颗粒特性对撞击分离器性能影响的实验与数值研究

颗粒-壁面撞击行为和气固相间作用对撞击分离器的性能具有重要影响。基于玻璃珠及煤粉的单颗粒撞击实验数据建立平均撞击恢复系数模型,采用非球形颗粒曳力模型对平板式撞击分离器的分离性能和气固流动开展数值研究。结果表明,采用基于实验的平均恢复系数模型以及考虑颗粒形状的曳力模型,能够准确地预测撞击式分离器的总分离效率和分级分离效率。颗粒分离过程中,Stokes数较大的颗粒对颗粒-壁面撞击模型比较敏感,Stokes数较小的颗粒对气固曳力模型比较敏感。     

颗粒特性对撞击分离器性能影响的实验与数值研究

颗粒-壁面撞击行为和气固相间作用对撞击分离器的性能具有重要影响。基于玻璃珠及煤粉的单颗粒撞击实验数据建立平均撞击恢复系数模型,采用非球形颗粒曳力模型对平板式撞击分离器的分离性能和气固流动开展数值研究。结果表明,采用基于实验的平均恢复系数模型以及考虑颗粒形状的曳力模型,能够准确地预测撞击式分离器的总分离效率和分级分离效率。颗粒分离过程中,Stokes数较大的颗粒对颗粒-壁面撞击模型比较敏感,Stokes数较小的颗粒对气固曳力模型比较敏感。     

稠密同轴气固射流的CFD模拟

同轴气固射流在能源领域具有广泛地应用,但大多数研究集中在颗粒浓度较低的工况。为了研究稠密同轴气固射流的流动特性,采用了稠密离散相模型(DDPM)耦合离散元模型(DEM)的方法对该体系进行计算流体动力学(CFD)模拟,该方法同时考虑了孔隙率对气固曳力的影响和颗粒间的碰撞作用。由于射流过程中,气体对颗粒的作用占主导,分别考虑了不同环形气体速度和气固曳力模型对气固流动的影响。模拟结果表明,该模型能合理地模拟在不同气速下稠密气固两相射流的颗粒弥散特性,与实验现象定性一致。在较高气速下,引入湍流模型对预测结果有显著影响,模拟得到的颗粒弥散程度较大。不同气固曳力模型对颗粒弥散的预测有明显影响,WenYu曳力模型下颗粒弥散程度较大,Gidaspow模型次之,SyamlalO’Brien模型给出的颗粒弥散程度较小。     

锥形分布板射流流化床CFD模拟及参数分析

采用双流体模型结合颗粒动力学理论,对锥形分布板射流流化床内气固流动行为进行了三维的计算流体力学(CFD)模拟研究,系统分析了曳力模型、恢复系数和颗粒间摩擦力对射流流化床膨胀高度和气泡动力学行为的影响.结果表明,Syamlal-O'Brien和Gidaspow曳力模型低估了床内实际曳力,从而导致模拟膨胀高度低于实验值,而Modified Syamlal-O'Brien曳力模型能更好地预测床内实际曳力,计算结果与实验值吻合得较好;恢复系数对于射流流化床内气泡动力学行为有着重要影响,气泡的大小、上升速率和产生频率均随着恢复系数的增加而减小;颗粒间的摩擦力也是影响床层膨胀高度和床内气泡产生的重要因素.     

非均匀气固流态化系统中颗粒流体相间作用的计算

曳力系数是双流体模型模拟气固两相流动的关键参数.文献中应用的关联式都基于平均方法,不再适用于模拟快速流态化系统的非均匀流动结构.本文试图阐明非均匀结构对曳力系数的影响,应用改进的能量最小多尺度模型提出一种计算微元体曳力系数的新方法.计算结果表明应用该模型计算出的曳力系数远小于基于平均方法关联式的计算结果,符合实验得出的结论.     

