Gas-particle Flow Research Articles

On modeling and physical laws of isentropic speed of sound in dense gas-particle two-phase compressible flows

Study of isentropic sound speed of two-phase or multiphase flow has theoretical significance and wide application background. As is well known, the speed of sound in fluid containing particles in suspension differs from that in the pure fluid. In the particular case of bubbly liquids (gas liquid two-phase flow), the researches find that the differences can be drastic. Up to now, the isentropic speed of sound in the flow field with a small volume fraction of bubbles (less than 1%), has been investigated fully both experimentally and theoretically. In this paper, we consider another situation, as the case with solid particles in gas, which is the so-called gas particle two-phase flow. Although many results have been obtained in gas liquid two-phase flow, there is still a lot of basic work to do due to the large differences in the flow structure and flow pattern between gas particle two-phase flow and gas liquid two-phase flow. Treating the gas particle suspension as the relaxed equilibrium, thermodynamic arguments are used to obtain the isentropic speed of sound. Unlike the existing work, we are dedicated to developing the computational model under dense condition. The space volume occupied by particle phase and the interaction between particles are overall considered, then a new formula of isentropic sound speed is derived. The new formula includes formulae of the pure gas flow and the already existing dilute gas particle two-phase flow as a special case. On the one hand, the correctness of our formula is verified. On the other hand, the new formula is more general. The variations of sound speed with different mass fractions of particle phase are analyzed. The theoretical calculation results show that the overall physical law of sound speed change is that with the increase of the particle mass fraction, the sound speed first decreases and then increases. The velocity of sound propagation in gas particle two-phase flow is far smaller than in pure gas in a wide range, so it is easy to reach the supersonic condition. When the particle volume fraction is below 10%, the result is consistent with Prandtl theoretical analysis. In this range, the influences of the particle phase pressure modeling parameters can be neglected. When the particle volume fraction is more than 10%, the particle phase pressure modeling parameters produce influences. Furthermore the corresponding physical principles and the mechanisms are discussed and revealed. The new formula and physical understandings obtained in this paper will provide a theoretical support for the researches of dense gas particle two-phase compressible flow and related engineering applications.

Experimental volcanic ash aggregation: Internal structuring of accretionary lapilli and the role of liquid bonding

Explosive volcanic eruptions can release vast quantities of pyroclastic material into Earth's atmosphere, including volcanic ash, particles with diameters less than two millimeters. Ash particles can cluster together to form aggregates, in some cases reaching up to several centimeters in size. Aggregation alters ash transport and settling behavior compared to un-aggregated particles, influencing ash distribution and deposit stratigraphy. Accretionary lapilli, the most commonly preserved type of aggregates within the geologic record, can exhibit complex internal stratigraphy. The processes involved in the formation and preservation of these aggregates remain poorly constrained quantitatively. In this study, we simulate the variable gas-particle flow conditions which may be encountered within eruption plumes and pyroclastic density currents via laboratory experiments using the ProCell Lab System® of Glatt Ingenieurtechnik GmbH. In this apparatus, solid particles are set into motion in a fluidized bed over a range of well-controlled boundary conditions (particle concentration, air flow rate, gas temperature, humidity, liquid composition). Experiments were conducted with soda-lime glass beads and natural volcanic ash particles under a range of experimental conditions. Both glass beads and volcanic ash exhibited the capacity for aggregation, but stable aggregates could only be produced when materials were coated with high but volcanically-relevant concentrations of NaCl. The growth and structure of aggregates was dependent on the initial granulometry, while the rate of aggregate formation increased exponentially with increasing relative humidity (12–45% RH), before overwetting promoted mud droplet formation. Notably, by use of a broad granulometry, we generated spherical, internally structured aggregates similar to some accretionary pellets found in volcanic deposits. Adaptation of a powder-technology model offers an explanation for the origin of natural accretionary pellets, suggesting them to be the result of a particular granulometry and fast-acting selective aggregation processes. For such aggregates to survive deposition and be preserved in the deposits of eruption plumes and pyroclastic density currents likely requires a significant pre-existing salt load on ash surfaces, and rapid aggregate drying prior to deposition or interaction with a more energetic environment. Our results carry clear benefits for future efforts to parameterize models of ash transport and deposition in the field.

