Draft Plates Research Articles

Simulation of spray coating in a spouted bed using recurrence CFD

Although numerical models such as the computational fluid dynamics–discrete element method (CFD–DEM) have enabled the accurate simulation of laboratory-scale apparatuses, the application of these methods to large-scale apparatuses with many particles and time scales ranging from minutes to hours remains a challenge. The recently developed recurrence CFD (rCFD) method seeks to overcome these issues in pseudo-periodic processes by extrapolating globally recurring patterns in a physically meaningful way and describing the transport and interaction of passive scalars using Lagrangian tracers. Spouted beds represent an interesting target because of the associated variety of flow regimes. They can be effectively described by CFD–DEM on the time scale of tens of seconds, whereas industrially relevant processes typically take hours. In this contribution, we established the validity of applying the Lagrangian rCFD method to spouted beds by demonstrating the accurate reproduction of the particle residence time distribution in a fictitious spray zone. The deposition of spray droplets onto tracer particles was simulated for 1h, and the particle surface coverage distribution was estimated using a statistical approach for both an unstabilized prismatic spouted bed and one stabilized by draft plates.

Investigation of hydrodynamics and heat transfer in pseudo 2D spouted beds with and without draft plates

Abstract In the present study, hydrodynamics and gas to particle heat transfer in pseudo two dimensional spouted beds (2DSB) with and without draft plates were investigated using the Eulerian-Eulerian approach. The main objective of the study was to provide an understanding of effects of the presence of draft plates on the hydrodynamics and heat transfer behavior of solid particles in the spouted beds. To validate the model, the predicted mean particle vertical velocity at the bed axis, the lateral profiles of vertical particle velocity at different bed heights for both systems, and the particle velocity vector fields in the beds were compared with the experimental measurements. A close agreement between the CFD results and the experimental data was found for both systems. The simulation results showed that the particle volume fraction in the spout and fountain regions of the spouted bed with draft plates is considerably lower than that in a conventional spouted bed (without draft plates). Simulation results also showed significant differences between the temperature distributions of gas and solid phases in spouted beds with and without draft plates.

CFD Simulation of Different Flow Regimes of the Spout Fluidized Bed with Draft Plates

Spout fluidized bed has shown promising for gas-solid contact operations with and without chemical reactions, such as drying, coating, granulation, gasification, pyrolysis, etc. This is because these beds combine features from both spouted and fluidized beds. The other point is the ability to treat chemical transformations involving both heat and mass transfer in combination with particles of various sizes. Therefore, it is extremely important the knowledge of fluid dynamic of the bed, mainly for scale-up projects, which makes computer simulation an essential tool. Researches using the Computation Fluid Dynamics (CFD) proved to be very effective in predicting of particles dynamic in this type of bed. In Computation Fluid Dynamics, the two phases are treated as interpenetration continuous, and these phases are described by equations of conservation of mass, momentum and energy. The goal of the present work was to simulate using CFD experimental fluid dynamics data of a spout fluidized bed. Eight distinct flow regimes were identified which showed up in good agreement with the regime map presented in literature. The results showed that the technique was efficient for the simulation of the hydrodynamic of the bed presented.

Impact of draft plate on the inter-chamber interaction in a two-chamber spout-fluid bed

The widespread application of the spout-fluid bed in the processing of thermoplastic composites, coal gasification, municipal solid waste gasification, and biomass gasification necessitates a deeply understanding on the flow mechanism of dense two-phase flow in the system in order to improve its design optimization and operation. By tracking the gas and solid phases respectively in the Eulerian and Lagrangian framework, the dense gas-solid flow in the two-chamber spout-fluid bed with and without draft plate was carried out using the computational fluid dynamics coupled with discrete element method (CFD-DEM) to evaluate the impact of the presence of draft plates and the plate length on inter-chamber interaction. The results demonstrate that the interaction of rising bubbles with the spouting channel gives rise to the unfavorable dancing spout phenomenon, which can be alleviated by draft plates. Insertion of the plates lowers the pressure drop magnitudes of both the spouting and background inlets, and the correlation coefficient of the two signals. Furthermore, the draft plates enhance the turbulent intensity of the solid phase in the fountain region and the vertical dispersion in the spout region. Increasing the draft plate length has a significant influence on gas-solid hydrodynamics and chamber interaction.

