Débit (physique) | EPFL Graph Search

Le débit est la quantité d'une grandeur qui traverse une surface donnée par unité de temps. Il permet de quantifier un déplacement de matière ou d'énergie. Le terme débit est le plus souvent associé au débit volumique : il quantifie alors le volume qui traverse une surface, une section, par unité de temps. Le débit massique caractérise la masse qui traverse la surface par unité de temps. Il s'agit de notions centrales dans une situation d'écoulement de fluide. Plus généralement, le débit peut désigner le flux des autres grandeurs déplacées par le fluide : débit de quantité de mouvement ou encore débit d’énergie. Le courant électrique est parfois considéré comme un débit de charges. Ceci se généralise en disant que le débit est l'intégrale sur une surface donnée de la projection du vecteur densité de flux sur la normale à la surface : D = \iint_S \vec{j}\cdot\vec{dS} avec \vec{j} le vecteur densité de flux de la quantité considérée (masse, énergie cinétique

Study of Microfluidic Mixing and Droplet Generation for 3D Printing of Nuclear Fuels

Internal gelation and 3D printing processes were proposed to combine in a process that is promising to produce nuclear fuels with simplified production route and enhanced in-core performance [1]. The process relies on in-situ mixing of feed solutions that bear actinides and uranium nitrates with the methenamine and urea mixture. These feed solutions should be thoroughly mixed and kept at low temperature up to an inkjet printing head, where an inkjet printing process is used to deposit controlled volumes of the mixed solution on the pellet under-fabrication. Microwave and/or laser-heating are then used to trigger the gelation reaction and to solidify these printed dots and later to drive out the excess water and pre-sinter the gelled oxides. The current study is an initial approach to optimize the upfront parameters of the 3D printing process. The feed solutions need be mixed in a passive microfluidic mixer, where the first optimization parameter is the minimum mixing time in the microfluidic mixer that will yield an acceptable mixing level. To do so, CFD modelling using OpenFOAM for different designs of microfluidic mixers was conducted to select the most efficient conception. To validate the CFD results, simulation of microfluidic mixers similar to some literature cases were performed and compared. The uncertainty of selected parameters in the CFD models was used to evaluate a conservative retention time and length. Secondly, the next printing step was modeled with OpenFOAM, which is the droplet generation process. The objective from these models was to correlate the feed parameters like the flow rate and the usage of an acoustic mechanical vibrator to the printed droplets size, generation rate, and the required stand-off distance of the printing head. To perform these CFD simulations with the right solution parameters, rheology and diffusivity measurements were conducted. A proposed empirical approach was investigated for the evaluation of the diffusivity of multi-species solutions. The resultant parameters from the current study are the mixing times and lengths for given microfluidic mixers that are industrially feasible, also, the rate and volume at which the droplets of the broth mixture are generated. These results can be used as guidelines for designing the inkjet printing head and to highlight areas where further investigations are required for an efficient production process.

EPFL2017

The effects of the tip clearance on the performance of small-scale turbopumps for ORC applications; analysis and modeling

Jürg Alexander Schiffmann, Sajjad Zakeralhoseini

In this paper, the influence of tip clearance on the performance of small-scale turbopumps is studied numerically on extensive parameter ranges suitable for organic Rankine cycle applications. A novel and fully parameterized design model is developed and used to generate a wide range of turbopumps and their fluid domains to carry out three-dimensional computations across the impeller stage. Impellers are investigated at different operating conditions and the accomplished results are analyzed to characterize the performance at design and off-design conditions. The CFD calculations demonstrate that the slip factor is dependent not only on its geometrical parameters as considered by most correlations but also on its operating conditions. The slip factor decreased almost linearly as the flow rate increased. In addition, the tip clearance ratio influences the slip factor and its influence is non-linear. The head rise decreases as the tip clearance ratios increased, however, for radial impellers, the higher head rise was observed for small tip clearance ratios.

Technical University of Munich2021

Trajectory Data Analysis on the Spatial and Temporal Influence of Pedestrian Flow on Path Planning Decision

Michel Bierlaire, Jeongyun Kim

The modeling of walking behavior and design of walk-friendly urban pathways have been of interest to many researchers over the past decades. One of the major issues in pedestrian modeling is path planning decision-making in a dynamic walking environment with different pedestrian flows. While previous studies have agreed that pedestrian flow influences path planning, only a few studies have dealt with the empirical data to show the relationship between pedestrian flow and path planning behavior. This study introduces a new methodology for analyzing pedestrian trajectory data to find the dynamic walking conditions that influence the path planning decision. The comparison of the pedestrians' path shows that the higher proportion of opposite flows are, the greater they influence the path selection decision. In this study, we investigate the relationship between the opposite flow changes and path planning behavior and find the spatial and temporal ranges of the opposite flow that affects the path planning behavior. Lastly, we find the ratio of pedestrians that update their paths with respect to the opposite flow rate.

MDPI2020

Study of Microfluidic Mixing and Droplet Generation for 3D Printing of Nuclear Fuels

EPFL2017