Water Model Experiments Research Articles

Transient Fluid Flow Phenomena in a Gas Stirred Liquid Bath with Top Oil Layer—Approach by Numerical Simulation and Water Model Experiments

The flow characteristics in a gas stirred ladle with oil layer were investigated with the help of water model experiments and numerical simulation. The oil layer has a great influence on the fluid flow and mixing behavior in the ladle. While the mixing time decreased with the increase of the gas flow rates, the oil layer over the top of the bath extended the mixing time in the whole range of gas flow rates, and at constant gas flow rate, the mixing time was extended with the increase of oil thickness. From the results of water model experiments and numerical simulations, transient formation of plume eye from the start of gas bubbling was matched well each other. Based on water model experiment the plume eye size was found to increase with the increase of gas flow rates and to decrease with the increase of the oil thickness. These were precisely confirmed with numerical simulated results. From the results of numerical simulation flow pattern without oil layer showed that bubbles rising eventually made a recirculation loop at the central area of the bath forming uniformly distributed velocity vectors in the bath. This flow pattern regarded as a good flow pattern for the better mixing behavior. However, flow pattern with oil layer showed distorted and localized recirculating loop near side wall below oil layer. This eventually gave extended mixing time in the bath with oil layer.

連続鋳造モールド内浸漬ノズル吐出口近傍におけるモールドパウダー巻き込みに関する水モデル実験

連続鋳造モールド内浸漬ノズル吐出口近傍におけるモールドパウダー巻き込みに関する水モデル実験

The Effect of Operating Parameters and Dimensions of the RH System on Melt Circulation Using Numerical Calculations.

Melt circulation as a function of the operating conditions and dimensions of the RH system were calculated using a 3-dimensional numerical model. This numerical model was developed with considering the driving force by gas blowing for melt circulation and was verified from both water model experiments and real RH process results. Using the numerical model, the influences of various parameters on melt circulation were investigated. The parameters can be divided into two categories, the operating conditions and dimensions of RH system. The effects of each operating parameters such as gas flow rate, the pressure of the vacuum vessel and submerged depth of the snorkel on the melt circulation rate were studied. In addition, the effects of the dimension parameters of the RH system such as the number of nozzles and snorkel diameter were also investigated. By synthesizing the calculated melt circulation for various conditions, a simple model, which can predict the melt circulation without calculating the three dimensional fluid flow, was developed. Using this prediction model, the effects of the various parameters on melt circulation could be calculated easily.

Mathematical Modeling of Iron and Steel Making Processes. Particle-Size-Grouping Method of lnclusion Agglomeration and its Application to Water Model Experiments.

Agglomeration of inclusions in liquid steel causes not only the enhancement of inclusion removal by flotation but also the increase in the number of large inclusions in final products. To clarify agglomeration behavior theoretically, a lot of studies have been made until now. However, the behavior is not clearly understood yet. In this study, a new particle-size-grouping (PSG) method has been established, which enables a simple calculation of the agglomeration by a small number of size groups with complete conservation in total particle volume. This method has been verified by the comparison with the exact solution of a revised population-balance equation. An experimental study of the agglomeration of polyvinyl-toluene latex (PVTL) in a stirred electrolyte solution has been made in an agitated vessel under a rapid agglomeration condition. An effective Hamaker constant of PVTL in water, A131, has been obtained by adjusting the measured agglomeration curve with the curve calculated by the PSG method. Good agreement has been obtained between observed and calculated agglomeration curves for A131=0.8×10-20J under a wide range of initial particle concentrations and agitation speeds. Numerical simulations of the fluid flow and particle transport in the vessel have been made to confirm the applicability of the PSG method. Computed agglomeration curves agree well with the theoretical curve if the energy dissipation rate averaged with the residence time of liquid in computational cells is used to calculate the dimensionless agglomeration time.

