Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

带内置水平管的大颗粒振动流化床流体力学研究

以大颗粒MAP（磷酸-铵）为试验物料对带内置水平管的大颗粒振动流化床流体力学进行了试验研究,结果表明,振动的引入对床层临界压降有明显的降低作用;同样床层条件下振动频率越大床层临界压降越低,振幅的改变对床层压降影响不大;低气速下振动使床层孔隙率降低而导致压降高于普通流化床;在带内置水平管的大颗粒振动流化床中,振动对大粒径颗粒的影响小于小粒径颗粒;由试验拟合出经验公式,与试验数据吻合良好。     

振动流化床中浅床层时床层与分布板之间的弹性作用

采用A类颗粒作为实验粉体,通过测量床层对分布板的作用力,分析了振动流化床中分布板与床层的作用机理,提出了振动流化床中在浅床层共振时分布板与床层的弹性作用模型. 实验表明,分布板与床层之间是连续的弹性作用,周期性的机械振动通过分布板与床层之间的弹性作用以波动形式在床层中传播. 该模型计算结果与实验测定结果符合的较好.     

振动流化床中流动结构的混沌分析

在内径90mm、静床高800mm的高床层流化床中,用动态压力传感器检测了不同气速条件下普通流化床和振动流化床中沿轴向的压力脉动信号,通过小波变换对信号除噪后,用混沌理论对信号进行了分析.通过关联维数和Kolmogorov熵定量表征振动流化床中的流动结构特征.结果表明:压力脉动信号的关联维数和Kolmogorov熵能够描述振动流化床中的流化状态;振动流化床中床层的流动结构存在两个区,在近分布板区域为射流区,床层主体部分为均匀流化区.     

Circulation of particles in a vibrated bed with an inner tube

The particle motion in a vibrated bed with an inner tube was simulated by the discrete element method (DEM). A height difference is observed in the vibrated particle bed between the interior and annulus of the inner tube. The bed height difference is strongly affected by the ratio of the cross section at the interior to that at the annulus of the inner tube. When the inner tube is immersed in the particle bed, the bed height difference causes the circulation of particles in the bed. The direction and velocity of particle circulation can be controlled by changing the inner tube diameter and the circulation velocity is also controlled with vibration conditions.     

振动流化床中大颗粒与水平管局部传热特性

在二维振动流化床中,以平均粒径1.83 mm的玻璃珠为物料,研究了大颗粒与水平管间局部传热规律;考察了气速、振动频率等因素对局部传热系数的影响,同时与小米和小玻璃珠实验结果进行对比。结果表明:大颗粒与小颗粒局部传热系数有很大差异;对于大颗粒,低速下局部传热系数随振动频率的增大先增加后减小,高速下局部传热系数随着振动频率的增加而降低;一定振动频率下,气速小时局部传热系数在60°左右达到最大,气速逐渐增加后,其最大值向90°转移。通过实验数据得到了计算大颗粒与水平管局部传热系数的关联式,计算值与实验值吻合较好,误差在±20%范围内。结果可为带浸没水平管的振动流化床设计和研究提供参考。     

RESIDENCE TIME DISTRIBUTION AND DRYING CHARACTERISTICS OF A CONTINOUS VIBRO-FLUIDIZED BED

An experimental study of the residence tine distribution and drying characteristics of a continous pilot-plant vibrated fluidized bed dryer(the air distributor plate is 1.50 m in length and 0.24 m in width) is presented and discussed. Operating variables in the study included: vibration intensity, mass flow rate of air, granules feed rate, inlet air temperature and particle size. It was inferred from the experimental results that the flow of particles in the continous VFB could be considered as plug flow. Among the variables considered, vibration intensity was identified as the most significant factor to affect particle mean residence time, particle diffusivity and constant drying rate for the bed of wheat particulates and instant pharmaceutical BYN granules.     

Effects of acoustic vibration on nano and sub-micron powders fluidization

Fluidization of nano and sub-micron powders with and without acoustic vibration was investigated. The effects of sound pressure level and frequency were studied. Loudspeakers located under the distributor plate were used as the sound source to disintegrate larger agglomerates concentrated at the bottom of the bed. Nanoparticles showed fluid-like behavior similar to Geldart's A group and application of sound vibration improved their fluidization quality. Submicron particles were hard to fluidize and their fluidization quality was partially improved by sound excitation. Bed compaction, caused by rearranging of the agglomerates, was observed for submicron particles at low gas velocities while the bed was fixed. Nanoparticles did not experience any bed compaction. Sound vibration led to a decrease in minimum fluidization velocity and an increase in bed pressure drop and bed expansion for both types of particles. The fluidization quality of both particles increased at low frequencies, while the reverse was observed at higher frequencies. Fluidization of these particles was improved by increasing sound pressure level. There was a critical sound pressure level of 110 dB, below which the effect of sound vibration was insignificant. A novel technique was employed to find the apparent minimum fluidization velocity from pressure drop signals.     

