Filtration characteristics of a binary multi-layer granular bed filter based on CFD-DEM coupling simulation

The coupled CFD-DEM method with the JKR (Johnson-Kendall-Roberts) model for describing the contact adhesion of dust to filter particles (FPs) is used to simulate the distribution pattern of dust particle deposition in the granular bed filter (GBF) with multi-layer media. The minimum inlet flow velocity must meet the requirement that the contact probability between dust and FPs is in the high contact probability region. The air flow forms vortices on the leeward side of the FPs and changes abruptly at the intersection of different particle size FPs layers. Dust particles form large deposits at the intersection of the first and second layers and the different particle size filter layers. Dual element multilayer GBF can further optimize the bed structure by interlacing filter layers with different particle sizes. Compared with single particle size multi-layer GBF, the bed pressure drop is reduced by 40.24%–50.65% and the dust removal efficiency is increased by 21.93%–55.09%.     

Experimental study on filtration performance of the moving bed granular filter with axial flow

The filtration performance of the moving bed granular filter with axial flow (MBGF-AF) is investigated through a large cold experiment. The effect of different operation parameters on the filtration performance (collection efficiency, pressure drop) of the axial-flow moving bed filter is investigated in combination with the dust deposition effect and the mechanism of trapping dust by the capturing particles. The results show that the collection efficiency of MBGF-AF is enhanced by decreasing the superficial gas velocity, increasing the inlet dust concentration properly, or decreasing the moving velocity of the capturing particles. A model covering the above operation parameters is established to calculate the collection efficiency of the moving bed granular filter. It is used in a wide range of operating parameters for the MBGFs.     

Formation of particle jetting in a cylindrical shock tube

A dense, solid particle flow is numerically studied at a mesoscale level for a cylindrical shock tube problem. The shock tube consists of a central high pressure gas driver section and an annular solid powder bed with air in void regions as a driven section with its far end adjacent to ambient air. Simulations are conducted to explore the fundamental phenomena, causing clustering of particles and formation of coherent particle jet structures in such a dense solid flow. The influence of a range of parameters is investigated, including driver pressure, particle morphology, particle distribution and powder bed configuration. The results indicate that the physical mechanism responsible for this phenomenon is twofold: the driver gas jet flow induced by the shock wave as it passes through the initial gaps between the particles in the innermost layer of the powder bed, and the chaining of solid particles by inelastic collision. The particle jet forming time is determined as the time when the motion of the outermost particle layer of the powder bed is first detected. The maximum number of particle jets is bounded by the total number of particles in the innermost layer of the powder bed. The number of particle jets is mainly a function of the number of particles in the innermost layer and the mass ratio of the powder bed to the gas in the driver section, or the ratio of powder bed mass (in dimensionless form) to the pressure ratio between the driver and driven sections.     

Numerical simulations of high pressure carbon dioxide fluid fluidized beds

Hydrodynamics of carbon dioxide fluid-particle mixtures are predicted using a low density ratio-based kinetic theory of granular flow in high pressure carbon dioxide fluid fluidized beds. A coexistence of particle waves and particle aggregates exists along bed height. The threshold to identify the occurrence of particle aggregates is suggested based on standard deviation of solid volume fractions in aggregative fluidization. The existence time fractions and frequencies of particle aggregates are predicted along axial direction. The effect of carbon dioxide fluid temperature and pressure on volume fraction and velocity distributions are analyzed at different inlet carbon dioxide velocities and particle densities in high pressure carbon dioxide fluidized beds. Simulated results indicate that the carbon dioxide-particles fluidization transits from particulate to aggregative states with the increase of inlet carbon dioxide velocities. The computed fluid volume fractions and heterogeneity indexes are close to the measurements in a high pressure carbon dioxide fluidized bed.     

Discrete particle modeling of granular temperature distribution in a bubbling fluidized bed

The discrete hard sphere particle model (DPM) is applied in this work to study numerically the distributions of particle and bubble granular temperatures in a bubbling fluidized bed. The dimensions of the bed and other parameters are set to correspond to those of Müller et al. (2008). Various drag models and operational parameters are investigated to find their influence on particle and bubble granular temperatures. Various inlet superficial gas velocities are used in this work to obtain their effect on flow characteristics. It is found that the superficial gas velocity has the most important effect on granular temperatures including bubble granular temperature, particle translational granular temperature and particle rotational granular temperature. The drag force model affects more seriously the large scale variables such as the bubble granular temperature. Restitution coefficient influences all granular temperatures to some degree. Simulation results are compared with experimental results by Müller et al. (2008) showing reasonable agreement.     

