Gas-solid Flow Research Articles

Visualization on the meso-scale particle flow in turbulent fluidized bed reactors with lower H0/D ratios via image processing

A two-step Otsu’s method was applied to process the images of gas-solids flow field at multiple axial levels in a TFB with a H0/D ratio from 0.5 to 2.0. The flow field was segmented to a discrete particle phase, a dense particle phase, and a cluster core and a cluster cloud phases. The solids holdup, volume and mass fractions, and the velocity vector field of each phase were analyzed by postprocessing. The frequency, size, shape, and velocity of the cluster cores were also extracted. The increasing H0/D ratio results in an increase in solids holdup in each phase and a decrease in the mass and volume fraction of cluster clouds. A strong disturbance in the local flow field was resulted from the formation and movement of cluster cores and a more dynamic velocity change was found inside cluster clouds. The clusters were found to travel downwardly and with equal chances to move towards the center and the wall of the TFB at a low H0/D ratio with a high Ug.

Impact of Particle Size Polydispersity on Chemical Looping Combustion of Biomass under a Continuous Injection Mode

Particle size polydispersity considerably influences the chemical looping combustion (CLC) of solid fuels. In the present work, a three-dimensional multiphase particle-in-cell (MP-PIC) method was established and applied to study the full-loop CLC of biomass under a continuous injection mode. After model validation, the gas–solid flow characteristics, solid circulation rate (SCR), particle mixing in a fuel reactor (FR), and biomass leakage at different oxygen carrier (OC) particle size distribution (PSD) widths and biomass densities were systematically explored. The results showed that OC particles significantly stratified within the FR and both loop seals. Moreover, the fluctuation amplitude of the SCR decreased as the OC PSD width increased. Small biomass particles were more likely to leak. The number fraction of total leaked biomass particles first decreased and then increased as the OC PSD width increased. In the FR, small OC particles were primarily distributed in the upper part of the bed, and large particles were mainly distributed in the lower part. In addition, within the test range, the magnitude of the density difference between biomass and OC particles positively correlated with the likelihood of biomass leaking. These observations are all beneficial for the design and optimization of the CLC process of biomass.

In Vitro and In Silico Investigations on Drug Delivery in the Mouth-Throat Models with Handihaler®.

This work aimed to evaluate the relative inhalation parameters that affect the deposition of inhaled aerosols, including mouth-throat morphology, airflow rate, and initial condition of emitted particles. In vitro experiments were conducted using the US Pharmacopeia (USP) throat and a realistic mouth-throat (RMT) with Handihaler®. Then, in silico study of the gas-solid flow was performed by computational fluid dynamics and discrete phase method. Results indicated that aerosol deposition in RMT was higher compared to that in USP throat at an airflow rate of 30 L/min, with 33.16 ± 7.84% and 21.11 ± 7.1% lung deposition in USP throat and RMT models, respectively, which showed a better correlation with in vivo data from the literature. Increasing airflow rate resulted in better drug aerosolization, while the fine particle dose trend ascended before declining, with the peak value obtained at a flow rate of 40 L/min. Overall, the effect of geometrical variation was more significant. Additionally, in silico results demonstrated clearly that the initial conditions of the emitted particles from inhalers affected the subsequent deposition. Larger momentum possessed by the central aerosol jet entering the mouth directly led to stronger impaction, which resulted in the deposition in the front region of mouth-throat models. This study is beneficial to develop an in silico method to understand the underlying mechanisms of in vivo mouth-throat deposition.

Numerical prediction of solid particle erosion in angle-cutting elbows with gas–solid flow

Angle-cutting elbows (ACEs) are important components of pipelines for fluid diversion and distribution. Severe erosion may occur on the internal walls of ACEs used to transport solid–gas/liquid flow. Erosion may cause pipeline failures. Therefore, the ACE erosion is a fundamental problem. However, the literature on the erosion characteristics and structural optimization of ACEs is rarely reported. This study conducts a numerical prediction of solid particle erosion in ACEs with gas–solid flow. Simulation models based on the two-way coupled Eulerian–Lagrangian approach and the E/CRC model are combined with particle-wall rebound models and a user defined function in the dispersed phase model. Simulation results are verified through a comparison with previously reported experimental results. The findings show that serious erosion occurs in the downstream part of the concave wall of ACE, whereas only slight erosion occurs on the convex wall of ACE. Therefore, anti-erosion measures should be adopted for the concave wall of ACEs. In addition, structural optimization of ACEs for erosion reduction is conducted. The optimal cutting angle is 40° > 50°, and the optimal non-dimensional cutting length is <1.05. The influence of fluid parameters on erosion characteristics is also determined. The erosion rate on the concave wall increases linearly with particle diameter and exponentially with inlet velocity.

