非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

气-液-固磁稳定床研究进展

综述了气-液-固磁稳定床的研究进展,涉及流体力学特性、传质过程、返混与扩散等.兼有固定床和流化床优点的磁稳定床具有压降低,传质系数高等优点,已在石油、化工及环境等领域获得了应用.指出今后应加强气-液-固磁稳定床的传热特性与传热机理、传质机理、流场模拟技术、固相摆动速度及其对流动、传热和传质的影响、局部流动稳态与瞬态性能、工业反应器用磁场发生装置优化设计的研究.     

颗粒群绕流传质特性的格子Boltzmann模拟

为考察气固两相流中颗粒团聚对气固传质过程的影响,采用耦合传质的格子Boltzmann模型对在空间均匀分布的颗粒结构和非均匀分布的颗粒团聚物结构条件下的绕流过程进行了直接数值模拟,得到了气体绕颗粒流动的速度分布和伴随流动的浓度分布,发现颗粒团聚物对绕流过程的速度分布和浓度分布都具有明显的影响。通过对颗粒绕流的气固传质过程进行定量分析发现,在两种结构条件下计算得到的传质舍伍德数均随着雷诺数的增大而呈指数函数形式增大,并且在均匀结构条件下的传质舍伍德数一般为非均匀结构条件下的3~5倍,颗粒团聚物的存在将严重影响颗粒绕流过程的气固传质效率。通过格子Boltzmann方法建立的气固相间传质模型,可以为研究气固两相流介尺度结构的传递特性提供理论依据。     

循环流化床气体-颗粒团聚物两相流体动力特性的研究

基于气固两相流体动力学,建立循环流化床内气体-颗粒团聚物两相流动模型,模拟计算循环流化床内气固两相流动.理论预测得到循环流化床内颗粒团聚物的形成,得到颗粒"环-核"流动结构,模拟计算颗粒相浓度和速度以及气相速度分布与前人实验结果的趋势基本吻合.     

湍流中粒子—流体间的传质

通过对已有各种两相湍流传质理论的剖析，指出颗粒与流体间相对速度的大小是确定传质系数的关键。通过分析湍流场中粒子的运动，认为在不同尺度涡旋中，液固间的相对速度是不同的，传质系数也不相同，提出用能谱加权的方法求取总传质系数。以此为基础得到的模型与本文及文献中不同体系的实验结果比较后发现，在流速、粒径、粒子密度、温度、浓度、管径等条件变化较大的情况下，模型均能较好地预测传质系数的变化规律。     

循环流化床多组分颗粒气固两相流动模型和数值模拟

基于稠密气体分子运动论和颗粒动力学,考虑多组分颗粒中颗粒组分与颗粒组分、颗粒组分内颗粒之间的相互作用以及气体与颗粒之间的相互作用,提出多组分颗粒非等温颗粒气固两相流动模型.以颗粒压力、径向分布函数、黏度、颗粒碰撞耗散等耦合各颗粒组分间和颗粒间的相间作用.采用大涡模拟方法模拟气相湍流流动.提出了多组分颗粒的径向分布函数计算方法.对循环流化床上升管中双组分颗粒气固两相流动特性进行了数值模拟,模拟结果揭示了上升管中双组分颗粒气固两相流动的环-核流动结构,得到了平均颗粒粒径的轴向和径向分布规律,计算结果与文献中实验结果相吻合.     

多段分级转化流化床颗粒浓度的数值模拟

多段分级转化流化床煤气化反应器是由鼓泡射流流化床和快速流化床耦合而成的新型气化炉,采用欧拉双流体模型对其气固两相流动行为进行了数值模拟。模拟结果表明,两种流型的耦合,并非两种流态形式的简单叠加,回料口下段床内轴向浓度呈"C"型分布,上段轴向浓度呈指数分布,与实验结果较接近。同时考察了局部位置的颗粒浓度分布及其随操作条件的变化情况,表明快速床稀相段边壁区域颗粒浓度波动较中心强烈,且对操作条件的变化敏感,回料口附近的偏流现象以及过渡渐缩段的颗粒弹溅现象均与实验观察一致。研究得到的规律可指导多段分级转化流化床的设计和优化。     

气固两相流内中空多孔催化剂性能的数值模拟

气固流态化过程中流体和颗粒分别聚集,形成稀密两相,严重限制其传质效率和反应速率的提高。针对此问题,本工作设计了一种中空多孔结构的催化剂颗粒,通过模拟方法研究该颗粒对稀密两相气相传质与反应的影响,及其在稀密相间转换的时间尺度。结果表明,一定的流动强度时,在颗粒稀密相转换的时间尺度内,中空多孔结构的颗粒能够有效地在稀相存储反应气体,并在密相释放,为密相提供额外的反应气体,增强体系的整体反应效率。当催化反应速率高于传质速率时,在所研究的流动条件下中空多孔颗粒体系的反应效率比实心球形颗粒体系高出26.92%~29.55%。可以预见在稀密相分布更广的大型气固流化床反应器中,中空多孔结构的催化剂颗粒能够更为有效地提高反应器的整体效率。     

倒置液固流化床内液固两相流动特性的数值模拟

基于欧拉-欧拉双流体模型,数值模拟倒置液固流化床内液固两相流动行为.数值模拟预测了床内颗粒的速度、浓度分布以及空隙率的变化.研究结果表明颗粒在床内分布呈现非均匀分布,床内形成局部高空隙率的流体团;随着床层高度增加,颗粒轴向速度增大:数值模拟床内空隙率与Renganthan等的实验结果相吻合.     

