Tatiana Gambaryan-Roisman

Drop impact, spreading, splashing, and penetration into electrospun nanofiber mats.

Experiments were conducted to study peculiarities of drop impact onto electrospun polymer nanofiber mats. The nanofiber cross-sectional diameters were of the order of several hundred nanometers, the pore sizes in the mats of about several micrometers, and the mat thicknesses of the order of 200 microm. Polyacrylonitrile (PAN), a polymer which is partially wettable by water, was used to electrospin nanofiber mats. The experiments revealed that drop impact onto nanotextured surfaces of nanofiber mats produce spreading similar to that on the impermeable surfaces. However, at the end of the spreading stage, the contact line is pinned and drop receding is prevented. At higher impact velocities, prompt splashing events with formation of tiny drops were observed. It was shown that the splash parameter K(d) = We(1/2) Re(1/4) (with We and Re being the Weber and Reynolds numbers, respectively) previously used to characterize the experiments with drop impact onto smooth impermeable dry substrates can be also used to describe the onset of splash on substrates coated by nanofiber mats. However its threshold value K(ds) (in particular, corresponding to the minimal impact velocity leading to generation of secondary droplets) for the nanotextured surfaces is higher than that for dry flat substrates. In addition, water penetration and spreading inside wettable nanofiber mats after drop impact was elucidated and quantified. The hydrodynamics of drop impact onto nanofiber mats is important for understanding effective spray cooling through nanofiber mats, recently introduced by the same group of authors.

Marangoni-induced deformation and rupture of a liquid film on a heated microstructured wall

Thermocapillary instability is one of the primary causes of a spontaneous rupture of thin films on heated walls. The film rupture may lead to an appearance of uncontrolled dry patches that significantly deteriorate the heat and mass transfer. In the present paper the thermocapillarity-induced film flow on a microstructured wall is studied in the framework of the long-wave theory. When the wall is heated or cooled, the solution predicts a film deformation caused by thermocapillarity. The linear stability analysis shows that the films on heated microstructured walls are less stable to the long-wave disturbances compared to the films on flat walls. The time-dependent film evolution is simulated and the effect of the wall structure on the film thinning and rupture is analyzed. It is shown that the wall topography exerts a profound effect on the dynamics of the film deformation and rupture, as well as on the size and the location of the dry patches. The full-scale direct volume-of-fluid simulations are used to verity the predictions of the long-wave theory. Good agreement is found for the small ratios between the groove depth and period. The agreement is further improved by including the effect of the convection heat transfer into the long-wave model.

Static and dynamic contact angles of evaporating liquids on heated surfaces

We studied both static and dynamic values of the apparent contact angle for gravity-driven flow of a volatile liquid down a heated inclined plane. The apparent contact line is modeled as the transition region between the macroscopic film and ultra-thin adsorbed film dominated by disjoining pressure effects. Four commonly used disjoining pressure models are investigated. The static contact angle is shown to increase with heater temperature, in qualitative agreement with experimental observations. The angle is less sensitive to the details of the disjoining pressure curves than in the isothermal regime. A generalization of the classical Frumkin-Derjaguin theory is proposed to explain this observation. The dynamic contact angle follows the Tanners law remarkably well over a range of evaporation conditions. However, deviations from the predictions based on the Tanners law are found when interface shape changes rapidly in response to rapid changes of the heater temperature. The Marangoni stresses are shown to result in increase of the values of apparent contact angles.

Marangoni convection and heat transfer in thin liquid films on heated walls with topography: Experiments and numerical study

Thermocapillary-induced motion in thin liquid films on a heated horizontal wall with parallel grooves on its upper surface is studied experimentally and numerically. The results of velocity and temperature measurements are reported. A numerical model for a liquid film on a structured wall is developed. The full incompressible Navier–Stokes equations and the energy equation are integrated by a finite difference algorithm, whereas the mobile gas-liquid interface is tracked by the volume-of-fluid method. The numerical model is verified by comparison with the experimental data showing a good agreement. The model is used to study flow patterns and film rupture caused by the thermocapillary forces. Heat transfer in the liquid is also investigated. In particular, it is found that the thermocapillary convection enhances heat transfer in liquid, though the effect depends on the shape of the wall surface.

Effect of Longitudinal Minigrooves on Flow Stability and Wave Characteristics of Falling Liquid Films

Falling liquid films are used in many industrial apparatuses. In many cases the film flow along a wall with topography is considered advantageous for intensification of the heat and mass transport. One of the promising types of the wall topography for the heat transfer intensification is comprised of minigrooves aligned along the main flow direction. The wall topography affects the development of wavy patterns on the liquid-gas interface. Linear stability analysis of the falling film flow based on the long-wave theory predicts that longitudinal grooves lead to the decrease in the disturbance growth rate and therefore stabilize the film. The linear stability analysis also predicts that the frequency of the fastest growing disturbance mode and the wave propagation velocity decrease on a wall with longitudinal minigrooves in comparison with a smooth wall. In the present work the effect of the longitudinal minigrooves on the falling film flow is studied experimentally. We use the shadow method and the confocal chromatic sensoring technique to study the wavy structure of falling films on smooth walls and on walls with longitudinal minigrooves. The measured film thickness profiles are used to quantify the effect of the wall topography on wave characteristics. The experimental results confirm the theoretical predictions.

Dynamics of the cavity and the surface film for impingements of single drops on liquid films of various thicknesses.

