Gas-particle Flow Research Articles

Investigation of erosion behaviors of sulfur-particle-laden gas flow in an elbow via a CFD-DEM coupling method

In the production and gathering process of natural gas with the high sulfur content, due to the variations in temperature, pressure, and other factors, sulfur dissolved in the gas may be precipitated as a solid particle in the gathering pipeline. Sulfur particles carried by high-speed flow impact elbow of pipelines, thus causing equipment malfunctions and even failures. In this study, a CFD-DEM-based erosion prediction model for gas-particle two-phase flow was proposed based on the consideration of the gas-particle, particle-particle and particle-wall interactions. The effects of secondary flow, vortices and particle trajectories on rare erosion scars were investigated. In addition, four kinds of polyhedral particles were modeled based on DEM framework to simulate erosion behaviors based on the consideration of particle shapes. The results indicate the V-shaped erosion scar is caused by the secondary collision. Two adjacent obvious erosion scars on the upper part of V-shaped scar are caused by direct collisions and sliding collisions. From the inlet to the outlet of the elbow, the turbulence intensity near the two sides of the wall increases and the secondary flows and vortices appear. Particle trajectories are affected by complex flow fluid, which causes rare erosion scars on the side walls of the downstream straight pipe near the elbow outlet. With the increase in the particle sphericity, the erosion rate decreases firstly and then increases. When the sphericity is <0.77, the erosion rate is mainly affected by the impact velocity and impact angle; when the sphericity is larger than 0.77, the influence of impact concentration on the erosion rate is more obvious.

Effects of specularity and particle-particle restitution coefficients on the hydrodynamic behavior of dispersed gas-particle flows through horizontal channels

Specularity coefficient (ϕ) and particle–particle restitution coefficient (e) are two important parameters governing the flow physics of dispersed gas-particle flows. In this work, a detailed numerical analysis is carried out to get an insight into the effects of these two parameters in the flow hydrodynamics of dispersed gas-particle flows through horizontal channels. Investigations have also been carried out to find the ϕ-e pair for which the phase velocities become an extremum. It has been found that at a particular value of e, both gas and particle velocities at the centerline of the channel increase with increase in the value of ϕ, whereas near the wall, they tend to decrease. At a fixed non-zero value of ϕ, both gas and particle velocities tend to increase with increase in the value of e. For ϕ equal to zero, which corresponds to free-slip boundary condition for particle velocity, there is no significant variations in gas and particle velocities with changes in e. Out of all combinations of values of ϕ and e investigated herein, it is found that both gas and particle velocities attain a maximum value when both the values of ϕ and e are maximum.

Assessment of gas-particle flow models for pseudo-2D fluidized bed applications

ABSTRACTThe aim of this work is to provide more insight into the general modeling criteria for simulating pseudo-2D bubbling fluidized beds. For this purpose, two experimental-based problems are studied. First, a fluidized bed with a high-speed central jet problem is analyzed. A qualitative study of the first bubble indicates that the bubble shape prediction is highly sensitive to the frictional model adopted. The most accurate results in terms of bubble shape and detachment time are given by a frictional model that relates the strain-rate fluctuations with the granular temperature. Second, a uniformly fluidized bed problem in bubbling regime is considered. For this case, the drag models and boundary conditions for the particulate phase are investigated. Time-averaged solid phase velocity profiles are compared with the results of the literature where it is found that no-slip conditions (or partial slip with a high specularity coefficient) are more appropriate than slip conditions at the walls for these regimes. Regarding the drag force, although none of the models presented could match the experimental velocity predictions for low gas velocities at the lower region of the bed, the Di Felice model produces the most accurate results for the whole range of regimes considered.

Influence of offset angle of mid-secondary air nozzles on gas-particle flow characteristics in a furnace.

A three-component particle-dynamics anemometer was used to measure the characteristics of two-phase gas-particle flows in a primary air nozzle using a gas/particle two-phase test facility. The jet trajectory of primary air velocities, Reynolds stress and particle concentration profiles were obtained. Based on this, the advantages and disadvantages of increasing the offset angle were analyzed to optimize performance. The results show that gas-particle slippage is not noticeable in the furnace, even though the mid-secondary air offset angle changes. Within the jet's free developing zone, the location of the peak concentration does not change with the mid-secondary air offset angles. Properly enlarging the offset angle of mid-secondary air nozzles will delay the mixing of the secondary air into the primary-air-and-coal mixture and realize horizontal-staged combustion in a furnace. It also can enhance the stability of airflow and form a better blended jet tail, meanwhile reducing the vortex at the back-fire side and avoiding a short circuiting of the airflow. It is favorable for increasing combustion efficiency and preventing slagging and high-temperature corrosion.

