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.     

Hydrodynamics in an Internal Loop Airlift Reactor with a Convergence‐Divergence Draft Tube

Gas holdup and liquid circulation of one conventional draft tube and three different convergence‐divergence draft tubes in an internal loop airlift reactor were investigated. Experiments were carried out in two‐phase systems with air‐water and air‐CMC (carboxyl methyl cellulose) solution and three‐phase system with air‐water‐resin particles. The two‐phase drift‐flux model was used to estimate gas holdup for three‐phase Newtonian and two‐phase non‐Newtonian systems. It is shown that gas holdup in convergence‐divergence draft tubes is higher than that in a conventional draft tube and increases with superficial gas velocity. Variation of the structural parameters of convergence‐divergence draft tubes has little effect on gas holdup in the two‐phase and three‐phase system. The mathematical model, which is based on a drift‐flux model, was developed to describe the liquid circulation velocity in the reactor satisfactorily.     

Modeling of Mixing and Reaction in Turbulent Multi‐Phase Flows with Distribution Functions

Numerical simulation of turbulent reacting or multiphase flows is gaining popularity as a tool for the analysis and optimization of many complex applications in process engineering. To make possible the accurate modeling of relevant reaction and transport processes, the respective distribution functions of mixture fraction or particle size must be considered in an adequate manner. In the present paper, novel approaches to make possible a more detailed yet efficient representation of distribution functions in turbulent, reacting multiphase flows are introduced. The application of the methods to the example of a system with mixing and reaction among three species is discussed.     

Experimental Study of Heat Transfer Intensification under Vibration Condition

Experimental and theoretical models for enhancement of heat transfer from a tube with rings rotating on the external surface were investigated. The rings were rotated on acting vibration forces (hula‐hoop phenomenon). The working fluid flowing into the tube was water. The Reynolds number ranged from 800 to 2000. The amplitude range of the parameters of vibration was 0.1 mm to 1 mm, and the frequency range was 10 to 120 Hz. On the basis of a dimensionless analysis, a mathematical model for the heat‐transfer process was developed. It was shown that the mean heat transfer coefficient became higher as the velocity of vibration increased. The experimental results were in good agreement with the theoretical model.     

Device Performance Improvement of Double‐Pass Concentric Circular Mass Exchangers under Uniform Wall Fluxes

A double‐pass concentric circular mass exchanger under uniform wall fluxes is produced by inserting a permeable barrier into a circular tube to improve the device performance. The mathematical formulation was developed theoretically for such double‐pass, forced‐convection, mass‐transfer problems which are referred to as conjugated Graetz problems. The analytical solutions are obtained by the linear superposition of an asymptotic solution and a homogeneous solution which are linear in the axial direction and solved with the use of an eigenfunction expansion in a power series. The analytical results show that the mass‐transfer rate of a double‐pass mass exchanger can be improved compared to that of a single‐pass mass exchanger by suitably adjusting the permeable barrier position. Moreover, the ratio of mass‐transfer efficiency improvement and power consumption increment is also shown to make good economic sense.     

Robust Direct Numerical Simulation of Viscoelastic Flows

Many technical processes involve viscoelastic flows, which makes the subject interesting for CFD. Despite the complex fluid rheology and related numerical problems in solving the constitutive equations, recent stabilization approaches allow for a robust simulation of viscoelastic flows in the technical relevant range at high degrees of fluid elasticity. A recent general‐purpose numerical stabilization framework, based on the finite‐volume method of OpenFOAM is presented and its capability for the robust simulation of viscoelastic single‐, as well as two‐phase flows is shown.     

An Algorithm for Cost Comparison of Optimized Shell‐and‐Tube Heat Exchangers with Tube Inserts and Plain Tubes

Heat transfer enhancement is currently allocated the most part of investigations to improve the thermal performance of shell and tube heat exchangers. Here an algorithm for cost comparison of an optimized enhanced tube heat exchanger to an optimized plain tubes has been developed. The results are shown in plots which allow the engineers to compare directly the capital cost of heat exchangers and to select the better options among the various tube inserts for a given duty.     

