Solutal Convection Research Articles

MHD Nonlinear Mixed Convection Flow of Micropolar Nanofluid over Nonisothermal Sphere

In this paper, two-dimensional steady laminar boundary layer flow of a nonlinear mixed convection flow of micropolar nanofluid with Soret and magnetic field effect over a nonisothermal sphere is evaluated. The mathematical formulation for the flow problem has been made with appropriate similarity transformation and dimensionless variables, and the main nonlinear boundary value problems were reduced into mixed high-order nonlinear ordinary differential equations. Solution for velocity, microrotation, temperature, and concentration has been obtained numerically. The equations were calculated using method bvp4c from Matlab software for various quantities of main parameters. The effects of various parameters on skin friction coefficient f″0, wall duo stress coefficient -G′0, and convection mass transfer coefficient -Φ′0 are analysed and presented through the graphs and tables. The convergence test has been maintained. For the number of points greater than the suitable mesh number of points, the precision is not influenced but the set time is increased. Moreover, a comparison with a previous paper, obtainable in the literature, has been presented and an excellent agreement is obtained. The findings indicate that an increase in the values of nonisothermal parameters (m, P), magnetic Ma, thermal and solutal nonlinear convection (λ, s) parameter, and Soret number is to enhance the temperature difference between the boundary layer and ambient fluid to diffuse which increases the velocity profile f′ζ and their boundary layer thicknesses near the surface of the sphere.

Simulation of CO2 Rayleigh Convection in Aqueous Solutions of NaCl, KCl, MgCl2 and CaCl2 Using Lattice Boltzmann Method

The Rayleigh convection induced by dissolved CO2 in saline aquifers is significant for CO2 permanent storage. In this paper, the lattice Boltzmann method (LBM) with external forces was established to simulate such the convection in NaCl, KCl, MgCl2 and CaCl2 solutions. The simulation was performed in two kinds of conditions, the one was the same concentration of salt, and another was the same concentration of Cl-, at 298.15 K, 1.00 atm. The results showed that the order of convection appearance in four salt solutions was: KCl＞NaCl > MgCl2>CaCl2 in 1 mol·kg-1 (M) salt solution. However, it was a little different in the condition of the same concentration of Cl-, the order was: KCl > MgCl2>CaCl2>NaCl. This was mainly because a bigger density difference between brine and CO2-saturated solution could enhance convective instability. The magnitude of convective velocity and mass transfer rate was 10-5 and 10-6 m∙s-1, respectively. Furthermore, the mean dissolution rate was around 2 times larger than that at the onset of convection. In addition, we found that the onset convection would be delayed as salt concentration increased. Our results have a practical guidance for a further experimental design and industrial application on CO2 geological separation in saline aquifers.

Microfluidic fabrication of stable collagen microgels with aligned microstructure using flow-driven co-deposition and ionic gelation

The controlled biofabrication of stable, aligned collagen hydrogels within microfluidic devices is critically important to the design of more physiologically accurate, longer-cultured on-chip models of tissue and organs. To address this goal, collagen-alginate microgels were formed in a microfluidic channel by calcium crosslinking of a flowing collagen-alginate solution through a cross-channel chitosan membrane spanning a pore allowing ion diffusion but not convection. The gels formed within seconds as isolated islands in a single channel, and their growth was self-limiting. Total gel thickness was controlled by altering the concentration of calcium and collagen-alginate flow rate to reach an equilibrium of calcium diffusion and solution convection at the gel boundary, for a desired thickness of 30–200 μm. Additionally, less calcium and higher flow produced greater compression of the gel, with regions farther from the pore compressing more. An aligned, stable collagen network was demonstrated by collagen birefringence, circumferential texture orientation, and little change in gel dimensions with de-chelation of calcium from alginate by prolonged flow of EDTA in the channel. Resultant gels were most stable and only slightly asymmetric when formed from solutions containing 8 mg ml−1 collagen. Diffusion of 4 kDa and 70 kDa fluorescently-labeled dextran indicated size-dependent diffusion across the gel, and accessibility of the construct to appropriately-sized bioactive molecules. This work demonstrates the physicochemical parameter control of collagen gel formation in microfluidic devices, with utility toward on-chip models of dense extracellular matrix invasion, cancer growth and drug delivery to cells within dense extracellular matrix bodies.

