Onset Of Convection Research Articles

Effect of fluid-particle interaction on 2D Rayleigh-Bénard laminar convection of a temperature-sensitive magnetic fluid

Rayleigh-Bénard (R–B) laminar convection of a temperature-sensitive magnetic fluid (TSMF) with an immersed nonmagnetic particle in a square cavity heated from the bottom and subject to an external magnetic field is investigated experimentally and numerically, which restricts to the initial constant temperature and stationary flow fields with perfect thermal insulated wall condition. The flow and particle behaviors as well as the heat transfer characteristic are examined at different Ra (Ra < 4 × 104) and Ram (Ram = 0, 1.89 × 108, and 3.14 × 108). The magnetic field slightly precipitates the onset of convection and the transition of the flow pattern. The particle may influence the early flow development and the final flow pattern, depending on the initial location of the particle, Ra, and Ram. Over certain ranges of the parameters, the particle stabilizes the flow by shifting the higher flow mode (bicellular) to the lower one (unicellular) and improves the heat transfer performance. The numerical results confirm the experimental findings and provide more detailed behaviors of the flow and temperature fields as well as the particle.

Variable gravity effects on penetrative porous convection

The effect of variable gravity on the onset of penetrative convection in a horizontal fluid-saturated porous layer is studied, performing the linear instability analysis and the nonlinear stability analysis (via the weighted energy method) of the thermal conduction solution. The linear and nonlinear critical Rayleigh numbers for the onset of penetrative porous convection are determined and their behaviour with respect to the fundamental physical parameters of the model is studied. In particular, we found that there exists a region of subcritical instabilities and that the increase of the upper bounding plane temperature and the decrease of gravity have a stabilizing effect on the onset of convection.

A NUMERICAL STUDY OF JOULE HEATING EFFECT IN ELECTRODEPOSITION PROCESS-CASE OF COPPER ELECTRODEPOSITION

The electrochemical process in the presence of heat dissipation effect (Joule heating effect) is highly detected in practical electroplating solutions such as 0.6 M CuSO4+0.5 M H2SO4 electrolyte, where currents passing through the cathode-anode-electrolyte increase temperature in the electrolyte and electrodes. Due to the complex nature of such systems and the diversity of physics describing it, a theoretical approach of appearance for natural convection on vertical and horizontal sides of a cathode in the presence and absence of Joule effect under unsteady state condition was observed using Comsol Multiphysics. It was found that the onset of natural convection on a vertical side depends on the Joule effect, and the time (tn) for appearance is longer on the edges than in other areas. The horizontal side of a cathode assists the appearance of natural convection in the presence and absence of the Joule effect. The Rayleigh (Ra) number has larger values on the vertical side than the horizontal side due to the effect of surface concentration distribution Cs. A Sherwood, Sh-Ra mathematical relation such that Sh=0.03Ra0.86 was proposed where natural convection had no pronounced effect on the deposition process. It was found that the Joule heating effect negatively affects the electrodeposition process by reducing the local current values.

Irrigation contrasts through the morning transition

AbstractThe Land surface Interactions with the Atmosphere over the Iberian Semi‐arid Environment (LIAISE) campaign was conducted in July 2021, primarily to investigate the role of irrigation in modulating the boundary‐layer evolution in the Catalan region of northeastern Spain. Contrasts in near‐surface meteorological parameters and boundary‐layer thermodynamic profiles at an irrigated and rainfed (arid) site were established during the morning transition. Evapotranspriation dominated the flux partitioning at the irrigated site (Bowen ratio of 0.07–1.1), whilst sensible heat flux dominated at the rainfed (arid) site (Bowen ratio greater than 10.0). The cumulative evapotranspiration during July 2021 was a factor of 10 greater at the irrigated site than at the rainfed (arid) site. The presence of irrigation was shown to modulate the vertical gradients of turbulence, temperature, and moisture. Irrigation is shown to have a significant effect on the development of the boundary layer, including during the morning transition. The morning transition mean buoyancy flux was 2.8 times smaller at the irrigated site (1.1 ms) compared with the rainfed (arid) site (3.1 ms), with a resultant delay in the near‐surface buoyancy‐flux crossover time (30–90 min) at the irrigated site. At the start of the morning transition (sunrise), the average screen‐level (50‐m) temperature was K ( K) colder at the irrigated site relative to the rainfed (arid) site. The colder temperatures at sunrise at the irrigated site are predominantly the result of a colder boundary‐layer thermodynamic profile from the previous day. At the end of the morning transition (convective onset), temperature differences between the two sites extend through much of the boundary layer and increase in magnitude. The average screen‐level (50‐m) temperature difference was K ( K) colder at the irrigated site relative to the rainfed (arid) site. There was considerable day‐to‐day variability in temperature contrasts at a regional level ( to K).

