Quasi-stagnant Zone Research Articles

The preliminary evaluation of design guidelines of stratum ventilation in a large tropical lecture hall

This article presents a study conducted to test the design guidelines of stratum ventilation (SV) for the conditions set in the tropics (Malaysia). Three different ventilation strategies and six different cases for SV were designed and examined by varying the layout of the return outlets and the air change per hour values. The ventilation strategy with a return outlet at the same level on opposite wall was found better than others. The simulation results of the medium-size room show that the thermal comfort is only favourable up to the centre region. Locations near the diffusers were noticed too cold and locations far away were too warm. In addition, the air flow pattern shows that the room has a high PPD due to quasi-stagnant zones that exist between the air jets from the diffusers. A similar condition does not exist in the small-size room, which was found under the acceptable range of PPD index (0%–25%). The ventilation strategy-1 (VS-1) is found with a greater energy saving with EUC 1.45 for the small room and 1.50 for the medium room compared to the other two ventilation strategies. The SV design guidelines are found adequate for the need of thermal comfort.

DEM simulation of particle flow in a parallel-hopper bell-less charging apparatus for blast furnace

The particle flow in a parallel-hopper bell-less charging apparatus has been studied by the physical experiments and DEM (discrete element method) simulations with mono-sized particles. The contact force chains, flow patterns, velocity distribution, stress distribution and wall pressure during the filling and discharge processes were analyzed numerically. The flow in the parallel-hopper is an asymmetric flow which consists of the quasi-stagnant zone, wall shear zone, deflecting flow zone, converging flow zone and the transitional flow zone. The particles in the quasi-stagnant zone, deflecting flow zone and converging flow zone play an important role in inducing the local heterogeneity of particle stresses in hopper. The asymmetry and difference of wall pressure on two side walls were also predicted numerically. The obtained asymmetrical stress distribution during discharge is clearly related to the unsymmetrical flow pattern and velocity distribution in the parallel-hopper as compared to the symmetrical hopper. The influence of the particle-wall sliding friction coefficient and rolling friction coefficient on the wall pressure was analyzed. In addition, the contact force chains within particles were investigated. It is demonstrated that the strong force chains mainly locate at the lower part of the hopper or near the sloping wall, whereas the weak force chains are distributed over the deflected flow zone.

DEM Simulation of Solid Flow Including Asymmetric Phenomena in COREX Shaft Furnace

Based on the principles of the discrete element method (DEM), a scaled-down model was established to analyze burden descending behavior, including asymmetric phenomena, throughout an entire COREX shaft furnace (SF). The applicability of the DEM model was validated by determining its accordance with a previous experiment. The effects of discharge rate and abnormal conditions on solid flow were described in terms of solid flow pattern and microscopic analysis. Results confirmed that the solid flow of the COREX SF can be divided into four different flow regions; the largest normal force exists at the top of the man-made dead zone, and the weak force network exists in the funnel flow region. The basic solid flow profile was identified as a clear Flat?U?W type. Increasing the discharge rate decreased the quasi-stagnant zone size, but did not affect the macroscopic motion of particles or the shape of patterns above the bustle. For asymmetric conditions, in which particles were discharged at different rates, the solid flow patterns were asymmetric. Under an abnormal condition where no particles were discharged from the left outlet, a sizeable stagnant zone was formed opposite to the working outlet, and “motionless” particles located in the left stagnant zone showed potential to increase the period of static contacts and sticking effect.

Analyses of solid flow in COREX shaft furnace with AGD by discrete element method

COREX is an industrially and commercially proven smelting reduction process. The shaft furnace (SF) for the direct reduction of iron ore is one of the two major reactors of COREX. In the new design of COREX-3000 SF in Baosteel, China, a new technique called areal gas distribution (AGD) has been adopted. With the installation of two AGD beams, the cross-sectional area varies in a complicated manner, which will affect the burden descending behaviour. This work uses a slot model to investigate the solid flow behaviour in the SF by discrete element method. The applicability of the discrete element method model is validated. The results confirm that the flow profile in SF with AGD evolves from a flat to wave and finally to W profile as the solids descend. A triangle shaped free area is observed under the AGD beam, which is the main channel for gas flow into the shaft centre. Increasing the discharging rate has an effect of decreasing the quasi-stagnant zone size, but does not affect the macroscopic motion of particles and flow pattern above the bustle. The cross-section of AGD channel increase with the increase in the discharging rate. Under the asymmetric condition, the solid flow pattern is asymmetric. The influence of AGD on macro- and microscope properties of solid flow in SF is also evaluated. This investigation reveals that AGD beams affect the particle uniform descending in bustle zone and increase the complexity of normal force distribution.

