Velocity Uniformity of Microchannels in a Laminated‐Sheet Structure Under Parallel and Series Methods

An integral microchannel device is generally formed by lamination of multiple microchannel sheets. Uniform velocity distribution among the microchannels in each sheet shows a large influence on the device's performance. The effects of microchannel and manifold structure on the velocity uniformity in each sheet with different structural parameters under parallel and series laminated structures are investigated. For the laminated‐sheet structure where each sheet has the same structural parameters, a more uniform velocity distribution exists as compared to where the structural parameters are different. There exists a direct correlation between the velocity values in each sheet and structural parameters, whereas an inverse correlation is found under series laminated structure.     

Three‐Dimensional Simulation of Bubble Formation Through a Microchannel T‐Junction

Three‐dimensional simulations of bubble formation in Newtonian and non‐Newtonian fluids through a microchannel T‐junction are conducted by the volume‐of‐fluid method. For Newtonian fluids, the critical capillary number Ca for the transition of the bubble breakup mechanism is dependent on the velocity ratio between the two phases and the microchannel dimension. For the power law fluid, the bubble diameter decreases and the generation frequency increases with higher viscosity parameter K and power law index n. For a Bingham fluid, the viscous force plays a more important role in microbubble formation. Due to the yield stress τ_y, a high‐viscous region is developed in the central area of the channel and bubbles deform to a flat ellipsoid shape in this region. The bubble diameter and generation frequency are almost independent of K.     

基于计算流体力学方法的啶虫脒结晶釜搅拌器优化设计

采用计算流体力学(CFD)方法对啶虫脒结晶釜进行了模拟与优化,考察了不同搅拌转速下的速度分布均匀性和晶体体积含量分布均匀性,结果表明,高效轴流型桨SP306对晶体悬浮的效果较好,搅拌转速对晶体体积含量分布均匀性有显著影响,确定了90 r/min为优化后的搅拌转速。经工业应用,验证了优化设计的合理性,产品粒径分布有明显改善。     

Effects of Distribution Channel Dimensions on Flow Distribution and Pressure Drop in a Plate‐Fin Heat Exchanger

A numerical investigation on flow distribution and pressure drop characteristics in a plate‐fin heat exchanger is presented. Influences from length and width of the distribution channel were particularly investigated. Flow distribution in the studied model was improved by increasing the length or reducing the width of the distribution channel, but at the cost of an increased pressure drop. The relationship of flow distribution and pressure drop was analyzed by the porous medium approach. A dynamic balance phenomenon was observed and further studied. Based on the results, a novel strategy for the attainment of flow uniformity, which causes negligible pressure drop variation, was proposed. Finally, a performance effectiveness factor was developed for predicting the effect of the proposed strategy on the performance of plate‐fin heat exchangers.     

Transient Simulation of Hollow‐Fiber Membrane Filtration with Nonuniform Permeability Distribution

Transient simulation of filtration in hollow‐fiber membranes with nonuniform permeability distribution was conducted. The diversity of permeability distributions caused different initial flux and transmembrane pressure distributions. Manipulating the permeability distribution enables a hollow‐fiber membrane to achieve its maximum volumetric flow rate. During solid‐liquid separation, the inter‐adjustment between flux and cake distributions improved their uniformities simultaneously. The reciprocal of the volumetric flow rate of the membranes all increased linearly with water production. Severely nonuniform permeability distribution caused low water production. The numerical results could be applicable to account for the non‐ideal performance of industrial hollow‐fiber membrane modules.     

Gas‐Solid Flow in an Airlift Loop Reactor: A Cluster Structure‐Dependent Drag Model

The hydrodynamic characteristics and heterogeneous structures in an airlift loop reactor (ALR) are analyzed by a computational fluid dynamics (CFD) approach. Based on the two‐fluid model, a modified cluster structure‐dependent (CSD) drag model under consideration of the cluster effects is applied to the prediction of the non‐uniform flow structure in the ALR. In comparison with experimental data and the traditional drag model, the present model enables a better prediction. The distributions of the local granular temperature in different regions of the ALR are indicated. The granular temperature in the particle diffluence region was found to be higher than that in the draft tube and annular zones.     

Three-Dimensional Numerical Modeling of Heat and Moisture Transfer in Natural Rubber Sheet Drying Process

The effects of heat and moisture transfer on rubber sheet drying were investigated by 3-D modeling using computational fluid dynamics (CFD). Complexities arose due to the inclusion of moisture transport. Local convective heat transfer coefficients for the sheets were predicted and then used to determine the mass transfer coefficients using the Chilton–Colburn analogy. Deviations between experimental and simulated temperatures were observed to range from 4.0 to 7.7°C. Temperature and moisture content of the rubber sheets were accurately predicted, and agreement between the experimental and simulated results was acceptable. This is useful for the design of an efficient rubber sheet drying chamber.     

