Interface Temperature Gradient Research Articles

Freezing Shrinkage Dynamics and Surface Dendritic Growth of Floating Refractory Alloy Droplets in Outer Space.

The freezing shrinkage and dendritic growth are of great importance for various alloys solidified from high-temperature liquids to solids since they dominate microstructure patterns and follow-up processing. However, the microgravity freezing shrinkage dynamics is scarcely explored on the ground as it is hard to suppress the strong natural convection inside liquid alloys. Here, a series of in-orbit solidification experiments is conducted aboard the China Space Station with a long-term stable 10-5 g0 microgravity condition. The highest temperature up to 2265K together with substantial liquid undercoolings far from a thermodynamically stable state are attained for both Nb82.7Si17.3 and Zr64V36 refractory alloys. Furthermore, the solidification under microgravity of a droplet is simulated to reveal the liquid-solid interface migration, temperature gradient, and flow field. The microgravity solidification process leads to freezing shrinkage cavities and distinctive surface dendritic microstructure patterns. The combined effects of shrinkage dynamics and liquid surface flow in outer space result in the dendrites growing not only along the tangential direction but also along the normal direction to the droplet surface. These space experimental results contribute to a further understanding of the solidification behavior of liquid alloys under a weaker convection condition, which is often masked by gravity on the ground.

Flow structure near three phase contact line of low-contact-angle evaporating droplets

Flow structure near three phase contact line (TPCL) of evaporating liquids plays a significant role in liquid wetting and dewetting, liquid film evaporation, and boiling. Despite the wide focus it receives, the interacting mechanisms therein remain elusive and in specific cases, controversial. Here, we reveal the profile of internal flow and elucidate the dominating mechanisms near TPCL of evaporating droplets, using mathematical modeling, trajectory analysis, and infrared thermography. We indicate that for less volatile liquids such as butanol, the flow pattern is dominated by capillary flow. With increasing liquid volatility, e.g., alcohol, the effect of evaporation cooling, under conditions, induces interfacial temperature gradient with cold droplet apex and warm edge. The temperature gradient leads to Marangoni flow that competes with outwarding capillary flow, resulting in the reversal of interfacial flow and the formation of a stagnation point near TPCL. The spatiotemporal variations of capillary velocity and Marangoni velocity are further quantified by mathematically decomposing the tangential velocity of interfacial flow. The conclusions can serve as a theoretical base for explaining deposition patterns from colloidal suspensions and can be utilized as a benchmark in analyzing more complex liquid systems.

Thermal boundary layer modelling for heat flux prediction of bubbles at saturation: A priori analysis based on fully-resolved simulations

This study presents and investigates uni-directional thermal sub-layer enhancement techniques within the context of an interface tracking method for simulating bubbly flows at saturation. Current discretisation methods on structured and fixed Cartesian grids tend to spread the bubbles' interface region, creating a trade-off between the freely moving nature of the bubble to the detriment of accurately capturing the discontinuous aspect of the interface and variations of properties and fluxes crossing it. Although robust techniques have been described in the literature to ensure energy conservation, less research has been undertaken to develop a methodology to retrieve the non-linear behaviour of quantities in the interface vicinity at a reasonable computational cost. In this study, the discretised bubble surface is used as a basis for addressing several quasi-static radial sub-problems that are bounded by an interfacial constant saturation temperature and a CFD temperature field value. A first approach, based on an analytical solution fitted at each time step using underlying Eulerian field values, has been developed to incorporate near-interface physics. This includes the tangential effect, incoming fluid velocity, and local mean curvature (first order surface approximation). A semi-analytical approach needs to meet certain assumptions to be valid. This is due to its derivation from a simplified plane boundary-layer development or from a spherical diffusion problem which limits its applicability range. Therefore, a second approach based on a uni-directional sub-resolution fed carefully by interpolated velocity and tangential source terms demonstrates promising results as it aligns with the principal variations of the solution. Both methodologies have been applied onto DNS data of a steady rising bubble configuration at low and moderate Reynolds {3.6;62.5} and Prandtl {1;2.5;5} numbers with a constant interfacial temperature after an extensive analysis of the advection-diffusion terms hierarchy. The key aspects to maximise the effectiveness of the sub-resolution method have been clearly identified and discussed. The Sub-resolution shows better applicability to our case study on moderately large thermal layers. The newly predicted interfacial temperature gradient and temperature profile could be re-employed for Eulerian fluxes correction.

