Buoyant Convection Research Articles

Numerical investigation on flow instability of sessile ethanol droplets evaporating in its pure vapor at low pressure

In order to understand the flow instability in the evaporation sessile droplet, a series of numerical simulations are carried out on the ethanol droplet evaporating in its pure vapor at low pressure. The contact radius of sessile droplet on the substrate is 2.5mm. The temperature of the ethanol vapor is fixed at 298K, and the corresponding saturated vapor pressure is 7615Pa. Results show that with the increase of the substrate temperature, different types of the thermal patterns appear in sequence. They are the steady axisymmetric pattern or multi-cell pattern driven by tangential temperature gradient, Bénard-Marangoni cells mainly induced by vertical temperature gradient, and longitudinal rolls caused by inclined temperature gradient. The relation between the evaporation rate and the contact angle is non-monotonic, which depends on the combined effects of Marangoni flow intensity, evaporating surface area as well as the length of heat transfer path from substrate to evaporation surface. The type of flow instability is independent on gravity. However, buoyancy convection changes the transition Marangoni number of the thermal patterns.

Thermosolutal convection and macrosegregation during directional solidification of TiAl alloys in centrifugal casting

Experiments of directional solidification of TiAl cylindrical samples were conducted within the frame of the ESA GRADECET project. The experiments were performed in the ESA “Large Diameter Centrifuge” using a furnace with a well defined thermal protocol. The furnace was mounted in the centrifuge and free to tilt in such a way that the total apparent gravity (sum of terrestrial gravity and centrifugal acceleration) was aligned to the cylinder centerline. Several centrifugation levels were investigated besides to one reference case out of the centrifuge. In this work, we present 3D numerical simulations of these experiments paying special attention in the liquid thermosolutal buoyancy convection and aluminum macrosegregation. The numerical model accounts for the non-inertial accelerations that appear in the rotating reference frame (centrifugal and Coriolis), motionless solid, themosolutal Boussinesq approximation and an infinitely fast microscopic diffusion model between the phases to depict the solid growth. The results showed that the Coriolis acceleration entirely modifies the liquid flow regime during solidification leading to a 3D aluminum segregation pattern with respect to the case solidified under normal terrestrial gravity conditions. Additionally the magnitude of aluminum segregation increases with the level of centrifugation.

Comparison of turbulent structures and energy fluxes over exposed and debris-covered glacier ice

AbstractWe present the first direct comparison of turbulence conditions measured simultaneously over exposed ice and a 0.08 m thick supraglacial debris cover on Suldenferner, a small glacier in the Italian Alps. Surface roughness, sensible heat fluxes (~20–50 W m−2), latent heat fluxes (~2–10 W m−2), topology and scale of turbulence are similar over both glacier surface types during katabatic and synoptically disturbed conditions. Exceptions are sunny days when buoyant convection becomes significant over debris-covered ice (sensible heat flux ~ −100 W m−2; latent heat flux ~ −30 W m−2) and prevailing katabatic conditions are rapidly broken down even over this thin debris cover. The similarity in turbulent properties implies that both surface types can be treated the same in terms of boundary layer similarity theory. The differences in turbulence between the two surface types on this glacier are dominated by the radiative and thermal contrasts, thus during sunny days debris cover alters both the local surface turbulent energy fluxes and the glacier component of valley circulation. These variations under different flow conditions should be accounted for when distributing temperature fields for modeling applications over partially debris-covered glaciers.

Developing buoyant convection in vertical porous annuli with unheated entry and exit

AbstractThe open‐ended vertical double‐passage annular space between three vertical concentric coaxial cylinders is an important physical configuration portraying many practical applications. Hence, in the present analysis, the developing buoyant convection in vertical double‐passage annuli filled with fluid‐saturated porous media is studied numerically by imposing unheated entry and unheated exit thermal boundary conditions. The numerical solutions of the mathematical model equations are found through finite difference technique. The velocity profiles in radial as well as axial directions and temperature profiles have been depicted for vast range of nondimensional numbers, baffle position, and heating and un‐heating ratio. The velocity and thermal gradients decreases as heating section length decreases. Maximum velocity and heat transport occurs in a narrow annular passage rather than equal or wider passages. The presence of porosity causes a reduction in flow velocities and thermal gradients.

