Turbulence flow and heat transfer of aluminum melt in tip cavity in process of thin-gauge high-speed casting

The uniformity of flow distribution of aluminum melt in tip cavity is a precondition to decide whether or not thin-gauge high-speed casting can be accomplished smoothly. The laws of aluminum melt flow and heat transfer in tip cavity can be found out through numerical simulation, which gives theoretical basis for solving the problem of the flow distribution of melt in tip cavity. A mathematical model with a low Reynolds number k--ε model for turbulence flow and heat transfer of aluminum melt in tip cavity was developed. The finite difference method was used to calculate the flow field and temperature field of aluminum melt in tip cavity. The phenomena and characteristics of turbulence flow and heat transfer were analyzed, including the characteristics of temperature distribution of turbulence similar to that of laminar flow. The simulation results are in good agreement with the experimental results for flow velocities and temperature at the exit of tip, which verifies the validity of the simulation results.     

金属快速凝固的非平衡超急速传热模型

运用非Fourier传热理论建立了金属快速凝固过程中的非平衡传热理论模型，包括非Fourier方程的建立、传热与相变模拟，模拟计算表明：（1）在溅射激冷条件下，界面换热系数越大，界面冷却速度和移动速度也越高。在界面换热系数相同时，计算得到的界面冷却速度随着固－液界面高度的提高呈现先上升而后下降的变化趋势；计算得到的冷却速度值明显小于Fourier定律的计算值。（2）在激光加热条件下，计算的界面移动速度在凝固开始时先急剧增加，然后渐趋平稳，计算还表明，金属的过热及过冷度与其热物性相关。     

单辊法制备非晶合金中的传热与熔体流动数值模拟

基于耦合的二维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ动量方程和热传导方程建立了单辊法制备非晶合金中的传热和熔体流体动力学模型。利用ＳＩＭＰＬＥ差分算法计算了熔池内的温度场和速度场,获得了不同辊速条件下熔体的平均冷却速度,与红外热像法测得的Ｆｅ７８Ｓｉ０Ｂ１２Ｍｏ非晶合金的冷却速度相比。     

Coupled heat/mass-balance model for analyzing correlation between excess AlF3 concentration and aluminum electrolyte temperature

The influence of aluminum electrolyte component on its temperature is an important issue within the field of aluminum reduction with pre-baked cells. The characteristic correlation between excess AlF3 concentration and aluminum electrolyte temperature was explored through the modeling of heat and mass transfer processes in industrial pre-baked aluminum reduction cells. A coupled heat/mass-balance model was derived theoretically from the mass and energy balance of an electrolysis cell, and then was simplified properly into a practical expression. The model demonstrates that if environmental temperature and Al2O3 concentration keep constant, the excess AlF3 concentration decreases with the aluminum electrolyte temperature linearly and its decrease rate is dependent on the heat transfer property of aluminum electrolyte, side wall and cell shell. Secondly, experiments were conducted on site with two industrial cells in an aluminum electrolysis plant. Excess AlF3 concentration and aluminum electrolyte temperature were obtained simultaneously together with other parameters such as Al2O3, CaF2, MgF2 and LiF concentrations. Results show that the maximum absolute error between the tested value and the calculated value of excess AlF3 concentration using the proposed model is less than 2%. This reveals that the coupled heat/mass-balance model can appropriately characterize the correlation between excess AlF3 concentration and aluminum electrolyte temperature with good accuracy and practicability.     

Investigation of the Ledge Structure in Aluminum Smelting Cells

In aluminum smelting cells, ledges freeze on to cell walls from the cryolitic bath when the temperature drops below the bath liquidus point. Modern cell design and control cause a suitable ledge profile to form and be maintained, in order to protect the cell walls from corrosive liquids (molten salts and Al metal) and ensure efficient current distribution and cell heat balance. During cell operation, a significant ledge, freezing and melting does occur following heat balance changes due to batch operations. The ledge formation mechanism has been studied at the laboratory scale in our previous work. It shows a linkage between the rate and directional nature of ledge growth and its structure as affected through a superheat change. An open ledge structure can dominate the laboratory ledge material growth or melt it out quickly when the superheat either decreases or increases, respectively. This paper begins the investigation of industrial ledge samples, in terms of structure and composition, primarily to identify whether the same ledge formation mechanism exists in industrial cells. In this study, as expected, the industrial ledge shows more complexity than the laboratory ledge; the open structure is different compared to the laboratory ledge due to the inclusion of carbon dust, a large thermal gradient across the ledge, and sufficient aging of the ledge in the cell. The comparison between the laboratory ledge and the industrial ledge has provided insight into the ledge growth mechanism in aluminum smelting cells.