CFD-DEM modeling of a three-dimensional prismatic spouted bed

In this contribution coupled CFD-DEM-simulations of a laboratory prismatic spouted bed with two horizontal gas inlets are presented. With regard to a feasible simulation time, the simulations are parallelized and a coarse-graining approach is used in order to handle a high amount of particles. Different drag models are used within the simulations to evaluate their applicability to represent the experimental results. The spouting stability is analyzed by means of the time-dependent pressure drop behavior. The influence of draft plates in the process chamber on the particle flow patterns and spouting stability is investigated both numerically and experimentally. By means of simulations, the residence times of particles within the spray zone are evaluated for different gas flow rates.

Computational Study of the Effect of Draft Plates on the Solid Behavior in a Spout-Fluid Bed

Using computational fluid dynamics coupled with the discrete element method (CFD-DEM), the gas–solid flow in the spout-fluid bed with or without draft plates is numerically simulated to quantify the effect of draft plates and plate length on the behavior of the solid phase. The mixing behavior, force, velocity, and dispersion of the solid phase in all three regions of the system are explored. The results demonstrate that the insertion of draft plates slows the mixing of the solid phase. Increasing the draft plate length leads to a lower fraction of particles in the spout and fountain regions, increases the solid cycle time, and the solid residence time in the annulus and fountain regions. Also, the collision force between particles is one order-of-magnitude larger than the fluid force. Moreover, increasing the draft plate length enhances the translational and rotational solid velocity, and the vertical solid dispersion in both the spout and fountain regions.

Numerical investigation on the effect of draft plates on spouting stability and gas–solid characteristics in a spout-fluid bed

Numerical simulation of dense gas–solid motion in a spout-fluid bed was carried out using the computational fluid dynamics coupled with discrete element method (CFD–DEM), in which the gas and solid motion are solved in the Eulerian and Lagrangian framework, respectively. After validating the simulated results with experimental data, the main cause of spouting instability was first identified, followed by evaluating the effect of draft plate length on spouting stability, pressure signals, and gas–solid hydrodynamics in the system. The results demonstrate that the onset of spout dancing, which is a type of spouting instability, is primarily due to the merging of the rising bubbles in the annulus with the spouting channel, which can be circumvented by the presence of draft plates. Increasing the draft plate length diminishes the mean pressure in the spouting inlet and the corresponding peak value of the power spectrum, and the correlation coefficient of pressure signals in the spout and background inlets. Regarding effect on the gas–solid hydrodynamics, a longer draft plate length leads to a more dilute upper spout, a higher spoutable height, higher voidage in the central fountain region, higher vertical gas flux (Fgz) and solid velocity (Usz) in the central axis, lower Fgz and Usz near the wall, and lower vertical solid flux (Fsz) overall. The optimization of the draft plate length depends on a balance between spout stability and solid circulation rate, since the former increases but the latter decreases with draft plate length.

CFD–DEM model for coupled heat and mass transfer in a spout fluidized bed with liquid injection

In this paper, we present a novel CFD–DEM model for coupled heat and mass transfer in a spout fluidized bed with liquid injection. In our CFD–DEM approach both particles and droplets are treated as discrete elements. Before actual simulations were conducted, a number of tests were performed to determine the optimum particle collision time step by analysing the error in the prescribed restitution coefficient (difference between defined and applied values). Subsequently we test the model for a selected case as a proof-of-principle. In this test case, the effect of liquid injection on dynamics of a pseudo-2D spout fluidized bed with draft plates was demonstrated for the fluidized bed-spouting-with-aeration (dispersed spout) regime with injection of water droplets. The computational results were compared with experimental results that were obtained through a unique combination of particle image velocimetry (PIV) and infrared thermography (IRT).

