Marangoni Convection Research Articles

Thermocapillary convection modes and phase change heat transfer characteristics of three-dimensional metal foam composited phase change materials with different aspect ratios under microgravity

This work aims to investigate the flow, heat transfer and phase change characteristics of porous media composite phase change materials with different aspect ratios in the thermocapillary mode under microgravity. A three-dimensional hydrodynamic model of thermocapillary-enhanced phase change heat transfer in the composite is developed. The model considers the shear stress boundary conditions on the porous free surface, employing the enthalpy-porosity method and the Darcy–Forchheimer model to describe the phase change process and fluid flow within the porous matrix, respectively, and the local thermal non-equilibrium model to describe the transient heat energy transfer between the paraffin and copper foam. The effects of different Marangoni numbers (Ma), aspect ratios (AR), Darcy numbers (Da) and porosities (ε) on the phase change heat transfer are compared. The results show that the increase of Ma from 6 × 104 to 2 × 105 accelerates the thermocapillary effect, and positively affect the thermal transport inside the cavity. The total melting time of the composite is reduced by 22.6 %, the heat storage efficiency is improved by 31.1 %, and secondary vortices appear in the convection zone. The large AR is favorable to the development of thermocapillary convection, but it also causes the reduced temperature gradient and heat transfer area of the heat source as well as the increased heat transfer distance. Compared to AR = 1, the total melting time increased by 34.4 % and 46.9 % for AR of 1.95 and 2.25 at a larger Ma, while the heat storage efficiency decreased by 26.5 % and 33.7 %, respectively. For a fixed ε, the decrease in Da hinders the development of convection, thus weakening the heat transfer and storage performance of the composite, and also causes heat conduction to dominate the phase change process in advance.

Symmetry-Breaking-Induced Internal Mixing Enhancement of Droplet Collision

Binary droplet collision is a basic fluid phenomenon for many spray processes in nature and industry involving lots of discrete droplets. It exists an inherent mirror symmetry between two colliding droplets. For specific cases of the collision between two identical droplets, the head-on collision and the off-center collision, respectively, show the axisymmetric and rotational symmetry characteristics, which is useful for the simplification of droplet collision modeling. However, for more general cases of the collision between two droplets involving the disparities of size ratio, surface tension, viscosity, and self-spin motions, the axisymmetric and rotational symmetry droplet deformation and inner flow tend to be broken, leading to many distinct phenomena that cannot occur for the collision between two identical droplets owing to the mirror symmetry. This review focused on interpreting the asymmetric droplet deformation and the collision-induced internal mixing that was affected by those symmetry breaking factors, such as size ratio effects, Marangoni Effects, non-Newtonian effects, and droplet self-spin motion. It helps to understand the droplet internal mixing for hypergolic propellants in the rocket engineering and microscale droplet reactors in the biological engineering, and the modeling of droplet collision in real combustion spray processes.

Prediction of crystal nucleation and growth behavior of Fe/Ni-based multi-materials deposited by laser directed energy deposition using a multi-area, multi-layer, and multi-scale phase field calculation (M3-PFC)