壁面条件对FCC颗粒湍动床气固流动影响的模拟

采用基于颗粒动力学的双流体模型对FCC颗粒湍动床和挡板床内气固流动进行模拟研究,考察颗粒相壁面边界条件对其气固流动的影响。研究结果表明,采用Johnson-Jackson壁面边界条件描述壁面与颗粒的相互作用时,壁面反射系数对气固流动行为影响较大,而颗粒-壁面碰撞恢复系数影响甚微。通过将模拟结果与实验数据对比,确定壁面反射系数取0.001时能较好地描述壁面与颗粒间的作用力。对于挡板床,挡板的加入增大了壁面面积,壁面对颗粒的作用力更加显著,挡板壁面采用部分滑移条件时,穿过导向挡板向上和向下运动的颗粒数量均较大于采用无滑移条件时的结果。挡板壁面的光滑程度对其在床层中的作用有一定影响。     

垂直气固两相流动体系中气固两相之间的协同效应探讨

垂直气固两相流动体系中,气体与颗粒之间的相互作用在一定的颗粒浓度范围内存在协同效应.这种协同效应的表现形式是颗粒群中每个颗粒的平均曳力系数CD与单颗粒曳力系数CDs之比CD/CDs＜1.0,协同效应的大小可以用准数Sy=1/(CD/CDs)描述.气固两相之间产生协同效应的微观机制是颗粒之间存在着团聚,这种颗粒团聚使气固两相流动消耗的能量降低,此时气固两相之间的动量和能量传递以能量消耗为最小的方式进行.     

垂直气固两相流动系统中颗粒群与气流之间的相互作用

本文对垂直气固两相流动系统中,从初始流态化到气力输送的整个区域内气固两相之间的相互作用规律进行了研究.结果表明,采用气体与颗粒群之间的曳力系数可以定量表征两者之间的能量交换过程.文章并讨论了不同操作条件下气固曳力系数的变化规律,并由此可见两相流系统中出现的不均匀状态(低气速时产生气泡,高气速时颗粒趋于聚集)均可用体系为使气流对颗粒群做功最小而采取的运动形态来解释.     

基于气泡和聚团的结构模型及其应用

综合考虑鼓泡流化床内气泡及聚团对床内细颗粒流动的影响,建立基于气泡和聚团的结构曳力模型及结构参数模型,同时,借助计算流体软件预测细颗粒在鼓泡床中流动状态。首先,基于细颗粒在鼓泡流化床的流动状态,在介观尺度上将床内气固流动结构划分为3个子结构,即气泡相、相间相及乳化相(聚团相)。然后,综合考虑细颗粒鼓泡流化床中气泡和聚团对气固流动的影响,根据力平衡、质量守恒建立基于气泡和聚团的结构参数模型及结构曳力模型。通过对结构参数模型封闭求解,得到11个结构参数值(f_b,U_b,d_b,U_(gb),f_i,U_(gi),ε_i,f_e,U_(ge),ε_e,d_c)。对结构参数计算结果进行分析,结构参数模型能够很好反映床内流动情况及床内各结构参数之间的关系,并能有效地预测颗粒聚团直径。此外借用非均匀因子,耦合结构曳力模型及结构参数模型到欧拉双流体模型对气固在床内流动行为进行数值模拟。模拟结果表明,使用基于气泡和聚团的结构曳力模型能够较好地预测细颗粒在鼓泡流化床中的流动行为。在对模拟结果中颗粒径向浓度比较时,可以发现,相对比基于气泡模型的结构曳力模型,使用基于气泡和聚团的结构曳力模型的模拟结果与实验结果更一致。     

分区曳力模型的2D鼓泡流化床流动特性模拟

曳力对两相流动特性具有很大影响,由于颗粒团聚的影响,传统曳力模型不能准确预测气固间曳力.为了提高两相流动特性的模拟精度,文中综合考虑不同曳力模型在特定浓度的曳力系数特性,分区确定曳力模型,并在模型断点处使用光滑函数,基于CFD开源平台OpenFOAM耦合TFM模型对2D鼓泡流化床进行流动特性模拟.结果表明:分区曳力模型...     

DEM simulation of gas-solid flow behaviors in spout-fluid bed

Three-dimensional gas and particle turbulent motions in a rectangular spout-fluid bed were simulated. The particle motion was modeled by discrete element method and the gas motion was modeled by k-ε two-equation turbulent model. Shear induced Saffman lift force, rotation induced Magnus lift force as well as drag force, contract force and gravitational force acting on individual particles were considered when establishing the mathematics models. A two-way coupling numerical iterative scheme was used to incorporate the effects of gas-particle interactions in volume fraction, momentum and kinetic energy. The gas-solid flow patterns, forces acting on particles, the particles mean velocities, jet penetration depths, gas turbulent intensities and particle turbulent intensities were discussed. Selected stimulation results were compared to some published experimental and simulation results.     