CPFD Simulation of Gas-Solid Flow in Dense Phase Zone of Pant-Leg Fluidized Bed with Secondary Air

Aggregation of fluidization media may appear at the dense phase region of the pant-leg fluidized bed near the incline walls. When the particles flow along the inclined wall, the friction and drag force will cause the particles to accumulate on the inclined wall, resulting in an uneven distribution of particles. The stagnant zones can be minimized by correctly arranging secondary air. Computational particle fluid dynamics (CPFD) method was used to simulate the gas-solid two-phase flow pattern in the dense phase region of pant-leg fluidized bed. Cold tests were performed on a benchtop pant-leg fluidized bed. A high speed imaging technology was used to monitor the flow pattern in the dense phase area, whereas the bubble size and residence time were compared to verify the accuracy of the simulation. The gas-solid flow patterns under various models were simulated. The influence of different secondary air velocities on the reduction of stagnant zone in the dense phase zone of the fluidized bed were predicted. The results indicated that the introduction of secondary air could effectively promote the mixing of particles, and weaken the accumulation of particles on the inclined wall surface. Moreover, secondary air can effectively promote the flow between the gas-solid two-phases and improve the combustion characteristics in the furnace.

Assessment of kinetic theory for gas–solid flows using discrete particle method

Kinetic theory is a common choice for closing the solid phase stress in the continuum theory for dilute and moderate dense gas–solid flows. In this article, methods are proposed for postprocessing the data obtained from discrete particle simulations, and the results are then used to critically assess the fundamental assumptions of kinetic theory. It is shown that (i) the fundamental assumptions and predictions of kinetic theory are, respectively, valid and accurate in homogeneous granular flows, as expected. Those results prove that the methods for data postprocessing are effective; (ii) in the case of nonequilibrium and heterogeneous gas–solid flows, nearly all fundamental assumptions get challenged, and the predictions of kinetic theory, in terms of collision frequency and particle pressure, deviate significantly from the statistical results of discrete particle simulation. Therefore, the standard kinetic theory is insufficient to provide the constitutive laws for continuum modeling of heterogeneous gas–solid flows.

Theoretical and numerical analysis of drag force at the interface between the dilute and dense phases

Resolving the sharp interface between the dilute and dense phases in gas–solid flows is a bottleneck in fine-grid simulations. This study addresses this issue through the theoretical analysis of an infinite gas–structure interface with arbitrary flow directions. The drag force at the interface is decomposed into three parts: the homogeneous drag forces of the dilute and dense regions and a stress divergence difference term. All the three parts are expressed as the functions of the solid volume fractions, particle Reynolds numbers, and stress divergences of the interface grid and its adjacent grids. The developed theoretical drag models at the interface are verified and improved based on particle-resolved direct numerical simulations (PR-DNSs) of flows past plug-like structures. The models are then tested against PR-DNSs of flows past bubble-containing, spherical, ellipsoidal structures. They yield significantly better performance than the traditional Beetstra et al.'s model [Beetstra et al., “Drag force of intermediate Reynolds number flow past mono- and bidisperse arrays of spheres,” AIChE J. 53(2), 489–501 (2007)].

