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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...

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.

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.

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.