Heat transfer in a quasi-one-dimensional Rayleigh–Bénard convection cell

We report an experimental and numerical study on Rayleigh–Bénard convection in a slender rectangular geometry with the aspect ratio $\varGamma$ varying from 0.05 to 0.3 and a Rayleigh number range of $10^5\leqslant Ra\leqslant 3\times 10^9$ . The Prandtl number is fixed at $Pr=4.38$ . It is found that the onset of convection is postponed when the convection domain approaches the quasi-one-dimensional limit. The onset Rayleigh number shows a $Ra_c=328\varGamma ^{-4.18}$ scaling for the experiment and a $Ra_c=810\varGamma ^{-3.95}$ scaling for the simulation, both consistent with a theoretical prediction of $Ra_c\sim \varGamma ^{-4}$ . Moreover, the effective Nusselt–Rayleigh scaling exponent $\beta =\partial (\log Nu)/\partial (\log Ra)$ near the onset of convection also shows a rapid increase with decreasing $\varGamma$ . Power-law fits to the experimental and numerical data yield $\beta =0.290\varGamma ^{-0.90}$ and $\beta =0.564\varGamma ^{-0.92}$ , respectively. Near onset, the flow shows a stretched cell structure. In this regime, the velocity and temperature variations in a horizontal cross-section are found to be almost invariant with height in the core region of a slender domain. As the Rayleigh number increases, the system evolves from the viscous dominant regime to a plume-controlled one, a feature of which is enhancement in the heat transport efficiency. Upon further increase of $Ra$ , the flow comes back to the classical boundary-layer-controlled regime, in which the quasi-one-dimensional geometry has no apparent effect on the global heat transfer.

Instability thresholds for penetrative porous convection with variable viscosity fluids

A comprehensive investigation discerning different viscosity-temperature variations on the onset of porous penetrative convection modeled through quadratic density-temperature law is undertaken. The fluid viscosity is assumed to vary linearly, quadratically and exponentially with temperature. The variable viscosity results in a coupling between stream function and temperature in the Darcy momentum balance equation. The linear instability of the basic state is analyzed numerically by examining small-amplitude disturbances in velocity and temperature fields. By carrying out numerical computations for a wide range of parametric regimes, it is established that the principle of exchange of stabilities (PES) is valid. The critical values of the Darcy-Rayleigh number and of the wave number are obtained. The analysis reveals that the combined influence of variable viscosity and the density-maximum characteristic is to expedite the onset of porous convection much faster compared to their isolation presence. The size of the convection cell decreases with an increase in the Darcy-modified Rayleigh number linked to the quadratic density variant. The outcomes obtained under the limiting cases are shown to exhibit consistency with previously published findings.

