Experimental study on secondary air mixing along the bed height in a circulating fluidized bed with a multitracer-gas method

A multitracer-gas method was proposed to study the secondary air(SA)mixing along the bed height in a circulating fluidized bed(CFB)using carbon monoxide(CO),oxygen(O_(2)),and carbon dioxide(CO_(2))as tracer gases.Experiments were carried out on a cold CFB test rig with a cross-section of 0.42 m×0.73 m and a height of 5.50 m.The effects of superficial velocity,SA ratio,bed inventory,and particle diameter on the SA mixing were investigated.The results indicate that there are some differences in the measurement results obtained using different tracer gases,wherein the deviation between CO and CO_(2) ranges from 42%to 66%and that between O_(2) and CO_(2) ranges from 45%to 71%in the lower part of the fluidized bed.However,these differences became less pronounced as the bed height increased.Besides,the high solid concentration and fine particle diameter in the CFB may weaken the difference.The measurement results of different tracer gases show the same trends under the variation of operating parameters.Increasing superficial velocity and SA ratio and decreasing particle diameter result in better mixing of the SA.The effect of bed inventory on SA mixing is not monotonic.

Particle velocity measurement of binary mixtures in the riser of a circulating fluidized bed by the combined use of electrostatic sensing and high-speed imaging

The flow dynamics of binary particle mixtures in the fluidized bed needs to be monitored in order to optimize the related industrial processes.In this paper,electrostatic sensing and high-speed imaging are applied to measure the velocities of polyethylene and sand particles in the binary particle mixtures in fluidization.Experimental studies were conducted on a lab-scale cold circulating fluidized bed.Correlation function between electrostatic signals from upstream and downstream electrodes placed along the riser shows two peaks that represent transit times for the two types of particles.To verify the above results,high-speed imaging was adopted to capture the flow images of particle mixtures.Particle Image Velocimetry and Particle Tracking Velocimetry algorithms were utilized to process the resulted images in order to measure the velocities of polyethylene and sand particles.The reasons for two-peak correlation functions are illustrated based on the frequency spectrums of the mono-solid-phase electrostatic signals and the velocity difference between polyethylene and sand particles.Finally,comparisons on the velocities obtained from electrostatic sensing and high-speed imaging demonstrate the electrostatic sensor can roughly estimate the particle velocity of binary particle mixtures in the near wall region of the circulating fluidized bed.

Incompatible thermal deformation of interlayers and corresponding damage mechanism of high-speed railway track structure

In the service period,the instability of ballastless track bed are mostly related to the damage of interlayers which are mainly resulted from the incompatible thermal deformation of interlayers.The temperature field within the ballastless track bed shows significant non-uniformity due to the large difference in the materials of various structure layers,leading to a considerable difference in the force bearing of different structure layers.Unit Ballastless Track Bed(UBTB)is most significantly affected by temperature gradient.The thermal deformation of interlayers within UBTB follows the trend of ellipsoid-shape buckling under the effect of the temperature gradient,resulting in a variation of the contact relationship between structure layers and a significant periodic irregularity on the rail.When the train travels on the periodically irregular rail,the structure layers are locally contacted,and the contact zone moves with the variation of the wheel position.This wheel-followed local contact greatly magnifies the interlayer stress,causes interlayer damage,and leads to a considerable increase in the bending moment of the track slab.Continuous Ballastless Track Bed(CBTB)is most significantly affected by the overall temperature variation,which may cause damage to the joint in CBTB.Under the combined action of the overall temperature rise and the temperature gradient,the interlayer damage continuously expands,resulting in bonding failure between structural layers.The thermal force in the continuous track slabs will cause the up-heave buckling and the sudden large deformation of the track slab,and the loss of constraint boundary of the horizontal stability.For the design of a ballastless track structure,the change of bearing status and structural damage related to the incompatible thermal deformation of interlayers should be considered.

Mixing and Segregation Characteristics of Binary Granular Material in Tapered Fluidized Bed: A CFD Study

The particle flow pattern, mixing and granule segregation in a tapered fluidized bed have been studied along with the hydrodynamics. At first the bed of varying total mass and granule fractions is fluidized then the bed is defluidized to freeze the composition, the bed is sectioned to layers and the composition in each layer is determined by sieving. Materials used in the present study are dolomite and glass beads with different B.S.S Sizes. A series of unsteady, three fluid CFD simulations were performed using FLUENTTM 6.2. Simulation parameters viz. solution technique, grid, maximum packing fraction and operating conditions like gas velocity were investigated for relative effects on particle mixing and segregation. Good arrangement of solid volume fraction profile was obtained between the experimental results and simulation results for regular particles.

