20Science and Engineering of Droplets

2.1.0ATOMIZATION OF NORMAL LIQUIDS

Atomization is defined as the disintegration of a liquid into small drops or droplets. The resultant suspension of fine droplets or solid particles in a surrounding gas is termed spray, mist, or aerosol. Atomization of a liquid into discrete droplets can be brought about by a variety of means: aerodynamically, mechanically, ultrasonically, or electrostatically, etc. For example, the breakup of a liquid into droplets can be achieved by the impingement with a gas in twofluid atomization, by centrifugal forces in rotary atomization, by ultrasonic vibrations utilizing a piezoelectric transducer in ultrasonic atomization, or by electrostatic/electromagnetic forces in an electric/ magnetic field in electrostatic/electromagnetic atomization. Atomization processes may also be classified according to the energy used to produce the instability on a liquid. For example, pressure energy is used for pressure atomization, centrifugal energy for rotary atomization, gaseous/fluid energy for two-fluid atomization, and vibratory energy for ultrasonic or acoustic atomization.

Most practical atomization processes for normal liquids include: (a) pressure atomization, (b) two-fluid atomization, and (c) rotary atomization. Many other useful atomization processes for normal liquids have been developed in special applications, including: (a) effervescent atomization, (b) electrostatic atomization, (c) ultrasonic atomization, (d) whistle atomization, etc. These processes may be loosely classified into two major categories in terms of the relative velocity between the liquid being atomized and the surrounding ambience. In the first category, a liquid at high velocity is discharged into a still or relatively slow-moving gas (air or other gases). Notable processes in this category include, for example, pressure atomization and rotary atomization. In the second category, a relatively slow-moving liquid is exposed to a stream of gas at high velocity. This category includes, for example, two-fluid atomization and whistle atomization.

Atomization of normal liquids has been long studied in the fields of spray combustion and spray drying. The most widespread application of the atomization of normal liquids is in spray

Droplet Generation 21

combustion processes. Generally, an injector or an atomizer is used to produce a spray of fuel inside a combustion chamber. With an injector, the fuel is introduced into the combustion chamber at high velocity, creating sufficient turbulence to generate a dispersion of fuel droplets. With an atomizer, the fuel is disintegrated into droplets either internally by spreading it on the surface of a swirl chamber or by an internal blast fluid, or externally by an external blast fluid, followed by the dispersion of the resultant droplets in the combustion chamber. Atomization of a fuel before injection into the combustion zone is critical to the combustion efficiency and performance of an engine or industrial furnace because atomization can promote the fuel evaporation and combustion by generating a very high surface-to-volume ratio in the liquid phase. A comprehensive theoretical and empirical background on the atomization of normal liquids has been established on the basis of the studies in these areas. A variety of atomizer designs, devices and techniques have been developed and tested for the atomization of normal liquids, the range of which is much wider than that for the atomization of melts.

The liquids involved in spray combustion, spray drying and other normal liquid processes are primarily hydrocarbon oils or aqueous liquids, although slurries, emulsions, and various nonNewtonian liquids have also been sprayed in some applications. The basic requirement in atomization applications is a narrow droplet size distribution, even a monosize distribution in some spray drying and aerosol spray applications. Small droplet size is desired in spray combustion for rapid heat transfer and vaporization. An ideal atomizer should possess the capability of providing energy-efficient and cost-effective atomization over a wide range of liquid flow rates and responding to changes in liquid flow rates. It should also be capable of scaling up for design flexibility, and not be susceptible to damages during manufacture, maintenance and installation. For spray combustion, an ideal atomizer should produce a uniform radial and circumferential fuel distribution, and be free of flow instabilities. It should not be susceptible to the gum formation by heat soakage and the blockage by contaminants or carbon buildup on the nozzle.

22 Science and Engineering of Droplets

This section describes the atomization processes and techniques for droplet generation of normal liquids. A comparison of the features of various atomization techniques is summarized in Table 2.1. Emphasis is placed on the atomization processes used in spray combustion and spray drying from which many atomization processes have evolved. Advantages and limitations of the atomization systems are discussed along with typical ranges of operation conditions, design characteristics, and actual and potential applications. The physical properties of some normal liquids are listed in Table 2.2, where T is the temperature,ρ is the density, µ is the viscosity, σ is the surface tension, and Pv is the vapor pressure. Depending upon the priority levels of application requirements, many factors need to be considered for the selection of an appropriate technique. These include, for example, (a) the cost-effectiveness, (b) the ease of operation and maintenance, and (c) the capability of producing a monodisperse spray, covering the required range of droplet sizes, accommodating various liquid properties, and achieving sufficient production rates.

