- •Foreword
- •1. General Introduction
- •2. Processes and Techniques for Droplet Generation
- •2.1.0 Atomization of Normal Liquids
- •2.1.1 Pressure Jet Atomization
- •2.1.3 Fan Spray Atomization
- •2.1.4 Two-Fluid Atomization
- •2.1.5 Rotary Atomization
- •2.1.6 Effervescent Atomization
- •2.1.7 Electrostatic Atomization
- •2.1.8 Vibration Atomization
- •2.1.9 Whistle Atomization
- •2.1.10 Vaporization-Condensation Technique
- •2.1.11 Other Atomization Methods
- •2.2.0 Atomization of Melts
- •2.2.1 Gas Atomization
- •2.2.2 Water Atomization
- •2.2.3 Oil Atomization
- •2.2.4 Vacuum Atomization
- •2.2.5 Rotating Electrode Atomization
- •2.2.7 Electron Beam Rotating Disk Atomization
- •2.2.9 Centrifugal Shot Casting Atomization
- •2.2.10 Centrifugal Impact Atomization
- •2.2.11 Spinning Cup Atomization
- •2.2.12 Laser Spin Atomization
- •2.2.14 Vibrating Electrode Atomization
- •2.2.15 Ultrasonic Atomization
- •2.2.16 Steam Atomization
- •2.2.17 Other Atomization Methods
- •3.1.0 Droplet Formation
- •3.1.1 Droplet Formation in Atomization of Normal Liquids
- •3.1.2 Secondary Atomization
- •3.1.3 Droplet Formation in Atomization of Melts
- •3.2.0 Droplet Deformation on a Surface
- •3.2.3 Droplet Deformation and Solidification on a Cold Surface
- •3.2.4 Droplet Deformation and Evaporation on a Hot Surface
- •3.2.5 Interaction, Spreading and Splashing of Multiple Droplets on a Surface
- •3.2.6 Sessile Droplet Deformation on a Surface
- •3.2.7 Spreading and Splashing of Droplets into Shallow and Deep Pools
- •4.1.0 Concept and Definitions of Droplet Size Distribution
- •4.2.0 Correlations for Droplet Sizes of Normal Liquids
- •4.2.1 Pressure Jet Atomization
- •4.2.5 Rotary Atomization
- •4.2.6 Effervescent Atomization
- •4.2.7 Electrostatic Atomization
- •4.2.8 Ultrasonic Atomization
- •4.3.0 Correlations for Droplet Sizes of Melts
- •4.3.1 Gas Atomization
- •4.3.2 Water Atomization
- •4.3.3 Centrifugal Atomization
- •4.3.4 Solidification and Spheroidization
- •4.4.0 Correlations for Droplet Deformation Characteristics on a Surface
- •4.4.1 Viscous Dissipation Domain
- •4.4.2 Surface Tension Domain
- •4.4.3 Solidification Domain
- •4.4.4 Partial Solidification Prior to Impact
- •5.1.0 Energy Requirements and Efficiency
- •5.2.0 Modeling of Droplet Processes of Normal Liquids
- •5.2.1 Theoretical Analyses and Modeling of Liquid Jet and Sheet Breakup
- •5.2.2 Modeling of Droplet Formation, Breakup, Collision and Coalescence in Sprays
- •5.2.3 Theories and Analyses of Spray Structures and Flow Regimes
- •5.2.5 Modeling of Multiphase Flows and Heat and Mass Transfer in Sprays
- •5.3.0 Modeling of Droplet Processes of Melts
- •5.3.4 Modeling of Multiphase Flows and Heat Transfer in Sprays
- •5.4.0 Modeling of Droplet Deformation on a Surface
- •5.4.1 Modeling of Deformation of a Single Droplet on a Flat Surface
- •5.4.2 Modeling of Droplet Deformation and Solidification on a Cold Surface
- •6. Measurement Techniques for Droplet Properties and Intelligent Control of Droplet Processes
- •6.1.0 Measurement Techniques for Droplet Size
- •6.1.1 Mechanical Methods
- •6.1.2 Electrical Methods
- •6.1.3 Optical Methods
- •6.1.4 Other Methods
- •6.2.0 Measurement Techniques for Droplet Velocity
- •6.3.0 Measurement Techniques for Droplet Number Density
- •6.4.0 Measurement Techniques for Droplet Temperature
- •6.5.0 Measurement Techniques for Droplet Deformation on a Surface
- •6.6.0 Intelligent Control of Droplet Processes
- •Index
Droplet Generation 63
modification and development into forms suitable for agricultural aviation applications in terms of the range of mean droplet size, monodispersity and capability of operating over a range of liquid properties. However, all of these techniques require some extension of their flow rate capabilities to match those required in agricultural aviation applications. Electrostatic atomization and vaporizationcondensation techniques were found not to be adequate for agricultural aviation applications despite of their capabilities of generating nearly uniform droplets.
It should be noted that some problems may arise in the techniques or devices for producing monodisperse or near-monodis- perse sprays. One of the problems is droplet coalescence. Initially uniform droplets may coalesce rapidly to create doublets or triplets, particularly in a dense and turbulent spray, deteriorating the monodispersity of the droplets. This problem may be lessened by using appropriate dispersion air around the spray.[88] Another problem is non-spherical droplet shapes that make estimations of monodispersity difficult.
2.1.11 Other Atomization Methods
In addition to the above-discussed techniques, some other atomization methods have also been developed for specific applications. These include, for example, solution atomization, liquefied gas atomization, and spark-discharge atomization,[88] etc.
Solution atomization involves dissolution of a relatively nonvolatile liquid (solute) in a volatile solvent and atomization of the solution. During the atomization, the solvent material will evaporate in surrounding medium (air), leaving only nucleus droplets of the nonvolatile solute. The final droplet size is a function of the initial droplet size, the mass concentration of the solute, and the density ratio of the solution to the solute. The limitation of this technique lies in that it requires the dissolution of the liquid to be dispersed in a solvent.
64 Science and Engineering of Droplets
A similar technique has been applied to the generation of monodisperse suspensions in water. This type of method was first used in medical field and then widely used to spray monodisperse solid particles such as polystyrene latex particles. Aerosols of solutes have also been produced by atomizing solutions of salt, sugar or methylene blue dye dissolved in water. In practical operations, a low concentration of solid particles in a solvent is recommended in order to avoid possible agglomeration of suspensions in the solvent.
Sprays of fine droplets can be generated by first mixing a liquid with liquefied gas under pressure and then expanding the mixture through a nozzle. This technique, referred to asliquefied gas atomization, has been used in many applications such as commercial aerosol cans. The mean droplet size generated with this technique is very small. In very few systematic studies, the measured droplet size distribution was found rather widely spread.[88] It is not clear, however, how the liquid amount, pressure, and nozzle design affect the mean droplet size and size distribution.
Fine droplets can also be generated by introducing electrodes into a liquid and applying a high potential across the electrodes. The disintegration of the liquid into droplets is due to spark discharge. The technique is therefore termedspark-discharge atomization. Small scale experiments have been performed to investigate the phenomena associated with the atomization process primarily for domestic burner application. The produced droplets exhibited a mass median diameter of about 100 µm, and the droplet size distribution was found quite widely dispersed.[88] No reliable or proven results seem to be available.
Some design concepts for generating uniform droplets have been proposed by Lee et al.[88] These include (a) centrifugal type chamber, (b) atomization by two opposing air-liquid jets, and (c) spinning disk coupled with an ultrasonic field. Some other conceptions include (d) rocket nozzle chamber, (e) frozen particles, (f) rotating brush, and (g) periodic vibrations using saw-tooth waves, etc.