Discrete particle simulation of solid flow in a three-dimensional blast furnace sector model

The application of the discrete element method (DEM) to industry processes often encounters difficulties because of the high computational demand. Two-dimensional slot models are often a preferable solution to reduce the demand; however it may experience significant deviations from the reality. The sector model with circumferential periodic boundary conditions is an alternative tool to solve this problem, and has been established in our previous work. In this work, such a sector model is used to examine the solid flow features in an ironmaking blast furnace, and the results are compared with those obtained from the corresponding slot model. Several significant differences of solid flow can be observed between the two models, such as the larger stagnant zone and quasi-stagnant zone, smaller funnel zones in the slot model and different layer shapes. The solid flow with the pre-set cohesive zone also confirms that the slot model is not accurate to describe the anisotropic features of solid flow in the tangential direction in a cylinder vessel. The gas flow in the raceway region also shows a slight difference between the two models. It has demonstrated that to realistically describe the three-dimensional solid flow characteristics, the sector model should be used in the future studies of multiphase flow in a blast furnace.

Wall-separation and vortex-breakdown zones in a solid-body rotation flow in a rotating finite-length straight circular pipe

AbstractThe incompressible, inviscid and axisymmetric dynamics of perturbations on a solid-body rotation flow with a uniform axial velocity in a rotating, finite-length, straight, circular pipe are studied via global analysis techniques and numerical simulations. The investigation establishes the coexistence of both axisymmetric wall-separation and vortex-breakdown zones above a critical swirl level, ${\it\omega}_{1}$. We first describe the bifurcation diagram of steady-state solutions of the flow problem as a function of the swirl ratio ${\it\omega}$. We prove that the base columnar flow is a unique steady-state solution when ${\it\omega}$ is below ${\it\omega}_{1}$. This state is asymptotically stable and a global attractor of the flow dynamics. However, when ${\it\omega}>{\it\omega}_{1}$, we reveal, in addition to the base columnar flow, the coexistence of states that describe swirling flows around either centreline stagnant breakdown zones or wall quasi-stagnant zones, where both the axial and radial velocities vanish. We demonstrate that when ${\it\omega}>{\it\omega}_{1}$, the base columnar flow is a min–max point of an energy functional that governs the problem, while the swirling flows around the quasi-stagnant and stagnant zones are global and local minimizer states and become attractors of the flow dynamics. We also find additional min–max states that are transient attractors of the flow dynamics. Numerical simulations describe the evolution of perturbations on above-critical columnar states to either the breakdown or the wall-separation states. The growth of perturbations in both cases is composed of a linear stage of the evolution, with growth rates accurately predicted by the analysis of Wang & Rusak (Phys. Fluids, vol. 8, 1996a, pp. 1007–1016), followed by a stage of saturation to either one of the separation zone states. The wall-separation states have the same chance of appearing as that of vortex-breakdown states and there is no hysteresis loop between them. This is strikingly different from the dynamics of vortices with medium or narrow vortical core size in a pipe.

Analyses of solid flow in latest design COREX shaft furnace by physical simulation

The present investigation was aimed at elucidating solid particle behaviour in a new design COREX shaft furnace using a new technique called areal gas distribution (AGD). The measurement of solid flow profile was performed with a semi-cylindrical model. To guarantee similarity of conditions, a modified Froude number was taken into consideration in the physical model constructed. The solid flow characteristics in this new furnace with different blast volumes were analysed, and the effect of discharge rate as well as abnormal conditions on solid flow behaviour have also been investigated. The work reveals that solid flow profile in the shaft furnace model progresses through a Flat→Wave→W shape profile as the burden descends. Blast volume has little influence on solid particle behaviour. However, increasing the discharge rate has an effect of decreasing the quasi-stagnant zone size and with the increasing discharge rate, the cross-section of AGD channel increases. For asymmetric flow, particles descend from the top and move to the discharging outlet and a very big stagnant zone is formed on the opposite side of the discharging outlet.

