Thermal Convection in an Annular Two-Layer System Under Combined Action of Buoyancy and Thermocapillary Forces

A set of two-dimensional numerical simulations of thermal convection of two immiscible liquid layers with a non-deformable interface was carried out in an annular cavity with the outer heated cylinder and the inner cooled cylinder using the finite-volume method. Bottom and top surfaces were bounded by two rigid and heat-insulated walls. The results show that the conversion of the thermal convection depends on the Marangoni number, the aspect ratio and the curvature of the annular cavity. For a sufficiently small Marangoni number, the flow is steady and there is one cell in each fluid layer. When the Marangoni number exceeds the critical value, the convective flow becomes instable and generates an unsteady multi-cell structure. In the case of the closed annular cavity, the value of the critical Marangoni number is higher than that in the infinite layer, and decreases with the increase of the aspect ratio.     

Two-Dimensional Numerical Simulation of Thermocapillary Convection in Annular Two-Layer System

In order to understand the characteristics of thermocapillary convection in the annular two-layer systems, a set of two-dimensional numerical simulations was carried out using the finite-volume method for the system of gallium arsenide and boron oxide with the aspect ratio 4, in a differentially heated annular pool with the outer heated cylinder and the inner cooled cylinder. The results show that liquid encapsulation can effectively suppress thermocapillary convection in the melt. When the buoyancy is considered, thermocapillary convection is reduced in liquid encapsulation, but enhanced in the melt.     

MHD Effect on Interfacial Deformation of Thermocapillary Convection in Two-Layer System

Steady thermocapillary convection with deformable interface in a two-layer system is simulated by the second-order projection method combined with the level set method, in which the three-stage Runge–Kutta technique and second-order semi-implicit Crank–Nicholson technique are employed to temporally update the convective and diffusion terms, respectively. The level set approach is employed to implicitly capture the interface. The continuum surface force tension model is used to simulate the Marangoni effect. Simulations are conducted for both fixed angle and fixed points at the contact between the interface and the end walls. The numerical results show that, the interface bulges out near the hot wall and bulges in near the cold wall, due to the Marangoni effect. With Marangoni number increasing, the deformability of interface increases. The contact condition of interface with the end walls is important for the prediction of thermocapillary convection characteristics, and the contact points fixed condition is more close to real condition.     

Numerical Simulation of Steady Thermocapillary Convection in a Two-Layer System Using Level Set Method

Steady thermocapillary convection with deformable interface in a two-layer system is simulated by the second-order projection method combined with the level set method, in which the three-stage Runge–Kutta technique and second-order semi-implicit Crank–Nicholson technique are employed to temporally update the convective and diffusion terms, respectively. The level set approach is employed to implicitly capture the interface. The continuum surface force tension model is used to simulate the Marangoni effect. Simulations are conducted for both fixed angle and fixed points at the contact between the interface and the end walls. The numerical results show that, the interface bulges out near the hot wall and bulges in near the cold wall, due to the Marangoni effect. With Marangoni number increasing, the deformability of interface increases. The contact condition of interface with the end walls is important for the prediction of thermocapillary convection characteristics, and the contact points fixed condition is more close to real condition.     

Long-wave Marangoni Convection in Two-layer Films Under the Action of Gravity Modulation

The influence of the gravity on the long-wave Marangoni patterns in two-layer films is considered. The numerical analysis is carried out in the lubrication approximation. Periodic boundary conditions have been applied on the boundaries of the computational region. The development of instabilities is investigated by means of nonlinear simulations. The cases of a constant gravity and a time-periodic gravity modulation are considered. In the case of a constant gravity, non-stationary three-dimensional and two-dimensional structures have been found. It is shown that the periodic modulations of the gravity force lead to the development of new three-dimensional spatially-periodic patterns. Outside the region of parameters, corresponding to three-dimensional structures, dynamical regime of two-dimensional traveling waves have been observed.     

Premixed Flames Under Microgravity and Normal Gravity Conditions

Premixed conical CH₄-air flames were studied experimentally and numerically under normal straight, reversed gravity conditions and microgravity. Low-gravity experiments were performed in Drop tower. Classical Bunsen-type burner was used to find out features of gravity influence on the combustion processes. Mixture equivalence ratio was varied from 0.8 to 1.3. Wide range of flow velocity allows to study both laminar and weakly turbulized flames. High-speed flame chemoluminescence video-recording was used as diagnostic. The investigations were performed at atmospheric pressure. As results normalized flame height, laminar flame speed were measured, also features of flame instabilities were shown. Low- and high-frequency flame-instabilities (oscillations) have a various nature as velocity fluctuations, preferential diffusion instability, hydrodynamic and Rayleigh-Taylor ones etc., that was explored and demonstrated.     

