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Challenging Paradigms By Optimizing Combustible Dust Separator
Abstract
A computational study was carried out to investigate the effects of internal geometry changes on the likelihood of solids buildup within, and the efficiency of, an industrial dust collector. Combustible solids held up in the unit pose a safety risk. The dust collector serves multiple functions, so the design requires a delicate balance. Particles should be separated from the incoming mixture and collected in the bottom of the unit. This particulate material should freely flow into a high-speed ejector (Mach 0.4) underneath. Gas must also flow freely to the top outlet, but sufficient gas must flow down to the ejector so that its motive gas augments the transport of particles back to the reactor (recirculation). Computational design evaluations included: (1) rod spacing, (2) ledge removal, and (3) rod cover plates. Testing on particle size distribution and density was carried out in-house to provide inputs to the computational fluid dynamics (CFD) model. Rod spacing reduction had a mixed effect on flow distribution. Plates were found to induce a negative effect on recirculation and a mixed effect on combustible solids accumulation. Removal of the ledge, however, offered slightly more recirculation along with completely alleviating stagnant solids accumulation. It is shown that, without consideration of detailed fluid physics, general separator design principals might be misguiding. © 2018 American Society of Mechanical Engineers (ASME). All rights reserved.
... An Eulerian-Eulerian approach to this multiphase problem is sought. Instead of assuming statistically small droplets (relative to the computational cell size) within a cell such as in [24], disintegrated liquid shapes will be spatiotemporally resolved explicitly. The continuity equation governing the mass balance of each phase is (1) The phase-averaged Reynolds-averaged linear momentum balance is (2) It can be seen that the gas and slurry share a common momentum field, and properties were phaseaveraged. ...
... . 11 shows time-averaged Sauter mean length scale versus distance from the injector face normalized24 by D O . Plot lines are created by connecting discrete value averages at various distances. ...
Past work involving validated “cold-flow” CFD modeling of self-generating and self-sustaining pulsating near-sonic non-Newtonian slurry atomization elucidated acoustic signatures, atomization mechanisms, and the effects of numerics and geometric permutations. The numerical method has now been incorporated with exothermic oxidation reaction kinetics relations along with radiation, i.e. no longer cold-flow. These models provide substantially increased model rigor and allow for new pulsing thermal measures which help assess injector thermal stresses. Twelve models have been run for extended periods of time in order to investigate the effects of dramatic changes in gas feed rate and prefilming (retraction) length. Given the new metrics and models, multiple statistically optimized designs are potentially available depending on the objective function(s) and their relative weightings in the overall value proposition to the project. In the case in which all metrics have equal value to the project and are simultaneously considered in a statistical model, the optimum design involves a mid-level of retraction and a mid-level gas feed rate. If, however, more relative weighting is placed on the importance of droplet size minimization and injector thermal management in lieu of feed passage pressure drop minimization, the optimum design involves a similar retraction but a very high level of gas feed rate.
... A transient Eulerian framework was prescribed where computation cells are smaller than the liquid region length scales, unlike the Eulerian-Lagrangian approach of (Strasser and Strasser, 2018), where the discrete phase is smaller than computational cells. The continuity equation governing the mass balance of each phase was ...
A detailed numerical study on a transonic self-sustaining pulsatile three-stream coaxial airblast injector provided new insight on turbulent pulsations that affected atomization. Unique to this investigation, slurry viscosity, slurry annular thickness, and how the annular thickness interacts with inner nozzle retraction (prefilming distance) were found to be paramount to atomizer performance. Narrower annular slurry passageways yielded a thinner slurry sheet and increased injector throughput, but the resulting droplets were unexpectedly larger. As anticipated, a lower slurry viscosity resulted in smaller droplets. Both the incremental impact of viscosity and the computed slurry droplet length scale matched open literature values. The use of a partial azimuthal model produced a circumferentially periodic outer sheath of pulsing spray ligaments, whereas modeling the full domain showed a highly randomized and broken outer band of ligaments. However, quantitatively the results between the two azimuthal constructs were similar, especially farther from the injector; therefore, it was proved that modeling a wedge with periodic circumferential boundaries can be used for screening exercises. Additionally, velocity point correlations revealed that an inertial subrange was difficult to find in any of the model permutations and that droplet length scales correlated with radial velocities. Lastly, droplet size and turbulence scale predictions for two literature cases were presented for the first time using computational fluid dynamics.
