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1.
The paper describes heat and mass balances, thermodynamic equilibria, and kinetics of the post-combustion reaction in iron-bath reactors and of the subsequent transfer of heat from gas to melt via metal and slag droplets suspended by the melt into the gas space above the melt. Heat and mass balances, equilibria, and kinetics of the reoxidation reaction of droplets, which accompanies the transfer of heat, are also described. Finally, the dynamics of droplet behaviour are presented. 相似文献
2.
A mathematical model to describe the solidification behavior of an atomized droplet during flight, in terms of nucleation
temperature, recalescence temperature, nucleation position, solid fraction at nucleation temperature, and droplet temperature
and velocity, is formulated. The concept of transient nucleation is applied to model the short nucleation event. A maximum
droplet velocity exists, beyond which the droplet velocity shows an inflection phenomenon during the flight. The velocity
of smaller droplets is higher at a shorter flight distance and lower at a longer flight distance. Variations of the gas flow
patterns have more effects on smaller droplets, and the effects are more significant at a longer flight distance. A minimum
surface heat-transfer coefficient exists as the droplet flies. Prior to nucleation or recalescence, smaller droplets have
lower temperature at a given flight distance. Smaller droplets have lower nucleation temperature. Medium-size (around 80-μm) droplets fly over the shortest flight distance before the nucleation starts. Smaller droplets have a larger solid fraction
at the end of recalescence. Atomization gas has more effects on the droplet momentum than on the heat content of the droplet. 相似文献
3.
Bacteria transfer from the water into the air may play an important role in bioaerosol cycle. Bubbles raising through the water column collect bacteria but also other suspended material and transport them towards water surface. When the bubble burst at the water surface collected material are skimmed off the bubble to become highly enriched in jet and film drops. After ejection airborne droplets can evaporate and as small droplets can be transported even to remote locations. Such a stream of aerosol droplets may carry stream of bacteria scavenged from the water column. The fate of bacteria in the air may possibly depend on the environmental conditions like intensity of sunlight or ambient air humidity. In addition the wind speed might be responsible for both wave/bubble mediated production of marine originated droplets and their transport in the atmosphere. The evidences that bacteria are transferred from the breaking waves, in particular in the coastal zone, were observed during several field experiments conducted in 1994 and 1995 over the Gulf of Gdansk and the Baltic Sea coast. Enhanced sea to air bacteria transfer were noticed over the polluted waters where in addition gas supersaturations in the water were recorded. Further laboratory investigations of bacteria scavenge via bubbles produced by single capillary and by plume of bubbles produced by ceramic stone indicated high enrichment within both mesophile and psychrophile bacteria categories. 相似文献
4.
J. I. Minchaca M A. H. Castillejos E F. A. Acosta G 《Metallurgical and Materials Transactions B》2011,42(3):500-515
Direct spray impingement of high temperature surfaces, 1473 K to 973 K (1200 °C to 700 °C), plays a critical role in the
secondary cooling of continuously cast thin steel slabs. It is known that the spray parameters affecting the local heat flux
are the water impact flux w as well as the droplet velocity and size. However, few works have been done to characterize the last two parameters in the
case of dense mists (i.e., mists with w in the range of 2 to 90 L/m2s). This makes it difficult to rationalize how the nozzle type and its operating conditions must be selected to control the
cooling process. In the present study, particle/droplet image analysis was used to determine the droplet size and velocity
distributions simultaneously at various locations along the major axis of the mist cross section at a distance where the steel
strand would stand. The measurements were carried out at room temperature for two standard commercial air-assisted nozzles
of fan-discharge type operating over a broad range of conditions of practical interest. To achieve statistically meaningful
samples, at least 6000 drops were analyzed at each location. Measuring the droplet size revealed that the number and volume
frequency distributions were fitted satisfactorily by the respective log-normal and Nukiyama–Tanasawa distributions. The correlation
of the parameters of the distribution functions with the water- and air-nozzle pressures allowed for reasonable estimation
of the mean values of the size of the droplets generated. The ensemble of measurements across the mist axis showed that the
relationship between the droplet velocity and the diameter exhibited a weak positive correlation. Additionally, increasing
the water flow rate at constant air pressure caused a decrease in the proportion of the water volume made of finer droplets,
whereas the volume proportion of faster droplets augmented until the water flow reached a certain value, after which it decreased.