Decision Tree-Supported Analysis of Gallium Arsenide Growth Using the LEC Method

In this study, an axisymmetric Czochralski furnace model for the LEC growth of gallium arsenide is presented. We produced 88 datasets through computational fluid dynamics simulations. Among the many parameters that affect crystal growth, a total of 13 input parameters were selected, including the geometry and material parameters of the hot zone (crucible, heaters, radiation shield, and crystal), as well as the process parameters (such as pulling and rotation rates, heating power, etc.). Voronkov criteria (v/Gn), interface deflection, and the average interface temperature gradient were selected as the output parameters. We carried out a correlation analysis between the variables and used decision trees to study the impact of the 13 input variables on the output variables. The results indicated that in the growth of gallium arsenide, the main factor affecting interface deflection and the average interface thermal gradients is the crucible rotation rate. For v/Gn, it is the pulling rate.

An investigation of phase change induced Marangoni-dominated flow patterns using the Constrained Vapor Bubble data from ISS experiments

Kinetic models of liquid-vapor phase change often implicitly assume that the interface is in equilibrium. This equilibrium assumption can be justified for large flat interfaces far from the source of thermal energy, but it breaks down when the liquid surface is near a solid wall, or there is significant interface curvature. The Constrained Vapor Bubble (CVB) experiments conducted on the International Space Station (ISS) provide a unique opportunity to probe this common assumption and also provide unique data and insight into phase change-driven flow physics. The CVB experiment consists of a quartz cuvette partially filled with pentane such that a vapor bubble is formed at the center. The setup is heated and cooled at opposite ends, resulting in simultaneous evaporation and condensation. CVB data from the NASA Physical Science Informatics (PSI) database was used to reconstruct the entire 3D interface shape using interferometric image analysis and obtain an estimate of the net heat input to the bubble. The reconstructed interface shape is used to develop a liquid-only CFD model embedded with a custom-built “active surface” method that sets a variable interfacial temperature/phase change flux boundary condition. Phase change flux varies in both the axial and transverse directions, leading to a small (∼1 K) but discernible temperature variation along the liquid-vapor interface. The positive phase change flux near the heater end (denoting evaporation) gradually reduces and becomes negative near the cooler end (denoting condensation), resulting in an axial bulk flow of liquid from the cold to the hot end. There is also a higher flux in the thin film as opposed to the thick film, resulting in a transverse bulk flow. However, the interfacial temperature gradients along both axial and transverse directions induce a separate thermocapillary flow in a direction opposite to the bulk flows, leading to complex “wavy” flows with recirculation. A qualitative analysis of the flow pattern is presented in this paper and correlated with optical signatures from experimental images.

Thermocapillary oscillations in liquid thin films: Interplay of substrate topography and thermal wave