Under certain conditions in preferred three-stream geometries, a non-Newtonian airblast atomization flowfield violently pulses (axially and radially) by self-generating and self-sustaining interfacial instability mechanisms. The pulsing is severe enough to send acoustic waves throughout feed piping networks. The most recent work on this system instructed that exothermic chemical reactions enhance this moderate Mach number atomization. Explored herein is the potential to further enhance reaction-assisted disintegration by independently superimposing both sinusoidal and randomized mass flow fluctuations of +/− 50% of the mean onto otherwise constant gas feed streams using surrogate models. Two nozzle geometries (low versus high prefilming distance) and multiple superimposed feed frequencies (ranging from below to above the naturally dominant tone) are considered for each gas stream, making twenty-one total long-running unsteady PLIC-VOF CFD models. Droplet size, plus nine other temporal measures, were considered for assessing atomizer performance in our energy production process. Results indicate that superimposed frequencies have potential to enhance chaotic atomization in a statistically significant manner. Depending on the geometry, the largest effect was about a 10% reduction in droplet size; however, some combinations experienced a droplet size increase. Only marginal differences were seen in the nine other measures, such as injector face heat exposure. In addition to the immediate industrial benefit from modulation, dramatic changes in acoustics were produced by imposed feed perturbations at frequencies lower than the natural tone. A detailed study of start-up flow reveals new mechanisms which explain performance differences.
Primary atomization within a transonic self-generating pulsatile three-stream injector has important industrial rele- vance; however, very few studies have explored the intricacies of these dynamic flows. Prior computational work used compressible axisymmetric (AS) models and incompressible 3D models for the purpose of obtaining spectral content and preliminary droplet size distributions, which was validated with experiments. The emphasis of the work herein shifts to compressible 3D computational models for a non-Newtonian slurry and a more inclusive computational do- main to further elucidate droplet size information. Effects of numerics, turbulence model, and geometric parameters are investigated. Lastly, links are discovered between responses in Sauter mean diameter and trends in AS model- ing metrics. As with prior air-water work and incompressible slurry simulations, higher gas inner flow rate reduced droplet size measurably. While the temporal mean droplet length scale was relatively insensitive to numerics, turbu- lence model, compressibility, and computational domain size, droplet size temporal variability responded very strongly to some of these effects. It was found that injector designs with less retraction (smaller prefilming region) produced smaller droplets and allowed increased process throughputs. Newly discovered correlation equations are provided and followed similar trends to AS work. Interestingly, it was also shown that droplet size can be correlated with spectral information from companion AS studies.
The effects of geometry, numerics, gas flow rate, and superimposed flow modulation on the self-generating pulsatile spray produced by an industrial scale three-stream coaxial airblast reactor injector have been studied for a non-Newtonian slurry and high-pressure gas (SH) system. A fully retracted design showed the most inner gas pulsation, and the spray character changed significantly between a flushed and retracted design; the flushed design showing more radially synchronized and focused pulsations. Pressure drop was found to be linearly proportional to retraction, and new correlations were introduced. Higher inner gas flows typically widened sprays for the base geometry only and lowered the droplet length scales, indicating that the lower droplet size limit was not set by viscosity limitations. Modulation of the inner gas at its dominant tone did not strongly affect many metrics, except that the inner gas pulsations substantially increased. Slurry video analyses provided spray angle directional trends so that a subset of the domain could be simulated to save computational time.
Relative to prior air-water (AW) studies, SH flow patterns and acoustics typically differed significantly, with the exception of the base geometry spray profiles at the higher inner gas flows, along with the droplet length scale. In general, SH simulations showed lower pressure drop, astoundingly lower pressure temporal variability, higher dominant tones, and less periodicity (more diffused spectra). Furthermore, the relationship between 3D SH droplet size and distance was of the form constant/distance; the constant was the same for both feed materials. It appears that acoustics cannot be linked between the two feed materials, but there is some connection in mean droplet size.
Although coaxial airblast primary atomization has been studied for decades, relatively little attention has been given to three-stream designs; this is especially true for transonic self-pulsating injectors. Herein, the effects of nozzle geometry, grid resolution, modulation, and gas flow rate on the acoustics and spray character within an industrial scale system were investigated computationally using axisymmetric (AS) and three-dimensional (3D) models. Metrics included stream pressure pulsations, spray lift-off, spray angle, and primary droplet length scale, along with the spectral alignment among these parameters. Strong interactions existed between geometry and inner gas (IG) feed rate. Additionally, inner nozzle retraction and outer stream meeting angle were intimately coupled. Particular attention was given to develop correlations for various metrics versus retraction; one such example is that injector flow capacity was found to be linearly proportional to retraction. Higher IG flows were found to widen sprays, bringing the spray in closer to the nozzle face, and reducing droplet length scales. Substantial forced modulation of the IG at its dominant tone did not strongly affect many metrics. Incompressible 3D results were similar to some of the AS results, which affirmed the predictive power by running AS simulations as surrogates. Lastly, normalized droplet size versus normalized distance from the injector followed a strikingly similar trend as that found from prior two-fluid air-slurry calibration work.