Effect of elevated pressure on gas-solid flow properties in a powder feeding system

Abstract In view of the powder feeding system, a multi-physical coupling model of the gas-powder-piston was established based on the Euler-Euler two-fluid model. The numerical simulation method was applied to explore the effects of dense gas-solid flow characteristics under different operating pressures. The results show that gas-solid pulsations at different operating pressures are mainly concentrated in the upper part of the powder tank. An elevated operating pressure efficiently decreases the powder layer area (εp = 0.1) fluctuation. As the operating pressure increases from 0.5 MPa to 3.0 MPa, the rising time and fluctuation rate of pressure are reduced by 71.4% and 62.3%, respectively, and the pressure in the tank has a long stabilization period. Meanwhile, the variation of the instantaneous powder flow rate is more stable and its average value is closer to the theoretical. A high-pressure environment is more conducive to the stable transportation of powder.

Erosion Behaviors of Gas–Solid Flow on an Assignment Plate using CFD–DEM

The erosion wear of a gas–solid flow is a major challenge for an S Zorb reactor; it affects the production safety and stability. This study aims to investigate the erosion wear problem of a gas–solid flow on an assignment plate in an S Zorb reactor using CFD–DEM. The erosion wear behavior of the assignment plate is studied through flow field and particle trajectory analyses. Moreover, five polyhedral particle models were established by DEM to study the effect of particle shape on erosion behavior. The numerical calculation results show that the special structure of the bubble cap affords a special serrated erosion wear area. According to the movement process of particles, the erosion wear of the assignment plate is divided into two stages: first a large range of slight erosion wear occurs and then seriously concentrated erosion wear occurs. With the decrease of particle sphericity φ, the erosion wear rate first decreases rapidly and then increases slowly. φ=0.85 is the critical value of change, and the assignment plate has the lowest erosion wear rate. The simulation studies are helpful for reducing the erosion wear of the assignment plate and improving its service life.

Development and verification of coarse‐grain CFD‐DEM for nonspherical particles in a gas–solid fluidized bed

AbstractComputational fluid dynamics coupled with discrete element method (CFD‐DEM) has been widely used to understand the complicated fundamentals inside gas–solid fluidized beds. To realize large‐scale simulations, CFD‐DEM integrated with coarse‐grain model (CG CFD‐DEM) provides a feasible solution, and has led to a recent upsurge of interest. However, when dealing with large‐scale simulations involving irregular‐shaped particles such as biomass particles featuring elongated shapes, current CG models cannot function as normal because they are all developed for spherical particles. To address this issue, a CG CFD‐DEM for nonspherical particles is proposed in this study, and the morphology of particles is characterized by the super‐ellipsoid model. The effectiveness and accuracy of CG CFD‐DEM for nonspherical particles are comprehensively evaluated by comparing the hydrodynamic behaviors with the results predicted by traditional CFD‐DEM in a gas–solid fluidized bed. It is demonstrated that the proposed model can accurately model gas–solid flow containing nonspherical particles, merely the particle dynamics are somewhat lost due to the scaleup of particle size. Finally, the calculation efficiency of CG CFD‐DEM is assessed, and the results show that CG CFD‐DEM can largely reduce computational costs mainly by improving the calculation efficiency of DEM. In general, the proposed CG CFD‐DEM for nonspherical particles strikes a good balance between efficiency and accuracy, and has shown its prospect as a high‐efficiency alternative to traditional CFD‐DEM for engineering applications involving nonspherical particles.

An inertial number regulated stress model for gas-particle flows with particle friction and volume fraction gradient

In this study, a solid stress model that can be used in both dilute and dense regimes for simulation of gas-particle flows with comprehensive considerations of friction and the effect of local solid volume fraction gradient is proposed. In the dilute regime, solid stress is closed by a modified kinetic theory that accounts for the effect of particle friction and volume fraction gradient. In the dense regime, solid stress is closed by the inertial number model. The transition from dilute to dense regimes is realized by using a dimensionless parameter χ , which is a function of the inertial number I s . This new model is validated with experimental data and discrete particle simulation from spout-fluid bed and bubbling fluidized bed. When compared with the traditional kinetic-frictional stress model, this new model improves the transition from dilute to dense regimes and the particle velocity predictions in both beds. • A solid stress model is proposed in both dilute and dense regimes for gas-solid flow. • The model comprehensively considers the effect of friction and volume fraction gradient. • The expression of excluded volume is derived from the generalized van der Waals theory. • The model gives a better prediction of spout-fluid bed and bubbling fluidized bed.

