Intensification of heat-transfer and mixing in heat exchanger-reactors by artificially generated streamwise vortices

International audienceCompact heat exchangers are well known for their ability to transfer a large amount of heat while retaining low volume and weight. The purpose of this paper is to study the potential of using this device as a mixer as well as a chemical reactor, generally called a multifunctional heat exchanger (MHE). Indeed, the question arises: can these geometries combine heat transfer and mixing in the same device? Such a technology would offer many potential advantages, such as better reaction control (through the thermal aspect [S. Ferrouillat, P. Tochon, H. Peerhossaini, D. Della Valle, Open-loop thermal control of exothermal chemical reactions in multifunctional heat exchangers, Int. J. Heat Mass Transfer, in press]), improved selectivity (through intensified mixing, more isothermal operation and shorter residence time, and sharper residence time distribution (RTD)), byproduct reduction, and enhanced safety.Several geometries of compact heat exchanger based on turbulence generation are available. This paper focuses on one type: vortex generators. The main objective is to contribute to the determination of turbulent flow inside various geometries by computational fluid dynamics methods. These enhanced industrial geometries are studied in terms of their thermal-hydraulic performance and macro-/micro-mixing ability [S. Ferrouillat, P. Tochon, H. Peerhossaini, Micromixing enhancement by turbulence: application to multifunctional heat exchangers, Chem. Eng. Process., in press]. The longitudinal vortices they generate in a channel flow turn the flow perpendicular to the main flow direction and enhance mixing between the fluid close to the fin and that in the middle of the channel. Two kinds of vortex generators are considered: a delta winglet pair and a rectangular winglet pair. For both, good agreement is obtained between numerical results and data in the literature. The vortex generator concept is found to be very efficient in terms of heat-transfer enhancement and macro-mixing. Nevertheless, the micro-mixing level is poor due to strong inhomogeneities: the vortex generator must be used as a heat-transfer enhancement device or as a static mixer for macro- and meso-mixing

Heat and mass fluxes across a density interfaces submitted to a grid-generated turbulence

The present study was first motivated by the industrial problem of the Liquefied Natural Gaz storage tank where the stratification between two different LNG layers could be destroyed by the increasing unstable thermal stratification. Then the choice was to reproduce the historical experiments of Turner with a thin stratified interface separating two homogeneous media mixed by oscillating grids. Comparisons between thermally or salinity stratified experiments exhibit the strong dependence of the flux on the molecular diffusivity. A phenomenological model is proposed for these transfer with a clear serial scheme which is different from the previous parallel ones. It gives a clear understanding of the influence of the molecular diffusivity on the flux across those density interfaces

Fluid Forces on a Circular Cylinder Moving Transversely in Cylindrical Confinement: Extension of the Fritz Model to Larger Amplitude Motions

International audienceThis paper is related to the fluid forces prediction on a rapidly moving circular cylinder in cylindrical confinement. The Fritz model, which mainly assumes infinitesimal motions of the inner cylinder in an inviscid fluid, is one of the simplest model available in the scientific literature and is often used by design engineers in the nuclear industry. In this paper, simple non-linear expressions of fluid forces are derived for the case of finite amplitude motions of the inner cylinder. Assuming a potential flow, advection term and geometrical deformations can be taken into account. The problem, formulated as a boundary-perturbation problem, is solved thanks to a regular expansion. The range of validity of the approximate analytical solution thus obtained is theoretically discussed. The results are also confronted to numerical simulations, which allows to emphasize some limits and advantages of the analytical approach

Rapic project: toward a new generation of inexpensive heat exchanger-reactors for process intensification

Process intensification (PI) in chemical production is a major concern of chemical manufacturers. This alternative technology involves transposing syntheses into continuous plug flow reactors with process intensification, leading to a multifunctional heat exchanger-reactor. In this context, the RAPIC R&D project aims to develop an innovative low-cost component (in the 10 kg/hour range). This project deals with the design from the local to the global scale and with testing, from elementary mock-ups to pilot scale. The present paper gives a detailed description of this research project and presents the main results on specification and definition of the reaction channel and the first simple mock-ups

Mesure du micromélange dans une turbulence quasi-homogène et isotrope

Le micromélange est mesuré expérimentalement dans un écoulement quasi-homogène et isotrope, sans écoulement moyen, à grand nombre de Schmidt et avec un nombre de Reynolds basé sur l'échelle de Taylor variant de 10 à 65. Cette turbulence est générée par deux grilles oscillant dans le plan vertical. La mesure du micromélange est effectuée par une « sonde chimique », sur le principe de deux réactions concurrentes qui consomment l'acide injecté localement au centre des grilles : le dosage des coproduits permet d'accéder à une mesure qualitative du micromélange. Deux modèles phénoménologiques 0D de micromélange sont testés pour des paramètres en concentration constante. Le modèle IEM ne peut fonctionner dans ce cas du fait d'une concentration d'acide trop forte. Le modèle d'engouffrement est validé à partir d'un nombre de Reynolds basé sur l'échelle de Taylor autour de 30

Etude expérimentale de l'effet du rayonnement solaire sur le seuil d'envol de particules dans une couche limite turbulente

Le transport éolien de particules par saltation est le processus responsable de l'avancée des déserts. L'initiation de ce phénomène est dépendante de nombreux paramètres (écoulement, diamètres des grains, humidité,...) qui peuvent favoriser ou retarder le seuil d'envol. Ce travail consiste en l'étude de l'influence du rayonnement solaire, important dans ces régions, sur la vitesse d'écoulement critique d'envol des particules. Dans ce but, une veine de soufflerie et un dispositif de chauffage infrarouge ont été conçus. Après avoir caractérisé l'écoulement sur parois lisse et rugueuses, des mesures de vitesse critique d'envol ont été réalisées, pour différents échantillons de particules, en isotherme puis sous l'influence d'un flux radiatif.Eolian transport of particles by saltation is responsible for the dunes displacement. The initiation of this phenomenon depends on several parameters (flow conditions, grains diameter, humidity,...) which can change the particles take-off threshold.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocRENNES-INSA (352382210) / SudocSudocFranceF

Cyanobacteria: Model Microorganisms and Beyond

In this review, the general background is provided on cyanobacteria, including morphology, cell membrane structure, and their photosynthesis pathway. The presence of cyanobacteria in nature, and their industrial applications are discussed, and their production of secondary metabolites are explained. Biofilm formation, as a common feature of microorganisms, is detailed and the role of cell diffusion in bacterial colonization is described. Then, the discussion is narrowed down to cyanobacterium Synechocystis, as a lab model microorganism. In this relation, the morphology of Synechocystis is discussed and its different elements are detailed. Type IV pili, the complex multi-protein apparatus for motility and cell-cell adhesion in Synechocystis is described and the underlying function of its different elements is detailed. The phototaxis behavior of the cells, in response to homogenous or directional illumination, is reported and its relation to the run and tumble statistics of the cells is emphasized. In Synechocystis suspensions, there may exist a reciprocal interaction between the cell and the carrying fluid. The effects of shear flow on the growth, doubling per day, biomass production, pigments, and lipid production of Synechocystis are reported. Reciprocally, the effects of Synechocystis presence and its motility on the rheological properties of cell suspensions are addressed. This review only takes up the general grounds of cyanobacteria and does not get into the detailed biological aspects per se. Thus, it is substantially more comprehensive in that sense than other reviews that have been published in the last two decades. It is also written not only for the researchers in the field, but for those in physics and engineering, who may find it interesting, useful, and related to their own research