Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes

First- and Second-order finite volume methods for the one-dimensional nonconservative Euler system

68 pagesGas flow in porous media with a nonconstant porosity function provides a nonconservative Euler system. We propose a new class of schemes for such a system for the one-dimensional situations based on nonconservative fluxes preserving the steady-state solutions. We derive a second-order scheme using a splitting of the porosity function into a discontinuous and a regular part where the regular part is treated as a source term while the discontinuous part is treated with the nonconservative fluxes. We then present a classification of all the configurations for the Riemann problem solutions. In particularly, we carefully study the resonant situations when two eigenvalues are superposed. Based on the classification, we deal with the inverse Riemann problem and present algorithms to compute the exact solutions. We finally propose new Sod problems to test the schemes for the resonant situations where numerical simulations are performed to compare with the theoretical solutions. The schemes accuracy (first- and second-order) is evaluated comparing with a nontrivial steady-state solution with the numerical approximation and convergence curves are established

A well-balanced approximate Riemann solver for compressible flows in variable cross-section ducts

International audienceA well-balanced approximate Riemann solver is introduced in this paper in order to compute approximations of one-dimensional Euler equations in variable cross-section ducts. The interface Riemann solver is grounded on VFRoe-ncv scheme, and it enforces the preservation of Riemann invariants of the steady wave. The main properties of the scheme are detailed. We provide numerical results to assess the validity of the scheme, even when the cross section is discontinuous. A first series is devoted to analytical test cases, and the last results correspond to the simulation of a bubble collapse

