Thesis defense of Miss Michelle NASSAR

Electrical devices, especially embedded ones, are becoming more and more compact and require higher power consumption. In fact, all these electrical components produce heat that must be evacuated to maintain an efficient operation. In this context, the electrofluidodynamics team at the Pprime Institute works on developing electrohydrodynamic (EHD) systems that have shown numerous advantages for both spatial and terrestrial applications. In order to achieve an optimized performance, EHD systems use ecofriendly dielectric liquids, such as Hydrofluoroethers (HFE), whose electrical behavior is not well defined yet. The objective of the current work is to characterize the variations of the electrical properties of HFE 7000 and 7100 in function of temperature. Several methods are used to analyze the electrical behavior of these liquids at low voltage. The first method is in accordance with the guidelines of the IEC 61620 standard and the second one is based on dielectric spectroscopy.

The study is then extended to the thermal behavior under high voltage. The analysis of the currentvoltage characteristics makes it possible to highlight the three typical zones of the electrical behavior: ohmic, quasiohmic and injection, and thus to define the limits of the conduction and injection (or iondrag) regimes. The study at high voltage also addresses, in compliance with the IEC 60156 standard, the problem of the dielectric strength of the two HFEs at different temperatures in both gas and liquid phases. Finally, an innovative preliminary study on the Kerr effect in HFE7100 is conducted. It shows

that this electrooptical effect can be used to study the development of charged layers at the HFE/electrode interfaces. In conclusion, the results obtained in this work contribute to the understanding of the electrical behavior of HFEs. This is necessary to improve and optimize the performance of EHD systems working with these liquids.

Thesis defense of ANH VO TRAN, on Wednesday June 23, 2021 at 4.30 p.m. in the Communication Room

Behavior of scratched hydrodynamic journal bearings: A finite volume numerical analysis

Résumé
Les paliers lisses hydrodynamiques sont des éléments de machine importants utilisés pour supporter les machines tournantes à hautes vitesses telles que les turbines et les compresseurs, en raison
de leur durabilité et de leur capacité de charge élevée. Sous des conditions sévères de fonctionnement, ils peuvent s’endommager par une perte de matière qui peut prendre différentes formes, entraînant une
chute de la capacité de charge et donc une réduction significative de l’épaisseur du film d’huile. Les rayures sont un type majeur de dommage de surface dans les paliers ; elles peuvent apparaître aussi bien sur la surface du coussinet que sur la surface de l’arbre, induisant de fortes discontinuités géométriques.
Peu de travaux existent dans la littérature mais tous montrent que la présence de rayures a une forte influence sur les performances des paliers. Pour simuler plus précisément et étudier le comportement
des paliers lisses rayés, il est donc nécessaire de disposer d’un modèle précis. L’analyse théorique est basée sur la résolution simultanée des équations de Reynolds, d’énergie et de transfert de chaleur dans les solides et dans les fluides. Le modèle thermohydrodynamique développé au moyen de la méthode des volumes finis est écrit en langage FORTRAN. Afin de valider et de compléter la modélisation élaborée, les résultats de la simulation ont été comparés à des données expérimentales de la littérature. Les résultats ont montré de bons accords pour la pression, les profils de température et les paramètres de performance globale du palier. En outre, des études numériques ont également été réalisées pour une large gamme de configurations avec différents nombres de rayures (une, deux et plusieurs), des profondeurs de rayures, des positions de rayures et différents fonctionnements. Les résultats obtenus ont permis d’approfondir les connaissances sur le comportement des paliers lisses rayés.

Mots-clés : Endommagements de paliers, rayures, lubrification hydrodynamique, paliers, simulations thermohydrodynamiques.

Abstract
Hydrodynamic plain bearings are important machine components used to support high-speed rotating machinery such as turbines and compressors, due to their durability and high load carrying
capacity. Under severe operating conditions, they can be damaged by material loss that can take many forms, resulting in a drop in load capacity and thus a significant reduction in oil film thickness. Scratches are a major type of surface damage in bearings; they can appear on both the bearing surface and the shaft surface, inducing strong geometric discontinuities.
Few works exist in the literature but all show that the presence of scratches has a strong influence on the performance of bearings. In order to simulate more precisely and study the behavior of
scratched plain bearings, it is therefore necessary to have an accurate model. The theoretical analysis is based on the simultaneous resolution of Reynolds, energy and heat transfer equations in solids and fluids. The thermohydrodynamic model developed using the finite volume method is written in FORTRAN language. In order to validate and complete the developed model, the simulation results were compared
with experimental data from the literature. The results showed good agreement for pressure, temperature profiles and overall bearing performance parameters. In addition, numerical studies were also performed
for a wide range of configurations with different numbers of scratches (one, two and many), scratch depths, scratch positions and different operation. The results obtained have provided further insight into the behavior of scratched plain bearings.

Keywords: bearing damages, surface scratches, hydrodynamic lubrication, journal bearings, thermohydrodynamic simulations.