Gas-solid Flow System Research Articles

Validation study on a scaling law model of the DEM in industrial gas-solid flows

Large-scale gas-solid flow systems, e.g., fluidized beds, cyclone separators and pneumatic conveyors, are often encountered in chemical engineering. Numerical modeling technologies are widely applied for design and understanding of complex phenomena in these gas-solid flow systems, for which the coupled model of the discrete element method (DEM) and computational fluid dynamics is generally employed. However, application of the numerical simulations for these systems is still limited because the number of the particles that can be modeled (about several hundreds of thousand) is quite small comparing with the immeasurable number of particles used in the industrial processes, and not sufficient to fully understand the complex behavior in these processes. The coarse graining DEM is then developed to provide an alternative approach for modeling the real industrial processes. Accuracy of the coarse graining DEM has been proven for simple systems so far. In the present study, applicability of the coarse graining DEM for complex shaped domains is explored, for which typical industrial processes, such as fluidization with inserted tubes, and powder flow into a confined space, are considered. In these calculations, signed distance functions (SDF) and immersed boundary method (IBM) are used to model an arbitrary shape wall boundary in a gas-solid flow. Both numerical modeling using the coarse graining DEM and experimental investigation are performed with a thorough comparison between the experimental and numerical results. It is demonstrated that the coarse graining DEM is capable of accurately modeling of industrial gas-solid two-phase systems. Besides, this numerical approach is shown to provide valuable information such as pressure profile during powder injection and interaction between bubbles and structures in a fluidized bed.

Discrete particle simulation of food grain drying in a fluidised bed

Drying is a common practice for post-harvest processing of food grains. Fluidised beds are often adopted for this purpose. It is of importance to understand the fluidised bed drying process for improving its energy efficiency. This work establishes a numerical drying model based on the combined approach of computational fluid dynamics and discrete element method for describing heat and mass transfer in the gas-solid flow system. Water evaporation is modelled in resemblance to a chemical reaction, thereby requiring fewer model parameters. The model is first described in detail. Then it is tested by comparing model predictions with those experimental data of corn kernel from the literature. General drying characteristics including grain and air moisture contents are reproduced qualitatively. The predicted drying rate curves are quantitatively comparable with those of experimental data. Finally, the effects of inlet air velocity and temperature are examined. The model predictions confirm that the drying rate increases with both the inlet air velocity and temperature. However, the drying product quality, here represented by the standard deviation of grain moisture distribution, increases with increasing air velocity or decreasing air temperature. This grain scale model would be useful to the design and control of the drying process.

Hydrodynamic modeling strategy for dense to dilute gas–solid fluidized beds

When investigating the hydrodynamic behavior of gas–solid flow systems, there are several options for the drag function, viscosity model, and other parameters. The low accuracy obtained with a random trial and error modeling strategy has led researchers to develop new drag models that are fine-tuned for their specific studies. However, besides the drag functions, an appropriate viscosity model together with radial distribution function have a great impact on the hydrodynamic modeling of fluidized beds. In this study, a detailed validation and verification task is conducted using three different experimental datasets to derive a modeling strategy for predicting hydrodynamic behavior in dense to dilute flow regimes of various fluidized beds. For this purpose, the steady-state Reynolds-averaged Navier–Stokes equations are solved in a finite volume scheme using the twoPhaseEulerFoam solver in the OpenFOAM 2.1.1 software. A comparative study of different drag and viscosity models enables an optimal modeling strategy to be determined for the accurate prediction of the bed pressure drop, bed expansion ratio, time-averaged solid hold-up, and bed height in various dense and dilute flow regimes. Our results show that the modeling strategy prescribed in this study is widely applicable for identifying the hydrodynamic characteristics of various gas–solid fluidized beds with different operating conditions.