Modeling and simulation of fixed-bed reactors made of metal foam pellets

Offenzellige Metallschäume werden häufig als Katalysatorträger für katalytische Gasphasenreaktionen verwendet, da sie hervorragende Transporteigenschaften aufweisen. Aktuelle Fortschritte in den Herstellungstechniken haben zur Entwicklung von legierten Schäumen (z. B. NiCrAl, FeCrAl) mit verbesserter thermischer Stabilität geführt, die zu Drop-in Pellets für Festbettreaktoren geformt werden können. Die Metallschaum-Pellets gelten als vorteilhafte Alternative zu keramischen Katalysatorträgern, auch für den Einsatz in Festbettrohrreaktoren für großtechnische Prozesse wie die Dampfreformierung von Methan. Die gewundene Zellstruktur, die Strömungen innerhalb und zwischen den Partikeln in Verbindung mit den lokalen Effekten der Festbettstrukturen führen jedoch zu komplexeren Transportphänomenen bei Festbetten aus Metallschaumpellets im Vergleich zu Feststoffpellets. Daher ist es wichtig, ein grundlegendes Verständnis der zugrunde liegenden Transportprozesse zu haben, um die optimale Form der Metallschaumpellets für eine spezifische Betriebsbedingung zu bestimmen. In dieser Arbeit wird eine modifizierte Version des Partikelaufgelösten numerische Strömungsmechanik-Ansatzes präsentiert, um die Transportprozesse, insbesondere die Strömung und den radialen Wärmetransport, in schlanken Festbettreaktoren aus Metallschaumpellets zu untersuchen. Das synthetische Festbett wird mit der Rigid Body Dynamics (RBD)-Methode generiert, und die Transportgrößen werden in den Zwischenräumen vollständig dreidimensional aufgelöst. Die Strömung und der Wärmetransport im Inneren der Metallschaumpellets werden jedoch durch den Ansatz über ein poröses Medium unter Berücksichtigung geeigneter Submodelle behandelt. Für die Durchführung von Experimenten zum Druckverlust und der Wärmeübertragung wurden Pilotmaßstab-Reaktoren gebaut. Die CFD-Simulationen zeigen eine sehr gute Übereinstimmung mit den experimentellen Daten. Als Ergebnis wurde eine virtuelle Designplattform entwickelt, die es ermöglicht, den Einfluss verschiedener Formen und Morphologien von Metallschaumpellets sowie von Betriebsbedingungen wie Durchflussraten, Einlass- und Wandtemperaturen auf die Transportprozesse in solchen Festbettreaktoren zu untersuchen. Zur Optimierung der Metallschaumpellets wird die Gesamtleistung verschiedener Pelletkonfigurationen auf der Grundlage der wünschenswerten Eigenschaften eines Festbettreaktors, darunter niedriger Druckverlust, hoher Wärmeübergangskoeffizient, vergrößerter Oberfläche sowie hohe Katalysatorbeladung, analysiert. Darüber hinaus erfolgt eine umfassende Analyse der zugrunde liegenden Wärmeübertragungsmechanismen mithilfe von experimentellen Daten und Simulationen. Dies ermöglicht die Entwicklung von Korrelationen für kritische Wärmetransportparameter wie die effektive radiale Bettleitfähigkeit und die Wand-Fluid-Nusselt-Zahl. Abschließend wird ein vereinfachter CFD-Ansatz zur Modellierung katalytischer Schaumpellets vorgestellt, der auch die externen und internen Stoffübergangswiderstände in einem beschichteten Schaumpellet berücksichtigt.Open-cell metal foams have been widely used as catalyst supports for gas-phase catalytic reactions, as they exhibit excellent transport characteristics. Recent advancements in manufacturing techniques have led to the development of alloyed foams (e.g., NiCrAl, FeCrAl) with improved thermal stability, and these can be shaped into drop-in pellets for fixed-bed reactors. The metal foam pellets are regarded as a beneficial alternative to ceramic catalyst supports, also for the use in tubular fixed-bed reactors for large-scale processes like steam methane reforming. However, the tortuous cellular structure, intraparticle and inter-particle flows, combined with local bed structure effects, result in more complex transport phenomena for fixed-beds made of metal foam pellets, compared with solid pellets. Therefore, a thorough understanding of the underlying transport processes is important to find the optimal metal foam pellet shape relevant to a particular operating condition. This thesis presents a modified version of the particle-resolved Computational Fluid Dynamics (PRCFD) approach to investigate the transport processes, particularly flow and radial heat transport, in slender fixed-bed reactors made of metal foam pellets. The synthetic bed structure is generated using the Rigid Body Dynamics (RBD) method, and the transport quantities are fully resolved three-dimensionally in the interstitial spaces. The flow and heat transport inside the metal foam pellets are modeled, however, by the porous-media approach with appropriate sub-models. Pilot-scale reactors were built to conduct pressure drop and heat transfer experiments. The CFD simulations show very good agreement with experimental data. As a result, a virtual design platform has been realized for exploring the influence of different shapes and morphologies of metal foam pellets, as well as operating conditions, such as flow rates, inlet and wall temperatures, on transport processes in such fixed-bed reactors. To optimize the foam pellet shape, the overall performance of different pellet configurations is analyzed, based on the desirable properties of a fixed-bed reactor, such as low pressure drop, high heat transfer coefficient, increased surface area, and high catalyst inventory. Furthermore, a thorough analysis of the underlying heat transfer mechanisms is carried out with the aid of experimental data and simulations. This results in the development of correlations for critical heat transport parameters such as effective radial bed conductivity and wall-fluid Nusselt number. Finally, a simplified CFD approach to model catalytic foam pellets is illustrated, which also considers the external and internal mass transfer resistances in a washcoated foam pellet

Contribution to the modeling of packed bed reactors under plugging conditions in single and two phase trickle flow

