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

Intensification des transferts de chaleur par convection forcée en conduite de section carrée avec des ferrofluides

Cet article présente une investigation expérimentale de l'étude du transfert de chaleur au sein d'un ferrofluide en convection forcée à flux thermique imposé sous champ magnétique. Le régime d'écoulement étudié est essentiellement laminaire (250 < Re <830). La géométrie du canal de test est carrée, ce qui nous permet d'étudier l'influence de la direction du champ par rapport à celle du flux de chaleur. Les résultats obtenus nous montrent une meilleure intensification des transferts thermiques de plus de 80% dans le cas où le champ magnétique est perpendiculaire au flux de chaleur.</p

Étude du micro-mélange pour la caractérisation des performances d'échangeurs-réacteurs compacts multifonctionnels

Ce travail a pour objectif la caractérisation des différents paramètres nécessaires à l'optimisation et au dimensionnement d'un échangeur-réacteur compact multifonctionnel et l'élaboration d'une méthodologie pour le dimensionnement de ce dernier. L'utilisation d'un échangeur de chaleur en tant que réacteur chimique constitue une évolution importante vers une nouvelle approche des procédés chimiques. Afin de répondre aux contraintes de fonctionnement, il est nécessaire d'avoir une connaissance précise des phénomènes régissant les transferts de matière et de chaleur. Par conséquent, une étude thermo-hydrodynamique ainsi qu'une étude du micro-mélange, basée sur une méthode physico-chimique, ont été réalisées afin de comparer les performances de transfert de matière et de chaleur des différentes géométries étudiées. La base de données établie lors de ces études et une modélisation du procédé a permis d'élaborer une méthodologie pour le dimensionnement d'un échangeur-réacteur.Various design parameters of compact heat exchanger-reactors have been characterized in order to develop a new methodology for their design and optimization. The use of compact heat exchanger as a chemical reactor constitutes a significant break trough towards a new approach of chemical processes. In order to answer the compelling operation requirements, heat and mass transfer phenomena must be precisely investigated. Therefore, by using an original physico-chemical method, micro-mixing and thermo-hydraulics of turbulent flows have been studied to compare heat and mass transfer performance of selected geometries. Based on the results of this study a substantial data base is built. Using this database a novel process modeling has led to a creative design of future compact heat-exchanger reactors.NANTES-BU Sciences (441092104) / SudocNANTES-Ecole Centrale (441092306) / SudocSudocFranceF

Micromixing enhancement by turbulence: application to compact heat exchanger-reactors

International audienceCompact heat exchangers are well-known for their ability to transfer large amounts of heat while retaining low volume and weight. This paper studies the use of this device as a chemical reactor, generally called a heat exchanger reactor (HEX reactor). Indeed, the question arises: can these geometries combine heat transfer and mixing in the same device? Such a technology would offer many advantages, such as better reaction control (through the thermal aspect), improved selectivity (through intensified mixing, more isothermal operation and shorter residence time, and sharper residence-time distribution), byproduct reduction, and enhanced safety.Several geometries of compact heat exchanger based on turbulence generation are available. This paper focuses on two types: offset strip fins (OSFs) and metallic foams. Our main objective is to contribute to the estimation of micromixing generated by these geometries by using an experimental method based on a unique parallel-competing reaction scheme proposed by Villermaux et al. The micromixing time, estimated according to the incorporation model, lets us compare the micromixing levels generated by duct channel, OSFs and metallic foams at volume flow rates ranging from 1 to 350 l h−1. The metallic foam concept is found to be very efficient in micromixing enhancement. Furthermore, OSFs make it possible to generate micromixing levels ranging between the duct channel and metallic foam level. Moreover, the results show that the fin micromixing level increases with fin thickness and ligament diameter. Finally, in an HEX reactor application, the residence time of chemical reactants must be considered in order to choose the best geometry for intensifying mass and heat transfer

Implantation of carbon nanotubes in photoresist micro-channels for heat transfer applications

One of the most important applied research field associated to microfluidics deals with heat exchangers, mixers or reactors. The future electronic devices will contain more and more components, causing an increasingly heat generation that can harm internal parts of the device and requiring innovative cooling methods. Since the publication of Tuckerman and Pease [1] who first used silicon microchannels as heat exchangers, there has been a lot of works for devices including microsystems that circulate a fluid to draw heat from chips. From a heat exchange point of view, using a liquid is better than air. But the reduction of the hydraulic diameter of the microchannels involves a dramatic increase of the pressure drops. So, to assume the flow rate of the liquid, the system has to be equipped with macroscopic pumps and the benefits of reducing the size of the exchanger are lost. Using nanofluids as cooling liquid systems is a new exciting alternated way studied by many researchers [2]. Nanofluids refer to a two-phase mixture, that is a liquid in which fine metallic nanoparticules less than 50 nm in size are dispersed. Of course, easily available carbon nanotubes (CNT) are an extensively used tool for such applications. A experiment under test is to incorporate CNT into thermal grease that sits between a microprocessor and a heat sink [3]. On the one hand, the CNT conduct heat extremely well, are very small, can be suspended in liquids or polymers. One the other hand, CNT are superior thermal conductors by themselves but because of high thermal boundary resistance between the tubes and the other elements, some authors argue that they cannot exhibit the same level of high conductivity when integrated into other materials [4]

Intégration pariétale de nanotubes de carbone sur les parois de microcanaux

Les nanotubes de carbone sont des vecteurs privilégiés de la diffusion des nanotechnologies dans des thématiques où la taille caractéristique des outils d'investigation peut varier de l'échelle micrométrique à l'échelle nanométrique. L'intégration pariétale de ces derniers sur les parois de micro échangeurs a pour objectif d'augmenter la surface d'échange et ainsi le coefficient d'échange thermique global. Deux procédés de microfabrication seront présentés

An Analytical Model for the Optimisation of Metal Foam for Power Electronics Cooling

International audienceWith regular technological breakthroughs in power electronics, there is an ever-increasing need for efficient thermal management. This has led to an increased focus on the study of metal foam enhanced heat sinks. The aim of this paper is to develop and validate an analytical model to calculate the total thermal resistance of a metal foam filled channel. The model will then be used to optimise the microstructure of the foam in order to minimise the thermal resistance of the channel. Foams with three different porosities (40, 60 and 80%) were compared with numerical simulations and the deviation in results was found to be less than 2%. When optimised under a fixed pressure loss of 100kPa, a thermal resistance of 0.023K/W was found