Aspect ratio of bubbles in different liquid media:a novel correlation

\u3cp\u3eThe bubble shape is a required parameter in the modeling and design of multiphase reactors. This communication contributes to the broader discussion and closes the knowledge gap by providing a practical correlation for the bubble shape. The correlation is based on a very large experimental dataset, encompassing a wide range of Morton numbers (Log\u3csub\u3e10\u3c/sub\u3e(Mo) in the range of −10.8 and 2.3), flow conditions (single bubbles and dense bubbly flows) and considering both gravity-driven flows and flows with an extra-external pressure gradient (counter-current flows). The experimental data were post-processed to derive a simple and physics-based correlation, relating the bubble aspect ratio to the bubble Reynolds and Eötvös numbers. This correlation provides a more accurate description and covers a wider range of applicability compared with literature correlations. As such, it can be helpful in the estimation of the interfacial area and velocity of a dispersed phase rising in a continuous phase.\u3c/p\u3

Direct numerical simulation of fluid flow accompanied by coupled mass and heat transfer in dense fluid-particle systems

In this paper we report the extension of an earlier reported DNS method (Deen et al., 2012 and Deen and Kuipers, 2013) based on a novel Immersed Boundary Method (IBM) which incorporates the fluid–solid coupling at the level of the discrete field equations. The extended method is used to study coupled mass and heat transport in dense fluid–particle systems where the coupling arises as a consequence of an exothermal chemical reaction proceeding at the exterior surface of the particles. Following a detailed verification (using an independent numerical technique) and validation (using established empirical correlations) we apply our DNS technique to study coupled mass and heat transfer in a dense fluid–particle system. In addition a comparison is made with results obtained from a simple one-dimensional (1D) heterogeneous reactor model which uses empirical closures for the fluid–particle mass and heat transfer coefficients. The main features of the complex transient temperature profiles obtained from our DNS agree quite well with the corresponding profiles obtained from the 1D heterogeneous reactor model

Two-phase PIV of bubbly flows: status and trends

No abstract

Discrete particle simulations of high pressure fluidization

Low density polyethylene and polypropylene are produced at large scale via the Unipol process. In this process catalyst particles are fluidized with monomer gas which reacts with the catalyst particles to form polymeric particles up to a size of 1 mm. The process is typically operated at pressures of 20 to 25 bar. Pressure impacts the hydrodynamics of the fluidized bed as it influences the bubble behaviour, particle mixing and heat transfer characteristics. Despite decades of research on fluid beds these effects are not completely understood. In order to gain more insight in the effects of operating pressure on the fluidization behaviour we have performed full 3D discrete particle simulations. We use a state-of-the art discrete particle model (DPM) to simulate fluidization behaviour at different pressures. In our model the gas phase is described by the volume-averaged Navier-Stokes equations, whereas the particles are described by the Newtonian equations of motion. The DPM accurately accounts for the gas-particle interaction, which is necessary for capturing the pressure effect. The simulation results were analysed with spectral analysis of the pressure drop fluctuations and analysis of the porosity field. In order to study the bubble behaviour, a sophisticated bubble detection algorithm was developed. From this algorithm, gas bubble characteristics, such as bubble velocity and bubble size are obtained. The simulation results show increasing emulsion porosity and decreasing bubble porosity with increasing pressure. In other words, the bubble-emulsion structure becomes less distinct. The determined bubble velocity is very well in accordance with empirical correlations for low pressures, and decreases at elevated pressures

Dense energy carrier assessment of four combustible metal powders

\u3cp\u3eMetal powders show great potential as dense energy carriers. This conceptual cycle for application presents a number of challenges which we address in this paper. In this study we narrowed down on four readily available promising candidates: aluminium, silicon, iron and zinc. Based on static power generation we estimated amounts required, transportation, cycle efficiency and physical explosion hazards. The scale required for transportation is much larger than in the current metal powder industry. The shipping requirements are comparable to coal. The handling hazards are only serious for aluminium. Iron and silicon emerge as the materials of choice.\u3c/p\u3

