Melt Convection Research Articles

Influence of Sulphur Content on Structuring Dynamics during Nanosecond Pulsed Direct Laser Interference Patterning.

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Interactions between hybrid nanosized particles and convection melting inside an enclosure with partially active walls: 2D lattice Boltzmann‐based numerical investigation

AbstractThe ice melting is investigated inside a square cavity with two isothermally partially active walls. The concept of dispersing hybrid alumina–Cu nanoparticles and hybrid silica–multiwalled carbon nanotubes (MWCNTs) nanoparticles is recommended for thermal performance enhancement in this thermal energy storage (TES) system. The two‐dimensional explicit lattice Boltzmann convection melting scheme in the single‐phase model is applied to account for the natural convection flow induced in the melt region and evolution of the solid–liquid interface. The complete melting time for the pure phase change material (PCM) using case (II) is 33.3% lower compared with other cases. If the price of hybrid Al2O3–Cu nanoparticles and heat storage capacity is important, the full melt time diminishes by 16.6% with a volume fraction of 0.01 in case (II). Once hybrid silica–MWCNT nanoparticles with a volume fraction of 0.01 are utilized inside case (II), the lowest charging time is achieved. The complete melting time abates by 23.66% in contrast to the pure PCM melting. The use of single/hybrid nanoparticles to enhance the PCM melting is not necessarily economical as efficient positions of active parts could further lessen the charging time. The efficiency of hybrid nanoparticles is linked to the type and weight proportions of nanoparticles, and positions of thermally active parts.

Lattice Boltzmann simulation of natural convection melting in a cubic cavity with an internal cylindrical heat source

In this work, natural convection melting in a cubic cavity with an internal cylindrical heat source is studied numerically by employing the two-relaxation-time lattice Boltzmann (LB) method. In numerical simulations, natural convective melting of pure Gallium in a three-dimensional enclosure is first considered to validate the present LB model. Then, effects of some key parameters, including the thermal boundary conditions at the walls of the outer enclosure, the location and the placement direction of the inner cylinder are numerically studied and analyzed. It is found that the full melting time obtained for the case of the constant temperature boundary condition is larger than that obtained for the case of the adiabatic boundary condition. In addition, the variation of the location of the inner cylinder plays an essential role in full melting time, and when the inner cylinder is located at the center of the cavity, the full melting time obtained in such a case is always smaller than the other cases. Further, the streamlines as well as the distributions of the temperature obtained for different conditions are also presented and discussed.

Natural convection melting in a cubic cavity with internal fins: A lattice Boltzmann study

In this paper, natural convection melting in a cubic cavity with internal fins is numerically investigated in detail by employing the lattice Boltzmann method. To have a better understand of the influence of the internal fins on melting process, the configurations of double fins and single tree shaped fin are both considered. In numerical simulations, a parametric study is carried out by varying the fin configuration in the premise of fixed total fins volume, and the influences of the fin configurations on contributing natural convection patterns, melting time as well as the interface area are discussed in detail. Based on the numerical results, it is observed that the existing of fins not only enhances the heat transfer performance, but also maintains similar total amount of stored energy compared with the no-fin case. In addition, the present work also illustrates how changing the fins structures offers a higher rate of heat transfer and a better energy storage capacity rather than increasing the number of fins.

The role of double-diffusive convection in basal melting of Antarctic ice shelves

The Antarctic Ice Sheet loses about half its mass through ocean-driven melting of its fringing ice shelves. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica's contribution to global sea level. We use high-resolution large-eddy simulation to examine ocean-driven melt, in a geophysical-scale model of the turbulent ice shelf-ocean boundary layer, focusing on the ocean conditions observed beneath the Ross Ice Shelf. We quantify the role of double-diffusive convection in determining ice shelf melt rates and oceanic mixed layer properties in relatively warm and low-velocity cavity environments. We demonstrate that double-diffusive convection is the first-order process controlling the melt rate and mixed layer evolution at these flow conditions, even more important than vertical shear due to a mean flow, and is responsible for the step-like temperature and salinity structure, or thermohaline staircase, observed beneath the ice. A robust feature of the multiday simulations is a growing saline diffusive sublayer that drives a time-dependent melt rate. This melt rate is lower than current ice-ocean parameterizations, which consider only shear-controlled turbulent melting, would predict. Our main finding is that double-diffusive convection is an important process beneath ice shelves, yet is currently neglected in ocean-climate models.

