共查询到20条相似文献,搜索用时 15 毫秒
1.
Diego Berzi 《Canadian Metallurgical Quarterly》2011,137(10):1200-1207
2.
Dong-Guan Seol Tirtharaj Bhaumik Christian Bergmann Scott A. Socolofsky 《Canadian Metallurgical Quarterly》2007,133(6):665-676
A direct measurement method for the velocity field in multiphase flows using the particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods is developed to study the flow characteristics of an unbounded bubble plume in quiescent, unstratified ambient conditions. A single camera is used to obtain images containing both bubbles and fluid tracer particles. Using gray-scale thresholding, phase-separated images of the bubbles are produced, and bubble velocities are obtained from these images using the standard PTV method. Regular PIV is applied to the mixed fluid images, and bubble vectors are removed using a velocity threshold and vector median filter that is calibrated to the PTV result. From the separate velocity fields, the time-averaged flow characteristics of a bubble plume are studied. Gaussian velocity profiles match the entrained fluid velocity, and top-hat velocity profiles match the bubble velocity. Time-averaged values are also presented of velocity, plume width, entrained fluid volume flux, and void fraction as a function of height. From these data, the entrainment coefficient for the entrained ambient fluid is calculated and lies between 0.08 near the plume source and 0.05 in the upper reaches. The results for the entrainment coefficient, together with those from the literature, are correlated to a nondimensional velocity, given by the ratio of the bubble slip velocity us to a characteristic velocity in the plume (B/z)1/3, where B = kinematic buoyancy flux and z is the height above the source. 相似文献
3.
Stephen E. Coleman Bruce W. Melville Lance Gore 《Canadian Metallurgical Quarterly》2003,129(11):872-884
Fluvial entrainment of fractured rock assessed in terms of bed shear stress, stream power, and time-averaged bed uplift pressures indicates that rock-block stability reduces with increasing protrusion and decreasing surface length (in the direction of flow), with protrusion of only a nominal portion of the block required to significantly decrease block stability. Variations in block uplift pressure coefficient with normalized block protrusion and block surface length can be used to predict the height of a block (of protrusion P and known surface lengths) at the point of entrainment for an open-channel flow (of average depth h and velocity U). Alternatively, entrainment of prismoidal particles of square section in plan by fully turbulent open-channel flows (of R?>100) can be predicted using (θC?0.002) = 0.0015?(Pvb/L)?1, where θC is the critical dimensionless shear stress, R? is the grain Reynolds number, L is the particle upper-surface side length, and Pvb is the particle protrusion relative to the virtual-bed level at which the average flow velocity is zero (approximately the tops of the supporting or surrounding particles for the present prismoidal blocks). Owing to the potential occurrence of cavitation on prototype block surfaces, it is recommended that quantitative scaling of the present results be conservatively limited to prototype average velocities (U) of less than approximately 6 m/s (with scale ratios λU2 = λL = 29). In contrast to existing practice, particle protrusion needs to be accounted for when assessing the erodibility of a channel bed using stream-power-based methods such as the erodibility index method reported by Annandale in 1995. 相似文献
4.
Bed sediment entrainment function is derived based on kinetic theory. The entrainment rate is expressed as an upward flux by integrating the particle velocity over all possible upward motions. The possibility of upward motion is determined by a sediment particle velocity distribution function, which is obtained by solving the Boltzmann equation instead of using a prior assumed distribution. External forces and turbulence intensity of the flow are shown to exert significant influences on the velocity distribution function and, in turn, the entrainment rate. Comparisons between available laboratory data and the entrainment function show that the calculated entrainment rates agree well with the observations. Applications of the entrainment function to the specification of the bottom boundary condition for convection-diffusion equation of suspended load are also presented, which show that the calculated concentration profiles are in good agreement with observations. The study also suggests that kinetic theory is a promising analytical approach for the study of sediment motion near a riverbed. 相似文献
5.
A numerical and experimental study was carried out to investigate the solidification process in a twin-belt (Hazelett) caster.
