HYDRODYNAMICS OF MAGNETOFLUIDIZED ADMIXTURE BEDS

Hydrodynamic behavior of a magnetofluidized bed is investigated over a wide range of magnetic-field intensity values as a function of superficial air velocity. The bed comprises of different proportions of copper and iron particles and is contained in a Plexiglas column of 0.102 m internal diameter. The uniform constant magnetic field is created by a Helmholtz electromagnet. In particular, the bed pressure drop is measured as a function of superficial air velocity and characteristic bed voidage and fluidizing velocities are determined at minimum fluidization and bubbling bed conditions. These characteristic properties of the magnetically stabilized fiuidized beds are reported as a function of magnetic-field intensity, and are correlated by suitable expressions. These will be useful for prediction purposes related to design and operation.     

HYDRODYNAMICS OF MAGNETOFLUIDIZED ADMIXTURE BEDS

Hydrodynamic behavior of a magnetofluidized bed is investigated over a wide range of magnetic-field intensity values as a function of superficial air velocity. The bed comprises of different proportions of copper and iron particles and is contained in a Plexiglas column of 0.102?m internal diameter. The uniform constant magnetic field is created by a Helmholtz electromagnet. In particular, the bed pressure drop is measured as a function of superficial air velocity and characteristic bed voidage and fluidizing velocities are determined at minimum fluidization and bubbling bed conditions. These characteristic properties of the magnetically stabilized fiuidized beds are reported as a function of magnetic-field intensity, and are correlated by suitable expressions. These will be useful for prediction purposes related to design and operation.     

Bed height and material density effects on fluidized bed hydrodynamics

Characterizing the hydrodynamics of a fluidized bed is of vital importance to understanding the behavior of this multiphase flow system. Minimum fluidization velocity and gas holdup are two of these key characteristics. Experimental studies addressing the effects of bed height and material density on the minimum fluidization velocity and gas holdup were carried out in this study using a 10.2 cm diameter cylindrical fluidized bed. Three different Geldart type-B particles were tested: glass beads, ground walnut shell, and ground corncob, with material densities of 2600, 1300, and 1000 kg/m³, respectively. The particle size range was selected to be the same for all three materials and corresponded to 500–600 μm. In this study, five different bed height-to-diameter ratios were investigated: H/D=0.5, 1, 1.5, 2, and 3. Minimum fluidization velocity was determined for each H/D ratio using pressure drop measurements. Local time-average gas holdup was determined using non-invasive X-ray computed tomography imaging. Results show that minimum fluidization velocity is not affected by the change in bed height. However, as the material density increased, the minimum fluidization velocity increased. Finally, local time-average gas holdup values revealed that bed hydrodynamics were similar for all bed heights, but differed when the material density was changed.     

Hydrodynamics of a superheated steam vacuum fluidized bed

Hydrodynamics of a superheated steam vacuum fluidized bed was experimentally studied. In these experiments, eight different types of large particles (1970–7430 μm) were used. In all cases, a behavior similar to that found in an air fluidized bed was observed. The minimum fluidization velocity was found to be increasing with decreasing operating pressure. In the case of employing superheated steam, the minimum fluidization conditions are established at a lower velocity than using air as the fluidizing medium. These tendencies are attributed to the variation of the mean free path of molecules. On the other hand, the experiments showed that the bed voidage in the minimum fluidization conditions is almost insensitive to the variation of the operating pressure. Several equations were developed to predict the minimum fluidization velocity. The values provided by these equations were compared with the experimental data as well as with the predictions of the correlations presented in the technical literature.     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。     

Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed

The fluidized behavior of binary mixtures of moist sawdust and glass spheres has been investigated. The sawdust alone was observed to fluidize poorly, with extensive channelling occurring. The addition of 0.322 and 0.516 mm glass spheres to the fluidized bed of sawdust improved the fluidization characteristics. The mixtures of sawdust and 0.322 mm spheres were completely mixed when fluidized. Mixtures of sawdust and 0.516 mm spheres were either partially or completely mixed, depending upon gas velocity in the fluidized bed. As the moisture content of the sawdust was increased, the minimum fluidization velocity of the binary mixture also increased. There was an upper limit to the moisture content of the sawdust at which fluidization could be achieved. When the moisture content of the sawdust exceeded 33 and 54 wt% on a dry basis, agglomeration and channelling occurred in the mixtures of sawdust and glass spheres, with sizes 0.322 and 0.516 mm, respectively. The moisture likely contributes to interparticle liquid bridging forces. Binary mixtures of larger 0.777 and 1.042 mm glass spheres and up to 82% moisture sawdust did not readily agglomerate, but the two components completely segregated during fluidization.     

