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°.     

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。     

喷射雾滴烟气流动蒸发特性

电站锅炉烟气脱硫废水喷入烟道蒸发是电厂废水零排放最经济可行的技术途径之一。针对该技术实际应用中存在的在烟道壁面上结垢腐蚀问题,以单台300 MW机组为对象,将针对连续相烟气的湍流流动传热欧拉方法,以及离散相雾化液滴群流动蒸发的拉格朗日方法相结合,建立物理数学模型。通过数值模拟方法,研究脱硫废水喷射雾滴在烟气中的流动蒸发特性及其影响因素,获得不同运行条件下喷雾的扩散范围和液滴在烟气内的运动轨迹。结果表明：烟气温度越高、雾化液滴群的直径越小,其完全蒸发所需的时间和距离越短;采用多喷嘴小流量的布置方式可以提高雾化液滴群的蒸发质量;喷嘴喷射方向的选择应该保证雾化液滴群与烟气相对运动增强的同时,保证液滴能在规定距离和时间内完全扩散并与烟气进行充分接触和换热,实现雾化液滴群蒸发质量的最大化;而液滴初速度、喷嘴的喷射全锥角、烟气速度对蒸发率的影响不大。研究结果可为火电厂脱硫废水烟气蒸发的工艺设计及性能调控提供依据。     

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.     

SNCR过程还原剂与烟气混合特性数值研究

以某台220t/h煤粉锅炉为研究对象,对选择性非催化还原过程中尿素溶液还原剂与烟气的混合问题进行数值研究.计算表明,还原剂液滴喷入炉内1 100℃左右的高温区后,迅速经历加热、沸腾和蒸发过程,液滴的停留时间很短,因此增加还原剂射流穿透深度可有效提高混合程度.以还原剂质量分数的相对标准偏差为混合程度的衡量指标,分析了还原剂喷射速度、流量、液滴粒径以及锅炉等因素的影响,发现降低锅炉负荷、提高还原剂喷射速度和流量均可有效提高混合程度,而液滴粒径对混合具有双重影响,一般存在最佳粒径范围.     

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.     

Simulation of a spray scrubber performance with Eulerian/Lagrangian approach in the aerosol removing process

In this study, a mathematical model has been developed to simulate the performance of a spray scrubber in an industrial ammonium nitrate plant. The model is based on the Lagrangian approach for the droplets movement and particle source in cell (PSI-CELL) model for calculating the droplet concentration distribution. Consequently, unlike former research, the emphasis is on the droplet dynamic behavior. In the current study, for approaching a realistic model, a droplet size distribution rather than average diameter, and also liquid film formation rather than uniform and constant droplet flow rate has been applied. Also, the Eulerian method has been used for the calculation of the particles removal efficiency and energy balance has been applied on the gas to estimate the droplet size distribution. In the experimental section, the concentration of particles and their size distribution in both inlet and outlet gas of the studied scrubber has been measured for the validation of the predicted particles collection efficiency. In addition, the temperature of the gas at inlet, outlet and in the middle of the tower has been measured for the confirmation of the predicted droplet size distribution in the tower. A good consistency between the model and data has been observed. After the model is validated, it is used to investigate the various variable profiles such as liquid film, total projected surface area of the droplets, velocity profile of the droplets and some of the other parameters in the spray scrubbers.     

Heat and mass transfer characteristics in a spray chamber

This paper presents a one-dimensional mathematical model for heat and mass transfer of water droplets in a spray chamber. The model includes drop size distribution and velocity of the droplets generated by a nozzle of inlet diameter 3.2 mm. By using the conservation of mass and energy, the changes in water temperature, air temperature and humidity along the spray cone in the spray chamber can be calculated. This model is tested with two different water mass flows. The results look reasonable from practical point of view and they also show that higher water mass flow results in a higher air temperature drop and higher humidity.     