Acoustics and ligament formation within a self-generating and self-sustaining pulsating three-stream injector are analyzed and discussed due to the importance of breakup and atomization of jets for agricultural, chemical, and energy-production industries. An extensive parametric study was carried out to evaluate the effects of simulation numerics and boundary conditions using various comparative metrics. Numerical considerations and boundary conditions made quite significant differences in some parameters, which stress the importance of using documented and consistent numerical discretization recipes when comparing various flow conditions and geometries. Validation exercises confirmed that correct droplet sizes could be produced computationally, the Sauter mean diameter (SMD) of droplets/ligaments could be quantified, and the trajectory of a droplet intersecting a shock wave could be accurately tracked. Swirl had a minor impact by slightly moving the ligaments away from the nozzle outlet and changing the spray to a hollow cone shape. Often, metrics were synchronized for a given simulation, indicating that a common driving mechanism was responsible for all the global instabilities, namely, liquid bridging and fountain production with shockletlike structures. Interestingly, both computational fluid dynamics (CFD) and the experimental non-Newtonian primary droplet size results, when normalized by distance from the injector, showed an inversely proportional relationship with injector distance. Another important outcome was the ability to apply the models developed to other nozzle geometries, liquid properties, and flow conditions or to other industrial applications.
In order to accurately predict the hydrodynamic behavior of gas and solid phases using an Eulerian-Eulerian approach, it is crucial to use appropriate drag models to capture the correct physics. In this study, the performance of seven drag models for fluidization of Geldart A particles of coal, poplar wood, and their mixtures was assessed. In spite of the previous findings that bode badly for using predominately Geldart B drag models for fine particles, the results of our study revealed that if static regions of mass in the fluidized beds are considered, these drag models work well with Geldart A particles. It was found that drag models derived from empirical relationships adopt better with Geldart A particles compared to drag models that were numerically developed. Overall, the Huilin-Gidaspow drag model showed the best performance for both single solid phases and binary mixtures, however, for binary mixtures, Wen-Yu model predictions were also accurate.
The gas-liquid separator is a key component in the gas removal system in thorium molten salt reactor (TMSR). In this paper, an experimental study focusing on the gas core formation in the gas-liquid separator was carried out. We observed that formation of the air core depends primarily on the back pressure in the separator. Gas core formation was visualized for a range of back pressures, swirl numbers, and Reynolds numbers. Analysis of flow patterns indicated that gas core formation may be defined as four stages: "air core with suction," "tadpole-shaped core," "cloudy core," and "rod core." When rod core is achieved, gas bubbles will be separated completely and that particular back pressure is defined as critical back pressure. The critical back pressure depends on swirl number and Reynolds number. The trends how the critical back pressures vary with the Reynolds number and the swirl number were analyzed.
Loss mechanisms in a scallop shrouded transonic power generation turbine blade passage at realistic engine conditions have been identified through a series of large-scale (typically 12 million finite volumes) simulations. All simulations are run with second-order discretization and viscous sublayer resolution, and they include the effects of viscous dissipation. The mesh (y+ near unity on all surfaces) is highly refined in the tip clearance region, casing recesses, and shroud region in order to fully capture complex interdependent flow physics and the associated losses. Aerodynamic losses, in order of their relative importance, are a result of the following: separation around the tip, recesses, and shroud; tip vortex creation; downstream mixing losses, localized shocks on the airfoil; and the passage vortex emanating from under the shroud. A number of helical lateral flows were established near the upper shroud surfaces as a result of lateral pressure gradients on the scalloped shroud. It was found that the tip leakage and passage losses increased approximately linearly with increasing tip clearance, both with and without the effect of the relative casing motion. For each tip clearance studied, scrubbing slightly reduced the tip leakage, but the overall production of entropy was increased by more than 50%. Also the overall passage mass flow rate, for a given inlet total pressure to exit static pressure ratio, increased almost linearly with increasing tip clearance. In addition, it was also found that there was slight positive and negative lift on the shroud, depending on the tip clearance. At the lowest tip clearance of 20 mils there was a negative lift on the shroud. In the 200-mil tip clearance case there was a positive lift on the shroud. The relative motion of the casing contributed positively to the lift at every tip clearance, affecting more at the lowest tip clearance where the casing is closest to the blade tip. Lastly, it was found that the computed entropy generation for the stationary 80-mils case using the SKE turbulence model was close to that of the 80-mils scrubbing case using the RKE turbulence model. In light of the proposed mechanisms and their relative contributions, suggested design considerations are posed.