Impact of flow-corrective insert on flow patterns in two-dimensional moving bed

This paper presents the study of the flow patterns inside a granular moving bed in a quasi-two-dimensional (2D) model of a granular moving bed filter (GMBF) vessel with vertical walls and a mass flow hopper. Fine quartz sand was chosen as the granular filter media in experiments. The related flow properties were measured by a Jenike Shear Tester. According to the flow properties of fine silica sand and Johanson's theory, a 2D moving-bed apparatus was built, with transparent walls and the flow-corrective insert to diminish the quasi-stagnant zones along the side walls. The flow patterns and velocity profiles of silica sand in the system with flow-corrective insert were studied experimentally, and the effect of the location of the flow-corrective insert on the internal flow patterns was investigated. The optimal placement of flow-corrective insert according to Johansson's theory was demonstrated to efficiently diminish the quasi-stagnant zones near the side walls. Little vertical shift of the flow-corrective insert from optimal position changed the stable and uniform mass flow pattern of moving bed into the funnel flow.

Three-dimensional modeling of biofouling and fluid dynamics in feed spacer channels of membrane devices

This study presents a new three-dimensional (3-d) computational model that couples fluid dynamics, solutes transport and biofouling by biofilm formation in NF and RO membrane modules. A computational domain of 3 × 5 feed spacer frames with geometry as applied in practice was used in the model. Comparing the hydrodynamics computed with the realistic spacer geometry and with a spacer made from straight cylindrical filaments, like in previous modeling studies, showed that cylindrical filament feed spacers are too simplified for representative modeling studies. The 3-d numerical simulations showed that biomass accumulation, by attachment and biofilm growth in time, strongly affected the feed channel pressure drop, liquid velocity distribution and residence time distribution. The main pressure drop is encountered by the flow passing over the spacer filaments. Simulations showed the development of a heterogeneous flow pattern and formation of preferential flow channels. This study indicates that the real impact of biofouling is on the flow regime leading to quasi-stagnant zones and an increase in the dispersion of the residence time distribution. The presented 3-d mathematical modeling approach in (bio)fouling of membrane modules may have significant implications for membrane system design and operation to have stable membrane installation performance at minimal costs.

Placement of a non-isosceles-triangle insert in an asymmetrical two-dimensional bin-hopper

This work presents a novel and simplified method for determining placement and size of a non-isosceles-triangle insert that eliminates the quasi-stagnant zone next to the wall in an asymmetrical bin-hopper. A two-dimensional model silo with a rectangular base was created to observe flow patterns, analyze flow regions and support this proposed technique for designing a non-isosceles-triangle insert. Placement of the non-isosceles-triangle insert for different hopper geometries and different insert apex angles was determined and tested. Moreover, a simple method was developed to plot the path lines of black particles that are in the same height at beginning. Accordingly, the area of the quasi-stagnant zone next to the wall of bin-hopper could be calculated properly, and these path lines also supported the discussion of flow regions in this investigation. Experimental results demonstrate that a non-isosceles-triangle insert designed using the proposed method effectively reduces the quasi-stagnant zone next to the wall in an asymmetrical bin-hopper.

Moving granular filter bed of quartz sand with louvered-walls and flow-corrective inserts

This study investigated the effect of a simple flow-corrective element, a vertical plate, on the flow patterns and wall stresses in a two-dimensional louvered-wall moving granular filter bed of quartz sand. The static wall stress distributions produced by the granular solids were measured, and compared with the theoretical prediction, using the differential slice and Runge–Kutta (order four) methods. The variations in the dynamic wall stresses over time were obtained using a two-directional pressure gauge. The results show that a vertical plate placed in each stage of the granular filter bed effectively diminished the quasi-stagnant zone adjacent to the louvered-wall, and reduced the wall pressure pulsation. For example, the ratio of the stagnant zone area in the second stage to the area of quartz sand in the second stage decreased from 50.66% (without an insert) to 27.86% (with a vertical plate). The maximum dynamic normal wall stress on the convergent section of third-stage louvered-wall (louver angle: 40°) declined remarkably from 9.95 kPa (without an insert) to 5.9 kPa (with a vertical plate), and then the reduction of the maximum dynamic normal wall stress was up to 40.7%. Furthermore, a vertical plate installed at an appropriate location satisfies the following requirements: (1) it shrinks the stagnant zone and reduces the wall stresses on the louvered wall of a moving granular filter bed; and (2) it solves the potential problem of a roof-type insert, on which the stagnant zone may sit.

