Experimental investigation of particle contact time at the wall of gas fluidized beds

The contact time of particles at the walls of gas fluidized beds has been studied using a radioactive particle tracking technique to monitor the position of a radioactive tracer. The solids used were sand or FCC particles fluidized by air at room temperature and atmospheric pressure at various superficial velocities, covering both bubbling and turbulent regimes of fluidization. Based on the analysis of tracer positions, the motion of individual particles near the walls of the fluidized bed was studied. The contact time, contact distance and contact frequency of the particles at the wall were evaluated from these experimental data. It was found that in a bed of sand particles, the mean wall contact time of the fluidized bed of sand particles decreases by increasing the gas velocity in the bubbling and increases in the turbulent fluidization. In other words, the particle-wall contact time is minimum at the onset of turbulent fluidization in the bed of sand particles. However, the mean wall contact time is almost constant in both regimes of fluidization in the bed of FCC particles. All the existing models in the literature predict a decreasing contact time when the gas velocity in the bed is increased. It was also shown that the contact distance increases monotonously by increasing the gas velocity in the bed of sand particles, while it is almost constant for the bed of FCC particles. Contact frequency has a trend similar to that of the contact time for both sand and FCC particles.     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

竖直管气固鼓泡流化床传热机理的CPFD模拟

针对细颗粒气固鼓泡流化床中床料与竖直传热管壁面间的传热行为,在前期实验的基础上,采用计算颗粒流体力学(CPFD)方法从颗粒在传热壁面更新的角度,深入分析了传热特性与壁面气固流动行为之间的关联性。结果表明,模拟得到的传热管壁面颗粒更新通量和基于颗粒团更新模型的颗粒团平均停留时间均能很好解释实验测得的传热系数变化规律,这证实颗粒团更新是影响传热过程的控制性因素。模拟还发现随加热管从床层中心向边壁的移动,加热管周向方向上颗粒更新通量和传热系数的不均匀性都呈增大趋势。随着表观气速的增大,气泡行为导致床层颗粒内循环流率增大,这是导致颗粒团在加热管壁面上的更新频率增大以及床层与壁面间传热系数增大的根源。     

Correlation of axial mixing of solids in fluidized beds by a dispersion coefficient

Axial mixing of solids in a 19.3 cm diameter bed was investigated using a tracer technique. Bed material and tracer consisted of ion-exchange resin particles of 0.846 mm and 0.645 mm diameter, respectively. Tracer concentration profiles were measured. The results were expressed as axial dispersion coefficients. An equation relating dispersion coefficient to superficial gas velocity is presented.     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

快速流化床内床层与浸润表面间的传热研究——传热系数的径向分布规律

本文对低温快速流化床内传热系数的径向分布规律进行了实验研究.推荐以下关联式来计算床内各轴、径向位置处的传热系数:式中a、n_1、n_2均为径向位置的函数,(?)为轴向位置的函数.     

Heat Transfer in the Downer and the Riser of a Circulating Fluidized Bed – A Comparative Study

The characteristics of heat transfer were studied in both a gas‐solids concurrent downflow fluidized bed (downer) and a gas‐solids concurrent upflow fluidized bed (riser) with FCC particles. The radial and axial distribution profiles of the heat transfer coefficient between a suspended surface and the gas‐solids flow suspension were obtained using a miniature heat transfer probe, under different operating conditions. Comprising the results of the heat transfer in the downer and the riser shows that there exists some significant distinction between the heat transfer processes in the two reactors. The characteristics of heat transfer in both cases are closely related to their hydrodynamics and the distinct flow structures determinate the different heat transfer behaviors. The results also indicate that the operating conditions present some different effects in the two beds.     

Analysis and modeling of particle-wall contact time in gas fluidized beds

Although extensive studies have been conducted on convective heat transfer from a heat exchanger surface to a gas fluidized bed, the contribution through particle convection has not been adequately described, especially in turbulent fluidized beds. In this study, the role that dense bed hydrodynamics play on particle convection has been outlined. The existing models in the literature suggest a constant decrease of particle-wall contact time with an increase in the gas velocity. It has been experimentally demonstrated, however, that the contact time increases, both in bubbling and turbulent regimes, upon increasing the gas velocity. A comprehensive model has been developed to represent such a trend and improve agreement with experimental data presented in literature. The proposed model includes two constants for taking into account the wall effect on bubbles and clusters. The constants of the model have been evaluated based on the radial profiles of the distribution of bubbles and clusters using a radioactive particle tracking technique. A comparison of the predicted results with the experimental data from the literature confirms the validity of the present model for the dense bed region of a fluidized bed of sand particles.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

气固循环流化床提升管环核区颗粒质量传递系数的估计

Based on analysis of energy dissipation in the core region of gas-solid fluidized bed risers,a simplified model for determination of core-annulus solids mass transfer coefficient was developed according to turbulent diffu- sion mechanism of particles.The simulation results are consistent with published experimental data.Core-annulus solids mass transfer coefficient decreases with increasing particle size,particle density and solids circulation rate, but generally increases with increasing superficial gas velocity and riser diameter.In the upper dilute region of gas-solid fiuidized bed risers,core-annulus solids mass transfer coefficient was found to change little with the axial coordinate in the bed.     

