THEORETICAL STUDY OF NONUNIFORM DROPLETS CONCENTRATION DISTRIBUTION ON VENTURI SCRUBBER PERFORMANCE

A new three-dimensional dispersion model has been developed and was used to study particulate removal operations in a venturi type scrubber. The model takes into account the effect of nonuniform droplets concentration distribution on the particulate removal efficiency of the scrubber of Brink and Contant (1958). Experimental data was used to test the results of the mathematical model. The results from the model are in good agreement with the experimental data. After validating the model, it was used to predict the effect of parameters such as liquid to gas flow rate ratio, gas throat velocity, and angle of the divergent section and nozzle diameter on the extent of nonuniformity of drops. Removal efficiency will increase with increasing uniformity of droplets concentration distribution. Thus, any factor that increases removal efficiency increases uniformity and vice versa. The factors affecting uniformity are: droplet diameter, liquid jet penetration length and size of scrubber.     

脱硫废水烟气喷射蒸发流动特性实验研究

确保喷雾液滴在接触烟道壁面前完全蒸发,是保障电站脱硫废水在锅炉尾部烟道内蒸发处理安全运行的关键。喷雾液滴的破碎、聚并等动力学行为,以及液滴群的粒径分布和速度等因素的影响机制,是喷雾蒸发的主要特性。设计搭建了热态风洞实验台,利用激光粒度分析仪和粒子图像测速仪（particle image velocimeter,PIV）,在不同的引射空气压力、喷嘴水流量,以及风速、加热空气温度等条件下,对喷雾液滴群的粒径变化和速度变化进行了测量和分析。实验结果表明：以大液滴形态离开喷嘴的射流在引射气流的携带作用下,因破碎而形成小液滴,而后液滴间聚并效果会显现出来。液滴初始粒径仅与引射气体压力和水流量有关;风速的提高一定程度上会促进液滴间的聚并。提高高压气体压力、温度、风速以及减小水流量均有助于提高液滴群速度,其中提高风速对液滴群的增速效果最为明显。研究结果为喷雾的数值模拟及工程应用改进方向提供了参考。     

Numerical simulation of gas-droplets mixing and spray evaporation in rotary spray desulfurization tower

Motion and evaporation of droplets significantly affect the semidry flue gas desulfurization efficiency and long-term operation. Both the flow field distribution and the heat and mass transfer in the spray towers are studied by numerical simulation, and the process of droplet motion and evaporation is analyzed in detail. Then, two indices, mixing variance and droplet mass-weighted life, are provided to quantify gas droplet mixing and the droplet group evaporation time. The simulation results show that the radial penetration distance of the droplets is longer with the diameter increase, and the appropriate swirl number improves the mixing between the flue gas and droplets. With the increase of droplet diameter and velocity, the droplet distribution in the tower is more widely, obtaining the optimum mixing variance. The droplet mass-weighted life is promoted linearly with the increase of average droplet diameter and the decrease of flue gas temperature. With flue gas temperature increase from 458 k to 488 K, the droplet mass-weighted life decreases linearly by 31%. In comparison, the initial droplet velocity and spray angle have a slight effect on the droplet mass-weighted life.     

Numerical simulation on gas-droplet flow characteristics and spray evaporation process in CFB-FGD tower

Nozzle arrangement in the nozzle spray system has a significant impact on the gas-droplet flow characteristics and the temperature distribution within the circulating fluidized bed flue gas desulphurization (CFB-FGD) tower, which is critical to the SO₂ removal efficiency. The effects of spray direction, nozzle number and nozzle spray angle on gas-droplet distribution and temperature distribution inside the FGD tower are investigated with numerical simulation based on a Eulerian-Lagrangian mathematical model. An optimal nozzle arrangement scheme is proposed to improve the contact between gas and water droplets and the flue gas temperature distribution. Results show that upward spray direction is beneficial to the interaction between water droplets, improving gas-droplet flow characteristics and spray evaporation process, and water droplets number trapped by tower wall could be reduced in the water droplets evaporation. With the increase in nozzle number, it is conducive to the contact between flue gas and water droplets to increase the evaporation efficiency of water droplets, as well as the uniformity of temperature distribution inside the tower. With nozzle spray angle increases from 30° to 120°, flue gas velocity decreases, water droplets number trapped by the tower wall increases. The temperature distribution at different cross-section is the most uniform when the nozzle spray angle is 60°.     

Droplet cooling in atomization sprays

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes.     

