Effects of Electrohydrodynamic Flow and Turbulent Diffusion on Collection Efficiency of an Electrostatic Precipitator with Cavity Walls

The effects of electrohydrodynamic (EHD) flow and turbulent diffusion on the collection efficiency of particles in a model ESP composed of the plates with a cavity were studied through numerical computation. Electric field and ion space charge density in the ESP were calculated by the Poisson equation of electric potential and the current continuity equation of ion space charge. The EHD flow field was solved by the continuity and momentum equations of gas phase, including the electrical body force induced by the movement of ions under the electric field. RNG k - l model was utilized to analyze turbulent flow. Particle concentration distribution was calculated from the convective diffusion equation of particle phase. As the ion space charge increased, the collection efficiency of charged particles increased because the electric potential increased over the entire domain in the ESP. The collection efficiency decreased as the EHD flow became stronger when the electrical migration velocity of charged particles was high. However, the collection efficiency could increase for the stronger EHD flow when the electrical migration velocity of charged particles was relatively lower. Also, the collection efficiency decreased as the turbulent diffusion of particles increased when the electrical migration velocity of particles was high. However, the collection efficiency could increase with the turbulent diffusion when the electrical migration velocity of particles was relatively lower.     

Electrohydrodynamic Flow and Particle Transport Mechanism in Electrostatic Precipitators with Cavity Walls

The electrohydrodynamic (EHD) flow induced by the corona wind was observed in a model electrostatic precipitator (ESP) of the simple geometry composed of the plates with a cavity. And the influence of the EHD flow and the turbulence condition of inlet cross-flow on the particle behavior inside the ESP and its collection efficiency were elucidated through experimental and numerical analysis. The profiles of streamwise gas velocities and turbulence intensities were measured in the ESP with a laser Doppler anemometer.A laser beam sheet visualized particle trajectories. Collection efficiencies were measured with a particle counter. In addition, numerical computations were performed to compare with the experimental results. The numerical results showed good agreement with the experimental data. As the corona voltage increased, the gas velocities of the core flow and the circulating flow inside the cavity increased due to the corona wind and the turbulence intensity increased near the cavity region. As the corona voltage increased for the low bulk gas velocity, corona wind prevented the particle transport into the cavity. And the particle transport into the cavity by turbulent dispersion was observed as the bulk gas velocity increased. When the flow with high turbulence intensity entered the ESP, the turbulent dispersion enhanced the transport of particles into the cavity; hence, the collection efficiency was higher compared with the case of the relatively lower inlet turbulence intensity below a critical corona voltage. However, the collection efficiency was slightly lower for the high inlet turbulence than for the low inlet turbulence above the critical corona voltage due to the turbulent diffusion of particles toward the centerline downstream from the corona wire.     

Numerical investigation of mass transfer enhancement through a porous body affected by electric field

The use of electrohydrodynamic (EHD) actuator has emerged recently as an effective mean to enhance mass transfer. However, the effect of nonthermal EHD-induced mass transfer in such porous body has remained unclear. Hence, in this paper, the mass transfer enhancement with EHD technique in a porous body is numerically investigated using the finite volume method. The flow patterns and the moisture content have been studied for different Reynolds numbers and the applied voltages. The numerical results show that the EHD flow field amplifies the mass transfer by a factor of 20.5 for Re?=?200 and 2.12 for the Re?=?4,000 and V?=?28?kV in comparison to the case without the electric field.     

Numerical investigation of the multi-pin electrohydrodynamic dryer: Effect of cross-flow air stream

Abstract

This article presents the results of numerical simulation and experimental study of a multi-pin electrohydrodynamic (EHD) dryer. Combined effect of EHD flow and the external air cross-flow on drying performance was investigated with 3-D numerical model, which accounts for electric field, electric charge transport, external air cross-flow and material-gas moisture transport. Effect of cross-flow air stream on drying was positive in the range of low velocities, changing to negative at high velocities due to counteracting with EHD flow. Numerical simulation predicted previously unknown effect of EHD flow on the cross-flow air stream, which was quantified as an increase of airway resistance. This prediction was fully validated by experiments. Both numerical simulation and experiment proved that for given intensity of EHD flow there is an optimum value of the cross-flow, resulting in maximum drying performance. The numerical model can be applied to determine the optimal operating parameters for multi-pin EHD dryer.     