Heat Transfer Characteristics of CO2 Desorption from N‐Methyldiethanolamine Solution in a Microchannel Reactor

The heat transfer performance and energy consumption of CO₂ desorption from rich N‐methyldiethanolamine (MDEA) solution were determined experimentally in a straight microchannel reactor. Nucleate boiling was found to be the dominant heat transfer mechanism in this experiment. The heat transfer coefficients were strongly dependent on the heat flux. The solution flow rate was the most influential factor on the heat flux, followed by desorption temperature, MDEA concentration, and CO₂ loading. In addition, an empirical correlation was proposed to predict the experimental heat transfer coefficients.     

基于Fluent的微通道天然气废气重整的数值模拟

利用Fluent软件对微通道反应器中天然气废气重整进行数值模拟。利用甲烷来代替天然气进行模拟。多通道反应器由具有平行通道的堇青石块组成,每个平行通道用Rh/Al_2O_3催化剂洗涂。由于堇青石的低热导率,通道之间的热传递被忽略。研究了进料温度,燃料组成和天然气中存在的丙烷对温度和产物分布的影响。通过模拟结果可得出:开始温度沿着通道良好地分布,并且没有形成明显的热点;增加进料温度有利于甲烷转化和氢气生产,但会导致温度分布不均;提高进料中蒸汽的量有助于增加氢气形成,但轻微地减弱了甲烷转化;提高入口处的O_2/C比值可导致甲烷转化率和温度成比例地增加。     

CFD Modeling of a Carbonation Reactor with a K‐Based Dry Sorbent for CO2 Capture

A 2D CFD simulation of the carbonation reactor is carried out to evaluate the performance of potassium‐based dry sorbent during the CO₂ capture process. A multiscale drag coefficient model is incorporated into the two‐fluid model to take the effects of clusters into account. The influence of several parameters on CO₂ removal is investigated. The results indicate that increasing the reactor height and reducing the gas velocity can lengthen the residence time of particles and enhance the CO₂ removal. The operating pressure has a significant influence on the performance of solid sorbents. A higher pressure will decrease the CO₂ removal efficiency.     

Steam Reforming of Methanol in a Compact Copper Microchannel Foam Reactor

Hydrogen production via steam reforming of methanol over a rare earth‐promoted Cu‐based catalyst washcoated on a microchannel foam reactor (MFR) was investigated. A low reforming temperature of 242 °C at a weight hourly space velocity for the methanol/catalyst of 10 h^–1 was observed in the MFR, which is lower than the 270 °C reforming temperature observed in a traditional packed‐bed reactor (PBR). According to a measurement of the reforming temperature distribution, the MFR made of Cu foam in this study exhibits extraordinary heat conductivity. The heat rate supplied from the external heating source can be transferred instantly to the reaction sites of the washcoated catalyst layer through a three‐dimensional framework of Cu microchannels. As a result, the cold spots normally encountered in a PBR are minimized effectively so that a high conversion of steam reforming of methanol is obtained. Moreover, the use of a high performance compact MFR with a volume of 4 mL as a portable hydrogen source is suggested. A hydrogen production rate of 280 mL min^–1 with a CO fraction of 1.5% was obtained, which can yield a practical power output of 25 W using a commercial proton exchange membrane fuel cell with an operational efficiency of 50%.     

Numerical Study on Bubble Formation of a Gas‐Liquid Flow in a T‐Junction Microchannel

Bubble emergence in a gas‐liquid flow in a T‐junction microchannel of 100 μm diameter, operated under a squeezing regime, was simulated with the computational fluid dynamics method. In general, bubble formation in channels includes three stages: expansion, collapse and pinching off. After analyzing and comparing quantitatively the three forces of pressure, surface tension and shear stress acting on the gas thread in the whole process, their effects in the different stages were identified. The collapse stage was the most important for bubble formation and was investigated in detail. It was found that the collapse process was mostly influenced by the liquid superficial velocity, and the rate and time of collapse can be correlated with empirical equations including the liquid superficial velocity, the capillary number and the Reynolds number.     

同向旋转双螺杆挤出机捏合区的速度分布

应用多普勒激光测速计测量同向旋转双螺杆挤出机捏合区域的速度分布。在９０ｒ／ｍｉｎ的转速下,测量了两螺杆捏合区中推力区和阻力区熔体的轴向速度和法和速度。     

Simulation and Experimental Validation of Two‐Phase Flow in an Aerosol Counter‐Flow Reactor using Computational Fluid Dynamics

A simulation of the hydrodynamic behavior of an aerosol‐counter flow reactor was conducted using an Euler‐Lagrange method. The simulation results were then verified with experiments. The process simulated was a separation process required during the production of biodiesel (fatty acid methyl ester). In this process, the liquid ester/glycerol phases are continuously injected through a hollow cone nozzle with an overpressure of 10⁶ Pa into the reactor, operated at 15000 Pa. The liquid is atomized because of the pressure drop and a liquid particle spray is generated with an inlet velocity of 44.72 m/s. Water vapor of temperature 333 K is injected tangentially through two side, gas inlets with an inlet velocity of 1.2 m/s. Excess methanol is subjected to a mass transfer from the liquid phase into the gas phase, which is withdrawn through the head of the reactor and condensed in an external condenser unit. The stripping of the methanol off the liquid leads to a sharp interface between the glycerol and the ester phase, which can then be easily separated by gravity or pumping. The gas velocity field, pressure field and the liquid particle trajectories were calculated successfully. Simulated dwell time distribution curves were derived and analyzed with the open‐open vessel dispersion model. Experimental dwell time distribution curves were measured, analyzed with the open‐open vessel dispersion model, and compared with the simulated curves. A good consistency between simulated and measured Bodenstein numbers was achieved, but 25 % of the simulated particles exited at the reactor's head, contrary to experimental observations. The difference between simulated and measured dwell times was within one order of magnitude.     

