Simulation numérique directe et analyse des transferts de chaleur dans les lits de particules fixes et mobiles

Ces travaux de recherche s'intéressent à la caractérisation des transferts thermiques dans les milieux fluide-particules, et en particulier, les lits fluidisés au sein desquels un solide divisé est mis en suspension par un fluide. La grande diversité d'échelles spatiales et temporelles dans ces procédés nécessite d'étudier les interactions hydrodynamiques, thermiques et/ou chimiques entre les particules et le fluide à l'aide d'une approche multi-échelles. Une étude des transferts thermiques dans des lits fixes puis fluidisés, est réalisée à deux échelles : locale (Particle Resolved Simulation) et moyennée (Discrete Element Method-Computional Fluids Dynamics). L'étude PRS permet de caractériser les couplages locaux des transferts thermiques entre particules ainsi que la dynamique de ces transferts dans les configurations fluidisées. Une étude comparative entre les échelles met en évidence les limites du modèle DEM-CFD à capter les fluctuations des transferts thermiques observées dans les simulations PRS. Dans un dernier temps, les fermetures du modèle DEM-CFD sont améliorées de manière à réintroduire les fluctuations perdues par le changement d'échelles

Direct numerical simulation of reactive flow through a fixed bed of catalyst particles

Many catalytic refining and petrochemical processes involve two-phase reactive systems in which the continuous phase is a fluid and the porous phase consists of rigid particles randomly stacked. Improving both the design and the operating conditions of these processes represents a major scientific and industrial challenge in a context of sustainable development. Thus, it is above all important to better understand all the intricate couplings at stake in these flows: hydrodynamic, chemical and thermal contributions. The objective of our work is to build up a multi-scale modelling approach of reactive particulate flows and at first to focus on the development of a microscopic-scale including heat and mass transfers and chemical reactions for the prediction of reactive flows through a dense or dilute fixed bed of catalyst particles. Please download the full abstract below

Micro/Meso simulations of a fluidized bed with heat transfer

Particulate flows encountered in fluidized beds are frequently used in industrial applications (refining, energy, chemicals/petrochemicals, pharmaceutics). The wide range of spatial scales and interactions between the phases in such systems yields a complex flow often coupled with mass and/or heat transfer. These transfer have been widely investigated in the past in an experimental and numerical manners for dilute (1, 2) and dense suspensions (3, 4). Multi-scale modeling is a numerical approach developed to understand these phenomena from the particle scale (microscale) to process unit (macroscale). An intermediate scale (mesoscale) ranging from 104 to 108 particles is also introduced. Fluid phase is resolved using an Eulerian description while particles may be followed in a Lagrangian or an Eulerian manner. Meso/macroscale require closure laws for momentum, heat/mass transfer that can be derived either from experiments or microscale Particle-Resolved simulations (PRS). In this work, numerical simulations of gas-solid fluidization with heat transfer are performed at the microscale (DLM/FD) and the mesoscale (Euler/Lagrange) with our massively parallel code PeliGRIFF (5). A soft-sphere model combined with a Discrete Element Method (DEM) to track particles trajectory and contacts is used at both scales and interphase drag/heat transfer closure laws derived from our own PRS are used at the mesoscale. We select a system that comprises a few thousands of particles and extract statistically averaged local and global heat transfer. We carry out a direct comparison of the predictions obtained at both scales and suggest how the mesoscale modeling might be improved to provide more accurate solutions. REFERENCES W.E. Ranz and W.R. Marshall. Evaporation from drops, Part I and I. Chemical Engineering Science, 48:141-146;173-180, 1952. Z.G. Feng and E.E. Michaelides. Heat transfer in particulate flows with Direct Numerical Simulation (DNS). International Journal of Heat and Mass Transfer, 52:777-786, 2009. D.J. Gunn. Transfer of heat or mass to particles in fixed and fluidized beds, International Journal of Heat and Mass Transfer, 21:467-476,1978. N.G. Deen and E.A.J.F. Peters, J.T. Padding and J.A.M. Kuipers. Review of direct numerical simulation of fluid-particle mass, momentum and heat transfer in dense gas-solid flows, Chemical Engineering Science, 116:710-724, 2014. A. Wachs, A. Hammouti, G. Vinay, and M. Rahmani. Accuracy of finite Volume/Staggered grid Distributed Lagrange Multipliers/Fictitious Domain simulations of particulate Flows. Computers & Fluids 115, 154–172, 2015

