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International Heat Transfer Conference 16

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

FLOW PATTERN AND INSTABILITIES IN VOLATILE SESSILE DROPLETS UNDER EVAPORATION

Sergey Semenov
Aix Marseille Universite, CNRS, IUSTI, Marseille, France; Aix Marseille Universite, CNRS, MADIREL, Marseille, France

Florian Carle
Laboratoire IUSTI - CNRS UMR 7343 Departement de Mecanique Energetique 5, rue Enrico Fermi - Technopole de Chateau Gombert 13453 Marseille Cedex 13 France; Dept. Mechanical Engineering and Materials Science, Yale University, Connecticut 06511, USA

Marc Medale
Aix Marseille Universite, CNRS, IUSTI, Marseille, France

David Brutin
Aix Marseille University, CNRS, IUSTI UMR 7343, 13013, Marseille, France; Institut Universitaire de France, 75231 Paris, France

DOI: 10.1615/IHTC16.cms.023515
pages 2031-2041


SCHLÜSSELWÖRTER: Boiling and evaporation, Convection, Droplets

Abstrakt

Internal flow pattern and related instabilities that occur in the course of droplets evaporation are not fully understood yet. We report results on an ethanol drop evaporating onto a heated substrate under microgravity conditions and with pinned contact line. They have been obtained from both experiments and 3D unsteady computations in order to determine what kind of instabilities develop. In one-sided model, appropriate boundary conditions for heat and mass transfer equations are required at the droplet surface. Such boundary conditions are obtained in present work based on a derived semi-empirical theoretical formula for the total droplet's evaporation rate, and on a two-parametric non-isothermal approximation of the local evaporation flux. The main purpose of these boundary conditions is to be applied in 3D one-sided numerical models in order to save a lot of computational time and resources by solving equations only in the droplet domain. Two parameters, needed for the non-isothermal approximation of the local evaporation flux, are obtained by fitting computational results of a 2D two-sided numerical model. To the best authors' knowledge, this is the first study which combines theoretical, experimental and computational approaches in convective evaporation of sessile droplets. The one-sided model demonstrates quantitative agreement with experiments and confirms that experimentally observed instabilities are driven by thermo-capillary stress and not by the gas convection. Post-processed infrared images drawn from computations led us to conclude that the experimentally observed thermo-convective instabilities, which look very similar to hydrothermal waves (HTWs) in the infrared spectrum, are actually nothing else than unsteady Benard-Marangoni instabilities.

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