Mesures de stéréo-PIV dans une cuve agitée

International audienceLes écoulements en cuve agitée se retrouvent dans de nombreuses applications industrielles comme l’agroalimentaire, le pharmaceutique, ou encore l’industrie chimique. Les écoulements présents dans ce type d’installation rotative sont largement tridimensionnels et particulièrement non symétriques lorsque la cuve est munie de contre pales. La modélisation de ces écoulements n’est donc pas aisée, et se complexifie d’autant plus lorsque plusieurs phases coexistent dans la cuve.L’objectif de cette étude conjointe entre l’Institut von Karman et le CETIM est de créer une base de don-nées expérimentale pour la validation des codes numériques destinés à la simulation d’écoulements en cuve agitée. Le projet comprend plusieurs étapes qui vont de mesures en écoulement monophasique, jusqu’à des mesures diphasiques de mélange solide – liquide et liquide – gaz. Cette communication traite des résultats expérimentaux de la première étape qui se focalise sur l’écoulement purement liquide.L’installation expérimentale est une cuve cylindre transparente en Plexiglass dont la hauteur et le diamètre font 638 mm. Elle comprend trois contre-pales, elles aussi en Plexiglass, séparées de 120°, et descendant jusqu’à quelques millimètres de son fond plat. L’axe central de rotation est muni une turbine à 4 pales. Deux turbines, de diamètre égal à 40% de celui de la cuve, sont testées ; la première est constituée de pales à 90°alors que dans la secondes les pales sont inclinées à 45°. La turbine se situe à 30% du fond de cuve.L’étude expérimentale utilise l’eau comme fluide pour les mesures en régime turbulents. Les trois composantes du champ de vitesse dans un plan horizontal situé au milieu des pales, et dans un plan vertical situé en aval d’une contre-pale sont mesurées par application de la stéréo-PIV (SPIV). Les écoulements créés par les deux types de pales avec sans contre-pales, et ce pour trois vitesses de rotation différentes (3 tpm, 15 tpm et 90tpm), sont comparés. L’analyse des mesures SPIV dans le plan horizontal ont nécessité l’implémentation d’un système dynamique de masque afin d’éliminer les erreurs dues à la présence de réflexions de particules sur les pales, du fait de leur caractère transparent.Les champs de vitesse moyen et de l’intensité de turbulence seront présentés. Du fait de la rotation des pales,deux définitions de valeur moyenne seront considérées ; tout d’abord une moyenne sur toutes les images mesurées, puis une moyenne effectuée uniquement sur les images correspondant à une position identique des pales.Enfin, les mesures effectuées à une vitesse de 15 tpm permettront de montrer, via une analyse fréquentielle des champs de vitesse, quelle partie de l’écoulement est directement influencée par la rotation des pales