Modelling and experimental study on agglomeration of particles from coal combustion in multistage spouted fluidized tower

An experimental platform of spray agglomeration has been designed and built for removing small fly ash particles (PM₁₀) from coal combustion. Agglomerating model of fine particles during the spray process was developed by combining rapid coagulation theory and analysis of the interaction between droplets and particulates. Systematic experiments were conducted in multistage spouted tower using several kinds of agglomerant solutions. The influencing factors such as surfactant, pH value, flow rate of agglomerant solutions and inlet flue gas temperature were analyzed. SEM was used to analyze the microstructure of the aggregation. Based on this model, coagulation rate constant was calculated and the influences of original parameters were observed. The spray agglomeration mechanism of particles can be analyzed from both macroscopical and microcosmic aspects. Preliminary results are presented and discussed in this work. Final results indicate that the special shape of multistage spouted fluidized tower has significant influences on the effect of agglomeration. The findings from this work will be helpful to form the basis and provide guidance for further studies on the control of fine particles such as PM_2.5 or even smaller ones.     

Performance estimation of a Venturi scrubber using a computational model for capturing dust particles with liquid spray

A Venturi scrubber has dispersed three-phase flow of gas, dust, and liquid. Atomization of a liquid jet and interaction between the phases has a large effect on the performance of Venturi scrubbers. In this study, a computational model for the interactive three-phase flow in a Venturi scrubber has been developed to estimate pressure drop and collection efficiency. The Eulerian-Lagrangian method is used to solve the model numerically. Gas flow is solved using the Eulerian approach by using the Navier-Stokes equations, and the motion of dust and liquid droplets, described by the Basset-Boussinesq-Oseen (B-B-O) equation, is solved using the Lagrangian approach. This model includes interaction between gas and droplets, atomization of a liquid jet, droplet deformation, breakup and collision of droplets, and capture of dust by droplets. A circular Pease-Anthony Venturi scrubber was simulated numerically with this new model. The numerical results were compared with earlier experimental data for pressure drop and collection efficiency, and gave good agreements.     

A simple transient numerical model for heat transfer and shape evolution during the production of rings by centrifugal spray deposition

Centrifugal spray deposition, the atomisation of a liquid metal by centrifugal force and the subsequent collection of the atomised droplets on a reciprocating collector, is currently being developed for the production of high performance Fe, Ni and Ti based ring-shaped components for use in aerospace and gas turbine containment applications. The process combines the technical, economic and metallurgical benefits of more conventional gas-assisted spray forming techniques with the advantage that it can easily operate under vacuum, reducing potential problems from gas entrapment and thermally induced porosity. In order to aid process development, understanding and optimisation, a transient numerical heat and mass transfer model has been developed that is capable of predicting the evolution of the deposit temperature distribution during spraying. The model has been validated experimentally using thermocouple measurements obtained during the production of 35 kg (340 mm diameter) IN718 rings and qualitative correlations have been observed between the predicted data and the type/distribution of porosity and second phase precipitates in the deposit. The model is currently being further developed and integrated with droplet size distribution and cooling models to provide a better insight into the physics and operational parameters which control deposit shape and microstructure.     

硝基胍连续喷雾干燥数值模拟研究

在对喷雾干燥塔内流动特点认识的基础上,采用较为成熟的计算流体力学（CFD）模型及算法,建立适用于硝基胍物料喷雾干燥的应用模型。利用Gambit软件对几何模型进行网格划分,导入Fluent中进行数值计算,预测塔内流场分布情况、液滴含水率及颗粒平均粒径等。通过测试样机干燥产品的含水率、产品粒度分布及场内温度分布等发现,预测结果与实验结果大致吻合,说明模拟结果有一定的准确度和可靠性。     

Non-equilibrium solidification of undercooled droplets during atomization process

Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid-liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.     

Simulation of an orifice scrubber performance based on Eulerian/Lagrangian method

A mathematical model based on Eulerian/Lagrangian method has been developed to predict particle collection efficiency from a gas stream in an orifice scrubber. This model takes into account Eulerian approach for particle dispersion, Lagrangian approach for droplet movement and particle-source-in-cell (PSI-CELL) model for calculating droplet concentration distribution. In order to compute fluid velocity profiles, the normal k− turbulent flow model with inclusion of body force due to drag force between fluid and droplets has been used. Experimental data of Taheri et al. [J. Air Pollut. Control Assoc. 23 (11) (1973) 963] have been used to test the results of the mathematical model. The results from the model are in good agreement with the experimental data. After validating the model the effect of operating parameters such as liquid to gas flow rate ratio, gas velocity at orifice opening, and particle diameter were obtained on the collection efficiency.     