Diminishing the air-to-water flow rates ratio, particularly below 10, resulted in mists of bigger and slower droplets with
low impinging Weber numbers. However, increasing the air pressure maintaining a constant water flow rate caused a greater
proportion of finer and faster drops with Weber numbers greater than 80, which suggests an increased probability of wet drop
contact with a hot surface that would intensify heat extraction. 相似文献
5.
Simulation of two‐phase flow in a copper flash smelting settler with simultaneous tapping of slag and matte is carried out using Eulerian‐Eulerian two‐fluid model and the flow and heat transfer performance of slag and matte is examined. Detailed velocity vectors, temperature and volume fraction distributions are obtained. The results show that the small copper droplets will be suspended in the slag. When the droplet diameter is large enough, the slag and matte layer are clearly formed and the dispersion layer between the slag and matte layers becomes thinner at larger droplet sizes and its thickness remains nearly unchanged when the copper droplet diameter is larger than about 500 μm. 相似文献
6.
《Acta Metallurgica Materialia》1991,39(12):3237-3249
Iron-cobalt alloys and pure cobalt have been processed in vacuum by electrohydrodynamic atomization (EHD) to produce submicron droplets. Pure iron was processed in previous work. Analytical electron microscopy is used to study the as-solidified spheres to determine the microstructure, composition, size, and crystal structure of the 10–300 nm diameter spheres. It is argued that homogeneous nucleation must be the predominant nucleation mechanism in EHD droplets. The crystal phase determinations show that bcc is produced as an alternative crystallization phase in alloys of up to 90 at.% cobalt content phase is, however, observed in Fe-60, 80 and 90 at.% Co and its proportion increases with cobalt content and droplet size. These findings are shown to be consistent with calculations based on classical nucleation theory. It is also found that under these extreme conditions of high cooling rate and very small volume, the smallest droplets solidify from the melt as an amorphous phase. The observed fraction of amorphous spheres in each size range are used as experimental input to an analytical simulation to determine the glass transition temperature of the metals. The calculated fraction of spheres that will solidfy to an amorphous structure agrees well with experimental findings. 相似文献
7.
Nucleation of solidification in liquid droplets 总被引:2,自引:0,他引:2
Analytical and numerical methods have been developed to analyze the solidification kinetics of a mass of liquid droplets dispersed
in a fluid or solid matrix using classical nucleation theory. The resulting analytical expressions and numerical calculations
can be compared directly with calorimetric measurements of the droplet solidification exotherms to obtain information about
the nucleation mechanism. With increasing contact angle at the solid-liquid-matrix triple point, the solidification onset,
peak, and end temperatures and exothermic peak height all decrease sharply and the droplet solidification exotherms become
broader. Decreasing either the droplet radius or the number of potential catalytic nucleation sites produces a similar but
smaller effect. Distributions in droplet radius, contact angle, and nucleation sites have no effect on the solidification
peak temperature, but the droplet solidification exotherms become broader and more symmetric. The solidification onset temperature
is independent of cooling rate in the calorimeter, but the solidification peak and end temperatures decrease and the exothermic
peak height increases with increasing cooling rate. Predicted droplet solidification exotherms are in excellent agreement
with detailed experimental measurements on 10-nm-radius Cd droplets embedded in a solid Al matrix. Analytical predictions
give best-fit values of 43 deg and 430 for the contact angle and the number of potential catalytic nucleation sites per droplet,
respectively; numerical predictions give best-fit values of 43 deg and 750 for the contact angle and the number of potential
catalytic nucleation sites per droplet, respectively. 相似文献
8.