喷雾半干法脱硫塔内射流卷吸特性的数值模拟

喷雾半干法烟气脱硫塔内射流卷吸特性对塔内烟气与吸收剂的混合以及流场分布均匀有着重要影响。该文通过数值模拟的方法,分析双流体雾化喷嘴的射流卷吸混合特性,得到喷雾半干法烟气脱硫塔内射流流场分布。结果表明：双流体雾化喷嘴在塔内流场呈现中间大两边小的对称分布,并且在靠近塔壁的两侧产生了回流;射流流体与周围气体发生动量交换,对周围气体有明显的卷吸作用,沿着射流轴线方向,射流卷吸量逐渐增加,随着伴随风量的增加,射流卷吸量也逐渐增加。     

Modeling of droplet dynamic and thermal behaviour during spray deposition

Mathematical modeling of supersonic gas atomization for spray forming has been investigated. Influence of the droplet dynamic and thermal behaviour on the resultant microstructure has been studied. Analytical models have been constructed taking into account the higher Reynolds number owing to supersonic gas flow. The impact velocity profiles of the droplets lend credence to the evolution of equiaxed grain morphology through dendrite fragmentation. The thermal history profile along with the fraction solid plot could yield optimized standoff distance to obtain a mushy droplet. A comparison of secondary dendrite arm spacing obtained from the mathematical model showed good agreement with experimental observations.     

MEASUREMENT OF THE KINETICS OF SOLUTION DROPLETS IN A CONTINUOUSLY MIXED CHAMBER

ABSTRACT

An experimental system is designed for the measurement of the evaporation and growth kinetics of individual solution droplets. Electrostatically charged droplets, nominally tens of micrometers in diameter, are suspended in a hyperboloidal electrodynamic chamber by balancing the droplet ueight against a uniform electrostatic field. By controlling the mixing dynamics in the chamber with specific flow configurations, a continuously mixed chamber is achieved. Consequently the Instantaneous chamber relative humidity is predicted from an exponential law with o characteristic relaxation time which is given by the ratio of the chamber volume and the gas volumetric flow rate. The evaporation and growth kinetics of phosphoric acid droplets is measured for relative humidity changes between 30 and 80%. Comparison between experimental and theoretical instantaneous droplet masses reveals less than a 5% deviation.The present system is employed to analyze the effect of adsorbed hexadecanol surfactant molecules on the evaporation and growth kinetics of phosphoric acid droplets. It is found a critical coverage exists which will result in a dramatic reduction in the evaporation kinetics of solution droplets. ConcomiCantly, the condensation coefficient is reduced from unity to 4.0 x 10-5 as droplet kinetics occur in the presence of a complete monolayer of hexadecanol.     

Study on ice slurry production by water spray

A theoretical and experimental study was performed to examine the water spray method of ice slurry production. First, the conditions for the formation of ice particles were investigated theoretically by the diffusion-controlled evaporation model. The prediction of the model was proved to agree relatively well with experiments in which we examined the conditions for a droplet of initial temperature 20°C and size 50 μm to change into an ice particle in a chamber of height 1.33 m. Second, the production of cold storage heat will increase almost proportionally to the number of spray nozzles because no substantial difference was found in the Sauter Mean Diameter (SMD) of sprays from single and twin nozzle. Third, an ice slurry was experimentally obtained by spraying droplets of 7% ethylene glycol aqueous solution in a vacuum chamber where pressure is maintained below the freezing point of the solution. Finally, based on the theoretical and experimental results, we propose an optimizing chart for providing the operating conditions to make ice slurry using the relations of the staying time of the droplet in the chamber, the injection pressure, the spray droplet size and the chamber pressure.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Transport and Deposition of Evaporating Droplets in a Ventilated Environment

In this study, droplet transport, dispersion, and deposition in a ventilated office with two manikins were studied using a computer-modeling approach. Different airflow distribution systems were used, and an Eulerian approach was employed for the airflow simulation. The trajectories of droplets were evaluated using the Lagrangian approach by solving the equation of droplet motion that included the inertial, viscous drag, Brownian, Saffman lift, and gravity forces. Droplet evaporation was also taken into account by solving the droplet heat and mass transfer equations, thus, allowing for the variation of the droplet size. Mixing and displacement air distribution systems were examined, and trajectories of droplets in the range of 1 to 100 microns emitted by one of the manikins were simulated under a range of conditions. The simulation results showed that the chance for small droplets to leave the room through the exhaust is relatively high. When the mixing air distribution system is used, the drop dispersion is higher than with the displacement distribution system. This in turn suggests that the chance of transmission of air borne diseases is relatively higher for the mixing ventilation system.     

