Horizontal slug flow pneumatic conveying: Numerical simulation and analysis of a thin slice approximation

Slug flow horizontal pneumatic conveying of granular materials is a complex phenomenon, involving many interacting properties. Investigating internal properties such as bulk density and bulk material stress, particularly via experiment, is limited by practical considerations. A combined 3D DEM + 2D CFD algorithm is introduced as a complementary means of investigating such properties. Important aspects of the simulation work include the extraction of bulk density, bulk material stress, stress transmission ratio and boundary stress data as functions of time and position in one and two dimensions. Such data is presented and analysed in the context of existing assumptions and a resulting one-dimensional model for bulk material stress within a slug.     

Evaluation of models and correlations for pressure drop estimation in dense phase pneumatic conveying and an experimental analysis

This study reviews the models and correlations for dense phase conveying in an effort to explore existing and new data on the subject and to provide guidance to the designer on the best pressure drop model. Using various data sets the Mi (Konrad)-based model was found to be best for predicting the pressure drop across dense phase plugs. A series of industrial scale tests also shows agreement with the Mi (Konrad)-based model.     

Analyzing threshold velocities for fluidization and pneumatic conveying

In previous works we have shown that by defining various threshold velocities in terms of the modified Reynolds number as a function of the modified Archimedes number a generalized master-curve can be plotted. Then, using this curve, the ratio between various threshold velocities can be easily determined. However, all the threshold velocities were defined for mono-sized particles. The purpose of this paper is to show how the threshold velocities can be used to design of particle-fluid systems for particle size distribution. The analysis provides practical guides for various engineering scenarios, such as selecting the appropriate fluidization velocity for maximum fluidization and minimum entrainment and determining the conveying velocity in pneumatic systems. In addition, the analysis provides a guide to determine whether the deposited layer of particles at the pipe bottom is stationary or moving, for cases where the superficial velocity is smaller than the saltation velocity.     

Signal modelling and algorithms for parameter estimation in pneumatic conveying

Capacitive measurement principles offer a non-invasive approach to determine particle velocity as well as particle concentration in pneumatic conveying. In order to assess the quality of a reconstruction method, it is mandatory to know the prevailing velocity and concentration profiles. For particulate bulk solids transportation, accurate reference systems are either not available or very expensive. Based on measurement data, a signal model of the particle flow is developed for dilute phase and dense phase conveying. These models are used to demonstrate the applicability of proposed algorithms for parameter determination — a cross-correlation technique for dilute phase and a two-step approach using the Fourier transform for dense phase.     

Experimental research of flow patterns and pressure signals in horizontal dense phase pneumatic conveying of pulverized coal

Understanding flow patterns and their variability is important for optimal design and trouble free dense phase pneumatic conveying of pulverized coal in a horizontal tube. Employing the electrical capacitance tomography (ECT), six flow patterns were identified and utilized for quantitative analysis based on the value and distribution of cross-sectional solid concentration. The dense-phase flow patterns in the horizontal tube of the pneumatic conveying system were somehow variable even when the operating conditions were unchanged. The probability calculation results suggest changing multiple flow patterns with one or two dominant flow for each of the seven sets of experimental conveying conditions and that a finite change in the dominant flow pattern would occur with an increasing superficial gas velocity. The power spectral density (PSD) function and the Hurst exponent of the pressure signals of the pulverized coal were well correlated with its flow patterns in a horizontal tube. The PSD functions and probability density functions (PDFs) of the void fraction signals from ECT are found to be related with flow patterns and can be used to quantitatively identify flow regimes. The ECT data may therefore be utilized for monitoring the flow patterns in a horizontal tube employed for pneumatic conveying of pulverized coal.     

Flow patterns in high-velocity fluidized beds and pneumatic conveying

Four flow patterns are identified for gas-solids vertical upward flows. Homogeneous dilute phase flow is characterized by the absence of both radial and axial solids segregation. Heterogeneous dilute phase flow (also called core-annulus flow) is characterized by the absence of axial solids segregation, with solids carried upward in the core and travelling downward near the outer wall due to the formation of particle streamers. Collapsed flow with a lower dense region and an upper dilute region is also referred to as the fast fluidization regime. In this case, the flow structure in the upper dilute region is similar to that in heterogeneous dilute phase flow, while the lower dense region resembles that in a turbulent fluidized bed. Dense phase flow can be reached when the riser is completely occupied by a relatively dense suspension with little axial density variation and no net solids downflow near the riser wall. The transition from fast fluidization to dense phase flow is still not clearly defined.     