利用离心喷雾干燥制备球形粉体的工艺因素研究

研究了铝矾土料浆的离心喷雾造粒过程,考察了料浆固含量、粘度、喷雾头的旋转速度对粉体干燥的影响.从颗粒内部水分迁移的距离、空气进出口温度两方面,分析了蘑菇头颗粒、变形颗粒、球形颗粒和破裂颗粒等颗粒形貌的形成原因,并优化了一组工艺参数:喷雾头转速为12000r/min,料浆固含量为50%,热风入口温度为300℃,废气出口温度为115℃,塔内负压为-350Pa,料浆流速为4L/h,料浆的粘度为14s,在此条件下能制备出球形度高、颗粒粒径集中分布在55μm左右的粉体.     

Equations of the kinetics of droplet fragmentation in a high-speed gas flow

For the conditions of a high-speed gas flow, within the framework of the model of quasi-continuous fragmentation of a droplet due to the mechanism of gradient instability, a differential equation of mass loss has been obtained. Within the approximation of the droplet sphericity, the law of variation of its mass, which depends on droplet acceleration by the gas stream, as well as the conditions and time of complete fragmentation of the droplet, have been found. A differential equation for the quantity of torn off droplets, has been obtained. In the event of equality between the rates of dispersion and relaxation equalization of the droplet and gas flow velocities, the size distribution functions of the number and mass of torn off droplets, as well as the values of the modal radius and total number of torn off droplets, have been found.     

Particle Formation from Solution and Slurry Sprays

ABSTRACT

The initial morphology of particles formed by spray drying or spray pyrolsis of solutions, sols or slurries, is determined by the relative velocity between the atomized droplet and the ambient gas in the furnace. A1 high relative velocities, the droplet can be disintegrated or flattened, whereas at low relative velocities the droplet remains spherical throughout the drying process. The time for respheroidization depends on the liquid properties and the solid content of the drying droplet. When the droplet reaches the percolation limit, the higher viscosity inhibits respheroidization, and the morphology of the deformed droplet is fixed. A new model based on the Weber number, percolation criteria, and the drying kinetics, is presented to predict particle morphology for various drying conditions. In addition, the effects of segregation during drying will be discussed in regard to its effect on particle formation during spray formulation. We will show how physical and chemical segregation processes occur in drying droplets, and how these segregation processes can be controlled to yield particles of designed morphology and internal phase distribution.     

Experimental investigation on the effect of liquid injection by multiple orifices in the formation of droplets in a Venturi scrubber

Venturi scrubbers are widely utilized in gas cleaning. The cleansing elements in these scrubbers are droplets formed from the atomization of a liquid into a dust-laden gas. In industrial scrubbers, this liquid is injected through several orifices so that the cloud of droplets can be evenly distributed throughout the duct. The interaction between droplets when injected through many orifices, where opposite clouds of atomized liquid can reach each other, is to be expected. This work presents experimental measurements of droplet size measured in situ and the evidence of cloud interaction within a Venturi scrubber operating with multi-orifice jet injection. The influence of gas velocity, liquid flow rate and droplet size variation in the axial position after the point of the injection of the liquid were also evaluated for the different injection configurations. The experimental results showed that an increase in the liquid flow rate generated greater interaction between jets. The number of orifices had a significant influence on droplet size. In general, the increase in the velocity of the liquid jet and in the gas velocity favored the atomization process by reducing the size of the droplets.     

Droplet breakup in electrostatic spray based on multiple physical fields

A droplet breakup model was proposed for simulating electrostatic spray in multiple physical fields. The static electricity, laminar flow and droplet atomization in COMSOL Multiphysics were coupled completely, and a two-dimensional simulation model was established. The process of droplet breakup and movement of electrostatic spray was revealed under the action of electric field, gravity field and air field. The electric field distribution under the needle ring electrode configuration was studied. The effects of different electrostatic voltage, needle ring distance and ring electrode diameter on droplet breakup characteristics, distribution uniformity and charge characteristics were analyzed. When the electrostatic voltage is -6 ～ -7 kV, the needle ring distance is 4 mm, and the ring electrode diameter is 30 mm, the electrostatic spray effect is better, and the density standard deviation is as low as 0.04528 /mm², 0.0559 /mm² and 0.06016 /mm², respectively. Electrostatic spray has the characteristics of refining droplets, improving the uniformity of droplets distribution and controlling spray morphology, which provides a strong basis for the application of electrostatic spray in surface film preparation, dust removal, fuel injection and other practical engineering fields.     