Dust transport behaviour in the Nanwenhe extra-large stepped underground metal mine stope

This paper is aimed at studying the dust behaviour of a super-large stepped stope, which is essential in reducing the dust hazards and setting up effective dust control facilities. In this paper, a three-dimensional model is established for the super-large stepped stope of Nanwenhe. Based on the theory of gas-solid two-phase flow, the Fluent software was applied to carry out a numerical simulation of the air flow distribution in the stope and the dust concentration distribution from multiple dust sources. The simulation results reveal that air flow is affected by the roof fault, resulting in it being diverted to the connecting roadway of the return air roadway, the roof of the lower stope and the interior of the stope. A part of the air flow flows back to the upper stope operation area at a speed of 0.3–0.6 m/s. The dust produced by the shovelling and dumping operations forms a high concentration dust belt with a length of 41 m, a width of 3.6 m and an average concentration of over 260 mg/m³. A portion of the dust flows back to the operation area by bypassing the ore pillar, with a concentration of 60–80 mg/m³. The reliability of our simulation method was verified by field measurements of the air flow velocity and dust concentration. Finally, a new intelligent wind-fog combined dust removal system was designed according to the dust concentration distribution, which achieved an overall dust removal efficiency of 76.9%. These research results provide certain references for the study of dust migration law in underground metal mines and the development of dust removal equipments.

Gas-Solid Flow and High-Efficiency and Low-NO x Combustion of a Novel Arch-Fired Boiler with Swirl Burners: Impact of Over-Fire Air

ABSTRACT For a 300 MWe anthracite- and arch-fired boiler with biased-swirling-secondary-air (BSSA) technology, the distribution law of gas/solid two-phase velocity in the furnace is obtained with the help of a 1:10 cold modeling experiment bench, and the combustion and NO x formation characteristics in the furnace are revealed by numerical calculation. The results show that, with the increase of over-fire air rate, the reflux velocity in the reflux zone under the arch decreases continuously, and when the main air flow from the furnace arch reaches the staged air region from the vent air region, the position of particle flow in the primary air turn upward is advanced, and the utilization rate of the space in the lower furnace will decrease continuously. In the regions of vent air and staged air, when the over-fire air rate increases from 15% to 25%, the maximum vertical velocity decreases by approximately 2 times, and the downward depth of air flow from the furnace arch decreases significantly. When the over-fire air rate is increased from 10% to 25%, the thermal resistance between the primary air of the burner and the high-temperature flue gas in the furnace center increases, the ignition of the primary pulverized coal air flow is advanced, the flue gas temperature in the center of the lower furnace is decreased. As a result, with the increase of over-fire air rate, the combustible content of fly ash increases continuously. Especially, when the over-fire air rate increases from 20% to 25%, the combustible content of fly ash increases significantly. With the increase of the over-fire air rate, the high CO concentration region in the primary combustion zone expands, the reducing atmosphere is strengthened, the formation of NO x is effectively restrained, and the NO x emission concentration at the furnace outlet continues to decreases. In particular, when the over-fire air rate increases from 15% to 20%, the NO x emission concentration decreases greatly. After the swirling secondary air of the arch-fired boiler is biased, the setting of over-fire air rate is recommended to be 20%, and the NO x emission concentration and combustible content of fly ash are 694 mg/m3(O2 = 6%) and 7.32%, respectively. Compared with a traditional swirling arch-fired boiler, the combustible content of fly ash and NO x emission concentration are reduced by 46.8% and 34.1%, respectively.

MP-PIC investigation of glycerol steam reforming in bubbling fluidized bed for high-quality hydrogen production

A reactive multiphase particle-in-cell approach is adopted to numerically investigate the physical and thermochemical characteristics of glycerol steam reforming process in a three-dimensional bubbling fluidized bed. Via the model verification, the simulated results are in acceptable agreement with the experimental measurements. The gas-solid flow hydrodynamics and the particle thermochemical properties are explored. In addition, the effects of different operating parameters (reaction temperature, particle diameter, steam/glycerol ratio, operating pressure) on the performance of the glycerol steam reforming system are discussed. The results reveal the significantly non-uniform spatial distribution of particle temperature and heat transfer coefficient (HTC) due to the intense endothermic reactions and nonlinear hydrodynamics. Comparatively large HTC of solid particles appears in the lower part of the bed. Enlarging the reaction temperature and operating pressure give rise to a larger particle HTC. The back-mixing and internal circulation behavior of particles highly influence the spatial distribution of particle velocity inside the bed. The slip velocity of particle accumulated around the gas feeding port is obviously large. The slip velocity and HTC of particle are almost positively correlated. Decreasing the steam/glycerol ratio enhances the particle thermochemical properties. Moreover, elevating the reaction temperature significantly promotes the yields of H2 and CO while the formation of CH4 is suppressed.

