Computational study of particle in-flight behavior in the HVOF thermal spray process

A computational framework is developed for the multiphase flow in a high velocity oxygen-fuel (HVOF) thermal spray coating process with steel powders as the feedstock. The numerical model includes continuum-type differential equations that describe the evolution of gas dynamics and multi-dimensional tracking of particle trajectories and temperature histories in the turbulent reacting flow field. The numerical study shows that the particle temperature is strongly affected by the injection position while the particle velocity is less dependent on this parameter. Moreover, both particle velocity and temperature at impact are strongly dependent on particle size, although the spatial variation of these two variables on the substrate is minimal. It is also found that not all the particles are deposited on the substrate perpendicularly (even if the spray angle is 90°), due to substantial radial fluid velocities near the stagnation point. A statistical distribution of particle velocity, temperature, impinging angle and position on the substrate as well as particle residence time is obtained in this work through independent random tracking of numerous particles by accounting for the distributed nature of particle size in the feedstock and injection position as well as the fluctuations in the turbulent gas flow.     

DEM-LES of coal combustion in a bubbling fluidized bed. Part I: gas-particle turbulent flow structure

The gas and particle motions in a bubbling fluidized bed both with and without chemical reactions are numerically simulated. The solid phase is modelled as Discrete Element Method (DEM) and the gas phase is modelled as 2-D Navier-Stokes equations for 2-phase flow with fluid turbulence calculated by large Eddy simulation (LES), in which the effect of particles on subgrid scale gas flow is taken into account. The gas/particle flow structure, the mean velocities and turbulent intensities can be predicted as a function of several operating parameters (particle size, bed temperature, and inlet gas velocity). The lower the inlet gas velocity, the higher the ratio of particle collision. The distributions of the particle anisotropic velocity show that the particles have no local equilibrium, and the distribution of gas kinetic energy corresponds to the distribution of gas-particle coupling moment in the fluidized bed. An intensive particle turbulent region exists near the wall, and the gas Reynolds stress is always much higher than the particle stress. The presence of the large reactive particles in the fluidized bed may affect significantly the gas and particle velocities and turbulent intensities. The effects of the bed temperature and inlet gas velocity on the gas particle flow structure, velocity, and turbulent intensity are also studied.     

MICROMIXING EFFECTS ON BARIUM SULFATE PRECIPITATION IN AN MSMPR REACTOR

The Effects of non-ideal and nonhomogeneous mixing on barium sulfate precipitation in an MSMPR reactor were observed experimentally and analyzed theoretically. To generate nonhomogeneous mixing the unmixed feed streams were fed to the reactor at the same location (joint feeding mode) or a plug flow reactor was connected to the MSMPR reactor. These nonhomogeneous mixing conditions resulted in significant reductions in particle size and increases in particle numbers. These non ideal mixing effects were dependent on the impeller speed, feed stream velocity and residence time in the connected plug flow reactor and are believed to result from elevated supersaturation levels in a premixing zone which are controlled by turbulent micromixing

To model the effect of nonhomogeneous mixing (premixing) in the MSM PR reactor a plug flow-stirred lank reactor series model was developed. The plug flow reactor represents the premixing region of the MSMPR reactor in which turbulent micromixing is important, and the stirred tank reactor describes the homogeneous mixing region of the MSMPR reactor where particle growth is important. The model predicts that the premixing effect is strongly dependent on micromixing of the feeds in the premixing region, and thus, as the turbulent mixing intensity in this region is increased, the particle size in the product suspension is reduced and the particle population is increased. These predictions of the model arc in good agreement with the experimental data. An interesting prediction of the model is that as the impeller speed increases, the precipitation of barium sulfate in an MSMPR reactor deviates increasingly from the precipitation in a perfectly mixed (ideal) reactor.     

Diagnosis of bubble distribution in a three-phase bubble column reactor for dehydration of ortho-boric acid

For the practical application of a three-phase bubble column as a reactor in the dehydration of ortho-boric acid, we investigated the bubble distribution and its effects on the reaction in a three-phase bubble column reactor (0.102 m IDx2.0 m in height) operating at relatively low pressure (below the atmospheric pressure). Effects of reaction time, temperature, gas velocity, particle size and gas injection mode (even, wall-side, central and asymmetric distribution) on the fractional conversion of the reaction were determined. The complicated bubble distribution as well as bubbling phenomena in the reactor were diagnosed and interpreted by means of the attractor trajectories and correlation dimension which were obtained from the resultant pressure fluctuations. The fractional conversion was closely related to the attractor shape or correlation dimension of the pressure fluctuations in the reactor. The fractional conversion in the case of even distribution of gas injection exhibited the highest value in all cases studied, at which the attractor of pressure fluctuations was less scattered in the phase space, while their correlation dimension had the lowest value. When the gas was injected by means of wall-side distribution, the conversion level was higher than that in case of central or asymmetric distribution mode. Although a fluid-solid heterogeneous reaction model can be applicable to the reaction, deviations from the model become considerable when the gas injection mode changes from even to wall-side, central or asymmetric mode, orderly.     

Modeling Oxidation of Soot Particles Within a Laminar Aerosol Flow Reactor Using Computational Fluid Dynamics

This work examines the measurement of surface specific soot oxidation rates with the High Temperature Oxidation-Tandem Differential Mobility Analyzer (HTO-TDMA) method. The Computational Fluid Dynamics package CFD-ACE⁺ is used to understand particle flow, oxidation and size dependent particle losses in the laminar aerosol flow reactor using an Eulerian-Lagrangian framework. Decrease of DMA selected mono-disperse particle size distribution due to oxidation within the aerosol tube is modeled using fitted kinetic soot oxidation parameters. The effects of Brownian diffusion and thermophoresis on particle flow and loss to the reactor walls are evaluated. The position of peak particle diameter, which is used as an indicator to determine oxidation rate, is found to be independent of diffusion, thermophoresis and secondary flow effects, thus validating its use in deriving kinetic soot oxidation parameters. Diffusion does not affect the evolution of particle size distribution within the reactor. However, thermophoresis is found to be the dominant mechanism influencing both shape of particle size distribution and particle loss to the walls of the aerosol reactor. Simulations show reduced effects of secondary recirculating flows on the particle flow trajectories in a vertical furnace as compared to horizontal furnace orientation. This work highlights the importance of making accurate measurements of temperature within the modeling domain. Since gas temperature within the flow tube could not be measured with high radial resolution using radiation shielded thermocouple, the derived soot oxidation rate may be uncertain by a factor of 2. Importantly, CFD simulations suggest that a distribution of temperature and size-dependent particle reactivities may be present in the reactor, requiring further theoretical and experimental investigation.     

Numerical simulation of flow field and residence time of nanoparticles in a 1000-ton industrial multi-jet combustion reactor

In this work, by establishing a three-dimensional physical model of a 1000-ton industrial multi-jet combustion reactor, a hexahedral structured grid was used to discretize the model. Combined with realizable k–ε model, eddy-dissipation-concept, discrete-ordinate radiation model, hydrogen 19-step detailed reaction mechanism, air age user-defined-function, velocity field, temperature field, concentration field and gas arrival time in the reactor were numerically simulated. The Euler–Lagrange method combined with the discrete-phase-model was used to reveal the flow characteristics of particles in the reactor, and based on this, the effects of the reactor aspect ratios, central jet gas velocity and particle size on the flow field characteristics and particle back-mixing degree in the reactor were investigated. The results show that with the decrease of aspect ratio in the combustion reactors, the velocity and temperature attenuation in the reactor are intensified, the vortex phenomenon is aggravated, and the residence time distribution of nanoparticles is more dispersed. With the increase in the central jet gas velocities in reactors, the vortex lengthens along the axis, the turbulence intensity increases, and the residence time of particles decreases. The back-mixing degree and residence time of particles in the reactor also decrease with the increase in particle size. The simulation results can provide reference for the structural regulation of nanoparticles and the structural design of combustion reactor in the process of gas combustion synthesis.     

An analysis of the falling film gas-liquid reactor

A mathematical model of the falling film reactor is developed to predict the conversion and temperature distribution in the reactor as a function of the gas and liquid flow rates, physical properties, the feed composition of the reactive gas and carrier gas and other parameters of the system. Transverse and axial temperature profiles are calculated for the laminar flow of the liquid phase with concurrent flow of a turbulent gas to establish the peak temperatures in the reactor as a function of the numerous parameters of the system. The reaction rate in the liquid film is considered to be controlled by the rate of transport of reactive gas from the turbulent gas mixture to the gas-liquid interface. The governing equations are solved by reformulating the problem in integral equation form employing Green's functions. The predicted reactor characteristics are shown to agree with large-scale reactor performance, and the effects of the most sensitive parameters are elucidated to provide a basis for reactor optimization. Numerical results are presented for realistic sulfonation reactor conditions.     

流化床粉煤气体热载体快速热解反应器的计算

在冷态模拟实验和煤热解动力学计算的基础上,对粉煤气体热载体快速热解提升管反应器的高度进行了计算。利用高速摄像粒子测速法结合互相关算法研究了不同气体流量和不同颗粒粒径时固体颗粒在热解提升管中的运动速度,通过求解神府煤热解动力学方程,得到了不同粒径神府煤颗粒热解挥发分析出的时间,从而确定了快速热解提升管反应器的高度。研究结果表明:当气体流量在850 m3/h,粉煤的粒径主要集中在0.7—3.0 mm时,提升管的高度应选择在10.0 m。     

下行床反应器内催化裂化过程的CFD模拟

耦合湍流气粒多相流模型和催化裂化集总动力学模型,建立了描述下行床内多相流动和催化裂化过程的反应器数学模型,并利用计算流体力学单元模拟软件CFX4.3对下行床内的催化裂化过程进行了数值模拟及分析.模型能预测出在工业应用中反应器内最受关注的诸多参数,如固含率、相间滑移速度、压降、气固相的加速区以及各组分浓度的分布情况.预测结果表明,气相反应的进行将导致反应器内的气粒流动行为发生较大变化,充分考虑反应与流动行为的耦合十分重要;而反应器床径的增大将导致转化率和各产物收率的下降.     

下行床煤拔头工艺的产品产率分布和液体组成

基于循环流化床锅炉燃烧技术和下行床技术相结合,在处理能力为8 kg/h的煤拔头工艺热态实验装置上,以内蒙古霍林河褐煤为原料,普通河砂为固体热载体,考察了反应温度和煤粉粒径对气液固产品产率分布和液体组成的影响规律. 结果表明,在实验温度范围内,随着温度的升高,气体和液体产品的产率增加;液体产率随粒径的增大而降低. 当反应温度为660℃、煤粉粒径小于0.28 mm、加料率为4.7 kg/h时,轻质焦油(焦油中的正己烷可溶物)的产率可达7.5%(干煤基,w),其中酚类占57.1%,粗汽油(脂肪烃类)占12.9%,芳香烃占21.4%,极性组分和其他组分占8.6%. 实验表明,下行床由于气固沿重力场并流向下流动的特点,是适合煤拔头工艺的比较理想的反应器.     

Analysis of iron particle growth in aerosol reactor by a discrete-sectional model

The growth of iron particles by thermal decomposition of Fe(CO)₅ in a tubular reactor was analyzed by using a one dimensional discrete-sectional model with the coalescence by sintering of neighboring particles incorporated in. A thermal decomposition of Fe(CO)₅ vapor to produce iron particles was carried out at reactor temperatures varying from 300 to 1,000°C, and the effect of reactor temperature on particle size was compared with model prediction. The prediction exhibited good agreement with experimental observation that the primary particle size of iron was the largest at an intermediate temperature of 800°C. Model prediction was also compared with Giesen et al.’s [1] experimental data on iron particle production from Fe(CO)₅. Good agreement was shown in primary particle size, but a considerable deviation was observed in primary particle size distribution. The deviation may be due to an inadequate understanding of the sintering mechanism for the particles within an agglomerate and to the assumption of an ideal plug flow in model reactor in contrast to the non-ideal dispersive flow in actual reactor.     

Pyrolytic characteristics of Jatropha seedshell cake in thermobalance and fluidized bed reactors

Pyrolytic kinetic parameters of Jatropha seedshell cake (JSC) were determined based on reaction mechanism approach under isothermal condition in a thermobalance reactor. Avrami-Erofeev reaction model represents the pyrolysis conversion of JSC waste well with activation energy of 36.4 kJ mol^?1 and frequency factor of 9.18 s^?1. The effects of reaction temperature, gas flow rate and feedstock particle size on the products distribution have been determined in a bubbling fluidized bed reactor. Pyrolytic bio-oil yield increases up to 42 wt% at 500 °C with the mean particle size of 1.7 mm and gas flow rate higher than 3U _mf , where the maximum heating value of bio-oil was obtained. The pyrolytic bio-oil is characterized by more oxygen, lower HHVs, less sulfur and more nitrogen than petroleum fuel oils. The pyrolytic oil showed plateaus around 360 °C in distribution of components’ boiling point due to high yields of fatty acid and glycerides.     

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model.

The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.     

MICROMIXING EFFECTS ON BARIUM SULFATE PRECIPITATION IN A DOUBLE-JET SEMI BATCH REACTOR

The effects of turbulent mixing on barium sulfate precipitation in an imperfectly mixed double jet semi batch reactor were investigated experimentally and theoretically. When two feed solutions in separate streams were fed into the semi batch reactor, the precipitation was significantly altered by the impeller speed and the feeding time. Generally, in the range of low impeller speed (below 400 rpm), the suspension was segregated vertically in the reactor and the average particle size increased with increasing impeller speed. However, in the range of high impeller speed (above 400 rpm) the suspension was homogeneously dispersed in the reactor, but the trend of the turbulent mixing effect on the precipitation was opposite to that in range of the low impeller speed. The precipitation in the semi batch reactor was controlled by particle mass transfer and micromixing of the feed streams, both of which were promoted by increasing the impeller speed. At low impeller speed the influence of the mass transfer was dominant so that the particle size increased with increasing impeller speed, but at high impeller speed it was surpassed by the influence of micromixing so that the trend was reversed because enhanced micromixing generates a large number of small particles in the reactor.