Etude expérimentale et modélisation de phénomènes aggravants, BLEVE et BOILOVER

The present thesis is conducted in the frame of a research project involving the ‘von Karman Institute (VKI Belgium)’ and the ‘Ecole des mines d’Alès (EMA France) with the support of the CEA Gramat. This project is about theoretical study, experimental characterization and modeling of hazards from pressurized or atmospheric reservoirs, containing liquids, flammable or not. The objective of this thesis is to study the apparition criteria and the consequences of an accident involving a container of pressure liquefied gas (BLEVE phenomenon) or liquid fuels (Boilover phenomenon). After a bibliographic research on the two phenomena, an experimental study in laboratory scale is conducted and from the results, the phenomena and their hazards are modeled. Small scale experiments are performed in the BABELs facility (Bleve And Boilover ExperimentaL setup) that consists of a cylindrical chamber of 2m diameter and 3m high, with round shape flanges, made out of steel with a rated pressure of 0.5 MPa. It has 3 series of 7 optical accesses, an entrance door, and an optional air venting system. A Boilover is a violent ejection of fuel due to the vaporization of a water sublayer, resulting in an enormous fire enlargement and formation of fireball and ground fire. Small scale experiments with cylindrical reservoirs of 0.08 to 0.3m diameter in glass or metal, filled with a mixture of diesel and oil have been performed. Instrumentation of the measurements consists of thermocouples rake, Medtherm radiometers, load cell and CCD or high-speed camera with a fisheye. During the quasi-steady combustion prior the Boilover, the typical variables describing a pool fire like burning rate, flame size, puffing frequency and radiation can be predicted with semi-empirical correlations available in the literature. At Boilover onset, high speed visualizations in glass reservoir show that the growth of one big bubble leads to a boiling front that propagates radially all along the fuel-water interface, ejecting the upper fuel layer and leading to the increase of flame size. LS-PIV technique applied to high-speed camera images shows that the flame enlargement is directly linked to the velocity of the flame.A BLEVE (or Boiling Liquid Expanding Vapour Explosion) is an explosion resulting from the catastrophic failure of a vessel containing a liquid at a temperature significantly above its boiling point at normal atmospheric pressure. Small scale experiments are performed with cylinders of 42g of propane, laid horizontally and heated from below by an electrical resistor. A groove of the reservoirs on the upper part allows better reproducibility of the rupture. High speed visualization and shadowgraphy are helping in visualizing the rupture and the content release. Thermocouples and PCB are also used to measure respectively the temperature and the blast wave overpressure. These experiments show that the fluid behavior during rupture differs with the size of the weakened part and therefore with the rupture pressure. The internal pressure measurement showed that the rupture pressure and temperature are supercritical, leading to the definition of a new type of BLEVE since there is no distinction between liquid and vapor phases prior rupture.Doctorat en Sciences de l'ingénieurinfo:eu-repo/semantics/nonPublishe

Heavy gas concentration prediction on complex terrain using CFD with Monin-Obukhov similarity theory

PresentationUnder stable atmospheric conditions (i.e., low wind speed and low heat radiation), once heavy gases (e.g., CO2, H2S, LNG) release to the atmosphere, the gas clouds tend to stay near the ground for long period of time, thereby causing high concentration zone and increasing the threats to the local population and the environment. Despite of the advanced development of computational fluid dynamic (CFD) modelling, or the Reynolds-Averaged Navier-Stokes (RANS) models with standard turbulence closures (e.g., standard , RNG and ), researchers have pointed out that these models are incompatible with the experimental data under stable atmospheric conditions. Therefore, there is an increasing interest in developing a robust mathematical model for heavy gas dispersion, especially in the field of turbulence modelling. This present study is to develop a CFD model with a two-equation turbulence model for heavy gas dispersion over complex geometry in stable atmospheric conditions. This two-equation turbulence model is a modified turbulence model based on the Monin-Obukhov similarity theory (MOST). The calculations from the modified turbulence model can maintain the homogeneity of the flow properties. The calculations from the CFD model with the modified model is compared with the experimental data collected from the Kit Fox experiment under stable atmospheric conditions (Class F). A robust and reliable model can provide potential guidelines for emergency mitigation planning for heavy gas leakage incidents

A stabilized finite element framework for anisotropic adaptive topology optimization of incompressible fluid flows

<p>Shape or topology optimization, aims to find the most convenient material distribution in order to minimize a given criteria, that is the cost function. In this work, we will demonstrate our application of topology optimization based on the Level Set method for reducing head losses. Anisotropic mesh adaptation was applied to capture the interface, and the sensitivity was calculated based on the continuous adjoint method. Three numerical examples were given, where we seek on one hand to validate the algorithm, and on the other hand, to demonstrate the effect of the starting geometry on the final shape.</p&gt

Quantitative Investigation of Ballistics Flow Fields by Background Oriented Schlieren Technique

