Gas-particle Flow Research Articles

Simulation of shock-powder interaction using kinetic theory of granular flow

Numerical simulation of the shock-powder interaction in a shock tube is carried out using the Eulerian-Eulerian approach, and special emphasis is placed on the particle phase behavior responding to the shock wave. The kinetic theory of granular flow is incorporated into the mathematical model of the compressible gas-particle flow. Since the particle phase pressure is provided by the kinetic theory, characteristics analysis demonstrates that the set of particle phase governing equations is a well-posed system, and the Roe scheme is then used in the numerical simulation process. The predicted results show that the particle phase is compressed by the shock wave, and the accumulation of particles increases the pressure of the particle phase. While, the reflected and transmitted shock waves are generated after the shock-powder interaction, the expansion waves in the particle powder show a decrease trend in the gas phase pressure. In addition, the passage of the shock wave gives rise to a sharp increase of the velocity slip between the gas phase and the particle phase. Performance of the particle kinetic theory in the present model is examined by the experimental data, which provides the numerical basis for implementing the particle kinetic theory in compressible multiphase flow problems.

Experimental investigation of non-stationary motion of single small spherical particles in an upward flow with different velocities

This experimental investigation focuses on carefully determining the drag coefficient of single spherical particles with considerable small diameters in non-stationary motion. A low-speed wind tunnel and a set of high-speed photography system are employed to confirm the displacement of each particle from its trajectory. Moreover, spherical particles of two different densities are used and the minimum diameter of these particles reaches 0.1mm. Meanwhile, the Basset force generated from the relative acceleration in the gas-particle flow is calculated approximately. The experimental results show that the maximum proportion of Basset force does not exceed 2.2%. In addition, the relationship of CD−Re for the experimental data is also determined. This research offers fundamental experimental results for industrial applications and also contributes to the research of non-stationary gas-particle flow.

A screw-brush feeding system for uniform fine zinc oxide powder feeding and obtaining a homogeneous gas–particle flow

The most important requirements of gas–solid two phase flow feeding are a very uniform mass flow of fine particles and providing a homogeneous dispersed cloud of them. In this study, a screw-brush feeding device has been developed to feed fine zinc oxide powders and to achieve a homogenous gas–particle two phase flow by a minimum gas flow rate. The results of these experiments indicated that the performance of this feeding system without gas flow was better than a screw feeder alone, to stabilize the feeding of fine ZnO powder at low flow rates. Moreover, the results of experimental micrographs showed that the screw-brush system with a minimal amount of gas flow rate is able to reduce the mean aggregate size of the particle size distribution and could provide a homogeneous gas–particle flow.

LES-DPM simulation of turbulent gas-particle flow on opposed round jets

A three-dimensional simulation was performed on the turbulent gas–solid flow in a cylindrical channel with opposed round jets. Large eddy simulation (LES) coupled with discrete phase model (DPM) was employed for carrier gas and laden particles respectively, to investigate the turbulent gas–solid behaviors. Simulations were focused on the effects of particle sizes (dp=5μm, 14μm and 40μm), Stokes numbers (St=0.125, 1 and 8), mass flux ratios (Zm=0.00128, 0.035 and 0.66) and initial gas velocities (U0=15m/s, 25m/s and 35m/s) on the vortex structure evolution, particle motion, time-averaged velocity profiles and turbulence intensity. It was found that the evolutions of vortex structure, including the destruction of large scale coherent structures to tiny vortices and the lateral spreading of vortices at the impingement zone, were augmented when using smaller particles and higher initial gas velocities. In addition, particles followed more closely and were easier to reach quasi-equilibrium state with the decrease of Stokes number (St<1), whereas more particles penetrated the impingement plane when increasing the Stokes number (St>1). And, with the increase of mass flux ratio, the momentum transfer was augmented as well as the turbulence intensity. Moreover, the position (rm) with maximum radial velocity was almost independent of the initial gas velocity on far-spaced opposed round jets.