Nonlinear Convective Flows in a Laterally Heated Two-Layer System with a Temperature-Dependent Heat Release/Consumption at the Interface

Nonlinear convective flows developed under the joint action of buoyant and thermocapillary effects in a laterally heated two-layer system filling the closed cavity, have been investigated. The influence of a temperature-dependent interfacial heat release/consumption on nonlinear steady and oscillatory regimes, has been studied. It is shown that sufficiently strong temperature dependence of interfacial heat sinks and heat sources can change the sequence of bifurcations and lead to the development of specific oscillatory regimes in the system.     

Asymptotic Solution of Thermocapillary Convection in a Differentially Heated Thin Annular Two-Layer Pool

The steady laminar two-dimensional thermocapillary convection of two immiscible liquid layers in a thin annular pool with one free surface, one liquid/liquid interface subjected to a radial temperature gradient was investigated using asymptotical analysis. The pool is heated from the inner cylindrical wall and cooled at the outer wall. Bottom and top surfaces are adiabatic. The asymptotic solution is obtained in the core region in the limit as the aspect ratio, which is defined as the ratio of the lower layer thickness to the gap width, trends to zero. The numerical experiments are also carried out to compare with the asymptotic solution of the steady two-dimensional thermocapillary convection. The results indicate that the expressions of velocity and temperature fields in the core region are valid in the limit of the small aspect ratio.     

Scaling Equation of State for a Nonequilibrium Solution Under Gravity Close to Phase Interface

Experimental investigations of equilibration kinetics for a methanol-hexane binary solution under gravity have been carried out at temperatures T below the critical consolute temperature, T<T _c , by using a refractometry technique. As a result of the experiment, both equilibrium n _z (z,t _e ) and nonequilibrium n _z (z,t) height dependences of the refractive index gradient n _z at different times t after the beginning of thermal equilibration have been obtained. Analysis of the data shows that the relaxation properties of the system at different fixed heights are determined not by a single relaxation time (z), but by a spectrum of relaxation times _i (z _j ,t). On the basis of the experimental data, the height dependence of the relaxation times has been analyzed for the studied solution in the course of its transition to equilibrium. The average relaxation time has been shown to decrease when nearing the phase interface (z=0). The relaxation time (z,t) at a certain height z has been shown to also decrease when the system approaches an equilibrium state. A dynamic nonequilibrium equation of state has been proposed on the basis of the fluctuation theory of phase transitions for a substance under gravity close to the phase interface of a binary solution. It is based on the assumption that for small solution concentrations, (c–c _c )/c _c <<1, every nonequilibrium height distribution n _z (z,t) corresponds to an equilibrium distribution n _z (z,T) at a certain temperature T=T–T _c . Here, c _c is the critical concentration of the solution.     

Shape of Diffusive Interface Under Periodic Excitations at Different Gravity Levels

This work discusses the role of gravity on the shape of interfacial waves between miscible liquids under horizontal vibrations. A big difference in the shape of an interfacial pattern has recently been observed in low-gravity experiments when compared to earth-based one. The evolution of an interfacial pattern from zero to normal gravity is discussed in the context of non-linear simulations in a confined system. The development of vibration-induced frozen waves with gravity is characterized by three distinct regimes that are associated with the wave height and the angle at the vertices of saw-tooth shape of the interface.     

Presynaptic Release of Glutamate by Heteroexchange Under Altered Gravity Conditions

The study focused on the release of glutamate by reversal of Na^?+?-dependent glutamate transporters that was investigated in cortical synaptosomes isolated from Wistar rats subjected to centrifuge-induced hypergravity of 10×g for 1 h. Transportable competitive inhibitor of glutamate transporters—DL-threo-beta-hydroxyaspartate (DL-THA) induced the release of L-[¹⁴C]glutamate via heteroexchange that consisted of 22.5 ± 1.7% of total label in normal gravity and 23.7 ± 1.7% in altered gravity conditions. Inhibition of the ability of synaptic vesicles to accumulate the neuromediator by preliminary treatment of synaptosomes with the protonophore (FCCP) augmented the release of cytosolic L-[¹⁴C]glutamate evoked by DL-THA from 44.0 ± 2.0% to 52.0 ± 2.3% of total label for G-loaded animals as compared to controls. Thus, combined application of DL-THA and FCCP revealed the increase in transporter-mediated release of glutamate that might be evidence for hypoxic injury of neurons.     

