Échangeur de chaleur à haute performance conçu sur la base du chaos Lagrangien

Les travaux présentés dans ce rapport d\u27activité ont été menés dans le cadre du programme Ecodev de l\u27ARC CNRS-GRETH "échangeurs thermiques". Ils portent sur l\u27étude d\u27un écoulement à travers une géométrie dite "en Dean alterné" qui est susceptible de générer des trajectoires de particules chaotiques, en régime laminaire. Ce régime d\u27écoulement laminaire particulier est désigné par "régime d\u27advection chaotique". Il présente des capacités de transferts similaires à celles des écoulements turbulents et les pertes de charges d\u27un écoulement laminaire. Il s\u27agit de mettre en oeuvre un modèle thermique simplifié permettant de simuler les transferts thermiques dans un échangeur tubulaire présentant deux configurations : hélicoïdales ou chaotiques

Effet des rangées de perturbateurs pariétaux sur les transferts de chaleur

L’étude numérique du transfert de chaleur dans un échangeur de type HEV (High Efficiency Vortices) permet d’expliquer les mécanismes de l’intensification induits par les perturbateurs de paroi. L’effet des différentes structures générées est ainsi mis en évidence. Les performances globales du HEV montrent qu’il affiche une meilleure efficacité énergétique par rapport à d’autres échangeurs du marché

Turbulent mixing of two immiscible fluids

International audienc

Kinematic mixing and heat transfer enhancement in chaotic split-and-recombine heat exchangers/reactors

Small system dimensions, low fluid velocity and high viscosity are all factors that hinder the production of turbulence. Enhancing mixing and heat transfer under these conditions, while keeping sufficient residence times and moderate pressure drops, constitutes a real challenge. Adapted to low-Reynolds flow regimes, Split-And-Recombine (SAR) static mixer and heat exchanger configurations are designed to exploit flow energy to produce chaotic advection and promote diffusion at the molecular level. The present work explores the hydrodynamic and thermal character of the SAR flow and compares, through CFD simulations, two such geometries namely SAR-1 and SAR-2, with two other reference configurations: a square three-dimensional continuous flow geometry (3D-Flow) and a plain square channel. Efficient convective heat transfer is achieved in deeply laminar creeping flow. Relative enhancements up to 1700% can be achieved compared to plain square channel flow, with a moderate increase in the pressure drop that does not exceed 17% for the SAR-2 configuration showing the better performance

Turbulence length scales in a vortical flow

Laser Doppler velocimetry is used to investigate the velocity spectra and turbulence length scales in a turbulent vortical flow. The turbulent vortical flow is ensured by vorticity generators (VGs) inserted into a straight circular pipe. Each VG generates a complex flow that is mainly the combination of a steady streamwise counter-rotating vortex pair and a periodic sequence of hairpin-like structures caused by the Kelvin- Helmholtz instability in the shear layer ejected from the VG trailing edges. These primary structures induce a secondary vorticity in the wake of the VG. The aim of the study is to analyze the velocity spectra and turbulent length scales for the different coherent structures in the flow. Thus, the Kolmogorov and Taylor microscales, the Liepmann-Taylor microscale and the viscous length scale are determined in different locations in the VG streamwise direction. The evolution of the length scales with respect to the Taylor-Reynolds number is compared with theoretical trends in a variety of flows in the open literature

Transport phenomena in chaotic flows: flux recombination HEX reactors

Rapid transport of heat and mass is required in many industrial processes. Mixing is a fundamental issue in chemical engineering applications and when exothermic reactions are involved, heat transfer capabilities of reactors and static mixers become an advantage and a necessity to ensure stable operating conditions and security standards. Enhancement of mixing and heat exchange is possible through turbulence, but vortical structures are often not feasible for highly viscous, non-Newtonian or shear sensitive fluids such as emulsions, pastes and slurries common in pharmaceutical, cosmetic and food industries. An alternative to improve transport within such materials is chaotic advection, where Lagrangian chaotic structures are induced by physical means in low-Reynolds laminar flows. Microfluidics is an increasingly active domain in which small dimensions and velocities render turbulent mixing extremely hard. Mixing by diffusion is one solution where topological mixing schemes exploiting the laminarity the flow to repeatedly fold the flow and exponentially increase the concentration gradients to obtain fast and efficient mixing by diffusion. This paper presents the first results of a study investigating laminar and turbulent mixing qualities of a Flux Recombination Hex reactor by using the chemical probe method. The geometry, exploiting a three-dimensional, steady flow configuration intended to mimic the baker’s map and enhance mixing by chaotic advection. First proposed by Chen & Meiners [1] for a microfluidic chip, it is here reproduced for investigation purposes using a stratified multiple plate manufacturing technique on a mini-scale where laminar and slightly turbulent regimes can be assessed