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Abstract: Numerical technology has been widely used for the study of the electrostatic precipitators (ESP) and the bag filters. This paper presents a numerical model for a scale hybrid particulate collector (HPC), which combines the ESP technology and the filtration technology together. The collection process of the HPC is unsteady as the pressure drop across the bag filter increases with the deposition of the particles. The physical processes of the model include the corona discharge, the fluid flow, the particle charging and the filtration. The corona discharge field is solved by using a finite volume method. For the fluid field, the unsteady and incompressible Navier–Stokes equations with the RNG κ–ε turbulence equations are solved. The effect of the electric field on the fluid field named electro-hydrodynamic is also considered. For the particle charging, the filed-diffusing combined model of Lawless (1996) is adopted. For the filtration, an unsteady cake formation model is proposed. The pressure drop across the cake is calculated according to the mass density of the cake. The coefficient between the pressure drop and the mass density of the cake comes from the experimental data. Applying the numerical model to the HPC, the influence of the hole diameter of the perforated-plate on the collection efficiency of the electrostatic zone is analyzed. Numerical results show that the collection efficiency of the electrostatic zone of the HPC has no certain relation with the hole diameter of the perforated plate. The effect of the hole diameter of the perforated-plate on the collection efficiency of the electrostatic zone becomes weaker with increasing the applied voltage. [Copyright &y& Elsevier]

Abstract: Four kinds of solid particles were captured by a single fiber. The particles included two kinds of fly ash particles and two kinds of ceramic particles under three different types of charging pretreatment. The single fiber was fixed across a square cross-section glass tube. A standard continuous aerosol generator was used to disperse particles to generate a uniform aerosol. The aerosol particles from the generator were tribocharged, polarized or charged, and then passed across the fiber. A microscope and a CCD camera were used to observe the capturing process and the shape distribution of dendrites. The results showed that the deposited particles developed in different ways. In the tribocharged case, dendrite formation can be classified into three distinct stages. In the prepolarized case, straight chains were formed at a uniform spacing interval, and had high binding intensity to support very long chains. Even though the long chains fell over they still had high capturing efficiency. In the precharged case, some straight chains with some branches were formed, but their binding intensity was low and they easily fell over and broke. The results also showed how the particle diameter and shape influenced the formation of dendrites and chains. [Copyright &y& Elsevier]

Hybrid structure significantly affects the collection mechanism and performance of a hybrid particulate collector. The electrostatic precipitation of particles in the wire-perforated plate structure of a full-scale compact hybrid particulate collector is numerically simulated in this study. The distributions of electric and flow fields and the charging, motion, and precipitation of particles in this specific structure are studied. The results show that the distribution of electric field in the final stage is asymmetrical because of the baffle plate at the end of the channel. The field distributions of other stages are identical to that of wire-plate electrostatic precipitator. The openings increase the electric field strength in the region adjacent to the perforated plate. The electric field passes through the openings, which is then distributed to the back side of the perforated plate. The aerosol cross flow rate along the perforated plate varies periodically under the effect of electric body force. Whereas, the overall cross flow rate of each stage is the same, except the first stage. Two counter-rotating eddies are formed behind the perforated plate between every two openings. Particle charge exceeds 80% of the final charge when the particles move to the position of the first wire. The 10 μm particles finish their charging processes faster than 1 μm particles with a final charge of approximately 100 times that of 1 μm particles. The charge acquired by a particle under the wire-perforated plate structure is 3% higher than that under the wire-plate structure. Particle trajectory result shows three modes of electrostatic precipitation in a compact hybrid particulate collector, namely, collection on the front, flank, and back sides of the perforated plate. Particle transport by eddies on the back of the perforated plate plays an important role in particle deposition on the back side. Variation in the capture probability of particles released from different positions corresponds to the opening structure. High and low probability areas separate with each other. The collection efficiency of 10 μm particle is higher than that of the 1 μm particle. The results can explain hybrid mechanism and optimize hybrid structure. [ABSTRACT FROM AUTHOR]