Experimental Investigation of Nucleate Boiling on a Thermal Capacitive Heater Under Variable Gravity Conditions

In the present work the behavior of single vapor bubbles of FC-72, generated on a thermal capacitive heater element, has been investigated during microgravity. A newly developed heater design allows temperature measurements by highspeed infrared thermography on the backside of the heater surface at a distance of approx. 800 nm from the fluid/heater-interface. The employed heater was manufactured by Physical Vapor Deposition (PVD) of a chromium based layer for better emissivity (Slomski et al., Mater Sci Technol 41:161?C165, 2010) and a pure chromium heating layer supplying the energy required for bubble generation and sustainment by electrical heating. The thermal diffusivity of the employed Calcium Flouride (CaF) heater substrate is comparable to the thermal diffusivity of stainless steel, which makes this heater design very close to technical applications. The acquired transient temperature fields of the heater surface allow numerical determination of the local heat flux from the heater surface to the fluid. A local temperature drop and high heat fluxes have been observed in the vicinity of the 3-phase contact line. This effect has already been reported by former publications for thin stainless steel foil heaters (Stephan and Hammer, Int J Heat Mass Transfer 30:119?C125, 1994; Wagner et al., Int J Heat Mass Transfer 42:875?C883, 2006) and is also confirmed for heaters with significantly higher thermal capacities.     

Amplitude Equations for Large-Scale Marangoni Convection in a Liquid Layer with Insoluble Surfactant on Deformable Free Surface

The numerical simulation has been carried out to investigate the motion of a droplet initially near a wall under gravity and confirm the existence of the wall repulsive force on the droplet. The numerical model is developed based on a mass conservation level set algorithm to capture the surface deformation of the droplet. The results show that the wall repulsive force on the droplet initially near the wall plays an important role in the droplet falling process, and the viscosity force affects the oscillatory trajectory of the falling droplet. In addition, the mutual repulsive effect between two droplets is also studied by settling symmetrically two droplets, and the oscillatory mechanism of droplet motion is discussed as well.     

Non-Lyapunov Type Stability in a Model of the Dewetted Bridgman Crystal Growth Under Zero Gravity Conditions

A numerical model has been developed based on a mass conservation level set algorithm considering a wall adhesion to predict the surface motion for a single and double dam break problems. The simulation for the single dam break shows that the wall adhesion force can be introduced through contact angle by modifying the level set function on the boundary in the level set method, and the magnitude of the contact angle has an effect on the moving location of the leading front of water–gas system. Moreover, the simulation for the double dam break presents clearly the processes of coalescence and break up of the free surface.     

Thermomagnetic Convection as a Tool for Heat and Mass Transfer Control in Nanosize Materials Under Microgravity Conditions

Magnetic colloids are relatively new man-made nanomaterials whose magnetic susceptibility is several orders of magnitude larger than that of natural substances. Experiments conducted in magnetic fluids show that strengthening or weakening of thermal convection in colloids is dictated by a competition between the gravitational and magnetic mechanisms as well as by the effect of the fluid density stratification due to gravitational sedimentation of magnetic particles and their aggregates. Therefore experiments in microgravity conditions are required to eliminate gravitational sedimentation. This will enable an accurate investigation of convection in magnetic fluids and unambiguous study of the interaction of a magnetic field with a magnetopolarized medium. Such experiments are also needed to perform an accurate measurement of fluid’s transport coefficients.     

Comparison of the Effect of Horizontal Vibrations on Interfacial Waves in a Two-Layer System of Inviscid Liquids to Effective Gravity Inversion

We study the waves at the interface between two thin horizontal layers of immiscible liquids subject to high-frequency tangential vibrations. Nonlinear governing equations are derived for the cases of two- and three-dimensional flows and arbitrary ratio of layer thicknesses. The derivation is performed within the framework of the long-wavelength approximation, which is relevant as the linear instability of a thin-layers system is long-wavelength. The dynamics of equations is integrable and the equations themselves can be compared to the Boussinesq equation for the gravity waves in shallow water, which allows one to compare the action of the vibrational field to the action of the gravity and its possible effective inversion.     

The Influence of the Gravity Force on Longwave Marangoni Patterns in Two-Layer Films

An Euler–Euler two-fluid model based on the second-order-moment closure approach and the granular kinetic theory of dense gas-particle flows was presented. Anisotropy of gas-solid two-phase stress and the interaction between two-phase stresses are fully considered by two-phase Reynolds stress model and the transport equation of two-phase stress correlation. Under the microgravity space environments, hydrodynamic characters and particle dispersion behaviors of dense gas-particle turbulence flows are numerically simulated. Simulation results of particle concentration and particle velocity are in good agreement with measurement data under earth gravity environment. Decreased gravity can decrease the particle dispersion and can weaken the particle–particle collision as well as it is in favor of producing isotropic flow structures. Moreover, axial–axial fluctuation velocity correlation of gas and particle in earth gravity is approximately 3.0 times greater than those of microgravity and it is smaller than axial particle velocity fluctuation due to larger particle inertia and the larger particle turbulence diffusions.