A 3D computational fluid dynamics investigation of particle-induced flow effects and liquid entrainment from an industrial-scale separator has been carried out using the Eulerian-Lagrangian two-way coupled multiphase approach. A differential Reynolds stress model was used to predict the gas phase turbulence field. The dispersed (liquid) phase was present at an intermediate mass loading (0.25) but low volume fraction (0.05). A discrete random walk method was used to track the paths of the liquid droplet releases. It was found that gas phase deformation and turbulence fields were significantly impacted by the presence of the liquid phase; these effects have been parametrically quantified. Substantial enhancement of both the turbulence and the anisotropy of the continuous phase by the liquid phase was demonstrated. It was also found that a large number (&1000) of independent liquid droplet release events were needed to make conclusions about liquid entrainment. Known plant run conditions and entrainment rates validated the numerical method.
This work presents a three-dimensional CFD study of a
two-phase flow field in a Gas-Liquid Cylindrical Cyclone
(GLCC) using CFX4.3TM, a commercial code based on the
finite volume method. The numerical analysis was made for
air-water mixtures at near atmospheric conditions, while both
liquid and gas flow rates were changed. The two-phase flow
behavior is modeled using an Eulerian-Eulerian approach,
considering both phases as an interpenetrating continuum. This
method computes the inter-phase phenomena by including a
source term in the momentum equation to consider the drag
between the liquid and gas phases. The gas phase is modeled as
a bimodal bubble size distribution to allow for the presence of
free- and entrapment gas, simultaneously. The results (interface
vortex shape and liquid angular velocity) show a reasonable
match with experimental data. The CFD technique here
proposed, demonstrates to satisfactorily reproduce angular
velocities of the phases and their spatial distribution inside the
GLCC. The experiments showed gas volume fractions smaller
than 150 ppm along the liquid exit, however it was not possible
to reproduce numerically these small volume fractions, since
they had approximately the same order of magnitude of the
error in the numerical method.
A moving-deforming grid study was carried out using a commercial computational fluid dynamics (CFD) solver, FLUENT® 6.2.16. The goal was to quantify the level of mixing of a lower-viscosity additive (at a mass concentration below 10%) into a higher-viscosity process fluid for a large-scale metering gear pump configuration typical in plastics manufacturing. Second-order upwinding and bounded central differencing schemes were used to reduce numerical diffusion. A maximum solver progression rate of 0.0003 revolutions per time step was required for an accurate solution. Fluid properties, additive feed arrangement, pump scale, and pump speed were systematically studied for their effects on mixing. For each additive feed arrangement studied, the additive was fed in individual stream(s) into the pump-intake. Pump intake additive variability, in terms of coefficient of spatial variation (COV), was >300% for all cases. The model indicated that the pump discharge additive COV ranged from 45% for a single centerline additive feed stream to 5.5% for multiple additive feed streams. It was found that viscous heating and thermal/shear-thinning characteristics in the process fluid slightly improved mixing, reducing the outlet COV to 3.2% for the multiple feed-stream case. The outlet COV fell to 2.0% for a half-scale arrangement with similar physics. Lastly, it was found that if the smaller unit's speed were halved, the outlet COV was reduced to 1.5%.
Induced draft fans extract coal-fired boiler exhaust gases in the form of a two-phase flow with a dispersed solid phase made of unburnt coal and fly ash; consequently fan blades are subject to erosion causing material wear at the leading edge, trailing edge, and blade surface. Erosion results in blade material loss, a reduction of blade chord, and effective camber that together degrade aerodynamic performance. This paper presents a numerical study of the erosive process in an induced draft fan carried out by simulating the particle laden flow using an original finite element Eulerian-Lagrangian solver. The particle trajectories are calculated using a particle cloud tracking technique that considers drifting near wall and an algebraic erosion model. The numerical study clarifies the influence of fan operation to the determination of the erosion regimes and patterns. In particular, the study investigates the role played by the size and mass distribution of the particles by considering a real composition of the flying ashes in the exhaust flow from a coal-fired boiler. The results illustrate the critical blade areas and erosion rates as given by the particle dynamics of different sizes. A specific analysis of the material wear at the blade leading edge is also given.