Numerical Investigation on Effect of Food Particle Mass on Spout Elevation of a Gas–Particle Spout Fluidized Bed in a Microwave–Vacuum Dryer

The effect of food particle mass on the instantaneous fluid dynamic characteristics of a gas–particle spout fluidized bed in a pulsed spouted microwave–vacuum drying (PSMVD) system was investigated by a numerical method. The spout fluidization process in a pseudo-2D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The effect of particle mass on the particle flow pattern, jet penetration depth, and pressure drop was evaluated. Under a specific particle mass, the spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effective transition of the flow pattern. With an increase in particle mass, the spout velocity tended to increase to obtain the expected gas–particle flow pattern and the correlation between the particle mass and particle flow pattern was revealed. This research on the fluid dynamic characteristics of a gas–particle spout fluidized bed could be used to improve the uniformity of particle mixing and microwave heating.

Numerical investigation of gas-particle flow in the primary air pipe of a low NOx swirl burner – The DEM-CFD method

The gas-particle flow in the primary air pipe (PAP) of a low NOx swirl burner was investigated using the computational fluid dynamics (CFD) coupled with the discrete element method (DEM). The mathematical models were validated using the measured values obtained at the outlet of the primary pipe through a phase Doppler anemometer (PDA) system. Particles of different Stokes numbers in the primary air pipe (PAP) were investigated, and the effects of the structure of the primary air pipe and the particle–particle interaction on particle dispersion were analyzed. The results indicate that particles under the combined effects of the Venturi pipe and the spindle body are concentrated into a narrow band area and that the PAP structure can more efficiently concentrate particles with large Stokes numbers. The formed fuel rich/lean jet persists for a long distance out of the burner, thereby favoring of air-staged combustion and NOx reduction. The particle collision frequency and its fluctuation range increase as the particle Stokes number increases. The collisions among particles result in an increase of the spanwise dispersion of particles. Experimental results indicate that the models that take particle–particle collision into consideration are more able to predict particle concentration.

CFD–DEM simulation of biomass gasification with steam in a fluidized bed reactor

A comprehensive CFD–DEM numerical model has been developed to simulate the biomass gasification process in a fluidized bed reactor. The methodology is based on an Eulerian–Lagrangian concept, which uses an Eulerian method for gas phase and a discrete element method (DEM) for particle phase. Each particle is individually tracked and associated with multiple physical (size, density, composition, and temperature) and thermo-chemical (reactive or inert) properties. Particle collisions, hydrodynamics of dense gas-particle flow in fluidized beds, turbulence, heat and mass transfer, radiation, particle shrinkage, pyrolysis, and homogeneous and heterogeneous chemical reactions are all considered during biomass gasification with steam. A sensitivity analysis is performed to test the integrated model׳s response to variations in three different operating parameters (reactor temperature, steam/biomass mass ratio, and biomass injection position). Simulation results are analyzed both qualitatively and quantitatively in terms of particle flow pattern, particle mixing and entrainment, bed pressure drop, product gas composition, and carbon conversion. Results show that higher temperatures are favorable for the products in endothermic reactions (e.g. H2 and CO). With the increase of steam/biomass mass ratio, H2 and CO2 concentrations increase while CO concentration decreases. The carbon conversion decreases as the height of injection point increases owing to both an increase of solid entrainment and a decrease of particle residence time and particle temperature. Meanwhile, the calculated results compare well with the experimental data available in the literature. This indicates that the proposed CFD–DEM model and simulations are successful and it can play an important role in the multi-scale modeling of biomass gasification or combustion in fluidized bed reactor.

Numerical modeling of electrostatic precipitation: Effect of Gas temperature

A Computational Fluid Dynamics model is presented to calculate the particle collection efficiency in electrostatic precipitators in terms of the interactions among gas-particle flow, dust resistivity and electric field. Various material properties and operation parameters are included to extend the model capability for process optimization. The current paper focuses on the effect of gas temperature for a cold-side precipitator. It is found that, as temperature increases in the range 90–120°C considered, the operation voltage drops significantly, due to increased dust resistivity. Behaviors of fine particles in the range from 0.05 to 25μm in diameter are simulated in a circular wire-plate configuration. A minimum efficiency is predicted for particle size around 0.5μm. Below this critical size, the increased collection efficiency results from relatively large surface charge density due to diffusion charging, and reduced inter-phase drag. For moist gas, temperature has significant non-linear effect on the fly ash resistivity. It is demonstrated that a same change in temperature from 120°C to 90°C is more effective than from 150°C to 120°C, as far as the collection efficiency is concerned.