2.1.1Pressure Jet Atomization

Pressure atomization is probably one of the most commonly used techniques in general application areas. In pressure atomization of a liquid, pressure is converted to kinetic energy to accelerate the liquid to a high velocity relative to surrounding ambience. Depending on the atomizer design and geometry, a liquid may be accelerated in different forms, such as a liquid jet in pressure jet atomization, a swirling liquid sheet in pressure-swirl atomization, or a fan liquid sheet in fan spray atomization. Pressure atomizers include: (a) plainorifice atomizer used in pressure jet atomization, (b) simplex, duplex, dual-orifice and spill-return atomizers in pressure-swirl atomization, and (c) fan spray atomizer in fan spray atomization. Pressure atomization has been long studied by many investigators, for example, Hiroyasu and Kadota,[90] Mock and Ganger,[91] Carey,[92] Radcliffe,[93] and Dombrowski et al.,[94][95] among others.

Droplet Generation 23

Table 2.1. Comparison of Features of Various Atomization Techniques for Normal Liquids[1][5]

	Method		Droplet	Application	Advantage	Limitation
			Size (μm)
				Diesel
		Plain-		engines, Jet	Simple,	Narrow spray
			25–250	engine	Rugged,	angle, Solid
		Orifice
				afterburners,	Cheap	spray cone
				Ramjets
						High supply
						pressure,
			20–200	Gas turbines,	Simple,	Varying spray
						angle with
		Simplex		Industrial	Cheap, Wide
						pressure
				furnaces	spray angle
						differential and
						ambient gas
						density
					Simple,
					Cheap, Wide
					spray angle,	Narrowing spray
					Good
				Gas turbine		angle with
		Duplex	20–200		atomization
				combustors		increasing liquid
					over a wide
						flow rate
					range of
					liquid flow
					rates
	Pressure				Good	Poor atomization
	Atomization					in transition
					atomization,
				A variety of		range,
		Dual-			Turndown
				aircraft and		Complexity in
			20–200		ratio 50:1,
		Orifice		industrial gas		design,
					Relatively
				turbines		Susceptibility of
					constant
						small passages to
					spray angle
						blockage
					Simple, Good
					atomi-zation
				A variety of	over entire
					range of	Varying spray
				combustors,
					liquid flow	angle with liquid
				Good
		Spill			rates, Very	flow rate, Higher
				potential for
			20–200		large	power require-
		Return		slurries and
					turndown	ments except at
				fuels of low
					ratio, Low	maximum
				thermal
					risk of	discharge
				stability
					blockage due
					to large
					passages
				High-	Good
		Fan		pressure	atomization,	High supply
			100–1000	painting/coat	Narrow
		Spray				pressure
				ing, Annular	elliptical
				combustors	spray pattern

24 Science and Engineering of Droplets

Table 2.1. (Cont’d.)

	Method		Droplet
			Size	Application	Advantage	Limitation
			(μm)
				Spray	Good mono-
				drying.	dispersity of	Satellite
				Aerial	droplets.
		Spinning	10–200	distribution	Independent	droplets,
		Disk		of	control of	360º spray
	Rotary			pesticides.	atomization	pattern
	Atomization			Chemical	quality and
				processing	liquid flow rate
		Rotary		Spray		Possible
			10–320	drying.	Capable of	requirement for
		Cup	[73][74]	Spray	handling slurries	air blast around
				cooling		periphery
					Good	Possible liquid
						backup into air
					atomization. Low
						line.
				Industrial	risk of clogging
						Requirements
		Internal	50–500	furnaces.	due to large
						for external
		Mixing	[75][76]	Industrial	passages.
						source of high
				gas turbines	Capable of
						pressure air &
					atomizing high-
						auxiliary
	Two-Fluid				viscosity liquids
						metering device
	Atomization
					Good atomi-
	Air-Assist				zation. Low risk
					of clogging due	Requirements
		External		Industrial	to large holes.	for external
			20–140	furnaces.	Capable of	source of high
		Mixing	[77][78]	Industrial	atomizing high-	pressure air.
				gas turbines	viscosity liquids.	Limited
					No risk of liquid	liquid/air ratios
					backup into air
					line
						Narrow spray
						angle.
			15–130	Industrial	Simple, cheap,	Atomizing
		Plain-Jet				performance
			[79]-[82]	gas turbines	good atomization
	Two-Fluid					inferior to
						prefilming air-
	Atomization
						blast type
	Air-Blast
				Wide range	Good atomi-	Poor
				of aircraft	zation especially
		Pre-	25–140			atomization at
				and	at high ambient
		filming	[83]-[86]			low air
				industrial	pressures. Wide
						velocities
				gas turbines	spray angle