Numerical simulation of heat transfer and fluid flow during nanosecond pulsed laser processing of Fe78Si9B13 amorphous alloys

Nanosecond pulsed laser processing is a promising method to modify the microstructure of amorphous alloy. However, the heat and mass transfer mechanisms during processing are still unclear, which prevent regulating the microstructure precisely. In this study, a 3D heat transfer and flow coupling model is established to investigate the heat transfer and fluid flow behaviors, and their effects on the surface morphology and microstructure formation during the nanosecond pulsed laser processing of Fe78Si9B13 amorphous alloy. The physical processes such as melting and solidification, melt evaporation, melt convection and heat conduction, as well as the main driving forces such as surface tension, recoil pressure and gravity are taken into account in this model. The simulation morphology is in good agreement with the experimental results, indicating the validity of this numerical model. Based on simulation data analysis, it is found that the evolution mechanism of surface morphology is mainly related to the recoil pressure caused by evaporation and the Marangoni effect induced by the surface tension spatial gradient. Combined the temperature field analysis with experimental characterization, the formation mechanism of new amorphous phase in the molten zone (cooling rate at the order of 107 K/s) and the precipitation behavior of α-Fe(Si)/Fe-B phase nanocrystals in the heat-affected zone are clarified.

Effect of Internal Radiation on Heat Transfer during Ti:sapphire Crystal Growth Process by Heat Exchanger Method

In this study, the effect of internal radiation on the heat transfer during titanium-doped sapphire (Ti:sapphire) crystal growth process by heat exchanger method (HEM) was comprehensively investigated based on a global heat transfer model. Radiation absorption, emission and scattering in the semitransparent melt and crystal were modeled with the rigorous finite volume method. The numerical results show that internal radiation strongly enhances the heat transport in the melt and crystal, strengthens the melt convection and reduces the temperature gradient in the central body of the crystal. However, it renders the isotherms intensively concentrated in the narrow bottom region of the crystal, resulting in a significant increase in the temperature gradient in this region compared to the case without internal radiation. What's more, internal radiation makes the melt-crystal interface deeply convex towards the melt, which could be responsible for the radial non-uniformity of titanium concentration in the large Ti:sapphire crystal by HEM. The sensitivity of internal radiation to the optical properties was also parametrically studied. It was found that the heat transfer is strongly depended on the absorption coefficients of the crystal and melt. Particularly, the interaction between internal radiation and heat conduction in the crystal is strongest at the intermediate value of the absorption coefficient of the crystal, leading to the largest values of conduction heat transfer rate and maximum temperature gradient at the bottom of the crystal. The influence of scattering in the crystal becomes significant only after the scattering coefficient is larger than absorption coefficient. This study suggested that the effect of internal radiation on the heat transfer during HEM Ti:sapphire crystal growth process shows some difference from that during Kyropoulos and Czochralski oxide crystal growth processes.

Numerical Study of the Upgraded Hot Zone in Silicon Directional Solidification Process

Abstract2D global transient model for generation‐six (G6) GT‐style furnace and upgraded generation‐seven (G7) ALD‐style furnace in which all types of heat transfer and flow are included is established to investigate the thermal field, melt convection, melt–crystal (m–c) interface shape, thermal stress, growth rate, and Voronkov ratios in the growing silicon ingot. The modeling is verified by the heater power and temperature experiment. Simulation results show that the melt flow is relatively stronger as the furnace upgrades. For G7, a relatively higher thermal stress and growth rate are found due to the higher temperature gradient both in the horizontal and axial directions. Furthermore, unlike the optimized G6, G7 shows the overly convex m–c interface in the initial stage and edge nucleation throughout crystal growth stage.