Laser directed energy deposition (LDED), based on the powder feeding technology, possesses the capability to fabricate complex structures within hollow structures. The arbitrary Lagrangian-Eulerian (ALE) method was innovatively applied to derive temporal interpolation functions for the free surface contour F (x, y, t) and the temperature distribution T (x, y, t) of the melt pool from a mesoscopic computational fluid dynamics (CFD) thermal model. These interpolation functions were not simply integrated but specifically designed to impart a quantified impetus for the growth of crystalline solidification within the microstructural phase field, effecting real-time adjustments to the computational domain of the phase field model. The multi-area, multi-layer and multi-scale phase field calculation (M3-PFC), which takes into account the bi-directional coupling of Marangoni flow and the phase field, was established to incorporate the transportation of elements within the multi-layer deposition and previously solidified material. Thermodynamics of multi-layer deposition was integrated into M3-PFC, enabling the prediction of multi-layer dendritic evolution using LDED process. Influence of Marangoni flow on microstructure evolution of multi-layer C276 deposited on 316 L-substrate utilizing M3-PFC was investigated during LDED process. It was observed that the nucleation of equiaxed and columnar crystals occurred at the top and bottom-middle of the melt pool. Both columnar and equiaxed crystals were preferentially produced along the opposite direction of Marangoni flow, impeding the apparent dendrite-growth in the counter direction. Partial remelting of the (N + 1)th layer was induced by the thermodynamics generated in the (N + 2)th deposition layer, promoting re-solidification of the liquid phase in the (N + 1)th layer. However, no liquid-solid transformation occurred in the Nth layer subjected to periodic heating. The partially remelted tissue in the (N + 1)th layer was shown in wider columnar crystals generated through the competitive growth, which were transformed into planar crystals and identified as the initial solidification conditions for the nucleation of the (N + 2)th layer. The columnar crystal growth, nucleated at the planar crystals of the (N + 1)th layer, was dominated in the (N + 2)th layer. Micropillar compression tests revealed that the columnar crystal had a compressive strength of 1102.78 MPa, while the equiaxed crystals displayed a higher compressive strength of 1464.34 MPa.

The Influence of Two-Dimensional Temperature Modulation on Floating Droplet Dynamics

We investigate the dynamics and instabilities of a droplet that floats on a liquid substrate. The substrate is cooled from below. In the framework of the slender droplet approximation and the precursor model, the problem is studied numerically. Oscillatory and stationary regimes of thermocapillary convection have been observed. The influence of a two-dimensional spatial inhomogeneity of temperature on the droplet dynamics is investigated. The two-dimensional spatial temperature inhomogeneity can suppress oscillations, changing the droplet’s shape. In a definite region of parameters, the two-dimensional spatial modulation can lead to the excitation of periodic oscillations. The influence of the Biot number on the shape of the droplets is studied.

Effect of static magnetic field on the molten pool dynamics during laser powder bed fusion of Inconel 718 superalloy

Recent researches show that a static magnetic field (SMF) has an impact on the molten pool dynamics and microstructure of laser powder bed fusion (L-PBF). This work is intended to the multiple effect of SMF on the molten pool dynamics during L-PBF of Inconel 718 superalloy. A three-dimensional (3D) multiphase computational fluid dynamics (CFD) model at the powder-bed scale was established by coupling the electromagnetic braking and Seebeck effect in a static vertically-distributed magnetic field. Results show that the magnetic damping force (MDF) of the order of 104 N/m3∼105 N/m3 within the whole molten pool suppresses the Marangoni convection. The thermoelectric magnetic force (TEMF) of the order of 106 N/m3∼107 N/m3 rotates in the mushy zone and further enhances the melt flow of the whole molten pool. The SMF will not transform the flow pattern and temperature distribution in the molten pool. SMFs of 0.1 T and 0.3 T will suppress the melt flow, while that of 0.5 T will enhance the melt flow. The width of molten pool first decreases and then increases when the magnetic flux density (MFD) increases from 0 T to 0.5 T. In addition, applying a SMF of 0.3 T can effectively inhibit pore defect and spatters. This work can provide insight into the control of molten pool by introducing a SMF during L-PBF.

Three-dimensional effects during thermocapillary-driven melting of PCMs in cuboidal containers in microgravity

The melting of a phase change material (PCM) in a cuboidal domain under microgravity conditions is investigated numerically. The upper surface of the PCM is free (in contact with air, for example) and variations in its surface tension drive thermocapillary convection in the liquid phase, which significantly enhances heat transfer and accelerates melting. Furthermore, the change in liquid fraction during melting is associated with transitions among various modes of thermocapillary dynamics, including an oscillatory instability to hydrothermal waves. While the characteristics of PCM melting and thermocapillary dynamics have previously been investigated in this system using a two-dimensional model, the current work examines the important question of transverse dynamics and their effect on the melting process. Careful quantitative comparisons are made between the three- and two-dimensional models in terms of melting times, solid/liquid interface evolution, thermal fields, and spectrograms. The results show that transverse modes are often, but not always, reflection symmetric about the midplane and that their influence on melting and PCM performance is relatively minor in most cases. Thus, two-dimensional models may be used to reduce computational costs while still providing a reasonable approximation of the melting process for high Prandtl number materials, especially when compared to the midplane of the full cuboidal domain.