CFD–DEM simulation of the pneumatic conveying of fine particles through a horizontal slit

Fine particles play a significant role in many industrial processes. To study the dynamic behavior of fine particle and their deposition in rock fractures, the pneumatic conveying of fine particles (approximately 100μm in diameter) through a small-scale horizontal slit (0.41m×0.025m) was studied, which is useful for the sealing technology of underground gas drainage in coal mining production. The CFD–DEM method was adopted to model the gas-particle two-phase flow; the gas phase was treated as a continuum and modeled using computational fluid dynamics (CFD), particle motion and collisions were simulated using the DEM code. Then, the bulk movement of fine particles through a small-scale horizontal slit was explored numerically, and the flow patterns were further investigated by visual inspection. The simulation results indicated that stratified flow or dune flow can be observed at low gas velocities. For intermediate gas velocities, the flow patterns showed pulsation phenomena, and dune flow reappeared in the tail section. Moreover, periodic flow regimes with alternating thick and sparse stream structures were observed at a high gas velocity. The simulation results of the bulk movement of fine particles were in good agreement with the experimental findings, which were obtained by video-imaging experiments. Furthermore, the calculated pressure drop versus gas velocity profile was investigated and compared with relative experimental findings, and the results showed good agreement. Furthermore, the particle velocity vectors and voidage distribution were numerically simulated. Selected stimulation results are presented and provide a reference for the further study of fine particles.

Influence of the gas particle flow characteristics of a low-NOx swirl burner on the formation of high temperature corrosion

In order to figure out the reasons for high temperature corrosion on side water walls, experimental investigations have been conducted on a 600MWe boiler. Samples collected from the side water walls were analyzed by XRD and SEM–EDS. The results revealed that the deposition of FeS and unoxidized fuel was responsible for the high temperature corrosion. Then the gas particle flow characteristic downstream of the HT-NR3 swirl burner was investigated using a three-component particle-dynamics anemometry (PDA) system. Velocities, particle concentrations and particle diameters were obtained. Results showed that there was an annular recirculation zone existing between the primary air and the secondary air. The mixing of the secondary air with the primary air downstream of the recirculation zone was very weak, so as the entrainment effect of the swirling outer secondary air on particles. Large particles tended to reach the water wall more easily. These gas particle flow characteristics strengthened the oxygen-deficient combustion for pulverized coal particles and increased the possibility of the deposition of FeS and unoxidized fuel.

CFD simulations of flow erosion and flow-induced deformation of needle valve: Effects of operation, structure and fluid parameters

A three-dimensional fluid–structure interaction (FSI) computational model coupling with a combined continuum and discrete model has been used to predict the flow erosion rate and flow-induced deformation of needle valve. Comparisons with measured data demonstrate good agreement with the predictions of erosion rate. The flow field distribution of gas-particle flow and the erosion rate and deformation of valve core are captured under different operating and structural conditions with different fluid parameters. The effects of inlet velocity, valve opening and inlet valve channel size, particle concentration, particle diameter and particle phase components are discussed in detail. The results indicate that valve tip has the most severe erosion and deformation, and flow field, erosion rate and deformation of valve are all sensitive to inlet condition changes, structural changes and fluid properties changes. The effect of particle diameter on erosion is the most significant, while the influence of inlet rate on deformation is the greatest one.