Modeling of wet jet in fluidized bed

A wet jet zone is established in many applications wherever feeding and dispersing a liquid, solution or slurry into fluidized bed by gases is needed. In the present study, a simple mathematical model has been developed to simulate the wet jet in fluidized bed. The different stages involved inside the jet zone have been estimated and analyzed.The evaporation stage of traveling droplets through the jet flare has been treated. The rates of evaporation of each size at all positions along the jet flare have been estimated according to the velocities and surrounding conditions. The final droplet sizes have been determined. Moreover, the total evaporation rate from traveling droplets, before collision either with entrained sand particles or flare boundaries, has been estimated. The traveling droplets, partially evaporated, may collide and settle on entrained sand particles. The model predicts the settlement rates of liquid droplets on entrained sand particles. The total part evaporated from settled liquid has been estimated as well.The study has been applied to the pneumatic feeding of liquid fuel into fluidized bed combustors operating at . The model has been utilized to predict the ratio of fuel vapor that releases inside the jet flare. The remaining part is assumed to evaporate inside the emulsion phase. Three different liquid fuels have been considered: a heavy oil, diesel fuel and gasoline. The main independent variables are those related to the injection conditions including the initial velocity of dispersing air, u₀, and air-to-liquid mass ratio, ALR.The model results demonstrate that only very small droplets completely evaporate inside the flare. The liquid settling over the entrained sand particles plays an essential role in the fuel evaporation inside the flare. The phenomenon is dominant at conditions that result in generation of droplets of larger sizes, i.e., heavier fuel, lower u₀, and greater ALR. The ratio of vapor fuel released in jet flare increases with lighter fuel, higher u₀ and lower ALR. At and ALR=1.0 nearly all-liquid fuel evaporates inside the flare.     

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas-solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.     

Dynamic characteristics of binary mixtures in a two-jet fluidized bed

Aiming to understand biomass-related pyrolysis and gasification processes, which have received particular interests recently due to increasing concerns over fossil fuels and carbon emissions, this work conducts a detailed experimental study of the fluidization characteristics and jet dynamics of biomass mixtures in a fluidized bed. Both single jet and two jets dynamics are investigated, and a parametric study of the influence of particle properties and operational conditions on the dynamic performance of the bed, especially on the jet penetration depth and jet frequency of the binary mixtures, is conducted. Detailed frame-by-frame image analysis coupled with physical parameters reveals many interesting phenomena, which includes multistage flow pattern development and a stochastic nature of jet dynamics, as well as a strong dependence of the jet penetration depth and jet frequency on the bed materials.     

Slug characteristics of polymer particles in a gas-solid fluidized bed

The slug characteristics (frequency, rising velocity and length) have been determined by analyzing pressure fluctuations in a fluidized bed (0.38 m-I.D.x4.4m-high) of linear-low-density-polyethylene (LLDPE) and polypropylene (PP) particles. The slug characteristics of LLDPE and PP particles have been determined as a function of gas velocity (0.6-1.2 m/s) and the axial height (0.65–1.15 m) from the distributor. The rising velocity and vertical length of slug increase with increasing superficial gas velocity and the axial height of the bed. The slug shape of LLDPE particles is found to be the square-nose whereas that of PP particles is the round-nose. The slug frequency and its length have been correlated in terms of the excess fluidizing velocity, column diameter and bed height based on the data from the present and previous studies.     

Friction measurement in dense phase plug flow analysis

Pneumatic conveying, employing the dense phase plug flow regimen, is largely used to transport bulk solids. This process permits the conveying of large amounts of material in economical manner with less particle and pipe degradation compared to dilute phase conveying. By using an experimental system with special measurement devices and different materials of construction and transport, the friction between the material being transported and the pipe wall, the actual motion of the particles was determined, and the degree of fluidization were estimated. This information permits more accurate modeling of dense phase plug flow providing basic parameters to insert in existing and developing models.     

Comparison of flow characteristics for dilute phase pneumatic conveying for two different plastic pellets

A series of dilute phase pneumatic conveying experiments using two different types of plastic pellets has led to the determination and development of distinguishing flow characteristics. Separate experiments on polystyrene and polyolefin pellets captured pressure-drop fluctuations and values at two different measuring points—one at the lower horizontal section of the transporting pipe and another at the upper section and at two different solid-loading ratios for each material.Also, comparison and analysis of the pressure-drop fluctuations and values obtained from the experiments were carried out under the same solid-loading ratio and blower rotational speed for both materials. Basic pressure drop calculations were made to find pressure drop due to pure gas, and that due to the presence of solids using a solid friction factor. In addition, the power spectral density analysis, and the wavelet analysis were conducted for both materials to evaluate the flow characteristics.     

