1

General Introduction

Droplets are encountered in nature and a wide range of science and engineering applications.[1]-[5] Natural droplets can be found in dew, fog,[6] rainbows,[7][8] clouds/cumuli,[9]-[11] rains, waterfall mists, and ocean sprays.[12] A dispersion of droplets in surrounding air can be produced by shower heads, garden hoses, hair sprays, paint sprays, and many other spray devices. A variety of important industrial processes involve discrete droplets, such as spray combus- tion,[1][13]-[15] spray drying,[2][16] spray cooling,[17] spray atomization,[4] spray deposition,[3] thermal spray,[18] spray cleaning/surface treatment, spray inhalation,[19] aerosol (mist) spray, crop spray, paint spray, etc. The related areas span automotive, aerospace, metallurgy, materials, chemicals, pharmaceuticals, paper, food processing, agriculture, meteorology, and power generation. Droplets are used to collect dust in charged droplet scrubbers[20] and to cool hot surfaces by droplet evaporation.[21] Liquid crystals with polymer-dispersed nematic droplets have found wide applications in optic display.[22]- [24] Droplet properties and deformation characteristics are of interest in liquid-liquid phase separation,[25][26] emulsion,[27] gas-liquid mass transfer, and ink-jet printing applications. In materials processing such as synthesis of some semiconductor thin films and composite materials, droplet formation may be a necessary step. For example,

1

2 Science and Engineering of Droplets

GaAs microcrystals are grown by Ga droplet formation and successive As (arsenic) supply in low pressure metalorganic chemical vapor deposition.[28] Silicon carbide whiskers are synthesized by the formation of silicon-rich liquid droplets in the vapor-liquid-solid growth process.[29] Fullerene is formed by droplet nucleation from supersaturated carbon vapor.[30] In some other processes, however, droplet formation is undesired. For example, in pulsed laser ablation and deposition of thin metal films[31] and YBa2Cu3O7 high temperature superconductor films,[32] droplet formation needs to be reduced by decreasing target etching rate. Further examples of science and engineering applications of discrete droplets of both normal liquids and melts are listed in Tables 1.1 and 1.2 for an overview.

Droplets may form by vapor condensation and deposi- tion,[33]-[36] liquid breakup,[37]-[41] or solid particle/wire melting.[42][43] The generation of droplets in most engineering applications involves the breakup of a liquid, i.e., atomization. Atomization can be achieved by a variety of means, aerodynamically, mechanically, ultrasonically, or electrostatically, etc. For example, a liquid jet or sheet can be disintegrated by the kinetic energy of the liquid itself, or by exposure to a high-velocity gas, or by the mechanical energy applied externally through a rotating or vibrating device. The liquids of practical interest may be classified into two fundamental categories, i.e., normal liquids and melts. Normal liquids are typically aqueous or oil-based liquids at near room temperature. Melts are usually molten metals, alloys or ceramics at high temperatures.

Atomization of normal liquids has been used in many fields. The typical applications include spray combustion, spray drying, and aerosol spray, as illustrated in Figs. 1.1, 1.2, and 1.3, respectively. Spray combustion involves atomization and burning of liquid fuels.[1] Spray drying is a process in which a solution or suspension is atomized into a spray of droplets in hot gaseous surroundings to produce solid powder of the solute.[2] Aerosol spray is among the effective and reliable technologies for fire suppression and application of agricultural chemicals.

General Introduction

3

Table 1.1. Science and Engineering Applications of Droplets of Normal Liquids

	Area	Application
		- Milk Powder Processing
	Spray Drying	- Food Processing
		- Chemical Processing
		- Diesel Engines
		- Spark Ignition Engines
	Spray	- Gas Turbines
	Combustion	- Rocket Engines
		- Industrial Furnaces
		- Domestic Heating Boiler
		- Cooling of Ingot in Continuous Casting
	Spray Cooling	- Cooling of Nuclear Cores
		- Cooling of Turbine Blades
		- Coke Quenching
	Spray Inhalation	- Vaporization of Volatile Anaesthetic
		Agents and Medication
		- Humidification/Air Conditioning
	Aerosol (Mist)	- Fire Fighting
		- Drenching Operations
	Spray
		- Lubricating
		- Pollution/Dust Control
	Crop Spray	- Applying Agricultural Chemicals
		- Spray Irrigation
	Paint Spray	- Surface Finishing
		- Surface Coating
		- Gas (Wet) Scrubbing
		- Gravel Washing
	Spray Cleaning	- Vegetable Cleaning
		- Surface Treatment
		- Car Washing

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Table 1.2. Science and Engineering Applications of Droplets of Melts

Area	Application

Powder Production:

-Dental Amalgams (AgSnCu Powder)

-Shot Blasting Grits (Steel or Iron Powder)

-Solid Rocket Fuels (Al Powder)

-Metallic Paints (Al, CuZn Powder)

-Filters (CuSn Powder)

-Solder Creams (PbSnAg, Bi57Sn43 Powder)

Spray

- Strips for Diamond Synthesis (Co Powder)

Atomization

-Flares (Mg Powder)

-Jewellery-Brazing Pastes (Pd, Au, Ag Powder)

-Dense Media for Mineral and Scrap Separation (FeSi Powder)