RETRACTED: Hybridisation of microstructures from three classes of titanium alloys

A ternary hybrid Ti alloy (HYTA) was produced by selective laser melting (SLM), combining all three classes of the titanium system. The hybridisation of the α CP Ti, α-β Ti-64 and β Ti-5553 gave rise to a highly heterogeneous microstructure encompassing all the possible phases and structures which do not co-exist in conventional monolithic Ti alloys. The details of the complex microstructure were comprehensively characterised, revealing the roles played by such important factors as the distribution of the ingredient alloy particles, the Marangoni convection and diffusion of solute during melting, and microsegregation of solute during solidification. The ternary HYTA exhibited an excellent combination of high tensile yield strength of 1000 MPa and enhanced ductility with uniform elongation of ∼15% and total elongation of ∼20%, superior to conventional Ti alloys of various types. Such exceptional performance is attributed to the high-degree heterogeneity and the operation of most major deformation mechanisms which coordinate effectively and transfer smoothly between various parts with a wide range of hardness (4–6 GPa). The outcome has further demonstrated the potential of hybridisation of microstructures as a new strategy for future alloy development.

On the suppression of oscillatory circulation inside an evaporating bi-component drop through acoustic streaming

The present study investigates the effect of acoustics on the oscillatory internal circulation of evaporating methanol – water drops. The circulation is termed oscillatory when a periodic boosting and deceleration of flow is observed. The acoustic signal generated, is a sine wave of different frequencies at constant amplitude, with and without added white noise. Minimum acoustic boundary layer thickness and maximum evaporation rate are observed at lowest acoustic frequency of 30 Hz, with and without, added white noise. The evaporation rate is enhanced for all volume fractions (vf) of methanol when compared to the absence of acoustics. It is observed that the oscillation frequency for a given drop decreases when subjected to an acoustic field, and is lowest at 30 Hz for all drops tested. The presence of intense acoustic streaming around the evaporating drop creates large velocity gradients on the drop surface due to higher air entrainment from the surroundings at low acoustic frequencies, sweeping out more amount of evaporated mass from the surrounding of drop and augmenting the heat and mass transfer from the drop surface. Further, the higher rate of air entrainment into the drop, (i) suppresses the oscillatory behavior of internal circulation and (ii) breakup of the circulation ligaments. The breakup of circulation ligaments accelerates the circulation velocity which results in enhancement of the evaporation rate. The enhanced evaporation raises solutal Marangoni and Rayleigh convection, and again forces the circulation to be steadier.

Effect of intentional convection on the passivity of an Fe–6Cr surface in pH 4.5 Na2SO4 solution

The anodic oxidation behavior of Fe–6Cr during potentio-dynamic polarization in pH 4.5 Na2SO4 solution was investigated by electrochemical ellipso-microscopy. Active, passive, and trans-passive states of the Fe–6Cr surface showed heterogeneous oxidation behaviors depending on the substrate micro-structure, electrode potential and solution convection. Enhancement of mass-transport by solution injection in the latter half of the active state resulted in acceleration of passivation of the Fe–6Cr surface. The passivity-current density of the surface passivated by intentional convection was reduced to almost half of the current density of the surface that had been in the passive state in a stagnant solution, and the surface passivated by intentional convection showed better resistance against pitting in a chloride-containing solution. The improvement of passivity of the surface was attributed to the formation of a Cr-enriched passive film by a decrease in the thickness of the diffusion layer in the latter half of the active state.

How non-Darcy effects influence scaling laws in Hele-Shaw convection experiments

We examine experimentally the influence of non-Darcy effects on convective dissolution in Hele-Shaw cells. We focus on buoyancy-driven convection, where the flow is controlled by the Rayleigh–Darcy number, , which measures the strength of convection compared to diffusion. The Hele-Shaw cell is suitable to mimic Darcy flows only under certain geometrical constraints, and a recent theoretical work (Letelier et al., J. Fluid Mech., vol. 864, 2019, pp. 746–767) demonstrated that a precise limit exists for the parameter – thickness-to-height ratio – beyond which the flow exhibits non-Darcy effects. In this work, we run experiments for solute convection in Rayleigh–Benard-like configuration. We examine a wide range of the parameters space and we clearly identify the application limits of Darcy flow assumptions. Besides confirming previous theoretical predictions, current results are of relevance in the context of porous media flows – which are often studied experimentally with Hele-Shaw set-ups. Using our original datasets, we have been able to explain and reconcile the discrepancies observed between scaling laws previously proposed for Rayleigh–Benard-like experiments and simulations in similar contexts. Specifically, we attribute an important role to the parameter , which clearly establishes thresholds beyond which Hele-Shaw experiment results are influenced by three-dimensional effects.

