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Influence de tensioactifs en atomisation assistée

Abstract : The mechanisms involved in assisted atomization have been the subject of detailed analyses. Yet, the role of surfactants on these processes has rarely been addressed, so that the impact of surfactants on the evolution of the liquid jet and on the production of drops remains poorly understood. In order to better understand their role, the atomization of a liquid jet (water and surfactant) driven by a coaxial gas stream (air) is experimentally studied, over wide ranges of liquid and gas injection velocities. Three surfactants (SDS, C12TAB, C16TAB) with various properties have been selected: used at different concentrations, they allow to explore a very wide range of characteristic migration times to the interface, evolving from 10-3 to 1 second. Attempts to characterize their rheological properties are also proposed.The injector used consists of a liquid tube of 5 mm diameter, and a coaxial gas tube with a 5 mm section. A decontamination procedure of the liquid circuit after surfactant injection has been implemented, ensuring the reproducibility of the measurements.We first characterized by image analysis the structure of the liquid jet before its breakup, as well as the frequencies of the instabilities leading to the droplet formation.Both SDS and C12TAB candidates significantly increase the breakup length of the liquid jet. These molecules exhibit adsorption dynamics faster than the characteristic times of liquid structure formation, allowing to significantly lowering the surface tension at the tip of the liquid jet prior to its breakup. Conversely, the surfactant with slow adsorption dynamics (C16TAB) does not change the structure of the liquid jet. A model for the breakup length, based on the dynamic surface tension, shows good agreement with the measurements.C12TAB induces additional effects: the reduction of the oscillation amplitude of the jet (up to 40%), and of its breakup frequency (up to 50%). Finally, addition of SDS and C12TAB leads to changes in the mode of liquid jet breakup at low gas injection velocities: from a dominant fiber/ligament type jet breakup, a majority of breakups with membrane formation is observed due to the reduction of the surface tension.Finally, the frequencies of shear and flapping instabilities of the liquid jet are not affected by surfactants, as these mechanisms do not depend on the surface tension.The characteristics of the droplets resulting from liquid jet assisted atomization were studied using three techniques: laser diffraction particle size analysis, fast imaging and Doppler optical probe. The Doppler optical probe is a new sensor: its use on sprays required developments and optimizations of the signal processing routines. This sensor have then been tested by comparing the integral of radial liquid flow profiles to the injected flow rate: the observed deviations are less than 20, demonstrating the reliability of the instrument.Finally, the spray was characterized in the presence of surfactants. Laser diffraction, which allows to measure a distribution of droplet sizes on a spray diameter. The latter remains constant beyond twice the average breakup length of the liquid jet, with or without surfactant. Then the volume size distributions show that surfactants having an impact on the structure of the liquid jet also induce a higher presence of small drops than with pure water. Finally, the optical Doppler probe is used to measure sizes, velocities and fluxes of drops at the center of the spray.We thereby demonstrate that the parameter governing the influence of a surfactant on the atomization process is the adsorption time, which depends on the nature of the surfactant and its concentration. If this time is low compared to the typical break-up times of the liquid jet, its structure is affected, as well as the downstream spray resulting from its atomisation.
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Submitted on : Monday, September 12, 2022 - 12:43:54 PM
Last modification on : Wednesday, September 14, 2022 - 3:11:10 PM

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  • HAL Id : tel-03775141, version 1

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Mathieu Alonzo. Influence de tensioactifs en atomisation assistée. Physique [physics]. Université Grenoble Alpes [2020-..], 2022. Français. ⟨NNT : 2022GRALI043⟩. ⟨tel-03775141⟩

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