Noise prediction of fan-tip/boundary layer interaction

International audienc

Noise prediction of fan-tip/boundary layer interaction

International audienc

Numerical Determination of the Volumetric Heat Transfer Coefficient in Fixed Beds of Wood Chips

International audienceThe aim of this work is to build CFD tools to study heat transfer in packed beds during the thermochemical upgrading process of woody biomass. For that purpose, a numerical workflow capable of investigating heat transfer phenomena inside a packed bed was developed. First, the bed is generated using a Discrete Element Method (DEM) code. Then, a steady-state Direct Numerical Simulation (DNS) solver is used to acquire the flow patterns inside the pores of the bed. Finally, the heat equation is solved for the particles and the fluid in a coupled manner. The results are thoroughly validated against two benchmark cases: flow around a sphere and flow through a packed bed of spheres. The importance of accounting for internal heat resistance for substrates with high Biot number (e.g. wood) is highlighted. Furthermore, the heat transfer phenomena inside a packed bed of torrefied wood chips are investigated. Values of the heat transfer coefficient are also provided (5 W/m/K, for Re ¡ 5, 7.7 for Re 10). Another interesting finding is the effect of channeling in this type of beds and its impact on the temperature field of the wood chips

Revisiting the influence of confinement on the pressure drop in fixed beds

International audienceWe study the non-turbulent pipe flow of a Newtonian fluid through a confined porous medium made of randomly arranged spherical particles in the situation where the ratio between the pipe diameter over the particle diameter (D/d) is less than 10. Using experiments and numerical simulations, we examine the relation between the flow rate and the mean pressure gradient as a function of the Reynolds number and particle size, and how it is affected by the presence of the walls. We investigate the intrinsic variability of the measurements in relation to the randomness of the particle arrangement and how such variability is linked to spatial fluctuations of pressure within the bed. We observe that as D/d decreases, the pressure gradient presents a stronger variability, particularly in relation to where measurements are taken within the pipe. The study also quantifies the difference between measuring the pressure drop at the wall versus averaging over the entire volume, finding a small difference of 2.5% at most. We examine how the mean pressure gradient is affected by the lateral walls, finding that the pressure drop follows a consistent 1/Re scaling regardless of the confinement of the bed. Finally, we observe that the pressure gradient balances the force exerted on the solid spheres with a very weak contribution of the wall friction, showing that the role of confinement corresponds to a global effect on the bed arrangement which in turns affects the mean pressure gradient

Hydrodynamics of gas-liquid dispersion in transparent Sulzer static mixers SMX TM

International audienceThe gas-liquid hydrodynamics in a Sulzer SMX TM static mixer was investigated in the present work through two different optical techniques: Backlight Shadowgraph Technique (BST) and Particle Image Velocimetry (PIV). 3D printed static mixers were manufactured using transparent plastic in order to provide optical access. The normal-heptane was used as the continuous liquid phase. Three different lengths of mixers and different gaseous nitrogen flow rates were investigated. The flow pattern in an empty tube without the mixing device was used as a reference. Bubble diameter distributions at the inlet and outlet of the SMX mixer were evaluated. The velocity fields inside the mixers were quantified. The gas holdup was also examined. These original results allow to appreciate the SMX static mixer'

Numerical prediction of Péclet number in small-sized fixed bed reactors of spheres

International audienceFluid dispersion in small fixed bed reactors under single phase laminar flow conditions is evaluated through accurate predictions of axial Péclet numbers Peax using Direct Numerical Simulations (DNS). The fixed bed packing is obtained through the Discrete Element Method (DEM) code Grains3D. Peax numbers are computed from steady state simulations of the flow. An additional set of equations solving the spatial distribution of the moments of the age distribution is used. This gives access to local information on dead zones or bypasses. Computations are realized with the OpenFOAM® open-source library. Results are close to the experimental data. The DEM-OpenFOAM® workflow benefits from an accurate control of the fluid physical properties and packing geometry, thus leading to a faster production of data than an experimental approach.We first explore the influence of the reactor-to-sphere diameter ratio δ (δ < 4), fluid velocity and molecular diffusivity on the Peax number. The behavior proves much more complex than any previous report, and it can be qualitatively explained by looking at the packing structure. Then, we use the DEM-OpenFOAM® workflow to quantify the uncertainty on the Peax number when repeating the packing of a δ = 4 fixed bed reactor, and we propose that the uncertainty on the Peax number decreases as a function of the reactor length to the power of −0.5