Interferometric laser imaging for droplet sizing: a method for droplet-size measurement in sparse spray systems

A full-field, time-resolved interferometric method for the characterization of sparse, polydisperse spray systems is reported. The method makes use of the angular intensity oscillations in the wide-angle forward-scatter region. A pulsed laser is used to illuminate a planar sheet through the spray, which is imaged, out of focus, from the 45°direction. The image consists of a set of out-of-focus spots, each of which represents an individual droplet, and superimposed on which is a set of fringes corresponding to the angular intensity oscillations of that droplet. Macrophotographic recording with high-resolution digitization for image analysis provides a full-field capability. The spatial frequency of fringes on each spot in the image plane is dependent on the diameter of the corresponding droplet in the object plane, and a simple geometric analysis is shown to be appropriate for the calculation of the spatial frequency of fringes as a function of droplet size. Images are analyzed automatically by a software suite that uses Gaussian blur, Canny edge detection, and Hough transforms to locate individual droplets in the image field. Fringe spatial frequency is then determined by least-squares fitting to a Chirp function. The method is applicable to droplets with diameters in the range of several millimeters to several hundred millimeters and number densities of up to 10(3) to 10(4). The accuracy of the method for droplet-size determination has been evaluated by measurements of monodisperse aerosols of known droplet size, and measurements of droplet-size distribution in a polydisperse aerosol produced by a gasoline fuel injector are also presented. An extension of the method, using high-speed photography to measure two components of velocity in addition to size and position, is discussed. A two-wavelength approach may also offer the capability to measure the concentration of model fuel additives in droplets, and the results of a feasibility study are described.     

MEASUREMENT OF THE KINETICS OF SOLUTION DROPLETS IN A CONTINUOUSLY MIXED CHAMBER

An experimental system is designed for the measurement of the evaporation and growth kinetics of individual solution droplets. Electrostatically charged droplets, nominally tens of micrometers in diameter, are suspended in a hyperboloidal electrodynamic chamber by balancing the droplet ueight against a uniform electrostatic field. By controlling the mixing dynamics in the chamber with specific flow configurations, a continuously mixed chamber is achieved. Consequently the Instantaneous chamber relative humidity is predicted from an exponential law with o characteristic relaxation time which is given by the ratio of the chamber volume and the gas volumetric flow rate. The evaporation and growth kinetics of phosphoric acid droplets is measured for relative humidity changes between 30 and 80%. Comparison between experimental and theoretical instantaneous droplet masses reveals less than a 5% deviation.The present system is employed to analyze the effect of adsorbed hexadecanol surfactant molecules on the evaporation and growth kinetics of phosphoric acid droplets. It is found a critical coverage exists which will result in a dramatic reduction in the evaporation kinetics of solution droplets. ConcomiCantly, the condensation coefficient is reduced from unity to 4.0 x 10-5 as droplet kinetics occur in the presence of a complete monolayer of hexadecanol.     

Observation of liquid hydrogen jet on flashing and evaporation characteristics

The evaporation speed of liquid hydrogen jet has been measured using high speed CCD camera. In the evaporation process the diameter of injected liquid hydrogen droplet plays important role. The experimental parameters for injection condition had been selected such as injection temperature and size of injection hole diameter which influences injected diameter of liquid droplet. Liquid hydrogen had been injected into the spray chamber that was filled with gaseous helium at room temperature. The liquid hydrogen jet moves in the horizontal direction and the images of the evaporation processes had been acquired from the observation window using high speed CCD camera. With this method it has been shown that evaporation speed of liquid hydrogen is influenced by injection temperature and size of injection hole diameter.     

Evaporation of additive droplets in a pulse MHD generator

Theoretical investigation into evaporation of additive droplets in the combustion chamber of a pulse MHD generator were undertaken. Flow in the chamber is considered as stationary and one-dimensional; mixing in a direction perpendicular to flow is believed to be ideal, and mixing is lacking in the flow direction. It is suggested that droplets are monodisperse, spherical, and motionless relative to the gas medium. The droplet evaporation can be taken as occurring in the diffusion mode. The specific heat c _p and heat conductivity coefficient are taken to be constant and independent of temperature and the concentration of components. The Lewis number is believed to be the unit value; and the Soret and Dufour effects, negligible. A formula for calculation of the droplet evaporation rate with allowance made for chemical reactions occurring in liquid and gas media is obtained.     

Certain features of the calculation of a two-phase flow in the droplet regime

A model of two-phase flow in the droplet regime has been considered as a particular case of a more general model with simplifying assumptions acceptable for the calculation of the evaporation of water in the case of injection into the channel of a gas-turbine engine. The chosen implicit integration scheme allows engineering calculations for a highly stiff polydisperse system appearing under the complete evaporation of a group of droplets. A procedure has been developed to avoid a critical point in the process of the simulation of evaporation (division by zero) at zero diameter of droplets or equal velocities of droplets and gas.     

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.