Hydrodynamics of a spouted bed with a porous draft tube containing a small amount of finer particles

A small-size spouted bed with a porous draft tube was employed to obtain hydrodynamic data of binary mixtures of glass beads for a range of operating conditions and design factors. In this case, a small amount of finer particles was added mostly to the large majority of coarser particles. Under this condition, minimum spouting velocity, bed pressure drop, hold-up of solid particles within a draft tube, gas flow rate through the annulus and solids circulation rate were determined by changing the total gas flow rate and the mass fraction of finer particles as operating parameters, and by changing the height of the entrainment zone and the draft tube diameter as geometric parameters. The results show that the gas flow rate through the annulus increases by increasing the distance between the gas inlet nozzle and the bottom of the draft tube, that is, the height of the entrainment zone, but decreases with increasing draft tube diameter and mass fraction of finer particles. The porous draft tube shows a higher gas flow rate through the annulus than the non-porous draft tube, particularly in the case of the low height of the entrainment zone. The solids circulation rate increases with increasing gas velocity, the height of the entrainment zone and the porous draft tube diameter. Moreover, the porous draft tube leads to a higher solids circulation rate than the non-porous draft tube.     

Solid circulation and gas bypassing in an internally circulating fluidized bed with an orifice-type draft Tube

The effects of orifice diameter in the draft tube, particle size, gas velocities and bed height on the circulation rate of solids and gas bypassing between the draft tube and annulus have been determined in an internally circulating fluidized bed (i.d., 0.3 m ; height, 2.5 m) with an orifice-type draft tube. A conical shape gas separator has been employed above the draft tube to facilitate the separation of gases from the two beds. The circulation rate of solids and the quantity of gas bypass from the annulus to draft tube show their minimums when the static bed height is around the bottom of the separator. The circulation rate of solids increases with an increase in orifice diameter in the draft tube. At fixed aeration to the annulus, gas bypassing from the draft tube to annulus sections decreases, whereas reverse gas bypassing from the annulus to the draft tube increases with increasing the inlet gas velocity to the draft tube. The obtained solids circulation rate has been correlated by a relationship developed for the cocurrent flow of gas and solid through the orifice.     

循环喷动流化床颗粒抛射高度的计算

通过对循环喷动流化床顶部封闭空间气体射流及气固运动的理论分析 ,得出了循环喷动流化床中颗粒出循环管后抛射高度的计算方法 ,所得计算结果与实测结果误差小于 8% ,为循环喷动流化床的设计提供理论依据     

大尺寸喷动床最低喷动速度的估算

最低喷动速度是喷动床设计的重要参数之一。过去人们习惯采用Mathur-Gisher公式来计算,但由于该公式是建立在小直径床层(D_c=0.076～0.3m)实验的基础上的,故在大直径床层上应用时,产生明显的偏差。本文通过实际数据分析,提出了大直径喷动床层最低喷动速度计算的经验公式。     

Solid circulation characteristics in an internally circulating fluidized bed with orifice-type draft tube

Effects of superficial gas velocities to a draft tube, to an annulus section and particle size on the solid circulation rate (G,) have been determined in an internally circulating fluidized bed (0.28 m I.D. × 2m high) with an orifice type draft tube. The solid circulation rate from the draft tube to an annulus section increases with increasing gas velocities to the draft tube(U _d ) and annulus section (U_a) and consequent increase in pressure drop across the orifice (ΔP_or). However, the values ofG _s decrease by 7–21% with increasing particle size from 86 to 288 μm. The pressure drop across the orifice increases with increasingU _d andU _a . However, ΔP_or decreases by 5–23% with increasing particle size. To predictG _s in an internally circulating fluidized bed, a correlation is proposed as a function of ΔP_or This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement Korea University.     

Effect of the annulus aeration on annulus leakage and particle circulation in a three-phase spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections of a liquid spout-fluid bed with a draft tube was analyzed in the presence of air flow through the annulus using a tracer response technique.The responses of the tracer injections prove that part of the flow originating from the annulus liquid inlet leaks into the draft tube when any amount of annulus liquid inlet flow is present and this annulus leakage increases when air flow is introduced into the annulus. This finding extends the recently acquired knowledge about the annulus leakage and cross flow in the entrance region below the draft tube in liquid spout-fluid beds to three phase spout-fluid beds with a draft tube where the annulus is aerated.The experimental system studied consisted of a semi-circular column of 80 mm diameter equipped with a semi-circular draft tube of 20 mm diameter. Liquid and air flows coexisted in the annulus. The liquid velocities were well above the minimum fluidization velocity and the particles were glass beads of 3 mm diameter.Experimentally determined values of the superficial annulus and draft tube liquid velocities from the tracer response analyses together with the experimental draft tube pressure gradients were used as input into Grbavcic et al.'s [Z.B. Grbavcic, R.V. Garic, D.V. Vukovic, Dz.E. Hadzismajlovic, H. Littman III, M.H. Morgan, S.Dj. Jovanovic, Hydrodynamic modeling of vertical liquid-solids flow. Powder Technol. 72, (1992)183-191] variational model for vertical non-accelerating liquid-solids flow to calculate the draft tube voidage and the particle mass flux assuming an air free draft tube. Results indicate that annulus aeration lowers the particle mass flux in this particular column geometry.     

