Search

This paper presents a numerical study of the complex multiphase flow of air, vapor, and particles in an innovative gas cyclone called CAP cyclone. The air–vapor flow is modeled as a mixture by the Reynolds‐averaged Navier‐Stokes equations with the mixture species transport and the Eulerian wall film model using FLUENT. Particle flow is modeled by the Lagrangian particle tracking method and the condensational growth of particle droplets is modeled via a user‐defined function. The model is validated by reaching good agreement with experimental results. Flow field analysis shows that the added vapor does not change the major vortex characteristics in the cyclone, but the vapor distribution is not uniform. The vapor concentration is much higher in the upper part than in the lower part of the cyclone, leading to insufficient condensational growth in the lower part. A secondary vapor injection is proposed to improve the vapor concentration in the lower part, which is shown to be effective in improving the collection efficiency. The model and the results are helpful to the understanding and optimization of the CAP cyclone technology, and also the vapor and particle droplet flow in turbulent flows. [ABSTRACT FROM AUTHOR]

• Electro-acoustic nebulizer was used to enhance the collection efficiency of cyclone. • Surfactants were added to the vapor to improve the wettability of fine particles. • Effects of three surfactants were studied and compared experimentally. • Both overall and grade removal efficiency can be improved by adding surfactants. • The agglomeration of particles was confirmed by morphological changes of particles. Using water vapor to improve the efficiency of dust removal in gas cyclones was recently proposed. However, some dust particles are not hydrophilic which can reduce the effectiveness of the vapor. This paper investigates using surfactants to overcome such a difficulty. In particular, surfactants are added to the water solution which is atomized into vapor and added in a cyclone dust collector. The effects of surfactant type and quantity on the removal efficiency of the cyclone are studied by a series of experiments. Three surfactants are used to change the wettability of the two types of fine particles to be collected: molecular sieve dust and white carbon black. The particles in the cyclone are found to form agglomerates, which are measured by using a laser particle size analyzer and scanning electron microscopy. The results show that the addition of surfactants can greatly enhance the agglomeration and thus improve the collection efficiency, especially for particles approximately two microns in size. This indicates that using the atomized vapor with containing surfactants is an effective way to enhance the collection of fine particles in a cyclone separator. [ABSTRACT FROM AUTHOR]

Gas cyclones are widely used to purify waste gases but are not effective in collecting fine particles. The cloud-air-purifying (CAP) technology was recently developed to overcome this deficiency by unitizing the supersaturated vapor in the cyclone. This paper presents a CFD study on the CAP cyclone. The condensational growth of particles is implemented in the CFD model. Its positive effect on the collection efficiency is confirmed and more clearly understood from microscopic investigations. Particles with the same initial size can grow into a narrow size distribution, yet the average growth ratio is increased with the increase of the supersaturation ratio and the decrease of the initial size, resulting in a significant improvement of the collection efficiency for fine particles, particularly in controlling those escaping through the middle-reverse flow. A predictive model is developed to consider the effect of the vapor. These results can help understand and optimize the CAP cyclone. Unlabelled Image • A numerical model developed to simulate gas cyclone with supersaturated vapor. • Condensational growth of particles implemented in CFD. • Positive effect of supersaturated vapor studied microscopically. • Effect of supersaturated vapor on different escaping particle flows analyzed. • A gas cyclone predictive model adapted to consider supersaturated vapor. [ABSTRACT FROM AUTHOR]

Gas cyclones are widely used for separating particles from gas, while recently various techniques have been developed to improve their performance. One is to use the supersaturated water vapor to increase the sizes of fine particles and hence their collection efficiency and another is to use the vortex-broken wing to decrease the pressure drop. This paper studies the synergistic effect of these two techniques. The supersaturated vapor, the vortex-broken wing and their combination were added to a conventional cyclone respectively and compared to the original cyclone by both experiments and numerical simulations. Through experiments, it was found that the removal efficiency of fine particles in the cyclone with the combined techniques was improved significantly while the pressure drop was reduced at the same time. The mechanism for such improvement was revealed through the computational fluid dynamics simulations. It was observed that the inner vortex can be effectively destroyed by the vortex-broken wing. This not only reduced the pressure drop, but also decreased the turbulence intensity under the vortex finder, which promoted the diffusion of water vapor and made the vapor distributes more uniformly. As demonstrated in the simulation, the improvement of the vapor distribution can enlarge the supersaturated region and enhance the condensational growth of fine particles. This was also testified by the online laser particle size analyzer in experiments. The findings establish a theoretical guidance for this new type cyclone and may provide new idea for the treatment of fine particles. Unlabelled Image • The collaborative effect of vapor and vortex broken wing was investigated. • The distribution of water vapor in cyclone has been greatly improved. • The particle-droplet diameter can be enhanced by the vortex broken wing. • The removal efficiency of fine particles was increased greatly. • The new cyclone technology showed obvious performance advantages. [ABSTRACT FROM AUTHOR]