The numerical model considers a generalized energy equation that is valid for the solid, liquid, and mushy zones in the cast.
Ak-ε turbulence model is used to calculate the turbulent viscosity in the melt pool. The process variables considered are the
belt speed, strip thickness, nozzle width, and heat removal rates at the belt-cast interface. From the computed flow and temperature
fields, the local cooling rates in the cast and trajectories of inclusions were computed. The cooling rate calculations were
used to predict the dendrite arm spacing in the cast. The inclusion trajectories agree with earlier findings on the distribution
of inclusion particles for near horizontally cast surfaces. This article also reports the results of an experimental study
of the measurement of heat flux values at the belt-cast interface during the solidification of steel and aluminum on a water-cooled
surface. High heat fluxes encountered during the solidification process warranted the use of a custom-made heat flux gage.
The heat flux data for the belt surface were used as a boundary condition for the numerical model. Objectives of the measurements
also included obtaining an estimate of the heat-transfer coefficient distribution at the water-cooled side of the caster belt.
Y.G. KIM, formerly Graduate Student, Materials Engineering Department, Drexel University. 相似文献
6.
While very fine sediments (colloids) are normally assumed to be readily transported downstream without deposition, recent evidence suggests that these particles will often deposit into streambeds due to a combination of physical and chemical mechanisms. This study investigates a regime of particle deposition where settling is unimportant and thus where particle deposition can only result from advective stream–subsurface exchange followed by deep-bed filtration. Laboratory flume experiments were conducted to examine the deposition of 0.45 μm diameter silica colloids into a silica sand bed. This system was selected for study because submicron sized colloids will not settle and silica colloid filtration by silica sand is generally quite low. Despite the lack of settling and the weak particle–particle interactions, the ongoing interfacial flux of colloids to the subsurface still produced significant filtration of silica colloids over the course of the experiments. Variation of the background ionic strength caused significant modification of filtration behavior and silica colloid deposition. In addition, cleaning the sand surface with mild acid and base washes reduced both filtration and net colloid exchange. These experimental results are interpreted in terms of a fundamentally based physicochemical model which predicts net particle deposition based on stream and subsurface hydrodynamic conditions and subsurface filtration. These results show that both particle surface chemical conditions and background water chemistry play a critical role in controlling the net transport and deposition of fine sediments. It is important to recognize the effects of physicochemical processes both when designing laboratory experiments and when analyzing environmental particle transport. 相似文献
7.
A room temperature model of a submerged gas-particle jet was used to investigate the hydrodynamics and gas-liquid mass transfer
in such systems. Air or CO2 was used to inject particles of silica sand into water. In some cases, the sand was coated with resin to produce a hydrophobic
surface. The flow regimes of behavior were observed: In the bubbling flow regime large bubbles of gas are formed and penetrated
by a stream of particles which did not entrain gas, and in the steady jet flow regime the gas and particles travel together
in a narrow cone. The second flow regime is favored by a high gas velocity, a small particle size, and a high ratio of particles
to gas in the jet. The surface characteristics of the injected particles do not appear to affect this transition. A CO2-NaOH solution model was used to determine the effects of inert particle injection of the rate of mass transfer from gas to
liquid. The rate of mass transfer was higher in steady cone jets, because under these conditions, the gas is dispersed into
finer bubbles and carried deeper in the bath.
Formerly Graduate Student in the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago 相似文献
8.