An instrumentation system for multi-parameter measurements of gas-solid two-phase flow based on Capacitance-Electrostatic sensor

In this paper, an instrumentation system for the measurements of local solid volumetric concentration, local solid velocity, local solid mass flowrate and solid mass flowrate in gas-solid two-phase flow system is developed. It is based on a new type of a Capacitance-Electrostatic sensor (CES). The CES sensor is mainly composed of a capacitance electrode array and two electrostatic electrode arrays. The optimum design of the sensor is achieved by finite element method. The capacitance electrode array is employed to detect the solid distribution over the cross-section of the pipe, and the local solid volumetric concentration measurement is further derived. The electrostatic electrode arrays are used to measure the local solid velocities in conjunction with cross-correlation method. From the local solid velocity and local volumetric concentration, the solid mass flowrate and the local solid mass flowrate can be achieved. The developed system for the local solid volumetric concentration measurement is verified through analogue simulation experiments and static experiments. Finally, the system is employed to measure the local solid volumetric concentration, local solid velocity, local solid mass flowrate and solid mass flowrate on a belt conveyor. The experimental results show that the measurement error of the local solid volumetric concentration measurement results are less than 10.43% for solid local volumetric concentration ranging from 0.02 to 0.56, the standard deviations of the local solid velocity measurement results are less than 0.42 for solid velocity ranging from 3.5 m/s to 15.0 m/s, and the relative error of the solid mass flowrate is within −19.6% to +14.9% for solid mass flowrate ranging from 0.006 kg/s to 0.103 kg/s, indicating that the system is capable of achieving multi-parameters measurement in gas-solid two-phase flow system.

Investigation on wet particle flow behavior in a riser using LES-DEM coupling approach

Hydrodynamic characteristics of dry particles and wet particles are obviously different in a dense gas-solid two-phase flow system. In this work, a modified LES-DEM (Large Eddy Simulation and Discrete Element Method) coupling approach has been used to investigate the flow behaviors of wet particles in a riser. The model of the simulated riser is similar to the one in Mathiesen et al.'s experiment with binary mixture of particles, and simulated results show good agreement with the experimental data. Different liquid contents of the particles are applied to find out the influence on particle behaviors. By contrast, bigger clusters are formed at the riser inlet and near the walls with increasing the liquid contents for the strong influence of liquid bridge on the cohesion between particles. Meanwhile, the translational axial velocities reduce with increasing the liquid content of the wet particles. Besides, the rotational axial velocity fluctuates in a wider range and distribute more randomly for the wet particles than that in the dry particle system. Moreover, the fluctuation of pressure drop increases with increasing the liquid content. Compared with the dry system, translational and rotational cluster granular temperatures for the wet particles distribute in a much wider range and the peak values increase. However, for both big particles and small particles the diameter distribution and particle volume fraction have a similar trend in both dry particle and wet particle systems.

CFD simulation of a fluidized bed using the EMMS approach for the gas-solid drag force

Drag force is one of the main contributing factors in describing the hydrodynamics of gas-solid flow systems. In fact, the drag force is the primary coupling force between the fluid and particulate phases. Therefore, accurate prediction of the drag is the goal of the present study. Flow of particulate solids in gas-solid flow systems is inherently heterogeneous due to different attraction and repulsion forces between the particles. The assumption of a homogeneous system leads to unrealistic predictions of system characteristics for different gas-solid regimes, e.g., bubbling fluidized beds and circulating fluidized beds. In the literature, several approaches have been developed to encounter the effect of heterogeneities inside the system, e.g., EMMS (Energy Minimization Multi-Scale) and subgrid filtering approaches. In this study, an EMMS approach was applied to a fluidized bed operating at the ranges between the bubbling regime and packed bed. The result of 2D simulations in a fluidized bed was compared with both experimental data and homogeneous models. Simulation results using the EMMS approach showed very good agreement with the Krishna [20] bed expansion experimental data, while the modeling using homogeneous drag force diverged considerably from the experimental data.