• Vortex eccentricity in gas cyclone studied by CFD. • Increasing inlet symmetry and velocity reduce vortex eccentricity. • Single inlet causes vortex precession center deviating from geometrical center. • Effect of vortex eccentricity on cyclone performance quantified. • Symmetrical inlets significantly reduce short-circuiting particles. This paper presents a numerical study of the vortex eccentricity in a gas cyclone and its effect on the performance of the cyclone. The gas flow in the cyclone was modeled as an unsteady flow by the Navier–Stokes equations with the Reynolds Stress Model (RSM) as the turbulence model. The particles were modelled by the Lagrangian particle tracking (LPT) approach in an unsteady gas flow. Gas cyclones with the same dimensions and total flow rates but different numbers of inlets were simulated with the inlet velocity varying from 12 to 20 m/s. The vortex eccentricities in different cases were analyzed in terms of radial deviation and angular deviation. In addition, the frequency of the precessing vortex core (PVC) was analyzed by the fast Fourier transform (FFT). The results show that the vortex center in the single inlet cyclone has a great eccentricity and its precession center is also different from the geometric center, which reduces the particle collection efficiency. The increase in the symmetry of the inlet causes only a very small increase in the pressure drop in the simulated cases, but it can significantly reduce the vortex eccentricity, particularly by eliminating the eccentricity of the PVC center. The improvement of the vortex eccentricity can generally increase the collection efficiency for particles greater than 2.0 µm. The increase of the collection efficiency is mainly because the symmetrical vortex can restrain the short-circuiting flow of particles. The results can improve the understanding of the vortex flow in gas cyclones which may guide the optimization of gas cyclones. [ABSTRACT FROM AUTHOR]

This paper presents a study on the combination of atomization and electrostatic charging to improve the collection efficiency of fine particles in a gas cyclone. A technology combining atomization and cyclones called cloud-air-purifying (CAP) has recently been developed, which utilizes the supersaturated vapor in the cyclone to realize the enlargement of dust particles and improve the collection efficiency. Here, the technology is further improved by charging the particles and droplets. Three different charging schemes are studied, including charging vapor droplets only, charging dust particles only, and charging both of them with opposite polarities. Experimental results indicate that all three charging schemes can significantly improve the collection efficiency. In particular, with charging, the collection efficiency of particles of approximately 2 μm is mostly improved. Consequently, the combination of the CAP technology and electrostatic charging can be a promising novel method to effectively remove PM 2.5. Unlabelled Image • Use atomization and electrostatic charging to enhance collection efficiency. • Experimental study on fine particles collection efficiency. • The collection efficiency for particles around 2 μm is mostly improved. • The particles grow larger due to the enhanced agglomeration. [ABSTRACT FROM AUTHOR]