ChemInform Abstract: CFD Simulation of Dense Particulate Reaction System: Approaches, Recent Advances and Applications

AbstractReview: CFD: computational fluid dynamics; 175 refs.

Analyzing the effect of wall roughness on gas–particle flow in confined channels based on a virtual-wall-group concept

Present work aims to develop a particle history effect model for describing the effect of wall roughness on particle–wall collision in confined flow channels. To solve the real distribution function of roughness angle, the effective distribution function is scrutinized with a virtual testing method, and a further virtual-wall-group concept is proposed to investigate the effect of particle motion experience. It is found that roughness angle distribution should be described by a particle history effect–probability distribution function. Rebound angle is rebuilt according to the coordinate rotation operation. Multiple collisions are divided into two types, namely Type I induced by negative rebound and Type II led by positive rebound. Besides, a new probability function is derived to distinguish multiple collisions from a single collision. A set of modified algorithm for multi-collision is built and applied to simulate the particle–rough wall collision, and the predictions are evaluated by the measurements. It reveals that a typical peak can be seen in the probability distributions of both types of multiple collisions, and the effect of wall roughness decreases with the increase of impact angle. The simulated results of confined planar gas–particle flow are good in accordance with the measured data.

Prediction of Black Powder distribution in junctions using the Discrete Phase Model

The present study consists in simulating turbulent gas–particle flows through pipe junctions under different geometrical and flow conditions. The purpose is to study the effects of the particles' size and orientation of the pipes with different degrees of asymmetry at junctions on phase split. Solid particles can be present in gas transmission networks as contaminants in the form of Black Powder to be eliminated. The simulation based on the standard k-ε turbulence model and the Discrete Phase Model (DPM) showed that the solid phase split can be considered to follow the air flow split closely for Stokes numbers small enough than unity (St=0.2). While for intermediate Stokes numbers (St=1) and slightly higher than unity (St=5), the particles gain some independence from the gaseous phase and for the later, the pipe orientation plays a significant role. Effects of the shape of the particles and their initial positions at the inlet are also considered.

Simulation of mass balance behavior in a large-scale circulating fluidized bed reactor

We determine using a compound model the influence of the mass of granular matter on the behavior of a supercritical circulating fluidized bed (CFB) reactor. Population balance enables a stationary-regime modeling of the mass flow of granular matter inside a CFB unit in a large-scale. The simulation includes some important dynamic processes of gas-particle flows in fluidized bed such as attrition, fragmentation, elutriation, and fuel combustion. Numerical calculations with full boiler loading were performed of operational parameters such as furnace temperature, furnace pressure, feeding materials mass flows, and excess air ratio. Furthermore, three bed inventory masses were adopted as experimental variables in the simulation model of mass balance. This approach enables a sensitivity study of mass flows of granular matter inside a CFB facility. Some computational results from this population balance model obtained for a supercritical CFB reactor are presented that show consistency with the operational data for large-scale CFB units.

Research on the Muzzle Blast Flow with Gas-Particle Mixtures Based on Eulerian-Eulerian Approach

ABSTRACTThe issue on the muzzle blast flow with gas-particle mixtures was numerically investigated in this paper. The propellant gas in the cannon was assumed to be gas-particle mixtures consisting of a variety of gaseous species and particles. The model made use of the Eulerian-Eulerican approach, where the particle were modeled as a second fluid with parameters like bulk density, velocity and temperature, interacting with the gas flow. A high-resolution upwind scheme(AUSMPW+) and detailed reaction kinetics model were employed to solve the chemical non-equilibrium Euler equations for gas phase. The Euler equations for particle phase were solved by MacCormack scheme. The particle diameter and the mass fraction of particle were tested to show their effects on the development process of muzzle blast flow with gas-particle mixtures. The distribution of the main flow parameters of both gas and particle were obtained at different time intervals. The results show the evolution of the muzzle blast flow with gas-particle mixtures and demonstrate the effects of key parameter on the flow field of the gas-particle flow. This paper is a significant investigation for understanding the muzzle blast flow with gas-particle mixtures, which can provide valuable reference for the research on the muzzle blast flow.