Simulated configurational temperature of particles and a model of constitutive relations of rapid-intermediate-dense granular flow based on generalized granular temperature

In gas–solid flat-base spout bed with a jet, the flow of particles must go through an intermediate regime where both kinetic/collisional and frictional contributions play a role. In this paper, the statistical framework is proposed to define the generalized granular temperature which sums up the configurational temperature and translational granular temperature. The configurational temperature, translational and rotational granular temperatures of particles are simulated by means of CFD-DEM (discrete element method) in a 3D flat-base spout bed with a jet. The configurational temperatures of particles are calculated from instantaneous overlaps of particles. The translational and rotational granular temperatures of particles are calculated from instantaneous translational and angular velocities of particles. Roughly, the simulated translational and rotational granular temperatures increase, reach maximum, and then decrease with the increase of solids volume fractions. However, the configurational temperature increases with the increase of solids volume fractions. At high solid volume fraction, the predicted configurational temperatures are larger than the translational and rotational granular temperatures, indicating that the rate of energy dissipation do contributes by contact deformation of elastic particles. The generalized granular temperature is proposed to show the relation between the variance of the fluctuation velocity of deformation and the variance of the translational fluctuation velocity of particles. The constitutive relations of particle pressure, viscosity, granular conductivity of fluctuating energy and energy dissipation in rapid-intermediate-dense granular flows are correlated to the generalized granular temperature. The variations of particle pressure, shear viscosity, energy dissipation and granular conductivity are analyzed on the basis of generalized granular temperature in a flat-base spout bed with a jet. The axial velocities of particles predicted by a gas–solid two-fluid model of rapid-intermediate-dense granular flows agree with experimental results in a spout bed.     

Dynamics of convective thermal waves in a porous continuum

A one-dimensional motion of an incompressible fluid (gas) through a fixed bed of a granular material under a short thermal pulse at the bed inlet is considered. An analytical dimensionless solution of the boundary-value problem is presented in quadratures. Specific features of the propagation of convective thermal waves due to the fluid (gas) flow are studied. The data on the propagation velocity, amplitude, temperature difference between the fluid (gas) and the bed, and the length of the zone of intense heat transfer are obtained. The examples of the analysis of a continuum flow in engineering apparatuses which use convective thermal waves are presented.     

ENERGY AND MASS TRANSPORT PROCESSES IN THE GRANULAR BED OF AN INDIRECTLY HEATED ROTARY KILN

The transport mechanisms of momentum,mass,species,and energy are invertigated in detail for the ro-tary kiln process.The residence time prediction of the granular bed is well improved by considerng differet flow patterns in the drum.Introducing a mixed flow pattern of the basic slipping and slumping behaviour has the most important effect on the improvement of the residence time prediction.The granular bed is assumed to hehave as a Bingham fluid in the active layer of the bed.The transport mechanisms of momentum,species,and energy are modelled on the basis of this assumption and using the kinetic gas theory.Additionally,a mathematical transformation is presented to save comput a-tional time.The model results of the temperature field are in very good agreement with experimental data.     

Experimental analysis of volatile liquid injection into a fluidized bed

Experiments were conducted on a lab-scale fluidized bed to study the distribution of liquid ethanol injected into fluidized catalyst particles. Electrical capacitance measurements were used to study the liquid distribution inside the bed, and a new method was developed to determine the liquid content inside fluidized beds of fluid catalytic cracking particles. The results shed light on the complex liquid injection region and reveal the strong effect of superficial gas velocity on liquid distribution inside the fluidized bed, which is also affected by the imbibition of liquid inside particle pores. Particle internal porosity was found to play a major role when the changing mass of liquid in the bed was monitored. The results also showed that the duration of liquid injection affected liquid–solid contact inside the bed and that liquid–solid mixing was not homogeneous during the limited liquid injection time.     

Hydrodynamics of a bubble-induced inverse fluidized bed reactor with a nanobubble tray

This paper reports on the hydrodynamics of a bubble-induced inverse fluidized bed reactor, using a nanobubble tray gas distributor, where solid particles are fluidized only by an upward gas flow. Increasing the gas velocity, the fixed layer of particles initially packed at the top of the liquid starts to move downwards, due to the rise of bubbles in this system, and then gradually expands downwards until fully suspended. The axial local pressure drops and standard deviation were examined to delineate the flow regime comprehensively under different superficial gas velocities. Four flow regimes (fixed bed regime, initial fluidization regime, expanded regime, and post-homogeneous regime) were observed and three transitional gas velocities (the initial fluidization velocity, minimum fluidization velocity, and homogeneous fluidization velocity) were identified to demarcate the flow regime. Three correlations were developed for the three transitional velocities. As the fine bubbles generated from the nanobubble tray gas distributor are well distributed in the entire column, the bed expansion process of the particles is relatively steady.     