A large-scale parametric air–water test stand (AWTS) study involving more than 40 evaluations was carried out for the purposes of three-stream airblast reactor feed injector characterization and optimization; a subset of seven air stream combinations is discussed here. The role of CFD as a supplement to, or a replacement for, air–water testing is of great industrial interest. To this end a set of CFD simulations was carried out to complement the AWTS study. Pressure responses, spray opening characteristics near the feed injector face, and spray distribution were primary measures for both the AWTS and CFD programs. It was found that, over the range of variables studied, there was somewhat of a match between CFD and AWTS results. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS (frequencies between 75 and 600Hz), and an interesting transition in spray bursting character occurred at moderate inner air flows. The oscillatory flow pattern mimics prior work in terms of the energy of the fluctuations, but the fact that the present fluctuations occur at an order of magnitude lower frequency is apparently related to the comparatively low gas/liquid momentum ratio in the current study. Overall, it is shown that the CFD method contained herein can be used to supplement, but not replace, air–water testing for said injector configuration.
The aim of the present paper is to introduce and to discuss inconsistency errors that may arise when Eulerian and Lagrangian models are coupled for the simulations of turbulent poly-dispersed two-phase flows. In these hybrid models, two turbulence models are implicitly used at the same time and it is important to check that they are consistent, in spite of their apparent different formulations. This issue is best revealed in the case of very small particles, or tracer-limit particles, where it is assessed that coupling inconsistent turbulence models (Eulerian and Lagrangian) can result in non-physical results, notably for second-order fluid velocity moments. Computations for fluid particles in a turbulent channel flow using several coupling strategies are presented to illustrate this question.
Dispersion of ellipsoidal particles in a simulated isotropic pseudo-turbulent field is studied. A procedure using Euler's four parameters in describing the particle orientations is used, and the governing equations for the translational and rotational motions of particles are outlined. Turbulence fluctuation velocity field is simulated by a Gaussian random model. Motions of ellipsoidal particles of different sizes and lengths are analyzed. Ensemble and time averagings are used for evaluating various statistics of particle motion. Effects of size, shape, and density ratio on the mean-square particle velocities and the relative particle diffusivities are studied. By applying the orientation-averaging procedure, an analytical model for estimating the mean-square particle velocities and the relative diffusivities is developed. The predictions of the approximate model are compared with the simulation results and discussed.
Mechanism of particle separation in a cyclone separator is fully clarified by one-way coupled numerical simulations of large eddy-simulation and particle tracking. The former resolves all the important vortical structures while the later inputs the computed flow fields and tracks trajectories of particles by considering Stokes drag as well as gravity. The computed axial and tangential velocities of the swirl flow in a cyclone well compare with the ones measured by particle image velocimetory (PIV). The precession frequency of the vortex rope computed for Stairmand cyclone also matches with the one measured by Darksen et al. The predicted collection efficiencies reasonably well agree with the measured equivalents for two cylindrical cyclones with different diameters and inflow conditions. Detailed investigations on the simulated vortical structures in the test cyclones and predicted trajectories of the particles have revealed that there are three major paths of trajectories for those particles that are not collected and exhausted from the cyclone. More than half of the exhausted particles are trapped by longitudinal vortices formed in the periphery of the vortex rope. Namely, the precession motion of the vortex rope generates a number of longitudinal vortices at its periphery, which trap particles and move them into the region of the upward swirl.
Temporally developing DNS for channel flow at Reτ = 180 and 590 are performed to understand the turbulence generation mechanism for wall bound flow transition. Simulations for Reτ = 180 were performed for initial turbulence intensities TI = 0.1%, 1%, 2.5% and 5% and for Reτ = 590 with TI = 1% and 5%. The results in the fully developed turbulent region were compared with Moser et al. (1999) DNS data to validate the predictions. The near-wall vortical structures, mean and turbulent stresses and energy spectra, and turbulent kinetic energy (TKE) and stress budget in the pre-transition, transition and turbulent regions are analyzed to understand the turbulence onset, growth and decay mechanism. Finally, molecular diffusion and pressure strain time scales in the pre-transition to turbulent regions are analyzed to evaluate the turbulence onset criteria.
