环栅式动力除尘器的两相流数值模拟

为了开发、设计高性能产品，针对环栅式动力除尘器进行了二维两相流数值模拟。模拟中使用了颗粒轨道模型。采用κ-ε模型对气相湍流进行模拟，用Stochas模型描述颗粒相的湍流扩散。对灰尘颗粒及气体的流动情况以及除尘器的特性进行了分析研究。     

气液两相流泵内气泡大小及分布规律研究

为了研究气液两相流离心泵内的气泡分布规律以及气泡大小对数值模拟的影响,采用高速摄像技术对泵内气液两相流动进行可视化试验,采用Fluent中的Eulerian模型和SST k-ω湍流模型对泵内的气液两相流动进行数值模拟。在转速为400 r/min、液相流量为10 m3/h和气相流量为0.5 L/min时,泵内流型为孤立泡状流,经统计测得叶轮内的气泡平均直径约为0.94 mm,且叶轮内气泡直径分布在0.1～2.0 mm之间,较蜗壳内的0.1～1.4 mm分布更广,蜗壳内的气泡平均直径沿着流道方向由0.55 mm逐渐增大到0.82 mm呈增大趋势,整体而言叶轮内的气泡平均直径大于蜗壳区域。通过设置不同直径的气泡对气液两相流泵进行模拟发现,气泡直径对泵内气相的集聚和分布范围有一定的影响,气泡直径越大,气相越容易聚集成高浓度分布,合理设置气泡直径能够提高数值模拟结果的准确性。     

流化床内多组分颗粒传质过程模拟

为了揭示活性颗粒的传质特性,采用Euler-Euler双流体模型,并结合气泡介尺度曳力模型和多组分传质模型,利用计算流体动力学(CFD)软件对存在惰性颗粒情况下多组分颗粒流化床内水蒸气吸附过程进行了数值模拟,得到流化床内固体颗粒体积分数和水蒸气质量分数分布,验证了气泡介尺度曳力模型的合理性,分析了入口表观气体速度、水蒸气质量分数和床层压强对水蒸气吸附过程的影响.结果表明:气泡介尺度曳力模型可以较好地再现水蒸气的吸附过程;减小入口表观气体速度及增大入口水蒸气质量分数有利于改善传质特性,有效增大了水蒸气吸附速率.     

高钙煤灰颗粒与液滴碰撞增湿脱硫过程的模型研究

为模拟增湿活化反应器内高钙煤灰颗粒的增湿脱硫过程，采用随机轨道模型分别跟踪颗粒群与液滴群，以相邻单元间26条颗粒流通道模型统计未增湿条件下的初始颗粒流量，以液滴群的空间单元捕捉效率为单元体内颗粒流量守恒方程的源项(汇)系数，来迭代各单元通道上的未增湿颗粒流量，耦合煤灰颗粒与液滴群的碰撞过程。并进一步考虑反应器内气、液、固多相湍流物理化学反应过程，建立了三维场内高钙煤灰低温增湿脱硫总体模型，初步预测了高钙煤灰增湿脱硫过程，模型预测与试验结果具有很好的吻合特性。图10参9。     

颗粒轨道模型对粉煤湍流燃烧计算结果影响的研究

本文对炉内粉煤两相湍流燃烧过程作数值模拟，着重研究了不同颗粒轨道模型对数值模拟结果的影响。研究结果表明，随机轨道模型能较好地模拟粉煤的运动规律，并得到较好的计算结果。     

颗粒扩散的三个效应分析及其数值模拟

在对均匀各相同性湍流中的颗粒进行研究时，目前通常采用拉氏方法描述两相湍流中的颗粒相运动，用的较多的就是基于颗粒所见气体速度的Langevin方程。此方程的封闭必须考虑颗粒扩散的轨道穿越效应、连续性效应和惯性效应。在仔细分析几个效应的基础上，提出了一种改进的漂移系效模型，综合考虑颗粒在均匀各向同性湍流内扩散所遇到的三种效应的影响，并进行数值模拟分析了几个效应对于效值模拟结果的影响。     

进口边界条件对边壁进风鼓泡床流动的影响研究

为了探寻速度进口边界条件对边壁进风鼓泡流化床床内气泡行为的影响,根据欧拉-欧拉双流体模型,应用标准kε-方程模型处理气体湍流流动,分别采用均匀进风速度边界条件、脉动项按正态分布和脉动项按正弦分布的脉动进风速度边界条件,对鼓泡床内的气固两相流动过程进行了数值模拟.结果表明：采用脉动进风速度边界条件时,在非射流孔道处气体可形成气泡,气泡的上升速度比匀速进风时小;气体能够充分地与固体颗粒相互作用,床层中的空隙率主频较小;在设置边界条件时考虑脉动能够更合理地预测和分析床内气泡的尺寸、速度等特性.     