On the interaction between shear dusty currents and buildings in vertical collapse: Theoretical aspects, experimental observations, and 3D numerical simulation

We investigate the behavior of vertical building collapses that, at impact on the ground, can generate shear dusty currents. These currents macroscopically resemble natural currents like dust storms and pyroclastic density currents, which may heavily interact with the surroundings while propagating. In particular, shear dusty currents are generated because of building collapse after pulverization, whereas pyroclastic density currents can be generated because of eruptive column or volcano collapse after fragmentation. Pyroclastic density currents can move for kilometers, and then load the surroundings by flow dynamic pressure; a similar dynamical behavior occurs in shear dusty currents that load buildings. We employed 3D engineering fluid dynamics to simulate the generation (by vertical collapse), and the propagation and building interaction of shear dusty currents. We used an Eulerian–Lagrangian multiphase approach to model the gas-particle flow, and an immersed boundary technique to mesh the domain, in order to account for sedimentary processes and complex 3D urban geometry in the computation. Results show that the local dynamic pressure of the shear current is amplified up to a factor ~10 because of flow-building interaction. Also, the surroundings consisting of multiple buildings and empty spaces make walls and streets as surfaces of particle accumulation, which from the collapse zone on can get thinner by exponential law. These results can help better assessing the intricate interaction between pyroclastic density currents and urban surroundings, as well as better link fragmentation, collapse and density current to each other.

Numerical model of the electro-gas-dynamics of a gas–particle system based on the equations of motion of a two-velocity two-temperature gas–particle mixture

A mathematical model of the electro-gas-dynamics of a gas–particle system is described. A numerical method for solving the system of equations is proposed, and an analysis is made of the motion of charged solid aerosol particles in gas–particle flow in the electric field produced by the corona electrode of the atomizer, the grounded surface on which deposition is performed, and the charge of the aerosol particles in the interelectrode space. The solution is based on the two-velocity two-temperature model of a monodisperse medium without phase transitions and coagulation assuming that only the carrier medium, described by the Navier–Stokes equations for a compressible gas, has viscosity. The dispersed phase is defined by the equation of conservation of mass, the equations of conservation of momentum components taking into account the Coulomb force and aerodynamic friction, and the equation of conservation of internal energy. The system is written in generalized coordinates in dimensionless form and solved using the explicit McCormack method with splitting over the spatial coordinates and a conservative correction scheme. The velocity and density fields of the gas–particle mixture were investigated in the interelectrode space and near the surface on which solid aerosol particles in the gas–particle flow are deposited.

Multi-scale simulation of chemical looping combustion in dual circulating fluidized bed

Abstract Chemical looping combustion (CLC) in an interconnected fluidized bed has attracted more and more attention owing to its novel technology with inherent separation of CO2. In recent years, some models have been developed to investigate the gas-particle flow and reactive characteristics during the CLC process. However, multi-scale structures in reactors make it complex to perform a simulation. In the current work, a multi-scale gas–solid flow-reaction coupled model is developed and applied to the simulation of the CLC process in a dual circulating fluidized bed (DCFB) system with consideration of the impact of multi-scale structures on chemical reactions, mass and heat transfer. By comparisons of gas pressure and gas components with experimental data, the present model shows a better prediction. The influence of clusters on the gas compositions and temperature field is analyzed.