Gas-particle interactions in dense gas-fluidized beds

The occurrence of heterogeneous flow structures in gas-particle flows seriously affects gas–solid contacting and transport processes in dense gas-fluidized beds. A computational study, using a discrete particle method based on Molecular Dynamics techniques, has been carried out to explore the mechanisms underlying the formation of heterogeneous flow structures. Based on energy budget analyses, the impact of non-linear drag force on the flow structure formation in gas-fluidized beds has been examined for both ideal particles (elastic collision, without inter-particle friction) and non-ideal particles (inelastic collision, with inter-particle friction). Meanwhile, the separate role of inter-particle inelastic collisions, accounted for in the model via the restitution coefficient (e) and friction coefficient (μ), has also been studied.It is demonstrated that heterogeneous flow structures exist in systems with both non-ideal particle-particle interaction and ideal particle-particle interaction. The heterogeneous structure in an ideal system, featured with looser packing, is purely caused by the non-linearity of the gas drag: the stronger the non-linearity of the gas drag force with respect to the voidage, the more heterogeneous flow structures develop. A weak dependence of drag on the voidage produces a homogenous flow structure. Collisional dissipation dramatically intensifies the formation of heterogeneous flow structures after the system equilibrium breaks. Quantitative comparisons of flow structures obtained by using various drag correlations in literature will also be reported.     

D类颗粒节涌流态化的实验和数值模拟

应用双流体气固两相流模型,对均一粒径分布的Geldart D类颗粒的节涌流态化过程进行了模拟. 研究了流化过程中流化状态转变、床层膨胀比以及压力脉动功率谱,从Froude准数和颗粒动能的角度分析了模拟结果中的流化特性,并与实验数据进行了对比. 模拟结果准确预测了Geldart D类颗粒的节涌流态化特性,对床层膨胀高度和功率谱的预测与实验基本吻合. Froude准数和颗粒动能随时间的周期性脉动较好地反映了流化床内节涌流态化的气泡行为.     

Modeling of cluster structure-dependent drag with Eulerian approach for circulating fluidized beds

Flow behavior of gas and solids is simulated in combination the gas-solid two-fluid model with a cluster structure-dependent (CSD) drag coefficient model. The dispersed phase is modeled by a Eulerian approach based upon the kinetic theory of granular flow (KTGF) including models for describing the dispersed phase interactions with the continuous phase. The drag forces of gas-solid phases are predicted from the local structure parameters of the dense and dilute phases based on the minimization of the energy consumed by heterogeneous drag. The cluster structure-dependent (CSD) drag coefficients are incorporated into the two-fluid model to simulate flow behavior of gas and particles in a riser. Simulation results indicate that the dynamic formation and dissolution of clusters can be captured with the cluster structure-dependent drag coefficient model. Simulated solid velocity and concentration of particles profiles are in reasonable agreement with experimental results.     

粗糙颗粒碰撞参数对鼓泡流化床内气固流动模拟的影响

结合粗糙颗粒动力学理论和双流体方法,数值模拟了碰撞参数对鼓泡流化床内稠密气固两相流动特性的影响. 结果表明,增大摩擦系数或减小法向弹性恢复系数会使床内颗粒分布更为不均,并增强床层膨胀及压力降脉动. 合理选取摩擦系数模拟得到时均气固流场分布,与实验吻合,罂粟籽颗粒的摩擦系数取0.3~0.6较合适. 法向弹性恢复系数改变不影响时均气固流场分布的基本形态,其取值敏感性不如摩擦系数;切向弹性恢复系数对鼓泡流化床动态特性及时均气固流场的影响相对较弱.     

一个气固两相流动阻力的新模型

为了解决现有经验气固阻力模型的普适性问题,合理描述颗粒团聚现象对气固阻力的严重影响,从理论分析入手,将传统的CFD方法与系统能量分析方法相结合,建立了计及颗粒团聚效应的气固阻力分析模型.与现有模型相比,新模型不仅合理地描述了气固两相相互作用的物理过程,而且避免了以往采用经验系数所导致的误差和局限性.经循环流化床数值模拟的检验证明,新模型的计算结果与实验数据吻合,从而在稠密气固两相流动的数值模拟中具有相当的优越性.     