Mixed convective flow of Maxwell nanofluid induced by vertically rotating cylinder

A study of mixed convection phenomenon in 3D flow of Maxwell nanofluid induced by vertically rotating and stretching cylinder is presented in current article. The Boussinesq approximation is used to predict the force of buoyancy due to which the free convection occurs in the fluid. Moreover, a popular Buongiorno model is utilized to reveal the influence of thermophoretic and Brownian forces for the transportation of energy in nanoliquid. The current physical problem of Maxwell nanofluid flow with energy transport is modeled under above consideration in form of partial differential equations (PDEs). Using the suitable flow similarities, the PDEs transformed into the set non-linear ordinary differential equations (ODEs) and then solved numerically by MATLAB built in scheme bvp4c. The outcomes of the problem are explored graphically and discussed with physical justification in rigorous way. In the flow and thermal analysis, it is noted that the higher values of both buoyancy and mixed convection parameters enhance the axial velocity and decline the swirl velocity, while these parameters decline the temperature and solutal fields, respectively. The higher value of thermophoretic force increases the thermal and solutal energy transport in the nanoliquid. Overall, it is revealed from the analysis that for large values of Reynolds number, the flow and energy transport decay exponentially faster to free stream conditions.

Numerical study on convective flow and heat transfer in 3D inclined enclosure with hot solid body and discrete cooling

Purpose This study aims to deal the numerical simulation on buoyant convection and energy transport in an inclined cubic box with diverse locations of the heater and coolers. Design/methodology/approach The left/right walls are cooled partially whereas the other walls are kept adiabatic. In the left/right walls, three different locations of the cooler are examined, whereas heater moves in three locations in the middle of the enclosed box. The governing models are numerically solved using the finite-element method. Findings The simulations are done on several values of the Rayleigh number and cavity inclination angles and different locations of the heater and coolers. The results are presented in the form of streamlines, isosurfaces and Nusselt numbers for different values of parameter involved here. It is recognized that the inclination of the box and the locations of the coolers strongly influence the stream and energy transport inside the enclosed domain. Research limitations/implications The present investigation is conducted for steady, laminar, three-dimensional natural convective flow in a box for different locations of cooler and tilting angles of a cavity. The study might be useful to the design of solar collectors, room ventilation systems and electronic cooling systems. Originality/value This work examines the effects of different locations of cooler and tilting angles of a cavity on convective heat transfer in a 3D cavity. The study is useful for thermal engineering applications.

Flexible Particle Focusing and Switching in Continuous Flow via Controllable Thermal Buoyancy Convection.

We present a novel approach that utilizes thermal buoyancy convection to achieve flexible particle focusing and switching in continuous flow of a microfluidic system. In this platform, three strip microheaters, A, B, and C, are symmetrically distributed at the bottom of microchannel, and they are isolated from the particle suspension by a thin glass slide. Continual transverse convection flow forms when the microheaters are energized by dc signals. The flow patterns are readily tuned by changing the energization strategies of the microheater array, leading to the modulation of the position of flow stagnation region. Accordingly, microparticles dispersed in fluids are rapidly focused to the flow stagnation region by the Stokes drag and thus form a continuous particle beam. The particle beam can also be switched to different lateral positions by adjusting the control voltages. This particle manipulation method is first demonstrated by respectively energizing these three microheaters and subsequently switching silica particles into different outlets. The lateral position of the particle beam then is flexibly controlled by simultaneously energizing microheaters A and B (or B and C) and adjusting the voltage applied on microheater A (or C). Furthermore, the versatility of this approach is proved by focusing and switching of microsized droplets, that is, oil-in-water and water-in-oil-in-water emulsion droplets. Finally, we use poly(ethylene glycol) diacrylate microgels, excellent reactant carriers, as an experimental sample and flexibly manipulate them in this microdevice, demonstrating this strategy's applicability for the cargo delivery. Therefore, this technique can be attractive for many particle preprocessing applications.

Melting Heat Transfer Analysis on Magnetohydrodynamics Buoyancy Convection in an Enclosure: A Numerical Study

Therollof melting heat transfer on magnetohydrodynamic natural convection in a square enclosurewithheatingof the bottom wall is examinednumericallyin this article.The dimensionlessgoverning partial differential equations are transformed into vorticity and stream functionformulationand then solved using the finite difference method(FDM). The effects of thermal Rayleigh number(Ra), melting parameter(M) and Hartmann number(Ha) are illustrated graphically.With an increasing melting parameter and Rayleigh number, the rate of fluid flow and temperature gradients are seen to increase. And in the presence of magnetic field, the temperature gradient reduces and hence the conductionmechanism dominated for larger Ha. Greater heat transfer rate is observed in the case of uniform heating compared with non-uniform case. The average Nusselt number reduces with increasing magnetic parameterin the both cases of heating of bottom wall.