Effect of ultrasonication on rising characteristics of a single gas bubble

In recent years, ultrasonication has been used to enhance flotation process. Gas bubble is the unique carrier during mineral flotation, and thus its characteristics play an important role. However, the bubble rising characteristics under ultrasound action are not clear. Therefore, this paper aims to investigate the effect of ultrasonication on bubble rising behaviors in deionized water and MIBC (Methyl Isobutyl Carbinol) solution. A laboratory visualization system of bubble motion with ultrasonication was designed based on the high-speed dynamic video technology to record the bubble rising process and analyze bubble rising characteristics. Bubble aspect ratio was found to increase with the surfactant concentration increasing, but the rising velocity was in opposite state. In each solution, a significantly higher bubble sphericity was observed with ultrasonication than that without ultrasonication, conversely, the bubble rising velocity was smaller under the action of ultrasonication. A roughly negative linear relationship was found between the bubble shape and rising velocity, and the ultrasonication diminished this relationship. Marangoni effect was considered as the cause of MIBC affecting the bubble motion, and possible explanations of ultrasonication affecting the bubble motion were given from the perspective of bubble surface wave, cavitation bubble oscillation and sound radiation force.

In-situ investigation of microstructure-dependent deformation behavior of an electron-beam welded Ni-Co based superalloy

In order to clarify the strengthening mechanism of an electron-beam welded (EBW) Ni-Co based superalloy, the local deformation behavior in different regions across the weld joint was investigated by the in-situ micro-tensile test in the scanning electron microscope and analyzed by the micro digital image correlation (μDIC). The microstructures across the weld joint were systematically investigated on multiscale levels. The micro-hardness profiles were also determined across the weld joint. Significant changes in the grain size, γ’ phase, MC carbides and dislocation configurations were observed within 2.0 mm from the weld line as a result of the steep temperature gradient and the resultant Marangoni flow occurring during EBW process. Furthermore, the micro-hardness profiles and microstructure-dependent mechanical properties were influenced by the variation of grain size, γ’ particle size and the dislocation density in the weld joint. The cracks preferred to initiate and propagate on the blocky MC carbides, rather than the ultrafine MC carbides and the liquefaction cracks. This study proposes a method for high throughput measurement of microstructure-dependent mechanical properties, and shows a certain guiding significance for optimization of microstructure and improvement of mechanical properties of electron-beam welded Ni-Co based superalloys.

Penetrative and Marangoni convection in a fluid film over a phase boundary

We study the effects of buoyancy, surface-tension gradients and phase boundary on the stability of a layer of water that is confined between air at the top and a layer of ice at the bottom. The temperature of the overlying air and flux condition at the free surface of the water layer are such that the layer is susceptible to both thermal and thermocapillary instabilities. We perform a linear stability analysis to identify these modes of instability and investigate the effects of the phase boundary on them. We find that with increasing thickness of the ice layer, the critical Rayleigh and Marangoni numbers for the instabilities are found to first decrease and then asymptote to constant values for ice thicknesses much larger than the thickness of the water layer. In the case of thermocapillary instability, we find that the thickness of the ice layer has negligible influence on the stability threshold for dimensionless wavenumber $k \gg 1$ , and that the presence of an unstably stratified liquid layer significantly alters the stability threshold for $k = O (1)$ . Furthermore, the inclusion of Marangoni stresses reduces the stability threshold of the thermal instability.