Numerical simulation of the characteristics of turbulent Taylor vortex flow

Turbulent Taylor vortex flow, which is contained between a rotating inner cylinder and a coaxial fixed outer cylinder with fixed ends, is simulated by applying the development in Reynolds stress equations mold (RSM) of the micro-perturbation. This resulted from the truncation error between the numerical solution and exact solution of the Reynolds stress equations. Based on the numerical simulation results of the turbulent Taylor vortex flow, its characteristics such as the fluctuation of the flow field, the precipitous drop of azimuthal velocity, the jet flow of radial velocity, the periodicity of axial velocity, the wave periodicity of pressure distribution, the polarization of shear stress on the walls, and the turbulence intensity in the jet region, are discussed. Comparing the pilot results measured by previous methods, the relative error of the characteristics predicted by simulation is less than 30%. Translated from Journal of East China University of Science and Technology (Natural Science Edition), 2006, 32(5), 617–622 [译自: 华东理工大学学报 (自然科学版)]     

A CFD-based design scheme for the perforated distributor with the control of radial flow

In the present study, flow homogenization by distributors in chemical apparatus is studied as a process of flow control and its mechanism is reconsidered from the model of resistance to the model of radial flow. This process is composed of four consecutive behaviors: the generation, distribution, conversion of the radial flow and the momentum transfer of axial flow. Based on these flow behaviors, the novel distributor is designed as the combination of perforated plate in the center area and vertical guiding baffles around. Taking the wire-screen catalytic reactor as a case study, numerical simulation is employed to optimize the structure of distributor and a CFD-based design scheme called “flow field analysis scheme” is proposed. Numerical simulation is conducted in the apparatus with a diffuser (inlet D₀ = 500 mm, main part D₁ = 3,000 mm) under the gas velocity of 3.6 m/s (corresponding Re ≈ 12,000). The numerical results from optimized distributor show that compared with the traditional perforated plate, the flow field adjusted by the novel distributor can achieve a better flow uniformity with lower energy consumption. The theoretical analysis and numerical results are also validated and proved by the experimental results.     

Scalar mixing in turbulent concentric round jets

The scalar mixing field of a free, turbulent concentric round jet has been examined using marker nephelometry. The flow conditions included velocity ratios between the centre and annular jet of 0.188, 0.519 and 0.911. The correlation function between the concentration fluctuations in the two jet streams increased from –1.0 near the nozzle to + 1.0 further downstream indicating a tendency to complete mixing between the jets. The initial mixing behaviour between the jets was better for the lower velocity ratio between the jets (U_i/U_o = 0.188) although further downstream there was better transverse mixing between the jets with the higher velocity ratio (U_i/U_o = 0.911).     

CPFD flow pattern simulation in downer reactors

This study contributes with a computational fluid dynamic simulation based on the numerical solution of continuity and momentum balance equations in a three‐dimensional (3‐D) framework. The proposed down flow gas–solid suspension model includes a unit configuration and C_D drag coefficients recommended for these units. Computational particle fluid dynamics (CPFD) calculations using suitable boundary conditions and a Barracuda (version: 14.5.2) software allow predicting local solid densification and asymmetric “wavy flows.” In addition, this model forecasts for the conditions of this study higher particle velocity than gas velocity, once the flow reaches 1 m from the gas injector. These findings are accompanied with observations about the intrinsic rotational character of the flow. CPFD numerical 3‐D calculations show that both gas and particle velocities involve the following: (a) an axial velocity component, (b) a radial velocity component (about 5% of axial velocity component), and (c) an angular velocity component. The calculated velocity components and the rotational flow pattern are established for a wide range of solid flux/gas flux ratios. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1635–1647, 2013     

Three-dimensional turbulent flow in agitated vessels with a nonisotropic viscosity turbulence model

Solutions of the time-averaged equations of motion with a nonisotropic k-e model were developed for the three-dimensional turbulent flow field in turbine stirred tanks. These results were validated with the measurements of three velocity components with a hot wire anemometer and literature data. The nonisotropic turbulence model considered the rotation and curvature effect of the turbulence with a turbulent Richardson number term and accounted for the important three-dimensional effects through the nonisotropy of the viscosity. Also, it was found that a frequently used isotropic k-? turbulence model did not describe this three-dimensional turbulent flow field.     

Effect of a Magnetic Field on a Micropolar Fluid Flow in the Vicinity of an Axisymmetric Stagnation Point on a Circular Cylinder

The effect of a magnetic field on a micropolar fluid flow in the vicinity of an axisymmetric stagnation point on a circular cylinder is studied numerically. The governing conservation equations of continuity, momentum and angular momentum are partial differential equations which are transformed into a system of ordinary differential equations by using the usual similarity transformations. The resulting system of coupled non‐linear ordinary differential equations is solved numerically by using the shooting method. The numerical results indicate the velocity, angular velocity and pressure distributions for different parameters of the problem including Reynolds number, magnetic parameter and dimensionless material properties, etc. In addition, the effect of the pertinent parameters on the local skin friction coefficient and the couple stress are discussed numerically and illustrated graphically.     