Abstract The ballistics field is known by the presence of several complex phenomena such as muzzles and flying projectiles flow fields. Consequently, numerical simulations are commonly used to model these complicated flows. However, the validation process of these codes has proven to be problematic due to the lack of experimental quantitative data. In this context, the present paper describes the application of the Background Oriented Schlieren technique (BOS) as a quantitative investigation tool in the ballistics field. We illustrate that BOS can accurately capture the main characteristics of the studied configurations: Firstly, we discuss the visualization and the density field reconstruction around a Bullet Simulated Projectile BSP flying at supersonic velocities and a sniper projectile flying at supersonic and transonic velocities. We demonstrate that these fields are in satisfactory agreement with the results of Taylor and Maccoll’s equation and numerical simulation. Then, the findings of the BOS visualization of the precursors and the propellant flow fields are presented. To this end, the salient features accurately captured by the BOS technique such as vortex rings, shock bottles, Mach, and blast wave are described both qualitatively and in terms of density profiles. Two improved approaches that are essential to the aforementioned analysis are proposed: the first is related to density field reconstruction based on Abel inversion and the second approach is a phase separation procedure.info:eu-repo/semantics/publishe

A new approach for the reconstruction of axisymmetric refractive index fields from background-oriented schlieren measurements

Background-oriented schlieren (BOS) is an optical visualization technique that reconstructs a whole-field flow based on its density gradient. Thanks to the cost-effectiveness of its optical setup and its unlimited field of view, BOS has become an attractive technique for laboratory and large-scale experiments. In BOS, the reconstruction of the refractive index field involves various mathematical calculations, which depend on the flow geometry. In this context, the present study presents a new method for the reconstruction of three-dimensional axisymmetric refractive index fields. The proposed method takes advantage of the bi-sensitivity quality of BOS to gain more accuracy in the reconstructed fields. We demonstrate the method’s precision and robustness against experimental noise throughout a synthetic axisymmetric refraction index field. The application of this new approach is not restricted to the BOS technique but can be extended to several other measurement techniques such as moiré deflectometry, laser speckle photography, and rainbow schlieren.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

On the Application of Background Oriented Schlieren Technique on Ballistics Field

The ballistics field is known by the presence of several complex phenomena such as muzzle and flying projectiles flow fields. Consequently, numerical simulations are commonly used to model these complicated flows. However, the validation process of these codes has proven to be problematic due to the lack of experimental quantitative data. In this context, the present paper describes the application of the Background Oriented Schlieren technique (BOS) as a quantitative investigation tool in the ballistics field. We illustrate that BOS can accurately capturethe main characteristics of the studied configurations: Firstly, we discuss the visualization and the density field reconstruction around a sniper projectile flying at supersonic and transonic velocities. We demonstrate that these fields are in satisfactory agreement with numerical simulation. Then, the findings of the BOS visualization of the precursors and the propellant flow fields are presented. To this end, the salient features accurately captured by the BOS technique such as vortex rings, shock bottles, Mach disk, and blast wave are described both qualitatively and in terms of density profiles. Two improved approaches that are essential to the aforementioned analysis are proposed: the first is related to density field reconstruction based on Abel inversion and the second approach is a phase separation procedure.info:eu-repo/semantics/publishe

An advanced aero-thermodynamic study of a heart-shaped dimpled pipe

The detailed flow field and local heat transfer enhancement in a pipe with heart-shaped dimples on the internal surface were experimentally studied. The flow structures were investigated at a flow regime of Re=20k in five different measurement planes around a dimple in the streamwise and spanwise directions using both planar PIV and stereo PIV. A ray tracing-based image correction method was utilized to overcome the important image distortions caused by the optically complex geometry of the dimple for the planar PIV measurements. Furthermore, the convective heat transfer coefficient on the wall of the dimple was investigated utilizing steady-state liquid crystal thermography (LCT) and infrared thermography within the flow regime range of Re=20k-60k. A comprehensive uncertainty analysis was conducted to account for the multiple measured quantities and corresponding error sources on such a complex geometry. The resulting heat transfer and flow fields revealed that maximum heat transfer enhancement is concentrated along the reattachment line downstream of the dimple where higher kinetic energy levels are observed. The enhancement factor, which is a measure of the heat transfer improvement relative to a smooth pipe, was evaluated on average at 1.7 across the dimpled surface, with local maxima reaching values up to 2.8. The dimpled pipe under examination had a relative skin friction coefficient of approximately 1.8 compared to a smooth pipe. This outperforms some of the roughness elements, such as ribs and spherical dimples, previously studied on similar circular channels in literatur

