A fundamental investigation of gas/solid mass transfer in open-cell foams using a combined experimental and CFD approach

In this work, we combine numerical (CFD) simulations and experimental measurements in a fundamental investigation of the fluid-solid mass transfer properties of open-cell foams, which are promising support for catalytic applications limited by external heat and mass transfer. CFD simulations are exploited to gain insight into the complex transport mechanisms and to enable a parametric analysis of the geometrical features by means of virtually-generated structures. Catalytic activity experiments under diffusion control are used to validate the CFD results and to extend the range of conditions and foam morphologies investigated. Analysis of the flow field by CFD simulations provides a rational basis for the choice of the average strut size as a physically sound characteristic length for mass transfer correlations. Results from both numerical simulations and experimental tests are interpreted according to a fully-theoretically based geometrical model for the prediction of the specific surface area, which accounts for the detailed node-strut geometry. The effects of cell size and strut shape are properly included in the functional dependence of the Sherwood number on the Reynolds number. The effect of porosity requires one additional dependence, wherein the Sherwood number is inversely proportional to the square of the void fraction. The resulting Sherwood–Reynolds correlation is in excellent agreement with experimental data and CFD simulations. It enables accurate (±15%) estimation of the external mass transfer coefficients for open-cell foams when coupled with the proposed geometrical model from two readily accessible pieces of geometrical information, i.e. the void fraction and either the cell size or the pore diameter of the foam. The derived correlation can be applied to the design of novel enhanced open-cell foam catalyst substrates and structured reactors

A systematic procedure for the virtual reconstruction of open-cell foams

Open-cell foams are considered a potential candidate as an innovative catalyst support in many processes of the chemical industry. In this respect, a deeper understanding of the transport phenomena in such structures can promote their extensive application. In this contribution, we propose a general procedure to recover a representative open-cell structure starting from some easily obtained information. In particular, we adopt a realistic description of the foam geometry by considering clusters of solid material at nodes and different strut-cross sectional shapes depending on the void fraction. The methodology avoids time-consuming and expensive measuring techniques, such as micro-computed tomography (μCT) or magnetic resonance imaging (MRI). Computational Fluid Dynamics (CFD) could be a powerful instrument to enable accurate analyses of the complex flow field and of the gas-to-solid heat and mass transport. The reconstructed geometry can be easily exploited to generate a suitable computational domain allowing for the detailed investigation of the transport properties on a realistic foam structure by means of CFD simulations. Moreover, the proposed methodology easily allows for parametric sensitivity analysis of the foam performances, thus being an instrument for the advanced design of these structures. The geometrical properties of the reconstructed foams are in good agreement with experimental measurements. The flow field established in complex tridimensional geometries reproduces the real foam behavior as proved by the comparison between numerical simulations and experiments

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

Influence of foam morphology on flow and heat transport in a random packed bed with metallic foam pellets: an investigation using CFD

Open-cell metallic foams used as catalyst supports exhibit excellent transport properties. In this work, a unique application of metallic foam, as pelletized catalyst in a packed bed reactor, is examined. By using a wall-segment Computational Fluid Dynamics (CFD) setup, parametric analyses are carried out to investigate the influence of foam morphologies (cell size φ = 0.45 –3 mm and porosity ε = 0.55–0.95) and intrinsic conductivity on flow and heat transport characteristics in a slender packed bed (N = D/dp = 6.78) made of cylindrical metallic foam pellets. The transport processes have been modeled using an extended version of conventional particle-resolved CFD, i.e., flow and energy in inter-particle spaces are fully resolved, whereas the porous-media model is used for the effective transport processes inside highly-porous foam pellets. Simulation inputs include the processing parameters relevant to Steam Methane Reforming (SMR), analyzed for low ( Rep ∼ 100) and high ( Rep ∼ 5000) flow regimes. The effect of foam morphologies on packed beds has shown that the desired requirements contradict each other, i.e., an increase in cell size and porosity favors the reduction in pressure drop, but, it reduces the heat transfer efficiency. A design study is also conducted to find the optimum foam morphology of a cylindrical foam pellet at a higher Rep ∼ 5000, which yields φ = 0.45, ε = 0.8. Suitable correlations to predict the friction factor and the overall heat transfer coefficient in a foam-packed bed have been presented, which consider the effect of different foam morphologies over a range of particle Reynolds number, 100 ≤ Rep ≤ 5000

Microtomography-based numerical simulations of heat transfer and fluid flow through β-SiC open-cell foams for catalysis

β-SiC open-cell foams are promising materials for catalytic supports with improved heat and mass transfer at moderate pressure drops. In this work, 3-dimensional (3D) models of a 30 ppi (pores per inch) β-SiC open-cell foam were generated using X-ray microtomography data. The resulting foam models were then used for finite element analysis (FEA) and computational fluid dynamics (CFD) simulations of heat transfer and fluid flow on the pore-scale. The FEA results demonstrate that (i) the overall effective thermal conductivity from direct simulations is comparable to the results estimated by experimental measurement, and are in the order of 10−1 W m−1 K−1 and (ii) thermal transport through fluid-saturated β-SiC foams depends on the solid-to-fluid conductivity ratio. By employing realistic foam models, pore-scale CFD simulations of fluid flows revealed the microscopic characteristics of laminar flow through open-cell foams. The anisotropic feature of realistic foam models promotes the axial and radial mixing of fluids in and after the foam element. The diffusion coefficient of laminar flow within foams was estimated at 10−4 m2 s−1, which is much larger than the molecular diffusion coefficient in a typical laminar flow in an open channel

Fluid dynamics and mass transfer in porous media: Modelling fluid flow and filtration inside open-cell foams

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Interface-Resolving Simulations of Gas-Liquid Two-Phase Flows in Solid Structures of Different Wettability

This PhD study is devoted to numerical investigations of two-phase flows on and through elementary and complex solid structures of varying wettability. The phase-field method is developed and implemented in OpenFOAM®. The numerical method/code is verified by a series of test cases of two-phase flows, and then applied to investigate: (1) droplet wetting on solid surfaces; (2) air bubble rising and interacting with cellular structures and (3) gas-liquid interfacial flows in foam structures

Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al2_{2}O3_{3} Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al$_{2}$O$_{3}$ layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEM$^{STAG}$. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat

CFD Investigation of an Innovative Additive Manufactured POCS Substrate as Electrical Heated Solution for After-Treatment Systems

In the last decade, additive manufacturing (AM) techniques have been progressively applied to the manufacturing of many mechanical components. Compared to traditional techniques, this technology is characterized by disruptive potential in terms of the complexity of the objects that can be produced. This opens new frontiers in terms of design flexibility, making it possible to create new components with optimized performances in terms of mechanical properties and weight. In this work, the focus is on a specific field of application: the development of novel porous media structures which can be the basis of advanced after-treatment systems for internal combustion engines. In particular, the possibility to design periodic open cellular structures (POCSs) that can be applied as catalytic substrates opens new perspectives in terms of flexibility and integrated functionalities. The present study investigates an innovative solution where the catalytic substrates are located in the pipes of the exhaust manifolds of a high-performance engine. A preliminary characterization of the pressure drop induced by the POCS structure is carried out, with a particular focus on the impact of the backpressure on the engine performances. Moreover, each POCS integrates an electrical circuit which is used to promote the heating of the device, with beneficial effects on the light-off of the catalytic reactions. An advanced CFD model is applied to evaluate the potential of the solution, comparing the pollutant conversion with that of the baseline configuration equipped with a standard after-treatment system solution