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。     

蛇管式换热器传热性能的数值模拟

利用Fluent对光滑蛇管换热器进行了模拟,将得出的结果与经典理论比较,发现数值模拟方法具有相当的可靠度,并在相同外部条件下对4种不同结构的波纹式蛇管进行模拟,得出湍流状态下管内流体的温度场和速度场,从微观上说明了波纹管强化传热机理。分析了波纹高度对波纹式蛇管传热效率的影响。     

鼓泡流化床风室及分布板区域流动特性的数值模拟

采用计算流体力学(CFD)软件对采用切向进风和垂直进风情况下不同开孔率气体分布 板的鼓泡流化床流态化特性进行研究,模拟风室及分布板区域的气体流动轨迹及其速度分布变化.结果表明:气体在切向进风形式下接近层流运动的上升状态,与垂直进风形式所产生的无序湍流上升状态相比,垂直进风形式下气体速度在空间分布更加均匀,受边界条件影响所导致的风室内局部风速过高情况较少;分布板开孔率在0.15％～1.0％变化时,随开孔率增大,气体压降逐渐减小,气体均匀性逐渐稳定;气体分布板开孔率为0.5％时,气体分布均匀性和分布板压降较合理.     

鼓泡流化床离散模拟中的一个局部空隙率模型

气固流化床离散颗粒模拟中通常采用面积加权平均法计算局部空隙率,不能较好地反映流化床中显著的非均匀结构特性.为了考虑非均匀结构对局部空隙率的影响,文中提出一个适用鼓泡流化床的局部空隙率模型.将流场划分为稀区(气泡区)和密区(乳相区);非均匀结构对密区局部空隙率的影响通过引入局部空隙率下限和非均匀影响系数描述;将提升管流动...     

基于LBM-DEM的鼓泡床内气泡-颗粒动力学数值模拟

将修正后的格子Boltzmann方法（LBM）与离散单元法（DEM）相结合,建立LBM-DEM四向耦合模型对单口射流鼓泡床中气泡运动进行模拟。其中,流体相采用格子Boltzmann方法中经典的D2Q9模型,颗粒相求解采用离散单元软球模型,颗粒曳力求解采用Gidaspow模型,流固耦合基于牛顿第三定律。应用Fortran语言编程对上述模型进行求解,模拟得到了鼓泡床内气泡演化过程,并与相关实验进行对比,有效验证了当前模型的准确性。同时,分析了床层内颗粒速度、颗粒体积分数以及能量分布。结果表明：颗粒时均速度分布不仅能体现颗粒运动强弱,也可以反映气泡运动过程;床内空隙率与颗粒体积分数分布在预测床层膨胀高度上具有高度的一致性;初始堆积效应使得床内颗粒势能始终大于颗粒动能;随颗粒密度增加,势能增大,动能逐渐减小。     

鼓泡流化床中气泡行为的模拟

应用气体拟颗粒模型模拟了单孔、双孔分布板流化床的鼓泡现象,动态地显示了气泡生成、上升、长大以及破裂的全过程,结果与已知的实验观测基本一致.     

鼓泡塔中水合物法分离混合气体的数值模拟

To develop a new technique for separating gas mixtures via hydrate formation,a set of medium-sized experimental bubble column reactor equipment was constructed.On the basis of the structure parameters of the ex- perimental bubble column reactor,assuming that the liquid phase was in the axial dispersion regime and the gas phase was in the plug flow regime,in the presence of hydrate promoter tetrahydrofuran（THF）,the rate of hydrogen enrichment for CH4＋H2 gas mixtures at different operational conditions（such as temperature,pressure,concentra- tion of gas components,gas flow rate,liquid flow rate）were simulated.The heat product of the hydrate reaction and its axial distribution under different operational conditions were also calculated.The results would be helpful not only to setting and optimizing operation conditions and design of multi-refrigeration equipment,but also to hydrate separation technique industrialization.     

不同管径水平管气液两相流流动特性数值模拟

为更好了解不同管径水平管气液两相流的压降特性,选用空气和水在管径分别为38.1,50.8,101.6 mm,长为9.525 m的水平管内进行气液两相流数值模拟,并结合理论分析了不同管径下水平管气液两相流压降的变化规律.利用Fluent软件研究结果表明:在同一管径下,当液速一定时,压降随气速的增大而增大;气速恒定时,压降...     

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...     

