Particle sedimentation during processing of liquid metal-matrix composites

A novel electrical resistance probe technique to measure thein situ volume fraction of ceramic particles in molten metals was applied to the measurement of sedimentation rates of 90-μm-diameter silicon carbide particles in molten aluminum. The results indicate that the rate strongly depends on volume fraction; the time to clarify a 0.15-m depth increased from approximately 60 to 500 seconds as the particle volume fraction increased from 0.05 to 0.30. Maps showing the changes in volume fraction throughout the melt were generated. A multiphase hydrodynamic model was developed to describe the sedimentation. Using volume fraction-dependent drag coefficients from work in aqueous systems, the model was able to simulate the experimental results remarkably well. The experimental and modeling results indicate that there was little agglomeration or network formation during sedimentation. The implications of the results for solidification and particle pushing are discussed. Formerly Graduate Student, McMaster University     

Particle incorporation by melt stirring for the production of metal-matrix composites

The production of ceramic particle-reinforced metal-matrix composites by the melt-stirring technique involves two different steps: firstly, the establishment of particle-melt contact, and secondly, the wetting of the particles by the melt. The wetting behaviour of metal-ceramic systems has been studied extensively, whereas the contacting problem has been largely neglected. In this paper, a novelin situ thermal method has been used to investigate the kinetics of particle-melt contacting, based on melt cooling during and immediately after the particle addition. An enthalpy balance relates the amount of cooling and the fraction of the particles contacting the melt. For the aluminium alloy (A356) with 15 vol% silicon carbide (17 m diameter), it was found that only 16% of the particles contacted the melt after mixing times of 15 min, but contact could be improved to 36% by adding magnesium to the melt. The fraction contacting the melt agreed well with the particle volume fraction calculated independently. Sampling of the froth layer on top of the melt revealed that a large portion of the particles resides there prior to incorporation into the melt.     

Settling of multisized clusters of alumina particles in liquid aluminum

The clustering of alumina particles during their incorporation into pure liquid aluminum by stirring was studied experimentally. It was found that these particles together with entrained oxide films formed clusters of various sizes and shapes. When the stirring stopped, these clusters settled to form a loose sediment (containing about 0.18 volume fraction of alumina) on the bottom of the crucible. A mathematical model was developed to describe the settling of multisized clusters. Information regarding the size distribution of the clusters was obtained by matching the model results with the experimental data. The size distributions of the alumina clusters were comparable to those estimated from metallographic measurement.     

A Study of Scrap Heating by Burners: Part II—Numerical Modeling

A computational fluid dynamics code was developed to model the heating of a bed of porous steel scrap by combustion gases from a burner. The code accounted for nonuniform void fraction in the bed; turbulent, non-Darcian flow, heat transfer from the gas to the scrap; and radiation. The measured bed porosity values were used in the code. Because steel scrap pieces are very irregular in shape and size, the effective particle diameter was fitted to measurements made in a 1-m³ capacity furnace, as reported in part I. It was found that the lower porosity of the scrap was the most beneficial in increasing the efficiency of heat transfer to the scrap bed because the interfacial area is larger. The effect of particle size was much smaller. It was found that the configurations that increased the residence time or path length of the gases increased the efficiency. The measured porosity of the bed approached unity at the walls, so this provided an easy path for the gas to short-circuit the bed, which limited the effectiveness of decreasing the porosity to increase heat-transfer efficiency. Similarly, simulations of nonuniform scrap distributions reduced efficiency of heat transfer due to short circuiting. The implications of the findings for industrial operations are discussed.     

Multiple-Angle Observations of Reflectance Anisotropy from an Airborne Linear Array Sensor

An airborne pointable imaging multispectral linear array (MLA) sensor has been developed for the multidirectional observation of surface reflectance anisotropy. The sensor design permits observations up to 450 off-nadir in three spectral bands (green, red, and near-infrared). Calibration permits the conversion of sensor data to radiance units with an absolute uncertainty of 6 percent. Observations of five field plots from seven view directions are discusseed. Calibration and atmospheric corrections are used to derive hemispherical-directional reflectance factors. A three-term reflectance model is fit to the reflectance factors for each plot to represent the continuous distribution of reflectance factors with view direction. The reflectance model is integrated over all view directions to calculate bihemispherical reflectance factors. The calculated bihemispherical factors differed by 1 to 25 percent from values based on an assumption of isotropic reflectance depending on spectral band and field plot. These calculations demonstrate the technologic and scientific capabilities required for the remote characterization of surface reflectance anisotropy. Remote multidirectional observations are both feasible and needed to fully evaluate land reflectance characteristics.     