Validation of filtered two-fluid models for gas–particle flows against experimental data from bubbling fluidized bed

Predictions of simulations based on filtered Two-Fluid Models (TFMs) with constitutive relations for filtered fluid–particle drag coefficient and filtered stresses proposed by Igci and Sundaresan [Ind. Eng. Chem. Res. 50 (2011) 13190–13201] and Milioli et al. [AIChE J. 59 (2013) 3265–3275] were compared against experimental data from a bubbling fluidized bed challenge problem put forward by the National Energy Technology Laboratory and Particulate Solids Research Inc. It is found that the most important correction to filtered models is a modification to the drag, and filtered stresses play a secondary role at best. As expected, coarse grid simulations using the kinetic-theory based TFM over-predicted the gas–particle drag force, yielding unphysical bed expansion. The filtered fluid–particle drag model proposed by Igci and Sundaresan that classifies the inhomogeneity in sub-filter scale flow structures using filter size and filtered particle volume fraction as markers also predicted unphysical bed expansion. Refined filtered drag models proposed by Milioli et al. based on filtered fluid–particle slip velocity as an additional marker led to good agreement with experimental data on bed expansion and the time-averaged gas pressure gradient. It was also observed that inadequate grid resolution in the region between gas distributor and the adjacent cylindrical wall of the test unit could lead to spurious asymmetric gas–particle flow predictions. With the inclusion of adequate inflation layer elements in that region, flow predictions became nearly symmetric with little to no effect on bed expansion predictions. However, it dramatically and qualitatively altered the details of gas–particle structures in the bed.

Microscopic flow characteristics in fluidized bed of cylinder-shaped particles

IBM (Immersed boundary method) and DEM (Discrete element method) coupling method were used to simulate the flow of cylinder-shaped particles in a fluidized bed. The greatest advantage of IBM-DEM is that it can reveal the microscopic characteristics of dense-phase gas-particle flow in Cartesian grids. Large cylinder-shaped particles are very difficult to fluidize, and slugging flow can be observed even if the static bed height is low. The gas flow field around the particle in fluidized bed is analyzed, and the formation and development of vortex behind the particle is affected obviously by the neighboring particles. Particle trajectory is obtained, and the effect of gas phase on particle rotation becomes active as particle size increases. Compared with the experimental results, the gas-solid force in simulation results is higher. This calculation error may be reduced by decreasing the grid size.

Flow of polydisperse gas-particle mixture in a duct followed by coagulation in a nonlinear wave field

Numerical simulation of the flow of an aerosol of polydisperse composition in a plane duct, where the resonance acoustic oscillations are generated, which are directed across the flow, has been carried out. The peculiarities of the flow, which is followed by coagulation and alteration of the distribution of particles over their sizes, have been described. The carrying medium has been modeled with the aid of the system of Navier-Stokes equations for compressible heat-conducting gas. The polydisperse phase dynamics is described by the systems of equations involving the equations of continuity, conservation of the momentum and internal energy. Equations of the motion of carrying medium and disperse fractions are written with allowance for interphase exchange by the momentum and energy. A Lagrangian model has been used to describe the coagulation process. The dispersion alteration in the gas-particle flow under the action of acoustic oscillations, which are resonant for the duct cross section, is analyzed.

Two-phase Turbulence Models in Eulerian-Eulerian Simulation of Gas-particle Flows and Coal Combustion