Experimental and numerical study of mini-riser geometry effect on drops residence time in the developing flow

In this study, velocity distribution and concentration of droplets for mini-riser with different geometries in the developing region is investigated experimentally and numerically to estimate the effects of riser shapes on the drops residence time. Since drops residence time affect on the drops size and consequently total separation efficiency by increasing of the condensation rate for moving droplets in the super-saturated carrier gas. Further, it increases the coagulation in mono dispersed dense gas-particles flow. In the experimental study, an ultrasonic atomizer produces small size water drops. Mini-risers with corners including triangular, rectangular and square shapes with the same hydraulic diameter are examined. Particle shadow velocimetry (PSV) technique based on shadow casting combined with the high-speed microscopic imaging is introduced for visualizing droplets motion and simultaneous velocity and concentration measurements near walls and corners. Such a technique is well suited for small water droplets measurements near surfaces especially in narrow geometries where conventional laser-based PIV methods are hampered by surface glare, interference of scattered lights and optical access. Effectively narrow-depth-of-field optical setup is employed for imaging a two dimensional plane within a flow volume. In addition, the numerical modeling is carried out for estimating general drops residence time in the riser. Results indicate that the drops concentration near wall is very low especially in the corners. Droplets residence time and concentration near the wall and corner is highest for triangular shape. Triangular shape enhances the average residence time of drops 50.6% more with respect to square one. Using of sharper corners and higher aspect ratio for the riser geometry increase droplets residence time in addition ease the separation process.

Analysis of gas-particle flow characteristics in impinging streams

A three dimensional Euler–Lagrange model for the gas-particle two-phase impinging streams (GPIS) is developed based on the direct simulation Monte Carlo (DSMC) method with consideration of particle rotation and collision. The gas-particle flow characteristics involved in GPIS as well as the effects of inlet gas velocity and particle rotation are analyzed. The results indicate that two pairs of counter-rotating gas vortices are developed at two sides of the opposite jet flows, which is able to entrain the particles and thus greatly weaken the deposition of particles. Interparticle collisions in the impingement zone produce two effects on the particle behaviors: the direct escaping of particles from impingement zone and the progressive accumulation of particles in impingement zone. Under the same inlet particle mass flow rate, the particle concentration in the impingement zone decreases with increasing inlet velocity of gas due to the increasing impinging reaction of interparticle collisions and growing entrainment of gas vortices. In addition, the rotation of particle provides an additional driving force to push the particles away from the impingement zone, leading to the higher speed of escaping particles and smaller maximum particle concentration at the center of impingement zone than those without particle rotation.

Large-eddy simulation of dispersed two-phase flows

Large-eddy simulation (LES) is under its rapid development. In comparison with the Reynolds-averaged Navier-Stokes modeling (RANS modeling) LES can give instantaneous flow and flame structures and in some cases may give more accurate statistical results than RANS modeling. The present author and his colleagues started to study the instantaneous flow structures of bubbly flows by LES in 2002. Subsequently, from 2005 up to now the studies on LES of gas-particle flows, the flow structure surrounding a single particle, and the flow and flame structure surrounding an evaporating and combusting droplet were carried out. In this paper, a review will be given for some recent studies by the present author and his colleagues on LES of dispersed two-phase flows, including the governing equations, sub-grid-scale models and numerical methods, main research results and their experimental validation.

Model study of the effect of bird’s nest on transport phenomena in the raceway of an ironmaking blast furnace

Ironmaking blast furnace is an important reactor in extractive metallurgy. Understanding complex phenomena in the raceway region of an ironmaking blast furnace is important for high efficiency production. A three-dimensional CFD model is described to simulate the flow and thermochemical behaviours in the raceway region of blast furnace. It includes two fuels: pulverized coal and coke bed. The effect of bird’s nest of coke bed is examined in terms of gas–particle flows and coal burnouts, where burnouts are investigated at two different locations, namely raceway endpoint and overall surface, respectively. It is found that the existence of bird’s nest impacts the length of the jet and extension of the raceway. More importantly, bird’s nest does not affect the coal burnouts at raceway endpoint much, but significantly affects the burnout over the raceway surface – lower burnout over the raceway surface when considering bird’s nest in the simulations. The model provides an effective tool for understanding and optimizing the operation of a blast furnace.