Scalar mixing in bubbly flows: Experimental investigation and diffusivity modelling

Transport properties of scalars, as concentrations of a solute or temperature, are important for scale-up and design of operation units. An appropriate description of convective and diffusive mechanisms is required to predict local concentrations in complex geometries. In the case of gas–liquid bubbly flows, which are present in many chemical- or bio-reactors, effective diffusivity of scalars results from three contributions: molecular diffusion, Shear-Induced Turbulence (S.I.T.), i.e. turbulence induced by gradients of velocity in the continuous phase, and Bubble-Induced Turbulence (B.I.T.), i.e. turbulence generated by interactions of bubble wakes. In a previous work (Alméras et al., 2014, 2015), the diffusion resulting from B.I.T. has been characterized. Based on experiments performed in a homogeneous bubble column, it has been shown that the transport can be modelled by an effective diffusion and a physical modelling has been proposed to predict the diffusion induced by B.I.T. when other contributions are negligible. In the present work, we investigate the transport of a passive scalar in a complex bubbly flow at moderate gas volume fraction (αgr3%), involving a large-scale flow recirculation responsible for the development of Shear-Induced Turbulence. Experimental mixing times measured by image processing under various operating conditions have been compared to numerical simulations of scalar transport. Simulations have been performed by means of an Eulerian RANS CFD model wherein the diffusion generated by B.I.T. modelled by Alméras et al. (2015) is implemented in addition to the diffusion resulting from the S.I.T. Results show that the diffusion caused by B.I.T. plays a major role in the mixing of scalars in the investigated flows. Neglecting this contribution leads to an important overestimation of the mixing time vunless assigning arbitrary low values to the turbulent Schmidt number Sct (o0.3) adapted a posteriori to the simulated cases. On the other hand, considering the scalar diffusivity by B.I.T leads to a good agreement between experiments and CFD simulations, with keeping the Schmidt number in the usual range adopted for mixing in S.I.T. [0.7–1]. The model is generic enough to reproduce the scalar transport for various gas injections, without any further user adaptation

Gas-solid fluidized bed simulations using the filtered approach: Validation against pilot-scale experiments

Numerical simulations of large-scale fluidized beds still remain challenging due to the computational limitation and experimental validation. In the present work, a CFD study of a large-scale fluidized bed is investigated using the NEPTUNE_CFD code based on an Eulerian n-fluid modeling approach. A SubGrid Scale (SGS) drag model based on the filtered approach is used to take into account the effect of very small solid structures unresolved with the coarse mesh. The numerical results are compared with the experimental data carried out in a pilot-scale cold-flow fluidized bed unit and provided by Particulate Solid Research Inc (PSRI). By applying the SGS drag model without any specific or empirical tuning, reasonably good grid-independence is achieved. The flow regimes inside the fluidized bed are well predicted for all the superficial gas velocities studied here. The bed density profiles and the solid entrainment fluxes are also in good agreement with the experimental measurement