Mathematical Model of Hot Metal Desulphurization by Powder Injection

A macro kinetic analysis of the hot metal desulphurization with powder injection has led to that there are three basic parameters which determine the desulphurization rate, as well as the power utilization efficiency. They are the penetration ratio (PR) of injected powder into the melt, the residence time distribution (RTD) of injected powder in the bath, as well as the homogeneous mixing time (HMT) of the bath. Both PR and HMT have been experimentally determined with water modeling, while the RTD has been determined from either flow field information or experimental measurements of water model with stimulus-response method, arriving at the correlation formulas between the parameter and the injection operation variables as well as powder characteristics. This has enabled the development of a mathematical model to predict the desulphurization process.Based on further analysis of the mechanism and macro kinetics of the process, a mathematical model has been developed, with the three basic parameters being taken into account, to simulate the variation of [S] in hot metal with time. Water-model experiment results, flow field information and industrial data are used to determine the model parameters and verify the model, leading to the result that the prediction from the model agrees well with practical results. In addition to CaO and CaC2 powders, the use of granular Mg has been particularly included and emphasized in the model for its more and more popular application. The coupling effect of hot metal desulphurization by injection of magnesium-CaC2 mixture was discussed.

Fluid flow and mixing behavior in gas stirred ladle with submerged lance

A numerical study for analyzing both fluid flow and mixing behavior in gas-stirred ladles with a submerged lance was performed. At first, the shape and volume of the plumes generated by the submerged nozzle were calculated. Subsequently, the fluid flow driven by these plumes, and the mass transfer in the ladle were calculated by a three dimensional turbulent simulation program. Water model experiments for the velocity measurements were performed to verify the accuracy of the calculation results. It was shown that as the gas flow rate increases, the downward velocity at the ladle wall increases and mixing time decreases. Mixing time is sensitive to the alloy addition position especially at a lower gas flow rate. The result suggested that the alloy should be added at the plume zone close to the center at the melt surface

Development of an interactive simulation system for die cavity fillingand its application to the operation of a low-pressure castingprocess

The purpose of this study is to develop an interactive die filling simulationsystem to assist in the design and operation of a low-pressure castingprocess. One of the keys to the success for the design and operation of thelow-pressure casting process is to obtain an optimal pressurization curve inorder to avoid flow related defects while maintaining productivity.The simulation system developed is based on the incorporation of acomputational fluid dynamics technique, named SOLA-MAC, which has thecapability of treating transient fluid flow problems with free surfaces. Theanalytical system with the necessary pre-processing and post-processingmodules is loaded on a personal computer. The accuracy and reliability of thefilling simulation system is then verified by water model experiments, whichare also conducted in this study. The capability of directly predicting flowrelated defects such as gas/dross entrapment during the transient fillingstage by using an air particle method is added to the simulation system. It isalso made interactive by allowing the casting engineer to view the transientfilling pattern and the corresponding gas entrapment situation on the computerscreen, interrupt the execution of the program, change the casting operationand restart the program from a previous instant. A process design algorithm isalso proposed in this study, such that the casting is divided into severalparts depending on their geometric characteristics and performancerequirements. By taking advantage of the interactive capability of the system,the proper gate velocity to fill each and every part of the cast can bedetermined. In turn, the necessary pressurization curve can be determined.The developed system is tested on an automotive wheel cast made by alow-pressure casting process. The casting is divided into three distinctiveparts. The proper gate velocity to fill each part is interactively determinedby directly examining the filling pattern and its resultant gas entrapmentcondition. With the optimal combination of the gate velocity, thepressurization curve for the whole filling stage can be obtained. It is foundthat the pressurization curve for the variable gate velocity shows only smalldeviation from that of employing a constant gate velocity throughout the wholefilling stage. It is consistent with the general knowledge in the low-pressurecasting industry: that the casting quality is rather sensitive to thepressurization curve employed in the process.

Mathematical Modeling of Fluid Flow Phenomena during Tundish Filling and Subsequent Initial Casting Operation in Steel Continuous Casting Process.