Mathematical modeling of postcombustion in a KOBM converter

A mathematical model has been developed to describe gas flow, combustion reactions, and heat transfer in converter-type steelmaking processes. Thek- ε two-equation turbulent model, a finite reaction model, and the DeMarco-Lockwood flux model have been incorporated in this model to deal with the turbulent flow, postcombustion reactions, and radiation heat transfer. Local gaseous flow patterns, temperature, and heat flux distributions were calculated for a 300 tonne Klöckner Oxygen Blowing Maximillanshuette (KOBM) converter. Comparison between the heattransfer fluxes calculated based on the model and those measured industrially indicates the validity of the model in this application. The postcombustion has been found to be determined by the decarburization rate (DCR) which is directly related to the hardness of blowing not by the entrainment of surrounding gas to the oxygen jet as previously reported. The model revealed that about 20 pct of what is normally considered to be recovered heat has actually been lost through the vessel wall and to the lance. It is shown that this model is useful in studying the detailed mechanisms of postcombustion to optimize operations in converter-type steelmaking processes.     

Measurement and modeling of turbulence in the gas/liquid two-phase zone during gas injection

Averaged and turbulent fluctuating liquid velocities in the gas/liquid plume zone of a gas-stirred water model ladle were measured with a combined laser Doppler anemometer (LDA) and elec-trical probe technique. The measured turbulence fields, void fraction distribution, and gas and liquid velocities in the plume zone were used for evaluation of various turbulence models. It was found that, among all of the turbulence models tested, only a modified k-ε model, with extra source terms to take into account the generation and dissipation resulting from the inter-action of the bubbles with the liquid, yielded good agreement with both the mean liquid flow field and the turbulent kinetic energy distribution. However, the values of the coefficients orig-inally proposed by their authors were found inapplicable to the bubbly plume situation; more appropriate values of the coefficients were determined based on comparison with experimental measurement.     

The modulo time plot-a useful data aquisition diagnostic tool

This paper illustrates the use of the Modulo Time Plot to facilitate diagnosis of data acquisition systems and components. While conventional techniques, involving spectral analysis and histograms, provide certain useful and necessary measures of performance, the use of reordered sample sets has gained considerable popularity in recent work aimed at characterizing analog-to-digital converter error mechanisms. Examples show that the Modulo Time Plot is useful for quick visual inspection of system performance including dynamic range, distortion and error plots, the detection of random bit errors, and timing errors between the test signal and the sample clock     

Mathematical Modeling of Impinging Gas Jets on Liquid Surfaces

In top-blowing operations, the gas jet is a major source of momentum, so to model the momentum exchange properly with the liquid, the full-stress boundary conditions must be applied. A new mathematical method for better representation of the surface boundary condition was developed by combining the Cartesian cut cell method and volume of fluid method. The computational code was validated with the broken dam problem and reported critical phenomena in wave generation. The model was applied to impinging jets on liquid surfaces in two dimensions. The cavity depth was in good agreement with experimental measurements. The process of ligament and droplet formation was reproduced. The extent of momentum transfer to the liquid was investigated, and the trends with lance height and gas flow rate were similar to experimental evidence. The following important aspects of momentum transfer were identified: surface roughness as well as the development of local pressure gradients around wave crests. Kelvin–Helmholtz instability theory was used to interpret the results with respect to critical velocity for the onset of droplet formation. These principles were extended to conditions relevant to basic oxygen furnace (BOF) steelmaking. The critical velocities for droplets were calculated as functions of the physical properties for the gas–steel, gas–slag, slag–steel interfaces. The implications for BOF steelmaking were discussed. The mathematical model was applied to a simplified configuration of full-scale BOF steelmaking, and the local force balance was well described.