Turbulent Fluid Flow Phenomena in a Water Model of an AOD System

Experimental measurements are reported regarding fluid flow and turbulence property measurements in a water model of an AOD vessel. Laser velocimetry was used to determine the time smoothed velocities, the turbulent kinetic energy, and the Reynolds stresses in the system; in addition, the rate of melting of immersed ice rods was also measured to determine the local heat transfer rates. The measurements have shown that for the model AOD studied both the velocity fields and the distribution of the turbulent kinetic energy were quite uniform; the absence of inactive or dead zones would render these systems ideal for mixing and for a range of ladle metallurgical operations. The rate at which immersed ice rods dissolved depended on both the local velocities and on the turbulence levels; a previously developed correlation could be employed to predict the appropriate heat transfer coefficients. Finally, the rate of turbulent energy dissipation per unit volume in real industrial AOD vessels was found to be much higher than in any other ladle metallurgy operations. This could raise interesting possibilities regarding the more widespread use of these systems for molten metals processing.     

Turbulent recirculating flow in induction furnaces: A comparison of measurements with predictions over a range of operating conditions

Experimental measurements and theoretical predictions are presented concerning the velocity fields, the maps of the turbulent kinetic energy, and the turbulent kinetic energy dissipation in an inductively stirred mercury pool. A single coil arrangement was used, and the frequencies examined ranged from 50 to 5000 Hz. A hot film anemometer and a direction probe were employed for characterizing the velocity fields. The theoretical predictions were based on the numerical solution of the turbulent Navier-Stokes equations. The technique of mutual inductances was employed to compute the magnetic field, while thek-ε model was used for calculating the turbulent viscosity. Overall, the theoretical predictions were in reasonable agreement with the measurements both regarding the velocities and the turbulence parameters. By presenting the results in a normalized, dimensionless form these findings were given a rather broader applicability than the actual numerical range explored. Formerly of the Department of Materials Science and Engineering at MIT     

The velocity field in the molten slag region of esr systems: a comparison of measurements in a model system with theoretical predictions

A comparison is presented between the experimentally measured velocity field in a room temperature model of an ESR system and theoretical predictions, obtained from the numerical solution of Maxwell’s equations and the turbulent Navier-Stokes equations. The experimental measurements were obtained in a horizontal trough, containing mercury, through which a current was being passedvia two electrodes. The velocity fields, which were measured, using a photographic technique were thought to model the electromagnetically driven component of the velocity field in the central plane of the slag phase in ESR systems. The agreement between the experimental measurements and the theoretical predictions is excellent, both regarding the absolute values of the velocities and the dependence of the velocity on the imposed current and on the electrode diameter. The calculations have shown that by the proper choice of the linear scale, and the current,. mercury may be used to model the electromagnetically driven flow in the slag phase of ESR systems. Furthermore, some general relationships have been developed showing the effect of the current on the velocity, the turbulence energy, and on the rate of turbulence energy dissipation. This work is thought to provide definite confirmation that the electromagnetically driven component of the velocity fields in ESR systems may be properly represented through the simultaneous solution of Maxwell’s equations and the turbulent NavierStokes equations.     

The measurement and prediction of the melt velocities in aturbulent,electromagnetically driven recirculating low melting alloy system

Experimental measurements are reported describing the velocity field in an inductively stirred low melting alloy. The molten metal was held in a cylindrical container, 500 mm high and having an inside diameter of 250 mm; induction stirring was supplied by a three phase coil, which provided a maximum field strength of 350 Gauss (0.035 Wb/m²). The velocities in the melt were measured by a mechanical force reaction probe and were found to range up to about 0.5 m/s. Theoretical prediction of the melt velocities was made by solving Maxwell’s equations, together with the turbulent Navier-Stokes equations, using a digital computer. The experimentally measured and theoretically predicted velocities were found to agree within about 30 pct, thus providing direct experimental proof for the validity of modelling electromagnetically driven flows using this technique.     