双效精馏流程的节能分析

探讨几种基本的双效精馏装置的流程，并从能源利用的角度研究和分析它们的特点及适用条件。     

纵流式管壳式换热器传热与流动特性的3D数值模拟研究

文章运用CFD软件Fluent分析了三种支撑结构对壳程性能的影响,可为工业用纵流式管壳式换热器优化及强化传热提供依据.文章探讨了水在三种不同支撑结构下的流阻与传热性能,得到不同支撑结构具有不同的传热效果,螺旋片相对其它两种支撑方式更有利于提高传热综合性能,并进一步对这三种换热器的传热强化机理进行了探讨.     

Extension of Rapid Sizing Algorithm for Shell‐and‐Tube Heat Exchangers with Tube‐Side Pressure Drop Constraint and Multipasses (Part B)

There are occasions in which process engineers need approximate estimates of the exchanger size rather than full detailed designs. These situations are often accompanied by a need for speed. This paper provides a simple procedure for exchanger sizing which is rapid and with reasonable accuracy. The methodology involved is described in detail here and it is shown that the procedure can be applied to pure counterflow and also for multipass exchangers.     

Experimental Investigation on the Holdup Distribution of Oil‐Water Two‐Phase Flow in Horizontal Parallel Tubes

An experimental investigation was conducted to study the holdup distribution of oil and water two‐phase flow in two parallel tubes with unequal tube diameter. Tests were performed using white oil (of viscosity 52 mPa s and density 860 kg/m³) and tap water as liquid phases at room temperature and atmospheric outlet pressure. Measurements were taken of water flow rates from 0.5 to 12.5 m³/h and input oil volume fractions from 3 to 94 %. Results showed that there were different flow pattern maps between the run and bypass tubes when oil‐water two‐phase flow is found in the parallel tubes. At low input fluid flow rates, a large deviation could be found on the average oil holdup between the bypass and the run tubes. However, with increased input oil fraction at constant water flow rate, the holdup at the bypass tube became close to that at the run tube. Furthermore, experimental data showed that there was no significant variation in flow pattern and holdup between the run and main tubes. In order to calculate the holdup in the form of segregated flow, the drift flux model has been used here.     

Prediction of Droplet Velocities and Rain Out in Horizontal Isothermal Free Jet Flows of Air and Viscous Liquid in Stagnant Ambient Air

Two‐dimensional phase Doppler anemometer measurements of droplet size and velocity conducted under several nozzle conditions and a systematic variation of the air mass flow quality and liquid phase viscosity show that the air entrainment process is enhanced when keeping all test conditions constant except for increasing the Newtonian liquid viscosity above of that of water. A two‐zone entrainment model based on a variable two‐phase entrainment coefficient is proposed with the normalized axial distance allowing for a change in the jet angle. Thus, the jet perimeter is lower and the breakup length is longer in the case of air/relatively higher viscosity liquid phase. It provides the most accurate reproduction of the experimental droplet velocity in comparison with that of other models in the literature and, hence, is recommended for the prediction of the droplet velocity in the case of two‐phase air/liquid phase free jet flow in stagnant ambient air. A model for predicting the droplet rain out, considering the droplet trajectories in the free jet flow, allows also for an adequate reproduction of the experimental data.     

Finite Volume Simulation of High Speed Combustion of Premixed Air‐Acetylene Mixtures in a Microchannel

High speed combustion characteristics of premixed stoichiometric air‐acetylene mixtures inside microchannels are numerically studied by solving a Navier‐Stokes (NS) system of equations with a single‐step chemistry model. A two dimensional explicit finite volume solver has been developed using modified advection upwind splitting methods (AUSM+) to predict the complex interactions among hydrodynamic processes, shock structures and combustion in microdimensions. The effects of channel aspect ratio and wall temperature on high speed microcombustion have been studied in this work. The increase in wall temperature due to wall friction in reduced dimensions initiates the chemical reaction of the combustible mixture near the wall region, and the reacted zone reaches the centerline for smaller height‐to‐length ratios of the microchannels. The wall temperature plays an important role in hypersonic combustion at the microscale.     

