【作者】 王芳；

【导师】 沈峰满；

【作者基本信息】 东北大学 ， 钢铁冶金， 2008， 博士

【摘要】 作为提高金属冶炼效率,改善产品质量的有效辅助手段,电磁场已在冶金领域得到广泛发展。行波磁场与旋转磁场均属于移动磁场范畴。本文运用现代流体力学的方法结合电磁场理论研究电磁场作用于冶金反应器内钢液流动的冶金特性,利用大涡模拟方法(Large eddy simulation-LES)揭示了不同磁场条件下冶金反应器内的流动规律。研究内容及取得的主要结果包括：(1)行波磁场作用下圆柱型反应器内液体的运动规律行波磁场被广泛应用于改善金属精炼与凝固质量,但对行波磁场作用下金属传输行为的认识尚不完善,制约了行波磁场的进一步应用。本文将描述行波磁场的麦克斯韦方程与流体流动N-S方程相结合,建立了计算行波磁场作用下圆柱型冶金反应器内三维流场的数学模型。通过对典型物理过程进行模拟分析,得到的结果与文献中提供的实验结果吻合良好,证明了本模型和编制的计算程序的可靠性。数值分析结果表明：行波磁场作用下圆柱型反应器内的主截面上形成了两个对称的旋涡。当径高比降低时,反应器内的流场结构没有改变,为两个对称的旋流,形状由圆形变成长方形。当电磁力增大时,轴向方向速度曲线则由平顶变为尖顶的抛物线,并且电磁力越大,波顶离底部壁面的距离越近。(2)离心式中间包钢液流动的大涡模拟和物理模型试验离心式中间包被用于特殊钢连铸工艺中,具有有效排除钢液中夹杂物的优点。但其操作特性仍未被充分掌握。并且电磁驱动旋流的效率较低,对此提出利用弯水口增大离心室内旋流强度的方法。采用自行设计的离心式中间包水模型装置,分析了旋流强度对中间包平均停留时间、死区体积分数等的影响,并对磁场强度进行了参数研究。结果表明：施加旋转磁场能显著地增长中间包的平均停留时间、缩小中间包的死区体积分数,有利于夹杂物的去除；选择离心室出口面积为0.75A时、叶轮转数为46 rpm有利于改善中间包内的流场。离心式中间包内液面下凹深度与叶轮转数的关系很重要。叶轮转速越大,液面下凹深度越大,越可能发生卷渣；但叶轮转速太小,旋流的强度又不够,降低了去除夹杂物的效率。通过拟合得到弯水口注流液面下凹深度和叶轮转速的关系式为：H=-3.17×107 n3+3.684×10-5 n2-0.0001521n,其中：H=h/D,h为下凹深度,D为离心室直径；n为叶轮转数,rpm。发展了大涡模拟方法求解离心式中间包内三维湍流流场。考察了只有弯水口注流,及旋转磁场作用下分别采用直水口、弯水口时离心式中间包内的流场结构和磁场强度对离心式中间包流场结构的影响。结果表明：当磁场强度从0.001T增大到0.004T时,旋流速度的最大值从0.012m/s左右增大到0.04m/s。随磁场强度的增加,旋流速度成线性增加。磁场强度的改变对流场结构的影响很小；本文模拟条件下,旋转电磁力和弯水口共同作用时可使由单纯电磁场产生的最大速度值增加约15%-19%。并得到了试验验证。(3)旋流场内底吹气气液两相流运动行为的数理研究喷气搅拌是炉外精炼中最广泛使用的技术,但突出的问题是氩气比较昂贵,且利用效率低。对此提出利用旋转磁场作用于底吹气冶金反应器的方法。采用室温模型实验,对冶金反应器内气液流动的混合特性进行了物理模型研究。结果表明：底吹气过程中施加旋转磁场能显著缩短反应器内液体的混匀时间,细化气泡并延长气泡在反应器内运动路程,更加高效地去除钢液中非金属夹杂物；选择吹气位置为0.5R,叶轮转速为60 rpm,吹气量为0.17m3/h为本试验的最佳值。建立了旋流场底吹气反应器内气-液两相流动模型。考察了底吹气位置和磁场强度对改善反应器内的流动形式和气泡运动行为的影响。结果表明：无底吹气体时,冶金反应器内竖直截面的流场结构为四个对称的旋涡；无旋流时,不断上升的气泡形成“倒锥形”的气柱。在旋转磁场(磁感应强度为0.007T)和底吹气共同作用时,主截面的流场结构发生改变,即旋涡的数量增加,上升的气泡形成了“螺旋状气柱”。当吹气位置离中心的距离从0增大到0.75R时,气泡在冶金反应器内运动的路程呈线性的增加,运动路程最大增加是无旋流时运动路程的1.54倍；当磁场强度从0增大到0.012T时,气泡在冶金反应器运动的路程呈指数形式的增加,运动路程最大增加是无旋流时运动路程的2.21倍。本工作中均采用FORTRAN语言,独自编程。采用有限体积法求解离散后的微分方程。用SIMPLEC法求解过滤后的非线性瞬态方程。利用ADI法和快修正法离散求解代数方程。更多还原