Flow patterns and wall stresses in a moving granular filter bed with an asymmetric louvered-wall and obstacles

This study investigated the flow patterns and wall stresses in a two-dimensional asymmetric louvered-wall moving granular filter bed with an obstacle placed at each stage of the granular filter bed. The obstacles utilized were: (1) a single vertical plate and (2) a wedge-shaped obstacle comprising a left-vertical-plate and a right-inclined-plate. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The instantaneous vertical velocity distribution at a certain height from the bottom of the third stage in a two-dimensional asymmetric louvered-wall granular filter bed was determined from two successive flow images. The static stresses and the variations in the dynamic wall stresses with time were obtained using a two-directional pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The preliminary theoretical study of the obstacle's placement in the asymmetric louvered-wall moving granular filter bed was proposed. The magnitude of the average dynamic normal stress on the convergent section of the louvered-wall greatly exceeds that on the vertical section of the louvered-wall. A wedge-shaped obstacle (or a single vertical plate) placed at a proper location, which causes two asymmetric flowing streams in the granular bed, may increase the velocity and refreshing rate of the filter granules in the quasi-stagnant zone near the louvered-wall and further the overall dust collection efficiency of the filter granules because the central flowing core above exit is not evident in the granular bed. Additionally, a wedge-shaped obstacle installed at an appropriate location, which consists of a left-vertical-plate and a right-inclined-plate, satisfies the following requirements: (1) it keeps the granules in the left part of the granular bed away from the obstacle, preventing interference therewith; (2) it shrinks stagnant zone and declines the normal stress on the convergent section of the louvered-wall in the right part of the granular bed; and (3) it solves the potential problem of a roof-type insert, on which the stagnant zone may sit.

Flow patterns and wall stresses in a moving granular filter bed with a curved symmetric louvered wall

Abstract This study investigated the flow patterns and wall stresses in a two-dimensional moving granular filter bed with a curved and symmetric louvered wall. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The numerical predictions of the flow patterns were implemented using the discrete element method and compared with the experimental results. The variations in the dynamic wall stresses with time were obtained using a pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The effects of the curvature of the louvered wall and the exit width upon the flow patterns and the wall stresses were also elucidated. For a fixed value of radius of curvature, the ratio of the quasi-stagnant zone (Q-SZ) area in the third stage to the granular bed area in the third stage increases with decreasing width of the exit, but the average dynamic normal stress on the curved louvered wall increases with increasing width of the exit. For a fixed value of the exit width, both the ratio of the Q-SZ area in the third stage to the granular bed area in the third stage and the average dynamic normal stress on the curved louvered wall increase with increasing radius of curvature. Additionally, the curved louvered wall can not only shrink the area of the Q-SZ in the granular bed, but also reduce the dynamic normal stress on the louvered wall.

The effect of a flow corrective insert upon flow patterns and wall stresses in a two-dimensional bin-hopper

Abstract The flow patterns and stresses on the wall in a two-dimensional bin-hopper with the placement of an equilateral triangle flow corrective insert were investigated. The geometric shape and location of five kinds of equilateral triangle insert used in this study were obtained using the method previously proposed by Johanson. The flow pattern histories of the quartz sand in the bin-hopper were recorded using a digital camcorder. The static wall stress distributions above the insert produced by the quartz sand were measured. The variations in the dynamic wall stresses with time were obtained. As the insert half-angle increased, the region of the quasi-stagnant zone, which sat on the equilateral triangle insert, became larger and the position of the static normal stress peak moved downward. In general, the values of the dynamic normal wall stress measured by the pressure gauge, which was not close to the insert, were smaller than those under the test without the placement of an insert during the most part of material withdrawal. Nevertheless, some of the dynamic normal wall stress peaks were measured by the pressure gauge, which was close to the insert, as the insert half-angle was between 30° and 50°.