Copyright © 2016 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
The first step in air pollution control is to determine the acceptable emission level for a specific pollutant. In most cases, the Environmental Protection Agency (EPA) has done this. A synopsis of EPA control requirements for various industrial applications presents simplified general guidelines. To obtain an operating permit, however, the industrial process user must submit a request that specifies his system's maximum design emission level. To save the company a great deal of time and money and to avoid complicated EPA formalities, the specifying engineer needs a reliable method of establishing acceptable emission levels.
It is well known that the hydrodynamic drag on particles is significantly enhanced close to a plane or curved boundary. This enhancement impedes the movement of the particles in both the parallel and the normal directions with respect to the wall. In the presence of a temperature gradient, the Brownian movement of particles induces the phenomenon of thermophoresis, which results in the steady motion of the particles toward the colder domains of the flow field. This paper examines the effect of the enhanced wall drag on the thermophoretic movement of the nanoparticles in a Newtonian fluid, at short distances (0-10 radii) from a flat, horizontal wall. The effect of the flow shear lift on the thermophoretic motion of the particles close to a horizontal wall is also examined. It is observed that the movement of the particles toward the plane wall is significantly retarded because of the enhanced drag and that it, actually, causes particle accumulation close to the plane wall. It is also observed that the lift, which is induced by the relative Brownian movement, does not have an effect on the average motion of particles toward the wall and does not play an important role on the deposition of particles.
Pneumatic transport of solids in a riser has a unique nonuniform flow structure, characterized by the core solids acceleration and the wall solids deceleration along the riser, which causes the down-flow of solids and hence back mixing. To predict this nonuniform flow structure, this paper presents a mechanistic model that includes two controlling mechanisms: the interparticle collision damping for axial transport of solids and the effects of collision-induced diffusion and turbulent convection for radial transport of solids. The model predictions are partially validated against available measurements, such as axial and radial distributions of concentration and velocity of solids.
The thickness measurements showed that boilers of the Shahid Rajaee power plant have a non-uniform thickness in some regions of the final reheater tubes after 80,000 h of operation. Experimental tests on areas such as thickness, hardness, metallography, and recorded temperature showed that high temperature erosion is the most obvious reason for thinning in these tubes. Therefore, two possible reasons for the non-uniform tube thinning (problems due to combustion and steam mal-distribution in the tubes) were explored. This paper presents simulations of combustion, flow distribution in reheater tubes, and heat exchange process in Pass 1 of the boiler using FLUENT software. Combustion simulation results showed temperature distribution and mass flux of combustion products are not uniform at the chamber outlet. But these non-uniformities are not proportional to the tubes’ thickness non-uniformity; whereas, simulations showed steam is mal-distributed in the tubes so that steam maldistribution is proportional to the tubes’ thickness non-uniformities. Because steam maldistribution was due to mal-feeding and offloading of headers (U-type headers), all possible ways of feeding and offloading of headers were studied, and the results showed that the H-type configuration has the most uniform flow distribution.
A method to assess computational fluid dynamics (CFD) models for polydisperse granular solids in a multifluid flow is developed. The proposed method evaluates a consistency constraint, or a condition that an Eulerian multiphase solution for a monodisperse material in a single carrier fluid is invariant to an arbitrary decomposition into a pseudo-polydisperse mixture of multiple, identical fluid phases. The intent of this condition is to develop tests to assist model development and testing for multiphase fluid flows. When applied to two common momentum exchange models, the constraint highlights model failures for polydisperse solids interacting with a multifluid flow. It is found that when inconsistency occurs at the algebraic level, model failure clearly extends to application. When the models are reformulated to satisfy the consistency constraint, simple tests and application-scale simulations no longer display consistency failure.
The aim of this work is to find an effective method to improve the collection efficiency of electrostatic precipitators (ESPs). A mathematic model of an ESP subjected to the external magnetic field was proposed. The model considered the coupled effects between the gas flow field, particle dynamic field and electromagnetic field. Particles following a Rosin-Rammler distribution were simulated under various conditions and the influence of the magnetic field density on the capture of fine particles was investigated. The collection efficiency and the escaped particle size distribution under different applied magnetic field intensities were discussed. Particle trajectories inside the ESP under aerodynamic and electromagnetic forces were also analyzed. Numerical results indicate that the collection efficiency increases with the increase of applied magnetic field. It was also found that a stronger applied magnetic field results in a larger particle deflection towards the dust collection plates. Furthermore, the average diameter of escaping particles decreases and the dispersion of dust particles with different sizes increases with the increasingly applied magnetic field. Finally, the average diameter decreases almost linearly with the magnetic field until it drops to a certain value. The model proposed in this work is able to obtain important information on the particle collection phenomena inside an industrial ESP under the applied magnetic field.
