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The effect of elevating the spout on the dynamics of a spout-fluidized bed, both numerically and experimentally is studied. The experiments were conducted in a pseudo-two-dimensional (2-D) and a cylindrical three dimensional (3-D) spout-fluidized bed, where positron emission particle tracking (PEPT) and particle image velocimetry (PIV) were applied to the pseudo-2-D bed, and PEPT and electrical capacitance tomography (ECT) to the cylindrical 3-D bed. A discrete particle model (DPM) was used to perform full 3-D simulations of the bed dynamics. Several cases were studied, that is, beds with spout heights of 0, 2, and 4 cm. In the pseudo-2-D bed, the spout-fluidization and jet-in-fluidized-bed regime, were considered first, and it was shown that in the spout-fluidization regime, the expected dead zones appear in the annulus near the bottom of the bed as the spout is elevated. However, in the jet-in-fluidized-bed regime, the circulation pattern of the particles is affected, without the development of stagnant zones. The jet-in-fluidized-bed regime was further investigated, and additionally the experimental results obtained with PIV and PEPT were compared with the DPM simulation results. The experimental results obtained with PIV and PEPT agreed mutually very well, and in addition agreed well wtih the DPM results, although the velocities in the annulus region were slightly over predicted. The latter is probably due to the particle-wall effects that are more dominant in pseudo-2-D systems compared with 3-D systems. In the jet-in-fluidized-bed regime, the background gas velocity is relatively high, producing bubbles in the annulus that interact with the spout channel. In the case of a non elevated spout, this interaction occurs near the bottom of the bed. As the spout is elevated, this interaction is shifted upwards in the bed, which allows the bubbles to remain undisturbed providing the motion of the particles in the annulus near the bottom of the bed. As a result, no dead zones are created and additionally, circulation patterns are vertically stretched. These findings were also obtained for the cylindrical 3-D bed; although, the effects were less pronounced. In the cylindrical 3-D bed the PEPT results show that the effect on the bed dynamics starts at hspout =1 4 cm, which is confirmed by the ECT results. Additionally, ECT measurements were conducted for hspout =1 6 cm to verify if indeed the effect happens at larger spout heights. The root mean square of the particle volume fraction slightly increased at hspout =1 2 cm, whereas a larger increase is found at hspout = 4 and 6 cm, showing that indeed more bubbles are formed. The presented results have not been reported so far and form valuable input information for improving industrial granulators. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2524-2535, 2012 [ABSTRACT FROM AUTHOR]

In this work, a numerical model for Marangoni flow around a growing hydrogen bubble on a microelectrode is developed and validated. The growth of the bubble is implemented using a body-fitted moving mesh method that moves the bubble interface based on the production of hydrogen at the cathode. The dissolved hydrogen that is produced at the cathode diffuses into the bubble. The geometry of the bubble is a spherical cap with a pinned contact line and a decreasing contact angle as the bubble grows larger. There are two cases: (1) a case in which the bubble grows on a microbubble carpet that grows during the evolution of the bubble, and (2) a case in which the carpet thickness is negligible. The purpose of this model is to simulate and investigate thermocapillary Marangoni convection during almost the entire bubble lifetime (nucleation and detachment are not included). We validate the model with data from two experiments reported in literature (Massing et al. 2019; Bashkatov et al. 2022; Bashkatov, 2022). The evolution of the bubble radius, the temperature profile around the bubble, and the profile of the Marangoni velocity, which is tangential to the bubble surface at 5 μ m from the bubble, near the lower half of the bubble are accurately predicted. For the case with a bubble carpet, we investigated models with a growing carpet and fixed carpet thickness. We found that for a given bubble size the Marangoni velocity reduces with increasing carpet thickness. Stagnant bubble models are less complicated and computationally cheaper than the growing bubble model. For the case without a carpet, we compare the growing bubble model with two stagnant bubble models: one using constant current and another one using a transient current. A stagnant bubble model is recommended if the growth rate d r b / d t < 0. 05 u M where d r b / d t is the instantaneous bubble growth rate and u M the average Marangoni velocity at the bubble interface. This is the case at a relatively late stage of the bubble evolution. At the early stage of the bubble evolution, the absence of bubble growth dynamics in the stagnant bubble models appears to have an effect on the velocity profile and the pressure part of the hydrodynamic force. • Numerical simulations of a growing H 2 bubble on a microelectrode are performed. • A body fitted method is developed and validated with experimental data from literature. • Two stagnant bubble models are considered and compared to the growing bubble model. • The stagnant bubble model with transient current is accurate after a characteristic time. • The Marangoni flow and hydrodynamic force on a bubble reduce with increasing carpet thickness. [ABSTRACT FROM AUTHOR]