低温振动流化床气固传热的实验研究

在低温粉碎工程中,振动流化床常用于固体颗粒的预冷。为了深入认识振动流化床低温下气固传热的特性,本文介绍了不同的振幅,气流速度和料层厚度下低温实验的结果,通过分析实验结果首次得到了振动流化床中低温气固的传热关联式与普通流化床相比,由于引入了振动,振动流化床气固传热换热系数大幅度提高。     

Density-dependent separation of dry fine coal in a vibrated fluidized bed

The main purpose of coal separation is to reduce ash, sulfur, mercury and other mineral contaminants in the coal to increase the calorific value and benefit the environment. Dry coal beneficiation has obvious advantages over the wet process although the latter is currently the predominant method in use throughout the world. A vibrated fluidized bed was constructed for separating dry fine coal particles from unwanted gangue particles. An experimental investigation of vibrational energy transmission, and the interaction between vibration and gas flow, was performed. The motivation for these experiments was a theoretical development of the principles involved in forming a dense-media vibrated fluidized bed (DMVFB). The mechanism of bubble breaking by vibration is discussed. A formula for calculating the critical vibration frequency at which bubbles can be efficiently broken and bubble formation restrained is proposed. The experimental results demonstrate that the density of a dense-media vibrated fluidized bed is uniform, with a maximum relative error of 1.68% under optimal technological and operating conditions. The < 6 mm fine coal was efficiently separated with a probable error E value of 0.07 t/m³. A lower limit of separation of 0.5 mm was achieved. The DMVFB separation efficiency is higher than wet jig with E value of 0.11 t/m³.     

Numerical simulation of particle motion in a gradient magnetically assisted fluidized bed

Flow behavior of magnetizable particles is simulated in a two-dimensional gradient magnetically assisted bubbling fluidized bed. The motion of particles is simulated by discrete element method (DEM) with the consideration of external magnetic forces at a constant gradient magnetic field along bed height. The distributions of velocity and concentration of magnetizable particles are analyzed at the different magnetic field intensities. The simulations show a significant effect on the motion of particles with vertical magnetic-fields applied. When the magnetic field strength is increased to a value at which the fluidization of strings starts, the particles are found to form straight-chain aggregates in the direction of the magnetic field. At very high magnetic field strengths, defluidization is observed. Gas pressure drop of bed decreases with the increase of magnetic-flux densities. The granular temperature of particles increases, reaches a maximum, and then decreases with the increase of magnetic-flux density. Through the analysis of the motion of particles, it is concluded that the moderate strength magnetic field gives a high fluctuation of particles and distribute gas more evenly in the bed.     

洗衣粉悬浮液在惰性粒子流化床中干燥的研究

针对气体分布板开直孔的惰性粒子流化床,开展了洗衣粉悬浮液在床内的干燥性能研究。测定了流化床的床层压降曲线,考察了进料量、进风温度、进风速度及惰性粒子直径对于流化床传热性能的影响,且与气体分布板开斜孔的传热性能进行了初步比较。结果表明,流化床的床层压降主要是由惰性粒子的流化阻力所致;适当增加进料量和进风速度,或减小惰性粒子直径,以及将气体分布板的孔道由直孔改为斜孔,均可提高流化床的传热性能,但过高的进风温度则可能导致传热性能的下降。     

Spouting of fine powder from vertically vibrated bed

Experimental and numerical studies of spouting of powder from vertically vibrated bed are performed. The powder flows out vigorously through a side orifice of the vertically vibrated vessel in which the powder is contained, provided that the vibration acceleration is greater than the gravitational one. The mechanism of the efflux and the relationship between the efflux rate and the vibration condition are examined. The interstitial air pressure in the bed is measured and is compared to the numerical analysis considering the relative motion of the powder bed to the vibrated vessel and percolation air flow. The outflow behavior of powder is observed by using a high-speed video camera. The experimental and the numerical results show that the powder spouts out intermittently with periodic air outflow, which is generated in response to the change of the gap between the bed and the vessel base. Furthermore, it is found that the efflux rate of powder is in proportion to the generated air pressure and in inverse proportion to the vibration frequency.     

Vibrated bed contact electrification and charge dissipation

Some characteristics of contact electrification and charge dissipation in a vibrated bed have been studied. The experimental variables of interest were the bulk particle resistivity, the amplitude of vibration and the system electrode geometry. Theoretical models have been proposed relating the measured variables of conduction current, and induced potential in the base plate, to the charging and dissipation processes. Air breakdown has been found to limit insulating particle charge levels, following a linear increase in bed charge with time after vibration. This behaviour has been predicted theoretically. The functional dependence of the induced potential and the conduction current on the vibrational amplitude predicted theoretically for conducting particles, is in agreement with observation.Charge production and dissipation in the vibrated bed simulate the electrification characteristics of many practical bulk particle handling situations. This allows conclusions of general relevance to be deduced from the present study.     