The turbulent flow within a cylinder-on-cone cyclone is highly three-dimensional and our knowledge of this flow has yet to be improved. This work aims to improve our understanding of the flow structure, with special attention to the swirl number effect. The three velocity components of the flow were measured using LDA and PIV. The Reynolds number, based on the inlet velocity and the cyclone cylindrical chamber diameter, was 7.4 x 10(4), and the swirl number examined was from 2.4 to 5.3. Three regions of the flow have been identified after careful analysis of the data, which are referred to as the core, the outer and the wall-affected regions, respectively; each is distinct from another in terms of the vorticity concentration, frequency of quasi-periodical coherent structure, the probability density function, and mean and variance of velocities. It has been found that the flow, including its Strouhal numbers and radial distributions of the mean and fluctuating velocities, depends considerably on the swirl number. [DOI: 10.1115/1.4005139]
The thickness measurements showed that boilers of the Shahid Rajaee power plant have
a non-uniform thickness in some regions of the final reheater tubes after 80,000 h of operation.
Experimental tests on areas such as thickness, hardness, metallography, and
recorded temperature showed that high temperature erosion is the most obvious reason
for thinning in these tubes. Therefore, two possible reasons for the non-uniform tube thinning
(problems due to combustion and steam mal-distribution in the tubes) were
explored. This paper presents simulations of combustion, flow distribution in reheater
tubes, and heat exchange process in Pass 1 of the boiler using FLUENT software. Combustion
simulation results showed temperature distribution and mass flux of combustion
products are not uniform at the chamber outlet. But these non-uniformities are not proportional
to the tubes’ thickness non-uniformity; whereas, simulations showed steam is
mal-distributed in the tubes so that steam maldistribution is proportional to the tubes’
thickness non-uniformities. Because steam maldistribution was due to mal-feeding and
offloading of headers (U-type headers), all possible ways of feeding and offloading of
headers were studied, and the results showed that the H-type configuration has the most
uniform flow distribution. [DOI: 10.1115/1.4004081]
Keywords: CFD simulation, combustion, flow mal-distribution
A moving-deforming grid study was carried out using a commercial CFD solver, Fluent® 6.2.16, in order to quantify the level of mixing of a lower viscosity additive (at a mass concentration below 10%) into a higher viscosity process fluid for a large-scale metering gear pump configuration typical in plastics manufacturing. Second order upwinding and bounded central differencing schemes were used to reduce numerical diffusion. A maximum solver progression rate of 0.0003 revolutions per timestep was required for an accurate solution. Fluid properties, additive feed arrangement, pump scale, and pump speed were systematically studied for their effects on mixing. For each additive feed arrangement studied, the additive was fed in individual stream(s) into the pump intake. Pump intake additive variability, in terms of coefficient of variation (COV), was > 300% for all cases. The model indicated that the pump discharge additive COV ranged from 32% for a single centerline additive feed stream to 3.9% for multiple additive feed streams. It was found that viscous heating and thermal/shear-thinning characteristics in the process fluid slightly improved mixing, reducing the outlet COV to 2.3% for the multiple feed stream case. The outlet COV fell to 1.4% for a half-scale arrangement with similar physics. Lastly, it was found that if the smaller unit’s speed were halved, the outlet COV was reduced to 1.1%.
To better understand how human movement causes particles to be resuspended from the ground, we performed flow visualization and PIV measurements on idealized human walking, a disk moved normal to the ground. The flow visualization indicates that particles are resuspended on both the down step and the up step of the walking process by a purely aerodynamic mechanism. The results suggest that a wall jet formed beneath the disk is responsible for particle resuspension, whereas large scale vortices created in the wake of the disk are responsible for the rapid redistribution of the resuspended particles.
Unsteady flow features of a plant-scale (>1.5 m diameter) cyclone-ejector system have been studied numerically and validated experimentally. Complexity arises from the fact that the transient pressure field within the Lapple cyclone governs the operation of the annular ejector, and vice versa. Eight geometric configurations for improving the system operation were evaluated. Simple geometric changes were shown numerically to make operational improvements while incrementally improving particle collection efficiency. It was also found that compressible, time-dependent CFD results were extremely sensitive to the pressure discretisation approach and to the differential Reynolds Stress pressure strain formulation.