Dense medium cyclones (DMCs) are widely used in coal mine sorting processes to improve the quality of coal products and reduce pollution. The conventional linear inlet DMCs has the problems of unstable flow field, high turbulence density and low separation efficiency. Based on these concerns, the volute inlet DMCs was proposed, however the nose angle as one of the most critical parameters has not yet been investigated in depth. In this paper, a DMC with volute inlet is investigated, and the effect of nose angle is studied by a computational fluid dynamics model in terms of flow field, particle motion and the forces acting on the particles. The performance of the DMC is evaluated with respect to the inlet nose angle. As the nose angle increases from 0 to 180 deg., the separation efficiency has a tendency to increase and then decrease with the inflection point founding when nose angle is 180 deg. Then the flow field, particle distribution and particle force are analyzed respectively. The results show that the symmetry and stability of the flow field can be significantly improved as nose angle increases from 0 to 180 deg., but they remain almost constant when nose angle is 270°. In addition, a quantification method of the short-circuiting flow is established and it has been found that increasing the nose angle can notably reduce the short-circuiting flow. In terms of particle phase, increasing the nose angle causes heavier particle distribution tends to concentrate. During this period, the variation pattern of forces on particles with different nose angles is also analyzed. The DMC with a nose angle of 180 deg. has a longer acceleration zone at the entrance height and an increased radial force on the particles, which results in better performance. This study provides a theoretical basis and reference for the structure optimization of dense medium cyclones. [Display omitted] • The effect of nose angle on the performance of volute inlet DMC was investigated. • A CFD model was built for the multiphase flow and particle motion in the DMC. • A quantification method of the short-circuiting flow in the DMC was established. • The particle distribution and analysis of forces on particles in DMC were studied. • The volute inlet DMC with 180 deg. nose angle has better operation and separation performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposing five cyclones with diverse tilt angles for enhanced flow characteristics. • The formation mechanism of short-circuit flow are analyzed by numerical simulation. • Comparing Euler numbers for various cyclones through experiments and CFD. • Cutoff diameter and grade efficiency of different cyclones are evaluated. Short-circuit flow and circulation flow are important secondary flows in a cyclone, which have a negative impact on separation performance. This work proposed new cyclones (VH cyclone), incorporating volute-helical inlets with five tilt angles based on the characteristics of short-circuit flow and circulation flow, in order to enhance separation efficiency without a significant pressure drop compared to a Lapple-type cyclone with a tangential inlet (TI cyclone). The separation performance and Euler number of six cyclones are obtained by experiments and analytical models. The simulation is utilized to explore characteristics of short-circuit flow and circulation flow, with a focus on the impact of these flow patterns on particle motion. The result shows that proportion of short-circuit flow increases initially and then decreases with the increase of the tilt angle, while circulation flow is continually weakened. The cutoff diameter increases first and then decreases while the trend of Euler number is opposite for new cyclones. By considering both separation performance and Euler number, VH4 cyclone (Tilt angle of volute-helical inlet is 6.5°) exhibits optimal performance, with a cutoff diameter 20.41% lower than that of TI cyclone, while their Euler numbers are similar. [ABSTRACT FROM AUTHOR]

• Surfactants were used in a dedusting technology combing heterogeneous condensation and cyclone. • Better condensable growth and removal effect for fine particles was obtained. • The MD simulation was used to explore the improvement mechanism of surfactants. • The solid–liquid interaction energy was reduced and mobility of water molecules was improved. • The spatial distribution of the head group and tail chain is an important influence factor. Using heterogeneous-condensation agglomeration to enhance removal of fine particles by conventional equipment is an economical and efficient method. Recently proposed Cloud-Air-Purifying (CAP) technology, which combines heterogeneous-condensation agglomeration with a centrifugal field, has significantly improved the removal efficiency of fine particles in gas cyclones. In this work, surfactants were used to further enhance the removal of fine particles. The effect of three surfactants on promoting the removal of fine particles was investigated experimentally. The results indicated that non-ionic surfactant octylphenol polyoxyethylene ether (10) (OP-10) had the best effect on promoting the removal of methylated silica (SiO 2 -CH 3) particles, because it improved heterogeneous condensation of fine particles and collision agglomeration between particles by improving the surface wettability, resulting in the formation of larger aggregates. Furthermore, the interaction energy, mean square displacement and hydrogen bond interactions in different simulation systems were calculated by molecular dynamics simulation to study the mechanism of surfactants on improving the surface wettability. The simulation results demonstrated the hydrophobic tail chains of OP-10 molecules spontaneously adsorbed on the nonpolar SiO 2 (0 0 1)–CH 3 surface through the Van der Waals interaction, which changed the surface polarity and reduced the solid–liquid interaction energy. Meanwhile, hydrogen bonds were formed between polar head groups and water molecules, which enhanced the interaction between water molecules and the surface. Moreover, the mobility of water molecules is enhanced by surfactant molecules. The spatial distribution of the head group and tail chain of surfactant is an important factor affecting the surface wettability. This work is of great importance for enhancing the removal of fine particles in the dedusting system with heterogeneous-condensation agglomeration technology. [ABSTRACT FROM AUTHOR]