Thermogravitational Convection in a Controlled Rotating Darcy-Brinkman Nanofluids Layer Saturated in an Anisotropic Porous Medium Subjected to Internal Heat Source

Thermogravitational convection in a controlled rotating Darcy-Brinkman nanofluids layer saturated in an anisotropic porous medium heated from below is Thermogravitational convection in a controlled rotating Darcy-Brinkman nanofluids layer saturated in an anisotropic porous medium heated from below is investigated. The presence of a uniformly distributed internal heat source and considers the Brinkman model for different boundary conditions: rigid-rigid, free-free, and lower-rigid and upper-free are considered. The effect of a control strategy involving sensors located at the top plate and actuators positioned at the bottom plate of the nanofluids layer is analysed. Linear stability analysis based on normal mode technique is employed. The resulting eigenvalue problem is solved numerically using the Galerkin method implemented with Maple software. The model used for the nanofluids associates with the mechanisms of Brownian motion and thermophoresis. The influences of the internal heat source strength, mechanical anisotropy parameter, modified diffusivity ratio, nanoparticles concentration Darcy-Rayleigh number and nanofluids Lewis number are found to advance the onset of convection. Conversely, the Darcy number, thermal anisotropy parameter, porosity, rotation, and controller effects are observed to slow down the process of convective instability.investigated. The presence of a uniformly distributed internal heat source and considers the Brinkman model for different boundary conditions: rigid-rigid, free-free, and lower-rigid and upper-free are considered. The effect of a control strategy involving sensors located at the top plate and actuators positioned at the bottom plate of the nanofluids layer is analysed. Linear stability analysis based on normal mode technique is employed. The resulting eigenvalue problem is solved numerically using the Galerkin method implemented with Maple software. The model used for the nanofluids associates with the mechanisms of Brownian motion and thermophoresis. The influences of the internal heat source strength, mechanical anisotropy parameter, modified diffusivity ratio, nanoparticles concentration Darcy-Rayleigh number and nanofluids Lewis number are found to advance the onset of convection. Conversely, the Darcy number, thermal anisotropy parameter, porosity, rotation, and controller effects are observed to slow down the process of convective instability.

Magneto-Rotatory Convection in Couple-Stress Fluid

Background: Thermal convection is the most convective instability when crystals are produced from a single element like silicon and the thermal instability of a fluid layer heated from below plays an important role in geophysics, oceanography, atmospheric physics, etc. The flow through porous media is of considerable interest for petroleum engineers, for geophysical fluid dynamicists and has importance in chemical technology and industry. Many of the flow problems in fluids with couple-stresses indicate some possible experiments, that could be used for determining the material constants, and the results are found to differ from those of Newtonian fluid. Keeping this in view, the present work was to study the effect of a uniform vertical magnetic field on the couple-stress fluid heated from below in the presence of a uniform vertical rotation through permeable media. Methodology: The present problem is studied using the linearized stability theory, Boussinesq approximation, normal mode analysis, and the dispersion relation is obtained. Results: The stationary convection, stability of the system, and oscillatory modes are discussed. In the case of stationary convection, the rotation postpones the onset of convection. The magnetic field and couple-stress may hasten the onset of convection in the presence of rotation while in the absence of rotation; they always postpone the onset of convection. The medium permeability hastens the onset of convection in the absence of rotation while in the presence of rotation, it may postpone the onset of convection. The rotation and magnetic field are found to introduce oscillatory modes in the system which was non-existent in their absence. A sufficient condition for the non-existence of overstability is also obtained.

Thermo-electric convection in a cylindrical annulus during a sounding rocket flight

An experiment on convective flows induced by the dielectrophoretic force was performed under the microgravity condition provided during a sounding rocket flight. The dielectrophoretic force possesses a non-conservative term that can be seen as resulting from an electric gravity. That gravity can be responsible for an electric Rayleigh–Bénard convection between a hot inner cylinder and a cold outer cylinder when an electric field is applied in the radial direction. Four cells with independent temperature and electric field controls allowed the investigation of eight different values of the electric Rayleigh number relatively close to the onset of the thermo-electric instability. A linear stability analysis is performed to predict the stability threshold and the evolution of the growth rate of the instability. The three-dimensional structure of the flow is captured by simultaneous particle image velocimetry and by shadowgraphy. The amplitude of the instability modes and the time evolution of the flow is analysed, and various methods are proposed to extrapolate the experimental critical value of the electric Rayleigh number for the onset of convection. The measured critical electric Rayleigh number is in agreement with the prediction of the linear stability theory. The comparison of the new experimental results with previous ones from parabolic flight campaigns highlights the importance of long-term microgravity for the achievement of thermal convection at low values of the control parameters.