The purpose of this study is to develop a mathematical model to analyze the fluid flow phenomena of molten steel in the tundish during its filling stage and subsequent initial casting operation in the continuous casting process of steel. The ultimate goal is to assure smooth initial casting operation without nozzle clogging by avoiding high deposition rate of inclusion on any of the tundish outlets during tundish filling and subsequent initial casting operation. The mathematical model is developed based on a computational fluid dynamics technique, named SOLA-MAC, and the k-e two-equation turbulence model. SOLA-MAC technique has the ability to handle the flow problem encountered in tundish filling, which is a transient flow problem with highly distorted free surfaces and the locations of the free surfaces are to be determined by theory. A fluid particle method is also employed in this study to analyze the distribution of inclusions in the molten steel and the extent of inclusion contamination in various strands of the continuous casting tundish. A water model that is one-fourth the scale of an actual billet continuous caster is also constructed in this study. Water model experiments are conducted to verify the accuracy and reliability of the mathematical model. The developed model is first tested on the water model to calculate the flow pattern of water in the tundish during the very early stage of filling operation. The simulated filling patterns are compared to the water model experiments. Good consistency is observed. The model is then tested on an actual billet continuous caster with four strands to simulate the fluid flow phenomena of molten steel in the tundish during the filling and subsequent initial casting operations. Inclusion distribution and the extents of inclusion contamination among the outlets of the various strands in the tundish are also analyzed. The simulated results show that for the left half of tundish, inclusion contamination in #2 strand is significantly more severe then that in #1 strand. This is confirmed by the actual experience on the shop floor of that particular billet caster that #2 strand experiences more difficulty in clogging problem during the initial casting operation than #1 strand does.

Investigation of transient fluid flow and heat transfer in a continuous casting tundish by numerical analysis verified with nonisothermal water model experiments

The tundish process is complicated due to the periodic nature of its operation during the changing of ladles, which makes it both a transient and nonisothermal process. A nonisothermal water model, with temperature variations in the inlet stream, is described in this article. A three-dimensional (3-D) transient mathematical model to simulate the fluid flow and heat transfer in this water model is also introduced. From the experimental and numerical results, it is shown that there is a significant thermal buoyancy force contributing to the flow in this nonisothermal water model.

Effects of surface flow control on fluid flow phenomena and mixing time in a bottom blown bath

The intensity of mixing in a molten metal bath stirred by bottom gas injection can be represented by the mixing time. According to previous water model experiments, the mixing time is known to be dependent on the operational variables such as the bath diameter, bath depth, location of a bottom nozzle, and gas flow rate. It is not easy to control the former three variables during processing, and the dependence of the mixing time on the gas flow rate is weak. In this study, the possibility of changing the mixing time drastically due to the control of the surface flow in the bath is examined. Three kinds of boundary conditions were imposed on the bath surface, and the relation between the fluid flow phenomena resulting from the surface flow control and the mixing time was investigated. The mixing time was found to be significantly influenced by the surface flow control. In particular, when the surface flow was suppressed by bringing a circular cylinder into contact with the bath surface, the mixing time became very long.

Water Model Experiments on Mixing Phenomena in a VOD Ladle.

Mixing phenomena in a VOD ladle were characterized by a watermodel simulation from the determination of mixing times by electrical conductivity measurements. Two mixing times defined for two degrees of mixing, viz., t95 (95 % mixing) and t99 (99 % mixing) were identified for each experiment. Various operating conditions as well as different placements of bottom nozzle with and without a top jet were simulated. Irrespective of whether the top blow is present or not, mixing rates were found to be increasing considerably as the nozzle position is moved from the center to the mid-radius position. The presence of a top jet, as in a VOD ladle, was found to weaken mixing for both a coaxial and non coaxial placement of the lance with the bubble plume. The results of watermodel experiments have been used to construct a simple two-tank model of mixing inside the actual ladle, in which the parameters are obtained as functions of process variables.

Influence of Wettability on the Behavior of Argon Bubbles and Fluid Flow inside the Nozzle and Mold.