Position and velocity of a large particle in a gas/solid riser using the radioactive particle tracking technique

The flow behavior of the solids phase in the fully developed region of a laboratory-scale circulating fluidized bed riser was studied using an assembly of sixteen NaI detectors to determine the position of a 500 μm radioactive particle, 100 times/s. The particle location was inferred from the number of γ-rays recorded by the assembly. The knowledge of the instantaneous positions enabled the determination of the instantaneous and mean velocity fields. Tests were conducted in a 0.082 m diameter, 7 m tall riser using 150 μm silica sand particles. Data were obtained at a gas superficial velocity of 4 m/s and solids mass fluxes from 23 to 75 kg/m²·s. Radial profiles of axial particle velocity showed that the solids velocity decreased with increasing solids circulation rates. Correspondingly, turbulent particle velocities and solids dispersion coefficient in the longitudinal direction were found to decrease as the solids circulation rate increased. The cross-sectional area where, on average, solids downflow took place, increased with increasing solids circulation rate.     

Heat transfer in a swirling fluidized bed with geldart type-D particles

A relatively new variant in fluidized bed technology, designated as the swirling fluidized bed (SFB), was investigated for its heat transfer characteristics when operating with Geldart type D particles. Unlike conventional fluidized beds, the SFB imparts secondary swirling motion to the bed to enhance lateral mixing. Despite its excellent hydrodynamics, its heat transfer characteristics have not been reported in the published literature. Hence, two different sizes of spherical PVC particles (2.61 mm and 3.65 mm) with the presence of a center body in the bed have been studied at different velocities of the fluidizing gas. The wall-to-bed heat transfer coefficients were measured by affixing a thin constantan foil heater on the bed wall. Thermocouples located at different heights on the foil show a decrease in the wall heat transfer coefficient with bed height. It was seen that only a discrete particle model which accounts for the conduction between the particle and the heat transfer surface and the gas-convective augmentation can adequately represent the mechanism of heat transfer in the swirling fluidized bed.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Surface-to-suspension heat transfer model in lean gas–solid freeboard flow

Heat transfer in dense fluidized beds have been extensively studied. However, there is not much detailed information about the mechanism of surface-to-suspension heat transfer in the freeboard region. In the present work, a newly designed heating plate was used to measure the plate-surface-to-particle-suspension heat transfer coefficients in the freeboard.The experimental unit consisted of a 30 cm i.d. fluidized bed reactor packed with fluidized catalytic particles of mean particle size 90 μm. Three types of plate orientations were used to test directional effects of surface on heat transfer rate. Height of the freeboard was 171 cm, and the superficial gas velocity was varied from 0.28 to 0.64 m/s. Local solids concentrations in the freeboard were also obtained by a nozzle-type sampling probe. Data on axial distribution of solids concentration were used to find out the solids kinematics in the freeboard region. Finally, a surface-to-suspension heat transfer model was developed to elucidate the surface to particle heat transfer mechanism in this lean phase system.The model is based on the transient gas-convective heating of single particles when sliding over the heating plate and the assumption of instantaneous attachment–detachment equilibrium between particles and the plate surface.     

带浸没管的惰性粒子振动流化床传热特性

采用带浸没加热管的惰性粒子振动流化床对膏状物料干燥进行了实验研究。考察了加料速率、进气温度、进气速度、加热管功率、振动强度等参数对床温和体积传热系数的影响,得出了计算体积传热系数的关联式。结果表明,在流化床中增设振动和浸没加热管装置,能大大强化传热传质,体积传热系数随加料量、振动强度、加热管功率、进风速度的增加而增大,随进气温度的增加而减小。其结果对惰性粒子流化床干燥器的设计和改进具有重要的指导意义。     

Computational Fluid Dynamics Study of Heat Transfer in Turbulent Fluidized Beds

The fluidization and heat transfer behaviors of a turbulent fluidized bed were investigated using computational fluid dynamics (CFD). The effects of inlet superficial velocity on heat transfer behaviors in a turbulent fluidized bed were analyzed and compared with those operated in other fluidization regimes. The effects of using particles belonging to different Geldart groups in a turbulent fluidized bed on fluidization and heat transfer behaviors were evaluated. For both fluidization regimes investigated, the solids temperature distribution during the heat transfer process became less uniform when the particle size was reduced.     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014     

Wall-to-bed heat transfer coefficient in gas—liquid—solid fluidized beds

Wall to bed heat transfer has been studied in three-phase fluidized beds with a cocurrent up-flow of water and air. Six sizes of glass beads, two sizes of activated carbon beads and one size of alumina beads, varying in average diameter from 0.61 to 6.9 mm and in density from 1330 to 3550 kg/m³, were fluidized in a 95.6 mm diameter brass column heated by a steam jacket. Complementary heat transfer experiments have been performed also for a gas–liquid cocurrent column and liquid–solid fluidized beds. The wall-to-bed coefficient for heat transfer in the gas–liquid–solid fluidized bed is evaluated on the basis of the axial dispersion model concept. The ratio of the wall-to-bed heat transfer coefficient in the gas–liquid–solid fluidized bed to that in the liquid–solid fluidized bed operated at the same liquid flow rate is correlated in terms of the ratio of the velocity of gas to that of liquid and the properties of solid particles. A correlation equation for estimating the wall-to-bed heat transfer coefficient in the liquid–solid fluidized bed is also developed.     

汽液固三相流动沸腾传热计算与实验研究

对汽液固三相循环流化床中流动沸腾传热进行了理论分析和实验研究 ,在此基础上结合渐进模型及表面更新机理建立了汽液固三相流动沸腾传热模型 ,模型计算值和实验数据吻合较好 ,最大偏差在 18%以内