Mixing and solid-liquid mass-transfer rates in a creusot-loire uddeholm vessel: A water model case study

The process of mixing and solid-liquid mass transfer in a one-fifth scale water model of a 100-ton Creusot-Loire Uddeholm (CLU) converter was investigated. The modified Froude number was used to relate gas flow rates between the model and its protoype. The influences of gas flow rate between 0.010 and 0.018 m³/s and bath height from 0.50 to 0.70 m on mixing time were examined. The results indicated that mixing time decreased with increasing gas flow rate and increased with increasing bath height. The mixing time results were evaluated in terms of specific energy input and the following correlation was proposed for estimating mixing times in the model CLU converter: T _mix=1.08Q ^−1.05 W ^0.35, where Q (m³/s) is the gas flow rate and W (tons) is the model bath weight. Solid-liquid mass-transfer rates from benzoic acid specimens immersed in the gas-agitated liquid phase were assessed by a weight loss measurement technique. The calculated mass-transfer coefficients were highest at the bath surface reaching a value of 6.40 × 10⁻⁵ m/s in the sprout region. Mass-transfer coefficients and turbulence parameters decreased with depth, reaching minimum values at the bottom of the vessel.     

Water model study of the frequency of bubble formation under reduced and elevated pressures

Air was injected vertically upward into a water bath through a bottom nozzle or a bottom orifice. The surface pressure was reduced or elevated from an atmospheric pressure in order to change the hydrostatic pressure around the nozzle and orifice. The gas delivery system was designed so that bubbles were generated in the middle and high gas flow rate regimes under a constant flow condition. The frequency of bubble formation, f _B , decreased as the surface pressure, P _s , decreased when the volumetric gas flow rate, Q _g , was kept constant. The measured f _B values were predicted satisfactorily by an empirical equation proposed previously by the present authors. This equation was derived originally to correlate the frequency of bubble formation both in aqueous and molten metal systems under an atmospheric surface pressure. The effect of surface pressure on the frequency of bubble formation was considered in terms of the density of gas, ρ _g , and the volumetric gas flow rate Q _g in the aforementioned empirical equation. These two quantities, ρ _g and Q _g , were evaluated at the nozzle exit by using the hydrostatic pressure there.     

Reduction of Fe1-xO in liquid state by H2+Ar mixtures

The investigations of reducing Fe_1-xO in liquid state with H₂+Ar mixtures, in top blowing stream of gas reactor, by using thermobalance method have been carried out. To make the system more homogeneous pure iron crucibles were used in the experiment. The experiments were carried out in the range of gas flow rate V = (5+80) · 10^?6 m³/s and in 1723 K. Reduction rate depends on hydrogen partial pressure and flow rate of gas mixture. The logarithm of apparent reduction rate constant is a linear function of logarithm of gas flow rate. In experimental conditions of this work, the initial rate of reduction satisfies the relation . Thus it has been shown that the reduction process is controlled by the mass transfer in the gaseous phase. Sherwood numbers obtained from observed values of reduction rates and empirical equation of the gas-phase mass transfer rate are correlated by equation . The values of mass transfer coefficient in gas phase calculated from Sherwood numbers are consistent with the values of apparent reduction rate constant and with estimated rate constant of chemical reaction of liquid F_e1-xO reduction.     

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.     

Effects of pore diameter, bath surface pressure, and nozzle diameter on the bubble formation from a porous nozzle

The effects of the pore diameter, bath surface pressure, and nozzle diameter on the bubble formation from a porous bottom nozzle placed in a water bath and on the behavior of rising bubbles were investigated with still and high-speed video cameras and a two-needle electroresistivity probe. Three types of bubble dispersion patterns were observed with respect to gas flow rate, and they were named the low, medium, and high gas flow rate regimes. The transition boundaries between these gas flow rate regimes were expressed in terms of the superficial velocity at the nozzle exit, i.e., the volumetric gas flow rate per unit nozzle surface area. These transition boundaries were dependent on the pore diameter but hardly dependent on the bath surface pressure and the porous nozzle diameter. The characteristics of rising bubbles in each gas flow rate regime were investigated as functions of the three parameters.     