Numerical simulation of particle motion in vibrated fluidized bed

The particle motion in vibrated fluidized bed was examined by using numerical simulation. A two-dimensional fluidized bed was used, and discrete element method (DEM) was used as for calculating the particle motion in vibrated fluidized bed. Geldart group B particle, which is regarded to have no effect of agglomerate, was used as the fluidizing particle. The vibration directions (vertical and horizontal) and vibration parameters (frequency and amplitude of vibration) were changed.In the case with vertical vibration, large bubbles caused by vibration gap (defined as the gap between particle bed and wall caused by vibration) appear in bed. When the vibration strength is high and the vibration frequency is low, it is difficult to fludize the particle bed in the case with horizontal vibration because the vibration gap acts like the channel. When the vibration frequency increases at the same vibration strength, the effect of vibration on the particle motion becomes small. The effect of vibration on the pressure loss in particle bed is large in the case with horizontal vibration.     

Numerical Simulation of Particle Segregation in Vibration Fluidized Beds

The influence of vibration parameters on the segregation phenomenon of a binary mixture in a vibration fluidized bed is investigated. Initially, the mixture composed of spherical balls with different densities but same diameter is in a perfect mixing state in the bed. The motion of particles is simulated based on the discrete element method. The effects of friction coefficient, vibration frequency, amplitudes, and gas velocity are analyzed. The coefficient of segregation to the degree of particle segregation is calculated for different operating conditions. The segregation degree in the vibration fluidized bed is found to be higher than that in the bed without vibration. The curve for the segregation degree exhibits a single peak value which represents the optimal segregation result.     

Influence of an electrochemically generated gas phase on pressure drop in a three-dimensional electrode and an inert bed

The influence of an electrochemically generated gas phase on the pressure drop in a three-dimensional electrode and an inert bed has been investigated. The electrode was composed of silvered glass particles, whereas the inert bed was composed of glass particles of the same diameter. During water electrolysis smaller gas bubbles are generated than by introducing gas into the system by means of a fluid distributor. Electrochemically generated bubbles with the liquid phase circulate upward through the electrode and the inert bed. The pressure drop for a monophase fluid was measured, as well as the change of pressure drop with time in the electrode and the inert bed for two-phase flow of fluid through an electrochemical cell. The steady and unsteady periods of the change of pressure drop have been discussed. Experiments were carried out at different electrolyte velocities and for different current densities. Higher electrolyte velocities cause an increase in the pressure drop in both beds. Also, increased current density causes an increase in gas evolution intensity at the electrode thereby increasing the pressure drop in both beds. A mathematical model describing the change of pressure drop with time has been proposed. The proposed model showed good agreement with experimental results as well as the results from the literature.     

Effect of distributors on Gas—Solid Fluidization

The efficient operation of a fluidized bed is very much dependent upon distributor performance, which in turn depends on its design parameters. The work reported here deals with the characteristics of such distributors as are commonly employed in laboratories, pilot plant and large scale operations. Specifically a porous plate distributor, two bubble cap distributors of different geometries and four Johnson screen distibutors of different percent open area have been investigated in a 30.5 cm by 30.5 cm square fluidized bed as a function of air fluidizing velocity and bed height. The pressure drop data for all the distributors have been correlated by a single equation with two unknown constants. The ratio of distributor pressure drop to bed pressure drop is found to increase rapidly with increase in fluidization velocity. The bed expansion ratio is found to increase with increase in excess fluidization velocity and distributor pressure drop but decreases with increase in bed height or weight.     

New investigation in regime transition from bubbling to turbulent fluidization

The regime transition velocity from bubbling to turbulent fluidization was investigated in a 0.267 m diameter fluidized bed with FCC particles. Different transition velocities and processes are found from the measurements of differential pressure fluctuations and local solids concentration. The measurements of differential pressure fluctuations shows a relatively quicker global regime transition, while the local regime transition obtained from the local solids concentration presents a more gradual process marked by two transition velocities. These transition velocities increase with initial static bed height. For the same initial static bed height, increasing the spacing between two pressure taps leads to lower pressure fluctuations and the appearance of two transition velocities.     

单组分颗粒振动流化床的流体力学研究（2）──床层膨胀和起始流化速度

本文在振动流化床中研究床层膨胀和颗粒的起始流化速度,分别研究颗粒物性、振动特性（频率、振幅）和静止床层高度对它们的影响,根据不同的振动条件下Ｇｅｌｄａｒｔ’Ａ、Ｂ、Ｄ类１３种物料起始流化速度的实验结果,关联了实验条件下起始流化速度的计算式,此计算式对振动流化床的设计具有指导意义。