Combustion of a Powder Layer of Methane Hydrate: The Influence of Layer Height and Air Velocity Above the Layer

The paper studies the dissociation and combustion of a layer of methane hydrate powder at a forced air flow over the upper surface of the layer (the air velocity is directed parallel to the upper surface of the layer). The influence of the layer thickness and air velocity on the combustion of gas hydrate is investigated. The calculated curves for the effect of the heat transfer coefficient, external convection and vapor concentration on the combustion temperature are obtained. The layer thickness and the air velocity significantly affect the dissociation rate of methane hydrate.

     

中心点火火焰在药床中传播规律的试验研究

为了研究中心点火管火焰在药床中的传播规律,设计了可视化模拟试验平台,开展了不同点火药量、不同装药结构的中心点传火试验。采用高速图像采集系统记录了中心点火管火焰在药床中的传播过程,采用瞬态压力记录仪记录膛内压力的时空变化。结果表明,点火药量为20 g时,出火时间为0.6 ms;点火药量为30 g时,出火时间为1.5 ms;杆状装药床的传火时间平均为2.2 ms,粒状装药床的传火时间平均为3.4 ms,而杆粒混装药床的传火时间为3.1 ms。可见,点火药量对药床出火时间影响显著,较大的点火药量导致药床出火时间延长;不同装药床结构传火性能差异较大,单一杆状装药床传火性能优于单一粒状装药和杆粒混装药床,并且粒状装药床易形成气体壅塞,膛内会出现明显的压力波动现象;根据火焰传播时序位置点,利用一阶指数衰减函数拟合建立了火焰传播过程数学模型,拟合优度大于0.98。     

Gas flow in rotary kilns

This paper describes an experimental investigation on the flow characteristics within a rotating cylinder containing a rolling bed of sand. The axis of the cylinder was horizontal and there was no axial bulk flow of particles. The velocity field of the gas flowing through the cylinder was measured by hot-wire anemometry. The measurements indicate that the velocity field is asymmetric with respect to a diameter perpendicular to the granular bed. CFD calculations confirm this finding. The gas velocity profiles are crucial in determining heat transfer from gas to solid.     

Equal quality distribution of gas–liquid two-phase flow by partial separation method

This paper proposes a new method for equal quality distribution of gas–liquid two-phase flow by partial separate-phase distribution with a dual-header distributor. The upper and liquid (lower) headers are interconnected with five vertical downward arms. A gas–liquid two-phase mixture enters the distributor from the upper header where most of the liquid of the mixture is removed through the downward arms into the liquid header. Hence, firstly, the remaining gas-rich fluid can be uniformly distributed into the outlet branches, and then secondly, the liquid collected in the liquid header can be uniformly re-distributed into the individual outlet branches. Because both distribution processes are conducted in the condition of single or near single-phase flow, mal-distribution of the two-phase flow is essentially eliminated, and a satisfactory equal quality distribution of gas–liquid two-phase flow is reached. Experiments were conducted in an air–water two-phase flow test loop. The inner diameter of the inlet pipe was 60 mm, the superficial velocity ranges of gas and liquid were 3–32 m/s and 0.02–0.17 m/s respectively, and the quality ranged from 0.02 to 0.44. The flow pattern in the inlet pipe included stratified flow, wavy stratified, slug flow, and annular flow. The experimental results showed that this new method could significantly improve the distribution performance of the two-phase flow. The maximum quality deviation between each outlet branch and the inlet pipe is less than ±1% under the conditions of stratified, wavy stratified and slug flows in the upper header, and less than ±5% in annular flow.     

Discrete particle simulation of mixed sand transport

An Eulerian/Lagrangian numerical simulation is performed on mixed sand transport. Volume averaged Navier–Stokes equations are solved to calculate gas motion, and particle motion is calculated using Newton's equation, involving a hard sphere model to describe particle-to-particle and particle-to-wall collisions. The influence of wall characteristics, size distribution of sand particles and boundary layer depth on vertical distribution of sand mass flux and particle mean horizontal velocity is analyzed, suggesting that all these three factors affect sand transport at different levels. In all cases, for small size groups, sand mass flux first increases with height and then decreases while for large size groups, it decreases exponentially with height and for middle size groups the behavior is in-between. The mean horizontal velocity for all size groups well fits experimental data, that is, increasing logarithmically with height in the middle height region. Wall characteristics greatly affects particle to wall collision and makes the flat bed similar to a Gobi surface and the rough bed similar to a sandy surface. Particle size distribution largely affects the sand mass flux and the highest heights they can reach especially for larger particles.     

Thermal and hydraulic performance of a three-phase fluidized bed cooling tower

An experimental study was made of the thermal and hydraulic characteristics of a three-phase fluidized bed cooling tower. The experiments were carried out in a packed tower of 200 mm diameter and 2.5 m height. The packing used was spongy rubber balls 12.7 mm in diameter and with a density of 375 kg/m³. The tower characteristic was evaluated. The air-side pressure drop and the minimum fluidization velocity were measured as a function of water/air mass flux ratio (0.4–2), static bed height (300–500 mm), and hot water inlet temperature (301–334 K).