Numerical simulation of local and global mixing/segregation characteristics in a gas–solid fluidized bed

Researches on solids mixing and segregation are of great significance for the operation and design of fluidized bed reactors.In this paper,the local and global mixing and segregation characteristics of binary mixtures were investigated in a gas–solid fluidized bed by computational fluid dynamics-discrete element method(CFD-DEM)coupled approach.A methodology based on solids mixing entropy was developed to quantitatively calculate the mixing degree and time of the bed.The mixing curves of global mixing entropy were acquired,and the distribution maps of local mixing entropy and mixing time were also obtained.By comparing different operating conditions,the effects of superficial gas velocity,particle density ratio and size ratio on mixing/segregation behavior were discussed.Results showed that for the partial mixing state,the fluidized bed can be divided into three parts along the bed height:complete segregation area,transition area and stable mixing area.These areas showed different mixing/segregation processes.Increasing gas velocity promoted the local and global mixing of binary mixtures.The increase in particle density ratio and size ratio enlarged the complete segregation area,reduced the mixing degree and increased the mixing time in the stable mixing area.

Segregation/Mixing Behavior of Binary Particles and Formation of Double-Density-Fluidized Bed

The current separator with air dense medium fluidized bed can only turn out two products at the same time with a single separating density. The double density fluidized bed means that two separating areas with different densities are formed in a fluidized cascade. In each separating area, the bed density is uniform and meets the technical requirement of coal preparation respectively. Therefore, the processed materials can be separated into three products according to density. In this paper, the fluidized behaviors of binary dense media were experimentally studied and the distribution characteristics of particle concentration and bed density in different bed structure were discussed. The segregation and mixing mechanisms of binary dense media were analyzed. It was testified that the pyramidal part designed in the bed structure played a key role in the formation of double density fluidized bed. The pyramidal part intensified the segregation of binary particles between two separating areas and strengthened the mixing in the low density area, which made for the density uniform of the area.

Theoretical analysis on ground vibration attenuation using sub-track asphalt layer in high-speed rails

Using a finite element method （FEM） program, a Portland cement concrete slab trackbed （So）, and a sub- track asphalt roadbed （RAC-S） were modeled under high- speed train loads to analyze their responses to ground vibration attenuation, by considering 10, 15, 20, 25, and 30 thick sub-track asphalt layer replaced on the top of the upper subgrade. FEM results show that the vibration amplitude of RAC-S is at least three times lower than the vibration for So. The maximum vibration amplitude of RAC-S is linearly increased with train speed. The vertical acceleration is found to be reduced by more than 10 % when the asphalt layer thickness is increased from 10 to 20 cm. However, the reduction in vertical acceleration is only about 1% when the thickness of the asphalt layer changes from 20 to 30 cm. The vibration level is slightly lower if the asphalt layer has higher resilient modulus in the seasons of autumn or winter. This theoretical analysis indicates that a railway substructure that consists of a 10-20 cm thick high modulus asphalt layer located at the top of trackbed shows a good performance in ground vibration control for high-speed rails.

FUNDAMENTAL STUDIES ON THE HYDRODYNAMICS AND MIXING OF GAS AND SOLID IN A DOWNER REACTOR

The effect of flow direction on hydrodynamics and mixing in the upflow and downflowcirculating fluidized beds is discussed in details.Similar profiles of gas and solids velocities andsolids concentration are found in both risers and downers.When the flow is in the direction ofgravity(downer),the radial profiles of gas and particle velocity are more uniform than that inthe riser,the solids mixing is very small and the flow pattern approaches plug flow,while theflow is against gravity(riser),the solids backmixing significantly increase and the flow pattern isfar from plug flow.Among many of factors the flow direction has the largest influence onhydrodynamics and axial mixing of gas and solids.

DRG背景下公立医院床位效率研究

目的分析医院各临床科室床位利用效率,为合理配置科室床位资源提供参考。方法以某三甲专科医院2022年度数据为基础,基于传统床位工作效率指标,引入CMI值进行调整,计算各科室床位的合理区间,结合数据包络分析法(DEA),综合评价各临床科室投入产出效率,得出床位配置优化方向。结果被统计的39个科室中,10个科室床位设置不合理,其中需要增加床位的有5个科室,需要减少床位的有5个科室。结论利用CMI调整病床工作效率指标,结合DEA方法,可为医院动态调整临床科室床位数提供科学依据;医院应合理利用床位资源,科学规划科室床位。

