Comparison and Uncertainty Quantification of Two-Fluid Models forBubbly Flows with NEPTUNE_CFD and STAR-CCM+

International audienceThe nuclear industry is interested in better understanding the behavior of turbulent boiling flowsand in using modern computational tools for the design and analysis of advanced fuels and reactorsand for simulation and study of mitigation strategies in accident scenarios. Such interests serve asdrivers for the advancement of the 3-dimensional multiphase Computational Fluid Dynamicsapproach. A pair of parallel efforts have been underway in Europe and in the United States, theNEPTUNE and CASL programs respectively, that aim at delivering advanced simulation tools thatwill enable improved safety and economy of operations of the reactor fleet. Results from acollaboration between these two efforts, aimed at advancing the understanding of multiphaseclosures for pressurized water reactor (PWR) application, are presented. Particular attention is paidto the assessment and analysis of the different physical models implemented in NEPTUNE_CFDand STAR-CCM+ codes used in the NEPTUNE and the CASL programs respectively, forapplication to turbulent two-phase bubbly flows. The experiments conducted by Liu and Bankoff(Liu, 1989; Liu and Bankoff 1993a and b) are selected for benchmarking, and predictions from thetwo codes are presented for a broad range of flow conditions and with void fractions varyingbetween 0 and 50percent. Comparison of the CFD simulations and experimental measurements revealsthat a similar level of accuracy is achieved in the two codes. The differences in both sets of closuremodels are analyzed, and their capability to capture the main features of the flow over a wide rangeof experimental conditions are discussed. This analysis paves the way for future improvements ofexisting two-fluid models. The benchmarks are further leveraged for a systematic study of thepropagation of model uncertainties. This provides insights into mechanisms that lead to complexinteractions between individual closures (of the different phenomena) in the multiphase CFDapproach. As such, it is seen that the multi-CFD-code approach and the principled uncertaintyquantification approach are both of great value in assessing the limitations and the level of maturityof multiphase hydrodynamic closures

Metal nanoparticle film–based room temperature Coulomb transistor

Single-electron transistors would represent an approach for less power consuming microelectronic devices if room-temperature operation and industry-compatible fabrication were possible. We present a concept based on stripes of small, self-assembled, colloidal, metal nanoparticles on a back-gate device architecture which leads to well-defined and well-controllable transistor characteristics. This Coulomb transistor has three main advantages: By employing the scalable Langmuir-Blodgett method we combine high-quality chemically synthesized metal nanoparticles with standard lithography techniques. The resulting transistors show on/off ratios above 90 %, reliable and sinusoidal Coulomb oscillations and room-temperature operation. Furthermore, this concept allows for versatile tuning of the device properties like Coulomb-energy gap, threshold voltage, as well as period, position and strength of the oscillations.Comment: 31 pages, 6 figure

A first assessment of the NEPTUNE_CFD code: Instabilities in a stratified flow comparison between the VOF method and a two-field approach

This paper presents some results concerning a first benchmark for the new European research code for thermal hydraulics computations: NEPTUNE-CFD. This benchmark relies on the Thorpe experiment to model the occurrence of instabilities in a stratified two-phase flow. The first part of this work is to create a numerical trial case with the VOF approach. The results, in terms of time of onset of the instability, critical wave-number or wave phase speed, are rather good compared to linear inviscid theory and experimental data. Additional numerical tests showed the effect of the surface tension and density ratio on the growing dynamics of the instability and the structure of the waves. In the second part, a code to code (VOF/multi-field) comparison is performed for a case with zero surface tension. The results showed some discrepancies in terms of wave amplitudes, growing rates and a time shifting in the global dynamics. Afterward, two surface tension formulations are proposed in the multi-field approach. Both formulations provided similar results. The time for onset of the instability, the most amplified wave-number and its amplitude were in rather good agreement with the linear analysis and VOF results. However, the time-shifted dynamics was still observed. (C) 2007 Elsevier Inc. All rights reserved

Low Temperature Characterization of Hole Mobility in Sub-14nm Gate Length Si 0.7 Ge 0.3 Tri-Gate pMOSFETs

International audienceWe performed low temperature characterization of hole mobility in SiGe Trigate nanowires (NW) with gate length scaled down to 10nm. Trigate NWs with high-k/metal gate stack were fabricated on SOI wafers using solely optical lithography to design the wires with width down to 15nm. Drain current measurements are conducted within a cryogenic probe station enabling study on a 80K-350K temperature range. From these measurements we extracted the temperature dependence of the low field mobility μ0 for a wide range of gate lengths and NW widths using the Y-function method in order to cancel the influence of the mobility attenuation factor due to series resistances. The impact of the temperature over the mobility can be used to identify the dominant scattering mechanism in the channel. Overall, we observed that hole transport is predominantly limited by the extra scattering due to neutral defects in all devices with a gate length under 40 nm. Source/Drain implantations trace the origins of these defects. Consequently, these particular process steps should gain special care in the design of further node generation as they critically hinder charge mobility at that scale

Statistical characterization of drain current local and global variability in sub 15nm Si/SiGe Trigate pMOSFETs

session A2L-E: Electrical Characterization of Advanced TechnologiesInternational audienceA detailed statistical characterization of drain current local and global variability in sub 15nm gate length Si/SiGe Trigate NW pMOSFETs is carried out. An analytical mismatch model is used to extract the main matching parameters. Our results indicate that, despite their very aggressive dimensions, such devices maintain relatively good variability performance

Cfd study of the transverse migration of bubbles in a narrow rectangular duct assessment of the neptune_cfd code and benchmark of various closure models

International audienceThis paper presents a computational fluid dynamics study of two-phase bubbly flow, and aims ata proper description of the spatial distribution of bubbles observed in narrow rectangular ductsfor various non-uniform inlet conditions. As such the study relies on the air-water experimentsperformed by Liu et al. (2012), and a comparison to their results allows an assessment of theaccuracy of different closure models, such as lift force, wall force and turbulent dispersion.Different interfacial area concentration models are also compared and appraised. The numericalcomputations are performed with the NEPTUNE_CFD code, developed together by AREVA,CEA, EDF and IRSN, and dedicated to the simulation of multiphase flows by means of anEulerian multi-fluid approach

Contribution a la conception d'alimentations continues de puissance a resonance : etude et realisation d'un convertisseur 50 kW, 20 kHz

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc