A computational investigation of non-isothermal gas-solid flow in a U-bend

The non-isothermal gas-solid flow through a U-bend of a pneumatic conveying dryer system is calculated using the commercial CFD program Fluent 6.1. Steady-state, incompressible and non-isothermal gas-solid flows are employed to simulate the cases. Variables studied include: particle diameter, particle density, solid loading ratio, feed gas temperature, heat flux through the wall, gas velocity and bend radius ratio on heat transfer phenomena between gas and solid particles. Validation is done by comparing calculation results with the available experimental data provided by Baughn et al. [J.W. Baughn, H. Iacovides, D.C. Jackson, B.E. Launder, Local heat transfer measurements in turbulent flow around a 180° pipe bend, Journal of Heat Transfer 109 (1) (1987) 43-48] and Depew and Farbar [C.A. Depew, L. Farbar, Heat transfer to pneumatically conveyed glass particles of fixed size, Journal of Heat Transfer 85 (1963) 164-172].In general, data validations of both cases show good agreement. The gas temperature decreases and the solid temperature increases along the axial direction of the pipe due to transfer of heat from the gas phase to the solid phase. The gas temperature decreases significantly at the outer bend wall due to an accumulation of particles, which causes much more energy to be transferred from the gas to solid phases. At the inner bend wall, the gas temperature decreases slightly but the solid temperature increases significantly due to a low concentration of particles. A U-bend significantly increases the local and area average Nu numbers, but not the mass average Nu number. The slip velocity and particle distribution are the major factors influencing the value of the mass average Nu number.     

网格尺寸对循环流化床内气固两相流动的影响

将基于能量最小多尺度方法(EMMS)的曳力模型耦合到双流体模型中,并针对循环流化床内的气固两流动进行了模拟研究。采用全滑移壁面边界条件处理颗粒相,考察了3种网格尺度对轴向空隙率和出口颗粒循环量等气固流动特性的影响。计算结果表明,应用EMMS曳力模型处理相间作用力,同时在采用全滑移壁面边界条件处理颗粒相时,双流体模型能够正确预测轴向空隙率分布。采用网格尺寸为2.325 mm×20 mm时,模拟结果和实测数据吻合较好,表明在循环流化床的数值模拟中选择恰当的网格尺度是极为重要的。     

Gas-solid flow in an ironmaking blast furnace-II: Discrete particle simulation

This paper presents a numerical study of the gas-solid flow in an ironmaking blast furnace by combining discrete particle simulation (DPS) with computational fluid dynamics (CFD). The conditions considered include different gas and solid flow rates, asymmetric conditions such as non-uniform gas and solid flow rates in blast furnace raceways, and existence of scabs on the side walls. The obtained results show that main gas-solid flow features under different conditions can be captured by this approach. The computed results are consistent with the experimental observations. Microscopic structures including the force structure are examined to analyze the effect of gas flow on the solid flow at a particle scale. Further, macroscopic properties such as solid pressure and porosity are obtained from the corresponding microscopic properties by an averaging method. It is shown that the solid pressure-porosity relationship in a blast furnace is complicated, varying with different flow zones. None of the literature correlations considered can fully describe such a feature. Based on the simulated results, two correlations are formulated to describe the solid pressure-porosity relationship covering different flow regimes. But their general application needs further tests in future work.     

DEM-CFD simulation of the particle dispersion in a gas-solid two-phase flow for a fuel-rich/lean burner

The objective of this study was to numerically investigate the particle dispersion mechanisms in the gas-solid two-phase jet for a fuel-rich/lean burner by means of coupling the discrete element method (DEM) with the computational fluid dynamics (CFD). The DEM was employed to deal with the particle-particle and the particle-wall interaction in the computation of solid flow; while gas flow was computed by CFD based on the commercial software package Fluent. The particles with various Stokes numbers equal to 0.1, 0.5, 1, 2 and 3 (corresponding to particle diameter 10.8, 24.17, 34.18, 48.38 and 59.21 μm, respectively) in the gas-solid fuel-rich/lean jet were investigated in this study. The particle-particle collision was simulated and its effect on the fuel-rich/lean separating performance was evaluated. The results show that the particle-particle collision occurred more frequently with the increasing of Stokes numbers from 0.1 to 3. The particle dispersion became more uniform between the fuel-rich side and the fuel-lean side for particles with small Stokes number; while for particles at St > 1, a better fuel-rich/lean separating performance was achieved. The efficiency of the DEM-CFD coupling method was validated by the corresponding experiments, and a good agreement between the simulation and experiments was achieved as a result of the particle-particle collision.     

Effect of pitched short blades on the flow characteristics in a stirred tank with long-short blades impeller

This work focuses on the design improvement of the long-short blades (LSB) impeller by using pitched short blades (SBs) to regulate the flow field in the stirred vessel. After mesh size evaluation and velocity field validation by the particle image velocimetry, large eddy simulation method coupled with sliding mesh approach was used to study the effect of the pitched SBs on the flow characteristics. We changed the inclined angles of the SBs from 30° to 60° and compared the flow characteristics when the impeller was operated in the down-pumping and up-pumping modes. In the case of down-pumping mode, the power number is relatively smaller and vortexes below the SBs are suppressed, leading to turbulence intensification in the bottom of the vessel. Whereas in the case of up-pumping mode, the axial flow rate in the center increased significantly with bigger power number, resulting in more efficient mass exchange between the axial and radial flows in the whole vessel. The LSB with 45° inclined angle of the SBs in the up-pumping mode has the most uniform distributions of flow field and turbulent kinetic energy compared with other impeller configurations.     

Numerical modelling of swirl flow induced by a three-lobed helical pipe

Increasingly in the process industries, the introduction of swirl flow upstream of bends is being used in particle-laden flow to reduce wear. This paper describes a computational fluid dynamics model of the swirl flow that is induced in an airflow passing through a horizontally mounted three-lobed helical pipe. It then goes on to discuss the results of the model verification and validation studies performed. The numerical model was used to further investigate the flows observed within and downstream of both a straight circular pipe and a three-lobed helical pipe that were previously studied during experiments conducted on a laboratory rig for a range of Reynolds numbers. A comparative analysis of the experimental and model predicted data concluded that the simulation results replicated well the experimental results for the control pipe. For the helical pipe, axial velocities were in reasonable agreement with experimental measurements across the range of Reynolds numbers studied. However, tangential velocities were under-predicted by an average of 35% in comparison to the experimental results. As a result, swirl intensity values were also under-predicted, but the values of the computed swirl decay rates were in agreement with those observed during the experiments.     

Effects of using two- versus three-dimensional computational modeling of fluidized beds: Part I, hydrodynamics

Simulations of fluidized beds are performed to study and determine the effect on the use of coordinate systems and geometrical configurations to model fluidized bed reactors. Computational fluid dynamics is employed for an Eulerian-Eulerian model, which represents each phase as an interspersed continuum. The transport equation for granular temperature is solved and a hyperbolic tangent function is used to provide a smooth transition between the plastic and viscous regimes for the solid phase. The aim of the present work is to show the range of validity for employing simulations based on a 2D Cartesian coordinate system to approximate both cylindrical and rectangular fluidized beds. Three different fluidization regimes, bubbling, slugging and turbulent regimes, are investigated and the results of 2D and 3D simulations are presented for both cylindrical and rectangular domains. The results demonstrate that a 2D Cartesian system can be used to successfully simulate and predict a bubbling regime. However, caution must be exercised when using 2D Cartesian coordinates for other fluidized regimes. A budget analysis that explains all the differences in detail is presented in Part II [N. Xie, F. Battaglia, S. Pannala, Effects of Using Two-Versus Three-Dimensional Computational Modeling of Fluidized Beds: Part II, budget analysis, 182 (1) (2007) 14] to complement the hydrodynamic theory of this paper.     

Numerical simulation of hydrodynamics in downers using a CFD-DEM coupled approach

The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows.     

Effects of using two- versus three-dimensional computational modeling of fluidized beds: Part II, budget analysis

The partial differential equations for modeling gas-solid flows using computational fluid dynamics are compared for different coordinate systems. The numerical results of 2D and 3D simulations for both cylindrical and rectangular domains are presented in Part I (N. Xie, F. Battaglia, S. Pannala, Effects of using two- versus three-dimensional computational modeling of fluidized beds: Part I, Hydrodynamics (2007-this volume), doi:10.1016/j.powtec.2007.07.005), comparing the hydrodynamic features of a fluidized bed. The individual terms of the governing equations in 2D and 3D simulations with the cylindrical and Cartesian coordinate systems are evaluated in this study through a budget analysis. The additional terms appearing in the 3D equations can be used to explain the discrepancies between 2D and 3D simulations. The values of the additional terms is shown to increase as inlet gas velocity increases. This explains the good agreement between 2D and 3D simulations that is observed for bubbling regimes with low gas velocity, and why the differences between 2D and 3D simulations increases for slugging and turbulent regimes.     

沉降离心机的性能分析

沉降离心机的生产能力取决于流体的运动形式与运动速率,因此对沉降离心机内部流场的研究有助于分析沉降离心机的性能。采用流体计算力学技术对两种沉降离心机在稳定工作状态下的内部流场进行数值仿真,解释在实际生产过程中出现的一些现象。并通过对两种沉降离心机转鼓内停留时间以及料液运动形式对比,得出从这些方面分析正杯离心机的性能要优于倒杯离心机的结论。     

Flow processes in a radiant tube burner: Isothermal flow

This paper presents the first part of a study of the combustion processes in an industrial radiant tube burner (RTB). The RTB is used typically in heat-treating furnaces. The work was initiated because of the need for improvements in burner lifetime and performance. The present paper is concerned with the flow of combustion air; a future paper will address the combusting flow. A detailed three-dimensional computational fluid dynamics model of the burner was developed, validated with experimental air flow velocity measurements using a split-film probe. Satisfactory agreement was achieved using the k-ε turbulence model. Various features along the air inlet passage were subsequently analysed. The effectiveness of the air recuperator swirler was found to be significantly compromised by the need for a generous assembly tolerance. Also, a substantial circumferential flow maldistribution introduced by the swirler is effectively removed by the positioning of a constriction in the downstream passage.     

Investigation of multiphase flow in sequencing batch reactor (SBR) by means of hybrid methods

An analysis of a sequencing batch reactor (SBR) from a fluid mechanical point of view, used in the cultivation of granular activated sludge (GAS), is carried out by experimental, computational fluid dynamics (CFD) and artificial neuronal networks (ANN) methods. Due to the complexity of the three-phase problem, new hybrid methods are developed in order to shorten the calculation time and to improve the numerical prediction. In the numeroexperimental hybrid, experimentally obtained velocities from particle image velocimetry (PIV) method are implemented as initial conditions for the numerical simulation. This operation causes improvement of multiphase flow results and save CPU time of about 40% in comparison with the standard calculation. In the neuronumerical hybrid, numerically obtained results and process parameters are employed for training of an ANN. With the trained ANN, several geometrical and physical entities are calculated for a range of SBRs. In this case, the acceleration, in the prediction of several process parameters, reaches the factor 1.0E+05.     

U形弯头单元内气液两相流型及压降波动特性

对空气-水两相流在内径16 mm、弯曲半径100 mm的横向U形弯头单元内向上流动时的流型进行了实验研究。利用流动可视化技术及其相应压降波动规律实现了流型的客观识别。总结了不同流型的压降波动特性并据此提出了定量识别流型的新方法。实验范围内发现了分层-搅拌流、塞状-泡状流、段塞-波形流、环状-波形流和环状弥散流等5种与水平直管和垂直直管不同的流型。相比标准偏差,压降波动的功率谱（PSD）分布能更好地反映U形弯头单元内不同流型的流态演变特征与动力学特性。PSD分布的偏度或峰度与气液表观流速比的结合可以定量客观地识别U形弯头单元内的流型,流型转变时的气液表观流速比为1和13。     

有机硅单体合成的气固流化床的三维数值模拟

为了探索有机硅单体合成气固流化床内硅粉颗粒的流化特性,作者利用计算流体力学CFD软件,采用双欧拉气固两相流模型及SIMPLE算法,模拟了三维的气固流化床内硅粉颗粒的流化特性;分析了气泡生成、长大和破裂的过程,及不同床层高度的固体颗粒运动速度矢量图,不同床层高度处横截面颗粒体积分数变化。结果表明:三维模拟能直观的表现颗粒在流化床中的流化状态,为工业生产及应用提供了有效的依据。     

Numerical Analysis of the Catalytic Combustion of Premixed Methane/Air Mixtures in Microtubes

The combustion characteristics and extinction limits for the catalytic combustion of a methane/air mixture in a microtube are investigated computationally using the commercial CFD code FLUENT coupled to an external subroutine DETCHEM. The effects of the microtube dimensions, conductivities of wall materials, external heat losses and flow velocity on the combustion stability, are also studied. The numerical model is set as either adiabatic or non‐adiabatic with a fixed exterior heat transfer coefficient. Numerical results indicate that thermal conductivity and wall thickness are vital to preheat the methane/air mixture through the conducting wall. Two types of extinction occur, i.e., thermal quenching and blow out. These extinction limits are characterized by wall surface temperature in the microtube and the ratio of Pt(s)/O(s).     

Numerical investigation of gas-solid heat transfer in rotating fluidized beds in a static geometry

Gas-solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas-solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas-solid slip velocities and resulting gas-solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed “width” to “height” allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas-solid slip velocities drastically increase the rate of gas-solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas-solid drag force being influenced by the fluidization gas flow rate in a similar way, rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed.     

Numerical simulations of the reactive mixing in a commercially operated stirred ethoxylation reactor

The computational fluid dynamics (CFD) technique was applied to describe the mixing and the chemical reactions in commercially operated stirred ethoxylation reactors. Two reactor sizes in the existing ethoxylation operations were studied in this work: a laboratory-scale autoclave with a single-Rushton turbine and an industrial-scale reactor with a dual-Rushton turbine. The ethoxylation reactor contents were described as an incompressible, turbulent single-phase liquid mixing regime with chemical species undergoing heat and mass transfer. Since the accurate experimental ethoxylation flow data could not be extracted from the industrial operations, the development of the CFD model for the ethoxylation process was undertaken in two stages. The first stage simulated a single-phase liquid agitation system based on the literature with experimental data on velocities, such as Wu and Patterson [1989. Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer. Chemical Engineering Science 44, 2207–2221], for a Rushton stirred reactor of standard configuration. Once validated, the numerical model was applied to compute the flow field in ethoxylation reactors. The second stage integrated the ethoxylation kinetics into the numerical model and simulated the ethoxylation process. In the simulation of the mean flow field, the qualitative features of the literature data were well reproduced. The computed results of both the ethylene oxide consumption and the temperature calculation compared very well with the measurements in the laboratory-autoclave operations. Reasonably good agreement was also reached between the simulated and experimental data on the time-dependent changes of ethylene oxide mass fraction in the bulk liquid in the industrial ethoxylation operations. These demonstrate that the CFD process model was capable of predicting the reaction behaviour and would be useful for exploration of any opportunity for increasing the ethoxylation capacity in the industrial operations.     

Numerical Study on Thermal Safety of Emulsion Matrix in Emulsifier

The purpose of this study is to research the effect of high speed stirring by emulsifier blades on the thermal safety of emulsion matrix. In order to analyze thermal safety of emulsion matrix in emulsifying process, the heating effect influenced by rotor speed and emulsify temperature was combined with self‐heat properties of emulsion matrix in this study. Computational fluid dynamics (CFD) is used for analysis. The emulsifier model adopts simplified CYJ type emulsifier, which is a kind of vertical emulsifier and has a large stirring area. The self‐heat properties of emulsion matrix are described by Arrhenius equation. The simulation results showed that heat could not accumulate in emulsion matrix due to the strong turbulence formed by stirring. But, because of the shear stress and friction, a high temperature region will be formed around the blades of the rotor in a short time. If the stirring speed reaches 10300 rpm, the temperature in this region would be able to reach the critical ignition temperature of emulsion matrix.     

NUMERICAL SIMULATION AND ANALYSIS OF FLOW IN A DTB CRYSTALLIZER

This research numerically simulates the two-phase (liquid and vapor) flow in a 1 m³ draft tube baffle (DTB) crystallizer with fines removal streams. The computational fluid dynamics (CFD) commercial software ANSYS CFX-10.0 was employed to perform 3-D simulation using the finite volume method with an unstructured mesh topology. The influence of hydrodynamics in the crystallizer, as characterized by the momentum source strength and fines removal flow, on the flow characteristics and the classification of crystals are investigated. The results showed the liquid flow is fully uniform in the main body of the crystallizer studied for momentum sources larger than or equal to 19.63 kg · m/s². The uniformity of the suspension will strongly affect the product crystal size distribution. Momentum source strengths and fines removal flow rates also have a significant effect on the fines removal cut-size due to varying up-flow velocities in the fines removal section, altering the size at which particles are carried out in the fines removal stream. The CFD predictions are compared with the experimental results from the literature and can be used for the optimization of commercial-scale DTB crystallizer design.