A novel experimental technique is introduced to study fluid-particle heat transfer in dense arrays of stationary particle. First a Constant Temperature Anemometer is reconfigured to a heat transfer probe. The thermal driving force is defined as the difference between the probe temperature and the initial fluid temperature and well known in our system. Second an abacus-like structure is employed to accurately control the solids volume fraction. The solids fraction was varied between 0 and 0.6 with increments of 0.1. The Reynolds number varied between 0 and 800. This combination of approaches allows for a very well-defined system, that can be studied both experimentally and numerically, and as such can serve as a validation of heat transfer studies with Direct Numerical Simulations, in fluid-particle systems. A single particle in unbounded flow, the effect of inter-particle distance and shielding effects for an inline array of three spheres as well as semi-structured arrays of particles are studied. [ABSTRACT FROM AUTHOR]

Gas-liquid-solid flows are frequently encountered in chemical, petrochemical and biochemical industries. To overcome the heat and mass transfer limitations in trickle bed reactors and bubble slurry columns, respectively, a micro-structured bubble column (MSBC) can serve as an attractive alternative. In a MSBC, wire meshes are introduced to cut the bubbles in smaller bubbles and enhance the surface renewal (and hence gas-liquid mass transfer) rates, by deformation of the bubbles. Earlier (Jain et al., 2013) modeling efforts using the Euler-Lagrange approach to simulate a micro-structured bubble column employed a bubble cutting closure based on purely geometrical considerations. To improve on this ad hoc procedure in this paper we explore the possibilities of Direct Numerical Simulations to gain more insight in this complex phenomenon with the ultimate aim to develop improved closures. A combined Volume of Fluid-Immersed Boundary method was applied to simulate the interactions between bubbles and wire meshes. When the bubbles are aligned with the opening of the wire mesh, cutting of the bubbles is not observed in our simulations, while cutting was expected based solely on geometrical considerations. When the Eötvös number, Eo , is larger than 4, the bubbles are highly deformable and squeeze themselves through the opening of the wire mesh. In addition, the bubble gets stuck underneath the mesh when the bubbles are small ( Eo ⩽ 4 ) and/or the opening is in the wire mesh is small. Almost all bubbles that hit the intersection of two crossing wires get stuck underneath the mesh, except for large bubbles ( Eo = 15 ), which get cut by the mesh. Based on these results, it is concluded that the cutting of bubbles depends on the Eötvös number, the opening of the wire mesh and geometrical considerations. However, the results also seem to indicate that the diameter of the wire mesh will also influence the cutting behavior. [ABSTRACT FROM AUTHOR]

Gas–liquid–solid three phase flows are commonly found in (bio-)chemical processes, e.g. in bubble slurry columns in the Fischer–Tropsch process. In order to facilitate the rational scale-up and the design of such columns, a detailed understanding of the complex phase interactions is required. In this work, we focus on the effective drag acting on particles and bubbles, using Direct Numerical Simulations. We combined the Front Tracking method of Roghair et al. (2013b) and the second order implicit Immersed Boundary method of Deen et al. (2012) . The resulting hybrid Front Tracking Immersed Boundary method is able to simulate dense three phase flows and is used to study swarm effects in terms of drag. A correlation has been obtained for the drag coefficient for a system consisting of 2 mm bubbles and 1 mm particles at several phase volume fractions. In future research, the developed methodology can be applied to study the effect of the bubble and particle size and their ratio. [ABSTRACT FROM AUTHOR]

This paper reports our study on fluidization of 5000 spherical particles in a pseudo-2D gas-fluidized bed by direct numerical simulations (DNS) and experiments as well. Simulations are performed using an immersed boundary method, together with the methodology developed in our earlier work for accurate prediction of gas–solid interactions at relatively low grid resolutions. This modelling approach provides detailed information on the gas flow and the motion of individual particles, which allows for a priori calculation of the bed hydrodynamics. Experimental measurements of solids mean motion are conducted using a combined technique of Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA). Further, the PIV technique is extended and applied for instantaneous measurements of the particle granular temperature, which is the key characteristics of particle velocity fluctuations. For the first time, this paper reports a direct comparison in great detail between DNS results and experimental data for realistic gas fluidization. The detailed comparison reveals a reasonably good agreement with respect to the time-averaged solids motion and the pressure fluctuations. In addition, the granular temperatures calculated from the simulations agree well with the experimental data, but provide more details with respect to the variations corresponding to bubble formation and eruption. From our investigation, it also becomes clear that attention should be paid on the measurement and interpretation of the granular temperature. [ABSTRACT FROM AUTHOR]