Influence of particle electrification on AC-based capacitance measurement and its elimination

Based on the variations of equivalent electrical permittivity with different phase concentrations in two-phase or multiphase flow, the capacitance sensor has been successfully used for the phase concentration measurement due to its advantages of non-invasion, low-cost, high reliability and non-radiation. However, solid particles are charged due to the collisions between particles and pipe wall in gas–solid flow system. The charge carried by solid particles will induce a certain amount of charge on the electrode of capacitance sensor, which causes an additional output in the capacitance measurement circuit. Hence, the particle electrification causes errors in capacitance measurements, and even leads to the failure of the measurement system for solid concentration and tomography system based on capacitance sensor. In this paper, the influence of particle electrification on the AC-based capacitance measurement circuit is theoretically and experimentally investigated based on a capacitance sensor with helical surface-plate electrodes. The results indicated that the capacitance signal and the electrostatic induction signal have different frequency band ranges. The influence of charge on capacitance measurements can be eliminated by rationally designing the capacitance measurement circuit, and further accurate and reliable capacitance measurement can be achieved.

Developing a High Speed Fiber-optic Endoscopic Technique for Measuring Particle Phase Characteristics in a Spouted Bed

A novel endoscopic photography technique coupling a high speed CCD camera and an intrusive fiber-optic endoscope has been extended to enable the investigation of particle flow behavior in a 3D spouted bed 1500mm in height and 200mm in diameter. A series of algorithms have been specifically optimized to realize the process of image processing and the calculation of the instantaneous particle velocity data. Then particle phase velocity and granular temperature were obtained respectively by statistical analysis and detailed study of the obtained data. The results showed that instantaneous particle velocities exhibited random complexity in both magnitude and direction. Although little information could be extracted from the raw data directly, the profile of particle phase velocity was able to show the mainstream particle flow patters along bed radius. Also, the computed granular temperature could be used to characterize the local particle fluctuation intensity. All experimental results could be useful to validate the CFD models for gas-solid flow systems.

Cartesian grid simulations of gas–solids flow systems with complex geometry

Complex geometries encountered in many applications of gas–solids flow need special treatment in most legacy multiphase flow solvers with Cartesian numerical grid. This paper briefly outlines the implementation of a cut cell technique in the open-source multiphase flow solver—MFIX for accurate representation of complex geometries. Specifically, applications of the Cartesian cut cell method to different gas–solids fluidization systems including a small scale bubbling fluidized bed with submerged tube bundle and a complete pilot-scale circulating fluidized bed will be presented. In addition to qualitative predictions on the general flow behaviors inside each system, quantitative comparison with the available experimental data will be presented. Furthermore, some results on extending the current cut-cell technique to Lagrangian–Eulerian simulations will be presented.

A spatial filtering velocimeter for solid particle velocity measurement based on linear electrostatic sensor array

A novel spatial filtering velocimeter for solid particle velocity measurement in gas-solid flow system is presented based on a Linear Electrostatic Sensor Array (LESA) in the paper. The fundamental spatial filtering principle of the LESA is theoretically described, and a differential filter based on two LESAs is further proposed to remove the pedestal component of the output signal from the LESA. Thus solid particle velocity can be measured by determining the central frequency of the periodic output signal of the differential filter. A spatial filtering velocimeter consisting of a measurement head, seven-channel differential amplifiers and a computer-based data acquisition and processing system is designed and its performance is evaluated on a purpose-built gravity-fed particle flow rig and a belt rig, respectively. Experimental results show that the system repeatability is within ±5.4% over the velocity range of 1.72–3.91 m/s for a particle concentration range 0.011–0.163 m3/m3.