Liquid flows over patterned and/or contoured substrates is utilized in many practical applications e.g., lubrication flow in MEMS (micro-electromechanical systems) /NEMS (nano-electromechanical systems) devices, cell sorting operations in biology, etc. The topological gradients in such geometries induce thermal gradients, which in turn generates Marangoni hydrodynamics. Here, we theoretically showcase the interplay of substrate topology, imposed wavy temperature stimulus, and chemically patterned wall slip to generate thermo-capillarity in thin liquid films. We obtain analytical solutions for the thermal and the hydrodynamic fields for low Marangoni Ma and Reynolds Re number thermocapillary effect. Vorticity is observed to generate and transport at the liquid-gas interface, and further transported to the bulk film by vertical diffusion and convection. We also study further to understand the significance of different geometric and physical parameters on the Marangoni thermo-fluid-dynamics. Interplay of constructive and destructive interference between the wave forms of the substrate topology and imposed periodic thermal stimulus may yield maximum interfacial temperature gradients and strong thermo-capillarity. It is also observed that the lower values of system Biot number Bi enables better diffusion of heat from the patterned substrate to obtain stable thermo-capillarity. Further, the presence of optimal slip patterning allows for stronger and tuned circulations in the liquid film.

PbF2–CdF2–SrF2 Ternary Solid Solution: Crystal Growth and Investigation

Crystals based on alkaline earth metal difluorides are widely used optical materials. In this study, in order to expand the range of optical matrices, multicomponent Pb1−x−yCdxSryF2 (0.27 < x < 0.55, 0.06 < y < 0.18) solid solution crystals with a fluorite structure (sp. gr. Fm-3m) were grown from melt using the vertical directional crystallization technique for the first time. The densities and refractive indices of the grown crystals vary depending on the quantitative content components (x and y) in the ranges of 6.6039(5)–7.5232(5) g/cm3 and 1.6403–1.7084, respectively. The optical transmission and electrochemical impedance spectra were studied. The homogenous composition regions of non-cellular crystallization of this ternary solid solution at a crystallization rate of 6 mm/h and an interface temperature gradient of 80 deg/cm were experimentally determined as 0.30 < x < 0.35, 0 < y < 0.6. These grown crystalline materials may be of interest as high-density highly refractive cubic isomorphic hosts and low-temperature ionic conductors (~2 × 10−5 S/cm at room temperature) for various applications.

Ultrafast and Nanoscale Energy Transduction Mechanisms and Coupled Thermal Transport across Interfaces.

The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.

Derivation and stability analysis of two-fluid model equations for bubbly flow with bubble oscillations and thermal damping

The two-fluid model with bubble oscillations, proposed by Egashira et al. (2004), can explain the properties of cavitating bubbly flow and pressure wave propagation in the bubbly liquid. However, the viscous effect as well as energy conservation leading to temperature changes inside the bubble with bubble oscillations have not yet been considered. Hence, this study aimed to incorporate the viscous (bulk viscosity and drag) and thermal effects to the previously proposed two-fluid model with bubble oscillations. Bulk viscosity was considered by averaging the shear stress term in the single-phase momentum conservation for a Newtonian fluid, and the drag was introduced by transforming the interfacial shear stress. We derived the averaged energy conservation for a general two-phase flow with a thermal conduction inside bubbles and heat transfer between the two phases, and limited this equation to that for a bubbly flow by closing the interfacial temperature gradient term via constitutive equations for a single bubble. Furthermore, we investigated the stability of our proposed one-dimensional model equations using the dispersion analysis. This analysis provided the following insight: (i) The difference in the temperature gradient models had a slight effect on the stability of the proposed model equations; (ii) the thermal conduction inside the bubbles was dominant in the thermal damping in bubbly flows rather than the heat transfer between the two phases; (iii) incorporating both the bulk viscosity and drag stabilized the proposed model equations. Our results provide insights into the development of mathematical models to investigate the thermal effects in bubbly flow with bubble oscillations, such as cavitating bubbly flow and wave propagation in bubbly liquids.

