CFD simulations of droplet and particle agglomeration in an industrial counter-current spray dryer

In this study, CFD simulations of particle and droplet agglomeration in an industrial counter-current spray dryer are presented. For this purpose, a modified form of the stochastic collision agglomeration model is proposed. This model takes into account droplet–droplet collision as well as wet and dry particle interaction. These events are coupled with heat, mass and momentum transfer. A comprehensive moisture evaporation model based on the concept of characteristic drying curve (CDC) was applied to predict the drying kinetics of the detergent slurry. Due to high instability in air flow inside the drying chamber, simulations were carried out under transient condition. A comparison between time-averaged simulation results and measurements, which were performed on an industrial spray drying installation, shows a good agreement. This finding proves the correctness of the developed agglomeration and drying models. The presented methodology of CFD simulations of agglomeration can be used to design or optimise spray-drying installations and to predict the final particle size distribution of the product.     

电弧喷涂快速成形技术研究现状

电弧喷涂成形是新近发展起来的一项重要金属快速成形技术。对比分析电弧喷涂快速成形技术与近终喷射成形和等离子喷涂成形技术之间的差异与特点,综述电弧喷涂在模具快速制造领域的研究应用现状,并讨论了电弧喷涂成形材料,以及喷涂成形雾化与沉积工艺过程中的传热传质、残余应力和氧化行为等关键性问题。最后,展望了电弧喷涂厚成形技术的重点研究方向与解决途径。     

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.     

Influence of internal refractive index gradients on size measurements of spherically symmetric particles by phase Doppler anemometry

A model based on geometric optics for predicting the response of interferometric (phase Doppler) instruments for size measurements of particles with radially symmetric but inhomogeneous internal refractive index profiles is developed. The model and results are important for applications in which heat or mass transfer from the particles or droplets is significant, for example, in liquid-fuel combustion. To quantify the magnitude of potential bias errors introduced by the classical assumption of uniform internal properties on phase Doppler measurements, we compute calibration curves for a sequence of times during the evaporation of a decane droplet immersed in an environment of T = 2000 K and p = 10 bars. The results reveal considerable effects on the relation between phase difference and droplet diameter caused by the refractive index gradients present. The model provides an important tool to assess sizing uncertainties that can be expected when applying conventional (based on uniform properties) phase Doppler calibration curves in spray combustion and similar processes.     

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.     

Computational simulation of thermally sprayed WC–Co powder

WC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 μm in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center.     

Laser-induced Spray Jet Cleaning Using Isopropyl Alcohol for Nanoparticle Removal from Solid Surfaces

Recent studies demonstrated that laser-induced spray jet cleaning (LSJC) based on optical breakdown of a water droplet is an effective way to remove nanoscale contaminant particles from solid surfaces with use of small amount of water. In this work, an LSJC process using isopropyl alcohol (IPA) as a non-water cleaning agent was developed. High-speed spray jet composed of atomized micro droplets of IPA was generated by inducing optical breakdown in the droplet. The particle removal efficiency was slightly lower than that of the LSJC using water droplets but it was high enough to remove 30 nm polystyrene latex particles completely and 10 nm gold particles partially from silicon wafers. Optical microscopy and secondary ion mass spectrometry confirmed that the LSJC process using IPA caused no watermark problem commonly observed in water-based cleaning processes without a special rinsing and drying process.     

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.     

Effect of particle spatial distribution on particle deposition in ventilation rooms

We used simulations and experimental tests to investigate indoor particle deposition during four commonly used ventilation modes, including ceiling supply, side-up supply, side-down supply and bottom supply. We used a condensation monodisperse aerosol generator to generate fine diethylhexyl sebacate (DEHS) particles of different sizes along with two optical particle counters that measured particle concentration at the exhaust opening and inside a three-dimensional ventilated test room. We then simulated particle deposition using the same ventilation modes with computational fluid dynamics (CFD) method. Our simulated results indicate that mean deposition velocity/rate for particles 0.5–10 μm (aerodynamic diameter) is not affected by different ventilation modes. However, both our experimental and simulated results indicate that the deposition loss factor, a parameter defined based on mass balance principle to reflect the influence of particle distribution on deposited particle quantity, differ significantly by ventilation mode. This indicates that ventilation plays an important role in determining particle deposition due to the apparent differences in the spatial distribution of particles. The particle loss factor during ventilation modes characterized by upward air flow in the room is smaller than that of mixing ventilation; however this trend was strongly influenced by the relative location of the inlets, outlets and aerosol source.     

