气固移动流化床干燥器流化与传热特性研究

以不同粒径的分子筛颗粒与炼焦煤颗粒为原料分别在气固移动流化床干燥器内进行了双组分颗粒分级流化行为特性与气固间传热特性的实验研究。考察操作气速、颗粒直径与不同粒径配比的变化以及搅拌转速对床层阻力的影响,并对流化状态加以描述分析,研究了干燥器气固间传热特性,得出了相应的表面传热系数关联式,计算值与实验值的误差小于15％。     

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。     

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。     

固--固换热器传热特性研究

在自制的固—固套管式换热器中,以催化裂化催化剂为原料,对系统的循环特性、传热系数进行了实验研究。分析了床层密度、提升风量、床层温度、催化剂粒径等因素对传热过程的影响。实验结果表明,随着提升风量增加,表观气速增大,床层密度下降,传热系数下降;提高床层温度,减小催化剂的粒径,传热系数增加。     

两段气升式气固环流取热器导流筒壁与床层间的传热特性研究

在一套φ600 mm×7000 mm半圆形的大型有机玻璃两段环流实验装置上,采用FCC催化剂,在导流筒区表观气速0.101～0.670 m/s,环形区表观气速分别为0.049 m/s和0.099 m/s范围内,测定了导流筒壁与颗粒床层之间的传热系数、床层密度和颗粒环流速度的分布规律.结果表明床层密度和颗粒环流速度是影响传热的主要因素,而这两者的变化规律完全由导流筒区和环形区的气速来调控.传热系数随导流筒区气速的增加而增加,适当增加环形区气速能显著改善传热效果.实验结果表明,两段环流取热器比目前工业催化裂化装置使用的外取热器有较高的传热系数.通过理论分析,给出了两段环流取热器的传热系数关联式.     

气固流化床外取热器内流动和换热特性分析

外取热器是维持催化裂化反应-再生系统热平衡和保持装置平稳运行的关键设备之一。外取热器的优化设计和合理调控，要求深入理解外取热器内的流动特性、换热特性及两者之间关系。在一套大型冷模热态实验装置上，分别考察了表观气速、颗粒质量流率对换热管附近的局部固含率和气泡频率、床层与换热管间传热系数的影响。结果表明：增加表观气速可以降低局部固含率、增加局部气泡频率、强化床层与换热管间换热；随着颗粒质量流率增加，局部固含率和局部气泡频率均增加；在较低表观气速下，增加颗粒质量流率不利于换热，而在较高表观气速下，传热系数随颗粒质量流率逐渐增加。不同流型下，气固流动特性对换热特性的影响不同。在鼓泡床流型下，过高的局部固含率不利于颗粒在换热表面的更新，增加换热管附近的局部气泡频率可以明显强化换热；而在湍流床流型下，换热管附近的局部固含率和气泡频率的增加，均使传热系数逐渐增大。建立了针对不同流型的换热经验关联式，预测值与实验值的平均相对偏差分别为6.9%和1.3%。     

离心流化床中气固传热特性的实验研究

本文使用玻璃细珠、黄砂和氧化铝球在不同操作条件下进行了离心流化床气固两相间传热特性的实验研究．测定了气体入口、出口和床层温度随时间的变化．分析了床层厚度、粒径、物料性质、床体转速和气体流速等因素对气固传热特性的影响．首次获得了离心流化床气固两相传热的准则方程．     

三相携带床的流体力学特性研究

研究了空气一水-黄沙三相系统在携带床反应器中的流体力学特性。反应器直径0．07m。实验考察了表观气速Ug、表观浆液流速UsL、固含率εs等因素对气含率εg和床层压降△P的影响以及三相携带床的操作特性。回归实验数据得到气含率及床层压降与各因素的关联式为εg=0.4084U△P=5783．672U研究结果为三相携带床工业反应器提供了流体力学依据。     

基于新型内热式移动-流化床干燥器的褐煤干燥过程

在传统流化床内,加入中空水平螺旋叶片,其内通入饱和蒸汽。对粒度分布很广的褐煤物料,借助气力和机械力联合作用,建立一种新型的内热式移动-流化床干燥模型。研究了操作气速、进口气体温度、机械力作用强度等因素对褐煤干燥过程的影响。采用因次分析的方法,建立了气固间传热系数关联式。实验结果表明,机械力场的加入,使床层内形成了稳定的大小颗粒共存的气固两相流场,并使干燥速率增大,传热系数增大。其中,叶片转速在3.33~5.40r/min范围时,传热系数增大了约35%。叶片内饱和蒸汽以内热源的形式,通过叶片传导换热,大大强化了传热和干燥效果。在其他条件不变的情况下,传热量增加了约45%。     

气-固流化床层与水平埋管壁面之间的传热

本文对气固流化床的床层与其中水平埋设的单管壁面之间的传热过程进行了传热机理的分析。进而将本文的实验结果与国外文献中所提出的传热系数关联式进行了比较。 本文实验中流化床层内埋设的水平单管(D_T=30毫米)采用了黄砂((?)_p=0.367毫米)与徐州煤((?)_p=1.355毫米)作为流化颗粒物料,对三种静止床高(H_o=35、73、90毫米),在不同流化质量速度(G=0.7～1.8公斤/米~2·秒)等情况下,进行了大量实验,整理后提出了可适用于黄砂、煤粒的关联式。 在有关工程设计计算中,采用本文推荐的关联式或Vreedenberg的关联式已够准确且较简便。     

INVESTIGATION ON THE INCIPIENT FLUIDIZATION AND HEAT TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Based on the continuum theory, a physical model of gas-solid two phase flow in a centrifugal fluidized bed has been proposed. A set of governing equations to describe the fluidization state are obtained and solved numerically after some simplifying. The quantitative experimental study on the characteristics of the incipient fluidization in the centrifugal fluidized bed is performed to examine the proposed model. Gas-solid two phase heat transfer in CFB during a drying process is also conducted. The influences of bed thickness, particle diameter, physical properties of particle, rotating speed of the bed and the gas superficial velocity on heat transfer characteristics are examined. A correlation that can be used to calculate the heat transfer coefficients in the drying process in CFB is obtained.     