Thermal-solutal-fluid flow of channel segregation during directional solidification of single-crystal nickel-based superalloys

Channel segregation – known as freckle – is one of the most complex defects which severely limit mechanical properties of investment cast single-crystal turbine blades. The freckle phenomenon is induced by the thermal-solutal-fluid flow instability during solidification process; it is therefore important to gain better understanding of the fluid flow, heat transfer and species transport during the investment casting process. In this work, a three-dimensional Eulerian two-phase model is developed to investigate chimney behaviours and the evolution of channel segregation in the directional solidification of nickel-based single crystal superalloy CMSX-4. The results reveal that the channels are merely distributed on the surface of ingot backed up with targeted experiments, and the morphology of the predicted channel segregation is also consistent with the observed freckle chains. By ignoring lateral heat flux, deviation in the prediction of channel segregation using Rayleigh number criterion is anticipated, because not all the chimneys can be developed into channel segregation under the significant perturbation. In the mushy zone, the perturbation of the melt convection from the horizontal direction to the casting direction leads to the extinction of existing chimneys, thereby exacerbating channel segregation. Once the stable channel segregation is formed, it will be developed steadily under further solute enrichment caused by thermal-solutal convection. Channel segregations is difficult to develop continuously in the centre of the representative rod, while channels near the lateral walls are more likely to develop stable flow along the casting direction. It is suggested that the weaker lateral heat flux can suppress the formation of channel segregation, providing an effective process design criterion for the freckle mitigation during the single crystal casting.

Study of Natural Convection Melting of Phase Change Material inside a Rectangular Cavity with Non-Uniformly Heated Wall

In the present numerical study, the convection diffusion phenomena associated with solid-liquid phase transition processes during phase change material (PCM) melting within a rectangular cavity is studied. The cavity is heated from left wall with a sinusoidal temperature distribution. Initially the enclosure was filled by solid gallium at melting temperature 29.78°C. The enthalpy-based lattice Boltzmann method (LBM) with D2Q9 particle velocity model is used to solve density, velocity and temperature fields. Influence of Rayleigh number ranging from 103 to 4×105 on streamlines, isotherms and liquid fraction is analyzed. The results indicate that natural convection of liquid phase change material (PCM) plays a significant role in the melting heat transfer of PCM. It is found that the rate of the melting increases with the increase in the values of the Rayleigh number.

Three-dimensional effect of high frequency vibration on convection in silicon melt

The effect of high-frequency vibrations with different orientations on the convective flow in a liquid metal inside a cubic cell heated from the side is investigated using the collocation spectral method. The vibration is in three dimensions with a vibration vector contained in one of the three principal planes of the cavity ($x\phantom{\rule{0}{0ex}}z$, $x\phantom{\rule{0}{0ex}}y$, and $y\phantom{\rule{0}{0ex}}z$ planes). In either weightlessness or gravity conditions, the influence of the vibrations on the flow is found to strongly depend on their direction: each vibration orientation has its own features and affects the flow intensity and pattern differently.

A general numerical procedure for solidification and melting in porous media and free fluids

A numerical model of the solid/liquid phase change problem in porous media and free fluids is presented in this paper for the first time by employing a modified version of the generalized porous medium model, in 2D and 3D, solved by means of the Characteristic-Based-Split (CBS) scheme. The proposed procedure is based on the solution of one set of equations to describe different phenomena: natural convection in porous media, solidification and melting both in porous media and in free fluids. The generalized model has been modified to incorporate the apparent heat capacity, source term and pseudo-viscosity methods in order to reproduce phase change phenomena.Two versions of the algorithm, the Semi-Implicit (SI) and the Quasi-Implicit (QI) schemes, have been developed to solve the governing equations, and validated against numerical and experimental data available in the literature. The two schemes have been compared in terms of accuracy and efficiency for the solution of the benchmark problems.