Characterizing solids mixing in DEM simulations

In the production and processing of granular matter, solids mixing plays an important role. Granular materials such as sand, polymeric particles and fertilizers are processed in different apparatus such as fluidized beds, rotary kilns and spouted beds. In the operation of these apparatus mixing often plays an important role, as it helps to prevent formation of hot-spots, off-spec products and undesired agglomerates. DEM can be used to simulate these granular systems and provide insight in mixing phenomena. Several methods to analyse and characterize mixing on basis of DEM data have been proposed in the past, but there is no general consensus on what method to use. In this paper we discuss various methods that are able to give quantitative information on the solids mixing state in granular systems based on DEM simulations. We apply the different methods to full 3D DEM simulations of a fluidized bed at different operating pressures. The following analysis methods will be investigated: average height method, Lacey index, nearest neighbours method, partner distance method and the sphere radius method. It is found that some of these methods are grid dependent, are not reproducible, are sensitive to macroscopic flow patters and/or are only able to calculate overall mixing indices, rather than indices for each direction. We compare some methods described in literature and in addition propose two new methods, which do not suffer from the disadvantages mentioned above. We applied each of these aforementioned methods to full 3D discrete particle simulations (DPM) with 280·103 particles and we performed simulations for seven different operating pressures. We found that, mixing improves with operating pressure caused by increased porosity and the increased granular temperature of the particulate phase

Bubble behaviour in fluidised beds at elevated pressures

Low density polyethylene and polypropylene are produced at large scale via the Unipol process. In this process catalyst particles are fluidised with monomer gas and grow into polymeric particles up to a size of 1¿mm. The process is operated at 20–25 bar. Pressure plays an important role in the hydrodynamics of the fluidised bed, which is reflected in bubble behaviour, particle mixing and heat transfer characteristics. Despite decades of research these effects are not completely understood. We use a state-of-the art discrete particle model (DPM) to simulate fluidisation behaviour at different pressures. The DPM is the most fundamental model suited for studying pressure effects in gas-fluidised beds, since it accurately represents the gas–particle interaction and particle–particle interaction. Fourier analysis of the bed pressure drop fluctuations and bubble properties (bubble velocity, bubble size, coalescence and break-up) obtained from a sophisticated bubble detection algorithm are presented

Numerical modeling of carbon dioxide chemisorption in sodium hydroxide solution in a micro-structured bubble column

\u3cp\u3eGas-liquid flows with solid catalyst particles are encountered in many applications in the chemical, petrochemical, and pharmaceutical industries. Most commonly, two reactor types, slurry bubble column (SBC) and trickle bed (TB) reactors are applied for large scale in the industry. Both of these types of reactors have some disadvantages limiting their efficiencies. To overcome the aforementioned disadvantages, a novel reactor type, micro-structured bubble column (MSBC), is proposed in Jain et al. (2013). In the MSBC, micro-structuring of the catalyst carrier is realized by introducing a static mesh of thin wires coated with catalyst inside the column. Wires also serve the purpose of cutting the bubbles, which in turn results in high interfacial area and enhanced interface dynamics. Moreover, the static catalytic mesh ensures lower cost by avoiding filtration of catalyst particles. In this paper, the MSBC is numerically studied using the hybrid volume of fluid - discrete bubble model (VOF-DBM) presented in Jain et al. (2014). The VOF-DBM is extended with a description of wire-meshes and the bubble cutting algorithm as introduced in Jain et al. (2013). Furthermore, a model for mass transfer with chemical reaction as developed by Darmana et al. (2007) is included in the model to study the impact of the wire-mesh and bubble cutting on the chemical reaction rate. In this work, first the model implementation and results are verified for a wide spectrum of parameters from the data available from previous studies, analytical results and experimental findings. Subsequently the model is applied to carry out a parameter study on the chemisorption of CO\u3csub\u3e2\u3c/sub\u3e in a NaOH solution, employing different mesh configurations. Since the reaction chosen here is very fast, mass transfer is the limiting step. It is found that the mass transfer can be considerably increased by stacking multiple meshes in the column.\u3c/p\u3