Investigations on the spouting stability in a prismatic spouted bed and apparatus optimization

This paper deals especially with spouting stability in a slot-rectangular (prismatic) spouted bed. The flow stability was characterized by the pressure drop evaluation (the uniformity and amplitude of the fluctuations) and visual observations (the flow symmetry). The effect of several geometrical conditions, such as the inlet design, prismatic angle and draft plates on the bed behaviour was investigated for different particles and bed inventories exposed to different air flow rates. The prismatic angle was found to have a strong impact on the spouting characteristics. A method to improve significantly the spouting quality and to make the spouting stability independent on the gas inflow velocity is presented. Compared to the reference geometry the prismatic angle was changed to a higher value and the apparatus was equipped with draft plates. Whereas, to achieve a stable spouting in a wide range of the air flow rates the design of draft plates should be customized to the bed inventory. Dependent on the static bed height full or open-sided draft plates should be used. In the proposed apparatus implementation, the maximum spouting velocity was almost eliminated for large particles (Geldart D), i.e. a highly coherent dense spouting can pass continuously into the dilute-like stable regime, without stability loss at intermediate gas velocities. The spouting of Geldart B particles is also improved.

Experimental study of hydrodynamics and thermal behavior of a pseudo‐2D spout‐fluidized bed with liquid injection

A novel nonintrusive technique is presented to investigate hydrodynamic and thermal behavior of gas–solid spout‐fluidized beds with liquid injection, by simultaneously capturing visual and infrared images. Experiments were performed in a pseudo‐2D bed with draft plates filled with glass or γ‐alumina particles to investigate the effect of liquid injection and particle properties on the flow characteristics. For the glass particles under dry and wet conditions, time‐averaged particle velocities show similar quasi‐steady‐state behavior. However, under wet conditions, lower particle velocities were observed in both spout and annulus as compared with the dry system. Whereas, γ‐alumina particles do not show considerable variation in the particle velocities under dry and wet conditions and fluidize well at higher liquid injection rates. Additionally, for the glass particles, the particle temperature significantly decreases as compared to the γ‐alumina particles. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1146–1159, 2015

Experimental and numerical investigations of a pseudo-2D spout fluidized bed with draft plates

Spout fluidized beds are often utilized for gas-solid contacting operations involving physical and/or chemical transformations with simultaneous heat and mass transfer such as drying, coating, granulation, combustion, gasification etc. This is because these beds combine advantages of both spouted and fluidized beds. Since the development of the spout fluidized bed, several geometrical modifications have been proposed to optimize the bed performance. One of these modifications often applied in granulation and coating industries includes a draft tube insertion inside the bed, which results in improved performance by providing a restriction on lateral particle flow providing clear distinction for wet spout and dry annulus zones. Moreover, the insertion of the draft tube leads to a stable spouting at lower flow rates, due to the reduced bypassing of the inlet gas (from spout to annulus).In this work, the hydrodynamic characteristics of a spout fluidized bed with draft plates was studied to identify the flow characteristics by constructing a flow regime map by image analysis and a fast Fourier transform of the measured pressure signal. In addition, the captured images were used to determine the particle velocity via particle image velocimetry (PIV). Furthermore, simulations were carried using a discrete particle model with a sub grid scale turbulence model for two regimes, namely the spouting-with-aeration and fluidized bed-spouting-with-aeration (dispersed spout), which are of most interest from an industrial view point. The obtained results were compared with previously obtained experimental data i.e. PIV. This study highlights various flow pattern observed during operation of spout fluidized bed with a draft plate over a wider operating conditions, which is useful to select proper operating conditions; whereas the experimental data can be used for computational fluid dynamic (CFD) model validation, which serve as a building block for design and scale-up at higher operational scales. Besides this, the quantitative information such as particle velocity, residence time distribution, solid mixing and circulation can be obtained after suitable post processing hence useful in optimizing the bed performance.