Measurement of thermophysical properties of molten Si-Cr and Si-Fe alloys for design of solution growth of SiC

To optimize the solution growth process, temperature and convection of the solvent, which are significantly affected by the thermophysical properties, have to be well controlled. In this study, the thermophysical properties of molten Si-Cr and Si-Fe alloys, which are important solvent systems for the solution growth of silicon carbide (SiC), were measured via electrostatic levitation and laser flash methods. The total hemispherical emissivity, spectral hemispherical emissivity, and heat capacity at constant pressure of the Si-Cr alloys at their liquidus temperatures were successfully obtained, and their compositional dependencies are discussed. The thermal diffusivity and thermal conductivity of the Si-Cr and Si-Fe alloys exhibited a linearly increasing trend with increasing temperature. Thermal conductivities of a few tens of W∙m−1∙K−1 were obtained, and the thermal conductivity markedly decreased by alloying for both the investigated alloy systems. Such a large drop would lead to a large difference of the temperature distribution during the solution growth of SiC, and hence the obtained properties can be utilized for optimization of the process.

Optimization of pulsed electric field conditions for sugar extraction from carrots.

Physical destruction and thermal treatment are pretreatment methods used to destroy cell membranes and facilitate the release of solute extraction. In this paper, sugar extraction from carrots under different pulsed electric field conditions (field strengths of 250, 750, and 1,250 V/cm, pulse numbers of 10, 45, and 80, and pulse frequency of 1 Hz) and simultaneous thermal treatments (at 20, 45, and 70°C) were studied based on full factorial design experiments with 27 runs. Carrot slices treated with PEF were suspended in water at the desired temperature and liquid‐to‐solid weight ratio of L/S = 2. Immediately after the PEF treatment, a significant increase in solute extraction was observed due to the permeability of cell membrane that could lead to the enhancement of solute convection on the surface of the tissue. Optimum extraction parameters were obtained as follows: PEF with the field intensity of 750 V/cm, 10 pulses, and temperature of 45°C.

Controlling mass transfer in a continuous-flow microreactor with a variable wall relief

Abstract As it is known, microreactors are an important development in chemical engineering since the pharmaceutical industry needs flexible production rather than a large amount of product yield. In this paper, we propose the design of a continuous-flow microreactor in the form of a narrow cell with an adjustable gap. By tuning the gap width in time and space, one can control the reaction rate and regulate the product yield. We show that the governing equation for the fluid flow can be reduced to the Darcy equation with the permeability varying in space and time. As a benchmark reaction, we consider the neutralization of nitric acid with sodium hydroxide resulting in the solutal convection in the presence of gravity. We demonstrate numerically that the prototyping spatially-distributed relief of the reactor walls can successfully separate the incoming and outgoing flows of reagents, control the mixing intensity, increase or decrease the product yield. We show also the dynamic control of the reactor in operation via real-time local changes in the gap width.

Fabrication of natural cellulose films with pattern by viscosity regulation of its solution

The structure and properties of polymer film are greatly affected by the patterns on its surface arising from solution convection. Therefore, it is necessary to explore more detail mechanism of pattern formation. Herein, we have successfully designed variable patterns on the surface of cellulose films via roller coating method. The cellulose with three degrees of polymerization was used as raw material, and a solution of N,N-dimethyl-acetamide/lithium chloride (DMAc-LiCl) was selected as the solvent system. The pattern variations were controlled by the solution viscosity, which was affected by the degree of polymerization of cellulose, concentration and temperature of solution. The formation process of the pattern on cellulose film was in situ tracked by video recording with the help of carbon black tracker. It was found that the solution moved from the edge of the cell pattern to the center at a relatively lower viscosity (η < 8.06 Pa s), and finally formed a polygon pattern with the convex center. With the increasing of the solution viscosity (η ≥ 8.06 Pa s), the direction of its convection was completely opposite and a circular or circular-like combination pattern with concave center appeared on the surface of the cellulose film. The higher the viscosity, the larger the concave area. Moreover, the film with polygon pattern demonstrated excellent mechanical properties, and also exhibited a potential anti-counterfeiting value. These results could provide guidance in perfecting the theory of polymer fluid, and regulating the structure and properties of polymer products.