A simplified mathematical model for gas?solid flow in a blast furnace

This paper presents a numerical study of the behaviour of solid flow in a two-dimensional blast furnace with or without gas flow. The mathematical model, as a simplified version of the more detailed model developed earlier, is similar to the so-called viscous flow model but the method to determine the stagnant zone profile is similar to that used in the kinematic model. The study shows that the simplified model is able to capture the key flow characteristics of solid flow in a blast furnace and describe reasonably the effects of gas and solid flowrates, and particle properties, although the predicted quasi-stagnant zone may be smaller. The advantage of the present approach is that it can be readily implemented in a full process model that needs to have a quick response to change for the purpose of control and optimisation.

The flow patterns and stresses on the wall in a symmetric louvered-wall moving granular filter bed

The flow patterns and stresses on the wall in a symmetric two-dimensional louvered-wall moving granular filter bed were investigated. The static wall stress distributions produced by the granular solids were measured and compared with the theoretical prediction using the differential slice and Runge–Kutta (order four) methods. The variations in the dynamic wall stresses with time in a moving granular filter bed were obtained. In addition, the effect of the louver angle upon the flow patterns and wall stresses was investigated. Four different flow regions were observed in a moving granular filter bed. As the angle of the louver decreases, the quasi-stagnant zone area adjacent to the side wall becomes smaller and the static normal stress acting on the convergent section of the side wall becomes larger. The magnitude of the static normal stress acting on the convergent section is approximately 10 times as large as that acting on the vertical section. When the normal stress measured by pressure gauge installed on the upper stage decreases to zero, the normal wall stress measured by pressure gauge installed on the adjacent lower stage then begins to descend and fluctuate under the static normal wall stress during granular material withdrawal. Employing the results obtained using stress measurements and image processing, the pressure pulsation phenomenon in a symmetric two-dimensional louvered-wall moving granular filter bed may be further understood.

Flow patterns and velocity fields of granules in Dorfan Impingo filters for gas cleanup

Inside a two-dimensional model of the louvered Dorfan Impingo panel with transparent front and rear walls the flow regions of filter granules without gas the cross-flow were observed. The white PE beads were used as filter granules. Colored PE beads served as tracers. Filter granules were discharged and circulated to the bed. The flow rate of filter medium was controlled by the belt conveyor. The image processing system was used to record the granular flow. The flow patterns and the quasi-stagnant zones history in the moving granular bed were evaluated. The experiment showed that there exists a fast central moving region (flowing core) of filter granules and two quasi-stagnant zones along the louver walls. The velocity fields in the moving bed in the Dorfan Impingo filter systems were also measured and analyzed. The results indicate that the velocity profiles of filter granules are influenced by the louver geometry.

Numerical simulation of flow patterns of disks in the asymmetric louvered-wall moving granular filter bed

Using the Discrete Element Method (DEM) model [P.A. Cundall, O. Strack, Geotechnique, 29 (1979) 47.], the granular solid flow pattern histories, velocity field and quasi-stagnant zone development close to the louvered walls of six kinds of 2-D asymmetrical moving granular filter beds were studied numerically. The numerical results reported here and the experimental results [J.T. Kuo, J. Smid, S.S. Hsiau, C.Y. Wang, C.S. Chou, Filtr. Sep., 35(6) (1998) 529.] provide fundamental and important information for designing moving granular bed high-temperature flue gas cleanup filters.

Numerical simulation of flow patterns of disks in the symmetric louvered-wall moving granular filter bed

Abstract Employing the stack disk method in the louvered-wall configuration described in the Appendix, the initial coordinates of the disks' center and the filter boundary were defined. The flow pattern histories of granular solids, velocity field and development of quasi-stagnant zones close to the louvered walls of three kinds of two-dimensional (2D) symmetrical moving granular filter beds and 2D hybrid moving granular filter beds were studied numerically. The discrete element method was used in this study. The numerical results reported here and experimental results provide fundamental and important information for designing moving granular bed for high-temperature flue gas clean-up filters.

Velocity profiles of granules in moving bed filters

The velocity fields in three symmetric and two asymmetric louver-walled moving bed systems were studied experimentally. The horizontal and vertical velocity distributions at five levels of the louver section were plotted. The results indicate that the velocity profiles of filter granules are influenced by the louver geometry. When the louver angle is steep, the velocity profile of filter granules is close to plug flow. When the louver section is shallow, the quasi-stagnant zones exist along the louver walls. The kinematic model for predicting the granule velocity distribution is used for analyzing the three symmetric louver configurations at different height levels.