Y. Bailly

Modification of the ultrasound induced activity by the presence of an electrode in a sonoreactor working at two low frequencies (20 and 40 kHz). Part I: Active zone visualization by laser tomography

Sonoelectrochemical experiments differ from sonochemical ones by the introduction of electrodes in the sonicated reaction vessel. The aim of the study is to characterize the changes of the ultrasonic activity induced by the presence of an electrode located in front of the transducer. The scope of our investigations concerns two low frequencies vibration modes: 20 and 40 kHz. For this purpose, two laser visualization techniques have been used. The first part of the study, described in the present paper (part I), deals with the laser tomography technique which provides an accurate picture of the reactor actives zones, related to numerous cavitation events. For each frequency, two parameters were studied: the electrical power supplied to the transducer and the electrode transducer distance. At both frequencies, without electrode, we can observe distinct zones corresponding to cavitation production and stationary waves establishment. When increasing the input power, bubble clouds tend to form a unique cloud near the transducer. In presence of the electrode, bubble cavitation clouds are largely influenced by the obstacle. The second part of the paper (part II) will describe the Particle Image Velocimetry (P.I.V.) technique which allows to measure the velocity vector field in the fluid portion between the horn and the electrode.

Characterization of HIFU transducers designed for sonochemistry application: Cavitation distribution and quantification

Acoustic field distribution was determined in HIFU sonoreactors as well as localization of cavitation activity by crossing different techniques: modeling, hydrophone measurements, laser tomography and SCL measurements. Particular care was taken with quantification of this last technique by pixels or photon counting. Cavitation bubbles generated by HIFU are mainly located on the outer layer of the propagation cone in the post-focal zone. Greatest acoustic activity is not located at the geometrical focal, but corresponds to a high concentration of bubbles zone. On the contrary, the main sonochemical activity shifts slightly toward the transducer, whereas quenching of inertial cavitation is observed directly at the focal. Finally, SCL thresholds have been determined.

Characterization of the activity of ultrasound emitted in a perpendicular liquid flow using Particle Image Velocimetry (PIV) and electrochemical mass transfer measurements

The present work is dedicated to the study of the interactions between a liquid circulation and a perpendicular acoustic wave propagation. A specific experimental setup was designed to study one transducer operating at 20 kHz, with the help of electrochemical mass transfer measurements combined with Particle Image Velocimetry (PIV) determination. Electrodes were located on the wall opposite to the acoustic emission. Experiments were performed for various Reynolds numbers: from 0 to 21700 (different liquid flow rates and viscosities). Both PIV and electrochemical measurements methods were found to be relevant, and had delivered complementary information. Even if PIV showed that the plume due to streaming was highly deflected by the additional flow, electrochemical measurements showed that there was still an activity, higher than in silent conditions, on the wall facing the transducer. Thus the ultrasound contribution remained noticeable on the surface opposite to the transducer even for a disturbed hydrodynamic environment due to the presence of a liquid circulation perpendicular to the wave propagation.

Flow Velocity Investigation by Particle Image Velocimetry in Supersonic Air Ejector

Ejectors are devices usually made of two convergent/divergent coaxial nozzles which are used to convert pressure energy into kinetic energy. These devices involve very complex phenomena which strongly affect their performance. Flow visualization methods are often used to provide precious information as for the nature of the flow within the ejectors and the comprehension of the physical phenomena encountered. Unfortunately, the visualization methods used successfully until now in these systems are primarily qualitative techniques. Some attempts at quantitative flow visualization by Particle Image Velocimetry have been carried out in quite specific applications but with mitigated results due to the complicated conditions of investigation. The objective of this paper is to present an attempt at PIV measurements in a supersonic air ejector. Several ejector operating conditions and flow seeding methods are taken into consideration. The velocity fields obtained are compared with CFD simulations of the flow and allow the rigorous validation of numerical models.

Évolution des techniques de micromesures thermiques au travers de quelques applications

Abstract The thermoelectric sensors made at present in our laboratory have junction dimensions from a few micrometers to 0.5 μm. Two techniques are used: welding of ultra-thin wires and vacuum deposition (PVD). Two applications related to the first one will be presented: the detection of periodic field of temperature for photothermal microscopy and acoustic resonator characterization. In the latter, the comparison of simultaneous pressure and temperature will lead us to improve the knowledge of thermoacoustic phenomena. The deposition technique allows us to create either simple thermoelectric couples, multiple as thermopiles; or heat (and light) flow sensors. Our work is presented in that field, with the development of a laser light sensor designed to image its light profile for large emission bandwidth and power densities. Finally, the description followed by an example of application is shown in the case of the heat flow measurement between a laminar gaseous flow and a wall. By comparison, we show that the use of such sensors could be worthwhile to replace the usual temperature sensors in the estimation of the heat transfer coefficient at the gas-wall boundary.

Optimization of the injection with a twin-fluid atomizer for suspension plasma spray process using three non-intrusive diagnostic tools

Suspension injection is a crucial point for suspension plasma spray, an emerging coating process. It must satisfy several requirements such as stability, reproducibility, high yield of suspension particles penetrating the plasma core. Indeed, the particles should follow a proper trajectory inside the plasma in order to be well treated and accelerated. Thus, the quality of the injection has a direct effect on the coating microstructure, therefore on the properties. This study focuses on the three-step characterization of a twin-fluid atomizer, without and within the plasma. While varying gas or suspension flow rates, the spray was firstly observed thanks to a shadowgraphy system. This system uses a short-pulsed diode laser in a back illumination configuration and a robust image processing software, allowing the detection of suspension droplets, even within a high emissive plasma. Secondly, droplet size measurements were performed via laser diffraction. Thirdly, droplet velocities were measured using a particle image velocimetry system. The combination of the results obtained by these techniques helped us to understand how gas and suspension flow rates impact on droplets size and velocity distributions and finally how one could select the best injection conditions. The injector characterization was completed by coating manufacturing and microstructure analysis.Graphical abstract

Effect of suspension characteristics on in-flight particle properties and coating microstructures achieved by suspension plasma spray

This paper focuses on the influence of suspension properties on the manufacturing of coatings by suspension plasma spraying (SPS). For this purpose, alumina suspensions were formulated with two different liquid phases: water and ethanol. Suspensions were atomized with a twin-fluid nozzle and injected in an atmospheric plasma jet. Suspension injection was optimized thanks to shadowgraphy observations and drop size distribution measurements performed by laser diffraction. In-flight particle velocities were evaluated by particle image velocimetry. In addition, splats were collected on glass substrates, with the same conditions as the ones used during the spray process. Scanning electron microscopy (SEM) and profilometry analyses were then performed to observe the splat morphology and thus to get information on plasma / suspension interactions, such as particle agglomeration. Finally, coatings were manufactured, characterized by SEM and compared to each other.