热喷涂液相合成粉末的雾化喷嘴设计

为了形成尺寸基本一致且在空间呈均匀分布的雾滴,以利于制备均匀性好、粒径分布窄的纳米颗粒,通过对单相流和两相流雾化喷嘴的对比,以及液滴的雾化过程分析,根据相关实验结果和经验公式,设计出了用于热喷涂液相合成粉末的雾化喷嘴,其雾化气输送管径为0.2mm,液料输送管径为2.0mm,二者夹角为90°。此雾化喷嘴在等离子喷涂制备TiO2纳米颗粒的实验中得到了成功应用。     

Droplet velocity and thermal state from hot gas atomization of steel melt: Impact on the quality of the spray-formed tubular deposit

The velocity and thermal behavior (temperature, enthalpy, solid fraction) of atomized droplets in a metal spray play the most important role in the spray forming process. These properties mainly determine the materials yield and the final product quality (e.g., porosity, microstructure) of the as-sprayed materials. Changing the gas temperature in the atomization process directly influences these droplet properties in the spray. To understand the droplet behavior in the spray at various atomization gas temperatures (i.e., room temperature RT 293 K, 573 K, 873 K), numerical simulations using computational fluid dynamics (CFD) techniques have been performed and validated by experiments. A series of atomization runs (powder production and spray-forming with AISI 52100 steel) has been conducted at different atomization gas temperatures and pressures with a close-coupled atomizer (CCA). The in-situ temperature detection of the deposit surface (pyrometer) and in the substrate (thermocouples) has been performed to observe the effect of particle properties on the deposit. The result shows that hot gas atomization provides smaller droplets with faster velocity in the spray, affecting the droplet impact and deformation time in the deposition zone. A higher solid fraction of the smaller droplets by hot gas atomization also reduces the deposit surface temperature. Increasing the substrate diameter further decreases the deposit surface temperature without compromising the deposit quality (i.e., porosity) and also refines the grain size. Pre-heating of the substrate up to 573 K results in lower porosity in the vicinity of the substrate.     

Preparation and characterization of spray-dried submicron particles for pharmaceutical application

The versatile use of submicron-sized particles (0.1–1?μm) requires new manufacturing methods. One possibility for the preparation of submicron-sized particles is spray drying. However, the generation of small droplets at a high production rate and the precipitation of submicron particles are quite challenging. In order to produce a sufficient amount of fine and uniform droplets, a two-fluid nozzle with internal mixing was combined with a cyclone droplet separator. The precipitation of particles was realized with an electrostatic precipitator. Considering the difficulty of electrostatic precipitation concerning explosion risks and to make it capable using organic solvents, the spray dryer was integrated in a pressure resistant vessel. Based on previous experiments, the now presented design is compact and the electrostatic precipitator is shortened. In addition, enhanced drying conditions ensured a controlled and reproducible preparation of submicron-sized particles. Thus, high separation efficiencies were shown. Spray-drying experiments were conducted with the model substance mannitol. With the cyclone droplet separator, a fine and uniform spray with a droplet size smaller 2?μm was produced. This robust atomizing technique is capable for high concentrations. For a 10?wt% mannitol solution, particles in the submicron range d_50,3?=?0.7?μm were produced.     

Characterization and verification of astigmatic interferometric particle imaging for volumetric droplet 3D position and size measurement

Sprays generated by atomization processes have been used in a wide range of fields. The size and spatial distribution of volumetric droplets in a spray are critical parameters in industrial applications. Astigmatic interferometric particle imaging (AIPI), an extension of the traditional interferometric particle imaging (IPI), is developed to simultaneously measure the size and 3D position of volumetric droplets in a sparse spray. Based on the generalized Huygens–Fresnel integral and the transfer matrix, the size and depth position of droplet can be respectively extracted from the fringe spacing and orientation of interferogram in AIPI. An AIPI setup is established to characterize droplets in a sparse spray generated by a nozzle with the AIPI calibration procedure adopted. The measured parameters by AIPI are compared with those obtained synchronously by digital inline holography, which is regarded as a standard measurement technique. Results show that the average deviation values of droplet size and depth position are respectively 3.8% and 6.8%. AIPI has been demonstrated with high accuracy in simultaneous 3D positions and size measurements.