Eulerian LES simulation of coal jet flame with a simplified DQMOM model

Eulerian methods are popular for describing particle dispersion and reaction in gas-particle two-phase flow simulations. However, for problems with wide Particle Size Distributions (PSD), such as those associated with coal combustion, the PSD has to be represented by a limited number of particle sizes in order to decrease the computational costs of the transport equations. The objective of this study is to investigate the effects of the number of particle sizes on the Eulerian LES simulation of a pulverized coal jet flame. In this study, experimental work on a Hencken-type entrained flow reactor was carried out to provide the validation data of flame ignition distances. A simplified Direct Quadrature Method of Moments (DQMOM) method was proposed in order to decrease the computation cost in the Eulerian framework. Several simulation cases with different quadrature nodes (particle sizes) were conducted. The flame ignition distance was well predicted when two and more quadrature nodes were adopted in the simulation. But it was overestimated when only one quadrature node was considered. So one particle size is not enough to represent a wide PSD of coal in the Eulerian LES simulation of pulverized coal flames. When 5 particle sizes were adopted in the simulation, the ignition distances and time-variant fluctuations can be well predicted when compared with experimental results under different operating conditions. This study shows that LES with the simplified DQMOM method can describe the ignition distance in gas-solid two-phase reacting flows even with only a few quadrature nodes.

A comprehensive assessment of different wall boundary conditions on the simulation of bubbling fluidized beds

There are different particle-wall boundary conditions available in the literature to account for the particle slip velocity and the granular energy flux at the wall. However, it is not yet clear how these different wall boundary conditions affect the simulated flow behavior, nor is it clear which are the most realistic. To this end, three different particle-wall boundary conditions are examined to assess their ability to predict the dynamics of a dense gas-particle flow inside a three-dimensional bubbling bed, using the two-fluid model. To understand how the wall models affect structural features of the flow, a quantitative analysis is performed on some important aspects of the mechanics of bubbling beds that have received relatively little attention in the literature. Accordingly, the effect of each wall model on the velocity field, three-dimensional bubble statistics, gas-pressure fluctuations, and particle resolved-scale Reynolds stress are investigated. Also, the predicted dominant mixing regions inside the bed are identified and visualized in order to quantitatively describe the bed performance. It is found that the more energetic bubbles result in a lower level of granular temperature, a less-expanded bed, and more extensive mixing regions inside the bed. It is also observed that, in the case of bubbling beds, the mixing caused by the Reynolds normal stress is much stronger than that caused by the Reynolds shear stress. Overall, the flows predicted by the three wall models are structurally similar. However, some specific features can differ in a systematic way that can be tracked to the effect of wall boundary condition on the bubble behavior. The numerical results are validated against published experimental data and demonstrate the significant role of the particle-wall boundary condition.

Verification, validation, and uncertainty quantification of a sub-grid model for heat transfer in gas-particle flows with immersed horizontal cylinders

In previous work we developed and implemented a sub-grid model for the efficient simulation of heat transfer in gas-particle flows around immersed horizontal cylinders. In this study we apply verification, validation, and uncertainty quantification methods to the developed model to rigorously examine its capabilities and limitations. Numerical verification with small, unit-cell problems shows excellent transient and steady-state behavior. Validation of a bubbling bed and a turbulent bed showed good agreement with high-resolution simulations. To quantify the error of the constitutive model predictions two methods were used to calculate confidence intervals, showing an error of approximately ±20%, well within the range of typical Nusselt number approximations. The sub-grid model was applied to a conceptual pilot-scale 1 MWe carbon capture reactor to compare with alternative modeling methods. Results show fair predictions of hydrodynamics, heat transfer, and carbon capture rates with significant savings in computational runtimes.

