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The computational fluid dynamics approach is applied to packed bed reactors with moderate tube-to-particle diameter ratios. In order to generate irregular particle arrangements within the reactors, a modified ballistic deposition method is used. While our previous studies were focused on pressure drop and mass transfer characteristics, the present paper uses the same framework to investigate the dispersion phenomena occurring in such reactors. Two different methods, namely the tracer and the post-processing method, are applied. The first method imitates the experimental procedure in which a non-diffusive tracer is injected into the packed bed. The residence time can be evaluated from the evolution of the tracer concentration. The second method is purely numerical and allows fast residence time estimation. Simulation results are successfully validated against experimental data from literature. The suggested numerical analysis is a valuable tool toward a deeper understanding of the fundamental transport phenomena in such reactors and, consequently, for the improvement of the packed bed performance.

Ergebnisse einer experimentellen und numerischen Untersuchung eines in einer wassrigen Phase aufsteigenden Toluol-Tropfens werden vorgestellt. Die experimentellen Untersuchungen wurden in einer Messzelle im Kleinstmasstab unter Zuhilfenahme von Hochgeschwindigkeitsmesstechnik durchgefuhrt. Die numerischen Untersuchungen erfolgten mithilfe des CFD-Programms COMCOL Multiphysics 3.3a und der dort implementierten Level-Set-Methode. Die gemessenen und die in Simulationen berechneten Tropfenaufstiegsgeschwindigkeiten zeigen eine sehr gute Ubereinstimmung und decken sich mit neueren Ergebnissen. Auch die Wiedergabe der Abweichung von der Kugelform bei verschiedenen Tropfengrosen war ausgezeichnet. Auf Basis der experimentell und numerisch erhaltenen Ergebnisse wird eine verbesserte Anpassung einer bestehenden Korrelation zur Berechnung der Tropfenaufstiegsgeschwindigkeit vorgestellt. The results of an experimental and numerical investigation of an organic toluene droplet rising in a continuous aqueous phase are presented. The experiments were carried out in a small-scale, high-speed measuring unit. The numerical simulations were performed with the aid of the level set method implemented in the commercial CFD tool COMSOL Multiphysics 3.3a. Both measured and simulated terminal velocities are found to be in excellent agreement and match recent results. Moreover, the deviation from the spherical form is captured very well for different droplet sizes. Based on the experimental and numerical data obtained, an improvement of an existing terminal velocity correlation is suggested.

A toluene droplet rising in a continuous aqueous phase is studied both experimentally and numerically. The experiments have been performed in a small scale, high-speed measuring unit with a high magnification device to image the process in all details. Numerical simulations are performed with the aid of the level set method implemented in the commercial CFD tool COMSOL Multiphysics 3.3a by COMSOL AB. A new technique for quantitative comparison of experimental and numerical results with respect to the droplet shape is presented. Both terminal velocity and droplet shape in experiments and simulations are found to be in excellent agreement. Based on the experimental and numerical data obtained, an improvement of an existing terminal velocity correlation is suggested. The adjusted correlation can be used for the design and optimisation of liquid–liquid extraction units.

A rigorous Computational Fluid Dynamics (CFD) based approach incorporating interfacial mass transfer at moving interfaces has been developed. The peculiarity of this approach is that it is able to govern multicomponent systems as well as interfacial boundary conditions in an arbitrary form. This is important in order to properly handle the typical concentration jump at the phase interface and to avoid an assumption of a constant distribution coefficient which is seldom met in real separation units. As a test system, the movement of a rising toluene droplet in quiescence aqueous phase is studied, whereas acetone is used as the transferring component. The level set method is applied for the description of the droplet movement. To give a first validation to the developed CFD model, the toluene droplet is first assumed to be stagnant and the mass transfer from/to the continuous aqueous phase is considered. A good agreement with analytical results from the literature is found in terms of the asymptotic value of mass transfer rate. Further, simulations for rising droplet are performed to incorporate the coupling between the momentum and mass transfer. The method provides concentration fields that are in qualitative agreement with the available data from the literature. Another test accomplished with an increased distribution coefficient also yields reasonable results.

The two-dimensional Kelvin-Helmholtz instability which arises in two immiscible co-currently moving horizontal liquid layers is studied numerically. Two different methods for capturing the free interface movement are applied, namely the volume-of-fluid method and the level-set method. Both methods are very popular and available within commercial computational fluid dynamics tools. The wave is initialized using two different perturbations implemented into the model. As a case study, a system comprising a toluene layer and an aqueous layer is chosen. Numerical results obtained by both methods are in a good agreement with the linear stability theory for small wave amplitudes. The application of both methods is compared and their advantages and drawbacks are highlighted.

In this work, the influence of the confining walls on pressure drop in packed beds is studied numerically for moderate tube/particle diameter ratios. Two different configuration types are investigated, a regular type and an irregular one. The regular configurations follow the atomic body-centered cubic and face-centered cubic structure in ideal crystals, whereas the modified ballistic deposition method is employed to generate the irregular configurations. To validate the simulation results, four experimental pressure drop correlations are used, namely by Ergun (1952) [1] , by Carman (1937) [2] , by Zhavoronkov et al. (1949) [3] and by Reichelt (1972) [4] . Simulation results for the regular configurations are in a good agreement with the Carman correlation. For the irregular configurations, which are closer to real packed beds, agreement with the correlations of Zhavoronkov et al. (1949) [3] and Reichelt (1972) [4] is better. This is explained by the fact that the latter two correlations take the wall effect into account. CFD simulations provide useful data on the flow field inside packed beds which can be used for the improvement and further optimization of the design and operation of packed bed reactors.

The reflection and transmission spectra of Pd capped MgyTi1-y thin films (y = 0.7, 0.8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepared and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a "black" absorbing one. The composition and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a MgyTi1-y switchable absorber in solar collectors allows to lower the stagnation temperature from 180 to 80 degrees C. The collector efficiency is affected only minimally. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

In this work, a computational fluid dynamics (CFD) approach is developed and applied to study the main design and operating parameters in packed bed reactors with moderate tube-to-particle diameter ratios. Packed bed reactors consist of irregular particle arrangements generated using a modified ballistic deposition method. The focus of these investigations is put on the pressure drop, mass transfer and dispersion phenomena in such reactors. Different numerical methods are applied and compared. The suggested numerical analysis represents a valuable tool towards deeper understanding of the fundamental transport phenomena in packed bed reactors and, consequently, for the improvement of the reactor performance.

The two-dimensional Kelvin-Helmholtz instability occurring in two immiscible co-currently moving horizontal liquid layers is studied numerically, using the volume-of-fluid method. To initialize the wave development, two different perturbation terms are implemented into the model. A three-component liquid system toluene-water-acetone is chosen, and both hydrodynamic and mass transfer studies are carried out. Numerical results are in a good agreement with experimental observations as well as the linear theory of instability.