B类颗粒在鼓泡流化床中流动特性的数值模拟

采用欧拉双流体模型对鼓泡流化床中气-固两相流动行为进行数值模拟。模拟结果表明,采用结构曳力模型能够较好地预测B类颗粒在鼓泡流化床中的流动行为。对比初始流态化颗粒浓度图和完全流态化颗粒浓度分布图,可以发现结构曳力模型能够较好地展现鼓泡流化床中气泡的运动特性。当比较不同曳力模型下的模拟结果时,结构曳力模型比传统曳力模型能够更好地预测颗粒的径向分布。     

网格尺寸对循环流化床内气固两相流动的影响

将基于能量最小多尺度方法(EMMS)的曳力模型耦合到双流体模型中,并针对循环流化床内的气固两流动进行了模拟研究。采用全滑移壁面边界条件处理颗粒相,考察了3种网格尺度对轴向空隙率和出口颗粒循环量等气固流动特性的影响。计算结果表明,应用EMMS曳力模型处理相间作用力,同时在采用全滑移壁面边界条件处理颗粒相时,双流体模型能够正确预测轴向空隙率分布。采用网格尺寸为2.325 mm×20 mm时,模拟结果和实测数据吻合较好,表明在循环流化床的数值模拟中选择恰当的网格尺度是极为重要的。     

三维循环流化床气化炉气固流动全回路模拟

基于欧拉双流体模型和颗粒流动理论对实验室规模的循环流化床气化炉进行了全三维模拟,考察了炉内压力和固体颗粒的分布特征,并进一步比较了操作条件对炉内气固两相循环流动的影响差异。模拟结果表明:提升管内呈上稀下浓的颗粒分布,径向处存在边壁浓中心稀的环合结构,下降管内能够形成堆积较为密实的料封,模拟得到的系统压力环路与试验值较吻合。藏量、粒径和入口气速均会影响炉内的固体颗粒浓度和压力分布,合适的粒径和入口气速条件与颗粒终端速度的匹配是影响炉内稳定循环流化的关键因素。     

35t/h内循环流化床锅炉流动数值模拟

采用基于颗粒动力学理论的欧拉双流体模型,对35t/h内循环流化床锅炉炉膛的气、固流动特性进行了数值模拟。结果表明:炉膛压差的模拟值与实验值吻合较好,说明欧拉双流体模型能够准确地描述炉膛内气、固流动特性;颗粒体积分数和速度分布特性既揭示了内循环流化床中颗粒的内循环流动机理,又为内循环流化床大型化和流动结构优化提供了参考。     

烟气循环流化床脱硫数值模拟进展及现状分析

介绍了循环流化床半干法烟气脱硫过程气固两相流动和反应特征及其数值模拟方法。对两相流常用的离散颗粒模型、双流体模型的研究进展进行论述,对于四步骤的脱硫模型进行说明,并介绍了耦合流动模型与脱硫模型过程模拟的优缺点。提出对循环流化床脱硫进一步数值模拟的工作重点应集中在颗粒动力学、曳力模型的修正和反应过程细化模拟方面。     

循环流化床气固流动行为的数值模拟

采用欧拉-欧拉双流体模型,在Fluent数值模拟平台上对循环湍动流化床进行了研究。采用Gidaspow模型分段描述密相湍动和稀相循环输送区气固的相互作用,考察了反应器内压力、颗粒浓度等气固湍动特性参数的变化。计算流体力学(CFD)的模拟结果表明:反应器中压力分布较为均匀;固体颗粒相和气体相间隙成絮状分布,在轴向中心区域颗粒浓度分布较为均匀;边壁区域颗粒浓度变化较大,存在强烈的气固相互作用。     

循环流化床气固流动行为的数值模拟

采用欧拉-欧拉双流体模型,在Fluent数值模拟平台上对循环湍动流化床进行了研究。采用Gidaspow模型分段描述密相湍动和稀相循环输送区气固的相互作用,考察了反应器内压力、颗粒浓度等气固湍动特性参数的变化。CFD的模拟结果表明反应器中压力分布较为均匀;固体颗粒相和气体相间隙成絮状分布,在轴向中心区域颗粒浓度分布较为均匀;边壁区域颗粒浓度变化较大,存在强烈的气固相互作用。     

耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

循环流化床脱硫反应器气固两相流动模拟

为加深对循环流化床脱硫反应器的流动特性的认识,本研究采用双流体模型,耦合非均匀曳力模型和颗粒动力学理论,使用非均匀的修正的Syamlal O?Brien曳力模型考虑颗粒团聚现象对气固流动的影响,对循环流化床脱硫反应器中的流体力学特性进行了数值模拟。模拟得到的时均颗粒浓度和压降值与实验数据具有较好的一致性,验证了模拟方法的可靠性。颗粒速度的时间序列和概率分布函数显示,在反应器壁面附近区域均存在颗粒的向上、向下运动,在床层内颗粒总体上向上运动,同时还存在微观的内循环运动。模拟结果为颗粒速度的径向非均一分布提供了合理的解释,较Gidaspow模型有更好的适用性。     

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.     