-Coatings for Welding Electrodes (Fe, FeMn, FeSi Powder)

-Explosives (Al Powder)

-Food Additives (Fe Powder)

-Batteries (Zn Powder)

		Net or Near-Net Shape Manufacturing:
	Spray	- Spray Forming
	Deposition	- Spray Casting
		- Spray Rolling
	Thermal	Surface Coating and Free-Standing Components:
		- Plasma Spray
	Spray
		- High Velocity Oxy-Fuel (HVOF)

8 Science and Engineering of Droplets

high surface tension liquids that are subsequently disintegrated into a dispersion of microto millimeter-sized droplets. Solidification of droplets always occurs simultaneously or subsequently, although a certain fraction of droplets may be in fully liquid state during impingement onto the deposition surface in a spray forming process. Rapid solidification that occurs in most atomization processes of melts is beneficial in promoting the metallurgical homogeneity and evenness of dendrite growth and distribution throughout the powders produced, leading to small grain size and elimination of macrosegregation in the end-products. Accordingly, the physical and mechanical properties of powder metallurgy (P/M) products or spraydeposited components are equivalent or usually superior to those of the parts manufactured via conventional routes. In addition, energy and cost savings are obvious advantages of P/M and spray atomization deposition processes. Hence, atomization of melts is a good choice for many manufacturing requirements.

In the applications associated with discrete droplets of both normal liquids and melts, large surface areas of droplets are utilized to achieve extremely high reaction rates, cooling and solidification rates, or evaporation rates. Therefore, droplet (particle) size and shape are the issues of most importance. As shown in Fig. 1.6, droplet (particle) size may vary from submicrometers, through tens of micrometers, to millimeters in different applications. Droplet (particle) shape may change from sphericity, through varying degrees of angularity, spongiform, to acicular, fibrous shapes, as illustrated in Fig. 1.7. By carefully selecting appropriate atomizers and atomization media, it is possible to obtain virtually any particular mean droplet (particle) size, size distribution, and any desired droplet (particle) shape. For example, high pressure gas atomization of melts with supersonic, close-coupled atomizers may generate fine metal droplets at high yields. Near-spheroidal droplets of metals can be obtained using gas atomization while water atomization of melts usually produces droplets of irregular shapes. In addition, efficient energy utilization, production of a narrow range of droplet (particle) size and shape, and droplet generation rate are some further issues of particular concern in the processes associated with discrete droplets.

General Introduction

9

(a)

Figure 1.4. (a) Schematic of powder production process; (b) a facility for powder production. (Courtesy of Atomizing Systems Ltd., UK.)

16 Science and Engineering of Droplets

Droplet deformation during impact onto a surface is an interesting subject in many scientific and engineering fields. The icing and erosion/ablation of aircraft surfaces may be caused by the impact of droplets in clouds and/or rains on the surfaces during flight.[44][45] The erosion of turbine blades[46] operating in wet steam and the erosion of terrestrial surfaces during rain are all related to droplet impact. The phenomena of droplet impinging and spreading on a surface are also encountered in a variety of engineering applications. These include, for example, spray combustion, spray cooling of surfaces, dispersed two-phase flow in once-through boilers, postcritical heat flux cooling, ink-jet printing,[47] metal welding/soldering, spray painting, lubrication, oil recovery from porous rocks,[48] and production of fine metal powders via impact atomization.[49] The spreading phenomenon is often accompanied by simultaneous heat transfer and solidification of droplets on deposition surfaces as observed in picoliter solder droplet dispensing for mounting of microelectronic components,[50] spray forming for near-net shape materials synthesis,[3] and thermal spray deposition for surface coating[18][51][52] (Fig. 1.8).

The extensive applications have stimulated the fundamental research and developments of the techniques associated with discrete droplets. In recent years, strong research efforts have led to significant strides in theoretical and experimental studies on droplet processes. The drastic developments in high speed, large memory, parallel processing computer systems and advanced computational and measurement techniques have enabled the direct numerical modeling and on-line, in-situ measurements of certain droplet properties, and largely advanced the atomizer design and the optimization of atomization and spray processes. These advances have significantly improved the fundamental understanding of the phenomena and processes associated with discrete droplets in general. Particularly, the design, testing and analysis of spray combustion in engine combustors have reached the highest levels in the overall field of spray science and technology.

18 Science and Engineering of Droplets

systematic survey of various processes and techniques for droplet generation, along with their applications and associated materials systems. It is followed by a thorough description of the fundamental phenomena and principles involved in droplet processes, with emphasis on the mechanisms of droplet formation and deformation in various processes. The empirical and analytical correlations, theoretical calculations and numerical modeling of droplet processes are discussed in detail to provide insight into the effects of process parameters on droplet properties and to summarize methodologies for analysis, design and optimization of droplet processes. Finally, the primary measurement techniques for droplet properties are outlined and the approaches to intelligent control over droplet processes are described along with discussions on recent developments. Detailed descriptions of the mechanisms involved in droplet condensa- tion[33]-[35][53] and evaporation,[54][55] vaporization,[56]-[59] heat and mass transfer,[60]-[69] and chemical reactions in spray combustion, spray drying and other droplet processes are beyond the scope of the present book.