Measurement and computation of drag forces in thermogravimetric studies

The upward drag force experienced by a hanging assembly in a thermogravimetric setup due to flowing gas has been measured using an electrobalance for two different gases, viz., argon and carbon dioxide, over a temperature range of 1000 °C to 1250 °C and at a gas flow rate of 1.42×10⁻⁶ to 3.33×10⁻⁶ m³/s (STP). Analytical estimates, assuming the assembly to be a solid sphere of equivalent surface area, predicted drag forces about two orders of magnitude lower than the corresponding experimental values. This discrepancy was resolved by numerical simulation employing the commercial computational fluid-dynamics (CFD) package called “FLUENT” and by employing the exact geometry of the reactor and assembly, with experimental and computed values differing by about 20 pct, on average. The duration of the initial transient flow upon starting the gas flow was very small (less than 5 seconds).     

Measurement of physical characteristics of bubbles in gas-liquid plumes: Part II. Local properties of turbulent air-water plumes in vertically injected jets

The structural development of air-water bubble plumes during upward injection into a ladle-shaped vessel has been measured under different conditions of air flow rate, orifice diameter, and bath depth. The measured radial profiles of gas fraction at different axial positions in the plume were found to exhibit good similarity, and the distribution of the phases in the plume was correlated to the modified Froude number. Different regions of flow behavior in the plume were identified by changes in bubble frequency, bubble velocity, and bubble pierced length which occur as bubbles rise in the plume. Measurement of bubble velocity indicates that close to the nozzle the motion of the gas phase is strongly affected by the injection velocity; at injection velocities below 41 m/s, the velocity of the bubbles along the centerline exhibits an increase with height, while above, the tendency reverses. High-speed film observations suggest that this effect is related to the nature of gas discharge,i.e., whether the gas discharge produces single bubbles or short jets. In this region of developing flow, measurement of bubble frequency and pierced length indicates that break-up of the discharging bubbles occurs until a nearly constant bubble-size distribution is established in a region of fully developed flow. In this largest zone of the plume the bubbles influence the flow only through buoyancy, and the spectra of bubble pierced length and diameter can be fitted to a log-normal distribution. Close to the bath surface, a third zone of bubble motion behavior is characterized by a faster decrease in bubble velocity as liquid flows radially outward from the plume.     

The kinetics of the reduction of chromium oxide by hydrogen

The reduction kinetics has been studied as function of hydrogen partial pressures,pH₂O/pH₂ ratio, gas flow rate, and temperature. The reduction follows a linear time law and is dependent on the gas flow rate below a value of approximately 10 cm · s^-1, since the rate is determined by the removal of the water vapor being formed. In this range the reduction rate may be calculated from gas dynamical data. At sufficiently high flow rates the phase boundary reaction is rate determining. The activation energy is 123 kJ · mol^-1. The reduction rate is proportional to the square root of hydrogen pressure and decreases with increasing water vapor content. Formerly Research Associate with Dechema-Institut     

氧气高炉炉身喷吹煤气在炉内的分布

通过二维冷态物理模型对氧气高炉炉身喷吹煤气在炉内分布进行了实验研究,分别研究了炉身煤气总量、辅助风口直径以及炉身喷吹煤气量与炉身煤气总量之比对炉身喷吹煤气在炉内分布的影响.结果表明,炉身喷吹煤气量与炉身煤气总量之比对炉身喷吹煤气在炉身分布起决定性作用,而炉身煤气总量和辅助风口直径的影响较小.同时,在炉身煤气上升过程中涡流扩散效应的影响也较小.通过对根据实验数据绘制的炉身等浓度分布图进行研究发现,炉身煤气分布主要分为两个不同的区域,一个是炉身喷吹煤气主流区,另一个是从高炉下部产生的上升煤气主流区.在炉身等浓度分布图的基础上通过回归分析的方法推导出炉身喷吹水平喷吹煤气的渗透公式.此外,辅助风口被安装在炉身下部有利于铁矿石在炉身的间接还原.      