Evaluation of turbulent forced convection of non-Newtonian aqueous solution of CMC/CuO nanofluid in a tube with twisted tape inserts

The present paper investigates the turbulent flow and heat transfer of non-Newtonian aqueous solution of Carboxymethyl cellulose (CMC) and CuO nanoparticles in a plain tube and also tube with twisted tape inserts. The aqueous solution of CMC and CuO/CMC nanofluid show a shear-thinning (pseudo-plastic) rheological behavior, resulting in a higher viscosity than that of water. The consistency index and the power law index are evaluated based on available experimental data. The single phase approach with temperature dependent thermo-physical properties is applied to simulate the nanofluid flow and heat transfer. Simulation results are presented at different nanoparticle concentrations and twisted tape ratios. Only an axial flow is identified in the plain tube whereas both axial and swirl flows are detected in the tube with twisted tape inserts. The turbulence kinetic energy in the tube with twisted tapes is significantly higher than that in the plain one, which is useful for non-Newtonian fluid with higher viscosity. Also, the temperature fields in the tube with twisted tapes are disturbed relative to those in the plain one, due to stronger turbulence intensity and better fluid mixing. Higher amounts of nanoparticles concentration and lower twist ratios, giving maximum values of total efficiency, display the advantage of using non-Newtonian nanofluid in the tube with twisted tape inserts rather than non-Newtonian base fluid in the plain one.

Switching Modes of Mixing Due to an Adjustable Gap in a Continuous-Flow Microreactor

Microreactors are an important development in chemical engineering since the pharmaceutical industry needs flexible production rather than a large amount of product yield. The size of the microreactor may be so small that it requires the development of non-mechanical methods for reagent mixing. In this paper, we propose the design of a continuous-flow microreactor in the form of a narrow cell with a variable gap. By tuning the gap width in time and space, one can control the reaction rate and regulate the product yield. We show that the governing equation for the fluid flow can be reduced to the Darcy equation with permeability varying in space and time. As a test reaction, we consider the neutralization of nitric acid with sodium hydroxide resulting in the solutal convection in the presence of gravity. We show numerically that the prototyping spatially-distributed relief of the reactor walls can successfully separate the incoming and outgoing flows of reagents, control the mixing intensity, increase or decrease the product yield. We demonstrate also the dynamic control of the reactor efficiency via real-time local changes in the gap width.

Unveiling the role of gas permeability in air cathodes and performance enhancement by waterproof membrane fabricating method

Air cathode is the most popular oxygen reduction reaction (ORR) cathode with various applications in energy conversion and environmental purification. Understanding the role of gas permeability in air cathodes is crucial for its fabrication and operation optimizations. Herein, the correlations between gas permeability and performance are investigated by two experimental setups. One measures the gas permeability and the other evaluates the pressure-dependent performance. The results show two distinct ORR performances with respect to the gas permeability. The performance in low gas permeability is shown to depend on the gas permeability, oxygen partial pressure and solution convection, with obvious limiting current observed. Once the gas permeability surpasses a threshold value, suggested as 2 × 10−12 m, the performance becomes less dependent on those factors, without limiting current observed. These observations are further mathematically demonstrated by the rate limiting process based on an oxygen transport model of air cathodes. An appropriately high gas permeability can eliminate the oxygen gas transport limitation and is vital for the performance enhancement. Following this principle, a novel waterproof membrane-based air cathode with high gas permeability is proposed, enabling cost savings in operation and production, and thus has great potential in practice.

Non-linear convection flow of micro polar nanofluid past an isothermal sphere

In this study, steady laminar two dimensional boundary layer flow of non-linear convection micropolar nanofluid over an isothermal sphere is examined. The mathematical developing for the flow problem has been made. By means of appropriate similarity transformation and dimensionless variable, the governing non-linear boundary value problems were reduced into combined high order non-linear ordinary differential equations. Then, solution for velocity, microrotation, temperature and concentration has been obtained numerically. The equations were calculated using method bvp4c from matlab software for various quantities of main parameters. The influences of diffident parameters on skin friction coefficient, wall duo stress coefficient the Nusselt number, Sherwood number as well as the velocities, temperature, and concentration are analyzed and discussed through the tables and plotted graphs. A comparison with previous paper obtainable in the literature has been performed and an excellent agreement is obtained. The finding results indicate the raise in either the values of thermal Grashof number Gr or Brownian motion parameter Nb or Schmidt number Sc allows to bringing down the kinematic viscosity of the fluid causes improve the temperature profile distribution within the boundary layer. On the other hand, it can be diminished by the growth in either the value of solutal Grashof number Gm or solutal non linear convection parameter s which agrees to enlarge fluid density.