内置扭带换热管数值模拟及结构参数优化研究

采用数值模拟方法对内插扭带式换热管换热过程进行模拟,分析了管内流动与传热特征,研究了扭带尺寸参数对管内流动与传热的影响。结果表明,随扭带余隙率增大,传热系数逐渐减小,传热性能降低,但过小的余隙率会增大摩擦阻力。传热系数随扭距增加先增大后减小,阻力系数随扭距增加逐渐减小。综合考虑扭带参数对传热与流动的影响,对换热管内置扭带的结构参数进行了优化。     

DEM-LES study of 3-D bubbling fluidized bed with immersed tubes

Based on Euler-Lagrange frame, a true three-dimensional numerical simulation of bubbling fluidized bed embedded with two immersed tubes is presented. The solid phase is composed of 178,200 particles of diameter and simulated by discrete element method (DEM, a soft-sphere approach). The gas phase is computed through solving the volume-averaged four-way coupling Navier-Stokes equations in which the Smagorinsky SGS tensor model is used in large eddy simulation (LES). Particle-tube collision is particularly treated as a transformation of DEM. The volume segmentation of a particle sphere for void fraction calculation is solved via a numerical sub-division approach. The numerical results are compared with the experimental results for validation. The results obtained with and without the LES model are also compared. The numerical results show a strong correlation between gas-particle interaction, particle-particle interaction, pressure drop, particle back mixing motion and bubble motion, and all of them follow a similar pattern of synchronous periodic variation though the periodicity may vary depending on different flow conditions. The effects of SGS tensor on evolution of fluidized bed are found in various aspects. Finally, the distribution of particle-tube impact frequency is given.     

静电效应对有无埋管气固鼓泡床内气泡特性的影响分析

采用基于双流体模型(TFM)耦合静电模型的方法,研究颗粒的静电对有无埋管气固鼓泡床内气固流动特性和气泡特性的影响。首先在无静电场存在的条件下,利用双流体模型对自由鼓泡床和埋管鼓泡床内的流动情况进行模拟并与实验结果进行对比;进一步耦合静电模型,考察静电对自由鼓泡床和埋管鼓泡床内床层的整体性质和气泡特性的影响。研究结果表明,在无静电场条件下采用双流体模型能较好地预测自由鼓泡床和埋管鼓泡床内的气固流动状况以及气泡的平均直径和气泡的上升速度。埋管的存在使鼓泡床内气固流动发生强烈扰动,并使气泡的平均直径和气泡的上升速度均呈振荡分布。静电的存在对自由鼓泡床和埋管鼓泡床内床层的平均固含率影响不大,但对气泡分布规律影响较大,使得自由鼓泡床内气泡数目减少,而埋管鼓泡床下部区域的气泡分布比较集中,上部有大气泡出现。     

Particle-scale simulation of fluidized bed with immersed tubes

In order to simulate gas-solids flows with complex geometry, the boundary element method was incorporated into the implementation of a combined model of computational fluid dynamics and discrete element method. The resulting method was employed to simulate hydrodynamics in a fluidized bed with immersed tubes. The transient simulation results showed particle and bubble dynamics. The bubble coalescence and break-up behavior when passing the immersed tubes was successfully predicted. The gassolid flow pattern in the fluidized bed is changed greatly because of the immersed tubes. As particles and gas are come in contact with the immersed tubes, the gas bubbles will be deformed. The collisions between particles and tubes will make the tubes surrounded by air pockets most of the time and this is unfavorable for the heat transfer between particles and tubes. __________ Translated from Chemical Engineering, 2007, 35(11): 21–24 [译自:化学工程]     

沉浸管式流化床的颗粒尺度模拟

为模拟具有复杂几何结构的气固流动系统,文中将计算流体力学和离散单元法与边界元方法结合起来,对沉浸管式流化床内颗粒及气泡的运动行为进行了数值模拟。模拟计算得到的瞬态流型图揭示了气泡绕流沉浸管束时出现的合并和破碎状态及颗粒群的详细运动行为,发现床内气固二相的流动受到沉浸管束存在的显著影响。当颗粒及气相的流动受到沉浸管的阻碍而绕管流动过程中气泡会发生变形,变得扭曲狭长且易被撕碎。同时颗粒与管道壁面碰撞会造成气固二相复杂的动态运动形式,床内的管道大部分时间会被气穴包围,将严重阻碍管道与颗粒之间的传热。     

Particle-scale simulation of fluidized bed with immersed tubes

In order to simulate gas-solids flows with complex geometry, the boundary element method was incorporated into the implementation of a combined model of computational fluid dynamics and discrete element method. The resulting method was employed to simulate hydrodynamics in a fluidized bed with immersed tubes. The transient simulation results showed particle and bubble dynamics. The bubble coalescence and break-up behavior when passing the immersed tubes was successfully predicted. The gas-solid flow pattern in the fluidized bed is changed greatly because of the immersed tubes. As particles and gas are come in contact with the immersed tubes, the gas bubbles will be deformed. The collisions between particles and tubes will make the tubes surrounded by air pockets most of the time and this is unfavorable for the heat transfer between particles and tubes.     