液-汽引射器内液汽混合过程压力及汽液分布特性研究

为了明确液-汽引射器在工作过程中压力及液汽分布特性,采用混合多相流模型及标准k-ε湍流模型,建立了液-汽引射器三维数值计算模型,对液-汽引射器内部的压力及液汽流动状态进行了分析模拟,得到了不同工作流体参数下引射器内部压力及汽液分布规律。研究表明:沿引射器水平中轴线方向,压力及汽相体积分数随工作流体压力的增大而减小,随工作流体温度及流量的变化基本不变;沿引射入口轴线方向,吸入室的压力随工作流体压力的增大而减小,随工作流体温度及流量的变化几乎不变;沿引射入口轴线方向,汽相体积分数随工作流体压力及温度的变化较小,随工作流体流量的增大而增大。研究将有助于掌握液汽引射器内部压力及汽相体积分数的变化规律,有助于引射器的科学设计和运行。     

水下排气两相流动及气泡粒径分布的数值模拟

对某内燃机水下排气管道的排气过程进行了数值模拟,以预测CO_2经过排气管道处理后的气泡直径分布,期为管道的优化设计提供参考。气液两相流动模型采用Euler模型,湍流模型采用Realizable k-ε模型,考虑了气泡的分裂与聚合的粒径分布采用群体平衡方程计算。计算结果表明:在所研究的几何条件与流动参数范围内,CO_2气泡在管道中的非连续流动会因为有浮力作用使其逐渐集中到管道上方从而导致气泡粒径变大,孔板下部开孔对气泡破碎效果有限。管道出口处粒径在4 mm以下的气泡体积分数的时均值为0.466。     

气固流化床的离散颗粒运动-碰撞解耦模型与模拟

基于分子动力学和气固两相流体动力学，建立流化床稠密气-固两相离散颗粒运动-碰撞解耦模型，采用硬球模拟方法处理颗粒与颗粒之间的碰撞，及大涡模拟方法处理气相湍流流动．单颗粒运动满足牛顿第二定律，颗粒相和气相相间相互作用的双向耦合由牛顿第三定律确定，数值模拟二维鼓泡流化床内稠密气-固两相流动，得到了气泡的形成、发展及颗粒的流化过程，计算结果表明颗粒弹性恢复系数影响气-固两相流动特性。     

微小通道内气泡逸出行为的VOF模拟

针对微小型直接甲醇燃料电池阳极流场,采用VOF(volume of fluid)方法模拟了液体通流微小通道内壁面逸出气泡的动态行为,讨论了液体物性、气体流速、逸出气孔直径对气泡形成、生长及脱离等过程以及流动阻力的影响.结果表明:随着甲醉溶液浓度的升高,单个气泡的脱离体积、脱离时间和流动阻力系数均减小;气体流速增加,气泡...     

Simulation of Convective Heat Transfer of Gas–Liquid Bubble Train Flow in Wet Microtube

In this paper, a direct numerical simulation of a two‐phase incompressible gas–liquid flow for simulation of bubble motion and convective heat transfer in a microtube is presented. The microtube radius is 10 μm. The interface between the two phases is tracked by the volume of fluid method with the continuous surface force model. Newtonian flows are solved using a finite volume scheme based on the PISO algorithm. Numerical simulation is done on an axisymmetric domain with a periodic boundary condition for different values of pressure gradient, void fraction, and bubble period. Mean pressure gradient is fixed for each simulation. The superficial Reynolds numbers of gas and liquid phases studied are 0.3 to 7 and 5 to 210, respectively. Numerical results are coincident with the Serizawa regime map, and there is a linear relation between the void fraction and gas flow ratio. Simulation shows local Nusselt number increases in the presence of a gas bubble.     

Bubble generation in quiescent and co-flowing liquids

A numerical simulation has been accomplished to analyze the problem of dynamic bubble formation from a submerged orifice in an immiscible Newtonian liquid under the condition of constant gas inflow. We have considered two cases for the surrounding liquid, namely the liquid in a quiescent condition and the liquid as a co-flowing stream with the gas. The full cycle, from formation to detachment of the bubbles and the corresponding bubble dynamics, was simulated numerically by using a coupled level-set and volume-of-fluid (CLSVOF) method. The role of the liquid to gas mean velocity ratio, the Bond number and the Weber number in the bubble formation process was studied and the order of magnitude of forces involved in bubble dynamics are presented. Our simulation results show that the minimum radius of the neck decreases with a power law behavior and the power law exponent in a co-flowing liquid is less than 1/2 as predicted by the Rayleigh–Plesset theory for quiescent inviscid liquids. Single periodic and double periodic bubbling (with pairing and coalescence) regimes are observed in the present investigation. It is identified that a moderate co-flowing liquid may inhibit the bubble coalescence. The volume of the bubble and the bubble formation time decrease with increasing liquid to gas mean velocity ratios. For small Bond numbers, significant differences pertaining to bubble dynamics are observed between the co-flowing liquid and the quiescent liquid. Furthermore, the generation and breakup of the Worthington jet after bubble pinch-off and formation of tiny drops inside the detached bubbles are observed.     