Mechanisms for removing impurities in a single crystal superalloy by electron beam drip melting

The development of clean and green manufacturing of superalloys has attracted widespread attention. The returned scrap of superalloys, however, contains a significant amount of oxygen (O), nitrogen (N), and inclusions, which cannot be used effectively, and thus resulting in substantial waste generation. In this study, the electron beam drip melting (EBDM) is used to deeply remove the O, N and inclusions in a single crystal superalloy. The concentration of O and N can be reduced to 0.57 and 0.55 ppmw after drip melting, with removal rates of 97.9 % and 83.8 % respectively. The number density of inclusions experienced a significant decrease, dropping from 12.90 to 0.18 mm−2, representing a reduction of 98.6 %. The intense Marangoni convection will gather the surface inclusions in the central area of the electron beam spot. The inclusions get close to each other to form cavities, and thus the aggregation reaction occurs to form inclusion clusters. Due to the dissolution and decomposition reactions caused by the local overheating and bombardment of electron beam, the inclusions can be removed effectively. The generated O and N are eliminated through mass transfer and evaporation reactions, while the remaining inclusions on the surface can be artificially removed. The electron beam drip melting can be used for the preparation of superalloys with extremely low levels of O, N and inclusions.

Evaporative phase separation in polymer microdroplets with confinement and internal flow

Evaporation can drive initially homogeneous multiphase liquid systems out of equilibrium to induce liquid-liquid phase separation (LLPS). Here, we demonstrate evaporative LLPS in microfluidic-generated emulsion microdroplets of polymer mixtures. The evaporation produces distinct polymer phases within the microdroplets. Phase separation occurs even with polymer combinations that do not form distinct phases in sessile droplet evaporation. We attribute this aspect to evaporation-driven solutal Marangoni flows and the interface capture accumulating the nuclei at the apex where the evaporation rate is the maximum. A fast coalescence and growth of the accumulated polymer nuclei occurs inside the droplets, unlike the capillary-flow-induced spread-out of the nuclei along the contact line in sessile drops. Our method of evaporation of the droplet cluster may facilitate studying LLPS in volume-limited environments and have implications for understanding LLPS in biological systems.

Improving Welding Penetration and Mechanical Properties via Activated-Flux Smearing by Tungsten Inert Gas Arc Welding

For the welding process of thick-walled structural components in liquid rocket engines, the activated-flux TIG method can effectively address issues such as the formation of intermetallic phases in the weld seams, thereby enhancing mechanical performance. The present study investigates the activated-flux TIG welding technique on 10mm thick 1Cr21Ni5Ti duplex stainless steel plates. Various activated-flux, including -SiO2, TiO2, V2O5, NiO, MnO2, CaO, AlCl3, CaF2, B2O3 Cr2O3, and Al2O3, were examined to understand their impact on the weld-bead geometry. The aim was to determine the optimal activator ratio for the effective welding of 1Cr21Ni5Ti duplex stainless steel. The weld-shift experiment confirmed that the deep penetration observed in flux-assisted welding is attributed to Marangoni convection in the molten pool. Comprehensive evaluations and analyses were performed on the microstructure and mechanical properties of the normal welded joint and the A-TIG welded joint. Finally, the study delves into a discussion on the factors influencing changes in the weld penetration, microstructure, and mechanical properties of the weld.

Numerical analysis of n-octadecane melting process in a rectangular cell under reboosting maneuver conditions

The improvement of heat management based on Phase Change Materials (PCMs) is of increasing importance in space environments. In this context, a future ESA project called “Effect of Marangoni Convection on heat transfer in Phase Change Materials (MarPCM)” will evaluate the degree of improvement in heat transport using thermocapillarity as convective activator of the liquid phase generated during the melting. Since this type of project needs to be performed onboard International Space Station, ISS, it is of outmost importance to know if the accelerometric environment of the Station could affect the experiment results.To do so, various 2D simulations of the solid-liquid phase change were carried out using n-octadecane as PCM material, by considering a pre-selected acceleration signal coming from a real ISS reboosting maneuver (June 24, 2021). Different gravity scenarios have been considered by changing both the intensity and the orientation of the reboosting maneuver, parallel (x direction) and perpendicular (y direction) to the thermal gradients. The acceleration levels were enhanced up to 1000 times the intensity achieved during the real reboosting in order to predict the safety margins of the ISS experiment.The results showed alterations of the liquid-solid interface, during the melting process in the high g-level scenarios considered. In these cases, the oscillatory flow pattern became more complex detecting sudden changes in the main frequency which were maintained approximately 1000 s after the reboosting ended. Nevertheless, applying real boosting maneuver no significant influence in the melting process was detected.