对烧结现场生产进行全流程取样,分析熔剂颗粒在烧结过程中的演变规律,及其对烧结过程的影响。结果表明,在烧结混合料制粒过程中,小于0.5 mm熔剂颗粒较铁矿粉颗粒更容易黏附至核颗粒表面形成新的颗粒,从而相对均匀地分布至混合料各粒级中。大于0.5 mm粒级熔剂颗粒作为核黏附一定厚度的黏附层形成新的颗粒,黏附层厚度均小于1 mm,因此,新颗粒直径仅在原始颗粒粒径基础上增大不超过2 mm。同时由于熔剂原始颗粒粒级较细,导致制粒后大于5 mm粒级混合料中熔剂含量较少。而在烧结台车布料过程中粒级存在偏析,大颗粒向下分布,最底层大于5 mm粒级颗粒分布最多,从而导致熔剂的偏析,混合料中大于5 mm粒级颗粒增多加大了熔剂的偏析,混合料中3~5 mm粒级颗粒增多减弱了熔剂的偏析。料层粒级偏析使烧结料层,最底层混合料中熔剂总量变少,大颗粒熔剂增多,熔剂颗粒数量减少,导致熔剂分布点变少,熔剂分布不均匀程度增加,局部高碱度环境变少,液相产生的难度增加。同时,由于颗粒的偏析,最底层混合料中大颗粒铁矿粉增加,出现更多的未熔原矿,最终导致烧结料层最底层烧结矿质量变差。 相似文献
9.
A model is developed for the mechanisms of densification of powder compacts of unequal sized powder particles subjected to
isostatic pressures at elevated temperature. The regimes of deformation mechanisms examined are those of athermal plastic
flow and power law creep. Specifically, a bimodal distribution of particle sizes is assumed and the model evaluates the magnitude
of the interparticle contact areas and the interparticle contact stresses are for each type of particle. It is found that
the smaller particles bear the burden of higher interparticle contact stresses and plastic strains on an average and, accordingly,
there is no longer a single boundary on a densification mechanism map between the regions of athermal plastic flow and power
law creep for the bimodal case but rather a separate boundary for each particle size. The results are discussed with respect
to a previous densification model for monosized particles, and the implications for a full size distribution are analyzed
based on the bimodal results. 相似文献
10.
S. Nourbakhsh 《Metallurgical and Materials Transactions A》1992,23(4):1181-1186
Interaction between a grain boundary and deformable particles has been modeled. It is shown that deformable particles pin
the grain boundaries more effectively than rigid spherical particles. The required driving force to unpin a grain boundary
increases with decreasing the interphase energy of the deformable particle. Deformable particles also reduce the rate of grain-boundary
migration more effectively than rigid spherical particles. The rate of grain boundary migration decreases with the interphase
energy of the deformable particle. The model allows the shape of a deformable particle to be determined as the particle is
dragged by a grain boundary. The particle shape allows prediction of the force acting on the particle. 相似文献
11.
Accounting for Soil Aging When Assessing Liquefaction Potential 总被引:1,自引:0,他引:1
Evangelia Leon Sarah L. Gassman Pradeep Talwani 《Canadian Metallurgical Quarterly》2006,132(3):363-377
It has been recognized that liquefaction resistance of sand increases with age due to processes such as cementation at particle contacts and increasing frictional resistance resulting from particle rearrangement and interlocking. As such, the currently available empirical correlations derived from liquefaction of young Holocene sand deposits, and used to determine liquefaction resistance of sand deposits from in situ soil indices [standard penetration test (SPT), cone penetration test (CPT), shear wave velocity test (Vs)], are not applicable for old sand deposits. To overcome this limitation, a methodology was developed to account for the effect of aging on the liquefaction resistance of old sand deposits. The methodology is based upon the currently existing empirical boundary curves for Holocene age soils and utilizes correction factors presented in the literature that comprise the effect of aging on the in situ soil indices as well as on the field cyclic strength (CRR). This paper describes how to combine currently recorded SPT, CPT, and Vs values with corresponding CRR values derived for aged soil deposits to generate new empirical boundary curves for aged soils. The method is illustrated using existing geotechnical data from four sites in the South Carolina Coastal Plain (SCCP) where sand boils associated with prehistoric earthquakes have been found. These sites involve sand deposits that are 200,000?to?450,000?years in age. This work shows that accounting for aging of soils in the SCCP yields less conservative results regarding the current liquefaction potential than when age is not considered. The modified boundary curves indicate that old sand deposits are more resistant to liquefaction than indicated by the existing empirical curves and can be used to evaluate the liquefaction potential at a specific site directly from the current in situ properties of the soil. 相似文献
12.