Visualization and modeling for water atomization of low melting point alloy

Liquid metal fragmentation by impinging fast water spray, so called water atomization, is widely used to produce metal powders efficiently. In the present paper, we conduct the high-speed visualization experiments and theoretical modeling for elucidating the mechanism of fragmentation and solidification processes, which are essentially important to control the metal powder characters. We successfully visualize the detailed sequential events from the water spray ejection, freely dropped molten metal of 42Sn-58Bi, followed by their collision, metal fragmentation in liquid phase, and solidification, leading to revealing the fragmentation processes as the impact of water spray and the vapor explosion. Quantified metal particle size convinces that the water atomization simultaneously proceeds fragmentation of metal in liquid phase with solidification. The experimental results of size distribution and mean diameter well validate the proposed physically-consistent theoretical modeling for the prediction of particle size.     

Using a coupled CFD – DPM approach to predict particle settling in a horizontal air stream

Preventing health and safety hazards such as dust explosions and respiratory exposure in the work force when handling and storing fine powders is a major challenge faced by plant operators [13]. Computational Fluid Dynamics (CFD) coupled with Discrete Phase Model (DPM) can be used as a tool to address this challenge by advancing the understanding of how particles deposit in a particular process. Particle settling, in air streams, is primarily dependent on the drag forces exerted on the individual particles through interactions with the suspension medium [2]. By improving the understanding of this interaction through repeatable experiments and simulations; more complex CFD – DPM simulations are possible, thus providing a significant step in reducing the risks associated with handling fine powders. To study the transport and settling of particles in air streams, an experiment was established where glass beads, alumina and iron ore dust were injected into a horizontal flow channel. The material was fed into the top of the test rig where it was then transported in a laminar air stream. Through this method particle settling, according to the particle size, can be observed by sampling different trays along the bottom of the test rig. Once the deposition of particles is analysed (using a particle size analyser) each diameter range can be tracked to determine the distance travelled. After evaluating these experiments a CFD coupled with DPM simulation was employed to predict particle deposition in the horizontal chamber. The results show a good agreement between experiments and CFD – DPM results.     

Predictive CFD modeling of whey protein denaturation in skim milk spray drying powder production

A new model of whey protein thermal inactivation has been combined with a CFD model developed for skim milk spray drying. Extensive evaporation and particle formation models were used to calculate particle moisture contents, temperatures and residence times. Calculated parameters were then used as input data for an experimentally developed quality model based on Williams-Landel-Ferry (WLF) equations for inactivation kinetics. The developed quality model was implemented into the CFD code and calculated in parallel to simulations of skim milk droplets evaporation based on the characteristic drying curve approach. The quality model and the simulation procedure were validated by comparison of protein activity levels obtained from the CFD with data obtained from differential scanning calorimetry (DSC) of milk powder samples collected during skim milk spray drying experiments. The simulations for different feed rates fit well with measurement results and show that the loss of whey protein activity is lower at higher feed rates, due to lower temperature fields in this case.     

