旋流中间包夹杂物碰撞去除的数值模拟

旋流中间包是在中间包注流区内设置旋流室，钢液经长水口从旋流室底部沿着切线方向流入中间包内，使重力势能转化为旋转动能，旋转的钢液促使夹杂物向旋流室中心聚集，促进夹杂物碰撞聚合长大。用ANSYS Fluent中的PBM模型模拟了夹杂物在旋流中间包内碰撞聚合长大，用DPM模型模拟不同粒径夹杂物的轨迹和去除率。结果表明，相同操作条件下，考虑夹杂物之间的碰撞聚合时，无旋流室中间包的夹杂物平均直径从3.93 μm增至4.25 μm，夹杂物去除率为40.07%；旋流中间包夹杂物的平均直径从3.93 μm增至4.35 μm，夹杂物去除率从30.09%提升至43.20%。旋流中间包对夹杂物的去除能力优于无旋流室中间包。     

15-7PH不锈钢电渣重熔过程夹杂物的行为

为了明晰15-7PH沉淀硬化不锈钢中夹杂物的行为,进一步提高其洁净度,采用超高功率电弧炉初炼→AOD (Argon Oxygen Decarburization)脱碳→LF (Ladle Furnace)精炼→模铸工艺制备了自耗电极,并用带压摆控制的气体保护电渣重熔炉进行重熔。采用HORIBA氧氮氢气体分析仪检测了电渣重熔前后氧、氮等含量的变化;ASPEX扫描电镜分析了夹杂物的尺寸、数目、化学成分、形貌等。结果表明,电渣重熔后15-7PH不锈钢中氧、氮含量有轻微的下降,但夹杂物的组成变化不大,主要由氮化物夹杂物(氮化铝+氮化钛)、氮化物-氧化物复合夹杂物、氧化物夹杂物、硫化物-氧/氮化物夹杂物组成,其中氮化物夹杂物尺寸最大、数量最多,明显高于其他夹杂物。电渣重熔对夹杂物的数量、尺寸有明显影响。重熔后夹杂数量大幅增加,氮化物尤为明显,但大颗粒夹杂物明显减少。氮化物夹杂物大量存在的主要原因在于钢中存在较高的Al, Ti, N等元素,而电渣过程由于熔渣吸附、部分夹杂物溶解,使大颗粒夹杂物减少,重熔过程的快速冷却抑制了夹杂物长大,最终结果是夹杂物尺寸更细小,但数量增加。     

板坯连铸结晶器内夹杂物运动行为的数值模拟

针对某钢厂板坯连铸结晶器的结构参数,建立了描述结晶器内钢液流动的三维数学模型,用Fluent软件对结晶器内钢液流动行为和夹杂物运动行为进行了模拟,分析了不同颗粒直径,不同颗粒密度,不同颗粒数量及不同加入位置对夹杂物上浮的影响.研究结果表明,夹杂物的轨迹与钢液的流线图相似;夹杂物颗粒数量对去除率影响不大;夹杂物密度为3500kg/m3、直径为400μm时可以完全去除;密度较小且靠近浸入式水口壁面的夹杂物易被除去.     

漩流中间包夹杂物去除水模实验

为更好地去除钢液中非金属夹杂物,在中间包内安放1个漩流室有利于夹杂物碰撞团聚上浮去除.通过水模实验对漩流中间包中央杂物的去除作了较为详细的模拟,对影响去除率的漩流室直径、高度、入口流量、入口方向和不同的夹杂物颗粒的加入量等凶素作了研究.结果表明,漩流室的引入使得夹杂物的去除率得到了提高,小粒径夹杂的上浮率最大提高了近10%,漩流中间包有利于夹杂物上浮去除.     

高碳连铸坯中夹杂物赋存规律的研究

利用金相法结合非水溶液电解法研究了高碳连铸坯中夹杂物的类型、尺寸、分布、化学成分和来源,确定了高碳连铸坯内部夹杂物情况。结果表明:高碳连铸坯中主要有Al2O3夹杂、硫化物以及Ca O·Al2O3、MgO·Al2O3等复合夹杂物,尺寸大部分小于20μm,在铸坯厚度方向内弧侧1/4处出现聚集,主要来源于采用Si-Ca-Ba系脱氧剂进行脱氧时产生的脱氧产物以及钢液对炉衬和耐火材料的冲刷。     