Annulus leakage and distribution of the fluid flow in a liquid spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections in a liquid phase spout-fluid bed with a draft tube was studied in a flat-based semi-circular column of diameter equipped with a semi-circular draft tube of diameter at superficial fluid velocities well above the minimum fluidization velocity. The particles used were glass spheres of diameter, and the spouting medium was tap water. A sodium chloride solution was injected into the reactor and the response recorded to determine the flow distribution between the draft tube and the annulus sections. The responses of the tracer injections prove that part of the flow originating from the annulus inlet leaks into the draft tube when any amount of annulus inlet flow is present. This finding makes a significant contribution to the understanding of the entrance region below a draft tube which has been modeled as a spout-fluid bed where fluid enters the annulus region based on the pressure distribution at the spout-annulus interface while the flow is in the radial direction from the spout into the annulus. This work shows that there is cross flow in the entrance region resulting in fluid exchange between the streams originating from the spouting inlet nozzle and the annulus inlet flow.The amount of leakage is found to increase with increasing inlet flow rates. An empirical correlation is developed to predict the fraction of the leakage of the annulus inlet flow. The superficial fluid velocity through the draft tube is found to vary linearly with the total flow rate through the bed under the experimental conditions studied. The pressure distributions in the entrance region favor the leakage of fluid from the annulus inlet flow into the draft tube.     

Hydrodynamics of a gas-driven gas-liquid-solid spouted bed with a draft tube

A cylindrical gas-liquid-solid spouted bed, driven exclusively by gas flow, has been developed with a high potential for use in biochemical processes, such as a biological wastewater treatment. A plexiglass column with a 152 mm inner diameter was used in combination with a 53 mm inner diameter plexiglass draft tube. Three particle types were studied with densities ranging from 1044 kg/m³-1485 kg/m³ and average particle sizes ranging from 0.7-2.5 mm. Four flow regimes were observed when increasing the gas velocity, including fixed bed, semispouted bed, full spouted bed, and internal circulating fluidized bed. The transition gas velocities between those regimes were experimentally measured and termed as minimum spouting velocity, full spouting velocity, and minimum circulating velocity, respectively. A measurement of the downward particle flux in the annulus was used to identify the minimum spouting velocity, while the particle velocity and dense phase retraction in the annulus were monitored for the full spouting and minimum circulating velocities. All regime transition velocities increased with more dense particles and longer draft tubes. The minimum spouting velocity and full spouting velocity were not affected when varying the nozzle-tube gap, while the minimum circulating velocity increased with longer nozzle-tube gaps. Experiments without a draft tube were carried, though the spouting stability was significantly reduced without the draft tube.     

循环喷动流化床固体循环速率的控制

通过研究操作条件对固体循环速率的影响 ,提出了新型的固体循环速率的双阀控制理论模型 ,基于循环喷动流化床中颗粒循环推动力的分析 ,得出了预测循环喷动流化床中固体循环速率最大值的方法     

Simulations of flow behavior of gas and particles in spouted bed with a porous draft tube

Flow behaviors of particles in a two-dimensional spouted bed with a porous draft plates are studied using a fluid dynamic computation with kinetic-frictional stresses models. Gas flow through the porous draft tube is simulated by Brinkman-Darcy-Forchheimer formulation. The distributions of concentration and velocities of particles are predicted. Simulated results predict the solid circulation rate and gas flux rate measured in the spouted bed with a porous draft tube (Ishikura et al., 2003). The solid circulation rate in the spouted bed with a porous draft tube is larger than that with a non-porous draft tube. The predicted bed pressure drop with the porous draft tube is high in comparison to the spouted bed with non-porous draft tube. The effect of the porosity of the porous draft tube on distributions of gas flux rate through the annulus and solid circulation rate are discussed.     

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.     

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.     

导向管直径对喷动床流动特性影响的计算颗粒流体力学数值模拟

导向管喷动床是较为常见的一种喷动床改进床型,通过阻断喷动区与环隙区气固接触来提高颗粒循环的规律性与稳定性。本文采用计算颗粒流体力学（CPFD）方法对于直径150mm的柱锥式导向管喷动床进行了数值模拟研究,考察了导向管直径对于喷动床内颗粒流动特性的影响,从环隙区死区分布、颗粒速度分布、固体循环量等方面分析了具有不同直径导向管喷动床的运行状态。结果表明,加入导向管在减少床内死区的同时也降低了运行时的固体循环量,对于本次采用的喷动床结构尺寸与运行参数,只有在导向管直径为40～60mm时才能保证床内具有良好喷动状态,综合考虑各因素,选用直径50～55mm的导向管最为合适。对于具有类似结构与运行条件的柱锥喷动床,导向管直径可考虑选为无导向管运行时喷动区直径的1.2～1.375倍。     

Discrete particle simulations for flow of binary particle mixture in a bubbling fluidized bed with a transport energy weighted averaging scheme

Flow behavior of small and big particles with the same particle density in a bubbling fluidized bed is modeled by a combined approach of discrete particle method and computational fluid dynamics (CFD-DPM). The collision time of a collision pair is computed by a quartic equation in which the effect of acceleration due to the different diameters is considered. A transport energy weighted averaging approach is proposed to determine the local gas velocity at a particle. The fluidization behavior of binary mixture differing in size is experimentally and numerically studied in the gas bubbling fluidized bed. The distributions of mass fraction of small and big particles along the bed height are simulated, and the profiles of the mean particle diameters of binary mixture are determined. The numerical results are in agreement with experimental data. The distributions of granular temperature, stresses, and shear viscosities of small and big particles are compared.