Laurent Girardot

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.

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

Exposure chambers for studying the partitioning of atmospheric PAHs in environmental compartments: validation and calibration using experimental and computational approaches.

The environmental partitioning of atmospheric polycyclic aromatic hydrocarbons (PAHs) conditions their entry into food chains and subsequent risks for human health. The need for new experimental exposure devices for elucidating the mechanisms governing ecosystemic PAH transfer motivated the elaboration of an original small-scale exposure chamber (EC). A dual approach pairing experimentation and computational fluid dynamics (CFD) was selected to provide comprehensive validation of this EC as a tool to study the transfer and biological effects of atmospheric PAH pollution in microsystems. Soil samples and passive air samplers (PASs) were exposed to atmospheric pollution by phenanthrene (PHE), a gaseous PAH, for 2 weeks in examples of the EC being tested, set up under different conditions. Dynamic concentrations of atmospheric PHE and its uptake by PASs were simulated with CFD, results showing homogeneous distribution and constant atmospheric PHE concentrations inside the ECs. This work provides insight into the setting of given concentrations and pollution levels when using such ECs. The combination of experimentation and CFD is a successful ECs calibration method that should be developed with other semivolatile organic pollutants, including those that tend to partition in the aerosol phase.

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.