Déformation de la surface libre en convection de Bénard-Marangoni

Free-surface deformations accompanying natural convection in a liquid layer heated uniformly from below are studied. Surface profiles are measured experimentally using a Poggendorf method, a Michelson method and the classical interferometric technique. The profile is concave when the liquid depth is small and convex when the layer is deep. For intermediate depths the profile is hybrid and little pronounced with a hollow in a hump. It is shown that the relief increases with the distance from the convective threshold. The relief is concave when surface tension forces predominate and it is convex when buoyancy forces are the most important forces.On étudie les déformations de la surface libre accompagnant la convection naturelle d'une couche de liquide chauffée uniformément par dessous. Les profils de la surface sont mesurés en utilisant une méthode de Poggendorf, une méthode de Michelson et une méthode d'interférences du « coin d'air ». Le profil est concave quand l'épaisseur de liquide est mince, convexe quand la couche est profonde. Pour des épaisseurs intermédiaires le profil est hybride et peu marqué avec un creux dans une bosse. On montre que le relief s'accentue avec l'écart au seuil convectif. On montre que le relief concave correspond à une prédominance des forces de tension superficielle, et le relief convexe à celle des forces de gravitation

Structural disorder in Benard-Marangoni convection

The influence of the vessel shape, the vessel size and heating on convective patterns in the Benard-Marangoni (B-M) instability has been analysed. These influences can be quantitatively studied by means of some theoretical tools, often used to describe melting in two dimensions. Experimental results show that a hexagonal geometry minimizes the disorder in the hexagonal pattern of B-M convection. This disorder is maximum when the aspect ratio is about 53. The disorder increases when the distance to the threshold increases.Une étude systématique de l'influence de l'écart au seuil, de la forme et de la taille du récipient sur la structure de l'instabilité de Benard-Marangoni (B-M) a été entreprise. Au moyen de certains outils théoriques, souvent utilisés pour décrire la fusion dans les milieux à deux dimensions, ces influences sont étudiées quantitativement. Les résultats expérimentaux montrent que le désordre est minimum lorsque le récipient est de forme hexagonale et que dans ce cas il passe par un maximum lorsque le rapport d'aspect est d'environ 53. Le désordre est une fonction croissante de l'écart au seuil

Benchmark study on fuel bundle degradation in the phebus FPT3 test using the severe accident codes ATHLET-CD, ICARE2, and MELCOR

This paper presents the results of pretest calculations of the Phebus fission product release experiment FPT3. The test scenario with the appropriate initial and boundary conditions was provided by the Institut de Radioprotection et de Sûreté Nucléaire. For the analyses, three severe accident codes were used: ATHLET-CD, ICARE2, and MELCOR. The calculations were focused on the main phenomena occurring in the bundle, such as the thermal behavior, the hydrogen production mainly due to cladding oxidation, the massive degradation of spent fuel and the release of fission products and control rod and structure materials. Using the predefined boundary and initial conditions, relatively small deviations between the code results were obtained, which demonstrates that the dominant processes occurring during a severe accident in the core of pressurized water reactors can be adequately simulated. By applying these codes to a large spectrum of integral tests as well as to plant analyses, one will obtain reliable results on the fuel bundle behavior. However, the spread in the calculated oxidized boron carbide masses indicates that modeling efforts are still necessary in all the codes in this respect

Benchmark study on fuel bundle degradation in the phebus FPT2 test using state-of-the-art severe accident analysis codes

This paper presents the results of posttest calculations of thephebus FPT2 experiment. While the exercise concentrates mainly on code-to-code benchmarking, a comparison is also made with selected experimental results. The test scenario with the appropriate initial and boundary conditions was provided by the Institut de Radioprotection et de SÛreté Nucléaire. For the analyses, seven severe accident analysis codes were used: ASTEC, ATHLET-CD, MELCOR, ICARE2, ICARE/CATHARE, SCDAP/RELAP5, and RELAP/SCDAPSIM. The calculations focused on the following phenomena occurring in the FPT2 bundle: thermal behavior; hydrogen production, mainly due to cladding oxidation; severe degradation of irradiated fuel; and the release of fission products, control rod, and structure materials. Using the same postdefined boundary and initial conditions, the code-data differences are typically within 10% for most parameters, and not more than 25%. More importantly, the codes were able to capture the major features of the transient evolution. Given that Phebus FPT2 exhibited almost all of the major low-pressure severe accident phenomena except for core cooling by water injection and late-phase core melt behavior in the lower head, the results engender a degree of confidence in the code predictive capability for sequences similar toFPT2

Piezo-generator integrating a vertical array of GaN nanowires

International audienceWe demonstrate the first piezo-generator integrating a vertical array of GaN NWs. We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows to establish for GaN NWs the relationship between the material properties and the piezo-generation and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy equipped with Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to a compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs gained from AFM analyses is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW/cm3. This value settles the new state of the art for piezo-generators based on GaN NWs and more generally on Nitride NWs, and offers promising prospects for the use of GaN NWs for high-efficiency ultra-compact energy harvesters

High Sensitivity Piezogenerator Based on GaN Nanowires

We report on a prototype of piezogenerator based on vertically-aligned GaN nanowires grown by Plasma-Assisted Molecule Beam Epitaxy (PAMBE). Our device generates a peak-to-peak voltage from 20 mV to 60 mV depending on the applied force. The output power density reaches 0.76 mW/cm3 which is of the same order of magnitude as the state-of-art value. Furthermore, the pressure applied to deform the nanowires is of the order of kPa, much less than that used in previous reports (which were of the order of MPa) utilizing ZnO nanowires for the power generation. Thus, an enhanced mechanic-electrical conversion efficiency has been achieved. Our GaN nanowire based devices are promising both for pressure sensing applications and for mechanic energy harvesting

Nitride Nanowires: From Rigid to Flexible Piezo-generators

International audienceHere we employ self-assembled Mg-doped GaN nanowires(NWs) grown by plasma assisted molecular beam epitaxy on Si(111) substrates to fabricate piezogenerators. We will first discuss the fabrication and testing of rigid nanowire-based generators and then a flexible generator prototype will be shown