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New experimental technique, based on IR thermography, is proposed to measure the surface pressure for dilute monomolecular films of surfactants on a liquid surface. The surfactant molecules are dis...

The application of high-resolution IR thermography for qualitative and quantitative determination of impurity concentrations in aqueous solutions is considered. Especially efficient turned out to be comparative analysis and analysis of contaminations: a film arises on the water surface in the presence of impurities, which can be seen well on an IR imager because of the difference in the surface temperatures and structures of convective vortices. It should be especially noted that the method makes it possible to determine low concentrations of impurities of neutral molecules, a case where no changes in the solution conductivity can be recorded using standard conductometric measurements. The problems related to the film simulation and influence of low-concentration impurities on the surface and volume properties of solution are considered.

The influence of Ar gas additives on ≪black silicon≫ formation is shown in this work. The way to achieve the conical shape of Si texture using low Ar dilution is demonstrated. Also, a possibility of silicon nanowire width reduction keeping a high density of array is shown. No damage to the Si structure caused by Ar plasma was detected. The introduction of Ar into the plasma also does not affect electrical properties. The lifetime value after cryogenic etching with 5 sccm Ar flow remains at the same level of 0.7 ms. The resulting black silicon has a low total reflectance of 1 ± 0.5% in the range of 450-1000 nm in the overall 100 mm Si wafer surface.

Natural convection driven by evaporation of heavier-than-air fluids from the surface of the heated horizontal and vertical flat plates is studied both experimentally and numerically. The combined buoyancy effects of temperature and vapor density are shown to result in either downward or upward flow depending on the plate temperature and evaporating substance volatility and molar mass. For some substances (e.g., butanol) flow reversals are observed: solutal buoyancy is dominant for low and high temperatures of the heated plate leading to downward flow, whereas at moderate temperatures thermal buoyancy prevails resulting in upward convection. Refractive index fields are measured using Background Oriented Schlieren (BOS). Similarity between the temperature and vapor density fields associated with heat and mass transfer analogy is used to obtain temperature and vapor density fields from experimental data. The validity of this approach is analyzed. Good agreement between experimental temperature and vapor density distributions and numerical simulations is obtained for ethanol vapor, which has Lewis number about 1.8. For butanol vapor, which has Lewis number about 2.5, similarity is violated and temperature and vapor density contributions to refractive index cannot be separated. This is also true for validity of simplified modeling using total expansion coefficient. Similarity is also shown to be violated for hot liquid evaporation from a tank with conductive walls. In this case boundary conditions are dissimilar and upward convective flow of warm clean air near the outer surface of the tank walls is observed, which entrains the heavy vapor.

Horizontal convection development in liquids with free surface nonuniformly heated by infrared radiation is investigated. Experimental measurements of velocity and temperature fields are performed by Particle Image Velocimetry (PIV) and Background Oriented Schlieren (BOS). Absence of Marangoni convection in distilled water due to presence of surface film leads to significant difference between the results obtained in distilled water and in ethanol. Strong Marangoni flow in ethanol results in quick dispersion of heat over the surface layer and acceleration of convective flow. In distilled water heat propagation along the surface is inhibited, because only weak secondary flow associated with 3D effects is observed at the surface. Experimental results are compared to numerical simulations with different boundary conditions for horizontal velocity at the free surface: no-slip condition for distilled water, representing stagnant surface film, and tangential-stress condition, taking into account Marangoni effect, for ethanol. Good agreement is obtained, demonstrating the importance of prescribing appropriate boundary condition, which corresponds to surface behavior of the considered liquid.

It is shown that convective structures that form upon heating a planar liquid layer from below are determined not only by standard similarity parameters, such as the Prandtl, Rayleigh, and Marangoni numbers, but also by the presence or absence of an elastic film on the surface of the liquid, which occurs because of impurities and stabilizes the surface. The level of impurities contained in distilled water is enough to prevent Marangoni convection, and only additional purification (deionization) of water allows one to induce the thermocapillary effect. Using the method of infrared surface thermography, the mean size of thermal structures that emerge on the surface in different liquids at different temperatures and layer thicknesses is determined. A convection theory that takes the impurities in the linear and nonlinear approximations into account is examined and good compliance of the theoretical calculations with the experimental data obtained in the present work is demonstrated.