Efficient technique for solution of the 2D buoyancy convection problems in vertical cylinders

Efficient technique for solution of the 2D buoyancy convection problems in vertical cylinders

Fully developed Darcy-Forchheimer mixed convective flow over a curved surface with activation energy and entropy generation

Background: Mixed convection (forced+natural convection) is frequently observed in exceptionally high output devices where the forced convection isn’t sufficient to dissipate all of the heat essential. At this point, consolidating natural convection with forced convection will frequently convey the ideal outcomes. Nuclear reactor technology and a few features of electronic cooling are the examples of these processes. Mixed convection problems are categorized by Richardson number (Ri), which is the ratio of Grashof number (for natural convection) and Reynolds number (for forced convection). For buoyancy or mixed convection the relative effect can be addressed by Richardson number. Typically, the natural convection is negligible when Richardson number is less than 0.1 (Ri < 0.1), forced convection is negligible when Richardson number is greater than 10 (Ri > 10) and neither is negligible when (0.1 < Ri < 10). It might be noticed that generally the forced convection is large comparative with natural convection except in case of remarkably low forced flow velocities. The current work gives significant insights regarding dissipative mixed convective Darcy-Forchheimer flow with entropy generation over a stretched curved surface. The energy equation is developed with respect to nonlinear radiation, dissipation and Ohmic heating (Joule heating). Binary reaction via activation energy is accounted.Method: Curvilinear transformations are utilized to change the nonlinear PDE’s into ordinary ones. Computational outcomes are obtained via NDSolve MATHEMATICA. The results are computed and discussed graphically.Results: Velocity decays for Forchheimer number. Entropy generation enhances for diffusion parameter and chemical reaction parameter. Concentration profile reduces chemical reaction parameter and enhances for activation parameter.

Onset of Thermal Buoyancy Convection in a Two-Layer System with Deformable Interface and Fixed Heat Flux at the Boundaries under Terrestrial and Microgravity Conditions

The stability of a conductive state of a system of two horizontal layers of immiscible fluids of close densities with a fixed heat flux at external boundaries is studied. The problem is solved in the framework of the generalized Boussinesq approximation, which correctly takes into account the deformations of the interface. The onset of convection at heating from above or below is investigated. Main attention is paid to the effect of layer thickness ratio and gravity level on instability. Two long-wave instability modes related the presence of a deformable interface and to a fixed heat flux at the external boundaries are found out. They are analyzed analytically. The behavior of the system with respect to monotonic and oscillatory perturbations of finite wavelength is studied numerically. The complete stability map is obtained. It is found that for very different layer thicknesses, as well as for single layers with fixed heat flux at the boundaries, a long-wavelength instability is observed. The presence of a long-wave instability domain at close thicknesses of layers is also found, which is associated with a strong stabilization of finite-wavelength instability in this parameter range. In the case of Earth gravity, the monotonic perturbations are most dangerous at any ratios of layer thicknesses, the oscillatory mode of instability is responsible for the onset of convection only in microgravity conditions.

Finite element method solution of mixed convection flow of Williamson nanofluid past a radially stretching sheet

AbstractThis computation reports the mixed convection flow of Williamson fluid past a radially stretching surface with nanoparticles under the effect of first‐order slip and convective boundary conditions. The coupled nonlinear ordinary differential equations (ODEs) are obtained from the partial differential equations, which are derived from the conservation of momentum, energy, and species. Then, utilizing suitable resemblance transformation, these ODEs were changed into dimensionless form and then solved numerically by means of a powerful numerical technique called the Galerkin finite element method. The effect of different parameters on velocity, temperature, and concentration profiles is inspected and thrashed out in depth by graphs and tables. The upshots exhibit that the velocity profile augments as the values of concentration buoyancy and mixed convection parameters are engorged. Also, the results demonstrated that both temperature and concentration profiles are improved with an enhancement in values of thermophoresis parameters. The outcomes also indicate that for finer values of magnetic field parameter and thermophoresis parameter, the numerical value of local Nusselt and Sherwood numbers is reduced.

Generalized regimes for the formation of stratified regions during freezing of multi-component mixtures

Stratified double-diffusive layers (DDLs) in fluidic mixtures such as oceans, magma, and latte typically contain alternating low gradient mixing regions separated by high gradient interfaces. The prior knowledge is restricted to the formation of layers, but the existence of DDLs, under prolonged freezing conditions, as well as in multicomponent mixtures, is not yet understood well. In this work, a new observation depicting the existence of a life-cycle for a double-diffusive layer is revealed with the help of real-time observations of unidirectional freezing of multicomponent mixtures. The observations showed a systematic occurrence of the onset, formation, disappearance, and recurrence of the DDLs when freezing conditions prevailed for longer durations of time. The results also include first-ever observations of compositional stratification in a ternary mixture, which depends on the regimes and nature of buoyant convection. The ternary experiments also demonstrated the formation of DDLs much closer to the solidifying mush, which shed light on retaining the stratified layers in the frozen state. Furthermore, the hypothesized life-cycle of the DDL was mapped to the regimes of occurrence and the nonexistence of DDLs in the mixture phase diagrams of binary and ternary systems, with a threshold composition difference and the corresponding critical Rayleigh number. This distinction of the regimes on the phase diagram shows a striking correlation with a reduced ternary phase diagram of igneous rocks, thus providing a suitable basis for explaining the formation of layered rocks.