Experimental measurement and numerical computation of velocity and turbulence parameters in a heated liquid metal system

Experimental measurements are reported describing the velocity field and the turbulence parameters in molten Woods metal due to the passage of a DC current between two electrodes immersed into the melt. The measurements were made using a hot film anemometer. A mathematical model has been developed to represent these measurements; in essence, this relied on the solution of Maxwell’s equations to represent the electromagnetic force field and turbulent Navier-Stokes equations to represent the fluid flow field. Three different turbulence models were examined; two were variants of thek-ε model, while the third was mixing length model type. In general, the velocities were well predicted by all three of these models, but there were significant discrepancies as far as the turbulence parameters were concerned. In the paper, a new criterion was suggested to predict the onset of turbulence in systems of this type.     

Experimental measurement and numerical computation of velocity and turbulence parameters in a heated liquid metal system

Experimental measurements are reported describing the velocity field and the turbulence parameters in molten Woods metal due to the passage of a DC current between two electrodes immersed into the melt. The measurements were made using a hot film anemometer. A mathematical model has been developed to represent these measurements; in essence, this relied on the solution of Maxwell’s equations to represent the electromagnetic force field and turbulent Navier-Stokes equations to represent the fluid flow field. Three different turbulence models were examined; two were variants of thek-ε model, while the third was mixing length model type. In general, the velocities were well predicted by all three of these models, but there were significant discrepancies as far as the turbulence parameters were concerned. In the paper, a new criterion was suggested to predict the onset of turbulence in systems of this type.     

Mass transfer in a bottom blowing cold model converter

In a model converter containing oil and water the volumetric mass transfer of caprylic acid was determined in dependence of the gas flow rate. Several other parameters were varied such as the number and configuration of nozzles, gas velocity, oil viscosity and volume and model size. Partially the results could be described by dimensionless equations. Visual observations of the bath surface with special regard to the wave formation complete the results of the measurements. By knowledge of the drop size spectrum independently determined, the mass transfer coefficient under dispersion conditions could be evaluated. These values were compared with mass transfer coefficients which had been directly measured by the single rising drop technique as well as the bubble stirred interface without dispersion. It seems that turbulence in dispersed systems does not enlarge the mass transfer coefficient.     

Fluid velocities in induction melting furnaces: Part I. Theory and laboratory experiments

Fluid flow in an induction furnace due to electromagnetic stirring forces is predicted theoretically from furnace design parameters by the simultaneous solution of the Maxwell and Navier Stokes equations. Streamline plots and velocity profiles are obtained and compared with surface velocities measured experimentally. The measurements were made on a mercury pool stirred inductively by a Tocco 30 kW 3 kHz induction melting unit. The agreement between the experimental measurements and theoretical predictions was good considering that no curve fitting by manipulation of adjustable parameters was involved. It is believed that such a model would be of value in the design and development of induction furnaces.     

A comprehensive study of the induced current,the electromagnetic force field,and the velocity field in a complex electromagnetically driven flow system

A mathematical formulation is developed to describe the electromagnetic parameters and the velocity fields in an inductively stirred melt, both in the presence and the absence of shields. The theoretically predicted results are compred with experimental measurements pertaining to the induced current, the components of the magnetic flux, and the phase angle between the current and the magnetic flux. In addition, the melt velocity was also determined, using a Vives probe. The excellent agreement obtained between measurements and predictions regarding both the individual input parameters and secondary quantities, such as the electromagnetic force field and the velocity field, shows a full validation of the technique employed. It has also been demonstrated that the use of magnetic shields may be a useful way of modifying the flow patterns in these systems. J.L. MEYER, formerly a Visiting Scientist in the Department of Materials Science and Engineering, MIT N. EL-KADDAH, formerly with the Department of Materials Science and Engineering, MIT     

On the numerical calculation and non-dimensional representation of velocity fields in bubble-stirred ladle systems