Hydrodynamics of gas stirred melts: Part I. Gas/liquid coupling

A hydrodynamic model of submerged gas injection systems and their effects on liquid metal stirring is presented. It is argued that hydrodynamic conditions at the nozzle, tuyere, or plug are not critical to flow recirculation produced in large cylindrical vessels(i.e., furnaces or ladles). An analysis of a buoyancy driven plume generated through gas injection shows that gas voidages are usually quite low (less than 10 pct). By equating the energy supplied by rising bubbles to turbulent energy losses within the bath, it is shown that mean plume velocities can be predicted using the relationship,U _p α (Q ^1/3 L ^1/3)/R^1/3 whereU _p equals mean plume velocity,Q is gas flow rate (at mean height and temperature),L is depth of liquid, andR is radius of the vessel. Associated rates of liquid turnover as a function of vessel dimensions and gas flow rate can also be predicted and these are similarly presented.     

Melting metal powder particles in an inductively coupled r.f. plasma torch

A numerical model is developed for the prediction of melting metal powder particles in an inductively coupled r.f. plasma torch. The model is developed for dilute spray conditions where the gas phase flow is not affected by the loading condition. The governing equation for the gas phase flow contains the source terms from the electromagnetic field. The theoretical calculations have shown that particle thermal history and its velocity are greatly affected by the plasma operating conditions (i.e., carrier gas flow rate, injector location, and power level,etc.). Without the proper control of particle trajectories, particles may bounce around the fireball and exit the torch as unmelted or resolidified solid particles. With the insertion of an injector or injecting particles with a high carrier gas flow rate, the predictions show that even relatively small size particles can be directed into the fireball and maintained in the molten state before they impact on the substrate. Consequently, more uniform and dense deposits can be achieved.     

Modeling of porosity during spray forming: Part II. Effects of atomization gas chemistry and alloy compositions

In this article, the effects of gas chemistry and alloy composition on the level of porosity in deposited materials are investigated by using a porosity model established in Part I of this article. The calculated results reveal that atomization gas chemistry has a significant influence on the level of porosity during spray forming, which can be rationalized on the basis of the influence of gas properties such as gas density, viscosity, and gas constant on the melt flow rate. The alloy properties that predominantly affect the variation of porosity with melt flow rate include melt viscosity, density, surface tension, solvent melting point, liquidus temperature, and equilibrium partition coefficient. A material property factor, μ _mγ_m/ρ ² _m , plays an important role in determining the processing conditions required to attain a minimum amount of porosity in deposited materials.     

Model study of mixing and mass transfer rates of slag-metal in top and bottom blown converters

Water model experiments have been conducted to clarify mixing rates of molten steel and mass transfer rates between slag and metal in LD and Q-BOP furnaces using six different circular tuyere arrangements. Splashing and ‘spitting’ were also examined with a view to finding a quiet bath with minimum mixing time and maximum mass transfer rate. Froude’s similarity criterion was fulfilled to determine gas flow rate and bath depth. Complete mixing time of water determined by tracer technique had been 0.9 second to 1.8 seconds for Q-BOP as compared to 6 seconds to 13 seconds for LD. This shows that the stirring intensity in Q-BOP is remarkably larger than that of LD. A simple relationship τ = 5.9(Q/N) ^−0.49 was obtained with gas flow rateQ and number of tuyereN. This indicates that flow rate of gas per tuyere should be intensified to realize better mixing. Mass transfer coefficient K_Ba for bottom blowing was found to be almost double that for top blowing. Of all the tuyere configurations studied for Q-BOP’s, a half circular tuyere arrangement was found to be the best considering all aspects of mixing, mass transfer, and bath agitation.     