Les réacteurs à lit fixe arrosé vers le bas en régime ruisselant se comportent comme des filtres en profondeur quand des liquides contaminés entrent en contact avec le lit. La rétention des solides de petite taille occasionne une augmentation progressive de la perte de charge. Éventuellement, l’opération du réacteur doit être interrompue et le lit colmaté est écarté, même si le matériel catalytique qui le constitue demeure encore actif occasionnant de la sorte des pertes économiques importantes. Cet ouvrage propose des méthodes et des modèles pour la simulation du colmatage du lit fixe avec des écoulements mono et biphasiques. Deux niveaux d’analyse sont présentés. Au niveau du lit complet, le modèle Eulérien-Eulérien, qui est une procédure de la mécanique des fluides numérique (CFD), permet l’inclusion des équations de fermeture pour le transfert de masse et de quantité de mouvement dans le contexte de la filtration en profondeur (deep bed filtration, DBF).. A l’échelle d’un seul élément de garnissage, l’analyse de trajectoire est couramment acceptée pour l’étude du taux de capture de particules dans le cadre de la filtration en profondeur dans le lit fixe. Dans le cas de l’écoulement monophasique, la capture de particules est calculée par l’expression de Rajagolapan & Tien (1976). L’insertion de cette expression dans le code CFD fourni des informations utiles à propos du comportement de la colonne en état transitoire. Dans le cas de l'écoulement biphasique en régime ruisselant, aucune procédure d’analyse de trajectoire n'est connue. En conséquence, une toute nouvelle adaptation de cette méthodologie est proposée. En utilisant un modèle de film pour représenter le réacteur à lit arrosé, l'analyse de TA est accomplie dans les cas suivants; monophasique et biphasique avec déposition monocouche et multicouche. Les tendances de TA concordent avec l'analyse de Rajagopalan et Tien (1976) démontrant que les mécanismes de capture sont du même type que ceux qui se présentent dans l'écoulement monophasique et qu’ils sont modifiés uniquement par la présence de la phase gazeuse. Les résultats ont été comparés aux données expérimentales de Gray et al. (2002). La rétention liquide statique (SLH) est un paramètre qui, selon des observations expérimentales, affecte sensiblement la capture en conditions multiphasiques. Une collection presque exhaustive des données de la SLH a été construite à partir de la littérature expérimentale disponible. Avec ces données de SLH et avec l'utilisation d'un algorithme considérant un minimum d'énergie de ménisque, des angles de contact moyennés pour une gamme de liquides et de garnissages ont été obtenus. En réinsérant les angles de contact calculés dans un logiciel de réseaux neuronaux, une corrélation qui surpasse toutes les corrélations disponibles a été obtenue. À l’avenir, il serait souhaitable que la rétention liquide statique soit incluse dans le modèle de colmatage, ou à tout le moins dans l’analyse des trajectoires.Trickle bed reactors (TBR) behave as deep bed filtration (DBF) units when the liquid feedstock is contaminated with fine particles. Solid retention causes an ever increasing pressure drop in the bed that leads to eventual halting of the installation. Industry response has been so far to change the plugged, but still active, catalytic bed with a fresh catalyst packing causing important profit losses of the process. In this work two levels of analysis are proposed for the DBF in single and two phase trickle flow conditions. At bed scale, an Eulerian-Eulerian CFD approach is used that provides the framework for the insertion of closure equations for the mass transfer in DBF. At pore scale, Trajectory Analysis (TA) is used as is an accepted procedure for the analysis of Deep Bed Filtration (DBF) in single-phase aqueous systems. In single phase flow through packed beds, the known TA based expression of Rajagolapan and Tien (1976) is used. By inserting this expression in the CFD approach it becomes possible to obtain valuable information about the transient structure and development of plugging. Benchmarking was obtained with the work of Narayan et al. (1997). In two phase trickle flow, no TA approach is known so far and an all new extension of this methodology is proposed in this work. Using a film model to represent the trickle bed reactor, TA analysis is performed in single phase, one-layer and multilayer deposition in TBR conditions. TA tendencies were akin to the analysis of Rajagopalan and Tien (1976) demonstrating that deposition mechanisms are of the same kind as in the single phase flow only modified by the presence of the gas phase. Results were compared with the data of Gray et al. (2002). Static liquid hold-up (SLH) is a parameter that, according to experimental observations, affects significantly solid deposition in multiphase conditions. An almost exhaustive collection of SLH values was constructed from the available experimental literature. With the SLH data and with the use of a minimum energy algorithm, average contact angles for a wide range of liquids and packing were obtained. Reinserting the calculated contact angles in neural network software, a correlation was obtained which outperforms all the available correlations. It is hoped that in future work, this last parameter, the SLH, will be included in the plugging model or at least in the trajectory analysis at the collector scale