The numerical simulation of axisymmetric gas stirred ladle systems has been considered and a mathematical model based on the dimensionless form of the turbulent Navier-Stokes equations developed. Embodying a simplified turbulence model in the calculation procedure, it is demonstrated from the first principles that non-dimensional velocity components at relatively low gas flow rates, follow identical distribution patterns at equivalent dimensionless bath depths and radii, provided values of the parameters L/R and Q/R^2.5 are similar for the various gas stirred systems being considered. The theoretical analysis has been substantiated through numerical experimentations and by considering a set of five independant experimental studies on liquid velocity measurements reported in the literature.     

Theoretical and experimental investigations on macrosegregation caused by mixing in the liquid bulk melt

An improved model describing the macrosegregation caused by mixing in the liquid bulk melt is presented. This model includes the mass transfer between the interdendritic liquid and the bulk melt which takes place additionally to the mass transfer via the boundary layer in front of the dendrite tips. Further, laboratory experiments on macrosegregation of iron-carbon alloys are described. The experiments were carried out in a unidirectional solidification device with 1,5 kg melts. During the solidification process the liquid melt was stirred by a ceramic blade stirrer with rotational speeds between 180 and 720 min^?1. The melts were supperheated above the liquidus temperature by amounts between 3 and 80 K. The solidification velocities were 2 to 6 mm/min. The experiments could satisfactorily be described by the model using an exchange factor α for the amount of additional mass transfer. The exchange factor increases with decreasing amounts of superheating and solidification velocity and with increasing stirring speed.     

Steel dissolution in quiescent and gas stirred Fe/C melts

The dissolution rates of commercial black iron rods in iron/carbon melts under isothermal conditions were measured. The effect of melt carbon content, temperature, natural convection, and gas stirred forced convection conditions were investigated. The experimental data under natural convection conditions (no external stirring) were fitted with a dimensionless correlation for vertical cylinders: Sh = 0.13(Gr . Sc)^0.34, representing mass transport control dominated by turbulent natural convection. Under bottom injection gas stirring conditions, it was found that the kinetic power input had little effect on the rod dissolution rates which were controlled by the total gas flow rate. Derived mass transport coefficients under gas stirring conditions were found to have the following dependence on the gas injection rates:k _m ∝Q ^0.21, wherek _m = mass transport coefficient andQ = gas flow rate. A comparison of the experimental results with previously measured mass transfer coefficients under forced convection conditions gave a plume velocity flow rate dependence ofU ∝Q ^0.3. A general discussion of gas stirring fluid dynamics and resulting mass transport effects is presented.     

Validation of a Three-Dimensional Numerical Code in the Simulation of Pseudo-Natural Meandering Flows

Validation of a three-dimensional finite volume code solving the Navier–Stokes equations with the standard k-ε turbulence model is conducted using a high quality and high spatial resolution data set. The data set was collected from a large-scale meandering channel with a self-formed fixed bed, and comprises detailed bed profiling and laser Doppler anemometer velocity measurements. Comparisons of the computed primary and secondary velocities are made with those observed and it is found that the lateral momentum transfer is generally under predicted. At the apices this results in the predicted position of the primary velocity maximum having a bias towards the channel center, compared to the position where it has been measured. Using a simplified two zone roughness distribution whereby a separate roughness height was prescribed for the channel center and channel sides relative to a single distributed roughness height, generally led to a slightly improved longitudinal velocity distribution; the higher velocities were located nearer to the outside of the bend. Improving both the free surface calculation and scheme for discretization of the convection terms led to no appreciable difference in the computed velocity distributions. A more detailed study involving turbulence measurements and bed form height distribution should discriminate whether using distributed roughness height is a precursor to using an anisotropic turbulence representation for the accurate prediction of three-dimensional river flows.     