13.
Mechanism of alumina buildup in tundish nozzles during continuous casting of aluminum-killed steels 总被引:3,自引:0,他引:3
S. N. Singh 《Metallurgical and Materials Transactions B》1974,5(10):2165-2178
During continuous casting of aluminum-killed steels, the pouring rate through the tundish nozzle often diminishes, posing
serious operating problems. This happens because a buildup of microscopic alumina particles at the nozzle orifice effectively
decreases the nozzle orifice diameter and causes constriction of the liquid-steel pouring stream. There are two major aspects
of the constriction problem: (1) the source of the depositing particles, and (2) the mechanism by which the particles deposit
at the nozzle orifice. Detailed microscopic examination of the buildup of alumina particles in nozzles from various casting
trials and petrographic examination of the nozzle refractory indicated that the particles depositing at the nozzle orifice
were already present in the melt because of deoxidation and reoxidation processes. A model proposed herein explains why and
how the alumina particles deposit and stay at the nozzle orifice. The model considers a microscopically thin boundary layer
at the nozzle bore where the velocity of the liquid steel approaches zero. Particles passing close to the refractory surface
and in the slow-moving boundary layer attach to the wall and to each other. The presence of eddy currents in and close to
the turbulent boundary layer increase the particle to particle collision. Such collisions of the particles thrown from outer
region of boundary layers with the already attached particles keep on dumping the particles toward the refractory surface.
The high interfacial energy of alumina inclusions in steel is a driving force for particles to attach to the refractory wall
and to each other, and high-température sintering then occurs to form a network of alumina particles. 相似文献
14.
《Acta Metallurgica Materialia》1992,40(2):275-283
An energetically unfavourable situation can develop when coherent particles are bypassed by migrating grain boundaries. This is due to the precipitates being exposed to incoherent interfaces in the new matrix. In this work, the rotation of precipitate particles to low energy, coherent orientations in the new matrix is shown to be one of several possible responses to this situation. A physical and kinetic model for the rotation is put forward and the results of calculations of rotation rate presented. It is shown that particle rotation is controlled by interfacial diffusion and depends upon alloy composition, time, temperature and particle size and shape. The possibility of particle rotation occurring during particle/boundary contact is also discussed. This is shown to depend upon the nature of the boundary type. Generally, boundaries moving under large driving pressures with high velocities (e.g. phase transformation interfaces), are less likely to allow rotation during particle/boundary contact than less mobile boundaries such as those in grain growth and recrystallization. Experimental results from SAD and TEM of a Ti stabilized austenitic stainless steel containing a dispersion of coherent TiC precipitates is also presented and supports the particle rotation model. 相似文献
15.
Anna Jensen Brett Brunk Gerhard Jirka Leonard W. Lion 《Canadian Metallurgical Quarterly》1999,125(4):351-359
Settling and entrainment are the dominant processes governing noncohesive particle concentration throughout the water column of salt-wedge estuaries. Determination of the relative contribution of these transport processes is complicated by vertical gradients in turbulence and fluid density. A differential-turbulence column (DTC) was designed to simulate a vertical section of a natural water column. With satisfactory characterization of turbulence dissipation and saltwater entrainment, the DTC facilitates controlled studies of suspended particles under estuarine conditions. The vertical decay of turbulence in the DTC was found to obey standard scaling law relations when the characteristic length scale for turbulence in the apparatus was incorporated. The entrainment rate of a density interface also followed established grid-stirred turbulence scaling laws. These relations were used to model the change in concentration of noncohesive particles above a density interface. Model simulations and experimental data from the DTC were consistent over the range of conditions encountered in natural salt-wedge estuaries. Results suggest that when the ratio of entrainment rate to particle settling velocity is small, sedimentation is the dominant transport process, while entrainment becomes significant as the ratio increases. 相似文献
16.