酒钢CSP流程SPCC钢夹杂形成机理

对酒钢CSP流程SPCC两个浇次在LF进站、LF中期、LF出站、中包、铸坯和轧制过程分别取钢样,用SEM-EDS分析其夹杂物及成分. 结果表明,在喂铝线后,钢液中很快形成大量形状不规则Al2O3夹杂物,随着精炼进行夹杂物存在两种变性路线：Al2O3-MgO·Al2O3-CaAlMg复合夹杂物和Al2O3-CA6-CA2-CA-低熔点夹杂物. 经过钙处理后大部分夹杂物可较好地变性到低熔点液相区或固液两相共存区. 夹杂物变性越好,则钢液中夹杂物球形化率越高,总量也越小,夹杂物成分对其尺寸也有重要影响. 分析了外层被钙铝酸盐和CaS包裹的双层夹杂物的形成机理,前者由于钢中Ca还原MgO·Al2O3尖晶石中Mg或Al2O3中Al;后者由于在铸坯凝固过程中温度降低及元素S的偏析,造成液芯中S浓度升高,其与Ca在已有的固体夹杂物核心的表面析出CaS. 在轧制过程中,前者变形能力较好,后者的外层CaS易与内部核心分离,甚至产生微裂纹.     

电渣重熔过程结晶器旋转对钢中夹杂物的影响

基于自行设计的双极串联结晶器旋转电渣重熔炉,采用ASPEX全自动夹杂物分析仪研究了结晶器转速对M2电渣锭洁净度的影响。结果表明,不论结晶器是否旋转,电渣锭中的夹杂物组成基本不变,主要由Al2O3, Al2O3–MnS, Al2O3–SiO2–CaO–MnS, MgO–Al2O3–SiO2–CaO–MnO, MgO–Al2O3–SiO2–CaO–TiO2–MnS, Al2O3–SiO2–CaO–MnO–TiO2组成,其中以Al2O3, Al2O3–SiO2–CaO–MnO–TiO2和Al2O3–MnS数量最多。结晶器静止电渣重熔时,钢中的夹杂物数量较多,且存在50 ?m以上的大颗粒夹杂物,而结晶器转速为6和13 r/min时,夹杂物数量减少,大颗粒夹杂含量大大降低;转速增至19 r/min时,夹杂物数量及尺寸又进一步增加,同时钢中全氧含量、氮含量明显增加。电渣锭中大颗粒夹杂物得以去除的主要原因是结晶器旋转导致金属自耗电极末端的熔融层变薄、熔滴尺寸变小,渣–金接触面积增大,促进了夹杂物被熔渣去除;过快的转速会增加自耗电极氧化、减少渣–金接触时间,从而降低电渣重熔过程的精炼能力。     

超声波功率对电渣钢锭中氧化铝夹杂物分布的影响

采用自行设计的带超声波振动的电渣炉,研究了超声波功率对轴承钢中氧化铝夹杂物分布及去除的影响. 结果表明,电渣重熔过程中超声波功率从0增至400和700 W,钢锭中的夹杂物最大尺寸分别是40, 36和14 mm,功率增至1000 W时,夹杂物最大尺寸增至36 mm. 电渣重熔过程中无超声波时,钢锭中夹杂物在试样边部与中心部位聚集,边部聚集尤为严重. 超声波功率增加,夹杂物聚集逐渐减弱直至均匀分布. 超声波去除夹杂物主要是其空化和声流效应改善了渣-金间反应的动力学条件,使夹杂物均匀分布于钢锭中. 但超声波功率过高会降低渣-金间的反应速率,引起金属熔池扰动,降低电渣精炼能力.     

外加磁场对电渣锭洁净度的影响研究

为进一步去除电渣锭中的大颗粒夹杂物,设计了带电磁搅拌的电渣重熔炉,研究了外加磁场及不同的电参数变化对电渣锭洁净度的影响,采用氧氮分析仪分析了气体含量的变化,采用扫描电镜分析了夹杂物形貌、组成、尺寸的变化。结果表明,不论是否施加磁场,电渣重熔后电渣锭中的氧含量明显增加,从自耗电极中的0.0007%增加最高至0.0052%,增幅高达7倍;氮含量略微下降。但夹杂物类型基本不变,主要由氧化铝、硫化锰、硫化锰?氧化物复合夹杂及氧化物等组成,且以氧化铝为主。外加磁场重熔以后,电极中最大夹杂物的直径从89.6 μm降为电渣锭中的12.1 μm (1.1 kA/108 Gs),小颗粒夹杂物所占的比例增加,大颗粒夹杂物的数量减少。但过高的电磁力会降低夹杂物的去除效果,当采用1.5 kA/108 Gs的工艺参数重熔时,最大的夹杂物尺寸为30.6 μm,超过了未施加磁场的电渣锭中的夹杂物。电渣重熔后氧含量增加主要受空气污染及渣系中不稳定氧化物的分解,而外加磁场后产生的电磁力增大了渣?金接触面积,从而吸收了电极中的大颗粒夹杂物。     