The advanced hybrid particulate collector (AHPC) is a highly promising and novel hybrid electrostatic filter technology that removes particles from flue gas. Perforated plates are used as collecting electrodes to attain better synergism between the electrostatic and bag zones. A laboratory-scale AHPC was built to investigate the dust collection performance of perforated plates with different percentages of open area (POA) and opening types. Particles were deposited on the front, flank, and back sides of the plate. Differences in cake packing density were observed at different locations. With the increase in POA, the collection efficiency of the front side decreases, whereas the collection efficiencies of the flank and back sides increase. The increase in the collection of the flank and back sides compensates for the decrease in the collection of the front side, which only changes slightly the total efficiency with the POA. The opening type exhibits minimal effect on total efficiency. A numerical method was established to simulate the dust collection process in the electrostatic zone. Simulation results indicate that the openings change the electric field near the plate and facilitate particle deposition on the flank and back sides, which result in the characteristics of collection efficiency. The difference in the cake structure results from the distribution of electric field strength on the plate surface. [ABSTRACT FROM AUTHOR]

The ash fouling during coal combustion is initialized by a deposition of ‘sticky’ inner layer composed of fine particulate matter (PM). In this work, we present a quantitative investigation on the built-up of the initial deposit layer and its roles in the capture of coarse fly ash particles. The fly ash and ash deposit are sampled in a 25 kW self-sustained pulverized coal combustor for various lignite samples including Zhundong and Hami lignites. The ash collection efficiency onto the probe is positively correlated to the ultrafine particle formation fraction. The deposition rate of fine particle PM 10 is modeled in a combined Eulerian-Lagrangian scheme, with an input particle concentration measured at the entrance of boundary layer. The predicted results divulge that the thermophoresis enhances the impaction efficiencies of submicron and micron sized particles by 1–2 orders. The deposited particles are more concentrated on the central top region of the probe, and the trend is even more remarkable for larger particles, consistent with experimental observations. For both Zhundong and Hami lignites, the deposits are largely composed of fine PM 10 and condensed matter during the initial stage of one minute, but the weight percentages of fine particle deposits over the total deposit mass decrease significantly in the subsequent 5–10 min. Consequently, the average sticking efficiency of the coarse mode PM 10 + increases asymptotically as the deposited PM 10 grows to ~ 1.5 g/m 2 . Zhundong lignite shows higher saturated sticking efficiency of PM 10 + than Hami lignite, which is possibly attributed to the different surface properties of the bulk ash particles. [ABSTRACT FROM AUTHOR]

As an important particle collection method, filtration is significantly affected by particle precharging. The filtration of differently charged fly ash particles using membrane and fibrous filters was experimentally studied. Particle charge was directly measured and quantitatively related to the enhancement of filter performance. Particle penetration, which was observed in the fibrous filter during the early stages of filtration, exponentially decreased with increasing particle charge when the filter was clean and rapidly decreased with increasing particle load. A penetration reduction factor ψ was defined to represent the effect of particle charge on penetration, and its correlation with particle charge was defined on the basis of the fitting experimental results. The average porosity of the dust cakes that were formed on both filters increased with particle charge, which in turn exponentially decreased the pressure drop. The electrostatic effect on pressure drop was less prominent than that on penetration. A pressure drop reduction factor θ was defined to quantify the effect of particle charge on pressure drop. Correlations for predicting the evolution of average cake porosity and pressure drop in consideration of particle charge were proposed on the basis of the fitting experimental results. [ABSTRACT FROM AUTHOR]

Abstract: The impaction-sticking mechanism of fine particulates plays a significant role in a wide range of applications from dust separation devices, thin-film deposition techniques to the astrophysics science, but the underlying physics still remains unclear. In this paper, a discrete element method (DEM) approach is established to investigate the impaction-sticking process during the initial deposition of fine particles on a single fiber. Starting from the JKR adhesive contact theory, the DEM well predicts the measured trend of single-fiber capture efficiency as a function of Stokes number in the literature, by the proper considerations of key dissipation terms including the first-contact energy loss, the linear-dashpot damping and the rolling friction resistance. The increasing work of adhesion not only dramatically increases the peak value of single fiber efficiency, but also causes the peak to move towards a much higher Stokes number. The DEM predictions further clarify that the sticking probability depends on the adhesion parameter, rather than Stokes number. An empirical power law between sticking probability and adhesion parameter is drawn as h =0.0558⋅Ad5/3 (for Ad<5.65). Finally, by using both adhesion parameter for the sticking and Stokes number for the impaction, four distinct deposition patterns during the initial stage of single fiber filtration are identified. [Copyright &y& Elsevier]