Investigation of a gas–particle flow with particle–particle and particle–wall collisions by immersed boundary method

Investigation of a gas–particle flow with particle–particle and particle–wall collisions by immersed boundary method

Modelling of Spouted and Spout-Fluid Beds: Key for Their Successful Scale Up

The development of robust mathematical models could provide the necessary tools for a more rapid, efficient, and reliable spouted bed technology development. Computer simulations can be very useful to aid this design and scale-up process: firstly, they can contribute to obtain a fundamental insight into their complex dynamic behavior by understanding the elementary physical principles such as drag, friction, dissipation etc.; secondly, the simulations can be used as a design tool where the ultimate goal is to have a numerical model with predictive capabilities for gas-particle flows at engineering scale. Clearly, one single simulation method will not be able to achieve this goal, but a hierarchy of methods modelling phenomena on different length and time scales can achieve this. The most fruitful approach will be when they are simultaneously followed, so that they can mutually benefit from each other. In this sense, this paper presents a review of the current state of the art of modelling on spouted and spout-fluid beds through an analysis of recent literature following a multiscale approach (molecular and particle, lab, plant and industrial scale). The main features of the different scales together with their current limits are discussed and specific topics are highlighted as paths that still need to be explored. In summary, the paper aims to define the theoretical setline and the basis of improvement that would lead to a robust multiscale model with solid links between micro and macroscopic phenomena. If done with the correct balance between accuracy and computational costs it will gear SB towards their reliable and successful implementation.

Numerical simulation study on the influence of incident position on erosion characteristics of gas-particle two-phase flow in 90° elbow

In gas gathering pipeline system, 90° elbow suffers constant and severe collision of gas-carrying particles, which causes serious erosion on the inner wall. In this study, models of standard elbows (carbon steel with 210 GPa in elasticity modulus and 8300 kg/m3 in density) whose diameter varies from 65 to 200 mm with curvature ratio 1.5 are established to predict erosion rate causes by silica sand (100 μm in diameter, 1550 kg/m3 in bulk density) under gas gathering operating conditions (5.0 MPa, 10 m/s). Unlike most of the previous studies which only focus on erosion result on the wall, in this article, author extracts sample particles as research objects and studies the motion of every single particle. It has been found that there are three typical types of particle motion patterns in the elbow, and a part of sand particles released from lower area of incident plane will not even cause erosion.

Towards filtered drag force model for non-cohesive and cohesive particle-gas flows

Euler-Lagrange simulations of gas-solid flows in unbounded domains have been performed to study sub-grid modeling of the filtered drag force for non-cohesive and cohesive particles. The filtered drag forces under various microstructures and flow conditions were analyzed in terms of various sub-grid quantities: the sub-grid drift velocity, which stems from the sub-grid correlation between the local fluid velocity and the local particle volume fraction, and the scalar variance of solid volume fraction, which is a measure to identify the degree of local inhomogeneity of volume fraction within a filter volume. The results show that the drift velocity and the scalar variance exert systematic effects on the filtered drag force. Effects of particle and domain sizes, gravitational accelerations, and mass loadings on the filtered drag are also studied, and it is shown that these effects can be captured by both sub-grid quantities. Additionally, the effect of cohesion force through the van der Waals interaction on the filtered drag force is investigated, and it is found that there is no significant difference on the dependence of the filtered drag coefficient of cohesive and non-cohesive particles on the sub-grid drift velocity or the scalar variance of solid volume fraction. The assessment of predictabilities of sub-grid quantities was performed by correlation coefficient analyses in a priori manner, and it is found that the drift velocity is superior. However, the drift velocity is not available in “coarse-grid” simulations and a specific closure is needed. A dynamic scale-similarity approach was used to model drift velocity but the predictability of that model is not entirely satisfactory. It is concluded that one must develop a more elaborate model for estimating the drift velocity in “coarse-grid” simulations.