Numerical investigation of airborne transmission in low-ceiling rooms under displacement ventilation

This study employs computational fluid dynamics (CFD) simulations to evaluate the risk of airborne transmission of COVID-19 in low-ceiling rooms, such as elevator cabins, under mechanical displacement ventilation. The simulations take into account the effects of the human body’s thermal environment and respiratory jet dynamics on the transmission of pathogens. The results of the study are used to propose a potential mitigation strategy based on ventilation thermal control to reduce the risk of airborne transmission in these types of enclosed indoor spaces. Our findings demonstrate that as the ventilation rate (Qv) increases, the efficiency of removing airborne particles (εp) initially increases rapidly, reaches a plateau (εp,c) at a critical ventilation rate (Qc), and subsequently increases at a slower rate beyond Qc. The Qc for low-ceiling rooms is lower compared to high-ceiling rooms due to the increased interaction between the thermal plume generated by the occupants or infectors and the ventilation. Further analysis of the flow and temperature fields reveals that εp is closely linked to the thermal stratification fields, as characterized by the thermal interface height and temperature gradient. When Qv < Qc, hT,20.7 < him (him is the height of infector’s mouth) and aerosol particles are injected into the upper warm layer. As Qv increases, the hti also increases following the 3/5 law, which helps displace the particles out of the room, resulting in a rapid increase of εp. However, when Qv > Qc, hT,20.7 > him and aerosol particles are injected into the lower cool layer. The hti deviates from 3/5 law and increases at a much slower rate, causing an aerosol particle lockup effect and the εp to plateau. In addition, as the Qc increases, the local flow recirculation above the infector head is also enhanced, which leads to the trapping of more particles in that area, contributing to the slower increase in εp. The simulations also indicate that the location of infector relative to ventilation inlet/outlet affects Qc and εp,c with higher Qc and lower εp,c observed when infector is in a corner due to potential formation of a local hot spot of high infection risk when infector is near the ventilation inlet. In conclusion, based on the simulations, we propose a potential ventilation thermal control strategy, by adjusting the ventilation temperature, to reduce the risk of airborne transmission in low-ceiling rooms. Our findings indicate that the thermal environment plays a critical role in the transmission of airborne diseases in confined spaces.

Orientation selection of particles growing in an alloy melt

The dynamics of orientation selection of a particle in an alloy melt is studied by using the method of multiple scales. The dynamic model accounts for two sources of anisotropic interfacial energy, which is characterized by two anisotropy parameters ε1 and ε2. The anisotropy parameters and the initial concentration are systematically changed to investigate the continuous orientation variations of the growing particle. In the <100> directions, as the anisotropy parameter ε1 increases, the interface temperature and concentration gradient increase, and the interface of the particle accelerates its growth and becomes locally convex, whereas in the <110> directions, the interface temperature and concentration gradient decrease, and the interface decelerates its growth and becomes locally concave. During this growth process, the particle changes continuously its preferred growth direction from <110> to <100>. As negative ε2 increases, a reversed process occurs that the particle changes continuously its preferred growth direction from <100> to <110>. In the preferred growth direction, the initial concentration inhibits the growth of the particle, whereas in other growth directions the initial concentration facilitates the growth of the particle. The orientation selection of the particle displays the rich interface morphologies of the particle with deformed petal-shaped patterns.

Enhanced both in-plane and through-thickness thermal conductivity of carbon fiber/epoxy composites by fabricating high thermal conductive coaxial PAN/PBO carbon fibers

PAN-based carbon fibers (PAN-CFs) have been widely used as structural reinforcements of various advanced composites due to their excellent mechanical properties. However, PAN-CFs have no obvious superiority in terms of the heat dissipation ability compared to mesophase pitch-based CFs and vapor grown CFs. In the present work, Poly- p -phenylene benzobisoxazole (PBO) macromolecules, which could contribute to the development of highly ordered graphite from conjugated aromatic backbone, were introduced onto PAN-CF surfaces to prepare coaxial PAN/PBO carbon fibers (PAN/PBO-CFs). Results showed that functional graphene oxide (GO) could result in uniform grafting of PBO macromolecules onto fiber surfaces owing to π–π conjugations between GO and benzoxazole backbone. The coaxial PAN/PBO-CFs were subsequently obtained through high-temperature graphitization and the rigid state could be also retained. Results by XRD and TEM demonstrated that PAN/PBO-CFs had smaller interlayer spacing and higher crystallinity compared to PAN-CF, and PBO derived graphite crystals could also be perpendicular to fiber surfaces. As for the thermal conductivity of composites, the in-plane thermal conductivity of PAN/PBO-CF reinforced epoxy composite was as high as 82.86 W/(m·K) which was almost 1.5 times of PAN-CF/epoxy composites, and the through-thickness thermal conductivity with the value of 2.54 W/(m·K) also increased by 65%. The introduction of PBO macromolecules could contribute new conductive paths and alleviate the problem of interfacial temperature gradients between CF and resin matrix . As a result, thermal conductivity properties of coaxial PAN/PBO-CF reinforced composites significantly improved.

