Convection | EPFL Graph Search

La convection est l'ensemble des mouvements internes (verticaux ou horizontaux) qui animent un fluide et qui impliquent alors le transport des propriétés des particules de ce fluide au cours de son déplacement. Elle peut être due à des différences de température, une agitation mécanique, un pompage etc. Ce transfert implique l'échange de chaleur entre une surface et un fluide mobile à son contact, ou le déplacement de chaleur au sein d'un fluide par le mouvement d'ensemble de ses molécules d'un point à un autre. La convection constitue, avec la conduction thermique et le transfert radiatif, l'un des trois modes d'échange de chaleur entre deux systèmes, et diffère de ces derniers par la méthode de transfert. Elle a aussi pour effet d'accélérer l'homogénéisation de la composition chimique du fluide, par rapport à la seule diffusion chimique. Ce phénomène physique très courant se produit dans de nombreux systèmes (casserole, circuits de chauffages, atmosphère, manteau terrestre, étoil

Long-range transport of pollution to Europe

Marion Auvray

Ozone (O3) and aerosols are harmful to human health. Long-range transport sources contribute to the background levels upon which local pollution builds. The goal of this thesis is to describe and quantify the respective contribution of local and long-range transported sources to the O3 and aerosol budget in the framework of European air quality management. This issue is examined using a global model of chemistry and transport, the GEOS-Chem model, constrained by several experimental datasets (e.g. trace gases and aerosol distributions, aerosol optical depth, particulate matter concentrations) taken from field experiments, ground-based sites and satellite. The capabilities of the model to reproduce O3 and aerosol patterns is first examined. The model reproduces well in general the distribution of O3 and related species over the North Atlantic/Europe area. The simulation of particulate matter smaller than 2.5 μm (PM2.5) and aerosol optical depth (AOD) over Europe is also satisfying in general, but this may reflect some compensating errors between individual components (e.g. underestimate of organic matter, and overestimate of sulphate and dust). Intercomparison between the GEOS-Chem and another global model (MOZECH) also reveals possible problems in the water vapour distribution associated with the processes which drive the vertical lifting of pollutant over continental boundary layers. Despite these (relative) deficiencies, the model provides anyway insights about the contribution of long-range transport on the O3 and aerosol loads over Europe. The impact of continental outflow on the O3 chemical tendencies over oceanic regions is quantified. Net O3 photochemical production in polluted air masses travelling over oceanic areas under the influence of continental outflow is enhanced all-year round compared to the background marine environment by 2 to 6 ppbv/day in the boundary layer and by 1 to 3 ppbv/day in the upper troposphere. The origin of O3 long-range sources and their impact on the European O3 budget, as well as the role of changing emissions on this budget over the pasts decades is further investigated. Import of North American O3 into Europe is mainly controlled by meteorological patterns and by photochemical production over the North Atlantic and thus reaches a maximum in summer. In addition, Asia contributes to long-range transported O3 pollution by the Indian summer monsoon easterly winds during summer. During the monsoon period, convection is strong and associated NOx lightning emissions also contribute to high O3 levels, especially in the Mediterranean basin. North American and Asian anthropogenic pollution contribute substantially to the annual O3 burden (integrated over the whole tropospheric column) over Europe, accounting for 11% and 8%, respectively while the European contribution only accounts for 9%. The increase in Asian emissions from 1980 to 1997 have compensated the local reduction of O3 precursors, especially in the free troposphere. Finally, the aerosol load over Europe and the contribution of long and medium-range sources including anthropogenic and biomass burning pollution from North America and mineral dust from North Africa is examined. The model was used to interpret MODIS (Moderate Resolution Imaging Spectroradiometer) AOD and observations provided by the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over the North Atlantic and European area. Trans-Atlantic transport of aerosol is controlled by meteorological patterns and scavenging processes and reaches a maximum in spring. North American uxes of aerosols reach Europe at flower altitudes than that of O3 because a larger fraction of aerosols are scavenged in the venting from the boundary layer by frontal passages and deep convection. During high vegetation fire events in the boreal forest of Alaska and Canada, biomass burning pollution could also reach Europe at high altitudes, because a fraction of fire emissions are emitted directly in the free troposphere. Finally, the Saharan region also contributes significantly to the aerosol load over Europe. European sources contribute by 58% and 48% to the surface PM2.5 and column AOD over Europe. The second main sources are the mineral dust which represent in average 20% of the surface PM2.5 and AOD. In summer, North American anthropogenic and biomass burning emissions represent between 2 to 5% of the surface PM2.5 and AOD. The PM2.5 daily standard levels of the World Health organization (WHO) (10 μg/m3) and of the U.S. Environmental Protection Agency (EPA) (65 μg/m3) are often exceeded during dust outbreaks, especially in southern Europe. Long-range transport of anthropogenic and natural pollution is thus an important issue which should be considered in the definition of air quality standards and regulation treaties.