Two-fluid LES and RANS modeling of sudden-expansion gas-particle flows

Sudden-expansion gas-particle flows are widely encountered in fluid and thermal engineering and are typical cases for basic studies. Both two-fluid large-eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) modeling using a unified second-order (USM) two-phase turbulence model are studied in this paper. The advantage of LES is showing the detailed flow structures and improving the simulation results. For LES most of investigators adopt Eulerian-Lagrangian (E-L) LES approach, in which the widely used are Smagorinsky eddy-viscosity model, Germano dynamic eddy-viscosity model and Kim's SGS energy equation model for single-phase flows and there are no particle SGS stress models. Some researchers proposed gas-phase SGS stress models accounting for the effect of particles in E-L LES. A few investigators proposed particle SGS stress models for two-fluid LES of gas-particle flows, however the particle SGS stress models are simply imitated from the gas SGS stress model, and the interaction between gas and particle SGS stresses is not taken into account. In this paper a kgs-kps-kpgs two-phase SGS stress model for two-fluid LES of gas-particle flows is proposed to simulate sudden-expansion gas-particle flows. Simultaneously, The USM-RANS modeling was made for sudden-expansion gas-particle flows. Also, measurements were made to validate both LES and RANS modeling results. It is shown that LES can reveal the detailed flow structures and surely in most regions of the flow field make some improvements over the USM-RANS modeling.

Impact zone dynamics of dilute mono- and polydisperse jets and their implications for the initial conditions of pyroclastic density currents

The fluid dynamics at the impact zone of an impinging jet dictate the initial conditions for the resulting downstream flow. Current understanding of this impact zone is limited to small scale, single phase, or monodisperse impinging jet experiments with a focus on industrial and engineering applications. Here, we present multifield numerical modeling results of mono- and polydisperse impinging jets with length scales and physical parameters relevant to large explosive volcanic eruptions. Our modeling shows that particle behavior is sensitive to whether a jet is monodisperse or polydisperse. For a monodisperse jet, the downstream flow behavior can be predicted by the coupling between the particles and gas, which we characterize by a form of the Stokes number (Stimp) where the time scale of changes in fluid motion is defined by the length scale and velocity change associated with vertical deceleration as a mixture approaches an impact surface. We find that the length scale of deceleration is sensitive to the mixture Mach number, as high Mach number flows produce standing shocks upstream of the impact site. For low Stimp monodisperse cases, the particles make the transition from axial to radial flow easily, and the flow continues downstream relatively unimpeded and well mixed. In contrast, in situations where the monodisperse particles are sufficiently poorly coupled (high Stimp), the particles rebound and/or concentrate at the impact zone, which results in a radial acceleration of gas as it is expelled from the concentrating mixture. In polydisperse jets, larger particles become better coupled to the gas in the free jet zone when in the presence of smaller, well-coupled particles due to particle-particle drag. However, the larger particles still lose significant momentum in the impact zone, which results in a lag effect where the smaller particles and gas are expelled and advance radially at a greater velocity. Although the simulations are simplified compared with real volcanic eruption scenarios, the results suggest that processes in the impact zone contribute directly to the formation of different types of gas-particle flows (concentrated versus dilute) that move outward across the ground.

Effect of particle stress tensor in simulations of dense gas–particle flows in fluidized beds

A two-fluid model based on the kinetic theory of granular flow for the rapid-flow regime and the Coulomb friction law for the quasi-static regime is applied to predict the hydrodynamics of dense gas–particle flow in a three-dimensional fluidized bed. Two different models for the particle stress tensor that use different constitutive equations in the elastic-inertial regime are examined to assess their ability to predict bed dynamics. To understand how particle stress models affect structural features of the flow, a quantitative analysis is performed on some important aspects of the mechanics of bubbling beds that have received relatively little attention in the literature. Accordingly, different flow regimes are identified in the context of fluidized beds through the dimensionless inertial number, and the main characteristics of each regime are discussed. In addition, how the particle stress tensor manifests itself in the bubble characteristics, natural frequency of the bed, and particle Reynolds stress are investigated, all of which help to better understand the complex dynamics of the fluidized bed. The numerical results are validated against published experimental data and demonstrate the significant role of the stress tensor in the elastic-inertial regime.

Turbulence modulation model for gas-particle flow based on probability density function approach**Project supported by the National Natural Science Foundation of China (Grant No. 51176044).

The paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function (PDF) approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data.

Suppression of the coalescence of gas bubbles in aqueous electrolyte solutions: dependence on the external pressure and velocity of gas flow through a column with liquid

An experiment on laser diagnostics of gas bubbles moving in aqueous solutions of different electrolytes has revealed that efficient suppression of gas bubble coalescence (collapse) occurs at certain concentrations of cation−anion pairs. It is experimentally proven that there is an optimal ion composition at which the coalescence is suppressed in a wide range of velocities of gas particle flow through a column with liquid. It is shown that the threshold ion concentrations corresponding to the onset of coalescence suppression decrease with an increase in the external pressure in the liquid. The results obtained can be used in oil production technologies to solve the problem of burning associated oil gas in torches.

