PREFACE: DROPS, BUBBLES, AND THIN FILMS, SPECIAL ISSUE HONORING PROFESSOR OLEG KABOV

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Dynamics of volatile liquid droplets on heated surfaces: theory versus experiment

We consider the evaporation of volatile liquid droplets deposited on a heated substrate in a pure saturated vapour environment. A mathematical model is developed that incorporates the effects of surface tension, evaporation, thermocapillarity, gravity, disjoining pressure, as well as unsteady heat conduction in the solid substrate. The apparent contact line is treated mathematically as a transition region between the macroscopic droplet shape and the adsorbed film of liquid on the heated substrate. Theoretical parametric studies are conducted to clarify the effects of thermocapillarity and wetting properties on the droplet dynamics. An experimental study is conducted in a closed container with de-ionized water droplets on a stainless steel foil heated by an electric current. The interface shapes are recorded together with the temperature profiles under the droplets, measured using thermochromic liquid crystals. Experiment and theory are in very good agreement as long as the conditions of applicability of our lubrication-type mathematical model are satisfied

Pressure drop and void fraction during flow boiling in rectangular minichannels in weightlessness

International audienceAn experimental investigation has been carried out on flow boiling in a minichannel to explain heat transfer enhancement observed experimentally under the conditions of weightlessness. The analysis is based on the local void fraction and frictional pressure loss measurements. Frictional pressure loss in two-phase flows in minichannels under terrestrial gravity is described by several well-known correlations. In weightlessness, however, few experimental results are available on the void fraction and the frictional pressure loss. The experiments for this study have been performed at constant heat flux supplied to the minichannel with inlet liquid mass velocity ranging between 30 and 248 kg s(-1) m(-2). The influence of hypergravity (gravity level of 1.8g) and microgravity (gravity level of +/- 0.05g) on the frictional pressure loss is observed and explained using the flow patterns visualization and experimental void fraction determination through image treatment. Pressure drops for two-phase flow in microgravity are found to be significantly higher than for single-phase flow under similar conditions; possible explanations for the difference are discussed. The experimental thermal measurements have been previously analyzed using inverse techniques which led to evaluation of the local heat transfer coefficient. The heat transfer enhancement observed during weightlessness is explained in the present work by investigating the differences in flow patterns and void fraction under different levels of gravity. (C) 2012 Elsevier Ltd. All rights reserved

Gravity effect on the locally heated liquid film driven by gas flow in an inclined minichannel

Thin nonisothermal liquid film flowing under action of gravity force and co-current gas flow, which create the tangential force on the gas-liquid interface, in an inclined minichannel is considered. 3D time dependant mathematical model has been developed. Effects of surface tension, temperature dependent viscosity and thermocapillarity are taken into account. The effect of gravity as well as the effect of gas speed has been studied to define main features of the film dynamics. In calculations vector of gravitational acceleration is oriented along the flow and is equal to the normal Earth gravity and Lunar gravity. Our investigations have shown that gravity has a significant effect on the film deformations. At the lower gravity conditions 3D liquid film pattern changes noticeably in spanwise direction and a middle stream between two main lateral waves appears. Also speed of film deformation is higher and stabilization time is longer. Variation of gas Reynolds number from 543 to 2000 does not change noticeably film pattern at normal gravity. At lower gravity conditions increasing of gas Reynolds number decreases significantly the width of the thermocapillary deformations and leads to a film stabilization.En ligne: http://www.springerlink.com/content/q281764097634023/info:eu-repo/semantics/publishe