This paper presents experimental and numerical investigations of single drop impacts onto liquid films of finite thickness. The dynamics of the drop impingement on liquid surface films, the shape of the cavity, the surface film dynamics and the residual film thickness are investigated and analysed. The shape of the penetrating cavity within the surface film is observed experimentally using a high-speed video system. Additionally, the thickness of the liquid film between the expanding, receding and retracting cavity and the solid wall is monitored in time using an optical sensor based on chromatic confocal imaging. The effects of various influencing parameters, such as the drop impingement velocity, liquid properties (surface tension and viscosity) and the initial liquid film thickness, on the time evolution of the cavity and film dynamics are investigated. Complementary to the experiments direct numerical simulations of the drop impacts and cavity expansion are performed using a volume-of-fluid free-surface capturing model in the framework of the finite volume numerical method. The numerical predictions of the film thickness dynamics agree well with the experiments for most phases of the impingement process. Finally, a scaling analysis of the residual film thickness between the cavity and the solid wall is performed for various impingement parameters.

Gravity effect on spray impact and spray cooling

An experimental study of the film produced by the spray impact on a heated target and of the spray cooling has been performed in normal gravity and in microgravity conditions during parabolic flights. A convex shape of the target allowed visualization of the film evolution and determination of the film characteristics using the image processing. The effects of the spray parameters and of the gravity level on heat transfer have been investigated. It has been found that the spray cooling efficiency depends on the water flow rate in a non-monotonous way. A range of spray parameters at which the effect of gravity level on heat transfer is significant has been determined. It has been found that the spray cooling is less effective in microgravity conditions in comparison with normal gravity and hypergravity.

Modulation of Marangoni convection in liquid films.

Non-isothermal liquid films are subject to short- and long-wave modes of Marangoni instability. The short-wave instability leads to the development of convection cells, whereas long-wave instability is one of the primary causes of the film rupture. In this paper different methods for modulation of Marangoni convection and Marangoni-induced interface deformation in non-isotherm liquid films are reviewed. These methods include modification of substrates through topographical features, using substrates with non-uniform thermal properties, non-uniform radiative heating of the liquid-gas interface and non-uniform heating of substrates. All these approaches aim at promotion of temperature gradients along the liquid-gas interface, which leads to emergence of thermocapillary stresses, to the development of vortices and to the interface deformation. Finally, Marangoni convection in a liquid film supported by a substrate with periodic temperature distribution is modeled by solution of steady state creeping flow equations. This approach is justified for low Reynolds numbers and for Marangoni convection in liquids with high Prandtl numbers. The model predicts interaction between Marangoni convection induced by non-uniform wall heating and the Marangoni short-wave instability.

Influence of gas emission on heat transfer in porous ceramics

It is known that thermal diffusivity, a, of several types of porous ceramic and refractory materials decreases with decreasing gas pressure. However, a of several ceramics (e.g., magnesite refractories with porosity about 25%) measured in vacuum by the monotonous heating exceeds the comparable data registered at atmospheric pressure. A similar effect was found for thermal diffusivity of several insulating materials. However, for some porous ceramics this phenomenon is absent or less prominent. It had been known that several heterogeneous physico-chemical processes take place on pore surfaces of ceramic materials. These processes include heterogeneous chemical reactions accompanied by emission of gaseous products. It had been conjectured that these processes affect thermophysical properties of ceramic materials, especially during fast heating or cooling. In this paper we substantiate this conjecture. Namely, we develop a quantitative model for the apparent thermal diffusivity, as measured by the nonstationary monotonous heating method. It takes into account the emission and adsorption of the gas on the opposite pore sides along the temperature gradient, the diffusive gas motion inside the pores and its removal from the pores due to the material gas permeability. The effect of these processes is shown to produce an additional heat flux inside the pore or crack and, hence, to increase the measured thermal diffusivity. In the presence of the passive gas, the rates of gas emission and its transport within the pore are significantly reduced, which leads to diminution of the effect of gas emission--adsorption on the heat transfer across the pore. Consequently, we show that this leads to a situation (observed in experiment) where thermal diffusivity of a material measured at high temperature in vacuum may exceed the comparable property at atmospheric pressure. When the reaction terminates due to the full conversion of the available solid reactant, the additional heat flow due to the gas emission and adsorption terminates, and the measured thermal diffusivity decreases. The rates of gas removal and of chemical conversion depend on the amount of reactant available within the specimen and on the heating rate. We show that as a result of this, the measured thermophysical properties depend on the material thermal history and heating parameters, and, hence, cannot be regarded as true material properties.

A numerical model for the thermocapillary flow and heat transfer in a thin liquid film on a microstructured wall

Purpose – This paper aims to study thermocapillarity‐induced flow of thin liquid films covering heated horizontal walls with 2D topography.Design/methodology/approach – A numerical model based on the 2D solution of heat and fluid flow within the liquid film, the gas above the film and the structured wall is developed. The full Navier‐Stokes equations are solved and coupled with the energy equation by a finite difference algorithm. The movable gas‐liquid interface is tracked by means of the volume‐of‐fluid method. The model is validated by comparison with theoretical and experimental data showing a good agreement.Findings – It is demonstrated that convective motion within a film on a structured wall exists at any nonzero Marangoni number. The motion is caused by surface tension gradients induced by temperature differences at the gas‐liquid interface due to the spatial structure of the heated wall. These simulations predict that the maximal flow velocity is practically independent from the film thickness, a...