POWDER COATING PROCESS PARAMETERS FOR A TRANSFER EFFICIENCY MODEL

The trajectories of charged powder particles in a powder coating system are governed by the electrostatic, gravitational and aerodynamic forces acting on the particles. A mathematical model of particle trajectories inside a powder coating booth must consider (1) the aerodynamic flow field, (2) particle size and charge distributions, (3) the electrostatic field distribution, and (4)the geometry of the target. Our approach is to employ a grid generation and flow solver to examine the air flow pattern and an iterative technique where the Charge Simulation Method can be used to compute the electric field strength and the Method of Characteristics can be used to compute the charge density in the gun-to-target region. The electrostatic forces due to the deposited powder layer and image charge are to be taken into account to determine if the particle will deposit on the substrate or not. The model is applied to the geometry of a high-voltage electrode consisting of a long thin rod with a hemispherical end cap and a grounded flat disk substrate. An experimental system to measure transfer efficiency, with the ability to control various parameters effecting transfer efficiency, has been developed to verify the theoretical model. The simulation results can provide valuable information concerning particle deposition and optimization of transfer efficiency. This paper describes (1) system parameters involved in modeling the transfer efficiency, (2) an approach to develop such a model with preliminary data on the simulation of particle track, and (3) experimental data on the real-time measurements of first pass transfer efficiency.     

The effect of ethanol on the formation and physico-chemical properties of particles generated from budesonide solution-based pressurized metered-dose inhalers

The aerosol performance of budesonide solution-based pressurized metered-dose inhalers (HFA 134a), with various amounts of ethanol (5–30%, w/w) as co-solvents, was evaluated using impaction and laser diffraction techniques. With the increase of ethanol concentration in a formulation, the mass median aerodynamic diameter was increased and the fine particle fraction showed a significant decline. Although data obtained from laser diffraction oversized that of the impaction measurements, good correlations were established between the two sets of data. Particles emitted from all the five formulations in this study were amorphous, with two different types of morphology – the majority had a smooth surface with a solid core and the others were internally porous with coral-like surface morphology. The addition of ethanol in the formulation decreased the percentage of such irregular-shape particles from 52% to 2.5% approximately, when the ethanol concentration was increased from 5% to 30%, respectively. A hypothesis regarding the possible particle formation mechanisms was also established. Due to the difference of droplet composition from the designed formulation during the atomization process, the two types of particle may have gone through distinct drying processes: both droplets will have a very short period of co-evaporation, droplets with less ethanol may be dried during such period; while the droplets containing more ethanol will undergo an extra condensation stage before the final particle formation.     

多层喷射共沉积制备 SiCP/Al-8.5Fe-1.3V-1.7Si复合材料

采用多层喷射沉积工艺制备SiC_P/Al-8.5Fe-1.3V-1.7Si复合材料, 研究了雾化及沉积工艺参数对沉积坯状态及SiC颗粒捕获的影响。结果表明, 液流直径大、雾化气体压力小、喷射高度小会导致沉积坯组织恶化, 反之则造成收得率低、致密度低。雾化器扫描不均匀则会造成沉积坯形状不均匀, 而且会由于热量集中导致显微组织恶化。SiC颗粒输送压力的提高有利于SiC颗粒的捕获以及颗粒的均匀分布。多层喷射沉积制备SiC_P/Al-8.5Fe-1.3V-1.7Si的优化工艺参数为: 液流直径3.6 mm, 雾化气体压力0.8 MPa, 喷射高度200 mm, SiC 颗粒输送压力0.5 MPa。 沉积坯存在两种SiC-Al界面: 晶态Si界面层与非晶态SiO₂界面层。     

A theoretical model for effective thermal conductivity (ETC) of particulate beds under compression

Thermal conductivity of particulate beds is an important property for many industrial handling processes as well as for storage of particulate materials. This paper presents a new theoretical model that is based on heat transfer between particles in three modes: heat conduction through contact area, heat conduction through voids and radiation through voids. The model is further adjusted in order to obtain effective thermal conductivity of a particulate bed by using empirical augmentation factors for the heat transfer coefficient of each one of the heat transfer mechanisms. Comparison of the results predicted by the semi-empirical model to our experimental results show good agreement. The theoretical model was investigated to examine the effect of various parameters (such as: particle elasticity and surface roughness, particle and gas thermal conductivity and particle diameter), on the effective thermal conductivity of various particulate beds. Our results show the significant effect of the contact area (that is a clear function of the compression load) between particles on the effective thermal conductivity.     