气力输送工艺在煤粉输送中的应用

针对工业煤粉输送技术效率低、不安全等问题,介绍了稀相正压输送、密相正压输送和负压输送3种气力输送工艺的原理和使用现状,研究了密相输送工艺的原理和目前主流的栓流式密相输送技术。分析了稀相正压输送、密相正压输送和负压输送工艺各自的技术特点,并在投资、能耗、环保等方面进行对比。结果表明负压输送输送距离较短,稀相正压输送能量损失大、易发热,而密相正压输送固气比高、能耗低、管道和物料磨损小、噪声低、输送过程不易发热,更适合煤粉输送。     

Effects of binary particle size distribution on minimum pick-up velocity in pneumatic conveying

Minimum pick-up velocities (U_pu) for entrainments of particle mixtures having binary particle size distributions (PSD) are measured in a horizontal pneumatic-conveying line using the weight-loss method. Geldart's groups A, B, and C glass beads having diameters of 400, 170, 40, and 5 μm are used. Variations in U_pu as a function of particle mass fraction (m) are examined. The capability of empirical correlations of monodisperse U_pu in predicting U_pu of binary mixtures is investigated. For group B particle mixtures (i.e. 400 & 170 μm), the particles are entrained separately resulting in linear U_pu variations with m, which is accurately predicted by the monodisperse U_pu correlation. For mixtures involving group A and B particles (i.e. 170 & 40, 400 & 40 μm), the two particles are collectively entrained resulting in U_pu that vary non-linearly with m and that cannot be predicted by the correlation. For mixtures involving group B and C particles (i.e. 400 & 5, 170 & 5 μm), U_pu are comparable to that of the monodisperse group B particles, therefore they are accurately predicted by the correlation. The significant impacts of binary PSD on U_pu found presently indicates that PSD effects on particle entrainment process warrants further investigations.     

Electrical capacitance tomography measurements on vertical and inclined pneumatic conveying of granular solids

Pneumatic conveying of granular solids in vertical and inclined risers was studied using electrical capacitance tomography (ECT). The focus of the study was on flow development past a smooth bend connecting the riser to a horizontal duct which brought the gas-particle mixture to the riser. In the vertical riser, dispersed flow manifested a core-annular structure, whose development is discussed. Three different time-dependent flow patterns were imaged. Slugging flow, which appeared to be intrinsic to riser flow, took the form of alternating bands of core-annular disperse flow and a slug with a particle-rich core. Averaging over these two structures yielded a composite distribution with high particle concentration both at the axis and the wall region. Pulsing flow, whose ECT fingerprint was similar to that of slugging flow, was largely an entrance effect. Stationary and moving annular capsules with a dilute core were also observed, and such flow patterns do not appear to have been reported previously. Our ECT measurements probing the development of disperse flow in an inclined riser past a bend revealed that the particle loading initially decreased, subsequently increased and then leveled off. Regimes such as eroding dune flow and flow over a settled layer could be easily imaged using ECT. The surface of the settled layer had a concave shape, suggesting that the particles were picked up from the settled layer by airflow at the center and deposited on the sides of the tube.     

Granular size and shape effect on electrostatics in pneumatic conveying systems

In solid processing systems, electrostatic problems are commonly observed for granules composed of various sizes and shapes. However, complete understanding about the functional dependence of electrostatic charge generation and transfer on the particle shape and size distributions has yet to be established. This observation has motivated the present study where novel methods are proposed to examine the effect of particle size and shape distributions on electrostatics. In this work, polyvinyl chloride (PVC) granule (original diameter 3.35-4.1 mm, in the shape of cylinders) was first discharged to remove any residual charges and subsequently its electrostatic charging was studied. Granular size and shape were varied by mechanical attrition conducted in a rotary valve jointly with a pneumatic conveying system. Characterised by induced current, particle charge density and equivalent current of the charged granular flow, granular electrostatics was found to increase with the extent of granular attrition in a continuous recycled pneumatic conveying process. In a separate setup, single particles (collected from the attrited granules formed in the rotary valve) were examined by correlating the extent of charge variation with size/shape. It was found that a dimensionless group, defined by the ratio of charge variation to size variation, is useful in describing the particle attrition process as this parameter increased with decreasing granule size. Smaller granules were found to be the main contributors in the enhancement of electrostatics charge density in bulk particles. By a separate shape analysis, it was uncovered that face shape requiring more shearing actions for its formation tended to give rise to a higher charge variation and so did column geometry. In this fashion, charge variation evaluated for whole attrited granules exhibited good agreement with the temporal variation of attrition weight; this applied for all air flow rates used in the conveying system. Furthermore, there is a reasonably good matching between results obtained by shape and size analyses.By the correlations presented above for single particle electrostatics either by size or shape analysis, charge variation of granular flow matched very well with that measured in the conveying system as well as the attrition process in the rotary valve. As such, the joint granule size and shape analysis has proven to be useful for characterisation of electrostatics in conveying systems where granules are made up of complex combinations of different particle sizes and shapes.     

Spherical particle movement in dilute pneumatic conveying

A numerical simulation of a particle in a horizontal pipe has been carried out, and the variation of aerodynamic forces is described. The major forces that control particle motion are drag in the axial direction, and lift due to air velocity gradient and due to spin in the transverse direction. An elastic contact model based on rigid body sliding has been incorporated, which avoids particle settlement without having to use any form of irregular bounce. The results from the simulation agree closely with experimental time-of-flight measurements.