Instability of thermocapillary–buoyancy convection in droplet migration

The instabilities of thermocapillary–buoyancy convection in droplet migration are examined by linear stability analysis. The droplet is flattened by gravity and placed on a unidirectional heated solid surface. The velocity and temperature distributions of basic flow are derived as a function of the migration velocity and the Bond number. The critical Marangoni number is obtained, which depends on the Prandtl number (Pr), the Bond number, and the migration velocity. The preferred modes at small and moderate Pr are oblique waves, which travel either upstream or downstream. For high Pr, the preferred modes include oblique and streamwise waves, while the amplitude of temperature on the surface is much smaller than that of the hot spot in the flow region. The instability mechanism is discussed and comparisons are made with liquid layers.

Continuous microfluidic mixing and the highly controlled nanoparticle synthesis using direct current-induced thermal buoyancy convection

We present a flexible and noninvasive approach for efficient continuous micromixing and microreaction based on direct current-induced thermal buoyancy convection in a single microfluidic unit. Theoretically, microfluids in this microsystem are unevenly heated by powering the asymmetrically arranged microheater. The thermal buoyancy convection is then formed to induce microvortices that cause effective fluidic interface disturbance, thereby promoting the diffusion and convective mass transfer. The temperature distribution and the convection flow in the microchip are first characterized and studied, which can be flexibly adjusted by changing the DC voltage. Then the mixing performance of the presented method is validated by joint numerical and experimental analyses. Specifically, at U = 7 V, the mixing efficiencies are higher than 90% as the flow rate is lower than Qv= 600 nL/s. So high-quality chemical or biochemical reactions needing both suitable heating and efficient mixing can be achieved using this method. Finally, as one example, we use this method to synthesize nano-sized cuprous oxide (Cu2O) particles by effectively mixing the Benedict’s solution and glucose buffer. Remarkably, the particle size can be tuned by changing the voltage and the concentration of Benedict’s solution. Therefore, this micromixer can be attractive for diverse applications needing homogeneous sample mixtures.

Buoyancy driven convection instability and related pulsing of continuous optical discharges

Optical discharges find now ever expanding applications in science, engineering and medicine. In present study authors discuss the phenomenon of periodic pulsing of a continuous optical discharge (COD) on the base of original experimental results. In the literature there is no general agreement regarding the origin of the considered instability. Authors have obtained the dependence of oscillation frequency on the gas pressure and proposed semi-empirical relations to estimate pulse frequency pressure dependence observed. Regular self-sustained oscillations of COD plasma were explained by the pulsations of heated gas bubble surrounding COD followed by buoyancy driven vortex formation that was detected on schlieren images of convection zone. This kind of buoyant convection instability was determined to obey the same similarity law as cycling frequency of puffing or flickering in diffusion and premixed flames. The instability discussed can affect the performance of COD in some important applications such as high brightness broadband light sources, for instance.

Two-dimensional gyrotactic microorganisms flow of hydromagnetic power law nanofluid past an elongated sheet

This article, describes two-dimensional magnetohydrodynamic steady incompressible viscous power law nanofluid comprising gyrotactic microorganisms adjacent to a vertical stretching sheet. The governing non-linear partial differential equations are lessened to a set of non-linear ordinary differential equation using similitude transformation. The non-dimensional boundary value problem is then solved under spectral relaxation method. The influences of different parameters such as buoyancy convection parameters [Formula: see text], magnetic field parameter M, power law parameter [Formula: see text], Prandtl number [Formula: see text], modified Prandtl number [Formula: see text], thermophoresis parameter [Formula: see text], Peclet number [Formula: see text], Lewis number [Formula: see text], Brownian motion parameter [Formula: see text], bioconvection Lewis number [Formula: see text], and bioconvection constant [Formula: see text] on flow convective characteristics phenomena are explored via plots and tables. The skin friction factor, rate of heat transfer, rate of mass transfer, and the density number of the motile microorganisms near the surface are also computed. Our results are compared with the existing results to support our model. Residual error analysis is determined for showing the convergence rate against iteration. Our result showed that the momentum thickness reduces as the value of [Formula: see text] induces and thermal boundary thickness increases as the value of [Formula: see text] induces. We also revealed that the density of the motile microorganisms [Formula: see text] is a reducing function of [Formula: see text] and concentration boundary layer induces with the increase of [Formula: see text], whereas its thickness close to the surface decreases with increasing of [Formula: see text]. Also, the stream line patterns are exhibited to the impact of physical sundry variables.