横向旋转磁场作用下的液固流态化

利用横向旋转磁场发生器研究了铁颗粒以及铁－不锈钢混合颗粒的流态化特征．在一定条件下,实现了铁颗粒成链自旋．研究表明,旋转频率、磁场强度和添加铁颗粒的比例是影响流化质量的主要因素．实验表明,横向旋转磁场的旋转频率和磁场强度对铁颗粒及铁－不锈钢混合颗粒的最小流化速度影响很小     

A comparative hydrodynamic study of two types of spouted bed reactor designs

In order to properly design and scale up spouted beds, one needs to predict the minimum spouting velocity of specific systems having different bed dimensions, and properties of particle and spouting gas. Because of inherent complexity of predicting minimum spouting velocity, the prevailing approach has been to use empirical correlations, a number of which are available in the literature. Central jet distributors are commonly used in the experimental studies reported in the literature. Circular slit distributor is a new concept in which air is supplied to the bed of particles through a circular slit. This paper presents results of an experimental study on the hydrodynamics of central jet and circular slit distributors. In this paper a fully connected feed-forward neural network model was used to predict the minimum spouting velocity of central jet and circular slit spouted beds. A neural network model was also developed to predict minimum fluidization velocity. The actual experimental data obtained from published literature and from the experiments carried out in this study were used for training and validating the models. The minimum spouting and fluidization velocities predicted using the neural network models developed in this study show a better approximation to the actual experimental values than those obtained from correlations available in the open literature. The position of flow regime of circular slit spouted bed was also established relative to the flow regimes of central jet spouted bed and fluidized bed.     

加压流态化：Ⅰ流化特性的研究

在建立４００ｍｍ压力为０．１～２．０ＭＰａ的冷模加压流化床试验装置基础上，对流化床的临界流化速度进行了研究，通过回归实验值得到了临界流化速度的计算式：Ｒｅｍｆ＝［２５．３２＋０．０４３４Ａｒ］１／２－２５．３，一般随压力的提高，临界流化速度减小。同时实验测定了ｄｐ＝５６２μｍ聚乙烯粉料的膨胀特性，通过对秦霁光［１］的床层膨胀计算式（超越方程）的简化得到比较简单而准确模拟实验的床层膨胀高度计算式Ｈ＝Ｈ０＋ＢＨａ０＋ａＡＢＨａ－１０－ＢＡａ１－ＢａＨａ－１０－ａＡＢ（ａ－１）Ｈａ－２０在本实验情况下，实验值与计算值的相对误差小于±６．３％，床层膨胀主要随气速的提高而增高，而压力对此不敏感。此外还测定了分布板的锐孔系数，Ｃ′ｄ＝０．６８６，可以作为设计加压流化床分布板的设计参数     

液－固喷动流化床的膨胀特性和液体混合

在φ１０４ｍｍ×１５００ｍｍ的液－固喷动流化床内，对２种不同孔径的喷嘴分别进行喷动流化，利用轴向扩散柱塞流模型得到了液体轴向扩散系数。讨论了流体速度、床层空隙率、颗粒物性和喷嘴孔径等因素对液体轴向扩散系数的影响，并得到了轴向扩散系数的关联式。另外，还研究了床层的膨胀特性，得到了床层空隙率的关联式。     

气-液-固三相逆流化床动力学研究

在内径0.152 m,高2.5 m的气-液-固三相逆流化床中系统研究了动力学特性。获得了气体和液体速度及聚乙烯和聚丙烯颗粒密度对各相含率和最小液体流化速度的影响规律。研究发现随着气体速度的增加,液体最小流化速度降低;随着气体或液体速度增加,气体、液体和固体含率均增加。     

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.     