Three-dimensional numerical simulation of a horizontal rotating fluidized bed

Three-dimensional numerical simulations of a horizontal rotating fluidized bed (RFB) containing glass bead particles (d_s = 82 μm, ρ_s = 2450 kg/m³) and washed alumina (d_s = 89 μm, ρ_s = 1550 kg/m³) were performed. FLUENT 6.1 software was used to carry out our simulation. The numerical results were compared with the experimental data of Qian and Pfeffer et al. [G.H. Qian, I. Bagyi, I.W. Burdick, R. Pfeffer, H. Shaw, Gas-Solid Fluidization in a Centrifugal Field.” AIChE J. 47 (5) (2001) 1022-1034]. The rotating speed of the RFB was set at 325 rpm (34 rad/s), which is equivalent to a centrifugal acceleration of 7 g.The flow behavior of the solid particles was analyzed; the bed thickness and the calculated pressure drop were compared with the experimental results. Our calculated pressure drop agreed very well with the experimental results.     

Effect of annular fins on heat transfer of a horizontal immersed tube in bubbling fluidized beds

Experiments were conducted in a bubbling air-fluidized bed to investigate the effect of annular fins of constant thickness on heat transfer. Steady state time averaged local heat transfer coefficient measurements were made by the local thermal simulation technique in a cold bubbling fluidized bed (90 mm ID, 260 mm tall) with horizontally immersed tube initially with no fin and then with three fixed annular fins of constant thickness. Silica sand of mean particle diameter 307 μm and 200 μm were used as the bed materials. The superficial velocity of air was from minimum fluidization conditions, u_mf, to approximately 3 × u_mf. The results indicate that, although the heat transfer coefficient falls with the use of fins, the total heat transfer rises as a result of the greater surface area. Increasing the particle diameter reduces the heat transfer coefficient not only for unfinned horizontal tube but also for annular finned horizontal tube at the same conditions of fluidized bed. Based on the experimental data, correlations are proposed for predicting heat transfer coefficient from fluidized bed to horizontally immersed tubes with and without fins.     

EMMS-based Eulerian simulation on the hydrodynamics of a bubbling fluidized bed with FCC particles

Although great progress has been made in modeling the bubbling fluidization of Geldart B and D particles using standard Eulerian approach, recent studies have shown that suitable sub-grid scale models should be introduced to improve the simulation on the hydrodynamics of Geldart A particles. In this study, the flow structures inside a bubbling fluidized bed of FCC particles are simulated in an Eulerian approach employing the energy minimization multi-scale (EMMS) model (Chemical Engineering Science, 2008, 63: 1553-1571) as the sub-grid scale model for effective inter-phase drag force, using an implicit cluster diameter expression. It was shown that the experimentally found axial and radial solid concentration profiles and radial particle velocity profiles can be well reproduced.     

Computational study of heat transfer in a bubbling fluidized bed with a horizontal tube

A combined approach of discrete particle simulation and computational fluid dynamics is used to study the heat transfer in a fluidized bed with a horizontal tube. The approach is first validated through the good agreement between the predicted distribution and magnitude of local heat transfer coefficient with those measured. Then, the effects of inlet fluid superficial velocity, tube temperature and main particle properties such as particle thermal conductivity and Young's modulus are investigated and explained mechanistically. The relative importance of various heat transfer mechanisms is analyzed. The convection is found to be an important heat transfer mode for all the studied conditions. A large convective heat flux corresponds to a large local porosity around the tube, and a large conductive heat flux corresponds to a large number of particle contacts with the tube. The heat transfer is enhanced by the increase of particle thermal conductivity while it is little affected by Young's modulus. Radiative heat transfer becomes increasingly important as the tube temperature is increased. The results are useful for temperature control and structural design of fluidized beds. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Numerical simulation of pulsed fluidized bed with immersed tubes using DEM-LES coupling method

The present work is a 2-D numerical simulation of pulsed fluidized bed with immersed tubes using DEM-LES coupling method. The pulsed inflow of gas phase is modeled as U₀(1+sin(2πft)), in which four pulsating frequencies of f=5, 10, 15 and 20 of velocity inflow are used. The discrete element method (DEM) simulation for particle motion coupled with the large eddy simulation (LES) for gas phase is used. The fluidized bed with five immersed tubes of staggered arrangement and six immersed tubes of in-line arrangement is simulated, respectively. It is found that the pressure drop, the mean drag force and the mean pressure gradient force experienced by particles are forced oscillated. The different effects of pulsed fluidization on the circumferential distribution of particle-tube collision on the outer surface of tubes at different pulse frequencies and modes of arrangement of immersed tubes are numerically analyzed. Finally, it is found that the pulsed motion of fluid with high frequency leads to suppression of particle fluctuating motion.