Steady flow of liquid aluminum in a rectangular-vertical ingot mold,thermally or electromagnetically activated

Hydrodynamic and thermal fields were studied in liquid aluminum circulating in a specially built parallelepipedic ingot mold. Fluid flow could be produced either by using an electromagnetic linear motor or by natural convection effects. Velocity measurements were performed using a magnetodynamic probe during steady state flow experiments. A theoretical model of heat transfer and fluid flow was developed and used to solve simultaneously Navier-Stokes and energy balance equations. The comparison of theoretical and experimental results is satisfactory regarding general distribution of velocity and temperature. It gives a better understanding of the effects of fluid flow in the melt produced either by external stirring or by natural convection.     

Kinetics of solution of hydrogen in liquid iron,nickel, and copper containing dissolved oxygen and sulfur

An investigation of the effect of surface active oxygen and sulfur on the rate of hydrogen solution in inductively stirred liquid iron, nickel, and copper was made using a modified constant-volume Sieverts’ method. The overall and initial rates of hydrogen solution in liquid iron and nickel were found to decrease with increasing oxygen content in concentration ranges found in the commercial refining of these metals. The results were consistent with a change in the rate controlling step from mass transport of hydrogen atoms in the metal phase to mixed control of a surface chemical reaction and a diffusion controlled process in the melt. For liquid copper the overall rate of hydrogen solution decreased with increasing oxygen content, but the initial rate could not be determined. It was believed that condensation of water vapor and melt surface turbulence interfered with measurement of the initial rate of hydrogen solution in liquid copper. The addition of sulfur to liquid iron, nickel, and copper lowered the overall rate of hydrogen solution, but the effect was not as pronounced as that for oxygen. Formerly Graduate Student, The University of Michigan, is Research Engineer, Noranda Research Center, Pointe Claire, Quebec, Canada     

The Solution kinetics of zirconium in liquid steel

Zirconium cylinders were immersed into liquid steel and their mass-transfer rates were measured under static and dynamic conditions. The cylinders were suspended from a load cell, and the apparent weight of the dissolving specimen as well as temperatures at various locations in the samples were measured continuously during immersion and recorded with a data acquisition System. From the weight measurements, the mass-transfer rate was deduced. In the assimilation of zirconium in liquid steel two periods were identified, i.e. the steel shell period and the free dissolution period. Immediately upon immersion the steel shell period Starts and when this period ends the free dissolution period commences. During the steel shell period, a shell of frozen steel wraps around the cylinder following its initial immersion. When zirconium cylinders were immersed into liquid steel, a reaction was detected at the steel shell/zirconium interface. This reaction took place almost immediately after immersion. The intermetallic Compounds Fe₂Zr and FeZr₂ were identified as reaction products. The experimentally measured mass transfer rates during the free dissolution, under quiescent and inductively stirred steel baths, were very similar. Under the experimental conditions used, the rate Controlling step of zirconium dissolution in liquid steel is reaction controlled.     

Study of Fluid Flow and Mixing Behaviour of a Vacuum Degasser

In recent times the demand of ultra-low carbon steel (ULCS) with improved mechanical properties such as good ductility and good workability has been increased as it is used to produce cold-rolled steel sheets for automobiles. For producing ULCS efficiently, it is necessary to improve the productivity of the vacuum degassers such as RH, DH and tank degasser. Recently, it has been claimed that using a new process, called REDA (revolutionary degassing activator), one can achieve the carbon content below 10?ppm in less time. As such, REDA process has not been studied thoroughly in terms of fluid flow and mass transfer which is a necessary precursor to understand and design this process. Therefore, momentum and mass transfer of the process has been studied by solving momentum and species balance equations along with k?C?? turbulent model in two-dimension (2D) for REDA process. Similarly, computational fluid dynamic studies have been made in 2D for tank and RH degassers to compare them with REDA process. Computational results have been validated with published experimental and theoretical data. It is found that REDA process is the most efficient among all these processes in terms of mixing efficiency. Fluid flow phenomena have been studied in details for REDA process by varying gas flow rate, depth of immersed snorkel in the steel, diameter of the snorkel and change in vacuum pressure. It is found that design of snorkel affects the melt circulation in the bath significantly.     