Investigation on Swirling Gas-particle Hydrodynamics Using an Improved Momentum Transfer Coefficient

Abstract Based on the Wen's 1966 empirical momentum transfer coefficient relation of gas-particle two-phase turbulent flows, a new model to make up for the discontinuous characteristics is developed using a three-stage continuous function with switch weight function. Incorporated into the kinetic theory of granular flows to consider particle collisions and the unified second-order moment two phase turbulent model, it is utilized to predict the hydrodynamics of swirl gas-particle turbulent flows and the validation from the experimental measurements by Sommerfeld and Qiu 1991 is also carried out as well. Numerical simulated results showed that the prediction results by this model are well in agreements with experimental results. The particle dispersion distributions have the distinctively anisotropic behaviors due to complicated effects of centrifugal force, turbulent diffusion and particle inertia, which are quietly different from gas flows. Meanwhile, distributions of axial-axial particle-gas correlated velocity fluctuation is always greater than axial-tangential particle-gas and the tangential-tangential particle-gas velocity fluctuation is also greater than tangential-axial particle-gas phase. At the stronger shear and the vortex flow regions, its results agreed with experiments are much better than Wen's model.

Implementation of Two-Fluid Model for Dilute Gas-Solid Flow in Pipes With Rough Walls

A numerical study was carried out to investigate the performance of a two-layer model for predicting turbulent gas-particle flows in rough pipes. An Eulerian–Eulerian two-fluid formulation was used to model both the gas and solid phases for turbulent gas-particle flow in a vertical tube. The stresses developed in the particle phase were calculated using the kinetic theory of granular flows while the gas-phase stresses were described using an eddy viscosity model. The two-fluid model typically uses a two-equation k-ɛ model to describe the gas phase turbulence, which includes the suppression and enhancement effects due to the presence of particles. For comparison, a two-layer model was also implemented since it has the capability to include surface roughness. The current study examines the predictions of the two-layer model for both clear gas and gas-solid flows in comparison to the results of a conventional low Reynolds number model. The paper specifically documents the effects of surface roughness on the turbulence kinetic energy and granular temperature for gas-particle flow in both smooth and rough pipes.

A multiphase turbulence theory for gas–solid flows: I. Continuity and momentum equations with Favre-averaging

CFD calculations for multiphase gas-particle flows would benefit from a multiphase turbulence theory which would allow the direct calculation of an average solution. A slightly truncated form of the two fluid equations, which are the basis of the MFIX code, have been formally averaged, introducing Favre-like averages for the phasic velocities. This process results in a set of equations which can be solved directly for the average variables describing the flow. This equation set has significantly fewer closure terms than would have occurred using standard Reynolds averaging. The full set of required closure relations is enumerated. Some simple forms for these closures are suggested, although no attempt is made to establish their validity.

Filtered models for bidisperse gas–particle flows

Using the microscopic multi-fluid model as a starting point we perform fine grid simulations of bidisperse gas–particle flows to examine the dependence of the filtered fluid–particle drag coefficient, particle–particle drag coefficient, and particle phase stress on the volume fractions and sizes of the particles and the filter size. Scalings are identified for the filtered fluid–particle drag coefficient, particle–particle drag coefficient, and particle phase pressure that capture the dependence of each filtered correction on the size and volume fraction ratios of the bidisperse assembly.

Numerical simulation of heat transfer in gas-particle two-phase flow with smoothed discrete particle hydrodynamics

Heat transfer between particles and that between gas phase and particle phase in gas-particle two-phase flow cannot be ignored. Smoothed discrete particle hydrodynamics, as a new method for solving the gas-particle two-phase flow, has been used in simulating the aerolian sand transport successfully. Based on the smoothed discrete particle hydrodynamics method, a heat conduction model is presented in this paper and is used to simulate the heat transfer processes and the particle evaporation in gas-particle two-phase flow. Firstly, the equations to be solved are presented in which the energy equations are introduced for each phase and the second derivative item in conduction is treated by combining a standard smoothed particle hydrodynamics first derivative with a finite difference approximation of a first derivative. The heat conduction between particle and gas is computed from temperature difference and heat transfer coefficient. The disc-type particle cluster problem and bubble fluidized bed are simulated and the results are in close agreement with the two fluid model simulation results. The vaporization law for discrete phase droplet is used to deal with the particle evaporation and then a jet evaporation is simulated. Numerical results all show a good agreement with the discrete particle model results. It is indicated that the new method is of good accuracy and practical applicability.