Compact 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-/micromixing 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 heattransfer enhancement device or as a static mixer for macro-and meso-mixing.
Compact 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.
The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power=110W and supplied thermal power=450W). In the same time cavitational activity due to 2MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.
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