     

Calculation of cooling rate of amorphous aluminum alloy melt-spun ribbons

Based on the heat transfer theory and liquid solidification theory, the heat transfer during the rapid solidification process of amorphous ribbons prepared by melt spinning was approximately modeled by one-dimensional heat conduction equation. Besides, integration with the temperature gradient, the relationship between the ribbon thickness and solidification time was derived according to the boundary conditions of ribbon-copper wheel. A simply theoretical model was obtained to calculate the cooling rates of aluminum amorphous ribbons. According to the above theoretical model, the critical cooling rate of aluminum amorphous ribbons by melt spinning is above 10^6 K/s, which proves that the aluminum based alloys belong to the marginal glass forming ability of alloys. The calculated results are in good agreement with other estimated values reported previously.     

铝合金厚板淬火表面换热系数的离散解析求法

为了快速准确求取铝合金厚板淬火过程的换热系数,对淬火热传导过程进行分析。首先,将换热系数解析过程假设为淬火温度离散化的,并且是相邻离散点可进行迭代优化的计算过程。然后,分步解析求解了各离散温度区间的换热系数,最后完成了数据修正和仿真计算还原。结果表明,该方法获得的换热系数,可以使实验冷却曲线与计算冷却曲线较好的吻合,从而证明这种计算方法的可行性,并在文末对该方法的误差来源和特点进行了分析。     

平面流铸工艺中冷却辊流场和传热三维稳态模拟

平面流铸法是一种制备非晶和纳米晶薄带的先进技术。这项技术的难点不仅在于快速冷却以抑制薄带的晶化,还在于获得高表面质量的薄带。提出了一种用三维稳态模拟模型预测工业生产中广泛应用的2种单辊冷却结构的流场和传热系数分布的新方法。利用FLUENT软件,结合能量和动量方程,计算了2种结构的流速分布。预测了2种结构的对流换热系数分布。结果表明,水槽结构的速度分布无均匀性,也无周期性;水缝结构的速度分布无均匀性,但具有周期性。2种结构的对流换热系数分布不是中心对称的,水缝结构的冷却特性更为规则。预测了水缝结构的3个对称分布的合适区域,并通过连续生产多次验证。根据热平衡原理,描述了平面流铸法工艺的传热过程。对流换热系数的均匀分布可以作为冷却辊结构设计的依据之一。     

一种无污染脱氧方法

将脱氧剂装入氧离子传导的固体电解质ＺｒＯ２（ＭｇＯ）管中,并用高温电子导电材料封堵料口。利用固体电解质电池短路脱氧的原理。金属液中的氧在氧化锆管外表面获取电子转变为氧离子,并通过固体电解质进入氧化锆管。在内表面与脱氧剂结合生成脱氧产物,产生的自由电子通过电子导电材料传递到外表面与其积累的正电荷中知,从而消除了阻碍氧离子继续迁移的电场,使该过程能够持续下去,直至达到脱氧平衡。由于脱氧产物保留在氧化锆     

多孔铂电极的蓄热传热性能及热稳定性研究

本研究应用固体物理理论和方法,研究了氧传感器多孔Pt电极材料的热容量、热导率等蓄热传热性能及其热稳定性随温度、时间和晶粒半径的变化规律,探讨了原子非简谐振动对电极材料蓄热传热性能及热稳定性的影响。研究表明,多孔Pt电极的定容热容量随温度的升高先增大后趋于恒定,随晶粒半径和时间的增大而减小;多孔Pt电极的蓄热性能热稳定性系数随温度的升高先急剧增大后迅速减小,最后趋于恒定,在温度约60 K时,其蓄热性能热稳定性最差;多孔Pt电极的热导率随温度的升高先急剧减小后趋于恒定,随晶粒半径的增加而增大,随时间的增长而减小;表面层对热导率的贡献随温度升高先急剧减小后趋于0;多孔Pt电极原子振动的非简谐效应使其热容量有所减小,而使蓄热性能热稳定性系数和热导率有所增大。本研究所得结果与其他文献的结果基本一致,其结论可为固体电解质氧传感器的稳定性问题提供理论指导。     