Hydrodynamic similarity in circulating fluidized bed risers

Hydrodynamic similarity in the fully developed zone of co-current upward gas–solid two-phase flow systems under different operating conditions was investigated by measuring the axial profiles of pressure gradient, radial profiles of solid concentration and particle velocity in two circulating fluidized bed (CFB) risers of 15.1 and 10.5 m high, with FCC and sand particles, respectively. The experimental data obtained from this work and in the literature show that when the scaling parameter, G s / ( ρ p U g ) , is modified as Fr D - 0.3 G s / ( ρ p U g ) , a detailed hydrodynamic similitude of the gas–solid flow in the fully developed zone of the risers under different operating conditions can be achieved. Furthermore, the experimental results from different gas–solid flow systems also show that as long as Fr D - 0.3 G s / ( ρ p U g ) remains constant, there is the same solid concentration in the fully developed zone of different CFB risers with different particles. With the same Fr D - 0.3 G s / ( ρ p U g ) , the local solid concentrations, the descending particle velocities, the cluster frequencies and the solid concentrations inside clusters in the fully developed zone of the risers all display the same axial and radial distribution, respectively. In other words, the empirical similarity parameter, Fr D - 0.3 G s / ( ρ p U g ) , appears to have incorporated the effects of operating parameters ( G s and U g ) , so that, the gas–solid flow in the fully developed zone of CFB risers under those different operating conditions but having the same Fr D - 0.3 G s / ( ρ p U g ) shows similar micro- and macro-hydrodynamic characteristics. The study shows that the empirical similarity parameter, Fr D - 0.3 G s / ( ρ p U g ) , is also independent of the upward gas–solid flow systems.

Integration of ECT measurements with hydrodynamic modelling of conventional gas–solid bubbling bed

The electrical capacitance tomography (ECT) provides fast images of the cross-sectional concentration distribution of solid–gas flow in a confined volume. This information can be integrated with numerical simulation to estimate some of the most important hydrodynamic quantities in solid–gas flow, such as the particles velocity, interstitial gas velocity and particle–particle contact forces. In this study, using the two-fluid approach, momentum and energy balance equations, along with the appropriate boundary conditions, have been solved by integrating the numerical procedure with the experimental data of the fluidised bed pressure drop and pixel distribution of particle concentration available from the ECT measurements. Preliminary results of time-dependent hydrodynamic features of the bed are presented. These results were analysed and assessed using the available experimental literature data on conventional bubbling fluidised bed. In general, it is demonstrated that the integration of ECT measurements with numerical modelling offers a unique and promising technique for comprehensive non-intrusive information on gas–solid flow systems.

APPLICATION OF THE DISCRETE ELEMENT MODELLING IN AIR DRYING OF PARTICULATE SOLIDS

ABSTRACT The Discrete Element Method (DEM) has been widely used as a mathematical tool for the study of flow characteristics involving particulate solids. One distinct advantage of this fast developing technique is the ability to compute trajectories of discrete particles. This provides the opportunity to evaluate the interactions between particle, fluid and boundary at the microscopic level using local gas parameters and properties, which is difficult to achieve using a continuum model. To date, most of these applications focus on the flow behaviour. This paper provides an overview of the application of DEM in gas–solids flow systems and discusses further development of this technique in the application of drying particulate solids. A number of sub-models, including momentum, energy and mass transfer, have been evaluated to describe the various transport phenomena. A numerical model has been developed to calculate the heat transfer in a gas–solids pneumatic transport line. This implementation has shown advantages of this method over conventional continuum approaches. Future application of this technique in drying technology is possible but experimental validation is crucial.

A transient method for determination of saturation carrying capacity

Clear definition of smooth and stable operating regimes is necessary if circulating fluid bed reactors are going to be adopted commercially for process applications that require load following capabilities. One significant source of instability or long-term transient occurs when traversing the transition from transporting in a lean phase fluidization regime to transporting in a turbulent or fast-fluidized bed regime. The gas saturation carrying capacity (SCC) is the key functional parameter required to define this transition. A simple method is presented to accurately determine the SCC in which the solids flux was abruptly halted and the loss in pressure signal monitored over time. The SCC was measured for PVC under a variety of conditions. Upon drawing from the analogous liquid–vapor to vapor phase transition, a predictive relationship is also presented to estimate SCC for various gas–solid flow systems, knowing the gas velocity and solids' terminal velocity in that gas.

Direct measurements of particle velocities in gas–solids suspension flow using a novel five-fiber optical probe

A new fiber-optic probe system was developed for the direct measurement of the local velocity of particles in gas–solids flow suspension. It consists of a five-fiber optical probe and a microcomputer data processor. This instrument is capable of measuring particle velocities in the range of 0.3 to 24 m/s and mean particle sizes in the range of 50 μm to 5 mm for solids hold-up between 0.1% and 25%. It is demonstrated that the use of the peak-to-peak detection technique in conjunction with the novel five-fiber optical probe is a promising system for measuring the movement of solid particles in the circulating fluidized bed and other gas–solids flow systems.