Dynamics of the surface layer in different liquids is examined by means of infrared thermography of the surface and simultaneous velocity fields measurements using surface and infrared Particle Image Velocimetry. This technique allows measurements and comparison of two velocity fields—at the surface and at small depth about 50–200 μm. In distilled water the velocity fields at the surface and at small depth exhibit significant dissimilarity. The flow field below the surface is essentially 3D, whereas the surface flow is characterized by vanishing 2D divergence of velocity, indicating predominantly planar motion. In contrast, in ethanol–butanol mixture two velocity fields are well correlated, both corresponding to 3D flow with continuous surface renewal. Thermal patterns, observed at the surface, and the flow field structure in different liquids are associated with different boundary conditions for velocity at the surface. Water surface is seldom renewed, which inhibits heat and mass exchange between the liquid and atmosphere. However, absence of vertical advection also enables organisms to live within the surface layer, to stand and walk on the free surface. This is illustrated by the difficulties a water strider faces on the surface of ultrapure water, which exhibits Marangoni convection.

The onset of convection in an initially homogeneous liquid layer cooled from above is investigated by means of a non-normal approach. This method is based on the analysis of the temporal evolution of small perturbations. The most amplified perturbation and the moment, when convection can be observed, are determined. Experimental measurements are performed for a layer of distilled water cooled by evaporation. Instantaneous surface temperature fields are obtained using infrared thermography. The effect of the surface mobility on the size and onset time of convective cells is examined. The comparison of experimental results with numerical simulations for different boundary conditions reveals the presence of a film of adsorbed contaminants on the surface of distilled water, which corresponds to the no-slip condition. In the presence of the film, boundary conditions for cooling from above and heating from below become the same, and the obtained results agree with the experimental data available for a liquid layer with a heated bottom. As the layer depth increases, the onset time and the average horizontal wavenumber of convective cells become independent from the layer depth. The condition for the transition to a deep water approximation is determined by the surface heat flux and thermophysical parameters of the liquid.

Accuracy of two infrared thermography techniques for air flow temperature measurements is evaluated for a benchmark test case of natural convection over a horizontal heated plate. The first technique, which can be called infrared thermography of optically thin media, involves observation of a distant screen with known temperature through the investigated air flow. It allows obtaining 2D temperature fields averaged along line-of-sight, like quantitative schlieren or interferometry techniques, using only software supplied with infrared camera. The second technique employs solid targets installed within the air flow. Temperature distributions, obtained with both methods, are compared to each other, to measurements using background oriented schlieren technique, to thermocouple measurements and to results of 2D and 3D numerical simulations. It is shown that solid target apparent temperature is affected by radiative heat exchange with environment, in this particular case with the heated plate. Simultane...

Measurements of liquid surface topography are performed using new modification of Background Oriented Schlieren technique, which is based on the apparent distortion of a background pattern image reflected by the deformed surface. The optical setup comprises a camera and a backlit background, both installed above the liquid surface. The camera images the background pattern reflection. The apparent displacement of the pattern features with respect to the reference image, taken for the unperturbed liquid surface, is proportional to the local slope of the surface. This enables one to obtain instantaneous surface relief map by solving Poisson equation. Compared to the popular free-surface synthetic Schlieren technique, our approach allows measurements of the surface topography over deep liquid layers and for opaque liquids or multiphase flows. It is also more sensitive, which enables reliable measurements of perturbations as small as 1 μ m . Experiments are performed for propagation of the circular surface waves, for convection driven by heating of a horizontal wire installed below the liquid surface and for Rayleigh-Benard-Marangoni convection in liquid heated from below. Attenuation coefficient of the gravity-capillary waves is determined for wave frequencies about 10 Hz. Comparison of the results, obtained in glycerol, distilled water and silicone oil, shows the remarkable difference between the surface deformation patterns, observed in liquids, which exhibit Marangoni convection, and liquids, where it is suppressed by the surface film. In distilled water and glycerol elevated surface is always observed in hot regions, whereas in silicone oil the effect of convection on surface relief depends on the competition between the buoyancy-driven and surface-tension-driven flows. free surface background oriented schlieren surface waves convection profilometry.