A356铝合金中夹杂物的研究

通过化学分离对A356铝合金中的夹杂物进行提取,并灰化处理,研究A356合金的微观组织及夹杂物的形貌和成份,并探讨了夹杂物形成与析氢之间的关系以及不同成分夹杂物的形成机制。结果表明:A356铝合金中的夹杂物大多呈无规则形态,尺寸在1~10μm左右,表面毛糙,且片状或团簇状存在。夹杂物成分以Al、Mg、Si、O、C等元素形成的氧化物、碳化物和金属间化合物。结合SEM图谱可知,A356铝合金中夹杂物多分布于气孔周围,可以推测夹杂伴随着析氢出现,夹杂物周围会增加析氢的几率。     

Interactions among metastable pits on heterogeneous electrodes

A previously developed spatiotemporal model of the interactions among metastable pits on homogeneous electrodes is extended to heterogeneous surfaces with randomly distributed inclusion sites in an otherwise passive electrode. The inclusions are treated as favored sites for pit initiation events. The effect of the spatial density of inclusions on susceptibility to explosive growth in pitting corrosion is investigated. It is shown that there exist critical values of the pit density for the explosive growth of metastable pits that leads to a transition to pitting corrosion. Convective mass transport is shown to alter the density required for explosive growth. The effect of a limitation on the number of times that an inclusion can initiate a pit on the transition to the pitting corrosion is also considered.     

Implementation of the population balance equation in CFD codes for modelling soot formation in turbulent flames

The simulation of soot formation in turbulent diffusion flames is carried out within a CFD code, by coupling kinetics and fluid dynamics computations with the solution of the population balance equation via the Direct Quadrature Method of Moments, a novel and efficient approach based on a quadrature approximation of the size distribution of soot particles. A turbulent non-premixed ethylene-air flame is used as the test case for validation of the model. Simplified kinetic expressions are employed for modelling nucleation, molecular growth and oxidation of particles, along with a Brownian aggregation kernel. A recently proposed approach for modelling the evolution of fractal dimension is used with a monovariate population balance to predict the morphological properties of aggregates.     

沉流式滤筒除尘器气固两相流动的数值模拟与分析

为掌握滤筒除尘器内部流动特征 ,应用FLUENT 5 .4 .8软件对DFT3 12型滤筒除尘器内部紊流气固两相流动进行了数值模拟 ,采用k ε紊流模型和壁面函数法模拟气相流动 ,采用双向耦合拉格朗日法追踪颗粒运动轨迹。对连续相速度和压力分布特征以及颗粒相运动轨迹进行了分析 ,得出了不同工况条件下的系统阻力和颗粒沉积量分布规律 ,对比分析了重力、布朗运动和紊流扩散作用对颗粒运动和沉积的影响     

Grain Growth in Two-Phase Ceramics: Al2O3 Inclusions in ZrO

The effect of Al₂O₃ inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO₂/Al₂O₃ composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO₂ grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al₂O₃ inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO₂ grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.     

Influence of laser wavelength on the critical energy density for initiation of energetic materials

Critical densities of the energy of laser initiation of PETN containing nanoscale aluminum inclusions at radiation wavelengths of 1064 and 532 nm were measured experimentally. The critical initiation-energy density that corresponds to a 50%th probability of explosion was 1.15 J/cm² for the first harmonic of a neodymium laser and 0.7 J/cm² for the second. The dependence of the efficiency of radiation absorption by aluminum on the size of metal nanoparticles for the first and second harmonics of a neodymium laser is calculated. It is shown that the particle diameter corresponding to the absorption efficiency maximum and the amplitude of the maximum depend on the radiation wavelength. The absorption efficiency maximum for the first harmonic is observed in an inclusion 204 nm in diameter, and for the second, in an inclusion 96 nm in diameter. The amplitude of the maximum increases from 0.351 at a wavelength of 1064 nm to 0.490 at a wavelength of 532 nm. Dependences of the critical initiation energy density for energetic materials on the radius of metallic nanoparticles are calculated. Qualitative agreement between theoretical and experimental results is shown.     