Influence of Marangoni convection, viscous dissipation, and variable fluid viscosity of nanofluid flow on stretching surface analytical analysis

AbstractThe current research paper investigates the viscous dissipation, influence of Marangoni convection (MC), and the variable fluid viscosity of nanofluid flow on stretching surfaces. The flow system takes into account variable viscosity with the impact of MC. Furthermore, velocity and temperature slips at the stretching surface are also considered in this study. To convert a collection of NLPDE to a NODE, we applied appropriate transformations. We utilize Homotopy analysis method (HAM) to solve this set of equations. The effects of temperature, tangential, and radial velocities on numerical perceptions involving Marangoni convection, nanoparticle volume friction, porosity parameter, Eckert number (EN) and heat source input factors are shown. A physical description is used to simulate and evaluate the structures of flow features such as velocity and temperature profiles in response to changes in developing factors. Based on the given result, we can observe that the temperature profile increases as EN and the heat source parameter increase, and that the velocity profile decreases as MC increases and increases with the porosity parameter.

Retraction Note: Marangoni convection in dissipative flow of nanofluid through porous space

Retraction Note: Marangoni convection in dissipative flow of nanofluid through porous space

Evaluation of the interfacial elasticity of surfactant monolayer at the CO2–water interface by molecular dynamics simulation: Screening surfactants to enhance the CO2 foam stability

Foam stability is adversely affected by the drainage process in the liquid film, which has been a great challenge for CO2 foam flooding in applications. The Gibbs-Marangoni effect, which is induced by the interfacial tension (IFT) gradient, provides a resisting force to the thinning of liquid films in foams under disturbance. Surfactants with higher interfacial elasticity enable rapid recovery to the equilibrium state, thereby improving the stability of the CO2 foam film. However, there is a lack of mechanistic studies aimed at elucidating the correlation between the interfacial elasticity of surfactant monolayers and the molecular structure of surfactants. In this paper, molecular dynamics simulations were employed to investigate the impact of the headgroups and linking groups of sodium polyoxyethylene alkyl ether sulfate (AES), sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDSn), and sodium laurate (SLA) surfactants on the interfacial morphology, monolayer properties, and surfactant behaviors at the CO2–water interface under reservoir conditions. The results show that the capacity to enhance CO2 foam stability follows the order AES > SDS > SDSn > SDBS > SLA, which is in good agreement with the available experiments. AES monolayers can reach the lowest IFT (∼0.2 mN/m) and the highest interfacial elasticity (30.4 mN/m). We show that the ethylene oxide groups (EO chains) can strongly interact with water molecules via hydrogen bonding and have affinity interactions with CO2 molecules, which is beneficial to the hydrophilic-CO2-philic balance (HCB). SDS monolayers show the second-lowest IFT (3.1 mN/m) and second-highest interfacial elasticity (26.0 mN/m). The linking oxygen atom in SDS facilitates bond rotation and is favorable to film stability. The phenyl group makes SDBS molecules too rigid and has poor HCB, leading to low stability with high IFT (16.0 mN/m) and low interfacial elasticity (13.1 mN/m). SLA has the highest IFT (22.1 mN/m) and lowest interfacial elasticity (6.3 mN/m), and also presents strong aggregation behaviors at the interface owing to interactions between the carboxyl groups. The simulation method and the microscopic insights delivered here are of great significance in screening surfactants to improve CO2 foam performance in oilfields.