以某电厂300 MW燃煤机组为例,对脱硫废水烟道喷射系统进行CFD数值模拟,研究了雾化粒径、烟气温度、烟气流速和喷枪布置方式对液滴蒸发的影响。结果表明:雾化粒径明显影响液滴群蒸发质量百分比和蒸发时间,在较低的烟温下(140℃),需要较小的雾化粒径(≤50μm)才能保证液滴完全蒸发;烟气温度对液滴群蒸发质量百分比和蒸发时间的影响显著,随着烟气温度升高,液滴蒸发速率加快,蒸发时间明显减少;烟气流速对液滴蒸发特性影响不大,随着烟气流速升高,烟气量增大,液滴群蒸发时间会降低,而液滴群蒸发质量百分比变化不大;在烟道同一截面处,不同喷枪布置方式对液滴蒸发影响小。 相似文献
9.
10.
The solidification of undercooled spherical droplets with a discrete melting temperature is analyzed using both a Newtonian
and a non-Newtonian (Enthalpy) model. Relationships are established between atomization parameters, the growth kinetics, the
interface velocity and undercooling, and other important solidification variables. A new mathematical formulation and solution
methodology is developed for simulating the solidification process in an undercooled droplet from a single nucleation event
occurring at its surface. The computational mesh used in the enthalpy model is defined on a superimposed bispherical coordinate
system. Numerical solutions for the solidification of pure aluminum droplets based on the enthalpy model are developed, and
their results are compared to the trends predicted from the Newtonian model. The implications of single vs multiple nucleation
events are also discussed. In general, the results indicate that when substantial undercoolings are achieved in a droplet
prior to nucleation, the thermal history consists of two distinct solidification regimes. In the first, the interface velocities
are high, the droplet absorbs most of the latent heat released, and the external cooling usually plays a minor role. The second
regime is one of slower growth, and strongly depends on the heat extraction at the droplet surface. The extent of “rapid solidification”,
as determined from the fraction of material solidified at temperatures below a certain critical undercooling, is a function
of the nucleation temperature, the particle size, a kinetic parameter, and the heat translow as 10~4.
Formerly a Research Associate at the University of Illinois, 相似文献
11.
连铸二冷区铸坯表面温度通常高于900 ℃,此时喷淋液滴接触高温铸坯时不会湿润铸坯表面,仅在其上形成一层蒸汽膜,阻碍了液滴与铸坯表面接触传热。针对以上问题,以国内某钢厂连铸二冷区的扁平型水喷嘴为原型,建立了喷嘴射流仿真计算模型,并对所建模型进行了理论和实验室验证;采用数值模拟的方法研究了喷嘴自由射流区的流场分布,运用连铸喷嘴冷却检测系统测量获得了射流液滴粒径演变规律;结合数值模拟和实验室测定结果,定量分析了喷嘴在不同水流量下射流液滴冲击铸坯表面蒸汽膜深度的变化规律。结果表明,该喷嘴的最大射流速度在喷嘴出口处,射流在喷嘴出口附近出能维持较大的射流速度,且随着水量的增加,射流保持高射流速度的距离也增长;整体射流的轴向速度占比均在80%以上。当喷淋水量越大时,射流液滴粒径变得越小;随着距喷嘴出口距离的增加,射流中心处的液滴粒径逐渐增大并达到最大值;当水流量为9和12 L/min时,液滴粒径基本相同,这表明当水流量增加到一定值时,冷却水量的增加不影响液滴粒径分布。在不同水流量下,随着喷淋距离的增加,液滴穿透铸坯表面蒸汽膜深度呈先增大后略微减小的变化规律,在喷射距离为100~200 mm范围内时,液滴穿透深度最大,这表明喷射高度在该范围时,喷淋冷却效果最好。 相似文献
12.
13.