					Droplet Generation		25
Table 2.1. (Cont’d.)
	Method	Droplet		Application	Advantage	Limitation
		Size (μm)
					Simple,
					reliable, very
					good
					atomization.
					Low risk of
	Effer-				plugging due	Need for
					to large holes.
						separate
	vescent	20–340			Easy
				Combustion		supply of
	Atomization	[87]			maintenance.
						atomizing
					Beneficial
						air
					effects on
					reducing soot
					formation &
					exhaust
					smoke.
					Cheap.
						Very low
	Electrostatic	0.1–1000	[88]	Paint		flow rates,
					Fine and	Strongly
				spraying,
	Atomization	300–600[88]			uniform	dependent
		100–250[126]		Printing, Oil	droplets	on liquid
				burner
						electrical
						properties
				Medical
		Nebulizers		spray.	Very fine
		1–5 (55kHz,		Humidi-		Incapable
	Ultrasonic				and uniform
		0.12 l/min)		fication.		of handling
	Atomization			Spray drying.	droplets,
		30–60				high liquid
				Acid etching.	Low spray
		(50 kHz)				flow rates
				Printing	rates
		1–200[88]		circuit.
				Combustion
		~50 (10kHz,		Atomization
	Whistle	≤75 l/min)		of liquid	Fine droplets,	Broad
		7 (>20 kHz,		metals	High gas	droplet size
	Atomization
		0.125kg/min		for powder	efficiency	distribution
		0.33 MPa)		production

26 Science and Engineering of Droplets

Table 2.2 Physical Properties of Some Normal Liquids[1][88][89]

	Liquid	T	ρ	µ	σ	Pv
		(K)	(kg/m3)	(Ns/m2)	(J/m2)	(mm Hg)
	Acetone	293	792	0.32×10-3	0.0237	195
	Aniline	288	1000	5.3×10-3	0.04	243a
	Benzene	293	880	0.65×10-3	0.029	80
	Carbon tetrachloride	293	1588	0.97×10-3	0.027	91
	Castor oil	293	970	1	0.039	-
	Chloroform	293	1489	0.58×10-3	0.02714	-
	Cottonseed oil	293	920	0.07	0.0354	-
	Diesel Fuel	293	890	0.01	0.03	-
	Ethanol	293	790	1.2×10-3	0.022	47
	Fuel oil (light)	313	916	0.047	0.023	-
	Fuel oil (medium)	313	936	0.215	0.023	-
	Fuel oil (heavy)	313	970	0.567	0.023	-
	Gas oil	313	863	3.3×10-3	0.023	-
	Gasoline	293	680	0.35×10-3	-	413
	Glycerin	293	1258	1.49	0.063	10-4
	Heptane	300	684	0.38×10-3	0.0194	1b
	Hexane	300	660	0.29×10-3	0.0176	162
	Kerosene	293	820	2.5×10-3	0.025	7c
	Lube oil SAE10	293	900	0.1	0.036	-
	Lube oil SAE30	293	900	0.3	0.036	1c
	Malathion (95%)	293	1230	0.045	0.032	4×10-5c
	Mercury	293	13550	1.53×10-3	0.48	10-3
	Methanol	293	800	0.6×10-3	0.022	100
	n-Octane	300	703	0.5×10-3	0.021	-
	Turpentine	293	867	1.49×10-3	0.027d	3
	Water	293	998	1×10-3	0.0728	18

a At 418k; b At233k; c At 303k; d At 283k

When passing a liquid of low viscosity through a small circular hole, i.e., a plain-orifice, the liquid can be readily atomized. At a low injection pressure, the liquid emerges at low velocity as a thin distorted- pencil-shaped stream. For injection pressures in excess of the ambient pressure by approximately 150 kPa, the liquid issuing from the orifice forms a high-velocity jet and disintegrates rapidly into droplets. The mean droplet size and size distribution depend on many variables, such as operation conditions, liquid properties, and nozzle geometry. Increasing the injection pressure can enhance the flow velocity of the liquid jet,

Droplet Generation 27

leading to an increase in both the level of turbulence in the liquid jet and the aerodynamic drag forces exerted by the surrounding medium, and thereby promoting the disintegration of the liquid jet. A high injection pressure, low ambient pressure and/or small orifice diameter lead to small droplet sizes. An increase in liquid viscosity and/or surface tension usually hinders the liquid disintegration. High viscosity causes large droplet sizes, poor atomization, high spray jet penetration, low evaporation rates, heterogeneous mixing, and imperfect combustion. Surface tension opposes the distortion of the liquid surface. Liquid density affects the spray penetration as the kinetic energy of a liquid jet is one of the primary factors determining the spray behavior. The angle of the spray cone formed by a plain-orifice atomizer ranges typically from 5° to 15°. This angle is mainly affected by the turbulence and properties (viscosity and surface tension) of the liquid jet, and to a lesser extent by the diameter and diameter-to-length ratio of the orifice. An increase in the turbulence enlarges the cone angle by increasing the ratio of radial-to-axial velocity components of the liquid jet.