湍流中粒子－流体间的传质

通过对已有各种两相湍流传质理论的剖析，指出颗粒与流体间相对速度的大小是确定传质系数的关键。通过分析湍流场中粒子的运动，认为在不同尺度涡旋中，液固间的相对速度是不同的，传质系数也不相同，提出用能谱加权的方法求取总传质系数。以此为基础得到的模型与本文及文献中不同体系的实验结果比较后发现，在流速、粒径、粒子密度、温度、浓度、管径等条件变化较大的情况下，模型均能较好地预测传质系数的变化规律     

循环流化床换热器中液固二相流的数值模拟

在循环流化床换热器的流体中加入固体粒子,可以对边界层有扰动作用,加快换热器中的传质与传热.对流化床换热器中的液固二相进行数值模拟,分别讨论了固体粒子对液体动量方程、k-ε方程及能量方程的影响,在单液相方程的基础上进行修正,建立了相应的液固二相流控制方程,确定了物理模型及边界条件.利用FLUENT进行数值计算,分别讨论了...     

液固磁稳定床研究进展

鉴于液固磁稳定床在石油化工、生化工程及环境工程等领域具有广阔的应用前景,本文从流体力学特性、传质过程、返混与扩散等几个方面,对液固磁稳定床的研究进展进行了综述。     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

循环流化床气固传质实验研究

This study is devoted to gas-solid mass transfer behavior in heterogeneous two-phase flow. Experiments were carried out in a cold circulating fluidized bed of 3.0m in height and 72mm in diameter with naphthalene particles. Axial and radial distributions of sublimated naphthalene concentration in air were measured with an online concentration monitoring system HP GC-MS. Mass transfer coefficients were obtained under various operating condition, showing that heterogeneous flow structure strongly influences the axial and radial profiles of mass transfer coefficients. In the bottom dense region, mass transfer rate is high due to intensive dynamic behavior and higher relative slip velocity between gas and clusters. In the middle transition region and the upper diluter region, as a result of low mass transfer driving force and the influence of flow structure, mass transfer rate distribution becomes non-uniform. In conclusion, among the operating parameters influencing mass transfer coefficients, the superficial gas velocity is the most important factor and the solid circulation rate should be also taken into account.     

新型催化裂解快速流化床内颗粒浓度分布实验研究

催化裂解反应器是石油深度加工的重要反应器,采用实验方法对新型快速床催化裂解反应器内气固两相流动特性进行了研究,测量了床层内颗粒浓度分布,考察了气体流量对床层轴向和径向上颗粒浓度分布的影响。实验结果表明,床层轴向上颗粒浓度呈现下部稠密上部稀疏的分布规律;当气体流量较低时轴向颗粒浓度呈S形分布,高气量下呈现指数函数形分布,即反应器上部区域的颗粒浓度分布影响较小;床层径向颗粒浓度分布呈现中心稀、边壁浓的特征,且增大空气流量,径向分布趋于均匀。在一定操作条件下,与传统提升管相比,新型快速床颗粒浓度显著提高。     

Heat and mass transfer characteristics in a gas-slurry-solid fluidized bed

The gas-slurry-solid fluidized bed is a unique operation where the upward flow of a liquid-solid suspension contacts with the concurrent up-flow of a gas, supporting a bed of coarser particles in a fluidized state. In the present study we measured the gas holdup, the coarse particle holdup, the cylinder-to-slurry heat transfer coefficient, and the cylinder-to-liquid mass transfer coefficient at controlled slurry concentrations. The slurry particles were sieved glass beads of 0.1 mm average diameter and their volumetric fraction was varied at 0, 0.01, 0.05 or 0.1. The slurry and the gas velocities were varied up to about 12 and 15 cm/s, respectively. The coarse particles fluidized were sieved glass beads of average diameters of 3.6 and 5.2 mm. The individual phase-holdup values were measured and served for use in correlating the heat and mass transfer coefficients. The heat and mass transfer coefficients in the slurry flow, gas-slurry transport bed, slurry-solid fluidized bed and gas-slurry-solid fluidized bed operations can be correlated well by dimensionless equations of a unified formula in terms of the Nusselt (Sherwood) number, the Prandtl (Schmidt) number and the specific power group including the energy dissipation rate per unit mass of slurry, with different numerical constants and exponent values, respectively, to the heat and mass transfer coefficients. The presence of an analogy between the heat and mass transfer from the vertically immersed cylinder in these slurry flow, gas-slurry transport bed and gas-slurry-solid fluidized bed systems is suggested.     

液固循环流化床两相流动模型

引　言流化床换热器具有防、除垢和强化传热等优点 ,在化工、食品、海水淡化、废水处理等领域具有广阔的应用前景[1].目前 ,流化床换热器历经散式流化床、内循环流化床 ,已发展到外循环流化床换热器[2 ],它要求在较稀的颗粒浓度 (颗粒浓度小于 5% )、较高的流速 ( 1～ 3ｍ·ｓ- 1)下操作 .流化床换热器中液体流动及颗粒运动状态的研究对流化床换热器的设计和操作具有重要意义 ,但人们对循环流化床换热器中颗粒运动情况的研究还很缺乏 .考虑到循环流化床换热器中的每根换热管都可作为一个独立的循环流化床对待[3].本文试图建立一滑移速度模型…     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.