Development of the molten pool and solidification characterization in single bead multilayer direct energy deposition

Although single-layer directed energy deposition has been extensively studied, an in-depth understanding of the cladding melting and solidification mechanism of single bead multilayer directed energy deposition is equally promising for industry, considering practical application scenarios. Based on the single-layer directed energy deposition, a numerical model of multilayer directed energy deposition was developed and validated to reveal the cladding forming process and solidification structure of multilayer directed energy deposition. The simulation results show the complex Marangoni convection pattern generated by the surface tension gradient in the melt pool. Moreover, the multilayer directed energy deposition enhances the penetration effect of the melt pool under the influence of the heat accumulation effect, resulting in a cross-layer mass transfer different from that of the single-layer directed energy deposition. The evolution of the multilayer cladding from columnar crystals at the bottom to equiaxed crystals at the top and the shift of its growth orientation were also analyzed based on the obtained solid-liquid interface temperature gradients and solidification velocities during the solidification of the melt pool. Finally, the relationship between cladding grain size and solidification properties was theoretically predicted by counting the grain size of different layers in the cladding, and the effect of grain growth on the mechanical properties was superficially discussed. • Penetration of multilayer deposition melt pools enhanced by heat accumulation effects. • Cross-layer mass transfer exists for multilayer deposition. • "C" or "U" type of grain growth orientation transition. • The recrystallization after solidification promotes the mechanical properties.

Analysis of faceted solidification in the horizontal ribbon growth crystallization process

Horizontal ribbon growth (HRG), in which a thin sheet of solidified material is pulled horizontally from the surface of a molten pool, is proposed as an efficient technique for growth of single-crystal silicon sheets. Despite recent results, some details of the process are still not understood, in particular the solidification mechanism at the triple junction point (TJP) where the solid, the liquid, and the surrounding gas meet. The solidification mechanism in the HRG process is investigated in this paper both analytically and numerically, incorporating the solidification kinetics that lead to faceted growth. The conventional solid–liquid problem in the HRG process is formulated analytically in the vicinity of the triple junction point (TJP). The temperature distribution is obtained for the liquid and solid regions as a function of the underlying parameters of the HRG process, such as the material properties, the ribbon pull speed, and the cooling heat fluxes. Using the analytical results, the TJP temperature, the facet length, interfacial temperature gradients, and liquid supercooling can be predicted. The analytical formulation is validated against accurate numerical simulations of the same problem, showing a good agreement in predicting the temperature gradients and the facet growth. The findings of this study suggest that using the analytical model, the behavior of the solid ribbon and the existence of a supercooled region in the liquid in the HRG process can be predicted without the need for numerical simulations. The model also gives criteria for optimal performance of the HRG process.