Relationship of Free Surface Area with Oxygen Concentration in Silicon Ingot Grown by Czochralski Method for High Efficiency Solar Cells

The relationship of free surface area with temperature, power, and oxygen concentration in a silicon ingot grown by Czochralski method for solar cells was investigated by simulating the growing process of 4-, 6-, and 8-inch diameter ingots with a 24-inch quartz crucible. It was confirmed that the temperature at auto-temperature-control (ATC) sensor of 4-inch diameter ingot was higher than 6- and 8-inch diameter ingot due to the longer distance between triple point and ATC sensor, i.e., larger area of the free surface. In addition, it was observed that the power consumption for growing 4-inch diameter ingot was greater than 6-and 8-inch diameter ingot because of its smaller heat capacity resulted from smaller charge size. In particular, it was confirmed that the oxygen concentration of 4-inch diameter ingot was lower than those of 6- and 8-inch diameter ingots since the larger free surface area and weaker melt convection led to the lower oxygen concentration in 4-inch ingot. This understanding of the relationship of free surface area with oxygen concentration in a silicon ingot can be applicable directly to control oxygen concentration in growing different diameter of silicon ingots.

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field (HMF) on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis (DTA). It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heating-cooling rates. Moreover, the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc. Regardless of magnetism, the nucleation frequency under an HMF is higher than that without an HMF. Furthermore, the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle, calculated by the functional relationship between the cooling rate and undercooling. This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc. Additionally, the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.

Influence of melt convection on distribution of indium inclusions in liquid-encapsulated Czochralski-grown indium phosphide crystals

Indium (In) inclusions have been found in 〈100〉 indium phosphide (InP) single crystals grown in In-rich melt by the liquid-encapsulated Czochralski (LEC) method. Two kinds of In inclusions with different morphologies, one is lath-like and the other is polyhedral, have been observed. Most of the In inclusions are lath-like and their long sides are always parallel to the $$[0\overline{1}1]$$ or $$[0{\overline{1}}{\overline{1}}]$$ orientation. In (100) InP wafers, these inclusions mainly distribute in two regions viz., center of the wafer and annular belt at a certain distance from the center. These special distributions of In inclusions in InP crystals were found in our experiments for the first time. In order to clarify the cause of the special distributions of In inclusions, numerical simulations have been carried out on the melt convection in the process of InP crystal growth. The results show that the rotations of crystal and crucible significantly affect the number and direction of convection cells in the melt, which have a great influence on the enrichment and distribution of excess In at the solid–liquid interface. Different melt convections modify the radial distributions of excess In atoms near the solid–liquid interface and result in different radial distributions of In inclusions in the crystals.

Phase-field lattice Boltzmann method with two-relaxation-time model for dendrite growth of a binary alloy with melt convection

To perform accurate, stable, and efficient phase-field lattice Boltzmann (PF-LB) simulations of the solidification of alloys with melt convection, a two-relaxation-time (TRT) model for calculating the collision term in the LB equation and an interpolated bounce-back (IBB) model for imposing a non-slip condition at a solid-liquid interface—(TRT-IBB model)—were introduced. The simulations of the plane Poiseuille flow and the dendrite growth of a binary alloy with forced convection were performed to confirm the computational accuracy and acceleration of PF-LB simulations upon introducing the TRT-IBB model. For dendrite growth with forced convection, the simulations of the TRT-IBB model were in good agreement with those of the conventional single-relaxation-time (SRT) model and the dissipative drag force (DDF) model for imposing the non-slip condition at the diffuse interface (SRT-DDF model). In addition, this study confirmed that the TRT-IBB model enabled stable simulations with ~500-times larger time increment than the SRT-DDF model. Thus, the developed TRT-IBB model was found to be extremely effective in the acceleration of PF-LB simulations for the solidification of alloys with melt convection.

Simulation on DC casting of magnesium alloy under out-of-phase pulsed magnetic field with different coil connection strategies

Four annular coils arranged vertically that have been loaded with two pulsed currents with different initial phases were used to create out-of-phase pulsed magnetic field (PMF) during direct-chill (DC) casting of magnesium alloy. A 2D axisymmetric mathematical model that couples electromagnetic field with fluid field and thermal field was established based on the finite element method by using COMSOL Multiphysics software. A two-step approach was adopted to handle the coupling problems, which the varying Lorentz force and Joule heat were treated as the averaged values during the steady calculation of DC casting process. Effects of coil connection strategies on the resulting Lorentz force, melt flow and solidification characteristics during DC casting of magnesium alloy under out-of-phase PMF were investigated and discussed. Results show that the application of magnetic field greatly strengthens the melt convection when compared to no magnetic field. Using out-of-phase PMF could control the range and intensity of melt convection by changing coil connection strategies, achieving lower overheating in the central part and higher temperature uniformity from the center to the edge or reducing the sump depth than a single PMF in which four coils are connected in series and loaded with the same pulsed current.