Toward Highly Efficient Solution‐Processable OLEDs: Inkjet Printing of TADF Emissive Layer

AbstractThe fabrication of optoelectronic devices using low‐cost inkjet printing techniques is a topic of great interest to the scientific and industrial community and represents a step toward the full deployment of solution‐processable organic light emitting diodes (OLEDs), particularly for commercial lighting and signaling applications. Herein, the inkjet printing of tBuCzDBA (9,10‐bis(4‐(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)−2,6‐dimethylphenyl)−9,10‐diboraanthracene) is reported, a high‐performing thermally activated delayed fluorescence (TADF) emitter for OLEDs. Optimizing the surface tension values of the ink formulations and the associated wetting behavior are crucial parameters for achieving a uniform and homogeneous printed thin film. In particular, it is observed that using a proper mixture of solvents with different surface tensions, it is possible to generate Marangoni flows inside the drop, which triggers a very fast drying process, ensuring optimized morphological and optical properties in the inkjet printed tBuCzDBA‐based film. OLEDs exploiting this film as an emissive layer are then fabricated, achieving a maximum luminance of 32 000 cd m−2, a current efficiency of 27.5 cd A−1, and an external quantum efficiencyof 10%. To the best of the knowledge, this is the highest efficiency reported to date for self‐hosted TADF inkjet‐printed OLEDs.

Photo-Activated Growth and Metastable Phase Transition in Metallic Solid Solutions.

Laser processing in metals is versatile yet limited by its reliance on phase transformation through heating rather than electronic excitation due to their low absorptivity, attributing from highly ordered structures. Metastable states (i.e., surfaces, glasses, undercooled liquids), however, present a unique platform, both energetically and structurally to enable energy landscape tuning through selective stimuli. Herein, this ansatz is demonstrated by exploiting thin passivating oxides to stabilize an undercooled state, followed by photo-perturbation of the near surface order to induce convective Marangoni flows, edge-coalescence and phase transition into a larger metastable solid bearing asymmetric composition between the near surface and core of the formed structure. The self-terminating nature of the process creates a perfectly contained system which can maintain a high relaxation energy barrier hence deep metastable states for extended periods of time.

Thermocapillary convection in shallow liquid layers for moderate Prandtl number liquids using different cylinder configurations

In this paper, the influence of thermocapillary convection in a shallow annular pool using different cylinder configurations is numerically investigated. An axisymmetric, two-dimensional, cylindrical geometry with two concentric cylinders kept at two different temperatures constitutes the model. The main cylinder has a radius of 40 mm and the secondary cylinder has a radius of 20 mm. The effects of Prandtl numbers (Pr = 6.7, 16.1, and 25.2) and layer depths (d = 3.0 mm and 11.0 mm) on the streamlines, isotherms, velocity, and temperature profiles at various locations of the flow are investigated. Depending on the position of the secondary cylinder, three different configurations: 1) an internal cylinder 2) an external cylinder, and a 3) central cylinder is analysed. The flow is characterized by a rapid inward flow along the hot outer wall towards the colder inner wall. In deeper pool at lower Prandtl numbers, flow patterns are characterized by a single rolling cell structure. Conversely, secondary cell formations appear in shallow pool regardless of the Prandtl number. Based on the comparative analysis of various cylinder configurations, it is clear that the external cylinder generates greater velocities and temperatures along its surface. The maximum disparity in surface velocity between an external cylinder and other configurations is 7%, 12%, and 13% for Prandtl numbers of 6.7, 16.1, and 25.2 respectively. Therefore, positioning the cylinder externally promotes swifter convection and more effective mixing of the fluid.

Dynamics of a silicone oil drop submerged in a stratified ethanol-water bath.

We analyze numerically the Marangoni flow around an immiscible droplet submerged in a stably stratified mixture of ethanol and water. The linear stability analysis shows that the base flow undergoes a supercritical Hopf bifurcation that leads to oscillations. The theoretical prediction for the critical droplet radius is consistent with previous experimental results. Ethanol diffusion in water is critical in the flow stability for both low and high droplet viscosity. Direct numerical simulations of the nonlinear oscillatory flow show that the frequency of those oscillations approximately equals that of the critical eigenmode. The nonlinear convective term of the ethanol diffusive-convective transport equationfixes the amplitude of the droplet oscillations. The viscous dissipation associated with the Marangoni flow inside the droplet considerably reduces the oscillation.