Comparative study of the single and double specularity coefficients on two-fluid modeling of a pseudo-2D gas–solid fluidized bed

The specularity coefficient is an unmeasurable parameter in the most popular wall boundary model during the two-fluid modeling of dense gas–solid flows. Using multiphaseEulerFoam solver, the influence of different specularity coefficient setting strategies on the gas–solid flow inside a pseudo-2D fluidized bed has been explored. It is found that the single specularity coefficient plays a regulatory role in the quantitative prediction. Increasing the specularity coefficient would cause a fluidization transition from freely bubbling to slugging, and the bed characteristics such as pressure drop and bed expansion present monotonic nonlinear changes. The double specularity coefficients approach is shown to significantly improve the predictive accuracy through verifying with the measured particle velocities, bubble diameter and rise velocity. In addition, the lognormal bubble size distribution and Gaussian bubble rise velocity distribution are observed. The specularity coefficient for walls in thickness direction is crucial and its different effects are unignorable. Overall, the present study provides a practical strategy of double specularity coefficients for the solid wall boundary conditions during two-fluid modeling.

Powder fuel transport process and mixing characteristics in cavity-based supersonic combustor with different injection schemes

The transport process and mixing characteristics of powder fuel in the supersonic airflow are essential for the working performance of a powder fueled scramjet. In the current investigation, the effect of different injection schemes on the powder fuel jet flow field and mixing characteristics are numerically studied in the cavity-based supersonic combustion chamber. The simulation is employed by the Eulerian-Lagrangian scheme coupling with the gas-solid two-phase Reynolds-average Navier-Stokes (RANS) method. Verification of numerical method and grid-independent is conducted firstly, which demonstrated the correctness and credibility of the method in this research. The results show that the powder fuel injection into the combustor induces typical gas-solid two-phase jet flow characteristics. With respect to the fuel injection schemes upstream of the cavity, the powder fuel moves across the cavity section and is continuously mixed with the mainstream along the flow direction. The mixing intensity in the combustion chamber is associated with the motion and distribution patterns of the particles. For the cavity internal fuel injection scheme, the particles are mainly distributed inside the cavity. Some of the particles can be coiled and sucked into the supersonic mainstream. The cavity shear layer and expansion section near the bottom wall is the main region with better mixing performance.

Research on casing erosion in reservoir section of gas storage wells based on Gas-solid Two-phase flow

Gas storage wells are characterized by large injection and production volumes and multi-cycle high-intensity cyclic injection and production, and the high-speed flowing sand particles have a strong erosion effect on the tubular column, threatening the safety of the gas storage wells' tubular column. In this paper, gas–solid two-phase flow erosion experiments and erosion simulation are used to study the effects of sand parameters, casing materials, and formation temperature on the erosion rate of reservoir section casing during gas recovery. The results show that due to the high temperature weakening the material strength of the casing in the reservoir section, the erosion rates of N80 and P110 casings increase with the growth of environment temperature. The casing erosion rate in the reservoir section increases with the raising of gas production volume, and the casing erosion rate increases rapidly when the gas production volume of the case well exceeds 162 × 104m3/d. The increases in sand particle diameter and mass flow rate also aggravate the casing erosion. The borehole dog-leg degree affects the erosion rate of the casing by influencing the attack angle of sand particles on the casing, and the erosion rate increases significantly when the borehole dog-leg degree exceeds 3°. The study results provide theoretical guidance for the building and operation of gas storage wells and are of great practical significance.

