Écoulement laminaire | EPFL Graph Search

En mécanique des fluides, l'écoulement laminaire est le mode d'écoulement d'un fluide où l'ensemble du fluide s'écoule plus ou moins dans la même direction, sans que les différences locales se contrarient (par opposition au régime turbulent, fait de tourbillons qui se contrarient mutuellement). L'écoulement laminaire est généralement celui qui est recherché lorsqu'on veut faire circuler un fluide dans un tuyau (car il crée moins de pertes de charge), ou faire voler un avion (car il est plus stable, et prévisible par les équations). Définition Point de vue microscopique Dans un écoulement laminaire, deux particules de fluide voisines à un instant donné restent voisines aux instants suivants. Ceci permet de décrire le champ de vitesses en utilisant les techniques classiques d'analyse mathématique. Quand l'écoulement devient turbulent, il est sans organisation apparente, et les techniques classiques ne suffisent plus. Point de vue macroscopique Les not

Numerical investigation of particle-laden thermally driven turbulent flows in enclosure

Riccardo Puragliesi

Nowadays, the global increase of energy demand and the necessity to satisfy high safety standards have led engineers and scientists to focus their efforts in order to understand and describe fundamental phenomena that are crucial for a correct design of the new generation nuclear power plants. In this framework, the present thesis aims at providing a first insight of the mechanisms of deposition of aerosol particles inside a closed geometry where relatively strong currents are present due to turbulent natural convective flows. Direct Numerical Simulations were conducted coupling high-order pseudo-spectral code with a Lagrangian particle tracker. Laminar flows were computed in two and three dimensions in order to benchmark the code with published reference data. A parametric study was performed for three different aerosol micro-size particle diameters and two super-critical Rayleigh numbers in a square cavity. An extended analysis of the turbulent flows is provided in terms of first and second order statistics, time-averaged momentum and energy budgets, and moreover, important terms appearing in the transport equations of turbulent kinetic energy and temperature variance are also briefly discussed. Furthermore, the evolution in time of particle concentration for the three different diameters is considered. The text provides information about the deposition velocity, the deposition patterns on the cavity surfaces, the influence of lift and thermophoretic forces and the fractal dimension. The same size dependent parametric study for the three different sets of micro-size particles was carried out in a fully three-dimensional closed cubic cavity for one super-critical Rayleigh number. A detailed investigation of the turbulence was performed by means of statistical quantities, signal processing and conditional averaging, in order to get a general view of the complexity of the flow and its characteristics. Further on, the sedimentation process is studied in the same way as for the two dimensional case. Finally a simple theoretical deposition model is provided in order to interpret the numerical results for the aerosol phase.

EPFL2010

Is the lattice Boltzmann method suited for aeroacoustic applications ?

Lionel Wilhelm

Lattice Boltzmann method has become a common tool for computational fluid dynamics. Thanks to the weak numerical dissipation of the numerical scheme, it seems very well fitted for aeroacoustics studies. However there exists only few validations in literature. In this semester project we propose to evaluate on a simple and well known case (laminar flow past one or two cylinders) the ability of the method to reproduce the aeroacoustic fields in a reliable manner. Firstly, we present the Lattice Boltzmann Method (LBM) and, in order to get familiar with LBM, a simple Matlab code is written and the flow past a 2D cylinder is simulated. Then, the ability of LBM to simulate aeroacoustics is tested. With this aim, the Palabos library (open-source and based on C++) is used to run the simulation in a more efficient way. The wake cylinder flow is one of the most studied and well known flow and it is therefore natural to validate the numerical scheme with this flow case. The LBM has to be validated for such particular case where numerical simulations tries to catch tiny variations of pressure. The simulations of the flow obtained with the LBM are compared to the results obtained in literature.

2015

Development of a novel microreactor-based calorimeter for the study of fast exothermal reactions in liquid phase

Marie-Agnès Schneider

Following several tragic chemical accidents that have occurred in recent years, some directives concerning chemical process safety have been issued. The Seveso directive, for example, requires a precise description of the consequences of a possible chemical accident assuming a worst-case scenario. However, to build such scenarios and, hence, make chemical processes safe, profound knowledge of the kinetic and thermal parameters of the reactions in question is necessary. These parameters are usually determined by calorimetric methods. However, in the existing commercial calorimeters the characterization of fast, exothermal reactions raises several problems. Indeed, on the one hand, it is difficult to maintain the isothermal conditions required to determine the kinetics simply and precisely. On the other hand, the calorimeter should be able to measure the heat flow generated as soon as the reactants are brought into contact. It should be able to do this without the need for a period of thermal equilibration that would perturb the measurements, and without any limitations due to the mixing of the reactants. The aim of this work is to develop a calorimetric method that is particularly adapted to studying fast exothermal reactions. The proposed system combines a microreactor with a commercially available microcalorimeter. In the first part of this project a flexible technique for constructing microreactors was chosen. The technique had to be precise, reproducible and inexpensive. The geometry of the microreactor had to fit into the cavity of the commercially available microcalorimeter. The technique chosen was the silk-screen printing of a thick film dielectric. This method makes it possible to build quickly and at low cost numerous microreactors that can be regarded as disposables. The geometry of the reaction channel can be varied and can be adapted to the type of reaction. Once the microreactors were built, the degree of mixing obtained in the microchannels was estimated first by simulation and then by experiment. The flow in the reaction channel was found to be purely laminar and the mixing time corresponded to the time for radial diffusion. Due to the small size of the channels, the mixing time was found to be adequate and not limiting for the characterization of fast reactions. The microreactor was then inserted into the cavity of the commercial calorimeter. The resulting microsystem was calibrated using the neutralization reaction of sulphuric acid with NaOH. This system was further modified to optimize the thermal transfer between the reaction channel and the sensor of the microcalorimeter and to increase its thermal efficiency. Finally, an electrical preheating system of the incoming liquids was put in place and tested. Once the quality of the thermal signal had been optimized, kinetic studies of chemical reactions could be undertaken. First, a model reaction was studied in order to validate the results obtained with the microsystem and to avoid the risk of systematic errors. In the second stage, a previously unknown fast exothermal reaction was characterized. The heat flows measured during the reaction reached 160'000 W·kg-1 but the conditions, however, remained completely isothermal. The global kinetics of this reaction as well as its activation energy were determined.

EPFL2004