Rayleigh-Benard Research Articles

Simulation of interfacial mass transfer process accompanied by Rayleigh convection in NaCl solution

Rayleigh convection caused by dissolved CO2 is critical for CO2 capture and storage technology in saline aquifers. However, the poor experimental measurements of theoretical model parameters limit a further analysis of the interfacial mass transfer process after the convection occurring. In this study, a two-dimensional lattice Boltzmann method (LBM) including electrostatic and volume forces is used to simulate Rayleigh convection for CO2 in a NaCl solution at 293.15 K, 1.00 atm. Through calculating the key parameter of the simulated onset time, it indicates that the CO2 interfacial concentration is only 15–20% of the saturated concentration at the true gas–liquid mass transfer interface. Besides, the simulated convective onset time would be delayed by increasing the salt concentration. The average instantaneous mass transfer factor is also decreased with the increased concentration and when the concentration of the NaCl solution is less than 3 mol L−1 (M), the mass transfer rate can be accelerated by more than 3.2 times. We propose a new method to obtain surface renewal time which is determined according to the concentration and velocity field simulated by LBM and find that changes in mass transfer rate follow the penetration theory in the molecular diffusion stage while abiding by the surface renewal theory in the interfacial convection stage. Our results can provide a valuable guide for relevant economic evaluation and appropriate geological sequestrated sites.

Two-layer thermally driven turbulence: mechanisms for interface breakup

Abstract

Investigation on the convection flow effect on the surface and depth of the evaporative polymeric solutions via PIV method

Convection is the main reason for surface roughness in both polymeric and non-polymeric films obtained by the drying process (solvent evaporation). It is important to find the origin to control this phenomenon which leads to the creation of a variety of surface patterns that are formed on the polystyrene/methyl ethyl ketone solution in various thicknesses and concentrations. In this research, the convection flow was observed and analyzed at the surface and depth of the polymeric fluids using Particle Image Velocimetry method. Counter-rotational flow was observed in the cross section of each cell and the flow direction was found to be ascending in the cell center and descending around it. The non-dimensional Rayleigh number (based on buoyancy) and Marangoni number (based on surface tension) were calculated to determine the convection dominant mechanism. The critical value of each number is the threshold for starting instabilities. According to the calculations based on initial assumptions and solving the one-dimensional mass transfer equation in a layer of the solution as well as controlling the effective parameters such as concentration, thickness and viscosity. Rayleigh convection was observed in a certain thickness and concentration of the samples, while concentration Marangoni convection was only observed in some of them. However, both concentration and thermal Marangoni were found in most samples.

Effect of Marangoni Convection on the Perovskite Thin Liquid Film Deposition.

Motivated by recent advances in the development of thin-film perovskite solar cells, the evaporation and deposition of a perovskite thin liquid film on a hydrophilic substrate, also in some cases subjected to ultrasonic vibrations, are studied in this paper. In practice, in the literature, the complexity of the underlying phenomenon has led to the study of a thick (macroscale) liquid film in the absence of solidification. Here, we investigate evaporation mechanics of a thin (microscale) liquid film of perovskite solution. We demonstrate flow fields within the film and study the reason behind the formation of such flow patterns within a thin liquid film and attribute such flows to surface-tension-induced Marangoni flows and rule out the possibility of the presence of buoyancy-induced Rayleigh-Benard convection. We show that perovskite deposition starts at the film contact lines and propagates toward the film center, creating two regions of the perovskite film with distinct characteristics. Our thermography results (temperature mapping) of the film surface show agreement between the temperature map and the flow patterns on the film surface. Moreover, we show that imposing ultrasonic vibration on an evaporating liquid film results in a more uniformly distributed flow pattern across the film due to micromixing enhancement achieved by ultrasonic vibrations.

Comparative studies on air, water and nanofluids based Rayleigh–Benard natural convection using lattice Boltzmann method: CFD and exergy analysis

The present study incorporates laminar natural convection and entropy generation in Rayleigh–Benard (R–B) convection with air, water and alumina–water nanofluid as working fluids. The fluid flow and energy equations are solved using D2Q9 and D2Q5 LBM models, respectively. The effects of Rayleigh numbers (Ra = 5 × 103, 104, 105) and volume fractions ( $$\emptyset$$ = 0 to 0.08) of nanoparticles on heat transfer and irreversibility are investigated. Results show that the heat transfer evaluated based on Nusselt number is enhanced due to addition of nanoparticles in the base fluid. The maximum enhancement in Nusselt number is found to be 13.93% at Ra = 105 with 8% of nanoparticle in base fluid. The various irreversibilities considered in this study are thermal, fluid flow and total irreversibility, where fluid flow and total irreversibilities in the study depend on irreversibility ratio. The irreversibility ratios taken into account are 10–2, 10–3, 10–4 and 10–5. One facet of study shows the deviation in onset of critical Rayleigh number for air is 1.58%. The other facet indicates dimensionless heat transfer, fluid flow and total irreversibility decrease with the increase in volume fraction of nanoparticles in the base fluid. The analyzed results of irreversibilities are presented in normalized form. In addition, dimensionless entropy generation maps and Bejan number contours are also plotted.