Numerical investigation on subcooled pool film boiling of liquid hydrogen in different gravities

A clear understanding of bubble dynamics and heat transfer characteristics of hydrogen boiling in microgravity is significant for achieving safe and high-efficiency utilization of liquid hydrogen in space. In the present paper, a numerical simulation model is developed to predict the subcooled pool film boiling for liquid hydrogen in different gravities. The computations are based on the volume of fluid method combined with Lee's phase change model. The results show that the bubble released from the wavy gas-liquid interface might grow to a larger size before departure with the decrease of gravity, and poor heat transfer performance is observed in reduced gravity. However, once the gravity level is low enough or the subcooling of liquid is sufficiently large, instead of bubble formation and release at the vapor-liquid interface, a thin gas film layer is almost observed and maintained in the surface of horizontal flat or wire heater.     

气化炉液池内单个高温气泡传热、传质的数值模拟

建立了气、液间传热传质数学模型,对单个热气泡在上升过程中与处于蒸发阶段的水之间的传热、传质规律进行了数值研究,获得了气泡半径、气泡温度、水蒸发速率以及蒸发量随时间的变化规律.研究表明:在水恒温蒸发阶段,由于传热传质的共同作用,气泡半径随着气泡的上升而变小,并逐渐趋于稳定;随着气、液间温度差的增大,气泡半径缩小得越快;在气泡与水开始接触时,水的蒸发速率及气泡内的水蒸汽增量最大;气泡温度在较短时间内急剧下降,并趋于稳定;随着气泡半径的缩小,气泡冷却速率提高;但随着接触时间的继续增加,对气体的冷却效果却无明显作用.     

Experimental and numerical study of downward bubbly flow in a pipe

An experimental and numerical study of the local structure of downward gas–liquid flow in a vertical pipe with 20-mm inner diameter is reported. In the experiment, the electrodiffusion technique was used in combination with electrical conductivity measurements. To examine the effect of gas-phase dispersion on flow characteristics, two different gas–liquid mixers were used capable of producing large-diameter (>1 mm) and small-diameter (<1 mm) gas bubbles at identical rate characteristics of the flow. The unified heterogeneous-medium mechanics approach was used to develop, in the Eulerian two-velocity approximation, a calculation model for downward turbulent liquid/air bubble flows. It is shown that, as the volumetric gas flow rate of the mixture at the inlet to the pipe increases, local maxima of continuous phase velocity and bubble concentration emerge in the near-wall zone of the flow, with liquid turbulence suppressed in the wall zone and enhanced in the core of the flow.     

黏性液体中锐孔处气泡的形成

考察一定流量气体，通过锐孔在静止黏性液体中连续溢出气泡的过程。应用动力学平衡半经验关系式，综合考虑气泡受力，分析气泡形成过程，给出合理假设，预测气泡直径。分析表面张力、气体流速、锐孔直径及液相物性对气泡脱离尺寸的影响，找到影响气泡脱离尺寸的主要因素。计算预报值与实验结果符合良好。     

Computational Fluid Dynamics–Population Balance Modeling of Gas–Liquid Two-Phase Flow in Bubble Column Reactors With an Improved Breakup Kernel Accounting for Bubble Shape Variations

Abstract

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model.     

CFD simulation of CO2 removal from hydrogen rich stream in a microchannel

The main object of this research is to perform computational fluid dynamics simulation of CO₂ capturing from hydrogen-rich streams by aqueous DEA solution in a T-Junction microchannel contactor with 250 μm diameter and 5 mm length at dynamic conditions. To develop a comprehensive mathematical framework to simulate the flow hydrodynamics and mass transfer characteristics of system, the continuity and Navier-Stokes equations, two phase transport, and reaction rate model are coupled in COMSOL Multiphysics software. The developed model is solved and the effects of gas and liquid velocities as well as amine concentration on the CO₂ absorption rate, hydrogen purification fraction, and flow hydrodynamic are investigated. The absorption process consists of CO₂ diffusion from bubble bulk toward the bubble boundary, CO₂ solubility in the liquid boundary, diffusion from the boundary into the liquid bulk, and reaction with the amine molecules. The results show that when the gas and liquid streams are mixed in the junction point to form a bubble, the gas cross-section area becomes narrow, and the fluid velocity increases due to the applied force on the bubble by the liquid layers. It appears that increasing the DEA concentration in the inlet from 5% to 20% increases hydrogen purification fraction from 42.3 to 66.4%, and up to 96.7% hydrogen purity is achieved by 20% aqueous solution of DEA.     

Numerical study of bubble growth and wall heat transfer during flow boiling in a microchannel

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier–Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. Experiments have been conducted to validate the numerical model. The bubble growth rate and shapes show good agreement between numerical and experimental results. The numerical results show that the wall heat transfer increases with wall superheat but stays almost unaffected by the liquid flow rate. The liquid vapor surface tension value has little influence on bubble growth and wall heat transfer. However, the bubble with the lowest contact angle resulted in the highest wall heat transfer.