基于DEFORM反传热模型表面换热系数的确定

以7075铝合金厚板淬火过程为对象,研究DEFORM反传热模型中控制参数对表面换热系数计算和温度预测精度的影响规律。结果表明,当选择实测温度曲线上的拐点温度作为温度控制点,且表面换热系数初始值接近平均换热系数时,采用反传热模型确定的表面换热系数所预测的冷却曲线与实测曲线吻合较好。在此基础上选取合理的控制参数,并确定了7075铝合金厚板淬火过程的表面换热系数,经冷却曲线预测结果与实测值对比表明,采用DEFORM反传热模型确定的表面换热系数所预测的温度场有较高精度,可以满足工程应用需要。     

电磁离心凝固过程熔体流动和传热的有限元数值模拟

利用电磁流体动力学有关理论,建立了电磁离心凝固过程熔体流动与热量传输的耦合分析模型,采用有限元法对电磁离心凝固过程进行了分析,得出了瞬态速度场和温度场。分析结果表明,电磁离心凝固过程中,正是由于电磁力引起的受迫对流运动,对金属熔体起电磁搅拌作用并使晶粒得到细化,从而提高铸件质量。     

Numerical modeling and analysis of the thermal behavior of copper molds in continuous casting

In this study, we establish a 3-D numerical analysis model to analyze the thermal behavior and to obtain the detailed heat transfer coefficient of a copper mold in a continuous casting system. This heat transfer coefficient changes according to variations in the mold geometry or cooling system. For increased flow speeds of the cooling water, the heat transfer coefficient also increases, but the rate of increase for the coefficient diminishes at higher flow speeds. As the thickness of the mold between the melt and the cooling water slots increases, the mold’s heat transfer coefficient decreases. However, the uniformity of the heat transfer coefficient improves with greater thickness. The effect of distance between cooling water slots on the mold’s heat transfer coefficient is also observed. Calculations show that greater distances between cooling water slots decrease the heat transfer coefficient.     

基于瞬态法的铝合金与模具钢接触换热实验研究

采用瞬态实验法研究了7050铝合金与5CrMnMo模具钢界面间接触换热过程,探讨了接触载荷与模具温度对接触换热系数的影响,对比了瞬态法与稳态法的实验结果。研究表明,接触换热系数随时间延长迅速增大到某一基本恒定值;瞬态法与稳态法的结果存在一定差异,但接触换热系数随接触载荷增大而增加的趋势是一致的;在160~340℃温度范围内,接触换热系数随模具温度升高而增大,为实际锻造过程中模具的预热提供了参考数据。     

400kA电解槽热场平衡及分析

利用多点热流计(HFM-215)和烟尘平行采样仪(TH-880F)对正常运行的两台400kA电解槽进行了热平衡测试,结合电解槽能量平衡理论与相关热量计算方法,对电解槽能量平衡情况进行了计算和分析。结果表明:400kA电解槽的侧部槽壳熔体区温度在195.1℃～308.8℃之间,不存在温度过高的区域,说明电解槽的侧部炉帮形成较好;槽底温度在55.5℃～116℃之间,整体保温较好;电解槽热平衡部分多余热量的60%以上从电解槽上部随烟气或槽盖板处散出,使槽壳的散热压力降低,有利于形成较好的炉帮,有利于电解槽高效稳定运行。400kA电解槽由于可在工作电压较低(约为3.91V)的条件下运行,能量利用率较高,约为49.02%。     

Exact and approximate methods for determining the thermal parameters of the forging process

Numerical simulation of the forged body plastic flow is a very important and indispensable technique in the mechanical forming. The correct simulation of changing the shape of a hot blank requires the use of very accurately determined physical parameters. There is thus a pressing need to create and supplement a database containing the exact physical parameters of forged materials. The following paper shows the results of development of methods to determine two thermal parameters of a solid body: the average value of the total emissivity and the average coefficient of heat transfer by convection. The methods of determining thermal parameters, described below, are considerably simpler than traditional ones, and they do not require the use of complicated equipment. The exact method is based only on processing of experimental data. In the approximate method, both the experimental data and theoretical values of thermal parameters available in the literature are used.