Experimental measurements of temperature field within liquid layer below the liquid–gas interface are performed simultaneously by means of Background Oriented Schlieren (BOS) and Infrared Thermal Imaging of the surface (IRTI). Completely different structures of the surface layer are observed in evaporating water and ethanol at room temperature. Ethanol has moving surface, whereas water surface is similar to stagnant elastic film. Transition from one type of the surface layer structure to another is observed in hot water or cold ethanol. The properties of the surface layer are examined with convective plume, coming from a heated wire and impinging the free surface from below. Heat propagation velocity along the surface is shown to be extremely sensitive to tangential velocity boundary condition at the interface. The stagnant state of liquid near the surface in water results in slow heat transfer. Direct comparison with numerical simulations shows that water flow corresponds to no-slip condition at the interface. In contrast, ethanol with moving surface exhibits fast heat propagation, described by boundary condition which takes into account Marangoni effect. Heat propagation velocity is over- or underestimated by factor of 5–15 if inappropriate boundary condition is used. Surface relief of the liquid layer heated with a wire is also simulated and discussed.

Forced convection cooling of photovoltaic modules mounted on the surface of moving solar car is considered. It is shown that the shape of the car should be optimized not only to reduce aerodynamic drag but also to enhance heat removal and to increase power generated by photovoltaic modules. The module is modeled by local energy balance equation providing boundary condition for equations of aerodynamics. Experimental data on surface temperature are obtained for a flat module and compared to engineering approximate relation and numerical simulations. Simplified approach is proposed based on solving energy equation separately from equations of momentum and continuity. Unlike analytical approximations, it provides accurate results if power generation depends on surface temperature. The results of numerical simulations for two different shapes of the solar car demonstrate that photovoltaic modules placed in flow separation regions should be treated as separate blocks, otherwise they can limit the performance of the whole system. Linear analysis of the power loss of photovoltaic array due to partial overheating or shading is also performed.

Constructing models of global heat exchange between the ocean and atmosphere requires information on boundary conditions at water–air interface. Experimental and theoretical studies of near-surface structures both in laboratory and in situ have been a part of geophysics for decades. Nowadays usage of modern CFD methods can be complemented by state-of-art experimental techniques providing visualization of small-scale phenomena. Temperature distributions near the liquid–gas interface for various evaporation regimes are measured in laboratory by Background Oriented Schlieren (BOS) and IR thermal imaging of the surface. The results, obtained by these two methods, are shown to coincide with accuracy about 0.1 K. Thanks to simplicity of experimental realization, both methods can be This is the e-book version of the article, published in the Russian Journal of Earth Sciences (doi:10.2205/2012ES000517). It is generated from the original source file using LaTeX’s epub.cls class. used also in situ. Thermal imaging yields not only the surface temperature field, but also the velocity gradient near the surface. It is shown to be much larger than vorticity of the bulk convective vortices. Possible separate numerical modeling of hydrodynamic processes in liquid and gas making use of thermal imaging data is discussed.

The parameters of the positive column of a glow discharge in neon are calculated with allowance for the induced hydrodynamic motion. It is shown that natural convection in the pressure range of ∼0.1 atm significantly affects the profiles of the parameters of the positive column and its current-voltage characteristic. The convection arising at large deposited energies improves heat removal, due to which the temperature in the central region of the discharge becomes lower than that calculated from the heat conduction equation. As a result, the current-voltage characteristic is shifted. With allowance for convection, the current-voltage characteristic changes at currents much lower than the critical current at which a transition into the constricted state is observed. This change is uniquely related to the Rayleigh number in the discharge. Thus, a simplified analysis of thermal conduction and diffusion, even with detailed account of kinetic processes occurring in the positive column, does not allow one to accurately calculate the current-voltage characteristic and other discharge parameters at intermediate gas pressures.