Abstract: A JKR-based dynamic model, with energy dissipation through both irreversible snap-on/snap-off process and viscoelastic effect, is developed to simulate the dynamics of low-velocity normal impact of micro-sized particle with a flat surface. The first-contact energy loss because of the incompletely reversible, quasi-static loading/unloading is incorporated in the dissipation model. The damping forces corresponding to both the attractive and repulsive components of the JKR contact forces are introduced to account for the hysteresis related to material viscoelasticity. The predicted particle behaviors for the impacts with incident velocities below or above the critical velocity are discussed. The predictions of both the critical velocity and the variation of restitution coefficient vs. incident velocity are compared with experiments in literature where reasonable agreement is obtained. The dependence of restitution coefficient on incident velocity is analyzed. Finally, the model sensitivity analysis such as the effect of the damping coefficient variation is discussed. The work provides a solid basis for the development of discrete-element-method approach of micro-sized particulate system. [ABSTRACT FROM AUTHOR]

Particle impaction on a droplet surface directly affects the particle capture by the droplet, and this phenomenon is the basic process of wet scrubbing and wet deposition. The experimental system is established to in-situ observe the behavior of micron particles impacting on the liquid surface. 50–200 μm hydrophobic PMMA and PS particles were used. The particles showed two types of motion behavior, namely, submergence and oscillation, after impaction onto the liquid surface. During particle sinking, the advancing contact angle remained constant. The shape of the liquid surface met the Young–Laplace equation under the quasi-static assumption. After the angle of the three-phase line reached the critical value, the liquid surface was closed, and the particle submerged. A small bubble formed, adhering to the particle trailing surface. If the particle velocity decreases to zero before the angle of the three-phase lines reaches the critical value, the particle will reverse the direction of its motion. When the particle moves to the horizontal liquid surface, the movement ceases. Reciprocating oscillation behavior was not observed. During particle reversion, the receding contact angle gradually decreased and then remained unchanged. The large contact angle hysteresis is the main cause of failure observation in rebound motion mode in the experiment. The changing rule of the critical submergence/oscillation velocity with the particle diameter and surface tension coefficient was studied. As the particle diameter increases, the critical submergence/oscillation velocity decreases. As the surface tension coefficient increases, the critical submergence/oscillation velocity increases. The observed critical submergence/oscillation velocity in this experiment demonstrated the accuracy of the proposed model. [ABSTRACT FROM AUTHOR]

Abstract: Nanoporous TiO2 thin films are deposited directly onto substrates by a one-step stagnation flame synthesis with organometallic precursors. The deposition mechanism in the stagnation-point boundary layer is intensively studied. For the first time, the radial profile of nanoparticle deposition flux is measured using a novel method of concentric collecting rings, which exhibits similar trend with the heat flux profile of stagnation-point flows. Then, we develop the mathematical model of nanoparticle transport and deposition in the stagnation-point boundary layer for further clarifying experimental results, especially the effects of substrate temperatures and in-situ produced particle sizes. Both thermophoresis in an inner part of boundary layer and thermal compression/expansion of the gas phase are found to play important roles in determining the deposition flux. The contribution of Brownian diffusion, determined by a thermophoretic Peclet number, is inappreciable compared to thermophoresis until particle diameter is as small as 2nm. The results in this work support a conclusion of size-independence of the thermophoretic velocity, implying that the rigid-body collision assumption of Waldmann''s formula is not accurate for small particles especially less than 10nm. This study can be generally applied to other deposition techniques of thin films. [Copyright &y& Elsevier]