Numerical Simulation of Gas‐Liquid Flow in a Stirred Tank with Swirl Modification

Although the standard k‐? model is most frequently used for turbulence modeling, it often leads to poor results for strongly swirling flows involved in stirred tanks and other processing devices. In this work, a swirling number, R_S, is introduced to modify the standard k‐? model. A Eulerian‐Eulerian model is employed to describe the gas‐liquid, two‐phase flow in a baffled stirred tank with a Rushton impeller. The momentum and the continuity equations are discretized using the finite difference method and solved by the SIMPLE algorithm. The inner‐outer iterative algorithm is used to account for the interaction between the rotating impeller and the static baffles. The predictions, both with and without R_S corrections, are compared with the literature data, which illustrates that the swirling modification could improve the numerical simulation of gas‐liquid turbulent flow in stirred tanks.     

Numerical simulation of the two-dimensional flow in high pressure catalytic combustor for gas turbine

The objective of this paper is modeling the mechanism of high pressure and high temperature catalytic oxidation of natural gas, or methane. The model is two-dimensional steady-state, and includes axial and radial convection and diffusion of mass, momentum and energy, as well as homogeneous (gas phase) and heterogeneous (gas surface) single step irreversible chemical reactions within a catalyst channel. Experimental investigations were also made of natural gas, or methane combustion in the presence of Mn-substituted hexaaluminate catalysts. Axial profiles of catalyst wall temperature, and gas temperature and gas composition for a range of gas turbine combustor operating conditions have been obtained for comparison with and development of a computer model of catalytic combustion. Numerical calculation results for atmospheric pressure agree well with experimental data. The calculations have been extended for high pressure (10 atm) operating conditions of gas turbine.     

失效20G高压炉管热应力数值模拟

炉管是注汽锅炉重要的换热元件,它的安全运行意义重大.由于炉膛内温度分布不均匀,当瞬时温差较大时炉管局部会产生较大热应力.本文从热应力的角度对高压锅炉管爆管原因进行分析.通过Ansys建立20G炉管裂纹模型,对50 ～ 350℃不同温差产生的热应力进行模拟.通过分析得出:热应力会加剧材料局部损伤,并且是导致爆管的重要因素...     

Numerical study on development of particle concentration profiles in a curved microchannel

The functional section of a microseparator/classifier is a semicircular microchannel whose downstream end bifurcates to separate/classify the particles in a slurry [Ookawara, S., Higashi, R., Street, D., Ogawa, K., 2004a. Feasibility study on concentration of slurry and classification of contained articles by microchannel. Chemical Engineering Journal 101, 171-178 and Ookawara, S., Higashi, R., Street, D., Ogawa, K., 2004b. The Influence of channel depth on the performance of a microseparator/classifier. Kagaku Kougaku Ronbunshu 30, 135-141.]. Previous numerical studies, based on an Eulerian-Eulerian approach, showed how the particle lift force was an indispensable factor for the separation/classification [Ookawara, S., Street, D., Ogawa, K., 2004c. A practical application of the Euler-granular model to a microseparator/classifier. In: Proceedings of the Fifth International Conference on Multiphase Flow, CD-ROM, #206.]. The present numerical study, by consistently employing the Eulerian-Eulerian approach, extensively examines the development of particle concentration profiles and the effects of feed concentration at various cross-sections in a curved microchannel for De=30(Re=450). The necessary arc length for particle concentration profiles to be fully established increases with the particle decreasing size. Particles become most concentrated at the centers of secondary Dean vortices. The dimensions of concentration region depend on the particle size and the feed concentration. In spite of the small particle relaxation time in water and the laminar flow nature, steep shear rates in a microchannel cause a collision interval comparable to the relaxation time of the particles that can be separated. To characterize the effect a newly defined Stokes number is based on the shear-induced particle-particle collisions in liquid laminar flow. A concentration efficiency is also defined as the normalized ratio of the maximum concentration to the feed concentration and it is approximately 1.0 below a Stokes number of 0.1. However, beyond the Stokes number of 0.1 the concentration efficiency decreases linearly as the log of the Stokes number increases independently of the particle size. This is because the particle to particles collision in a concentrated slurry adversely influences the efficiency of the separator.     

小型往复式压缩机等温压缩过程的数值分析和工程实现方法研究

等温压缩过程理论上可大幅降低制冷压缩机的功耗,通过对压缩机分别进行绝热压缩过程和近似等温压缩过程的数值模拟,明确了等温压缩可以节省的压缩机输入功耗。研究了通过润滑油系统冷却压缩机实现近似等温的工程方法,并通过润滑油冷却系统的试验确定最优的注油量,为提高压缩机的性能和节能减耗提供有价值的参考。