The problem of conservation of magnitudes is considered for a vortex in a relaxing compressible medium. Heat release due to the relaxation of a nonequilibrium medium leads to the propagation of compression waves, which remove material. Traditional integrals of motion are inapplicable in this case. We pro-pose the concept of integral quantity, which is conserved with an arbitrary degree of accuracy despite the fact that waves cross the boundary of the integration domain. Based on this concept, a broad class of conservation laws is derived for axisymmetric disturbances of columnar vortices, including conservation of the circulation and total angular momentum of the vortex. For nonaxisymmetric disturbances, it is shown that the total angular momentum and properly defined energy integral are conserved. Numerical verification of the derived conservation laws is performed and the perspectives for using these conservation laws in numerical simulations are discussed.

The validity of using simplified models of compressible fluids for calculations of unstationary convection flows inside closed cavities is tested for the problem of hot gas cooling in a rectangular cavity with cold walls. The Oberbeck-Boussinesq approximation and other simplified models are shown to yield incorrect values of pressure, density, and temperature if average pressure change is not negligible. Modification of the numerical method is proposed, which allows one to describe the temporal dependence of pressure correctly without loss of computational efficiency. The results obtained by using a modified Oberbeck-Boussinesq approximation and the complete equations for a fully compressible fluid are compared.

The problem of the evolution of a single vortex in a nonequilibrium medium with a temperature-or density-dependent energy release was solved. In a medium that is stable against small perturbations, the vortex passes to a new stationary state, and no vortex breakdown occurs. For a temperature-dependent energy release, analytical solutions that define changes in the characteristics of the vortex were obtained for high and low relaxation times. The solutions were compared with the results of numerical simulation by the Godunov method.

The paper contains a brief review of the modern status of nanotechnology. We present a general description of nanotechnology research, viz., the financing, growth of the number of publications, and the contribution of Russian scientists. The methods of nanomaterial manufacturing and diagnostics, properties of nanomaterials, and their applications are considered separately. The main attention is devoted to carbon nanomaterials: carbon nanotubes and fullerenes. We stress the close connection between nanotechnology and molecular physics.

A general analysis of the stability of the flame with respect to small linear two-dimensional hydrodynamic perturbations was performed in the high activation energy approximation with consideration given to perturbations of both hydrodynamic and thermal parameters.

The problem of a thermal explosion in the cylinder has been solved taking into account the natural convection. Both the classical solution of the Frank-Kamenetskii model and a realistic model of a thermal emission which takes into consideration reverse endothermic processes have been considered. It is shown that in this case the solution exists for any value of the energy influx into system. However, under certain conditions a sharp transition into the area of high temperatures still occurs which corresponds to the thermal explosion. It is shown that convection leads to significant change of the parameters of the thermal explosion. The results of this consideration are of interest for the theory of lasers and discharges where the Rayleigh numbers are rather large and the influence of convection appears to be essential.

The stability of an inhomogeneous subsonic vibrationally-nonequilibrium flow is examined in the linear approximation. It is shown that the nonequilibrium flow stability is reduced with increase in the initial flow velocity and the energy pumping zone width. This is attributed to the effect of the feedback due to acoustic disturbances propagating counter to the flow. Calculations are carried out for different pumping models.

Natural convection of gas ( Pr = 0.7) between two horizontal coaxial cylinders with uniform internal heat generation is numerically investigated. Such a problem plays an important role in the analysis of lasers in which a similar circuit of a heat-conducting path is used. It has been found that the behavior of the system critically depends on three parameters including the inverse relative gap width σ (=diameter of the inner cylinder/gap width), the Rayleigh number Ra and the modified Rayleigh number Ra T which describes heat generation. In the case Ra T = 0 our results coincide with the known ones. It has been established that in such a system there exist two types of fluid flow for low Rayleigh numbers with different vortex structure. Optimization of the corresponding coaxial laser system has been analyzed.

The problem of vibrationally nonequilibrium gas flow past a cylinder with the formation of a Karman street is considered. The effects of relaxation on the flow parameters, namely, on vortex damping and deformation, is analyzed on the basis of a numerical simulation. A theoretical explanation of these effects is proposed.

The general type of boundary conditions in heat exchange problems, including heat exchange in laser systems, is discussed. The appearance of convection in a plane layer of a nonequilibrium gas with the volume energy release and its temperature dependence is briefly considered. Convection in a cylindrical system and in a system of coaxial cylinders with a cooled central part is considered in detail. Such systems simulate real laser devices. It is shown that the maximum temperature in the cylindrical system decreases due to convection, whereas the maximum temperature in the system of coaxial cylinders increases, i.e., the analysis of heat removal in a laser system reveals a very important role of convection.