Reducing energy loss is one of the goals of optimizing gas cyclones, which can be benefitted from a deep understanding of the energy loss mechanism in gas cyclones. In this study, the gas-solid flow in a gas cyclone was simulated using the combined computational fluid dynamics (CFD) and the discrete element method (DEM). An energy loss model for analyzing the spatial distribution and proportion of the mean mechanical energy loss of the gas cyclone was adopted and validated in terms of the total pressure drop. It is found that the near-wall region generally has high rates for both viscous dissipation and turbulent production, especially the vortex finder, the conical part, and the discharger. The effects of the inlet velocity, the wall roughness, and the particle loading ratio on the energy loss are studied. The findings reveal that increasing the inlet velocity mainly increases the viscous dissipation of the wall and the turbulent production of the core region, resulting in an increase in mechanical energy loss. In contrast to the energy loss mechanism of the pipe, the mechanical energy loss of the gas cyclone decreases slightly with increasing wall roughness, which is mainly caused by the reduction of the viscous dissipation of the wall. By injecting particles, the spatial distribution of the energy loss rate is greatly altered, and the mechanical energy loss is also reduced. Further analysis shows that increasing the particle loading ratio reduces both viscous dissipation and turbulent production. [Display omitted] • Gas-solid flow of a gas cyclone is simulated by CFD-DEM method. • Model for analyzing energy loss is adopted, developed and validated. • Distribution of energy loss rate provides guidance for structural optimization. • Effects of inlet velocity, wall roughness, and particle loading are revealed. [ABSTRACT FROM AUTHOR]

This paper characterizes the vortex flow field in a gas cyclone based on different vortex identification methods, including vorticity method, Q criterion, λ 2 criterion, Ω method and Liutex method. Analysis and comparison of isosurfaces of different vortex identification methods indicate that except vorticity method, the vortex core, the eddy flow and the short-circuit flow can be identified directly by other methods. But the small scale vortices, such as top ash ring, can only be fully recognized by Liutex method. The contours demonstrate that the magnitude of five vortex parameters is the largest in the vortex core, followed by the short-circuit flow, and the eddy flow is the smallest. But only Liutex method can predict the vortex core center and identify the most severe vortex eccentricity is in the bottom of vortex finder, the lower part of the cone and the hopper. The work not only improves the understanding of the vortex structure in a gas cyclone, which can critically affects cyclone performance, but also reveals the differences in the commonly used vortex characterization methods. [Display omitted] • The vortex flow field was analyzed based on five vortex identification methods. • The vortex core, eddy flow and short-circuit flow was identified directly. • The isosurfaces of different vortex identification methods was compared. • The differences of five vortex characterization methods was revealed. • Vortex eccentricity in a gas cyclone was discussed by Liutex method. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new type of double-stage tandem nesting cyclone was designed. • The removal efficiency of fine particles was significantly improved. • The new double cyclone has great potential in the classification for three products. • The numerical model for the double cyclone was established and verified. This paper proposes a new type of gas cyclone separator, which nests an inner cyclone with a double-spiral inlet in an outer cyclone. Experiments were conducted to compare the separation performance of the new cyclone to that of the traditional Lapple cyclone. In addition, the flow fields and the particle collection in the two cyclones were simulated by the computational fluid dynamics (CFD) and compared. Compared with the traditional Lapple cyclone, the new double cyclone can present significantly improved collection efficiency, resulting from the two-stage separation and better symmetry of the flow field. On the other hand, the pressure drop of the new cyclone was only slightly elevated because of the low velocity in the outer cyclone. Moreover, this new double-stage cyclone separator can be further developed for particle classification, as the particles collected in the three outlets of the cyclone are in different size ranges. [ABSTRACT FROM AUTHOR]

• The performance of new cyclone for different particles was investigated. • The particles with better hydrophilicity showed higher efficiencies. • The nucleation rate and activation probability of used particles were calculated. • An analytical model was established to quantify the effect of hydrophilicity. • The "fish-hook" can be significantly depressed due to the added vapor. Using water mist or vapor to help collect fine particles has been attracting increasing attention in separation and purification. A new fine particle separation technology which combines the centrifugal force field with the supersaturated vapor was recently proposed to realize the high collection efficiency of PM2.5 in gas cyclones. In this and similar technologies the interaction between particles and vapor is vital to the collection efficiency. Therefore, this paper investigated the effect of the dust hydrophilicity in such purification process. Six kinds of particles with different hydrophilicities were tested in the new cyclone. The overall and grade separation efficiencies were measured, which were significantly improved when the vapor was added, and the particles with better hydrophilicity showed a greater improvement. The improvement can be attributed to the particle growth in the supersaturated vapor and the gas flow in the cyclone, which were shown by the SEM analysis of the morphology change of the particles. Additionally, theoretical analysis shows that the nucleation rate and activation probability of heterogeneous condensation are both higher for the particles with better hydrophilicity, thus these particles should be easier to be activated as condensation nucleus and hence grow in size. Based on the condensational growth, an analytical model was established which can quantify the effect of hydrophilicity on the grade efficiency curve of the new cyclone. [ABSTRACT FROM AUTHOR]