Electromagnetic and fluid flow phenomena in a mercury model system of the electromagnetic casting of aluminum

A mathematical formulation has been developed to represent the electromagnetic force field and the velocity field in the melt for the electromagnetic casting of aluminum. The theoretical predictions based on fundamental considerations are compared with experimental measurements obtained on a physical model system. The measurements and predictions were found to be in good agreement, regarding both the velocity fields and the electromagnetic force fields. The principal conclusion emerging from this work is of critical importance in achieving the dual objective, that is providing a restraining force, while minimizing the melt velocity perpendicular to the free surface. The mathematical formulation presented in the paper provides the theoretical framework for quantitatively defining these conditions in terms of the coil and the shield parameters. J.L. Meyer, formerly with the Department of Materials Science and Engineering at Massachusetts Institute of Technology N. EL-KADDAH, formerly with the Department of Materials Science and Engineering at MIT     

Dissolution rates of coals and graphite in Fe-C-S melts in direct ironmaking: Influence of melt carbon and sulfur on carbon dissolution

Carbon dissolution from graphite and coals was investigated by using a carburizer cover technique in an induction furnace. The intent of the study was to investigate the influence of factors governing the rate of carbon dissolution from carbonaceous materials, especially coals, into Fe-C-S melts. The factors studied were the initial melt carbon and sulfur concentrations and the wettability between carbonaceous materials and the melt. It was found that graphite dissolves markedly faster than coal. The rate of carbon dissolution from graphite could be decreased by increasing the sulfur in the melt. Also, poor wetting could retard the rate of carbon dissolution by reducing the surface area for mass transfer. Carbon dissolution from graphite is controlled by mass transfer in the liquid boundary layer adjacent to the solid/liquid interface. The rate of carbon dissolution from coal is more sensitive to the molten iron composition. A higher initial melt carbon and sulfur content retards the rate of carbon dissolution from coal more significantly than from graphite. However, the rate constant of coal char dissolution does not show a strong dependence on the wettability. Carbon dissolution from coals is most likely governed by a mixed-control mechanism that includes liquid-side mass transfer. The mechanisms underlying the influence of bath sulfur on carbon dissolution from graphite and coals are discussed.     

Modeling of spray deposition: Measurements of particle size,gas velocity,particle velocity,and spray temperature in Gas-Atomized sprays

Spray deposition is a novel manufacturing process which is currently being developed for producing near-net-shape preforms. Spray deposition involves the creation of a spray of droplets by a gas atomizer and the consolidation of these droplets on a substrate to create a preform. In order to maximize the metallurgical benefits of spray deposition, a thorough characterization of momentum and heat transfer in the gas-atomized spray is required. The present paper describes measurements of particle size, gas velocity, particle velocity, and spray temperature in gasatomized Sn-Pb sprays. Measurements were performed on steady-state axisymmetric sprays which were generated using a close-coupled gas atomizer with Sn-5 wt p t Pb and Sn-38 wt p t Pb alloys, atomizer gas flow rates of 2.5 g/s (0.56 MPa) and 3.4 g/s (1.04 MPa), melt flow rates of 35 and 61 g/s, and atomizer-substrate distances of 180 and 360 mm. Gas velocities in the range to 4 m/s were measured using Pitot tube and laser Doppler anemometry (LDA). Droplet velocities in the range 3 to 32 m/s were determined from photographic streak-length measurements and LDA. Oil calorimetry of the spray enthalpy indicated that the spray temperature decreased with increasing axial distance from the gas atomizer, increasing gas flow rate, and decreasing melt flow rate. Formerly Doctoral Student, Department of Metallurgy and Science of Materials, Oxford University