A correct formulation of the problem of the structure and stability of relaxation waves is given for arbitrary Lewis (Le) and Prandtl (Pr) numbers. The high activation-energy approximation in which the reaction to heating zone length ratio is assumed to be small is analyzed in detail.

The convective instability of a flat layer of a nonequilibrium gas is investigated under different conditions of energy pumping. The conditions under which convective, thermal, and acoustic instabilities arise are compared, and the regions of thermodynamic parameters at which a specific type of instability is dominant are determined. The effect of boundary conditions and of vibrational thermal conductivity (diffusion of vibrational quanta) on the convective instability of a system in the region of thermal explosion is revealed.

A mechanism for the flash ignition of a carbon nanotube (CNT) is proposed. At the first stage (during flash ignition) oxidation of an ultradispersed impurity of metals in the CNT occurs. At the second stage, heating by chemical reactions occurs. The final stage is the initiation of the combustion wave. Qualitative descriptions of processes at each stage are given.

Within the framework of linear nonequilibrium thermodynamics, a generalization of the mass action law to the case of two-temperature reacting gases is performed. Concrete calculations are performed for the dissociation of diatomic molecules and ionization of atoms under conditions when the translational temperature of heavy particles differs from the vibrational temperature of molecules and from the temperature of electrons.

We consider the stability problem for a plane gas layer whose nonequilibrium state is maintained by pumping energy into the molecular vibrational degrees of freedom and by heat removal through the walls. Two approaches to the study of gas stability are discussed, which consider the evolution of small hydrodynamic perturbations and thermal explosion. We have studied in detail the Rayleigh–Benard problem of convection instability and the Semenov–Frank-Kamenetskii problem of thermal explosion, and generalised them to the case of a nonequilibrium gas. Some unsolved problems of the physical hydrodynamics of a nonequilibrium gas have been outlined as well.

We review the propagation of optical and hydrodynamic perturbations in a gas having nonequilibrium internal degrees of freedom. We discuss the nonequilibrium processes that lead to variation of the refractive index and self-focusing via the effect of kinetic cooling and nonequilibrium vibrational excitation of molecules. We analyze the propagation of small and nonlinear perturbations, as well as shock waves, in nonequilibrium gases.

Temperature fields in evaporating liquids are measured by simultaneous use of Background Oriented Schlieren (BOS) technique for the side view and IR thermal imaging for the surface distribution. Good agree- ment between the two methods is obtained with typical measurement error less than 0.1 K. Two configurations of surface layer are observed: thermocapillary convection state with moving liquid surface and small thermal cells, associated with Marangoni convection, and "cool skin" with negligible velocity at the surface, larger cells and dramatic increase of velocity within 0.1 mm layer beneath the surface. These configurations are shown to be formed in various liquids (water with various degrees of purification, ethanol, butanol, decane, kerosene, glyc- erine) depending rather on initial conditions and ambient parameters than on the liquid. Water, which has been considered as the liquid without observable Marangoni convection, actually can exhibit both kinds of behavior during the same experimental run. Evaporation is also studied by means of numerical simulations. Separate prob- lems in air and liquid are considered, with thermal imaging data of surface temperature making the separation possible. It is shown that evaporation rate can be predicted by numerical simulation of the air side with ap- propriate boundary conditions. Comparison is made with known empirical correlations for Sherwood-Rayleigh relationship. Numerical simulations of water-side problem reveal the issue of velocity boundary conditions at the free surface, determining the structure of surface layer. Flow field similar to observed in the experiments is obtained with special boundary conditions of third kind, presenting a combination of no-slip and surface tension boundary conditions.

The propagation of non-linear hydrodynamic perturbations in a vibrationally non-equilibrium gas has been considered in the two limiting cases of low and high frequencies. It is shown that non-equilibrium accelerates shock wave formation. Conditions have been determined under which relaxational amplification is compensated by viscous dissipation resulting in the formation of a stationary regime.