A methodology to integrate melt pool convection with rapid solidification and undercooling kinetics in laser spot welding

In laser spot welding because of small melt pool and short process duration, the welding process takes place rapidly with high cooling rates. As a result the analysis of solidification in laser spot welding becomes challenging. The solidification takes place generally with non-equilibrium kinetics involving undercooling and recalescence. In this work, a methodology for handling the rapid solidification in laser spot welding is developed by solving the coupled transient conservation equations of mass, momentum and energy. Undercooling and recalescence are coupled with the melt pool convection and heat transfer using an interface tracking algorithm which predicts the rapidly evolving interface temperature and interface speed. The melt pool result is first validated with the published results. Thereafter, the effect of undercooling is investigated by presenting a detailed comparison of results obtained from the rapid (non-equilibrium) solidification model and the conventional (equilibrium) solidification model. The results conclude that undercooling and recalescence have profound effect on melt pool dimension, temperature, solidification time and melt pool convection. The rapid solidification behavior is delineated further with the help of solidification parameters, such as interface temperature, solidified thickness, temperature gradient (G) and solidification rate (R). The weld microstructure is estimated using the combined values of GR and G/R.

Evaporation of Initially Heated Sessile Droplets and the Resultant Dried Colloidal Deposits on Substrates Held at Ambient Temperature.

The present study experimentally and numerically investigates the evaporation and resultant patterns of dried deposits of aqueous colloidal sessile droplets when the droplets are initially elevated to a high temperature before being placed on a substrate held at ambient temperature. The system is then released for natural evaporation without applying any external perturbation. Infrared thermography and optical profilometry are used as essential tools for interfacial temperature measurements and quantification of coffee-ring dimensions, respectively. Initially, a significant temperature gradient exists along the liquid-gas interface as soon as the droplet is deposited on the substrate, which triggers a Marangoni stress-induced recirculation flow directed from the top of the droplet toward the contact line along the liquid-gas interface. Thus, the flow is in the reverse direction to that seen in the conventional substrate heating case. Interestingly, this temperature gradient decays rapidly within the first 10% of the total evaporation time and the droplet-substrate system reaches thermal equilibrium with ambient thereafter. Despite the fast decay of the temperature gradient, the coffee-ring dimensions significantly diminish, leading to an inner deposit. A reduction of 50-70% in the coffee-ring dimensions is recorded by elevating the initial droplet temperature from 25 to 75 °C for suspended particle concentration varying between 0.05 and 1.0% v/v. This suppression of the coffee-ring effect is attributed to the fact that the initial Marangoni stress-induced recirculation flow continues until the last stage of evaporation, even after the interfacial temperature gradient vanishes. This is essentially a consequence of liquid inertia. Finally, a finite-element-based two-dimensional modeling in axisymmetric geometry is found to capture the measurements with reasonable fidelity and the hypothesis considered in the present study corroborates well with a first approximation qualitative scaling analysis. Overall, together with a new experimental condition, the present investigation discloses a distinct nature of Marangoni stress-induced flow in a drying droplet and its role in influencing the associated colloidal deposits, which was not explored previously. The insights gained from this study are useful to advance technical applications such as spray cooling, inkjet printing, bioassays, etc.

Numerical Study of Thermocapillary-Buoyancy Convection of Volatile Liquid Layer in an Enclosed Cavity

In this paper, steady thermocapillary-buoyancy convection of a volatile liquid layer in the case of a two-dimensional enclosed cavity subjected to horizontal temperature gradient is numerically investigated by using the finite difference method. A two-phase model with convective diffusion of vapor in consideration is fully developed. Kinetic theory of gases and vapor-diffusion-limited phase change are used to describe mass flux on the liquid-gas interface, and the latter is validated to play a dominating role in phase change. Numerical results reveal that transition from unicellular flow to multicellular flow in liquid layer is due to the increasing thermocapillary effect, while increasing buoyancy effect has a major impact on gas flow, which can separate the convection in gas phase into a two-layer flow. Evaporation and condensation occur at the interface and the mass flux distribution is bound up with the convective patterns in liquid layer. For a large imposed temperature gradient, the interfacial mass flux fluctuates in the core region with its amplitudes gradually amplifying from the cold to hot side. Thermal boundary effect on temperature and mass flux distribution close to the end walls is discussed. We also simulate a corresponding model ignoring phase change and vapor transport, and find the phase change tends to stabilize the thermocapillary-buoyancy flows due to the reduction of interfacial temperature gradient caused by the latent heat absorbed and released along the interface.