Ray tracing-based PIV of turbulent flows in roughened circular channels

Particle image velocimetry (PIV) is more and more used as a reference method for the measurement of velocity fields. However, this technique requires optical access and the current distortion correction methods are efficient only for small optical distortions. The motivation of the present study is to properly determine the velocity field of the turbulent flow in a channel with optically complex-shaped obstacles, designed for heat transfer enhancement. A ray tracing-based image correction method is employed to eliminate high-level image distortions on PIV images induced by heart-shaped dimples. To reduce the uncertainties in the application of the method, an optimization algorithm is built for artificially recreating the PIV calibration image using rendering software. The positions, material properties and dimensions of the objects in the experimental setup, which construct the 3D model, are considered as the design parameters. The artificial image was obtained with a standard deviation of 0.13 pixels from the actual calibration image in 4-5 h. In the calibration process, the ray tracing-based correction with the optimized artificial image provided a standard deviation of 0.32 pixels from the reference grid while the third-order polynomials had provided 9.6 pixels. To illustrate the approach, measurements were acquired on the center plane of a circular channel with the heart-shaped dimples in the streamwise direction. The 2D velocity and turbulent kinetic energy field obtained at a Reynolds number of around 20,000 showed that the flow separates as it reached the leading edge of this dimple whereas the reattachment point was captured at the trailing edge. The highest amounts of turbulent kinetic energy were found just downstream of the dimple where the best heat transfer was expected

Multi-Objective Topology Optimization of Conjugate Heat Transfer Using Level Sets and Anisotropic Mesh Adaptation

This study proposes a new computational framework for the multi-objective topology optimization of conjugate heat transfer systems using a continuous adjoint approach. It relies on a monolithic solver for the coupled steady-state Navier–Stokes and heat equations, which combines finite elements stabilized by the variational multi-scale method, level set representations of the fluid–solid interfaces and immersed modeling of heterogeneous materials (fluid–solid) to ensure that the proper amount of heat is exchanged to the ambient fluid by solid objects in arbitrary geometry. At each optimization iteration, anisotropic mesh adaptation is applied in near-wall regions automatically captured by the level set. This considerably cuts the computational effort associated with calling the finite element solver, in comparison to traditional topology optimization algorithms operating on isotropic grids with a comparable refinement level. Given that we operate within the constraint of a specified number of nodes in the mesh, this allows not only to improve the accuracy of interface representation and motion but also to retain the high fidelity of the numerical solutions at the grid points just adjacent to the interface. Finally, the remeshing and resolution steps both run within a highly parallel environment, which makes it possible for the proposed algorithm to tackle large-scale problems in three dimensions with several tens of millions of state degrees of freedom. The developed solver is validated first by minimizing dissipation in a flow splitter device, for which the method delivers relevant optimal designs over a wide range of volume constraints and flow rate distributions over the multiple outlet orifices but yields better accuracy compared to reference data from literature obtained using uniform meshes (in the sense that the layouts are more smooth, and the solutions are better resolved). The scheme is then applied to a two-dimensional heat transfer problem, using bi-objective cost functionals combining flow resistance and thermal recoverable power. A comprehensive parametric study reveals a complex arrangement of optimal solutions on the Pareto front, with multiple branches of symmetric and asymmetric designs, some of them previously unreported. Finally, the algorithmic developments are substantiated with several three-dimensional numerical examples tackled under fixed weights for heat transfer and flow resistance, for which we show that the optimal layouts computed at low Reynolds number, that are intrinsically relevant to a broad range of microfluidic application, can also serve as smooth solutions to high-Reynolds-number engineering problems of practical interest