In this work we study heat transfer from a complex geometry consisting of crossing cylinders subject to forced convection. For several sizes of a wire-mesh inserts direct numerical simulations (DNS) are performed using an implicit implementation of the immersed boundary method (IBM). The local heat flux is studied and compared to the total heat flux. A heat transfer correlation is derived describing the Nusselt number as a function of the Reynolds number and the ratio of the pitch to the cylinder diameter, p / D . [ABSTRACT FROM AUTHOR]

Abstract: In blast furnaces, particles like coke, sinter and pellets enter from a hopper and are distributed on the burden surface by a rotating chute. Such particulate flows suffer occasionally from particle segregation during transportation caused by differences in density or size. To get a more fundamental insight into these effects, we started an experimental study to investigate the effect of rotation on such particulate flows. Here, as a first step, we present an experimental study of granular flow of monodisperse 3mm spherical glass particles flowing with a constant mass rate through a rotating smooth rectangular chute, which is inclined at a fixed angle. Experiments are performed for a sufficiently long time to study steady (but streamwise accelerating) flow. Particle image velocimetry (PIV), electronic ultrasonic height sensors, and a dynamic weighing scale are used to measure the surface velocity of the particle stream, the particle bed height and mass flow rate in the chute, respectively. The influence of rotation speed and angle of inclination of the chute is studied. We find an interesting interplay between the Coriolis force, which pushes the granular flow to the sidewall of the chute and tends to diminish the acceleration of the flow, and centrifugal forces that accelerate the flow. The velocity components display a complex dependence on the spanwise and streamwise position in the chute. The bed height in the chute is also influenced by these effects of system rotation. These measurements provide a well-defined set of observations for refining and validating computer simulations of granular flows, and point out some important limitations of physical experiments. We present preliminary three-dimensional discrete particle simulations, which show that the experimental measurements of bed height and surface particle velocity in a chute inclined at 30° can be predicted nearly quantitatively both without and with rotation of the chute. [Copyright &y& Elsevier]

Abstract: Bubble column reactors suffer from bubble coalescence resulting in decreasing efficiency. In order to increase the mass transfer rate in bubble column reactors, a novel reactor is proposed, the micro-structured bubble column (MSBC). The new reactor makes use of a wire mesh serving the purpose of cutting bubbles into smaller bubbles. This increases bubble surface area and enhancing the mass transfer coefficient, due to the enhanced surface dynamics. The first step is understanding of fluid–structure interaction in the limiting case of single-phase flow past crossing cylinders not found in literature. To this end Direct Numerical Simulations (DNS) are performed, where the presence of the wire mesh consisting of intersecting cylinders is accounted for using an Immersed Boundary Method (IBM). The IBM is verified with existing literature data for flow past a single cylinder. Finally a new set of simulations is done for intersecting cylinders varying in cylinder diameter and Reynolds numbers. From these simulations a new drag relation is derived describing the drag as function of the Reynolds number and the pitch to cylinder diameter, p/D, ratio. [Copyright &y& Elsevier]

Abstract: The present paper describes an Euler–Lagrange model utilizing a drag closure derived from direct numerical simulations (front-tracking model) for (i) single isolated bubbles and (ii) bubbles rising in bubble swarms, expressed as a function of the local gas fraction. The model is applied to the prediction of an air/water system in a bubble column and for which experimental data is available. The effect of variation in size of the mapping window for the interphase coupling between the Eulerian and Lagrangian framework is investigated for both closure relations. It is found that the drag closure as a function of the local gas fraction is an improvement over the use of the drag closure for isolated single bubbles for the prediction of bubbly flow. [Copyright &y& Elsevier]

Abstract: Gas–liquid–solid three phase flows are encountered in for example the Fischer–Tropsch process for the production of synthetic fuels in bubble slurry columns. To predict the hydrodynamics in large slurry bubble columns a multi-scale modeling approach can be used, which accounts for the large variation in time and length scales. In this paper, the smallest scale model has been developed using the Front Tracking model of Dijkhuizen et al. (2010b) and the Immersed Boundary model of Kriebitzsch (2011). In the Front Tracking model, each bubble is tracked separately. Furthermore, the Immersed Boundary method introduces the particle–fluid and the particle–particle (via a hard sphere model) interactions in the model. The resulting hybrid Front Tracking Immersed Boundary model is able to simulate dense three-phase flows and accounts for swarm effects in a fundamental manner. From our simulations we found that the relative drag coefficient of bubbles in three-phase flows seems to increase with increasing solids volume fraction. However, longer averaging periods are needed to derive a fully predictive correlation for the relative drag coefficient with respect to the solid volume fraction. [Copyright &y& Elsevier]