It is well known that the occurrence of the defects has a close relation with the behavior of the bubbles and the flow pattern of the molten steel inside the mold. In this research, the behavior of argon gas inside the flowing water and the influence of wettability between the water and the porous refractory through which argon gas is injected were investigated. In addition, the influence of the wettability on the behavior of injected gas in molten iron was investigated experimentally. Furthermore, a water model simulating the submerged entry nozzle and the mold of the continuous casting process was established. The behavior of argon gas that was injected into water flow through a porous refractory in nozzle and its influence on the flow pattern of the water inside the mold were studied in the cases of different wettability between water and the porous refractory. Water model experiment demonstrated quite different behaviors of the bubbles and flow pattern of water when the wettability was changed. The result in the case of poor wettability is thought to be similar to those of the continuous casting process in which molten steel does not wet the refractory.

Modeling studies of fluid flow below flash-smelting burners including transient behavior

Water-model experiments were carried out on 1:14-scale models of venturi, distributor, and jet-flow burners to ascertain flow patterns at varying Reynolds numbers (60,000 to 507,000) using time-lapse streak photography and video streak photography. Digital particle image velocimetry (DPIV) was used to determine the axial and radial velocities and to estimate the turbulence kinetic-energy field beneath the distributor burner. In the DPIV experiments, a temporal instability in the main jet exiting from the burner occurred at a Reynolds number=104,000, a Strouhal number≈3×10−3, and a large expansion ratio (shaft/burner-diameter ratio=10). The main jet usually pointed away from the burner inlet but was also observed to fluctuate and precess in a quasi-random fashion. Recom-mendations are made for improving flash-smelting burner performance by promoting conditions to eliminate precessing. The use of higher Reynolds numbers was recommended to improve both the use of shaft volume and the mixing of the concentrate particles and gas stream. A three-dimensional (3-D) mathematical model was used to simulate the water flow through the distributor burner, shaft, and settler. The predicted velocity field consisted of a main jet pointing away from the burner inlet and a large recirculation zone in the center of the shaft. The predicted and measured velocity magnitudes compared well in the recirculation zone, but the steady-state mathematical model predicted higher velocity values in the main jet than were experimentally determined.

A numerical simulation for mini-mill Tundish design —effects of FC pad on tundish flow characteristics

A numerical analysis of fluid flow in a mini-mill continuous casting tundish has been carried out. The major concern has been focussed on validating the possibility of the use of an FC (Flow Control) pad under the ladle shroud in the tundish. For this purpose, steady state modeling to find flow patterns and non-steady state approaches calculating residence time distribution (RTD) curves for different tundish configurations have been chosen in the present study. Both calculated flow fields and predictions of tracer dispersion have been compared with the results of water model experiments.

Development of a Laser Optical Sensor for Measuring Bubble Characteristics in an Oily Liquid Bath.

Oily liquids such as silicone oil and normal pentane have been commonly used as the model for top slag to investigate the role of the top slag on the mixing in the bath of various refining processes. An electroresistivity probe is a powerful tool for measuring bubble characteristics in a bath filled with a liquid of high electrical conductivity and it is extensively used in water model experiments. Unfortunately, the electrical conductivity of oily liquids is very low, and accordingly, the electroresistivity probe looses its validity in oily liquid baths. Considering these circumstances, a two-channel laser optical sensor was developed to measure the bubble characteristics in oily liquid baths. Gas holdup, mean bubble rising velocity and mean bubble chord length were measured accurately, whereas measurement of bubble frequency was disturbed by Doppler signals originating from the bubble surface.

Removal of Inclusion through Bubble Curtain Created by Swirl Motion in Submerged Entry Nozzle.