The developed methods were applied to determine thermal parameters of test bodies of SAE 1045 steel and ABNT 6061 aluminum alloy.     

Simulation of thermal behavior of a CNC machine tool spindle

The thermal deformations of a CNC machine tool spindle are the major contributor of thermal error. It is very significant both theoretically and practically to study how to accurately simulate the thermal error of the spindle. Firstly, this paper proposes a method for computing the coefficient of convection heat transfer of the spindle surface by referencing the theory on computing the coefficient of convection heat transfer of a flat plate when air flows along it. Secondly, the temperature field and thermal errors of the spindle are dynamically simulated under the actions of thermal loads using the finite element method. Thirdly, the characteristics of heat flow and thermal deformation within the spindle are analyzed according to the simulation results. Fourthly, the selection principle of thermal key points, which are indispensable for building a robust thermal error model, is provided based on the thermal error sensitivity technology. At last, a verification experiment is implemented on a CNC turning center, and the results show the simulation results are satisfying to replace the experiment results for further studies.     

Numerical analysis of thermal fluid transportation during electron beam welding of 2219 aluminum alloy plate and experimental validation

A three-dimensional mathematical model using volume-of-fluid method is developed to investigate the heat transfer,fluid flow and keyhole dynamics during electron beam welding of 2219 aluminum alloy plate.In the model,an adaptive heat source is employed to simulate the heating process of electron beam.Fluid flow is mainly driven by surface tension,thermo-capillary force,recoil pressure,hydrostatic pressure and thermal buoyancy.The thermal-fluid transport behaviors of welding pool during the drilling and backfilling stages of keyhole and the formation reason of the nail-shaped weld with an arc crater are systematically analyzed.Finally,all calculation results are validated by experiments and show good agreements.     

填充纯铝颗粒对热态粘性介质传热性能的影响

针对粘性介质温热成形过程中导热率低的问题,提出了在粘性介质中填充一定量的高导热率颗粒的方法来提高其导热率。以纯铝颗粒为研究对象,通过纯铝颗粒在粘性介质中的沉降试验、不同填充参数的粘性介质导热率测量试验及传热过程的有限元分析,确定了纯铝颗粒粒径大小和质量分数对粘性介质传热过程的影响。研究结果表明:由于纯铝颗粒密度大于粘性介质密度,纯铝颗粒在粘性介质中存在沉降现象,沉降速度随温度的升高和粒径的增大而增加;同时,随着纯铝颗粒粒径的增大和填充质量分数的增加,粘性介质的导热率增大,有利于外部热量向粘性介质内部的传递;一定温度条件下,选择合适粒径和填充质量分数的纯铝颗粒作为粘性介质的填充物,对粘性介质传热性能的改善效果较为明显。     

Development and application of coupling model of aluminum thin-gauge high-speed casting

Based on the analyses of aluminum melt flow, solidification, heat transfer during the process of twin-roll casting, a coupling mathematical model of aluminum thin-gauge high-speed casting was developed, which included the casting roller shell. At the same time, Galerkin method was adopted to solve the coupling model. The fluid field and temperature field of aluminum melt in casting zone, the temperature field and thermal stress field of roller shells were simulated by the coupling model. When the casting velocity is 7 m/min, and the thickness of strip is 2 mm, the circumfluent area comes into being in the casting zone, and the mushy zone dominates the casting zone, while the temperature of melt decreases rapidly as it approaches the rollers. The temperature of the roller shell varies periodically with the rotation of roller, and reaches the highest temperature in the casting zone, while the temperature of roller shell decreases gradually as it leaves the casting zone. The difference of thermal stress between the inner surface and outer surface of the roller shell is very large, and the outer surface suffers tensile-compressive stress.