Computational Fluid Dynamics Modeling of Gas‐Liquid Two‐Phase Flow around a Spherical Particle

Microscale studies, which can provide basic information for meso‐ and macroscale studies, are essential for the realization of flow characteristics of a packed bed. In the present study, the effects of gas velocity, liquid velocity, liquid‐solid contact angle, and liquid viscosity on the flow behavior were parametrically investigated for gas‐liquid two‐phase flow around a spherical particle, using computational fluid dynamics (CFD) methodology in combination with the volume‐of‐fluid (VOF) model. The VOF model was first validated and proved to be in good agreement with the experimental data. The simulation results show that the film thickness decreases with increasing gas velocity. This trend is more obvious with increasing operating pressure. With increasing liquid velocity, the film thickness tends to be uniform on the particle surface. The flow regime can change from film flow to transition flow to bubble flow with increasing contact angle. In addition, only at relatively high values does the liquid viscosity affect the residence time of the liquid on the particle surface.     

3D CFD Simulation of Reaction Cells,Cooling Cells,and Manifolds of a Flatbed Reactor for CO2 Methanation

The methanation of CO₂ has attracted great interest in recent years as a technology to generate renewable synthetic natural gas and to recycle CO₂ from different sectors. The actual development state of a flatbed reactor for the methanation of pure stoichiometric feed gas is presented. Additionally, computational fluid dynamics (CFD)-based design strategies are introduced which can be applied for the development and optimization of different processing units. The results of the reactor development demonstrate a good heat exchange and flow distribution in the reactor.     

Computational Fluid Dynamics Study of a Styrene Polymerization Reactor

The computational fluid dynamics (CFD) approach was adopted to simulate benzoyl peroxide (BPO)‐initiated styrene polymerization in a laboratory‐scale continuous stirred‐tank reactor (CSTR). The CFD results revealed the effects of non‐homogeneity and the short‐circuiting of the unreacted styrene and initiator on the reactor performance. The study also investigated the effects of the impeller speed and the residence time on the conversion and the flow behavior of the system. The CFD simulation showed that intense mixing remained confined to a small region near the impeller. With increasing impeller speed, it was found that the perfectly mixed region near the impeller expanded, thus reducing non‐homogeneity. Different contours were generated and exhibited the effect of the mixing parameters on the propagation rate and styrene conversion. The monomer and initiator conversions predicted with the CFD model were compared to those obtained with a CSTR model. The CFD model accounts for the non‐ideality behavior of the polymerization reactor, and hence conversion predictions are more realistic.     

微通道壁面刻蚀孔缺陷对通道内部流场的影响

Microchannel reactors are commonly used in micro-chemical technology. The performance of microreactors is greatly affected by the velocity field in the microchannel. The flow field is disturbed by the cylindrical etch holes caused by air dust on the microchannel surface during its processing procedure. In this approach, a two-dimensional computational fluid dynamics (CFD) model is put forward to study the effect of etch holes on flow field. The influenced area of single or two concave etch holes is studied for the case of laminar flow. The hole diameter, the Reynolds number and the distance between the center of holes are found to have influences on the flow field. Numerical results indicate that the effects of etch hole on the flow field should be evaluated and the way of choosing the economic class of cleanroom for microreactor manufacture is presented.     

Design and Analysis of a Submerged Membrane Reactor by CFD Simulation

CFD was applied to demonstrate the effect of reactor configurations on the fluid flow pattern in submerged membrane reactors. A mixture model, a realizable k‐? model, and the multiple reference frame (MRF) technique were employed to simulate the solid‐liquid turbulent flow. Influences of the introduction of a ceramic membrane, the relative position between ceramic membrane and impeller, and the types of impeller on velocity profiles and concentration distributions were systematically discussed. These simulation results were validated qualitatively with experimental data for various reactor configurations.     

Model for Predicting Solids Velocity Fluctuations in Sedimenting Suspensions

Computational fluid dynamics (CFD) simulations of sedimenting suspensions in two‐dimensional periodic domains using a Eulerian two‐fluid model were performed with the commercial software ANSYS Fluent®. Three particle systems belonging to Geldart groups A and B were selected to gather data on the solids velocity fluctuations. It is proposed that solids velocity fluctuations are composed of local and global parts. Therefore, the solids velocity signal was segregated into low‐frequency and high‐frequency fluctuations using the FFT solver in MATLAB®. A model is proposed to predict the velocity fluctuations of a solid phase at low volume fractions and the model results are compared with the CFD results. The model is capable of capturing the solid particles fluctuation in the dilute limit.