To model our hypothesis for the effects of imperfect mixing on the reaction precipitation in the semi batch reactor, a micromixing-limited plug flow-ideal semi batch series reactor model was developed. The model predicts that enhanced micromixing created high supersaturation levels in the premixing region (plug flow reactor) which reduces the average particle size. The model also predicts the effect of feeding time on the precipitation in the semi batch reactor. These predictions are in excellent agreement with the experimental data. An interesting prediction of our model is that micromixing in the premixing region plays an important role in the overall reaction precipitation and its effect is greatly intensified as the turbulent mixing intensity is increased, which is opposite to our common sense.     

气相氧化反应器内错流射流场数值模拟

气相法制备颗粒材料的管式氧化反应器中 ,原料气通过管壁的开孔或环缝喷入反应器内与轴向流动的高温氧气流混合并反应 ,混合质量是决定颗粒产品质量的关键因素。采用k ε湍流模型对气体通过环缝喷入管内轴向气流形成的错流射流场进行了数值模拟 ,模拟结果与实验数据及有关文献进行了比较。结果表明 ,k ε湍流模型能较好地模拟管内错流射流流场的基本特征 ,可以用来预测气相氧化反应器内错流射流场的有关数值。     

Simulation of nanoparticle synthesis in an aerosol flame reactor using a coupled flame dynamics–monodisperse population balance model

Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamics–monodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.     

硅油光-热老化气升式反应器数值模拟

为进行液浸聚光光伏系统中液浸液体即二甲基硅油在光热条件下的老化实验研究,设计制作了气升式外环流光催化反应器。采用Fluent软件欧拉多相流模型和RNG k-ε湍流模型对反应器进行了三维全尺寸数值模拟。分析了结构参数、操作参数和硅油物性参数对反应器中气液两相流动的影响,得到反应器内循环液速和气含率等参数的分布。模拟结果与实验结果有较好的一致性。模拟结果表明:硅油黏度越小,循环液速越大,气含率越小;反应器适宜操作气速不应大于0.02 m/s;反应器最适宜高径比和环隙面积比分别为47.6和0.31~0.41。     

分布孔大小对环流反应器内流动影响数值模拟

采用k-ε二方程模型和欧拉多相流模型,对一种气升式环流反应器内的湍流气液二相流进行了全尺寸的数值模拟研究,考察了采用具有不同大小分布孔气体分布器时反应器内气含率和流速分布的细节。模拟结果表明采用小分布孔的反应器内的平均气含率较高,气液二相接触效果较好,对于反应过程有利。计算所得的整体气含率与实测的整体气含率进行了对比,吻合较好。     

Powder processing in a plasma jet: A proposed model

A theoretical study is reported on the use of d.c. and r.f. plasma jets as chemical reactors for the processing of minerals in the form of a fine powder. The temperature and flow fields of the jet are calculated by solving the integral boundary layer equations. Single particles trajectories are obtained by solving the Basset-Odar equations. A multi-particle model is then developed for a feed of known particle size and injection velocity distributions under low loading conditions. Calculators are made on the thermal decomposition of molybdenum disulphide (5 to 30-micron equivalent diameter). The parameters investigated are the free jet velocity, the mean injection velocity, and the reactor ambient conditions. The results are presented as the probability density distributions of the gas loading, particle temperature, and conversion at different levels downstream of the nozzle.     

乙烯裂解炉耦合模拟中湍流模型的影响分析

配备底部烧嘴和侧壁烧嘴的乙烯裂解炉应用越来越广泛，不同燃烧模式影响着炉膛内湍流流动状态，考虑到裂解炉中湍流流动与燃气喷料、燃烧和传热有较强的非线性耦合作用，为此探究不同湍流模型在裂解炉/反应器耦合模拟中的影响对于裂解炉的精确设计和优化至关重要。针对不同湍流模型对某十万吨工业乙烯裂解炉进行了耦合模拟，利用CFD数值模拟对采用标准k-ε模型、RNG k-ε和Realizable k-ε模型所建立的湍流流动模型进行评估。将三种湍流模型的模拟结果与工业数据进行比较，重点分析了裂解炉内的速度、温度、湍流能力等参数的分布情况，表明Realizable k-ε模型在火焰稳定性、反应效率等方面优于其他两种模型，且基于Realizable k-ε湍流方程的反应管模型在热通量、炉管外壁温度分布计算结果更接近实际工况。