The large-scale footprint in small-scale Rayleigh–Bénard turbulence

Abstract

A model for universal spatial variations of temperature fluctuations in turbulent Rayleigh-Bénard convection

We propose a theoretical model for spatial variations of the temperature variance σ2(z,r) (z is the distance from the sample bottom and r the radial coordinate) in turbulent Rayleigh-Bénard convection (RBC). Adapting the “attached-eddy” model of shear flow to the plumes of RBC, we derived an equation for σ2 which is based on the universal scaling of the normalized RBC temperature spectra. This equation includes both logarithmic and power-law dependences on z/λth, where λth is the thermal boundary layer thickness. The equation parameters depend on r and the Prandtl number Pr, but have only an extremely weak dependence on the Rayleigh number Ra Thus our model provides a near-universal equation for the temperature variance profile in turbulent RBC.

Stepwise transitions in spin-up of rotating Rayleigh–Bénard convection

Abstract

The -dependence of the critical roughness height in two-dimensional turbulent Rayleigh–Bénard convection

Abstract

Model of the thermal processes during controlled deposition of island nanofilms

A model of the influence of thermal factors in the Rayleigh-Benard convection approximation is proposed. The system of equations of the model was solved using the Chorin projection scheme taking into account the correction based on Ri – Chou interpolation.

Low dimensional models of dynamo action in rotating magnetoconvection

Abstract Two low-dimensional models for nonlinear dynamo action in Rayleigh-Benard convection in presence of rigid body rotation about vertical axis are constructed for metallic fluids with finite magnetic Prandtl number ( P m ) and small (or zero) thermal Prandtl number ( P r ). Dynamo effect is seen for P m ≥ 0.75 with P r = 0.025 and Taylor number, 0 T a ≤ 1000 . The value of reduced Rayleigh number at the dynamo onset ( r d ) varies with P m , P r and T a . The value of r d decreases with increase in P m and T a separately, if the other two parameters are kept fixed to some small values. However r d increases slightly if P r is increased, the value being minimum for P r = 0 . When 0.75 ≤ P m 4 , dynamo onset appears as intermittent chaotic burst. The intermittent burst changes to continuous chaos with rise in P m . The probability mass of the height of peak in average magnetic energy follows power law when P m is small. For 4 ≤ P m 6 and 600 ≤ T a 1000 dynamo effect starts at the onset of convection as finite oscillation. This effect continues in a small window of r and then disappears. The dynamo action again appears as quasi-periodic wave or chaotic wave for further increase in r .

ОБ УПРАВЛЕНИИ ИНТЕНСИВНОСТЬЮ КОНВЕКЦИИ ХИМИЧЕСКИ РЕАГИРУЮЩЕГО ГАЗА

A linear analysis of the stability of Rayleigh-Benard static regime convection is performed in a chemically reacting equilibrium hydrogen-oxygen gas mixture with added chemically inert microparticles of aluminum oxide. It is shown in the Boussinesq approach that a suitable choice of temperature can intensify convection and adding chemically inert microparticles can suppress convective motion. It is established that the isobaric convection regime is realized when the height of the region is less than the critical value, and when the height of the region exceeds this critical value, the convection regime is superadiabatic. During convection in the superadiabatic regime, its adiabatic suppression is observed. Within the framework of the isobaric convection regime, the limits of applicability of the Boussinesq approach are determined.

The effect of Prandtl number on turbulent sheared thermal convection

Abstract

Nanofluid and nanoemulsion absorbents for the enhancement of CO2 absorption performance