The influence of cavitation on the flow characteristics of liquid nitrogen through spray nozzles: A CFD study

Spray cooling with cryogen could achieve lower temperature level than refrigerant spray. The internal flow conditions within spray nozzles have crucial impacts on the mass flow rate, particle size, spray angle and spray penetration, thereby influencing the cooling performance. In this paper, CFD simulations based on mixture model are performed to study the cavitating flow of liquid nitrogen in spray nozzles. The cavitation model is verified using the experimental results of liquid nitrogen flow over hydrofoil. The numerical models of spray nozzle are validated against the experimental data of the mass flow rate of liquid nitrogen flow through different types of nozzles including the pressure swirl nozzle and the simple convergent nozzle. The numerical studies are performed under a wide range of pressure difference and inflow temperature, and the vapor volume fraction distribution, outlet vapor quality, mass flow rate and discharge coefficient are obtained. The results show that the outlet diameter, the pressure difference, and the inflow temperature significantly influence the mass flow rate of spray nozzles. The increase of the inflow temperature leads to higher saturation pressure, higher cavitation intensity, and more vapor at nozzle outlet, which can significantly reduce mass flow rate. While the discharge coefficient is mainly determined by the inflow temperature and has little dependence on the pressure difference and outlet diameter. Based on the numerical results, correlations of discharge coefficient are proposed for pressure swirl nozzle and simple convergent nozzles, respectively, and the deviation is less than 20% for 93% of data.     

Fragments spawning and interaction models for DEM breakage simulation

Particles breakage occurs in many industrial applications. During the last decade many works have been devoted for modelling and simulating such processes. A new and innovative procedure of empirical comminution functions for Discrete Element Method (DEM) simulations (Kalman et al. in Granul Matter 11(4):253–266, 2009) posed the question how to introduce the fragments of the broken particle back into the computational domain. Daughter particles (Fragments) spawning and interaction imposes several problems during DEM simulation. Some of the main problems are: seeding (allocating) daughter particles and their initial conditions i.e. fragments locations, velocities and physical properties. This work focuses on the daughter particles seeding and the interaction between “sibling” particles for spherical particles. Fragments spawning and interaction algorithm for particle breakage during DEM simulation was developed. The algorithm enables prediction of particle comminution/attrition processes using DEM applications. The new algorithm can utilize any breakage function allowing unlimited fragment size fractions. In the proposed model, sibling particles can overlap without increasing the energy of the system in the simulation. Particle-particle and particle-wall interactions are calculated using the standard DEM calculations. Daughter particles interactions were calculated using the developed temporary contact radius model. The model was utilized to predict particle comminution in jet milling and particle attrition during pneumatic conveying with great successes.     

铝铜合金雾化沉积快速凝固过程的传热计算

建立了雾化沉积快速凝固过程热量传输的理论模型，对Ａｌ－４．５％Ｃｕ合金在雾化沉积过程的颗粒运动动力学以及颗粒与沉积层的温度，固相分数和冷却速度等凝固参量的变化规律进行了数值计算。     

Aerosol particle deposition and distribution in bifurcating ventilation ducts

The quantitative information gained from detailed studies of particle deposition in ducts is important, for example, to evaluate human exposure to particles within buildings, implement cleaning strategies for ventilation ducts and also understand particulate deposition in the respiratory tree. For this purpose, an experimental study for aerosol particles of diameters ranging from 8.1 to 23.2 microm was conducted in a curved bifurcating ventilation duct. At the bend segment of the duct, the particle size, bend angle, curvature ratio and Reynolds number affect aerosol deposition significantly. On the other hand, tests conducted on the bifurcating segments show that deposition increases with particle size and Reynolds number. Accumulation of particles occurs mainly around the bend segment and the ridge of carina of the bifurcation. In all segments of the duct models, particle deposition is found to be enhanced with increasing humidity which increases from 66 to 95% (i.e., close the saturation). A physical interpretation of the results obtained is also presented.