In light of environmental differences (such as gravitational fields, surface temperatures, atmospheric pressures, etc.), the mechanical behavior of the subsurface soil on the Moon is expected to be different from that on the Earth. Before any construction on the Moon can be envisaged, a proper understanding of soil properties and its mechanical behavior in these different environmental conditions is essential. This paper investigates the possible effect of surface-energy forces on the shear strength of lunar soil. All materials, with or without a net surface charge, exhibit surface-energy forces, which act at a very short range. Although, these forces are negligible for usual sand or silty sand on Earth, they may be important for surface activated particles under extremely low lunar atmospheric pressure. This paper describes a constitutive modeling method for granular material considering particle level interactions. Comparisons of numerical simulations and experimental results on Hostun sand show that the model can accurately reproduce the overall mechanical behavior of soils under terrestrial conditions. The model is then extended to include surface-energy forces between particles in order to describe the possible behavior of lunar soil under extremely low atmospheric pressure conditions. Under these conditions, the model shows that soil has an increase of shear strength due to the effect of surface-energy forces. The magnitude of increased shear strength is in reasonable agreement with the observations of lunar soil made on the Moon’s surface. 相似文献
17.
18.
Kristen L. Jellison Richard I. Dick Monroe L. Weber-Shirk 《Canadian Metallurgical Quarterly》2000,126(12):1153-1157
While successful in removing turbidity and pathogens from drinking water, slow sand filters require ripening periods at the beginning of each filter run. The premise of this research was that it should be possible to enhance the ripening of slow sand filters. Potential ripening agents were screened by assessing their interaction with the surface of filtration media and turbidity particles. Four natural organic polymers and nine synthetic polymers were investigated for their potential to enhance filter ripening. Of the 13 modifying agents considered, none conclusively sorbed to the filter media, and only one, a synthetic polymer, interacted with kaolin particles. A filter modified with continuous feed of the polymer ripened successfully and produced water with turbidity below 1.0 NTU in about 24 h. Most turbidity removal in the treated filter occurred in the schmutzdecke rather than within the depth of the filter bed. Hence, the mechanism of enhanced ripening in this case probably was particle agglomeration with resulting acceleration of particle deposition at the filter surface accompanied by straining or attachment to previously removed particles. 相似文献
19.
在广义热弹性扩散理论框架下建立非等径两颗粒系统三维有限元模型,研究颗粒系统温度场和浓度场的分布规律,分析场分布对脉冲电流烧结初期迁移驱动力的影响。结果表明,颗粒颈部空位浓度梯度、温度梯度、由温度场和应力场产生的浓度梯度是颗粒颈部物质迁移的共同驱动力。烧结颈部的温度会产生两次突变,烧结过程中小颗粒一直保持高温状态;温度变化会引起浓度改变,使得颈部浓度高于边缘浓度;热扩散占总扩散通量的2/3,浓度扩散占1/3,因此烧结颈部的热扩散驱动力和浓度扩散驱动力是脉冲电流烧结过程的主导驱动力,提高热扩散能力和浓度扩散通量可显著提高烧结过程驱动力。非等径颗粒的烧结驱动力远远大于等径颗粒,为非等径颗粒的烧结比等径颗粒更为迅速提供了理论依据。 相似文献
20.
A model is presented that describes the wall-to-bed heat transfer in a circulating fluidized bed (CFB) used for the prereduction
of iron ore in the smelting-reduction iron-making process. The model incorporates the core-annulus type flow structure and
the wall emulsion layer growing downward along the surface. Model predictions showed good agreements with measured data taken
from the literature. The hydrodynamic behavior near the wall surface was able to be properly described by the core-annulus
flow structure. A higher heat-transfer coefficient with higher solid circulation flux was obtained in the upper part of the
bed because of the heat input caused by the lateral diffusion of particles from the core. The predicted and measured data
also showed the minima in the heat-transfer coefficients in the lower part of the bed. Model predictions indicated that in
the CFB for the reduction of iron ore particles, it is important to properly control the inlet temperature of the reducing
gas, rather than that of the solid particles. The implications of the behavior of heat transfer in the CFB are discussed for
the reduction of iron oxides. 相似文献