Research on CO 2 capture using nanofluids and nanoemulsions has been actively conducted in recent decades, and numerous studies have achieved improvements in the CO 2 absorption performance of nanofluids and nanoemulsions. In this study, to analyze the enhancement of CO 2 absorption performance achieved by nanofluids and nanoemulsions, experiments are conducted for visualizing the diffusion of CO 2 inside absorbents using the shadowgraph method. The nanofluid absorbents are prepared using SiO 2 solid particles, and the nanoemulsion absorbents are prepared using dodecane as the dispersed phase. The effects of concentration are analyzed through the absorption experiments performed for each concentration. The diffusion coefficient is calculated experimentally and theoretically based on the results of the visualization tests. The absorption of CO 2 begins at the absorbent’s upper interface when the CO 2 gas is stationary or moving. The highest absorption is observed at 0.05 vol% of nanoparticles. The absorption performances of the nanofluid and nanoemulsion absorbents are improved by 23.05% and 26.80%, respectively. As CO 2 is absorbed, the absorbent density changes and the Rayleigh convection becomes prominent, resulting in a plume-like flow. The plume formation and growth stages are subdivided into four stages, and both absorbents are compared. The visualization test results indicate that the hydrodynamic effect is a dominant factor in improving nanofluids’ mass transfer and nanoemulsions. Hydrodynamic effect model. • Enhancement mechanism of CO 2 absorption performance by nanoabsorbents is firstly clarified. • As CO 2 absorption starts, the Rayleigh convection becomes prominent, resulting in a plume-like flow. • The absorption performances of the nanofluid and nanoemulsion absorbents are improved by 23.05% and 26.80%, respectively. • Hydrodynamic-effect model plays an important role in improving the absorption performance.

On the well-posedness for the 2D micropolar Rayleigh–Bénard convection problem

The article is devoted to the study of Cauchy problem to the Rayleigh–Benard convection model for the micropolar fluid in two dimensions. We first prove the unique local solvability of smooth solution to the system when the system has only velocity dissipation, and then establish a criterion for the breakdown of smooth solutions imposed only the maximum norm of the gradient of scalar temperature field. Finally, we show the global regularity of the system with zero angular viscosity.

Optimal Decay Estimates for 2D Boussinesq Equations with Partial Dissipation

Buoyancy-driven fluids such as many atmospheric and oceanic flows and the Rayleigh–Benard convection are modeled by the Boussinesq systems. By rigorously estimating the large-time behavior of solutions to a special Boussinesq system, this paper reveals a fascinating phenomenon on buoyancy-driven fluids that the temperature can actually stabilize the fluids. The Boussinesq system concerned here governs the motion of perturbations near the hydrostatic equilibrium. When the buoyancy forcing is not present, the velocity of the fluid obeys the 2D Navier–Stokes equation with only vertical dissipation and its Sobolev norm could potentially grow even though its precise large-time behavior remains open. This paper shows that the temperature through the coupling and interaction tames and regularizes the fluids, and causes the velocity (measured in Sobolev norms) to decay in time. Optimal decay rates are obtained.

Thermal boundary layer structure in low-Prandtl-number turbulent convection

Abstract

Experimental Observations of Lagrangian Coherent Structures and Fluid Transports in Perturbed Rayleigh-Bénard Convection

Abstract In this paper, we make experimental observations of the perturbed Rayleigh-Benard convection to detect the Lagrangian coherent structures (LCSs), which correspond to the invariant manifolds of time-dependent systems and also to investigate the associated fluid transports by numerically integrating the two-dimensional velocity field obtained by Particle Image Velocimetry (PIV). We show the chaotic fluid transports that appear in the system, though some particles are transported almost periodically with time period T or 3T, where T = 17.3 s is the period of the perturbation. We finally propose a novel perturbed Hamiltonian system that enables to recover qualitatively global behaviors observed in experimental results and we also explore the periodic fluid transport that appear in the system.

Experimental Observations of Perturbed Rayleigh-Benard Convection and Analysis of Chaotic Fluid Transport

Experimental Observations of Perturbed Rayleigh-Benard Convection and Analysis of Chaotic Fluid Transport

Darcy–Carreau Model and Nonlinear Natural Convection for Pseudoplastic and Dilatant Fluids in Porous Media

The linear and weakly nonlinear stability analyses are carried out to study instabilities in Darcy–Benard convection for non-Newtonian inelastic fluids. The rheological model considered here is the Darcy–Carreau model, which is an extension to porous media of Carreau rheological model usually used in clear fluid media. The linear stability approach showed that the critical Rayleigh number and wave number corresponding to the onset of convection are the same as for Newtonian fluids. By employing weakly nonlinear theory, we derived a cubic Landau equation that describes the temporal evolution of the amplitude of convection rolls in the unstable regime. It is found that the bifurcation from the conduction state to convection rolls is always supercritical for dilatant fluids. For pseudoplastic fluids, however, the interplay between the macroscale properties of the porous media and the rheological characteristics of the fluid determines the supercritical or subcritical nature of the bifurcation. In the parameter range where the bifurcation is supercritical, we determined and discussed the combined effects of the fluid properties and the porous medium characteristics on the amplitude of convection rolls and the corresponding average heat transfer for both pseudoplastic and dilatant fluids. Remarkably, we found that the curves describing these effects collapse onto the universal curve for Newtonian fluids, provided the average apparent viscosity is used to define Rayleigh number.