Reaction rates for sulfur fixation with iron at 1100 to 1275 K

The rates of sulfidizing iron in a simulated coal gasification atmosphere were studied. Mixtures of H₂S and CO were passed through fixed beds of coal char and prereduced iron ore, and effluent gas compositions were measured as a function of time. These mixtures ranged from 2.5 pct to 10 pct H₂S at various flow rates, with temperatures from 1100 K to 1275 K and iron ore sizes from 10 mesh down to 100 mesh. Experimental conditions were established to form a steady state reaction profile in the fixed bed. Analysis of the exit gas provided a measurement of the profile. The slope of the profile was used directly as a measure of the reactivity of the solids in the bed. The development of this experimental technique and its experimental design requirements are discussed. The observed sulfidization rate of thein situ reduced iron ore is characterized by a single rate constantm (minutes^-1), which varies primarily with temperature and particle size and is substantially independent of gas flow rate, bed configuration, and H₂S content of the incoming gas. Accordingly, the rate constant m can be applied in the design of a combined sulfur fixation, coal gasification reactor to estimate the solids retention time, and the minimum mass of iron required per cross sectional area of reactor. CRAIG B. SHUMAKER, formerly a Graduate Student in the School of Materials Engineering, Purdue University     

A numerical study of high-velocity oxygen fuel thermal spraying process. Part I: Gas phase dynamics

A mathematical model is formulated to simulate the effect of operational parameters on the gas dynamics that occur during high-velocity oxygen fuel (HVOF) thermal spraying. Computational fluid dynamic techniques are implemented to solve the Favre-averaged mass, momentum, and energy conservation equations. The renormalization group (RNG) κ-ɛ turbulence model is used to account for the effect of turbulence, and high-order interpolation schemes are employed to resolve compressibility effects in the supersonic jets. The calculated results show that the most sensitive parameters affecting the process are propylene flow rate, total flow rate of oxygen and propylene (oxyfuel flow rate), total inlet gas flow rate, and barrel length. The results show that increasing the total inlet gas flow rate has limited effect on the gas velocity and temperature inside the nozzle for the parameter range investigated in the present study. However, increasing the total inlet gas flow rate increases the total thermal inertia and momentum inertia; moreover, under these conditions the flame gas is retained at a high velocity and temperature for a longer distance. Increasing the oxyfuel flow rate significantly increases flame velocity and temperature, particularly after exiting the nozzle. The effect of propylene flow rate is significant and complex. In order to minimize the extent of the oxidation of the sprayed powder particles and to achieve a high flame temperature and velocity, the overall injected stream should be adjusted to be propylene-rich. The nitrogen flow rate significantly affects the gas flow inside the gun. On the basis of the calculated results, it is evident that, in order to obtain maximum gas velocity and temperature, the nitrogen flow rate should be kept to a minimum, provided that particles can be delivered to the gun in a smooth manner. By minimizing the entrainment of the surrounding air, a nozzle with a longer barrel length achieves a relatively high gas velocity and temperature for a longer distance than does a nozzle with a shorter barrel length. The calculated results are in good agreement with available experimental results.     

Studies on the chlorination of zircon: Part I. Static bed investigations

Carbochlorination is an important unit operation in the processing of zirconium resources. In the article, the use of different reducing agents in zircon chlorination, to produce zirconium tetrachloride, has been examined on thermodynamic and other considerations. While numerous workers have investigated zircon chlorination, a literature survey shows that there is a wide variation in the reported effect of various process parameters on the chlorination rate and a wide scatter in the values for kinetic parameters such as order of reaction, activation energy, rate constant as also the rate law expression. This work is an extensive study on zircon chlorination and the article discusses the effect of process parameters such as charge particle size, gas and solid composition, gas flow rate, temperature, reaction duration, etc. on the chlorination rate, over a much wider range of the parameter values. During investigations in the static bed chlorinator, it was noticed that the initial rate and the total extent of chlorination are proportional to the exposed surface of the solid zircon-coke charge but independent of the depth or amount of the charge. Further, the stalled chlorination could be reactivated by remixing the solid charge. Also, while the reaction rate in general increased as the charge became finer, the effect of zircon particle size was much more predominant. The activation energy value for the chlorination showed a wide variation with other operating conditions. Likewise, the order of reaction with respect to chlorine decreased from two to zero as the chlorine concentration in the gaseous atmosphere increased. Interestingly, the chlorination rate initially increased with gas flow rate, then decreased, before finally becoming independent of the gas flow rate. Results also indicated that there is an optimum charge composition that yields the maximum chlorination rate and the article discusses the effect of the zircon to coke particle number ratio in the initial charge on the chlorination kinetics. With the help of these observations, it is possible to explain the wide variation in the reported effect of the various process parameters on zircon chlorination.     