Effect of Robin boundary conditions on the onset of convective torsional flows in rotating fluid spheres

Torsional flows are preferred at the onset of thermal convection in fluid spheres with stress-free and perfectly conducting boundary conditions, in a narrow region of the parameter space for Prandtl numbers Pr≲0.9 and ratios Pr/Ek=O(10), with Ek being the Ekman number. In this case, the transport of heat to the exterior is supposed instantaneous. When the thermal conductivity of the internal fluid is large, and the external convective heat transfer or radiative emissivity is low, the heat transmission is less efficient, and the thermal energy retained in the interior increases, enhancing the onset of convection. This study is devoted to analyzing the combined influence of the thermal conductivity and external conditions (temperature and resistance to heat transport) on the onset of the torsional convection by taking a Robin boundary condition for the temperature at the surface of the sphere. It is shown, by means of the numerical computation of the curves of simultaneous transitions to torsional flows and Rossby waves, that when the heat flux through the boundary decreases, the region where the axisymmetric flows are preferred shrinks, but it never strangles to an empty set. It has been found that with adequate scalings the curves delimiting the transition to torsional flows, and those of the critical Rayleigh number, Rac, and the frequencies of the modes vs Ek become almost independent of the parameter of the Robin boundary condition.

Rayleigh-Bénard-Marangoni Convection of Mono and Hybrid Nanoliquids in an Inclined Plane and Solution by Shooting Method

Unprecedented study on Rayleigh-Bénard-Marangoni convection in mono and hybrid nanoliquids in a region confined between two infinite inclined parallel planes. Linear stability analysis is conducted to investigate the stability of longitudinal and transverse rolls. The shooting method is used to obtain the eigenvalues of the boundary value problem with complex coefficients in the case of four different boundary conditions. The inclination angle is chosen in the range [0, 45] and the Rayleigh number is chosen in such a way that the critical Rayleigh number is greater than 0. The thermophysical properties measured at 300 K of twelve nanoliquids and thirty hybrid nanoliquids having a total volume fraction of 0.5% are evaluated using phenomenological laws and mixture theory, and prediction on the onset of convection is made in all cases. C2H6O2-SWCNT (F = 0.972881) advances the onset of convection the most among nanoliquids and C2H6O2-Ag-SWCNT and C2H6O2-Cu-SWCNT (F = 0.972875) among hybrid nanoliquids. Rayleigh-Bénard-Marangoni convective system in an inclined plane is more stable than that in a horizontal plane.

Effect of viscous dissipation on the onset of jeffery fluid porous convection in the presence of throughflow and electric field

Effect of viscous dissipation on the onset of jeffery fluid porous convection in the presence of throughflow and electric field

Global stability of Bénard–Marangoni convection in an anisotropic porous medium

Surface tension is essential in many industrial applications, especially where the liquid surface is in contact with the environment, such as crystal growth, semiconductor manufacturing, and welding. The present article reports a numerical analysis of convection induced by the combined effects of buoyancy force and surface tension in an infinitely extended horizontal fluid-saturated anisotropic porous layer with high permeability. We assume that the bottom boundary is rigid and the top is exposed to the air. Biot numbers establish general thermal conditions at both ends instead of commonly used adiabatic and isothermal boundary conditions. The anisotropy of the porous structure results in thermal and mechanical anisotropy parameters while examining layer's stability. The Chebyshev Tau technique yields the critical Marangoni number, MaLc and MaEc, representing linear and energy stability boundaries. We compare the constraints obtained from linear and energy analyses and conclude that the energy bounds for the current problem are less than linear bounds, indicating subcritical instabilities may exist. It is also observed that thermal anisotropy and Biot numbers stabilize the system. In contrast, mechanical anisotropy and the Darcy number advance the onset of convection. The existing results of limiting cases of the present problem are recovered with remarkable accuracy.