Localization of combustion in a periodic velocity field

A direct numerical simulation of combustion-front propagation in a specified periodic velocity field of a medium was performed within the framework of a two-dimensional thermal diffusion formulation. The calculations showed that as the velocity amplitude is increased, the flame front separates into discrete burning segments, i.e., spatial localization of the combustion takes place. Only when heat losses are introduced into the model, flame quenching is observed at a certain amplitude of the medium’s velocity. The level of heat losses required to extinguish the combustion becomes lower with increase in the amplitude of velocity perturbations. On the whole, the obtained numerical results agree with results of an asymptotic analysis. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 19–28, May–June 1998.     

Self‐sustained oscillations in opposed impinging jets in an enclosure

The flow of jets in confining enclosures has significant application in many engineering processes. In particular, the impingement of axisymmetric jets in a confined space has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. Several flow regions were found; stable steady, regular oscillatory, and irregular oscillatory. Initially, a steady flow field existed for all arrangements for Re_d < ?90 (based on the nozzle diameter d, the fluid kinematic viscosity v and the volumetric flow rate Q through the nozzle (Q = πd²/4U_avg)) but subsequent increments in the fluid velocity caused a regularly oscillating flow field to emerge. The onset of the oscillations and the upper limit of finite oscillations were found to be a function of the Re_d, and the nozzle diameter to chamber dimension ratio. Steady numerical simulations predicted the steady flow field well and good agreement was obtained in unsteady simulations of the oscillating flow field. The oscillating flow field is considered to be a class of self‐sustaining oscillations where instabilities in the jet shear layer are amplified because of feed back from pressure disturbances in the impingement region.     

MODELLING GAS-SOLID FLOW IN A PNEUMATIC-FLASH DRYER

ABSTRACT

Investigations of aerodynamics of gas-solid flow in a pneumatic-flash dryer in semiindustrial scale have been carried out. Apparatus was composed of three elements with varying cross-sectional area connected together, i.e. expanding cone, decreasing cone and a vertical pipe with constant diameter. A mathematical model of the dryer is based on the continuity equations for both gas and solid phase and on differential equations for momentum balance of the gas-solid mixture and momentum balance of the solid phase. The model has been solved by means of Gear's numerical method. The effect of various empirical correlations for the solid-wall friction factor has been shown. Distributions, resulting from the model, for pressure, gas velocity, panicle velocity, voidage and residence time of panicle along the axis of apparatus have been presented. The results of numerical calculations have been verified on the basis of measurements in pan.     

Detonation waves in a reactive supersonic flow

A supersonic hydrogen—air flow is studied in detail, in particular, the fields of gas-dynamic parameters and chemical homogeneity of the mixture in various cross sections of the duct. The processes of excitation and propagation of a detonation wave in the downstream and upstream directions are considered. The detonation-wave velocity with respect to the mixture flow is found to differ from the nominal Chapman—Jouguet velocity for a quiescent mixture: the detonation-wave velocity is higher if the wave moves upstream and lower if the wave moves downstream. Some hypotheses on the reasons for these deviations of the experimentally measured velocity from the nominal value are given. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 85–100, September–October, 2006.     

Separation of greenhouse gases from gas mixtures using nanoporous polymeric membranes

Removal of greenhouse gases from gas streams using porous membranes was carried out in this work. Theoretical studies were performed in terms of mathematical modeling and numerical simulation of CO₂ capture in a flat‐sheet membrane contactor. Numerical simulation was performed using computational fluid dynamics (CFD) of mass and momentum transfer in the membrane module for laminar flow conditions. Physical absorption was considered in the simulations for absorption of CO₂ in pure water. CO₂ concentration distribution in the membrane module was determined through numerical solution of continuity equation coupled with the Navier‐Stokes equations. The modeling predictions indicated that the CO₂ concentration difference is not appreciable in the membrane direction. Moreover, velocity distribution was determined in the liquid side of membrane contactor. CFD also represents a design and optimization tool for membrane gas separation processes. POLYM. ENG. SCI., 55:975–980, 2015. © 2014 Society of Plastics Engineers     

Modeling and simulation of conditionally volume averaged viscoelastic two‐phase flows