A two-dimensional phase-field study on dendritic growth competition under convective conditions

In the present work, the growth competition of columnar dendrites is investigated in the presence of melt convection. By using a multiphase-field model, the microstructural evolution of unidirectionally solidified dendrites with different crystal orientations is addressed in detail. Generally, for converging bicrystal grains, the dendritic growth competition follows a conventional overgrowth model, where a favorably oriented dendrite eliminates an unfavorably oriented dendrite at the grain boundary. However, in the presence of lateral melt flow in the liquid phase, we discover that the conventional overgrowth mechanism transforms into an unusual overgrowth mechanism as the melt velocity gradually increases. Recently observed in bi-crystal studies, the unfavorably oriented dendrites overgrow at the expense of the favorably oriented dendrites during an unusual overgrowth behavior. From our simulations we elucidate that the presence of an additional convective transport in the liquid phase modifies the solute distribution at the grain boundary, which in turn affects the overgrowth mechanism of converging dendrites. We show that there exists a critical melt velocity to determine the growth competition at the grain boundary. In addition, the role of interfacial anisotropy on growth competition is analyzed in detail, where we discern that the conventional overgrowth behavior is dominant at large anisotropic strengths. Lastly, a morphological selection map is depicted to predict the crossover region between the two overgrowth behaviors.

Experimental and numerical investigation of the performance of bogie chassis heater deicing systems

In winter, a large amount of icing and snowing occurs in outdoor equipment and systems, which adversely affects their lifespan. The melting process is divided into two parts: convection melting and gravity shedding melting. This paper is based on an experiment with a real high-speed train unit, and it establishes a three-dimensional computational fluid dynamics model of the bogie area to validate the mathematical model of the winter ice melting experiment. A numerical simulation was used to calculate the airflow in the baffle-enclosed space and to predict the effects of interactions between air and the ice body on heat transfer and phase change. The influence of air velocity and temperature on the heat transfer was analyzed. The computational simulation effectively quantifies the amount of heat transfer and energy consumption under different conditions. This paper also presents a research method for the simulation of gravity shedding in complex models. These models provide information for the construction of similar ice melting models. This paper has suggested that for thin ice bodies, convection melting was the dominant strategy. For thick ice bodies, however, gravity shedding melting became dominant. The study also confirmed that enclosed hot-air ice-melting systems gave a better energy performance than unenclosed systems.

Comparative study on natural convection melting in square cavity using lattice Boltzmann method

The phase change process is of significant importance in the application of phase change material (PCM). In this paper, the natural convection melting in a square cavity was investigated and the enthalpy-based lattice Boltzmann model (TLBM) combined with the pseudo-potential LB model was developed to trace the solid–liquid interface. Four cases with different boundary conditions were calculated and the corresponding temperature contours, velocity vectors and average liquid fraction at various time were compared.Results show that TLBM has good accuracy in simulating the solid-liquid phase transition process. Variation in boundary conditions has appreciable impact on the natural convection process and melting behavior. Comparing model.1 (single heating wall) with model.2 (double adjacent heating walls with cold wall), the maximum liquid fraction is increased 16.7% in model.2 and the additional top heating wall acts to accelerate the average melting rate by 16.7%. The comparison between model.3 (double opposite heating walls) and model.4 (double adjacent heating walls without cold wall) shows that the natural convection heat transfer is intensive in model.3 and resulting in a 280% faster melting rate than that in model.4. Comparing the melting rate between the model.5 (all heating walls) and model.3, the increase of 54% due to the more heating walls in model.5 is observed.