Internal heat source effects on thermosolutal convection of Casson nanofluids embedded by Darcy-Brinkman model

The current study investigates Darcy-Brinkman convective instability for a non-Newtonian binary nanofluid layer in the presence of internal heat source. Non-Newtonian fluid behavior is characterized by Casson model. Normal mode technique is employed to convert the relevant PDEs into ODEs and Boussinesq approximation is used. The phenomenon is explored for both stationary as well as oscillatory modes of convection. The novelty and significance of the work lie in the fact that oscillatory motions have come into existence due to the presence of internal heat source term in the thermal energy equation which were non-existent otherwise for top-heavy configuration of nanoparticles. The impact of various nanofluid and solute parameters except solute Rayleigh number is found to destabilize the nanofluid layer while solute Rayleigh number, Darcy number and porosity parameters are found to postpone the onset of convection. Non-Newtonian Casson parameter and internal heat source strength are found to hasten the convection for the present arrangement of nanoparticles.

The role of magnetic waves in tangent cylinder convection

The secular variation of the geomagnetic field suggests that there are anticyclonic polar vortices in the Earth's core. Under the influence of a magnetic field, the polar azimuthal flow is thought to be produced by one or more coherent upwellings within the tangent cylinder, offset from the rotation axis. In this study, convection within the tangent cylinder in rapidly rotating dynamos is investigated through the analysis of forced magnetic waves. The first part of the study investigates the evolution of an isolated buoyancy disturbance in an unstably stratified rotating fluid subjected to an axial magnetic field. It is shown that the axial flow intensity of the slow Magnetic-Archimedean-Coriolis (MAC) waves becomes comparable to that of the fast MAC waves when |ωM/ωC|∼0.1, where ωM and ωC are the Alfvén wave and inertial wave frequencies respectively. In spherical shell dynamo simulations, the isolated upwellings within the tangent cylinder are shown to originate from the localized excitation of slow MAC waves in the dipole-dominated regime. Axial flow measurements in turn reveal the approximate parity between the slow and fast wave intensities in this regime, which corresponds to the existence of strong polar vortices in the Earth's core. To obtain the observed peak azimuthal motions of 0.6–0.9∘yr−1, the Rayleigh number in the low-inertia geodynamo must be ∼103 times the Rayleigh number for the onset of nonmagnetic convection. However, if the forcing is so strong as to cause polarity reversals, the field within the tangent cylinder decays away, and the convection takes the form of an ensemble of plumes supported entirely by the fast waves of frequency ω∼ωC. The resulting weak polar circulation is comparable to that obtained in nonmagnetic convection.

Rayleigh–Bénard convection in mono and hybrid nanoliquids in an inclined slot

Linear stability analysis is conducted to investigate the longitudinal and transverse rolls (TRs) generated in Rayleigh–Bénard convection in mono and hybrid nanoliquids confined between two infinite inclined parallel slots. Thermophysical properties of six mono nanoliquids and fifteen hybrid nanoliquids are calculated for different volume fractions (0.5%, 1%, 2%) using phenomenological laws and mixture theory. The shooting method is used to solve boundary eigenvalue problems to obtain the eigenvalues for 16 different boundary conditions. It is observed that as the inclination angle is increased, it delays the onset of longitudinal rolls in the case of all boundary conditions. However, it advances the onset of TRs except when the lower plate is adiabatic. The addition of mono and hybrid nanoparticles results in the advancement of the onset of convection. The addition of SWCNT and SWCNT— accelerates the onset of convection the most while Cu and Cu-Ag accelerates the onset of convection the least amongst the mono and hybrid nanoparticles considered in the study.

Effect of variable gravity on thermal convection in Jeffrey nanofluid: Darcy-Brinkman model

The subject under consideration in this research has various geophysical and astrophysical applications. Specifically, we investigate the impact of variable gravity on the onset of thermal instability within a layer of Jeffrey nanofluid confined in a Darcy-Brinkman porous medium. The solution of the fluid layer, which is positioned between two free-free boundaries, is determined using a linear stability analysis employing the normal mode technique. The Rayleigh number on the onset of convection is derived by using the Galerkin method. For stationary convection, the effects of different variable gravity parameters on the Jeffrey parameter, Darcy-Brinkman number, Lewis number, moderated diffusivity ratio, porosity of porous media and nanoparticle Rayleigh number are analyzed and presented graphically. The choices of using Jeffrey nanofluid as the base fluid for the study add novelty. Non-Newtonian fluids encompass a diverse range of substances, such as engine oils, oil extraction, wallpaper paste, and various biological liquids like blood etc.