Conditional volume averaging is used to develop a model capable of simulating two‐phase flows of viscoelastic fluids with surface tension effects. The study is started with the single‐phase mass and momentum balances, which are subsequently conditionally volume averaged. In doing so, we arrive at a set of equations having unclosed interfacial terms, for which closure relations for viscoelastic fluids are presented. The resulting equations possess a structure similar to the single‐phase equations; however, separate conservation equations are solved for each phase. As a result, each phase has its own pressure and velocity over the entire domain. Next, our numerical implementation is briefly outlined. We find that a Poiseuille single‐phase flow is predicted correctly. The closure terms are examined by considering a two‐phase shearing flow and a quiescient cylinder with surface tension. A convergence analysis is performed for a steady stratified two‐phase flow with both phases being viscoelastic. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3914–3927, 2013     

Binding Energy of Hydrogen-Like Impurities in Quantum Well Wires of InSb/GaAs in a Magnetic Field

The binding energy of a hydrogen-like impurity in a thin size-quantized wire of the InSb/GaAs semiconductors with Kane’s dispersion law in a magnetic field B parallel to the wire axis has been calculated as a function of the radius of the wire and magnitude of B, using a variational approach. It is shown that when wire radius is less than the Bohr radius of the impurity, the nonparabolicity of dispersion law of charge carriers leads to a considerable increase of the binding energy in the magnetic field, as well as to a more rapid growth of binding energy with growth of B.     

Fireball during combustion of hydrocarbon fuel releases. I. Structure and lift dynamics

The formation, combustion, and thermal interaction of the fireballs which develop upon ignition of a cloud of hydrocarbon fuel near the Earth’s surface are simulated numerically. The axisymmetric nonstationary flow is described by a system of Favre averaged conservation equations invoking a (k−ε)-turbulence model, a model for turbulent combustion, and a global-kinetic scheme for formation and burnup of soot particles. The optical properties of the mixture of combustion products and soot are modeled by a weighted sum of gray gases. The radiation field is calculated using a combination of a volume emission approximation and a diffusion approximation. Calculations are done for fireballs formed during vertical releases of gaseous propane masses of 1 g to 10³ kg with ignition near the release point. The internal structure of a fireball is analyzed in detail at various stages of its evolution. The lift dynamics of a fireball is illustrated for release velocities corresponding to Froude numbers (defined as the square of the ratio of the linear outflow velocity to the characteristic velocity owing to buoyancy forces) ranging from 5–250. The temperature, concentrations, and reaction rates in the fireball are determined as functions of time. It is shown that for these ranges of fuel mass and release velocity, the dimensionless parameters introduced here can be used for scaling the results and using the calculated dependences obtained here in a unified fashion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 7–19, May–June 1999.     

MODELLING GAS-SOLID FLOW IN A PNEUMATIC-FLASH DRYER

Investigations of aerodynamics of gas-solid flow in a pneumatic-flash dryer in semiindustrial scale have been carried out. Apparatus was composed of three elements with varying cross-sectional area connected together, i.e. expanding cone, decreasing cone and a vertical pipe with constant diameter. A mathematical model of the dryer is based on the continuity equations for both gas and solid phase and on differential equations for momentum balance of the gas-solid mixture and momentum balance of the solid phase. The model has been solved by means of Gear's numerical method. The effect of various empirical correlations for the solid-wall friction factor has been shown. Distributions, resulting from the model, for pressure, gas velocity, panicle velocity, voidage and residence time of panicle along the axis of apparatus have been presented. The results of numerical calculations have been verified on the basis of measurements in pan.     

Ignition of porous materials by gas filtration (unsteady downstream filtration)

The wave theory of ignition is used to create an analytic method for calculating the temporal characteristics of ignition in porous samples by a flow of hot gas into the material (unsteady downstream filtration). When classical dimensionless variables are used, characteristic ignition times are found to have an anomalous dependence on the parameter β=RT _ign/E: as β is increased, the duration of the ignition stages is reduced and does not increase, as it does for conductive heating of the material. A scale is found for the gas density, such that the temporal ignition characteristics have the customary dependence on β when it is used. It is shown that the equations for isothermal filtration can be used to determine the mass feed rate of the gas. Numerical calculations confirm the validity of the basic assumptions of the theory regarding the existence of stages in the ignition process and the wave mechanism for heating the material. Good quantitative agreement is obtained between an approximate analysis and the numerical calculations. The error in determining the time to develop a zero temperature gradient at the sample boundary and the thermal explosion time is less than 50%. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 1, pp. 49–59, January–February 1999.