In the steel-making processes, it is well known that suspended non-metallic inclusions can be removed by Ar gas bubbles injected in the steel baths. This study aims to remove the inclusions through a bubble accumulated zone (namely, bubble curtain) created by swirling motion imposed on the flow in a submerged entry nozzle. The results obtained through water model experiments and numerical calculations (Eulerian/Lagragian, Eulerian/Eulerian method) show as follows; the swirling motion works effectively to form a bubble curtain through that all the flow pass, and the half vertical angle of the bubble curtain increases with the increasing swirl strength. On the other hand, inclusions pass nearly parallel to the axis of the submerged entry nozzle, the trajectories of them being similar to the liquid-phase flow pattern. Therefore, a chance for a bubble and an inclusion to meet with each other would significantly increase.

Bubble formation from nonwetted slot nozzles

Experiments examining gas injection and bubble formation from nonwetted slot nozzles are presented in this work. Gas was injected into water and mercury. Slot nozzles used for the water model experiments were made of Teflon plates which are not wetted by water. The slot lengths were 50 mm and 200 mm, and the slot widths were between 0.05 mm and 1 mm. For mercury model experiments, a slot nozzle was made of acrylic resin. The slot length and width were 80 mm and 0.05 mm, respectively. The size of the bubbles and the number of bubble sources were measured from pictures taken with a high speed video camera. It was found that the bubbles formed at a series of sources distributed along the slot length where each source acted like a nonwetted orifice. The number of bubble sources were less than observed previously for gas injection through welted slot nozzles. The source spacing was attributed to RayleighTaylor instability. The measured bubble size was explained well by a simple dynamic model for a point source orifice. The line of bubble formation sites along the slot can generate a wide bubble plume which may offer high efficiency for metal refining processes.

Water Model Study on Convection Pattern of Molten Steel Flow in Continuous Casting Tundish.

Natural convection will obviously influence the liquid steel flow in continuous casting (CC) tundish, which cannot be neglected in tundish metallurgical process. Through the theoretical analysis, the dimensionless number, Gr/Re 2 , is adopted to determine the convection pattern in tundish system. Validity of this criteria in a reduced scale water model are also discussed in this paper. Non-isothermal water model experiment with the temperature variation of inlet stream, has been simulated to see the effect of temperature variation of ladle stream. The convection pattern of molten steel flow in continuous casting tundish is also studied numerically by using the commercialized computational fluid dynamic (CFD) software package, CFX4. From the results of theoretical analysis, physical and mathematical model simulation, it can be stated that the convection pattern of molten steel flow in tundish is controlled by the combined nature convection and forced convection.

Study of the fluid flow in the mould for high quality slab casting

Using a combination of water model experiments, numerical simulations and plant measurements, the flow of argon and steel in the mould of a continuous caster is investigated. Insight is gained in the influence of argon injection on the flow patterns in the mould and in the way in which the flow of argon and steel determines slab quality.

Model study of turbulence structure in a bottom blown bath with top slag using conditional sampling

Water model experiments were carried out to study the effect of top slag on the turbulence structure in a molten steel bath agitated by bottom gas injection. Water and silicone oil were used as models of molten steel and slag, respectively. Air was injected through a single-hole bottom nozzle so that the reverse emulsification of the silicone oil occurred at the silicone oil-water interface. Silicone oil droplets thus generated were carried deeply into the lower water layer. Turbulence measurements were made using a two-channel laser Doppler velocimeter (LDV) in the presence and absence of the top oil layer. A conditional sampling method called the four-quadrant classification method was applied to detect large scale coherent motions in the vertical bubbling jet as well as in the recirculation region. The structure and intensity of turbulence inside the bubbling jet were strongly affected by bubbles. In the absence of the top oil layer, higher momentum fluid motions directed from the centerline of the bubbling jet to the sidewall of the vessel were mainly responsible for the turbulence production in the bubbling jet, while lower momentum fluid motions directed from the sidewall toward the centerline governed the turbulence production in the recirculation region. On the other hand, in the presence of the top oil layer, the coherent motion in the bubbling jet was also affected slightly by the top layer except for the center of the bubbling jet, whereas any distinguished coherent motion was not observed in the recirculation region, and hence, turbulence production was weak there.