two other turbulence models were tried; the k-w model of Wilcox 29) and the Reynolds Stress model (RSM) which ...
Fluid flow of water in a model of a slab caster has been simulated using the large eddy simulation (LES) computational approach, and the simulated results are compared with experimental measurements performed using digital particle image velocimetry (DPIV) techniques. Simulation results agree acceptably well with the experimental measurements of instantaneous velocity fields. Flow patterns change with time as a consequence of the vertical oscillation of the jet core. These oscillations are originated by the residual Reynolds stresses that characterize turbulent flows. The asymmetry of fluid flows caused by these stresses provides biased flows. Thus, turbulence originates natural biasing effects without the influence of other operating factors such as the slide gate opening, gas bubbling, or inclusions clogging of the submerged entry nozzle (SEN). Instantaneous velocities follow periodical behaviors with time whose frequencies increase with increases of flow rate of liquid. Periodical flow changes originate velocity spikes, at some given casting speed, which are physically and mathematically identified. These sudden changes of fluid velocities are responsible of unsteady phenomena associated with fluid dynamics during steady operations of the mold.
Two-phase flow in a water-air model of a continuous casting slab mold is studied using Particle Image Velocimetry technology. At low gas-loads (mass flow rate of gas/mass flow rate of liquid) fluid flow patterns of phases, gas and liquid, are different and with increases of this parameter both flow fields become similar. In the liquid phase, angles of the jet-root (in front of the SEN's ports) and jet core (main jet-body) are complex functions of the gas flow and casting rates. The first is decreased well below the angle of the SEN's port and the second is increased well above the same angle for all gas-loads. The jet-root angle increases, from small values, while the jet-core angle observes a maximum with the gas flow rate at any casting rate. The jet-core angle approaches to the angle of the SEN's port at high gas flow rates. Accumulation of bubbles is observed in the mold cavity when the casting rate is high at low or high flow rates of gas. Averaged bubble sizes depend on the coalescence-breakup kinetics, which vary with the gas-load. Liquid entrainment by gas to the flux is greatly increased with the casting rate even at low gas-loads. Further understanding of the two-phase flow dynamics should be attained in order to improve the boundary conditions of mathematical models.
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