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Newly Developed Austenitic Heat-Resistant Alloy

The hot deformation characteristics, microstructure evolution, and dynamic recrystallization (DRX) mechanism of the newly developed austenitic heat-resistant steel Fe–18Cr–10Ni–0.3Nb–2.5Cu were systematically investigated by thermal compression tests combined with microstructure characterizations. The activation energy (Q) map, Zener–Hollomon parameter (Z) map, and processing map were plotted according to the stress–strain curves to reveal the inherent connection between the three maps and the hot deformation characteristics of this alloy. The high η region in the processing map does not precisely correspond to the region where DRX developed. Nevertheless, the flow instability map accurately predicts the microstructure. The variation pattern of Z corresponded more closely to the hot deformation microstructure evolution than did the variation pattern of Q. The degree of DRX increases with decreasing Z. The optimal process parameters are 1000 °C/0.01 s⁻¹/0.8 and 1100 °C/10 s⁻¹/0.8 (temperature/strain rate/strain), and they result in complete DRX and a narrow range of Z values. The DRX mechanism at high strain rate is characterized by the combined enhancement of discontinuous DRX (DDRX), continuous DRX (CDRX), and twin-DRX (TDRX). The dominance of the particle-stimulated nucleation (PSN) mechanism at intermediate strain rate results in the formation of incompletely recrystallized microstructures with approximate orientation. Sufficient time at low strain rate promotes the development of DDRX and CDRX.

     

Mixing time and correlation for ladles stirred with dual porous plugs

Bulk mixing times up to a degree of 95 pct were measured in three different, cylindrical-shaped water model ladles (D=0.60 m, 0.45 m, and 0.30 m, respectively) in which, water was agitated by air introduced through two tuyeres/nozzles placed diametrically opposite at the base of the vessels at ±1/2 R positions. To this end, the electrical conductivity measurement technique was applied. A range of gas flow rates and liquid depths were investigated (viz. 0.7≤L/D≤1.2 and 0.002≤ɛ _m (watt/kg)≤0.01) and these were so chosen to conform to the practical ladle refining conditions. In the beginning, extensive experimental trials were carried out to assess the reliability of the measurement technique. In addition, some experiments were carried out to determine the location of the probe in the vessel such that measured mixing times could be interpreted as the bulk mixing times. It was observed that for smaller gas flow rates (or specific energy input rates), 95 pct bulk mixing times tend to decrease appreciably with increasing gas flow rates (e.g., τ _mix∞Q ^−0.58. However, for relatively higher flow rates, the dependence was found to be less pronounced, mixing times decreasing nearly in proportion to a third power of gas flow rates. Similarly, it was found that there exists a critical gas flow rate for any given vessel beyond which mixing times in dual plug stirred configuration are somewhat shorter than those in equivalent axi-symmetrical systems. A dimensional analysis followed by multiple regression of the experimental data (for ɛ _m ≥0.07 W/kg) indicated that mixing times in ladles fitted with dual plugs located diametrically opposite at ±R/2 locations could be reasonably described via τ _{mix, 95 pct}=15Q ^−0.38 L ^−0.56 R ^2.0 in which L is the depth of liquid (m), R is the vessel radius (m), and Q is the ambient flow rate (referenced to mean height and temperature of the liquid). Finally, the adequacy and appropriateness of the correlation was demonstrated with reference to the experimental data derived from a 0.20 scale, tapered cylindrical-shaped water model of a 140 T industrial ladle as well as scaling equations and modeling criteria reported in the literature.     

Fluid flow behaviour of liquid in cylindrical vessels stirred by one or two air jets

In the present paper, based on the two‐phase model (Eulerian‐Eulerian model), the three‐dimensional fluid flow in water system and liquid steel system stirred by one or two gas jets is simulated. A new modified k‐? turbulence model is introduced to consider the bubbles movement contribution to k and ?. The mathematical simulation agrees well with the experimental results. Calculation indicates that the distance of the two jet nozzles has a big effect on the fluid flow behaviour. Placing two gas injection nozzles at the half radii of one diameter of the bottom generates a much better mixing than that injected by only one nozzle with the same total gas flow rate.