A computational investigation of transient heat transfer in pneumatic transport of granular particles

The Distinct Element Method (DEM) has been employed to model a range of gas–solids flow systems such as solids flow in hoppers, gas–solids pipe flow and particulate fluidized bed. But little has been reported on the application of this technique to heat transfer in gas–solids flows. This paper describes a numerical model for gas–solids two-phase flow with heat transfer in pneumatic transport pipes. The gas phase is modelled as a continuum and the solids phase is modelled by DEM. Convective heat transfer between gas and particles and between gas and pipe wall, and conductive heat transfer between particles and between particles and pipe wall are modelled. The influence of particle concentration on the predicted performance of the system has been compared with experimental data. This comparison has shown a good agreement between the DEM simulation and experiment for both pressure drop and pipe wall temperature profile at developed flow region.

1998 Thomas Baron Award Lecture: Learning Multiphase flow

This article is the summary of the 1998 Thomas Baron Award Lecture prepared by one of the pioneers and creative researchers in the gas–solid flow systems area, the late Professor S.L. Soo. Unfortunately, Professor Soo passed away before he was able to present this lecture.

管内固気混相流装置における脈動の減衰特性と流量計測誤差

It is known that pressure fluctuation produces an error in flow volume. Many researchers tried to remove the pressure fluctuation by using a surge-tank, thus the effect of the surge-tank was investigated. However, their research was insufficient to explain the characteristics of the local pressure fluctuation damping process and the error in the flow volume of the gas solid suspension flow system in the pipe. First, the author used a surge tank and heat exchanger to damp on the pressure fluctuation. The pressure fluctuation at the local position of air flow was by a memory recorder, and the flow volume was measured by a quarter circle nozzle. Then, the characteristics of pressure fluctuation damning and the error in the flow volume were analysed.

Dilute fluidized cracking catalyst particles-gas flow behavior in the riser of a circulating fluidized bed

Laser Doppler anemometry (LDA) was used to measure velocity, fluctuating velocity and size distribution in the riser of a circulating fluidized bed. Fluidized cracking catalyst (FCC) particles with volume average size of 70 μm and density of 1650 kg/m 3, in a dilute gas-solid flow system with solid volume fraction ranging from 0.2% to 2.5% and solid mass flux of 10–120 kg/m 2 s were experimentally studied at the test section which was 1.07 meters away from the entrance zone. More particle collision and less slip velocity between particles of different size were observed at higher solids loadings and superficial gas velocities. At lower superficial gas velocities and lower solids loadings, instantaneous reversal flow of particles and lower particles fluctuating velocities and larger particles/agglomerates were measured along the wall. The radial distribution of axial particles velocity profile was flatter than a parabolic shape with maximum velocity of particles of different size in most cases away from the center. The particles fluctuating velocities was about 20%–30% of the particle velocity. However, it decreased at the wall region due to less probability of collision with moving particles and significant collision with the wall. In contrast, the turbulence intensity increased at the wall region due to lower particles velocities in this zone.

Hydrodynamic analysis of dilute gas—solids flow in a vertical pipe

Dilute gas-solids systems are widely used as a proper reactor in many chemical, gas, food processing and waste treatment industries. The optimal utilization of this technology requires a good understanding of gas and particle flow patterns and the capability to predict such behavior. To respond to this need, the Two-Dimensional Steady State Two-Phase Hydrodynamic Model is developed to describe gas—solids flow behavior in dilute gas—solids flow systems. The model considers particles of same size and their boundary layers as a separate continuum phase. Furthermore, viscous dissipation forces and phase interaction forces are incorporated in the model. Numerical solution was obtained using the Method of Lines which is incorporated in FORSIM computer code. The predicted system variables agreed with the observed behavior of the experiments reported in the literature. The predicted average of the pressure drop profile using this model agreed well with the available experimental data in the literature.