Assessment of Different Discrete Particle Methods Ability To Predict Gas-Particle Flow in a Small-Scale Fluidized Bed

Several discrete particle methods exist in the open literature to simulate fluidized bed systems, such as discrete element method (DEM), time-driven hard sphere (TDHS), coarse-grained particle method (CGPM), coarse grained hard sphere (CGHS), and multiphase particle-in-cell (MP-PIC). The main difference between these methods is in the treatment of particle–particle interactions: by calculating collision forces (DEM and CGPM), using momentum conservation laws (TDHS and CGHS), or based on the particle stress model (MP-PIC). Here, these methods are compared by simulating the same small-scale fluidized bed with the same open-source code MFIX. The results indicate that both modeling the particle–particle collision by TDHS and lumping a few particles in a parcel increase the computation speed with little loss in accuracy. However, the MP-PIC method predicts an unphysical particle–particle overlap, which results in incorrect overall bed hydrodynamics. These results suggest using the CGHS method for fluidized bed...

A solution algorithm for fluid–particle flows across all flow regimes

Many fluid–particle flows occurring in nature and in technological applications exhibit large variations in the local particle volume fraction. For example, in circulating fluidized beds there are regions where the particles are close-packed as well as very dilute regions where particle–particle collisions are rare. Thus, in order to simulate such fluid–particle systems, it is necessary to design a flow solver that can accurately treat all flow regimes occurring simultaneously in the same flow domain. In this work, a solution algorithm is proposed for this purpose. The algorithm is based on splitting the free-transport flux solver dynamically and locally in the flow. In close-packed to moderately dense regions, a hydrodynamic solver is employed, while in dilute to very dilute regions a kinetic-based finite-volume solver is used in conjunction with quadrature-based moment methods. To illustrate the accuracy and robustness of the proposed solution algorithm, it is implemented in OpenFOAM for particle velocity moments up to second order, and applied to simulate gravity-driven, gas–particle flows exhibiting cluster-induced turbulence. By varying the average particle volume fraction in the flow domain, it is demonstrated that the flow solver can handle seamlessly all flow regimes present in fluid–particle flows.

Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) Study of Mass-Transfer Mechanisms in Riser Flow.

We report a computational fluid dynamics–discrete element method (CFD-DEM) simulation study on the interplay between mass transfer and a heterogeneous catalyzed chemical reaction in cocurrent gas-particle flows as encountered in risers. Slip velocity, axial gas dispersion, gas bypassing, and particle mixing phenomena have been evaluated under riser flow conditions to study the complex system behavior in detail. The most important factors are found to be directly related to particle cluster formation. Low air-to-solids flux ratios lead to more heterogeneous systems, where the cluster formation is more pronounced and mass transfer more influenced. Falling clusters can be partially circumvented by the gas phase, which therefore does not fully interact with the cluster particles, leading to poor gas–solid contact efficiencies. Cluster gas–solid contact efficiencies are quantified at several gas superficial velocities, reaction rates, and dilution factors in order to gain more insight regarding the influence of clustering phenomena on the performance of riser reactors.

Euler–euler anisotropic gaussian mesoscale simulation of homogeneous cluster‐induced gas–particle turbulence

An Euler–Euler anisotropic Gaussian approach (EE‐AG) for simulating gas–particle flows, in which particle velocities are assumed to follow a multivariate anisotropic Gaussian distribution, is used to perform mesoscale simulations of homogeneous cluster‐induced turbulence (CIT). A three‐dimensional Gauss–Hermite quadrature formulation is used to calculate the kinetic flux for 10 velocity moments in a finite‐volume framework. The particle‐phase volume‐fraction and momentum equations are coupled with the Eulerian solver for the gas phase. This approach is implemented in an open‐source CFD package, OpenFOAM, and detailed simulation results are compared with previous Euler–Lagrange simulations in a domain size study of CIT. The results demonstrate that the proposed EE‐AG methodology is able to produce comparable results to EL simulations, and this moment‐based methodology can be used to perform accurate mesoscale simulations of dilute gas–particle flows. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2630–2643, 2017