提升管内气固流动行为的数值模拟

应用计算流体力学软件Fluent,对空气为连续相、固相为催化裂化反应催化剂的循环流化床提升管内的气固流动行为进行模拟。采用用户自定义函数引入颗粒与壁面的恢复系数和颗粒的镜面反射系数,对颗粒在边壁处的部分滑移运动进行描述。采用不同的计算动力学模型及参数,数值模拟了径向颗粒浓度、轴向床层压降的空间分布,以及用以描述颗粒脉动动能的颗粒温度与固含率的关系,并与文献报道的实验和数值模拟结果进行对比分析。结果表明,选取的颗粒动力学理论模型及参数、颗粒部分滑移边界条件及气固曳力模型,可计算得到合理的颗粒轴向及径向分布,验证了提升管中存在典型的径向环核流动结构和轴向压降分布。进一步分析表明固含率显著影响颗粒温度,当固含率为0.05~0.1,颗粒温度存在转折区。     

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.     

基于颗粒流运动论的喷动流化床气固流动行为三维模拟

1 INTRODUCTION Spout-fluid beds have been of increasing interest in the petrochemical, chemical and metallurgic indus-tries since spout-fluid beds can reduce some of the limitations of both spouting and fluidization by su-perimposing the two type of systems[1―4]. In recent years, spout-fluid beds have become an alternative for gas/solid contactors in coal gasification. Spout-fluid bed coal gasifiers have been adopted for APFBC-CC (advanced pressurized fluidized bed combus-tion-combined…     

Geldart A类颗粒节涌床气固流动特性的实验及模拟

针对气固节涌床,在实验基础上,基于欧拉?欧拉双流体模型结合颗粒动力学理论,考虑Geldart A类颗粒聚团对气固间曳力的影响,采用修正后的Gidaspow曳力模型对气固节涌床进行数值模拟。结果表明,通过与实验结果及经验公式进行对比,修正的模型可准确合理地模拟流化床内节涌特性。表观气速0.09 m/s≤Ug≤0.39 m/s时,床层内部压力脉动标准偏差随表观气速增加而增加,流型由鼓泡转变为节涌直至节涌程度最大,床内气固流动主要受轴对称栓运动特性影响,床内压降、床层膨胀比、气栓平均上升速度、最大轴对称栓长度随表观气速增加而增加,最大轴对称栓产生位置随表观气速增加而降低;Ug>0.39 m/s后,床内压力脉动标准偏差随表观气速增加而降低,节涌程度降低至向湍动流态化流型转变,床内气固流动主要受壁面栓运动特性影响,增加表观气速,节涌床内压降变化幅度较小,气栓平均上升速度增加幅度加大,床层膨胀比及最大轴对称栓长度降低,最大轴对称栓产生的位置略有升高。     

CFB中环核结构的模拟

In this paper, the stochastic particle-trajectory model is proposed for simulating the dynamic behavior of circulating fluidized bed (CFB). In our model, the motion of solid phase is obtained by calculating the individual particle trajectory while gas flow is obtained by solving the Navier-Stokes Equation including two-phase interaction. For the calculation of solid phase, the motion of each particle is decomposed into a collision process and a suspension process. In suspension process, the less important and/or unclear forces are described as a random force considering gravity, drag force and pressure gradient. As a result, the proposed model gives some numerical simulations of CFB. It indicates that the stochastic particle-trajectory model can be used to simulate qualitatively the annulus-core structure of CFB and the influences of stochastic factors cannot be ignored. In a CFB, the coupling of stochastic factors between two phases makes the radial voidage decreased. Moreover, the upward motion of particles is mitigated by both stochastic factors and turbulence between two phases.     

气固循环流化床全回路数值模拟研究进展

气固循环流化床具有良好的混合、传热、传质、反应特性,同时还具有处理量大、可连续生产等优点,在众多领域均有广泛应用。气固循环流化床是一个由多个单元连接组合形成的循环回路,各单元间相互耦合、相互影响。对整个循环流化床系统进行全回路数值模拟,不仅能够获得更全面和详实的结果,而且在揭示系统流动规律和探究各单元内、单元间、单元与系统间的相互作用上具有独特优势。近十年来,以气固循环流化床全系统为模拟对象的全回路数值模拟研究逐渐兴起。本文对气固循环流化床全回路数值模拟方法的研究进展进行综述,对该方法的应用情况进行详细介绍,并对方法中采用的模型及相应特点进行逐一分析。循环流化床全回路系统同时存在多种流态,有待于建立适用于全回路系统的多流态物理模型（气固曳力模型与固相应力模型）。随着计算能力的提高以及物理建模的不断发展,全回路模拟方法将不断完善并发挥出更大的作用。     

采用改进的曳力模型模拟2D鼓泡流化床的流化特性

针对特定颗粒浓度范围颗粒团聚作用导致的曳力下降问题,基于传统曳力模型在不同颗粒浓度段的特性分析,选择适用于不同颗粒浓度区间的曳力模型,通过光滑函数得到改进的曳力模型,并耦合欧拉双流体模型对2D鼓泡流化床进行数值模拟. 结果表明,与Gidaspow和Syamlal-O'Brien模型相比,改进的曳力模型对床层局部压降的预测结果更好;随表观气速增加,改进的曳力模型能更准确地预测床层膨胀;当表观气速Ug=0.46 m/s时,改进的曳力模型对径向颗粒浓度分布的模拟结果明显好于Syamlal-O'Brien模型.     

Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009