气固微型流化床反应分析仪的理想流型判据分析

微型流化床因具有气体返混小和可操作性强等优点,在反应动力学测量等领域备受关注。获得流型随操作参数的变化规律,才能实现微型流化床理想流型的有效调控。采用双流体模型耦合考虑结构的相间曳力(又称多尺度CFD)模拟了一系列A类和B类颗粒在不同操作条件下的流化行为,结合颗粒浓度和气体停留时间分布(RTD)特征考察气速、床径和初始床高对气体返混行为的影响。研究表明,当微型流化床在初始鼓泡到湍动流态化之间操作时,才逼近颗粒全混流和气体近似平推流的运动状态。进一步分析气体RTD曲线形状特征(如拖尾、多峰等),提出采用斜度S<0.6作为微型流化床平推流判据,弥补了原判据[满足方差σ_(t)^(2)<0.25且峰高E(t)h>1]的不足,为微型反应分析仪的流型调控奠定了基础。

1000 MW超超临界机组精处理系统改造及智能化升级

某电厂2×1000 MW超超临界机组锅炉给水采用加氧处理(OT),利用西安热工研究院“火电厂高速混床运行性能诊断及优化专家系统”对精处理高速混床和再生系统进行诊断,发现运行末期泄漏离子、再生系统缺乏智能监控装置及再生酸碱用量大等问题。应用树脂输送图像智能识别及控制仪(IRIC)和双层多孔板布水装置改造后,树脂体外再生过程实现了智能控制,树脂输送终点识别准确率100%;高速混床平均周期制水量增幅108%,出水Na^(+)和Cl^(-)含量均优于《火力发电机组及蒸汽动力设备水汽质量》(GB/T 12145—2016)要求,经济效益和安全效益显著。

钱塘江河口盐官段混合式丁坝修建对水沙及河床影响的实测研究

钱塘江北岸古海塘堤脚加固工程中,辅助围堰施工的临时性管袋黏土混合式丁坝对工程的顺利进行、提高施工效率起着至关重要的作用,然而目前对其周围水沙运动的研究较少,且缺乏相关实测资料。考虑混合式丁坝修建的动态过程,利用无人机拍摄水流视频结合LSPIV(粒子图像测速)处理技术对丁坝修建前、中、后期周围的流场进行分析,并对丁坝附近的地形进行跟踪测量,同时对不同潮差的水流含沙量进行现场测定。结果表明,丁坝具有改变水流方向,减少水流对河岸冲刷的作用,且丁坝越长,挑流作用越明显,为围堰施工提供了较为平缓的水流条件;混合式丁坝对坝后处的促淤效果明显,且坝后地形变化与含沙量密切相关,潮差越大,水体含沙量越高,丁坝后河床淤积幅度越大,有利于提高围堰的施工效率。研究成果对之后相关物理模型和数值模拟的研究提供数据支持。

气液混合强化在固定床加氢过程中的应用进展

炼油工业作为国民经济的支柱,在创造大量财富的同时,往往存在高能耗、高物耗和高污染的问题。固定床加氢技术是重要清洁炼油技术,在油品质量升级、产品结构调整、原油资源高效利用、生产过程清洁化进程中发挥了重要的作用。提高固定床加氢效率有助于充分利用石油资源、生产清洁燃料和实现生产过程的节能降耗。本文从固定床反应器滴流床加氢和液相加氢过程的氢油两相物料混合特性出发,综述了通过开发新型混氢设备和加氢工艺,强化气液混合过程,提高固定床多相催化加氢效率的应用进展,并提出固定床加氢反应过程气液混合强化技术发展趋势,为炼油化工生产过程提质增效、节能降碳新技术的开发提供参考。

混床运行周期降低的原因及处理

陕西延长中煤榆林能源化工股份有限公司(简称“榆能化公司”)是中国污水近“零排放”企业之一,原水水质复杂,由地表水、回用水、再生水及少量的污水站产水混合而成。各类回用水占比较大,部分难去除离子长期在水系统内富集,导致脱盐水混床运行周期不稳定、树脂再生频繁、酸碱单耗增加,严重影响装置稳定运行。文章通过对混床进水离子含量、树脂交换容量及再生操作等方面原因分析,发现混床进水电导率是影响混床运行周期的关键因素,并通过对各类回用水进行分质利用等措施,使混床运行周期降低问题得到了有效解决。

脉动流化床内青刀豆颗粒的混合特性数值模拟

青刀豆一般采用流化床装置进行速冻。为了解决其床层堵塞和聚集不良的问题,进一步提高青刀豆的混合效率,课题组建立了三维混合脉动流化床模型,将计算流体力学与离散单元法(computational fluid dynamics-discrete element method,CFD-DEM)相结合,探究1,2,4和8 Hz 4种不同频率的脉动进口风速下青刀豆颗粒的流化混合情况。仿真结果表明:脉动流化床对于青刀豆颗粒混合具有一定改善作用,青刀豆颗粒的轴向混合强于径向混合,4 Hz时颗粒混合标准偏差值最小;当进口风速的频率接近颗粒的自主振动频率时,床层膨胀最明显,出现类似“共振”的现象;2 Hz时颗粒径向、轴向混合效果最佳,频率过大或过小对于流态化增强效果不明显。