Draft of innovation and research roadmap

sepponen2014ano abstrac

Draft of innovation and research roadmap

sepponen2014ano abstrac

Innovation and research roadmap

hukkalainen2015aThe READY4SmartCities (R4SC) project examines the adaption of Information and CommunicationTechnologies (ICT) in energy systems, in order to improve their sustainability and energy efficiency insmart cities. This deliverable presents an innovation and research roadmap, suggesting thedevelopment needs of ICTs in short, medium and long term for the holistic design, planning andoperation of energy systems. The focus is on large energy systems at the city level: centralized anddistributed energy systems with connections to both national level energy systems and to theneighbourhood and building level energy systems.The roadmap is divided into five roadmap sections: citizens, the building sector, the energy sector,municipality and energy data. Each roadmap sector introduces drivers, needs and requirements,visions, barriers, expected impacts and key stakeholders. In the following, the goals of the differentroadmap sections are specified from the viewpoints of key stakeholders. The role of ICTs and energydata in enabling these goals is also identified.The involvement of citizens in decision making related to energy aspects should be increased. Citizensshould take an active role in the operation and use of energy to improve their energy behaviour. ICTscould help citizens to improve their energy behaviour by making them aware of the impacts of theiractions.Buildings should become connected objects operating actively with energy networks and are optimizedto balance the energy behaviour and thereby maximize the comfort of the inhabitants. Efficient energyuse and on-site renewable energy production in the buildings is expected to be of high importance.Buildings could also be able to act as energy providers. This requires the smart use of data from thebuilt environment, energy grids, the weather etc., implying that interoperability is ensured at differentlevels.The energy supply in cities should rely both on distributed and centralised energy production with usingmany renewable and local energy sources. Cities would become large power plants and virtual storage,reacting flexibly on the availability of renewables. ICT standards are needed for the communicationbetween all the energy systems.Municipalities should foster the integration of different city systems to maximize their synergy impacts.Efficient energy use and supply could be realized through appropriate decision making, energyplanning, development projects and daily operation within cities. Energy supply and use are integratedto other city operations with various ICT solutions.Access to open energy data would enable the sharing of cross-domain data between differentstakeholders, leading to the consolidation of energy-related knowledge in cities. The use of energy datawould also give the stakeholders a holistic view of the energy systems.The repeating theme throughout the roadmap is a strong need for broad collaboration, communicationand interoperability within all the stakeholder networks. This requires the standardisation of bothinterfaces and systems themselves, to enable cross-organisational operation

Direct numerical simulations and analysis of heat transfer through fixed and fluidized beds

Ces travaux de recherche s'intéressent à la caractérisation des transferts thermiques dans les milieux fluide-particules, et en particulier, les lits fluidisés au sein desquels un solide divisé est mis en suspension par un fluide. La grande diversité d'échelles spatiales et temporelles dans ces procédés nécessite d'étudier les interactions hydrodynamiques, thermiques et/ou chimiques entre les particules et le fluide à l'aide d'une approche multi-échelles. Une étude des transferts thermiques dans des lits fixes puis fluidisés, est réalisée à deux échelles : locale (Particle Resolved Simulation) et moyennée (Discrete Element Method-Computional Fluids Dynamics). L'étude PRS permet de caractériser les couplages locaux des transferts thermiques entre particules ainsi que la dynamique de ces transferts dans les configurations fluidisées. Une étude comparative entre les échelles met en évidence les limites du modèle DEM-CFD à capter les fluctuations des transferts thermiques observées dans les simulations PRS. Dans un dernier temps, les fermetures du modèle DEM-CFD sont améliorées de manière à réintroduire les fluctuations perdues par le changement d'échelles.This work aims at characterizing heat transfer into fluid-solid flows, and more particularly fluidized beds, into which a solid phase is suspended by a flowing fluid. The wide range of spatial and temporal scales present in such processes encourage to study hydrodynamic, thermal and/or chemical interactions between the particles and the fluid through a multi-scale strategy. The analysis of thermal interactions was first carried out for fixed bed configurations and then, fluidized beds at two overlapping scales: local (PRS; Particle Resolved Simulation) and mesoscopic (DEMCFD; Discrete Element Method-Computional Fluids Dynamics). The PRS approach accounts for the local coupling of heat transfer between the particles and its dynamics into fluidized beds. A comparative study of the two scales indicated the limits of the DEM-CFD model to capture the heat transfer fluctuations observed into PRS. In a last step, the closure laws for DEM-CFD were improved to reintroduce the fluctuations lost at this scale