Numerical investigations of a pseudo-2D spout fluidized bed with draft plates using a scaled discrete particle model

Spout fluidized beds are often utilized for the gas–solid operations involving physical and/or chemical transformations such as drying, coating, granulation, combustion, gasification, etc. This is because these beds combine advantages of both spouted and fluidized beds. Following the spout fluidized bed development, several geometrical modifications have been proposed to optimize the bed performance. One of the modifications includes a draft tube insertion inside the bed, which results in improved performance by providing a restriction on particle cross flow. Moreover, the draft tube insertion leads to a stable spouting at lower flow rates, due to the reduced inlet gas bypassing (from spout to the annulus).In this work, a discrete particle model (DPM) with a sub-grid scale (SGS) turbulence model was used to simulate the bed dynamics and analyze the effects of the draft plates and particles physical properties by considering two regimes namely, the spouting-with-aeration and fluidized bed-spouting-with-aeration (dispersed spout), which are of interest from an industrial point of view. The obtained results were compared with our experimental data (Sutkar et al., 2013b) from particle image velocimetry (PIV) and digital image analysis (DIA). Furthermore, an alternative simulation approach has been used to simulate dynamics of the industrial scale gas–solid contactors consisting very large particles with reduced total computational time by changing the physical properties of the particles to maintain constant Archimedes and Reynolds numbers. The obtained results were compared with the experimental data, as well as with simulation data obtained with the original physical properties.

Experimental investigations of a pseudo-2D spout fluidized bed with draft plates

Spout fluidized beds often prove promising for gas–solid contacting operations involving chemical reactions with changing particle size. However, difficulties may arise during operation, such as unstable spouting, dead zones formation and limited flexibility in adjustment of maximum spoutable height and residence time. This can be improved by inserting draft tubes or plates. In this work, the hydrodynamics of a pseudo-2D spout fluidized bed with draft plates was studied by changing the background velocity from 0 to 1.75m/s with increments of 0.25m/s and the spout velocity from 10 to 50m/s with increments of 5m/s. The distinct flow regimes were identified by capturing high-speed images and Fast Fourier transformation (FFT) of measured pressure signals. In addition, the effects of the entrainment height (in the range of 0.03–0.05m), static bed aspect ratio (in the range of 1–1.5) and particle properties (for glass and γ-aluminum oxide particles) on the hydrodynamics were investigated by determining the time-averaged particle volumetric flux by combined particle image velocimetry (PIV) and digital image analysis (DIA) for two regimes namely, spouting-with-aeration (IV) and fluidized bed-spouting-with-aeration (dispersed spout) (VIII).

Simulations of flow behavior of gas and particles in spouted bed with a porous draft tube

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic–frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman–Darcy–Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube (Ishikura et al., 2003). The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.

Numerical simulation of two-dimensional spouted bed with draft plates by discrete element method

A discrete element method (DEM)-computational fluid dynamics (CFD) two-way coupling method was employed to simulate the hydrodynamics in a two-dimensional spouted bed with draft plates. The motion of particles was modeled by the DEM and the gas flow was modeled by the Navier-Stokes equation. The interactions between gas and particles were considered using a twoway coupling method. The motion of particles in the spouted bed with complex geometry was solved by combining DEM and boundary element method (BEM). The minimal spouted velocity was obtained by the BEMDEM-CFD simulation and the variation of the flow pattern in the bed with different superficial gas velocity was studied. The relationship between the pressure drop of the spouted bed and the superficial gas velocity was achieved from the simulations. The radial profile of the averaged vertical velocities of particles and the profile of the averaged void fraction in the spout and the annulus were statistically analyzed. The flow characteristics of the gas-solid system in the two-dimensional spouted bed were clearly described by the simulation results.

