Transient mixed convection with internal heat generation and oscillating plate temperature

Summary. Oscillating plate temperature effects on transient mixed convection heat transfer from a porous vertical surface with internal heat generation or depletion are considered. The governing equations are transformed into dimensionless form by a set of variables and then solved using the finite element method. It is found that the velocity inside the boundary layer increases and the temperature decreases as time passes. In addition, it is found that when the energy generation is increased, the temperature near the wall will be higher than the wall temperature, and the velocity inside the boundary layer will increase due to an increase of buoyancy forces. Increasing the energy depletion term decreases the velocity inside the boundary layer and increases the heat transfer rate. Different temperature and velocity profiles are drawn for different dimensionless groups. Numerical values of Nusselt number as well as local skin friction coefficient are tabulated. Comparison with previous works shows complete agreement.     

Effect of three-dimensional melt pool convection on process characteristics during laser cladding

A three-dimensional heat transfer model is developed to simulate the cladding process that include the different physical phenomena such as heat transfer, phase changes, addition of powder particles and fluid flow due to Marangoni–Rayleigh–Benard convection. It is found that the Rayleigh–Benard convection is insignificant and Marangoni–Benard convection is dominant for the studied cases. By varying the scanning speed and Marangoni number the melt pool size and strength of convection are changed and its influence on clad built-up geometry, dilution level, maximum and average melt pool temperatures and the form and scale of the microstructure of the solidified clad track has been studied.     

Convection phenomena at reduced gravity of importance in space processing of materials

The basic aspects of convection processes are delineated. It is shown that even in weak gravitational fields buoyancy can induce fluid motions. Furthermore, at reduced-gravity other non-gravity forces such as surface or interfacial tensions,g-jitter, thermal-volume expansions, density differences due to phase changes, and magnetic and electric fields can induce fluid motions. The various types of flow possible with these various driving forces are described and criteria for determining the extent and nature of the resulting flows and heat transfer are presented. The various physical mechanisms that can occur separately and in combination are indicated and the present state of knowledge of each of the phenomena is outlined.     

Effect of electromagnetic stirring on melt pool free surface dynamics during vacuum arc remelting

The deformation of the free surface of the liquid pool in the VAR process may play a key role in explaining the apparition of surface defects on the ingot skin. In the present study, the deformation of the surface induced by electromagnetic stirring is investigated, for a simplified geometry, using CFD-based simulations, combining models for the turbulent fluid flow, heat transfer, free surface movements and electromagnetic forces. Particular attention is given to the implementation of thermal and electrical boundary conditions at the moving free surface. Verification of the developed model is carried out by comparison with the dedicated code SOLAR. The free surface dynamics of a liquid zirconium pool is then quantitatively analysed for different stirring procedures, and the effects of the stirring parameters (magnetic induction, reversal time) are predicted and discussed. The obtained results provide some insights into mechanisms by which surface deformation may influence the ingot skin solidification, leading to poor ingot surface quality.     

Research on distribution of aluminum in electron beam melted silicon ingot

The purification of metallurgical grade silicon, especially the removal of aluminum, was investigated by electron beam melting and solidification. Small amounts of silicon raw materials were melted in an electron beam furnace with same melting time and different solidification time to obtain the distribution of Al in silicon ingot. The removal mechanisms in different stages were also discussed. The results show that the removal of Al during melting process only depends on evaporation and that during solidification process depends on both segregation and evaporation. The distribution of Al shows an obvious increasing trend from the bottom to the top of the silicon ingot when solidification time is 600 s. The removal efficiency in most area is close to that in the ingot solidified instantaneously, but the energy consumption is less, which is considered to be an effective way for the purification of silicon.     

真空自耗电弧重熔凝固过程的计算机模拟

真空电弧重熔(Vacuum Arc Remelting,VAR)工艺参数显著影响铸锭质量,采用计算机模拟技术可以有效预测各种工艺参数对铸锭凝固过程的影响.本工作介绍了国外对VAR工艺中计算机模拟熔池形状、流场、温度场及铸锭微观组织演化的最新进展.其模拟结果和实验值具有较高的吻合度.     

Pb-Sn合金侧向凝固过程A偏析的数值模拟

基于二元系凝固过程热溶质的传输行为,建立了描述A偏析形成及演化的数学模型,给出了固相分数与温度场及浓度场的耦合关系.先用已有的实验结果验证了模型的正确性,然后模拟计算了Pb-Sn合金侧向凝固过程A偏析的形成及演化过程,并研究了浮力数对A偏析形成位置及偏析程度的影响.结果表明,在糊状区中双扩散对流引起的密度变化,导致局部流动,形成偏析通道;为了维持偏析通道中局部液体的流动,枝晶间的液体通过糊状区从液相区得到补充.在相同的凝固条件下,浮力数越小,A偏析的形成时间愈迟,偏析的程度也越小.     