Local linear stability of plumes generated along vertical heated cylinders in stratified environments

The linear temporal and absolute/convective stability characteristics of a thermal plume generated along a heated vertical cylinder are investigated theoretically under the Boussinesq approximation. Special focus is given to the uniform-wall-buoyancy-flux case whereby the cylinder surface sustains the same linear temperature gradient as the environment. A competition between the axisymmetric and helical modes is a remarkable feature of the instability, distinguishing these ‘annular plumes’ from free plumes/jets for which the helical mode is generally dominant. It is found that higher surface curvature stabilises the temporal axisymmetric mode significantly, but only has moderate effects on the helical mode. The most temporally unstable perturbation mode switches from a helical into an axisymmetric mode when the Prandtl number increases beyond a critical value. Both the roles of shear and buoyancy during the destabilisation are identified through an energy analysis which indicates that, while the shear work is usually a major source of perturbation energy, the buoyancy work manifests for long-wave axisymmetric perturbation modes, and for thin cylinders and high Prandtl numbers. For the specific temperature configuration considered herein, an annular plume is always convectively unstable whereas decreasing the cylinder radius from the planar limiting case first decreases and then increases the tendency of the flow towards being absolutely unstable. The helical mode is especially susceptible to being absolutely unstable on very thin cylinders. Several conditions for the onset of cellular thermal convection and plume detrainment are proposed based on our results and a hypothesis which connects the absolute instability to the detrainment phenomenon.

The Onset of Darcy–Brinkman Convection in a Porous Layer with Mutual Impact of Thermal Non-Equilibrium and Non-Uniform Temperature Gradients

The two-field thermal conditions of local thermal nonequilibrium (LTNE) were used to investigate linear stability of thermal convection in a liquid-saturated, porous layer via the extended Brinkman–Darcy model for different non-uniform basic thermal gradients. The critical values were numerically computed by the Galerkin method for rigid isothermal boundaries. The impact of LTNE and different forms of non-uniform basic temperature gradients on the onset of porous convection was examined. The porosity modified conductivity ratio has no influence on system stability at a small inter-phase heat transport coefficient limit. However, for higher values of the inter-phase heat transport coefficient, an increase in the porosity modified conductivity ratio hastens the onset of convection. An increase in the Darcy number delays the convective motions. The results for different basic temperature profiles are symmetric qualitatively. In addition, the possibility of control of convection by a basic temperature profile was studied in detail.

Modeling the Impact of Electrolyte Flow on Heat Management in a Li-Ion Convection Cell

In response to challenges in the thermal management of lithium-ion batteries (LIBs), we investigate the concept of circulating electrolyte through the porous electrodes and separator to facilitate effective, uniform, and real-time temperature regulation. We show, through physics-based electrothermal modeling and dimensional analysis of a single, planar LIB cell, that electrolyte convection can simultaneously draw heat from the cell and suppress heat generation from entropy change, charge-transfer, and ohmic losses, and that the cell temperature rise can be effectively mitigated when heat removal matches or exceeds heat generation. These findings distinguish internal convection from external surface cooling approaches used in conventional thermal management that often lead to a tradeoff between heat and mass transport. In a simulated exemplary 5.7-C case, a LIB cell with stationary electrolyte must stop discharging at only 54% of its capacity due to cell temperature rise to an upper threshold (325 K); with sufficient electrolyte flow (∼1 μm s−1 for a single cell, or a residence time of ∼200 s), the cell can be maintained below 315 K while delivering 98% of its capacity. Finally, to illustrate the potential for dynamic temperature regulation, we simulate scenarios where cells already experiencing self-heating can instantly arrest temperature rise with the onset of convection.