A method was developed to compute continuous-cooling-transformation (CCT) diagrams for the heterogeneous nucleation of alloy
droplets from a few experimental data. The developed model addresses both oxidation-catalyzed surface nucleation and internal
nucleation caused by another catalyst. Droplet surface oxidation is regarded as a first-order reaction in order to account
for the effects of the gradual increase in surface oxidation on the kinetics of surface nucleation. CCT curves were computed
for Sn-5 mass pct Pb droplets cooling in atmospheres with various oxidation potentials using data determined with monosize
droplets produced by capillary jet breakup. The developed model may be used to predict droplet nucleation kinetics in industrial
thermal spraying processes. 相似文献
14.
在前人对单个液滴碰撞传热解析研究的基础上,通过实验与理论分析,建立了气-水喷雾冷却在膜态沸腾区的传热数学模型。采用此模型,可以在已知气-水喷嘴的冷态参数(流量密度、液滴粒径、液滴速度等)的情况下,模拟计算出该喷嘴的传热特性。计算结果与实验数据在可比的范围内基本吻合。 相似文献
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17.
报导了一种气—水雾化喷嘴的测定结果,并与一种水喷嘴进行了比较。研究表明,采用气—水雾化喷嘴时,铸坯二冷的传热系数不仅和水流密度有关,而且与喷射状况,诸如水滴直径,水流密度、气-水雾化喷嘴的用气量等条件有关。 相似文献
18.
Gas atomization of melt is widely used to produce metal powders and spray formed parts. In this process heat and momentum transfer of metal droplets take place during their flight in the gas medium. It is quite important to know the values of various parameters related to these two modes of transport phenomena. In the present paper mathematical models were developed and experimentally verified to predict values of various parameters, such as droplet and gas velocity, droplet temperature with time and distance travelled, percentage release of latent heat by the droplet. Four metals, lead, zinc, aluminium and tin were atomized to collect experimental data. 相似文献
19.
Baolong Zheng Yaojun Lin Yizhang Zhou Enrique J. Lavernia 《Metallurgical and Materials Transactions B》2009,40(5):768-778
In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powders were studied
via numerical simulations. Modeling simulations were based on the assumption of Newtonian cooling with forced convection,
as well as an energy balance, which involves gas dynamics, droplet dynamics, and heat transfer between gas and droplet. To
render the problem tractable, phase transformations, crystal nucleation, and growth were not taken into account in the analysis
of the solidification of Al droplets; instead, an energy balance approach was formulated and used. The numerical results and
associated analysis were used to optimize processing parameters during gas atomization of Al-based amorphous powder. The results
showed that the cooling rate of droplets increases with decreasing powder size and can reach in excess of 105 K/s for powder <20 μm in diameter. Gas composition has a more significant influence on cooling rate than gas pressure, and 100 pct He has the
highest cooling effect. The results also showed that the cooling rate increases with increasing melt superheat temperature. 相似文献
20.
K Richter 《Canadian Metallurgical Quarterly》1994,53(3):237-249
Mouse liver tissue was rapidly cooled by high-pressure freezing at a nominal pressure of 2100 bar. Ultrathin cryosections were examined at 110 K with a cryoelectron microscope and the state of water was studied on micrographs and electron diffraction patterns. The results are compared with those from liver specimens cryofixed at ambient pressure by plunge freezing. The high-pressure frozen specimens contained crystalline frozen regions as well as areas which were vitreous. The spot diffraction patterns from crystalline regions are found to differ from those known for hexagonal ice. The comparison with powder diffraction data reveals the presence of the high-density morphologies Ice II and Ice III. Vitreous areas of biological samples frozen at ambient pressure are optically denser than areas where the water crystallised as cubic or hexagonal ice. However, on micrographs from high-pressure frozen samples, no significant contrast is found. This observation is explained by the higher specific mass of high-pressure ice compared to Ice I, an interpretation which requires a higher specific mass for high-pressure vitreous water compared to vitreous water produced at ambient pressure. The larger diffuse first ring of the diffraction pattern from high-pressure vitreous water confirms this argument. 相似文献