Eulerian-Eulerian (two-fluid) simulation of gas-particle flows and coal combustion are widely used because of its convenience in simulating large-size facilities. The key point is that it needs more complex closure models, compared to those in Eulerian gas-Lagrangian DEM modeling of particles. This keynote lecture will give a brief review on our many-year studies to solve these problems. The first one is the particle turbulence model. To overcome the shortcomings of the Hinze-Tchen's “particle-tracking-fluid” model, about 20 years ago, a transport equation of particle turbulent kinetic energy and transport equations of both gas and particle Reynolds stresses were proposed by us and subsequently constitute the so-called “k-ɛ-kp”, “unified second-order moment (USM)” and “non-linear k-ɛ-kp” two-phase turbulence models. Furthermore, for simulating reacting gas-particle flows and coal combustion, a full two-fluid model and a combined two-fluid-trajectory model, accounting for both particle turbulent diffusion and particle history effect due to moisture evaporation, devolatilization and char oxidation were proposed. The next is the particle-rough wall interaction. A particle-wall collision model accounting for wall roughness was proposed. Then, to overcome the limitation of the well-known kinetic theory of dense gas-particle flows, an anisotropic two-phase turbulence model, called “USM-Θ” model, accounting for both particle turbulence (large-scale fluctuation) and inter-particle collision (small-scale fluctuation) was proposed. Next, the particle-wake effect on gas turbulence modulation was studied to construct a sub-model and was added to the two-fluid modeling. At last, in recently developed two-fluid large-eddy simulation of gas-particle flows and combustion the particle sub-grid-scale (SGS) stress model is insufficiently studied. Some of them are based on a simple extension of the gas Smagorinsky SGS model without theoretical justification. Therefore, a USM-SGS two-phase stress model was proposed by us, properly accounting for the anisotropy of two-phase SGS stresses and the interaction between them.

Gas-particle Flow Modeling: Beyond the Dilute Limit

The flow of pulverized fuel in a power plant is one example of gas-particle flows in the energy and process industries. Computational Fluid Dynamics (CFD) is a widely used trouble shooting and engineering design tool increasingly deployed within industry to predict the behavior of such flows. Despite wide use, traditional models are often inadequate for solving industrial problems. An example is the concentration of the particulate phase within a coal burner's fuel feed due to particle inertia and inter-particle collisions. This local concentration of the particles can be advantageous, but it is difficult to characterize the flow beyond the dilute limit (volume fraction of 0.001). To address this and other short comings a number of two-phase flow models have been investigated and their applicability and accuracy assessed by comparison with experiments in the literature for a vertical pipe flow with relatively high mass loadings of the discrete phase. The physics investigated includes: particle-wall collisions, particle-particle collisions and structure dependent drag. Models have been implemented in the commercial CFD software ANSYS FLUENT R.14.5 for the Discrete Phase Model. The results show a significant improvement over industry best practice and provide an indication as to the key physics and the effects of scale on confined gas-particle flows. Furthermore, the modeling approach will be applicable in a number of other industrial areas, such as biomass conveying. This paper provides an overview of CFD models for the application to pulverized fuel flows within a coal fired power plant and discusses the gap between academic development and industrial adoption of advanced CFD models.

Numerical simulation of gas-solid flow in an interconnected fluidized bed

The gas-particles flow in an interconnected bubbling fluidized cold model is simulated using a commercial CFD package by Ansys. Conservation equations of mass and momentum are solved using the Eulerian granular multiphase model. Bubbles formation and their paths are analyzed to investigate the behaviour of the bed at different gas velocities. Experimental tests, carried out by the cold model, are compared with simulation runs to study the fluidization quality and to estimate the circulation of solid particles in the bed.

Numerical simulation of fuel dispersal into cloud and its combustion and explosion with smoothed discrete particle hydrodynamics

A fuel air cloud is formed under the driving force of the explosive detonation and then it’s ignited to explosion to attack the target. The existing numerical simulations are mainly limited to the fuel dispersal processes which are all based on mesh methods. The fuel particles in the air cloud are difficult to traced. Otherwise, the computing process is complex and could not be solved by the exiting methods for the chemical reaction and the forming and propagation of shock waves are both involved in the fuel combustion and explosion. Smoothed discrete particle hydrodynamics (SDPH), as a new method to solve the gas-particle two-phase flow, has been successfully used to simulate the aeolian sand transport, heat transfer and evaporation. Based on the previous work, the Jones-Wilkins-Lee (JWL) function is imported to describe the explosive detonation to expansion and it is solved by finite volume method. The fuel drops dispersed by explosion are traced by the improved smoothed particle hydrodynamics. The drop evaporation model and the EBU-Arrhenius combustion model for gas high-speed combustion are introduced to describe the combustion and detonation of fuel drops. Then we build a new SDPH method to simulate the warhead initiation, fuel dispersal, and the fuel second explosion. Firstly, we design a test that is the dispersal of circular fuel drops drove by explosive detonation to validate our new method. The changing of the explosive detonation pressure and the velocity fields of explosive and particles are analyzed and they are consistent with the theory. And then, the forming and developing of FAE cloud are simulated. Through comparing with the experiments, the shapes of the cloud by the two methods coincide with each other. The effects of different initiations on the cloud forming are also analyzed. Finally, based on the cloud group forming, the evaporation and combustion models are introduced to study the combustion and explosion of FAE. We obtain the velocity field and the distribution of combustion product. The result indicates that the fuel dispersal into cloud and its explosion can be simulated better with the mathematical model and computational method built in this paper. This finding supplies a more effective numerical method for the design and research on this type of weapon equipments.