利用强磁场控制过共晶铝硅合金的凝固组织

研究了静磁场和梯度磁场的强度和方向对Al-15.7%Si合金宏观和微观凝固组织的影响.结果表明,在不同的磁场条件下,从过共晶合金中析出的初晶硅粒的分布状况和共晶硅的形态和密度有显著不同.通过改变磁感应强度和磁场梯度的大小和方向可有效控制初晶硅的分布;合理控制强磁场的操作参数可达到细化铝硅共晶体的目的.强磁场的磁化力和洛伦兹力通过控制初晶硅颗粒迁移行为来改变其在合金基体中的分布状态,通过影响凝固过程中的对流现象改变合金的凝固组织.     

Effect of Welding Parameters on GTA Weld Shape for Pure Iron Plate under Ar-O2 Mixed Shielding

Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc (GTA) welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the argon base shielding gas can effectively adjust the oxygen adsorption to the molten pool. An inward Marangoni convection occurs on the pool surface when the oxygen content in the weld pool is over the critical value, 80×10-6, for pure iron plate under Ar-0.3%O2 mixed shielding. Low oxygen content in the weld pool changes the inward Marangoni to an outward direction under the Ar-0.1%O2 shielding. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the content of surface active element, oxygen, in the weld pool and the welding parameters. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension (dσ/dT) and the temperature gradient (dT/dr) on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.     

An experimental study of surface-tension induced convection at reduced gravity

Fluid motions are induced by surface-tension gradients on the free surface of a liquid in a cylindrical container which is in free fall in a drop tower. Identical experiments are conducted in a normal gravitational environment. The motion in that case is due to natural convection. The ratio of surface-tension to buoyancy forces in the tests is of the order of 10⁴ at reduced gravity conditions and 0·13 under normal gravity. In each case a two way flow occurs throughout most of the liquid. Comparison of the results from the two types of experiments is made in order to indicate differences in the flow details.     

Thermocapillary Convection and Buoyant-Thermocapillary Convection in the Annular Pools of Silicon Melt and Silicone Oil

The thermocapillary convection and buoyant-thermocapillary convection in the annular pool of silicon melt (Pr=0.011) and silicone oil (Pr=6.7) with depth d=10 mm differentially heated at the outer wall and cooled at the inner wall are investigated by 2-D numerical simulation. The numerical results exhibit that the thermocapillary flow is enhanced by buoyancy force for silicon melt while it is weakened for silicone oil. Linear stability analysis indicates that the buoyancy force destabilizes the thermocapillary convection, which is different from that for silicone oil. The detailed reason of different influence of buoyancy force on the thermocapillary flow with different Pr numbers is explained according to present numerical results.     

Composition variations in directionally solidified InSb-GaSb alloys

Melts containing 10%, 30% and 50% InSb were directionally solidified horizontally, vertically and in Skylab 3 and 4. The resulting concentration profiles were much more uniform in the ingots solidified horizontally. Fluctuations in composition occurred sooner as the InSb content increased and later in the horizontally processed ingots. In the 10% InSb ingot processed in SL-3 a large orientation-dependent segregation was observed at about 2 cm from the initial position of the interface. A radial variation in composition was also observed, and tentatively attributed to surface-tension driven convection.     

Boundary domain integral method for the study of double diffusive natural convection in porous media

The main purpose of this paper is to present a boundary domain integral method (BDIM) for the solution of natural convection in porous media driven by combining thermal and solutal buoyancy forces. The Brinkman extension of the classical Darcy equation is used for the momentum conservation equation. The numerical scheme was tested on a natural convection problem within a square porous cavity, where different temperature and concentration values are applied on the vertical walls, while the horizontal walls are adiabatic and impermeable. The results for different governing parameters (Rayleigh number, Darcy number, buoyancy ratio and Lewis number) are presented and compared with published work. There is a good agreement between those results obtained using the presented numerical scheme and reported studies using other numerical methods.     

Numerical investigation of droplet impact on the welding pool in gas metal arc welding

A transient three‐dimensional model that describes physical phenomena inside a welding pool during gas–metal arc welding process is presented. The model considers such phenomena as heat‐mass transfer, electromagnetics, hydrodynamic processes and deformation of the weld pool free surface. The fluid flow in the weld pool is induced due to the presence of the mechanical impact of the droplets, thermo‐capillary surface tension, thermal buoyancy and electromagnetic forces. The weld pool surface deformation is calculated by considering arc pressure and droplet impact force. A comparative analysis of the impact of the electric current of the welding arc and different force factors causing the motion of liquid metal in the weld pool on the shape of the welded seam was carried out and discussed.     