Flow patterns of solids in a two-dimensional spouted bed with draft plates: PIV measurement and DEM simulations

Particle flow behaviors in a two-dimensional spouted bed (2DSB) with draft plates were studied using both the particle image velocimetry (PIV) and the combined technique of discrete element method and fluid dynamic computation (DEM-CFD) while considering the gas turbulence effect. The bed consisted of a rectangular column, 152 mm wide and 15 mm deep, a conical section with an included 60° angle and two draft plates with a distance of 15 mm. Images of particle flow were recorded by a high speed CCD camera and analyzed using a self-developed PIV algorithm to obtain a time-averaged particle velocity field. Experiments predict that the addition of draft plates not only makes the streamline of particles in the annulus steeper, but the velocity magnitude is made smaller as well. DEM results predict well the longitudinal profile of the particle vertical velocity along the bed centerline, especially during the rapid acceleration stage at the lower part of the spout. Finally, the distributions of drag forces and net forces are introduced in this paper to explain the particle velocity profiles by PIV measurement.

Characteristics of heat transfer in two-dimensional spouted bed

Heat transfer characteristics in a two-dimensional spouted bed with draft plates were studied using three agricultural materials, paddy, corn and soybean. Increases in inlet superficial velocity and bed porosity result in the increased superficial velocity in downcomer. Heat transfer coefficients for the spout and downcomer were founded to be a function of the superficial velocity and bed depth-to-particle diameter ratio. Two equations correlating heat transfer coefficients for both regions have been developed.

Prediction of gas-particle dynamics and heat transfer in a two-dimensional spouted bed

Abstract The aerodynamics of particles and heat transfer of gas-to-particles in a two-dimensional spouted bed (2DSB) with draft plates are investigated by the discrete element method (DEM). The physical properties of the particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the empirical correlations proposed by Kudra et al. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. The draft plates can reduce the minimum spouting velocity and pressure drop. They also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated. The mixing of a 2DSB without draft plates of 10 000 particles is better than that of 26 000 particles. In our simulation, the gas-to-particle heat transfer is investigated. The Ranz–Marshall correlation and the correlation of Sartori et al. are applicable in the spout region and the downcomer region, respectively. The gas-to-particle heat transfer occurs mainly in the central or spout region, as reported by Freitas and Freire.

Investigation of Fluid and Coarse‐Particle Dynamics in a Two‐Dimensional Spouted Bed

AbstractThe aerodynamics of particles and gas flow in a two‐dimensional spouted bed (2DSB) with draft plates is investigated with the aid of the discrete element method. The geometry of the 2DSB with draft plates is set as close as possible to the experimental apparatus of Kudra [1] and Kalwar [2]. The physical properties of the coarse particles are similar to those of shelled corn. The calculated minimum spouting velocity and pressure drop agree well with the correlations of Kudra [1] and Kalwar [2]. In the spout region, the particle vertical velocities are found to decrease as the height increases. The fluid velocity in the downcomer region decreases as the superficial gas velocity increases. The particle circulation rate increases when the friction coefficient decreases or the separation height increases. At the minimum spouting velocity, the bed height does not affect the particle circulation rate in the 2DSB with draft plates. The draft plates not only reduce the minimum spouting velocity and pressure drop but also increase the maximum spoutable bed height. The effect of taking out the draft plates on the spouting phenomenon is investigated and the effect of putting in a deflector on the possible breakage of the particles is also estimated.

Flow Characteristics in Slot‐Rectangular Spouted Beds with Draft Plates

AbstractAn experimental investigation was conducted of slot‐rectangular spouted beds with air entry slots spanning the full thickness of the column and vertical draft plates intended to help control the solids circulation rate. With increasing superficial gas velocity, the flow between the draft plates changed from bubbling to slugging and then to spouting with dilute pneumatic between the plates and moving‐bed downward motion on both sides. However, there was difficulty maintaining stability and symmetrical flow on the two sides. Once spouting is established, pressure drops and local voidages vary with gas velocity, particle size and gas entry size in broadly similar manners as for conventional spouted beds