Modeling of calcium-based sorbent reactions with sulfur dioxide

A mathematical model of calcium sorbent reactions for simulation of sulfur dioxide reduction from pulverized coal combustion fl e gasses is developed, implemented within numerical code and validated against available measurements under controlled conditions. The model attempts to closely resemble reactions of calcination, sintering and sulfation, occurring during the sorbent particles motion in the furnace. The sulfation is based on PSSM (Partially Sintered Spheres Model), coupled with simulated particle calcination and sintering. Complex geometry of the particle is taken into account, with the assumption that it consists of spherical grains in contact with each other. Numerical simulations of drop down tube reactors were performed for both CaCO3 and Ca(OH)2 sorbent particles and results were compared with available experimental data from literature. The sorbent reactions model will be further used for simulations of desulfurization reactions in turbulent gas-particle flow under coalcombustion conditions.

Numerical Modelling of ESP for Design Optimization

Electrostatic precipitators (ESP) are widely used for collection of particulate matters. In the present paper, a Computational Fluid Dynamics (CFD) model, based on Eulerian-Lagrangian framework, is used to simulate the gas-particle flow under electric field. A typical two-stage ESP is chosen as a case study and its collection performance is assessed under a series of modified geometric conditions, including inlet pipework, perforated plate, addition of flow baffles, and spatial extent of electric field. The results show that the collection efficiency for PM2.5 can be increased by 20% after optimization.

CFD Study of the Gas-particle Flow in a Horizontal Duct: The Impact of the Solids Wall Boundary Conditions

The Johnson-Jackson boundary conditions are included in the twoPhaseEulerFoam solver of OpenFOAM-2.2.x in order to consider the wall-friction effect on the movement of particles. A straight conveying line using Johnson-Jackson boundary conditions is simulated. The results show that the particles’ velocity at the wall is in better agreement with the experimental results [1], compared to the results of the original solver neglecting wall-friction. However, the model is valid only for a specific adjusted value of the specularity coefficient. Thus, Johnson-Jackson model is revisited based on the work of Li and Benyahia [2]. To improve the accuracy of the granular temperature contribution, a new boundary condition, which incorporates sliding and non-sliding collisions, proposed by Schneiderbauer et al. [3], is included in the solver. The velocity profiles of the particles, using the latter boundary conditions, are compared to that of the Johnson-Jackson cases as well as the experimental results. It appears that the boundary conditions of Schneiderbauer et al. [3] yield better agreement with experimental data since these are based on the physical properties and incorporate the coulomb limit to particle-wall collision. Furthermore, based on the experimental study of Sommerfeld and Kussin [1], wall-roughness appears to be an important factor for the transfer of tangential momentum in vertical direction. Accordingly, wall-roughness and shadow effect [4] are studied in order to propose a new model for including wall-roughness in TFM simulations. Finally, the proposed model is implemented in twoPhaseEulerFoam, and the effect of this phenomenon on the particle's velocity profile is studied. The results show that wall-roughness has a strong effect on rebound and redispersion of the particles and accordingly changes the particles’ concentration in the section of conveying line significantly.