Inclusions in diamonds with solubility changes and phase transformations

Liquid-metal inclusions are sealed into diamond crystals during crystal growth at high temperatures and pressures. The pressure and temperature of these inclusions change as the diamond is lowered to room temperature and atmospheric pressure and during subsequent heating and cooling cycles at atmospheric pressure. Liquid-metal inclusions that dissolve carbon follow different P–T curves than liquid metal inclusions that do not. A discontinuous change of the pressure in the inclusion occurs when the liquid metal solidifies during cooling. This discontinuous change is caused by the molar volume and carbon solubility difference between the liquid and solid metal. Contrary to common belief, the residual pressure in these inclusions can be large and positive at room temperature when graphite precipitates from supersaturated solid metal inclusions. The formation of carbides in the inclusion following solidification can also have a significant effect on the pressure in the inclusion. Internal stresses are generated in the diamond that are proportional to the difference between the pressure in the inclusion and the external pressure around the diamond. These stresses increase as the cube of the inclusion diameter and fall off as the cube of the distance from the inclusion. Small positive or negative temperature fluctuations (±30 °C) while the diamond is growing can generate large stresses that produce plastic flow around inclusions. Plastic flow produces an increased atomic defect density around the inclusion as well as permanent internal elastic stresses in the diamond far from the inclusion. These internal stresses may affect the mechanical properties of the diamond. When the pressure in the inclusion falls below the diamond-graphite equilibrium line, graphitization can occur on the walls of the inclusion. Because graphite has a molar volume 50% greater than diamond, such graphitization reduces the volume of the inclusion cavity and increases the pressure in the inclusion. Multiple inclusions generate stresses that are proportional to the inclusion concentration.     

膜分散微结构反应器制备纳米氧化锌的实验及模型

以硫酸锌和氢氧化钠为原料,在膜分散微结构反应器中通过快速均匀混合制备得到颗粒平均尺寸为10～20 nm的氧化锌,并利用混合尺度模型模拟了微反应器内纳米颗粒的成核、生长和团聚过程。模拟计算结果表明,在最初的0.6 ms内颗粒成核占主导地位,在1.6 ms以后以生长为主,同时由于颗粒数密度较大,颗粒运动碰撞造成团聚效应,使得颗粒尺寸具有一定的分布。混合尺度和反应物浓度对颗粒直径和分布有很大影响。模拟结果表明当混合尺度从50 μm减少到5 μm,纳米氧化锌颗粒从19 nm降低到12 nm。微反应器制备实验结果表明,随着膜孔径的减小,混合强度增加,纳米氧化锌颗粒平均直径从20 nm 降低至11 nm,当初始反应物浓度从0.05 mol·L^-1提高到0.20 mol·L^-1,氧化锌纳米颗粒尺寸由10 nm增大至16 nm。颗粒平均直径及分布的模拟值与实验值相符较好。     

Particle agglomeration and capture as a result of synergism between inertial impaction and eddy diffusion

Particle capture experiments conducted in turbulent cross flow with various aerosols involving liquid and/or solid particulates have resulted in collection efficiencies which are in excess of the values predicted by the various known models of particle capture, i.e., inertial impaction, interception and Brownian diffusion. In one type of experiment the turbulent air stream, carrying submicron dust particles, is flowing past a cylindrical collector (such as a piece of wire) with its axis orientated perpendicular to the direction of flow. Collecting efficiencies ranging up to about 20% have been found under conditions where the conventional models of particle capture predict practically zero collection efficiencies. In another type of experiment involving injecting a fog (2–80 μm diameter water droplets) into the dusty gas stream carrying submicron size dust particles which subsequently enters a slow turning fan. While passing through the fan, the fog agglomerates into raindrops while scavenging most of the dust particles. For example, a 0.8 μm median particle size aluminum silicate pigment was collected with 97–99.5% efficiency, the exact value depending on the operating conditions. Theoretical analysis of these phenomena may be based on the idea of synergism involving inertial impaction and eddy diffusion: the smaller dust particles/drops are captured by the larger drops in the fan and the dust particles are captured by the wires because (a) there is a significant relative velocity between them and (b) because the particles undergo eddy diffusion.