Efficient particle and droplet manipulation utilizing the combined thermal buoyancy convection and temperature-enhanced rotating induced-charge electroosmotic flow

Efficient granular sample manipulation is crucial for various microfluidic-based applications such as material synthesis and drug delivery. Herein we present a novel method to efficiently manipulate microbeads and droplets using the combined thermal buoyancy convection and temperature-enhanced rotating induced-charge electroosmotic flow. Within the granular fluid, a pair of counter-rotating microvortices is formed above the floating electrode, leading to the formation of a flow stagnation region at the bottom center. Granular samples then can be effectively transported to this region by the Stokes drag, and the concentration performance can be flexibly manipulated by adjusting the energization strategies of the chip. The contributions of fluid convection, dielectrophoresis, thermophoresis, and gravity force to particle migration are first studied and compared, proving that the convection flow and gravity force are mainly responsible for particle migration and deposition respectively. Then the systematic enriching experiments of 4-μm silica particles demonstrate that the particle migration velocity can be highly improved by the combined thermal-electrical field. Finally, the effective concentration of nanocopper particles and the assembly of oil-in-water/water-in-oil-in-water droplets indicate that this approach is capable of manipulating diverse granular samples. Therefore, this strategy can be attractive for lots of microfluidic-based applications because of its high efficiency and simplicity.

Theoretical analysis of tangent hyperbolic nanoparticles with combined electrical MHD, activation energy and Wu’s slip features: a mathematical model

Current study deals with the rheology of non-Newtonian material over moving surface by utilizing the electrical, magnetohydrodynamics and activation energy effects. A famous tangent hyperbolic viscoelastic fluid model has been used in presence of nanoparticles which may extremely useful to enhance the transportation phenomenon. The second order slip effects (Wu’s slip) are also entertained in the boundary conditions. Further, the energy equation occupied the nonlinear thermal radiation while the activation energy expression is carried out in the concentration equation. The highly nonlinear boundary value problem is metamorphosed into ordinary differential equations by means of the suitable variables. These transmuted equations results the self-similar solution which is computed numerically by shooting technique. First, the reported solution is justified after comparing with available resources and detail graphical analysis have been executed for parameters like Weissenberg number mixed convection buoyancy parameters the power law index thermophoresis Brownian constraint and slip parameters Further, the impact of suggested parameters on local Nusselt number and Sherwood number is determined in tabular form. It is found that the slip parameters resulted the weaker momentum boundary layer. The nanoparticles temperature is enhanced by the increasing values of thermophoresis and Brownian motion parameters, Weissenberg and thermal Biot numbers. Further, the convection parameters decrease the nanoparticles concentration while presence of activation energy slightly boosts up the nanoparticles concentration.

Integrated Fluxes in Magneto-Hydrodynamic Mixed Convection in a Cavity Sustained by Conjugate Heat Transfer

Abstract A numerical study of magneto-hydrodynamic mixed convection in a cavity has been conducted to investigate the influence of magnetic field on integrated flux of thermal energy, linear momentum, and kinetic energy. Shear force through lid motion establishes the forced convection effect and buoyancy force due to differential heating of the moving lid and the stationary interface ensures the natural convection phenomenon. Additionally, conduction through the solid slab with prescribed temperature at the outer surface attached to the cavity induces conjugate heat transfer. Further, the top and bottom walls throughout the domain are kept insulated and a uniform horizontal magnetic field is applied on the interface toward left. Fluid flow and heat transfer characteristics are examined for a range of Hartmann number (Ha): 0, 10, 50, and 120 at fixed values of Reynolds number, Grashof number, and Prandtl number of 300, 9 × 104 and 0.71, respectively. The result shows that the transport of heat in the near wall regions of the fluid domain is primarily governed by diffusion, whereas advection appears stronger in the central region of the cavity. Increase in magnetic field strength from Ha = 0 to 120 gradually suppresses the recirculating structure of the flow signifying a reduction in advective strength as depicted by the decrease in the value of total integrated heat flux from 25.15×10-3 to 6.0×10-3. The reduction in heat flux, momentum fluxes, and kinetic energy fluxes with increase in magnetic field has been well correlated in the range of 0≤Ha≤120.