Heat, water and solute transfer in saline loess under uniaxial freezing condition

Salt weathering of loess slopes in the seasonally frozen region is closely related to water and salt migration induced by temperature gradient. This paper carried out uniaxial freezing tests on sodium sulfate saline loess with three temperature gradients considered. Visible cracks are developed in the frozen domain during testing while little change noted in lower unfrozen part. The lower the cold-end temperature, or the higher the temperature gradient, the less developed cracks emerge. At higher temperature gradients, the freezing front descends more rapidly, corresponding to higher freezing rates and larger maximum frozen depths. Synchronous variations were found in water and salinity profiles, with peak values near the frozen fringe. An improved Harlan model was then established for heat-water transfer in freezing soils, incorporating an impedance factor. Considering the convection and diffusion of solute, the models for salinity field were proposed based on Darcy’s and Fick’s law. Numerical simulations were carried out on the coupled heat-water-solute transfer in freezing saline loess columns. Comparisons of calculated and measured data prove the rationality of the models. Finally, dynamic changes of unfrozen water content and the influence of freezing point on thermal regime, water and salinity profiles were discussed.

Convective dissolution in porous media: experimental investigation in Hele‐Shaw cell

AbstractThe process of solutal convection in porous media is here studied experimentally in a Hele‐Shaw cell. Motivated by the carbon sequestration process, potassium permanganate and water are used to mimic the dissolution of carbon dioxide in brine in underground geological formations. An optical method is used to accurately reconstruct the solute concentration. In this work, we examine the influence of different calibration conditions on the measurement of the solute dissolution rate and we provide indications to improve the accuracy of the technique.

Solutal convection induced by dissolution

The dissolution of minerals into water becomes significant in geomorphology, when the erosion rate is controlled by the hydrodynamics transport of the solute. Even in absence of an external flow, dissolution itself can induce a convection flow due to the action of gravity. Here we perform a study of the physics of solutal convection induced by dissolution. We simulate numerically the hydrodynamics and the solute transport, in a 2D geometry, corresponding to the case, where a soluble body is suddenly immersed in a quiescent solvent. We identify three regimes. At short timescale, a concentrated boundary layer grows by diffusion at the interface. After a finite onset time, the thickness and the density reach critical values which starts the destabilization of the boundary layer. Finally, the destabilization is such as we observe the emission of intermittent plumes. This last regime is quasi-stationnary: the structure of the boundary layer as well as the erosion rate fluctuate around constant values. Assuming that the destabilization of the boundary layer occurs at a specific value of the solutal Rayleigh number, we derive scaling laws both for fast and slow dissolution kinetics. Our simulations confirm this scenario by validating the scaling laws both for onset, and the quasi-stationary regime. We find a constant value of the Rayleigh number during the quasi-stationary regime showing that the structure of the boundary layer is well controlled by the solutal convection. Finally, we apply the scaling laws previously established to the case of real dissolving minerals. We predicts the typical dissolution rate in presence of solutal convection. Our results suggest that solutal convection could occur in more natural situations than expected. Even for minerals with a quite low saturation concentration, the erosion rate would increase as the dissolution would be controlled by the hydrodynamics.

Influence of silicon melt convection on interface instability in large-size silicon carbide solution growth

The interface instability in large-size silicon carbide solution growth has been investigated with a focus on the influence of silicon solution convection. Indications have shown that solution convection produces a substantial effect on the instability of the growth interface. As the SiC size increases, the instability becomes more evident. Therefore, improved solution convection is required for growing large-sized silicon carbide ingots by the solution method.

Mass transport induced asymmetry in charge/discharge behavior of liquid metal batteries

Mass and charge transport in liquid metal batteries are closely intertwined because of the fully liquid interior of the cells. We found that charging and discharging cycles may show pronounced asymmetries. They are caused by the presence (charge) or absence (discharge) of solutal convection. While the direction of thermal gradients in the positive electrode of a liquid metal battery depends on boundary conditions and thermodynamics in a non-trivial manner, the solutal gradient predictably changes direction from charge to discharge. The unstable density distribution during charge drives a flow strong enough to prevent any concentration polarization. In contrast, during discharge, the stable density gradient suppresses convection and leads to a substantial mass transport overvoltage. We illustrate this scenario by experimental data, numerical simulations and a physical model.