Deep-penetration welding-brazing of aluminum/steel dissimilar metal using laser-MIG arc hybrid heat source

Deep-penetration welding-brazing process without gap of steel/aluminum dissimilar metal butt joint with a thickness of 4 and 6 mm was achieved using laser-MIG hybrid heat source. The welding parameters were optimized to obtain a sound joint and the microstructure of the interfacial layer was observed by scanning electron microscopy (SEM) and experimental validation. The joint temperature distribution of three welding methods: MIG welding-brazing, laser welding-brazing, and laser-MIG hybrid deep-penetration welding-brazing were investigated by finite element method (FEM) numerical simulation. Moreover, the effect of laser deflection angle on the temperature distribution of the laser-MIG deep-penetration welding-brazed joint was investigated. The results indicated that the temperature distribution of laser-MIG hybrid deep-penetration welding-brazed joint was more uniform than MIG welding-brazing and laser welding-brazing. The temperature of the lower part of the joint was increased, which was beneficial to improve wetting and spreading of the liquid metal on the stainless steel and obtain better weld formation. In the laser-MIG hybrid deep-penetration welding-brazing process, the temperature of the lower part of the joint interface and the interface temperature gradient could be improved by adjusting the laser deflection angle, which was beneficial to the formation of uniform intermetallic compound layer. Typical welding-brazing joint characteristic with the thickness of 4 mm using laser-MIG hybrid heat source was found. The main phases of interface layer compounds were composed of Fe4Al13 and Al8Fe2Si, and the weld metal consisted of α-Al and Al-Si eutectic. The 6-mm steel/aluminum joint was also achieved by using laser-MIG hybrid heat source, but some cracks at the interface were observed.

Simulation on the Marangoni flow and heat transfer in a laser-heated suspended droplet

In this study, the Marangoni flow and heat transfer inside a laser-heated water droplet suspended in the immiscible oil phase is numerically investigated. It is shown that the Marangoni flow is induced by non-uniform temperature distribution across the droplet interface caused by the localized heating source. Effects of the laser power, laser beam size and surface tension temperature coefficient are investigated. Simulation results show that the temperature rising rate and average temperature rising linearly increase with laser power due to more heat generated by photothermal effect and the Marangoni flow is intensified due to larger interface temperature gradient. With increasing laser beam diameter, the temperature distribution becomes more uniform due to more uniform laser energy distribution. The Marangoni flow is reverse when the surface tension temperature coefficient changes from the positive to the negative and vice versa, resulting in the transformation of the Marangoni flow and temperature distribution.

Stability analysis of a gravity-driven thermoviscous liquid film flowing over a heated flat substrate.

Two-dimensional steady-state solutions and their stability analysis are presented for a gravity-driven thin film of a thermoviscous liquid. The governing equations and boundary conditions are simplified using the lubrication approximation. The analytically obtained film thickness evolution equation consists of various dimensionless parameters such as the Marangoni number, Biot number and thermoviscosity number. The viscosity of the liquid is assumed as an exponential function of temperature. The viscosity decreases within the liquid film as the temperature increases. Due to localized heating interfacial temperature gradients generate surface tension gradient which results into thermocapillary or Marangoni stress. The Marangoni stress opposes the fluid flow at the leading edge of heater leading to an increase in the film thickness locally. This locally thick structure becomes unstable beyond critical values of the parameters that leads to formation of rivulets in the transverse direction. Using the linear stability analysis it is found that the Marangoni stress and the thermoviscous effect have a destabilizing effect on the thin-film flow. At much higher values of the thermoviscosity number another mode of instability appears which is known as thermocapillary instability which leads to oscillating film profiles. For streamwise perturbations, the destabilizing effect of the thermoviscosity number for localized and uniform heating remains consistent.