Turbulent Properties of Stationary Flows in Porous Media

International audienceIn this study, we investigate the flow dynamics in a fixed bed of hydrogel beads using particle trackingvelocimetry to compute the velocity field in the middle of the bed for moderate Reynolds numbers(Re = 124, 169, 203, 211). We discover that even though the flow is stationary at the larger scales, itexhibits complex multiscale spatial dynamics reminiscent of those observed in classical turbulence. Wefind evidence of the presence of an inertial range and a direct energy cascade, and are able to obtain a valuefor a “porous” Kolmogorov constant of C2 = 3.1 + ou - 0.3. This analogy with turbulence opens up newpossibilities for understanding mixing and global transport properties in porous media

Shear improved smagorinsky model for large eddy simulation of flow in a stirred tank with a rushton disk turbine

International audienceLarge eddy simulation (LES) is a very attractive method for simulations of reactive flows for a wide range of Reynolds number. In this approach, the effects observed at the large scales (for velocity and concentrations) are directly computed using modelled interactions with the small subgrid scales. Small scales tend to be more isotropic than the large ones, so it is easier to predict their behaviour using simpler and more universal models than RANS ones and called subgrid-scale (SGS) models. In this work, computational fluid dynamics (CFD) and particle image velocimetry (PIV) techniques have been used to describe the flow field in standard stirred tank equipped with a Rushton disk turbine (RDT). A new efficient and cost effective SGS model called shear improved Smagorinsky model (SISM) for large eddy simulation (LES) has been successfully implemented and validated in the open source CFD code “OpenFOAM”. The shear improved Smagorinsky model is capable of predicting turbulent near wall region accurately without any wall function. The model is based on results concerning mean-shear effects in wall-bounded turbulence. It takes into account the mean shear arising due to anisotropy of the flow. The proposed model, in addition to being physically sound and consistent with the scale-by-scale energy budget of locally homogeneous shear turbulence, has a low computational cost when compared to the original Smagorinsky model and possesses a high potential for generalisation to complex non-homogeneous turbulent flows since no geometrical argument enters in the definition of the eddy-viscosity. However, an appropriate average must be specified in the absence of homogeneity directions. For this purpose we use an adaptive Kalman filter. There was good agreement between the CFD simulations and PIV experimental results

Living Labs and other participatory approaches applied to research on multiple environnemental exposures and chronic risks

International audienceBACKGROUND AND AIM: Participatory research in environmental health is still rare in France. The objectives of environmental health research, in general, can be very diverse (e.g.: identifying situations associated with risks estimating exposures and effects, testing the effectiveness of preventive actions) and related methods are diverse as well. Opportunities for greater implication of the civil society and related challenges differ at each step of research activities. These aspects need to be better known and shared collectively. The LILAS project aimed to 1) co-construct, among institutional researchers, academics and civil society representatives, a mutual understanding of the main problematics and research methods in environmental health, their stakes for different actors, but also the requirements, strengths and limitations of these methods 2) identify expected benefits and points of vigilance related to stronger degrees of participation as part of such environmental health research projects. METHODS: LILAS gathered institutional researchers, academics and civil society representatives interested in multiple exposures (chemical, radiological). Several meetings allowed to collectively identify different types of study (including environmental epidemiological studies) and reflect about the added value, limitations, and methodological principles related to the introduction of growing participation as part of such studies. An analysis matrix was co-created and filled by participants. RESULTS:For different types of studies (studies for assessment of environmental exposures, identification of their determinants, interventions on these exposures, development of sensors, quantitative risk assessment, environmental epidemiological studies, experimental research, studies on the health of ecosystems…), the matrix lists expected benefits for several categories of stakeholders, fundamental methodological principles and practical constraints, advantages and limitations related to the use of participatory approaches (such as the living lab one) or more “classical” approaches. CONCLUSIONS:LILAS has allowed, through a cross-acculturation process, to develop consolidated grounds for the co-construction of future participatory research projects on multiple environmental exposures