Axial gas mixing in conical spouted beds with high density particles

Conical spouted beds operating with high-density particles(ρp>2500 kg/m^(3))have recently gained attention because of their potential use as nuclear fuel coaters for next-generation nuclear reactors.In the literature,the number of axial gas mixing studies in conical and conical-cylindrical spouted beds is very limited and all axial mixing studies were carried out with relatively light particles(ρp≤2500 kg/m^(3)).Therefore,the objective of this study was to generate experimental data that can be used to explain the gas axial mixing behavior in conical spouted beds operating with both low-and high-density particles.Experiments were conducted in two(γ=30°,60°)conical spouted beds with three different types of particles:zirconia(ρp=6050 kg/m^(3)),zirconia toughened alumina(ρp=3700 kg/m^(3))and glass beads(ρp=2460 kg/m^(3)).In order to be able to compare experimental data obtained at different conditions,a 1-D convection-diffusion gas mixing model originally developed by San Joséet al.(1995)was implemented to determine the axial dispersion coefficients.The results show that the axial dispersion coefficients range between m^(2)/s and m^(2)/s,increase with superficial gas velocity and are higher than the corresponding dispersion coefficients of fixed beds,lower than the dispersion coefficients of fluidized beds and in the same range with the cylindrical spouted beds reported in the literature.The corresponding Peclet numbers were in the range of 0.6–7.8 for all operating conditions and slightly higher Peclet numbers were obtained with glass beads indicating the relative importance of gas convective transport over gas dispersion for light particles compared to heavy particles.

Effects of Airflow Field on Droplets Diameter inside the Corrugated Packing of a Rotating Packed Bed

Rotating packing bed(RPB) has a better mixing performance than traditional mixers and shows potential application in the petroleum industry. However, acquisition of information about the mixing process directly through experiments is difficult because of the compact structure and complex multiphase flow pattern in RPB. To study the mixing characteristic, Fluent, the computational fluid dynamics(CFD) software, was used to explore the effect of airflow field on droplet diameter. For conducting calculations, the gas-liquid two-phase flow inside the packing was simulated with the RNG k-ε turbulence model and the Lagrange Discrete Phase Model(DPM), respectively. The numerical calculation results showed that coalescence and breakup of droplets can take place in the gas phase flow inside the packing and can be strengthened with increased rotating speed, thereby leading to the enlargement of the average diameter.

Enhancing the activity of MoS_(2)/SiO_(2)-Al_(2)O_(3) bifunctional catalysts for suspended-bed hydrocracking of heavy oils by doping with Zr atoms

Developing catalysts with not only hydrogenation activity but also cracking activity is very important for the advancement of suspended-bed hydrocracking technology.Within this respect,MoS_(2)/SiO_(2)-Al_(2)O_(3)bifunctional catalyst is a kind of typical catalysts with both hydrogenation and cracking activity.Herein,a series of Zr-doped SiO_(2)-Al_(2)O_(3)mixed oxides were synthesized by a sol-gel coupled with hydrothermal method.The synthesized mixed oxides were characterized for chemical structures and acidic properties.It is found that doping SiO_(2)-Al_(2)O_(3)with Zr atoms significantly increases the numbers of acidic sites.The Zr-doped SiO_(2)-Al_(2)O_(3)mixed oxides were then combined with dispersed MoS_(2),which was in-situ produced from oil-soluble Mo precursors,to fabricate a novel kind of bifunctional catalysts for suspended-bed hydrocracking of heavy oils.Owing to the significantly increased numbers of acidic sites in Zr-doped SiO_(2)-Al_(2)O_(3)mixed oxides,corresponding bifunctional catalysts demonstrate much enhanced activity for suspended-bed hydrocracking of heavy oils in relative to MoS_(2)/SiO_(2)-Al_(2)O_(3)bifunctional catalysts.

Modeling of the Precipitation Process in a Rotating Packed Bed and Its Experimental Validation

A mixing-precipitation model based on the modified coalescence-redispersion model was presented to describe the flow,mixing,nucleation and growth in a rotating packed bed(RPB) .The model was coupled with population balance,mass balance and crystallization kinetics.It predicted well the influence of coalescence prob-ability,which represents the mixing intensity among droplets,on the particle number density,supersaturation and mean particle size of the produced precipitates.The effects of the radial thickness of packing,liquid flow rate and rotating speed on the product particle size were also investigated.The results indicate that the needed radial length of packing is short for sparingly soluble substance precipitation(about 40-50 mm in this work) ,and the mean parti-cle size of precipitates decreases with the increase of rotating speed and liquid flow rate,respectively.The validity of this model was verified by experiment on BaSO4 precipitation in RPB.