MHD 3D free convective flow of nanofluid over an exponentially stretching sheet with chemical reaction

This paper deals with a problem where the effect of variable magnetic field and chemical reaction on free convective flow of an electrically conducting incompressible water based nanofluid over an exponentially stretching sheet has been investigated. In the present study, Buongiorno model associated with Brownian motion and thermophoretic diffusion is employed to describe the heat transfer enhancement of nanofluids. Some suitable similarity transformations reduced the governing boundary layer non-linear partial differential equations into a set of ordinary non-linear differential equations. The transformed equations are then solved numerically using fourth order Runga-Kutta method along with Shooting technique. The major outcomes of the present study is that the magnetic field impedes the fluid motion while thermal as well as mass buoyancy forces accelerate it, the thermophoretic diffusion enhances dimensionless fluid temperature as well as concentration leading to thicker thermal and concentration boundary layers. On the other hand, concentration exponent, Brownian motion parameter and chemical reaction parameter exhibit reverse trend on temperature and concentration. In addition, the presence of magnetic field under the influence of thermal as well as mass buoyancies supports to reduce the rate of heat transfer as well as wall shear stress while the first order chemical reaction develops a thinner concentration boundary layer.     

Effects of slip on Graetz problem of power law fluids with second kind boundary condition

Abstract

The linear convective instabilities of a fluid layer of binary alloy, cooled from above and consequently frozen at the bottom, are considered. Due to the density jump across the freezing interface, some light material is then released and diffused by pressure and composition gradients. As a result of a low cooling rate, the effect of thermal buoyancy is insignificant and the freezing interface advances upward at a slow speed by accumulating the solidified binary alloy. As Schmidt number P_L approaches infinity, instabilities set in stationarily at the marginal state. Cellular convective modes are possible, provided a destabilizing compositional profile occurs in the fluid layer, while morphological modes, associated with non‐cellular convection, require a constitutional supercooling near the freezing interface. In the absence of a constitutional supercooling, morphological modes are not important and cellular convective modes become dominant.     

Fine equiaxed dendritic structure of a medium carbon steel cast using pulsed magneto-oscillation melt treatment

The application of a pulsed magneto-oscillation (PMO) technique during the solidification of a commercial high melting point medium carbon steel ingot (φ140 mm 9 450 mm) produced fully equiaxed grains in the cast ingot, indicating that the PMO process significantly promotes heterogeneous nucleation near the solid-liquid interface. The vigorous convection induced by PMO forced the partly solidified grains to move from the solid-liquid interface and became randomly distributed throughout the melt, which resulted in the formation of uniformly sized equiaxed dendrites throughout the whole ingot. Building on the developed nucleation mechanism and a flow field simulation of pure aluminum, a PMO-induced grain refinement model for steel is proposed.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-017-0206-5     

Methods of low gravity simulation

The dependence on gravity forces of both static and dynamic fluid behaviour is explained, and dimensionless numbers are defined which express the relative influence of gravitational and various other forces. Methods of reducing gravitational effects are outlined, including neutral buoyancy, free fall and orbital flight and their relative merits, in studies of various types of fluid behaviour, are compared.     

Purifying multicrystalline silicon by directional solidification method with induced electromagnetic field

Abstract

As an improved directional solidification (DS) method, the complex directional solidification (CDS) method is used for purifying and preparing multicrystalline silicon ingot in this experiment. The induced electromagnetic field is imposed to control refining and solidification process. An integral silicon ingot with the diameter of 130 mm, the length of 130 mm and the weight of 4 kg is successfully fabricated in a self-designed CDS furnace. Metallographic analyses reveal that the direction of the most grains is parallel to the axial of silicon ingot. Analyses proved that the distribution of impurities in the cross-section is more homogeneously, the distribution in axial is improved and the effective length of silicon ingot is increased. Theoretical calculations indicate that the effect of solidified rate on the removal of impurities is limited and the impurities can be removed effectively after more than two times directional solidification process.     

Numerical simulation of microparticles transport in a concentric annulus by Lattice Boltzmann Method

Dispersion and removal of microaerosol particles are investigated numerically in a horizontal concentric annulus by Lattice Boltzmann Method and Lagrangian Runge–Kutta procedure with the assumption of one-way coupling. Drag, buoyancy, gravity, shear lift, Brownian motion and thermophoretic are forces that are included in particle equation of motion. All simulations were performed at Rayleigh number of 10⁴ and particles specific density of 1000. The effect of aspect ratio and particles diameter were determined on particles behavior such as removal and dispersion. Results show that recirculation power increases by decreasing of cylinders gap. Particles move in a thinner quasi-equilibrium region by increasing of their diameter and decreasing of cylinders gap. Brownian motion is dominant removal mechanism in particle with diameter of 1 μm.