Prediction of minimum bubbling velocity, fluidization index and range of particulate fluidization for gas-solid fluidization in cylindrical and non-cylindrical beds

A uniform fluidization exists between minimum fluidization velocity and minimum bubbling velocity. Experimental investigations have been carried out for determination of minimum bubbling velocity and fluidization index for non-spherical particles in cylindrical and non-cylindrical beds. In the present paper equations have been developed for the prediction of minimum bubbling velocity for gas-solid fluidization in cylindrical and non-cylindrical (viz. semi-cylindrical, hexagonal and square) beds for non-spherical particles fluidized by air at ambient conditions. A fairly good agreement has been obtained between calculated and experimental values. Based on the experimental data it is concluded that under similar operating conditions minimum bubbling velocity and the fluidization index are maximum in case of either semi-cylindrical conduit or hexagonal conduit for most of the operating conditions and minimum in case of square one. It is further observed that the range of uniform (particulate) fluidization is maximum in case of semi-cylindrical bed for identical operating conditions.     

Transition velocity and bed expansion of two-phase (liquid-solid) fluidization systems

Hydrodynamic transition experiments for two-phase (liquid-solid), both upward and downward, liquid flow systems were performed in a 127-mm diameter column. The particles were 3.2-mm polymer (1,280 kg/m³), 5.8mm polyethylene (910, 930, 946 kg/m³), 5.5-mm polystyrene (1,021 kg/m³) and 6.0-mm glass (2,230 kg/m³) spheres, with water, aqueous glycerol solution and silicone oil as liquids. The dimensionless pressure gradient increases initially with increasing liquid velocity, but decreases gradually with increasing liquid velocity beyond U_lmf due to bed expansion. The non-dimensionalized pressure gradient using the liquid/solid mixture density increases with increasing liquid velocity and then reaches a constant value close to unity beyond U_lmf. The minimum fluidization Reynolds number for liquid-solid system increases with increasing Archimedes number including both heavier and lighter than the density of the liquid phase. U_lmf should be the same for both upward and downward fluidization systems since the Ergun equation is based on the main assumption that drag force of the superficial liquid velocity, U_lmf, is equal to the net difference between gravitational and buoyancy forces.     

单组分球形颗粒最小流化速度的简便计算

对Ｇｅｌｄａｒｔｓ的Ｂ．Ｄ两类物料建立了一个新的采用倾斜量筒的随意松散堆积法来简易可行地确定该单分物料随意散堆积空隙率。实验数据表明，这一随意松散堆积空隙率可用来代替最小流化空隙的率的数值，并用Ｅｒｇｕｎ方程式，求取最小流化速度。     

The fluidization process of binary mixtures of solids: Development of the approach based on the fluidization velocity interval

The main characteristic of the fluidization process of particulate mixtures is the interaction between bed suspension and segregation-remixing dynamics of their components.On addressing the fluidization properties of binary beds of solids differing either in particle density or diameter, the paper shows how interpretations based on definition of a minimum fluidization velocity of the mixture can lead to erroneous conclusions about the influence played by system variables on their behaviour. An alternative method of investigation is followed which takes into consideration the existence of a finite velocity interval, bounded by the “initial” and “final fluidization velocity” of the mixture, along which the whole process of fluidization has place. The results of a wide series of experiments demonstrate that this alternative approach allows recognizing the independent variables of binary fluidization; they also highlight the differences of behaviour between density- and size-segregating beds.     

A Draft-Tube Conical Spouted Bed for Drying Fine Particles

A study has been conducted on the performance of a draft tube conical spouted bed for drying fine particles. Batch operation has been performed with nonporous, porous, and open-sided draft tubes in order to ascertain the optimum configuration of this internal device. The nonporous draft tube requires the lowest minimum spouting velocity. Nevertheless, the solid circulation rate and the drying efficiency of the open-sided draft tube are superior to any other spouted bed configuration. Moreover, it allows for reducing the height of the fountain and, consequently, the height of the drying equipment.     

A Draft-Tube Conical Spouted Bed for Drying Fine Particles

A study has been conducted on the performance of a draft tube conical spouted bed for drying fine particles. Batch operation has been performed with nonporous, porous, and open-sided draft tubes in order to ascertain the optimum configuration of this internal device. The nonporous draft tube requires the lowest minimum spouting velocity. Nevertheless, the solid circulation rate and the drying efficiency of the open-sided draft tube are superior to any other spouted bed configuration. Moreover, it allows for reducing the height of the fountain and, consequently, the height of the drying equipment.