Turbulent flow structure in a cylinder-on-cone cyclone was experimentally investigated. Measurements were conducted at a fixed geometrical swirl number. Experiments were performed at a swirl number of 3 and Reynolds numbers from 37,100 to 74,200, based on the inlet velocity and the cyclone body diameter. The flow field in planes normal to and through the cyclone axis was measured in detail using a two-component laser Doppler velocimetry (LDA) and a particle imaging velocimetry (PTV). Two dominant frequencies of vortical structures were identified based on LDA-measured tangential and axial velocity spectra. Although one of them agreed quite well with those in literature, the other was reported for the first time. One explanation was proposed.
The hydrodynamics of multiphase flow in a Liquid-Liquid Cylindrical Cyclone (LLCC) compact separator have been studied experimentally and theoretically for evaluation of its performance as a free water knockout device. In the LLCC, no complete oil-water separation occurs. Rather, it performs as a free water knockout, delivering a clean water stream in the underflow and an oil rich stream in the overflow. A total of 260 runs have been conducted for the LLCC for water-dominated flow conditions. Four different flow patterns in the inlet have been identified, namely, Stratified flow, Oil-in-Water Dispersion and Water Layer flow, Double Oil-in-Water Dispersion flow, and Oil-in-Water Dispersion flow. For all runs, an optimal split ratio (underflow to inlet flow rate ratio) exists, where the flow rate in the water stream is maximum with 100% water cut. The value of the optimal split ratio depends upon the existing inlet flow pattern, varying between 60% (for Stratified and Oil-in-Water Dispersion and Water Layer flow patterns) to 20% for the other inlet flow patterns. For split ratios higher than the optimal one, the water cut in the underflow stream decreases as the split ratio increases. A novel mechanistic model has been developed for the prediction of the complex flow behavior and the separation efficiency in the LLCC. The model consists of several sub-models, including inlet analysis, nozzle analysis, droplet size distribution model, and separation model based on droplet trajectories in swirling flow. Comparisons between the experimental data and the LLCC model predictions show excellent agreement. The model is capable of predicting both the trend of the experimental data as well as the absolute measured values. The developed model can be utilized for the design and performance analysis of the LLCC.
The performance of a newly developed cyclone dryer is investigated using RANS-based single-phase computational fluid dynamics (CFD) and experimental model studies. The cyclone dryer is a cylindrical tower, divided by conical orifices into several chambers; recirculation of the flow within individual chambers ensures adequate retention time for drying of the transported solid material. Numerical calculations are performed using the commercial CFD code CFX5.7 for different mesh types, turbulence models, advection schemes, and mesh resolution. Results of the simulation are compared with data from experimental model studies. The RNG k- turbulence model with hexahedral mesh gives satisfactory results. A significant improvement in CFD prediction is obtained when using a second order accurate advection scheme. Useful descriptions of the axial and tangen- tial velocity distributions are obtained, and the pressure drop across the cyclone dryer chamber is predicted with an error of approximately 10%. The optimized numerical model is used to predict the influence of orifice diameter and chamber height on total pressure drop coefficient. DOI: 10.1115/1.2354523
This paper describes a theoretical investigation into (i) the response of a spherical particle to a one-dimensional fluid flow, (ii) the motion of a spherical particle in a uniform two-dimensional fluid flow about a circular cylinder and (iii) the motion of a particle about a lifting aerofoil section. In all three cases the drag of the particle is allowed to vary with (instantaneous) Reynolds number by using an analytical approximation to the standard experimental drag-Reynolds-number relationship for spherical particles.
A stochastic inter-particle collision model for particle-laden flows to be applied in the frame of the Euler/Lagrange approach is introduced. The model relies on the generation of a fictitious collision partner with a given size and velocity, whereby no information is required on the actual position and direction of motion of the surrounding real particles. However, the fictitious particle is a representative of the local particle phase properties. In sampling the velocity of the fictitious particle correlation with the velocity of the real particle as a consequence of turbulence is accounted for. The occurrence of a collision is decided based on the collision probability according to kinetic theory. For validating the collision model, results from large eddy simulations (LES) are used for monodisperse particles being dispersed in a homogeneous isotropic turbulence and a binary mixture of particles. In the case of the binary mixture two situations are considered; a granular medium without particle-flow interaction and two fractions of particles settling under the action of gravity in an isotropic homogeneous turbulence. For all the considered test cases the agreement of the model calculations with the results obtained by LES was found to be very good.
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