Total and gas convective heat transfer from a vertical tube to a mixed particle gas—solid fluidized bed

Experimental data were obtained for the average gas convective and total heat transfer coefficients for a vertical tube immersed in an air-fluidized bed of narrowly as well as widely distributed particle size mixtures. The gas convective heat transfer coefficient was determined by measuring the rate of mass loss from a vertical naphthalene tube 0.0262 m in diameter and 0.1012 m in length and using a heat and mass transfer analogy. These data were obtained at a bed temperature of about 330 K and superficial velocity of 0.1 to 1.1 m/s. The total heat transfer coefficients were measured under identical conditions using an electrically heated vertical tube. The total heat transfer coefficient decreased with an increase in particle diameter from 0.237 to 1.35 mm. The addition of fines was found to increase the total heat transfer coefficient. The gas convective heat transfer coefficient increased with increase in particle size and fluidizing velocity. The dependence of the gas convective heat transfer coefficient on gas velocity was more pronounced for large particles. The addition of fines resulted in decrease in gas convective coefficient. The relative contribution of the gas convective component of heat transfer coefficient was found to increase with increase in particle diameter. Its dependency on fluidizing velocity was found to be more complex. The experimental data were compared with the existing heat transfer models and correlations.     

INVESTIGATION ON THE INCIPIENT FLUIDIZATION AND HEAT TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Abstract

Based on the continuum theory, a physical model of gas-solid two phase flow in a centrifugal fluidized bed has been proposed. A set of governing equations to describe the fluidization state are obtained and solved numerically after some simplifying. The quantitative experimental study on the characteristics of the incipient fluidization in the centrifugal fluidized bed is performed to examine the proposed model. Gas-solid two phase heat transfer in CFB during a drying process is also conducted. The influences of bed thickness, particle diameter, physical properties of particle, rotating speed of the bed and the gas superficial velocity on heat transfer characteristics are examined. A correlation that can be used to calculate the heat transfer coefficients in the drying process in CFB is obtained.     

鼓泡流化床内高灰煤气化过程的颗粒尺度特性研究

基于多相流体质点网格方法(MP-PIC)对高灰煤在三维鼓泡流化床气化过程进行了数值模拟研究。在欧拉-拉格朗日框架下将气相和固相分别视作连续介质和离散相处理。首先,将模拟得到的出口处气体组分结果与实验数据进行对比,实验数据与模拟结果具有良好的一致性。其次,研究了煤颗粒在气化炉内的温度、传热系数、速度和停留时间,从颗粒尺度揭示了鼓泡流化床气化炉内的颗粒分布特性和气固流动特征。结果表明:在气化炉入口附近煤颗粒与床层温差最大,传热系数最大;由于流化床内强非线性的气固流动,床中煤温度和传热系数的空间分布不均匀;煤颗粒和床料的瞬时速度具有稳定的波动幅度,其中垂直方向速度波动最明显,且煤颗粒的瞬时速度比床料的瞬时速度略大;由于颗粒间的剧烈碰撞,延长了煤颗粒停留时间。此外,对鼓泡流化床中煤气化过程颗粒尺度的研究,有助于深入了解固体颗粒的流动行为以及气固相相互作用特性,对鼓泡流化床反应器的设计优化具有重要意义。     

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.     

Modeling of grid region heat transfer in a shallow gas-solid fluidized bed

Heat transfer coefficients between the bed and an immersed horizontal tube in the grid-region of a shallow gas-solid fluidized bed were experimentally and theoretically studied. Experiments were carried out in two fluidized bed columns with inside diameters of 88 and 137 mm, respectively. The fluidized particles tested were sand, limestone and glass beads. Experimental parameters also included particle size, superficial gas velocity, tube diameter, tube location and distributor design. A mechanistic model considering the contributions of jet phase, emulsion phase and dead phase was derived for estimating the grid-region heat transfer coefficients. Most of the model predictions were found to be within 25% of the experimentally observed data.     

A particle-to-particle heat transfer model for dense gas-solid fluidized bed of binary mixture

A particle-to-particle collisional heat transfer model in the frame of Eulerian-Eulerian approach was proposed in this paper. By incorporating it into the multi-fluid model to close the enthalpy equations, the heat transfer between different particle classes in a gas bubbling fluidized bed of binary mixture was investigated, based on the CFD simulations of particle mixing in literature (Cooper and Coronella, 2005). The results showed that the particle-to-particle heat exchange coefficient between different particle classes increases with increasing the size of large particle class and the superficial gas velocity. The ratios of the particle-to-particle heat transfer to the gas-to-particle heat transfer range from 8.04% to 15.0% for various calculating conditions. In order to better understand the heat transfer behavior in a dense gas-solid fluidized of binary mixture, it is important to take the particle-to-particle heat transfer into account.     

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。