【Abstract】 The electromagnetic field has been widely used in metallurgy as a effective auxiliary method of improving product quality, which would increase efficency of metal refining. Alternative magnetic filed includes traveling magnetic field and rotating magnetic field. On the basis of advanced fluid mechanics and theory of electromagnetic field, turbulence of molten steel affected by different electromagnetic field has been studied using large eddy simulation (LES). The main works and conclusions are presented as follows.(1)Flow field of molten metal affected by traveling magnetic field in cylindrical reactorThe traveling magnetic field has been widely used in metal refining and solidification. But the knowldgement of transport behivor of molten metal affected by traveling magnetic field is lack, which will hold back the further application of traveling magnetic field. By coupling the traveling magnetic field theory and N-S equation, the three dimensional flow field is produced in cylindrical reactor. The new formulation of electromagnetic force affected by traveling magnetic field is introduced. The simulation result is accord with the experiment result made by Gerbeth, which testifies the self-developed program and model are reliable. The results show that the flow structure of main section in cylindrical reactor affected by traveling magnetic field is made of two symmetrical vortices; when the aspect ratios decreased, the flow structure has no changed, only the shape changed from round to rectangular; when the electromagnetic force increased, the curve of axial direction velocity changes from flat to steeple and the peek and moves close to the bottom wall.(2) Studied on flow structure of molten steel in centrifugal flow tundish with the method of large eddy simulation and physical model experimentCentrifugal flow tundish is used in continuous casting of producing special steel, which has the advantange of effectively removing non-metal inclusion. But the principle operation condition is not clear. For the strength of swirling flow density affected by magnetic field is low, the method of using bend nozzle to increase swirling flow density in rotation chamber is suggested. The water model of centrifugal flow tundish is self-designed. The residence time distribution, dead volume fraction etc have been studied with varying magnetic field density. Results show that the use of rotating magnetic field can increase average residence time and reduce dead volume fraction, which is helpful to exclusion the non-metal inclusion. The optimum condition in the experiment is the oulet area of rotation chamer of 0.75A, and rotating speed of 46 rpm. Through the fitting of the data, the expression of sink depth of fluid level with rotating speed is as follows:H=-3.17×107n3+3.684×10-5n2-0.0001521n, where:H=h/D, H is sink depth of fluid level, D is diameter of rotating chamber; n is rotating speed, rpm.Large eddy simulation (LES) method is developed to resolve the three-dimensional flow structure of centrifugal flow tundish. In the present works, flow field in three cases are simulated and analyzed, i.e. the swirling flow is produced by (a) electromagnetic force with the direct nozzle (b) bending nozzle using height potential energy of molten steel (c) combination of electromagnetic force and bending nozzle. The effect of magnetic field density on flow structure of centrifugal flow tundish is considered. Results show that when the magnetic field density increased from 0.001T to 0.004T, The maximum velocity increased from 0.012m/s to 0.04m/s,the maximum velocity increased linearly when the the magnetic field density increased. The effect of magnetic field density on flow structure is small; the effectiveness of bending nozzle is validated by 15%-19% increment of maximum swirling velocity in rotation chamber which have verified by water experiment.(3) The physical and mathematical research of gas-liquid two phase flow behavior with bottom gas stirring in rotating magnetic fieldArgon gas blowing is the most used tecnology in refining, but the pop problem is that argon gas is very expensive and its efficency is very low. It is suggested that using rotating magnetic field can extend the gas bubble moving distance in reactor. The water model is designed by myself and mixing character of gas-fluid in reactor has been studied. The results show that the rotating magnetic field used in the process of bottom gas stirring can greatly shorten the mixing time in reactor, refine the size of gas bubble, extend the gas bubble moving distance, enlarge the collision area of gas bubble and nonmetal inclusion and is beneficial to removal inclusion. The optimum condition is the position of nozzle at 0.5R, rotating speed at 60 rpm and gas quantity of 0.17m3/h. The three dimensional gas-liquid two phase mathematical model in rotating magnetic field is established. The effect of nozzle position and magnetic field density on flow structure and gas bubble behavior of reactor is considered. The results show that if there is no gas stirring, the flow structure of main section in reactor is made of four symmetrical vortices. If there is only gas stirring, the shape of gas plume is inverted cone, When gas stirring and rotating magnetic field applied together, the flow structure of main section in reactor have changed, which takes more vortices than no gas stirring. The chape of gas plume became spiral and the shape of gas plume simulated is the same with the experiment observation. When the distance between reactor center and nozzle position increased, the distance of gas bubble moving increased in linear form. If the distance between reactor center and nozzle position changed from 0 to 0.75R, the channel length wthich gas bubbles float to free surface is prolonger to 1.54 times at most than without swirling flow. When the magnetic density changed from 0 to 0.012T, the channel length wthich gas bubbles float to free surface is prolonger to 2.21 times at most than without swirling flow.The program is self-developed using FORTRAN language. A finite volume method has been chosen for this calculation of complicated system. The filtered non-linear simultaneous equations are solved by using SIMPLEC algorithm. A combination of Alternating-Direction-semi-Implicit iteration scheme (ADI) and block correction is used to solve the discrete algebraic equations.更多还原