The experimental results indicate that the tower characteristics KaV/L increases with increases in the bed static height and hot water inlet temperature and with decreases in the water/air mass flux ratio. It is also shown that the air-side pressure drop increases very slowly with increases in air velocity. The minimum, fluidization velocity was found to be independent of the static bed height.

The data obtained were used to develop a correlation between the tower characteristics, hot water inlet temperature, static bed height, and the water/air mass flux ratio. The mass transfer coefficient of the three-phase fluidized bed cooling tower is much higher than that of packed-bed cooling towers with higher packing height.     

Euler-Euler CFD modeling of fluidized bed＂ Influence of specularity coefficient on hydrodynamic behavior

Euler-Euler two-fluid model is used to simulate the hydrodynamics of gas-solid flow in a bubbling flu- idized bed with Geldert B particles where the solid property is calculated by applying the kinetic theory of granular flow （KTGF）. Johnson and Jackson wall boundary condition is used for the particle phase, and different amount of slip between particle and wall is given by varying the specularity coefficient （φ） from 0 to 1. The simulated particle velocity, granular temperature and particle volume fraction are compared to investigate the effect of different wall boundary conditions on the hydrodynamic behavior, Some of the results are also compared with the available experimental data from the literature. It was found that the model predictions are sensitive to the specularity coefficient. The hydrodynamic behavior deviated sig- nificantly for φ = 0 and φ = 0.01 with maximum deviation found at φ = 0 i.e. free-slip condition. However, the overall bed height predicted by all the conditions is similar.     

Forming and breaking the ceiling of inlet gas velocity regarding to separation efficiency of cyclone

The maximum-efficiency inlet velocity (MEIV) is a ceiling of inlet gas velocity that defines separation efficiency during cyclone design and operation. Experiment and computational fluid dynamics (CFD) simulation exhibited that an apex cone at the dust outlet can break the ceiling and improve the separation efficiency. The phenomenon is closely related to the effect of excessive high inlet gas velocity on the back-mixing escape of fine particles, which is the final result of back mixing, entrainment by the rapid upward airflow, and secondary separation of the inner vortex. In the center of the inner vortex, the airflow rotates slowly and moves rapidly upward. This elevator type of airflow delivers re-entrained particles to the vortex finder. A higher inlet gas velocity accelerates the elevator, causing more entrained particles to escape. This explains the decrease in efficiency at an excessively high inlet gas velocity. When an apex cone is installed at the dust outlet, the back-mixing is significantly weakened because the vortex core is bounded to the center of separator, while the transport effect of rapid upward airflow is weakened by the decrease in axial velocity in the center. Therefore, particle escape is weakened even at excessive high inlet gas velocities. Instead, the centrifugal effect is enhanced because of increased tangential velocity of the gas and particles. Consequently, the ceiling of inlet gas velocity is broken.     

An analysis of the effect of the solid layer for the modified chemical vapor deposition process

The heat transfer problem relative to the modified chemical vapor deposition process has been analyzed and the effects of solid layer thickness, torch speed and tube rotation are studied. The quasi-steady three-dimensional energy equations have been solved for the temperature fields in the gas and the solid layer with a Gaussian heat flux boundary condition on the outer surface. Of particular interest is the effect of the solid layer thickness and the torch speed on inner surface temperature, gas temperature and thermophoretic velocity. The large change of the axial temperature distribution of the surface occurs for different solid layer thicknesses or torch speeds. The presence of the solid layer and tube rotation reduce the effects of nonuniform torch heating in the circumferential direction and the resulting surface temperatures are very uniform in this direction.     

Solid transport measurements through image processing

The paper describes a novel technique which enables evaluation of sediment fluxes on the upper layer of a granular bed by means of separate measurements of concentration and velocity of the moving particles. Specific elaboration techniques based on digital image processing were applied to films of the solid fluxes, allowing for automatic measurement. Sediment concentration was measured via a technique based on image subtraction and following proper filtering procedures, while grain velocity was measured by Particle Image Velocimetry. The method was applied in laboratory experiments of one dimensional bed load; the solid discharges measured by the proposed image processing technique were compared to those obtained by manual count of the grains passing over a fixed plate used as a sight. After calibration of the automatic technique, dimensionless solid discharges ranging from 1.0 × 10⁻³ to 1.2 × 10⁻¹ were measured with a maximum error as large as 25%. The technique proposed also enables measurement of the time variation of the quantities and the two dimensional direction of sediment motion, for a complete characterization of grain kinematics in solid transport processes.