磷石膏颗粒湍动流化特性实验及模拟

针对磷石膏颗粒湍动流化体系曳力变化的问题,在实验的基础上,考虑非均匀结构对曳力的影响,引入修正因子φ修正Gidaspow曳力模型,对2D流化床进行了数值模拟研究。通过将Gidaspow模型在不同φ值下的模拟结果与实验结果进行对比,研究φ值的改变对模拟结果的影响规律及一定气速范围内磷石膏颗粒湍动流化体系曳力变化特性。结果表明,Gidaspow模型高估了实验体系曳力,对体系流化特性的预测效果较差;适当φ值的引入能明显提高Gidaspow模型对床层膨胀、压降及体系非均匀度的模拟精度。模拟结果反映出φ值越小,床层膨胀高度越低,床内颗粒浓度分布越不均匀,床层压降波动性越大。随着气速的升高(0.144~0.240 m/s),颗粒沿水平方向上的聚集程度加剧,φ值呈非线性减小(0.31~0.24)。流化体系的非均匀度随着气速增加而增大,颗粒浓度沿径向存在较大梯度,两侧边壁处附近出现环-核结构且流场分布对称性较差。     

颗粒旋转对鼓泡流化床内气固两相流动特性影响的数值模拟

运用考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散影响的颗粒动理学方法,建立鼓泡流化床气固两相Euler-Euler双流体模型,数值模拟流化床内气体颗粒两相流动特性。分析表明,颗粒平动温度与旋转温度之比是法向和切向颗粒弹性恢复系数和摩擦系数的函数。与不考虑颗粒旋转效应计算结果相比,考虑颗粒旋转效应后床内较容易形成气泡,颗粒自旋转运动将导致床内非均匀结构更明显。并且床层平均空隙率和床层膨胀高度增加,床中心区域颗粒轴向速度提高,床内颗粒平动温度下降。考虑颗粒旋转效应后预测的颗粒轴向速度和颗粒脉动速度与文献实验结果基本吻合。考虑颗粒旋转效应后获得的气泡直径更接近于前人经验关联式。     

气-固环流反应器内瞬态流体力学特性的数值模拟

采用双流体模型和颗粒动力学理论,并考虑颗粒团聚现象对气固相间曳力的影响,对气-固环流反应器内的流体力学特性进行了数值模拟。模拟的时均固含率和颗粒速度与实验数据具有较好一致性,验证了模拟方法的可靠性。模拟结果表明：气-固环流反应器内瞬态固含率的分布具有典型聚式流态化的非均匀特征;压力脉动沿床层轴向的分布在一定程度上定性反映了气泡运动的信息;颗粒速度的时间序列和概率密度分布函数表明,床层各径向位置均存在颗粒的向上、向下运动,颗粒主体在床层内向上运动的同时还存在微观的内循环运动,模拟值为颗粒时均速度的径向非均一性宏观分布提供了合理的微观解释。     

Hydrodynamic modelling of dense gas-fluidised beds: comparison and validation of 3D discrete particle and continuum models

A critical comparison of a hard-sphere discrete particle model, a two-fluid model with kinetic theory closure equations and experiments performed in a pseudo-two-dimensional gas-fluidised bed is made. Bubble patterns, time-averaged particle distributions and bed expansion dynamics measured with a nonintrusive digital image analysis technique are compared to simulation results obtained at three different fluidisation velocities. For both CFD models, the simulated flow fields and granular temperature profiles are compared. The effects of grid refinement, particle-wall interaction, long-term particle contacts, particle rotation and gas-particle drag are studied. The mechanical energy balance for the suspended particles is introduced, and the energy household for both CFD models is compared. The most critical comparison between experiments and model results is given by analysis of the bed expansion dynamics. Though both models predict the right fluidisation regime and trends in bubble sizes and bed expansion, the predicted bed expansion dynamics differ significantly from the experimental results. Alternative gas-particle drag models result in significantly different bed dynamics, but the gap between model and experimental results cannot be closed. In comparison with the experimental results, the discrete particle model gives superior resemblance. The main difference between both CFD models is caused by the neglect of particle rotation in the kinetic theory closure equations embedded in the two-fluid model. Energy balance analysis demonstrates that over 80% of the total energy is dissipated by sliding friction. Introduction of an effective restitution coefficient that incorporates the additional dissipation due to frictional interactions significantly improves the agreement between both models.