Continuous wavelet analysis and proper orthogonal decomposition on particle dynamics in a horizontal self-exited gas-solid two-phase pipe flow

The high-speed particle image velocimetry is applied to measure the particle velocities of a horizontal self-exited gas-solid two-phase pipe flow with soft fins at the air velocity of the minimum pressure drop. The detailed particle dynamics of using fins' cases are analyzed and compared with a conventional non-fin case. It is found that the particle accelerating process is delayed in the acceleration regime and the vertical particle velocity is decreased near the top part of fully-developed regime due to the effect of fins' oscillations. The continuous wavelet transform of axial particle fluctuation velocity suggests that the fins make the particle motions fall into a lower frequency range in the acceleration regime. In the fully-developed regime, the dominating frequencies of large-scale particle flows are decreased and the streaks appearing in the range of high frequency are weakened by using fins. The energy distributions of proper orthogonal decomposition (POD) modes indicate that the relative energy of POD mode1 is increased by using fins in both acceleration and fully-developed regimes, and the dominance is significantly enhanced by using fins in the fully-developed regime. The auto-correlation coefficients and central frequencies of POD modes exhibit that the dominating large-scale particle motion is enhanced by using fins, correspondingly, small-scale particle motion is suppressed. • The particle velocity of a horizontal self-exited gas-solid two-phase pipe flow is measured at minimum air velocity by PIV. • POD and continuous wavelet transform is adopted to analyze the particle fluctuation velocities. • The dominating frequencies of large-scale particle flows are decreased. • The streaks appears in the range of high frequency are weakened by using fins. • The dominance of POD mode 1 is significantly enhanced by using fins in the fully-developed regime.

On the comprehension of the gas split in loop seal devices

Gas distribution through the chambers of a loop seal in circulating fluidized bed (CFB) units is still difficult to predict and often misunderstood. Dedicated experiments were carried out in an isolated loop seal to assess how the gas fed into a loop seal is distributed under different aeration modes (through the recycle chamber, the supply chamber and both chambers), aeration flowrates and solids fluxes. Experimental results shed light on the non-ideality of the gas-solids flows occurring in these units and, in combination with theoretical considerations, it is demonstrated why semi-empirical models cannot effectively describe the performance of loop seals operating as non-mechanical valves. Recommendations are given for the optimization of the operation of a loop seal as a non-mechanical valve in a solids circulating loop. A method is proposed to characterize the performance of a loop seal in a CFB unit from measurements of that loop seal in isolation mode. • Gas splitting into the chambers of a loop seal is a key issue difficult to predict. • The gas distribution was experimentally assessed in an isolated loop seal. • Loop seal operation as non-mechanical valves leads to non-ideal gas-solids flows. • Non-ideal gas-solids flow pattern establishes the gas splitting into the chambers. • The study gives insights on how to control the solids circulation with a loop seal.

Investigation of particle flow characteristics in a CFB recirculation system using electrical capacitance volume tomography

Gas-solid fluidized beds have complex flow characteristics and opacity, and the particle motion characteristics in the recirculation system of circulating fluidized bed (CFB) have not been studied thoroughly. In order to study the motion of gas-solid two-phase flow in a CFB recirculation system, a 32-electrode electrical capacitance volume tomography (ECVT) system was developed with an imaging speed of 120 frames per second. Under various working conditions, the experimental platform of CFB was also built to measure the particle flow in the CFB recirculation system with the 32-electrode ECVT system, including the solid concentration and the flow velocity of particles in the branch pipe. When the velocity of fluidized gas U f is adjusted from 2.73 m/s to 1.76 m/s, the average solid concentration in the riser is roughly distribution of upper dilute and lower concentrated along the axial direction. With the decrease of U f , the average solid concentration generally showed an increasing trend, and the difference of solid concentration between different planes in the branch pipe gradually decreased. The slug length in the imaging area decreases, and the rising velocity of the slug in the riser above the branch pipe decreases. ECVT continuous images (U f = 2.73 m/s). • The 3-D solid concentration distribution in the CFB recirculation system was investigated. • ECVT continuous images under three different working conditions are given. • Solid concentration in the CFB recirculation system was acquired from the ECVT images. • Solid velocity was gained by analyzing the time lag of the neighbour planes.