EPFL2006

Replicated microcellular aluminium structures for thermal management

Alexandros Athanasiou-Ioannou

Open-pore aluminium foams are of interest in thermal management of power electronics due to their large specific surface area, low weight and relatively high thermal conductivity. The replication process is a low-cost and simple foam manufacturing method that offers many opportunities for the investigation of the influence exerted by the microstructure (porosity, pore size) and geometry or their combination in the performance of metal foams in heat-exchange applications. In this study, we explored replicated aluminium foams as a part of an integrated heat sink manufactured in one step that seamlessly combines metal and ceramic phases. Replicated aluminium foams were manufactured and tested first under natural and then under forced convection. A dedicated test apparatus was built to test these samples under both natural and forced convection, using the guarding ring technique. In the case of natural convection, the aluminium foams had different pore sizes (125-180 µm, 400-450 µm and 5 mm), porosities (78 to 86%) and geometry (disk, cylinder and finned structures). In all cases, replicated aluminium foams were found to dissipate the same amount of heat as their bulk aluminium (alloy) equivalents. The thermal heat exchange performance of replicated microcellular aluminium was also measured under forced convection. The test rig was modified to measure the pressure drop and volumetric air flow in a cylindrically symmetric jet impingement configuration. Foams of different pore size (125-180, 400-450 and 900-1300 µm), porosity (between 75 and 85 %), height (1 and 2.8 cm) and infiltrated at various pressures (between 2.7 and 70 bars) were manufactured for thermal testing, all having a hollow central channel, through which air is injected at a range of flow rates. The results show that within the covered parameter range foams of higher porosities, larger pore sizes and lower infiltration pressures dissipate more heat at lower cost in terms of pumping power. Shorter heights also dissipate more heat for a given volumetric flow but do so at the cost of an increased pressure drop and hence greater pumping power expenditure. A numerical model, developed and written by Dr. D. Ingram, using the finite-volumes technique, was employed to elucidate how the various parameters influence the heat transfer. The model solves the Darcy-Forchheimer formulation of fluid flow in porous media, which is then coupled to a convective/conductive heat transfer model of the same structure. These simulations were used to perform parametric studies on the influence of height and pore size with regard to the thermal behaviour of replicated foams. Furthermore, the model was benchmarked against the experimental results; overall, despite some discrepancies with measured temperature profiles, the model captures data trends well, and provides insight on the physical phenomena that underlie the experimental data and observations. In particular, the presence and influence of recirculating air flow along the inner portion of the heat exchanger structures was identified. Finally, we successfully fabricated the proposed integrated multimaterial structure and tested it under forced convection, using the same conditions as the replicated aluminium foams. Results demonstrate that this type of structures can be effectively used to cool power electronics.

EPFL2017

Accurate buoyancy and drag force models to predict particle segregation in vibrofluidized beds

Mehrdad Kiani Oshtorjani

The segregation of large intruders in an agitated granular system is of high practical relevance, yet the accurate modeling of the segregation (lift) force is challenging as a general formulation of a granular equivalent of a buoyancy force remains elusive. Here, we critically assess the validity of a granular buoyancy model using a generalization of the Archimedean formulation that has been proposed very recently for chute flows. The first model system studied is a convection-free vibrated system, allowing us to calculate the buoyancy force through three different approaches, i.e., a generalization of the Archimedean formulation, the spring force of a virtual spring, and through the granular pressure field. The buoyancy forces obtained through these three approaches agree very well, providing strong evidence for the validity of the generalization of the Archimedean formulation of the buoyancy force which only requires an expression for the solid fraction of the intruder, hence allowing for a computationally less demanding calculation of the buoyancy force as coarse graining is avoided. In a second step, convection is introduced as a further complication to the granular system. In such a system, the lift force is composed of granular buoyancy and a drag force. Using a drag model for the slow-velocity regime, the lift force, directly measured through a virtual spring, can be predicted accurately by adding a granular drag force to the generalization of the Archimedean formulation of the granular buoyancy. The developed lift force model allows us to rationalize the dependence of the lift force on the density of the bed particles and the intruder diameter, the independence of the lift force on the intruder diameter, and the independence of the lift force on the intruder density and the vibration strength (once a critical value is exceeded).

AMER PHYSICAL SOC2021