The flow of the surrounding gas or air has an important effect on pressure jet atomization with a plain-orifice. This is because the atomization process is not completed after a jet leaves the orifice. In fact, it continues in the surrounding medium until the droplet size falls below a critical value so that no further breakup may occur. For a given liquid, this critical size is dependent on the relative velocity between the liquid and the surrounding medium. If both the liquid and gas are flowing in the same direction, the penetration is augmented and atomization is retarded, leading to an increase in the mean droplet size. If the flows are in opposite directions, the penetration is decreased with a concomitant increase in the spray cone angle and the quality of atomization. The gas velocity affects the formation and development of a spray and the quality and degree of atomization. It should be noted that the effect of the gas flow on spray characteristics using a plain-orifice atomizer is relevant only to those operations in which the gas velocity is not high enough to change the nature of the atomization process. If the gas velocity is sufficiently high, the mechanism of liquid jet breakup may change to that of air-blast atomization.

28 Science and Engineering of Droplets

Plain-orifice atomizers are widely used for injecting liquids into a flow stream of air or gas. The injection may occur in a co-flow, a contra-flow, or a cross-flow stream. The best known application of plain-orifice atomizers is perhaps diesel injectors. This type of injectors is designed to provide a pulsed or intermittent supply of fuel to the combustion zone for each power stroke of the piston. As the air in the combustion zone is compressed by the piston to a high pressure, a very high pressure (83–103 MPa) is required to allow the fuel to penetrate into the combustion zone and disintegrate into a well-atomized spray.

Another important application of plain-orifice atomizers is jet engine afterburner injectors. The fuel injection system typically consists of one or more circular manifolds supported by struts in a jet pipe. The fuel is supplied to the manifold by feed pipes in the support struts and sprayed into the combustion zone through the orifices in the manifold. Increasing the number of orifices and/or using a ringlike manifold may promote uniform distribution of liquid. To reduce the risk of blockage of orifices, a minimum orifice size of 0.5 mm is usually regarded as practical for kerosene-type fuels.

The third important application of plain-orifice atomizers is rocket injectors. The jets of liquid fuel and oxygen are designed to impinge each other out of the nozzles. At low injection velocities and large impingement angles, a well-defined sheet forms at right angles to the plane of the two jets and breaks up into droplets. The sheet becomes progressively less pronounced as the liquid velocities are increased and/or the impingement angle is reduced. Under typical operation conditions, atomization occurs via a process resembling the disintegration of a plain jet and by somewhat ill-defined sheet formation at the intersection of the two jets.

2.1.2Pressure-Swirl Atomization

One of the limitations of plain-orifice atomizers is the narrow spray cone generated. For most practical applications, large spray cone angles are desired. To achieve a wide spray cone, a simplex, i.e.,

Droplet Generation 29

a pressure-swirl atomizer[96] can be used. A pressure-swirl atomizer consists of a conical swirl chamber with a small orifice at the vertex. During operation, a liquid is introduced into the swirl chamber through tangential ports and allowed to swirl. If the liquid pressure is sufficiently high, a high angular velocity is attained and an air-cored vortex is created. The swirling liquid then flows through the outlet of the swirl chamber and spreads out of the orifice under the action of both axial and radial forces, forming a tulip-shaped or conical sheet beneath the orifice. The sheet subsequently disintegrates into droplets. The liquid-air interaction, liquid surface tension and viscous forces are the primary factors governing the liquid breakup process. The spray cone angle may range from 30° to almost 180°, depending on the relative magnitude of the tangential and axial velocity components at the nozzle exit, and hence can be controlled by adjusting these variables. The droplet size is a function of liquid pressure and swirl chamber dimensions. The smaller the swirl chamber is, the finer the resultant droplets become. However, there exist quite restrictive flow limits for small droplets. Another undesirable feature is the instability to get sharp cutoff of spray due to dribbling.

Swirling flows are used extensively in atomizers and burners to generate recirculation zones with high shear rates. This type of atomizers makes use of centrifugal forces. The tangential and radial velocity components are of the same order or higher than the axial velocity component. The designs of pressure-swirl atomizers developed to date may be grouped into two basic types in terms of the spray pattern generated, namely, solid-cone spray and hollow-cone spray. In the first type of designs, droplets are distributed fairly uniformly throughout the volume of the spray cone, hence referred to as solid-cone spray. In the second type of designs, droplets are concentrated in the outer periphery of the spray cone, thereby termed hollow-cone spray.

A hollow-cone spray can be generated via a simplex atomizer. The spray pattern varies depending on the injection pressure. At very low pressures, liquid dribbles from the nozzle orifice. With increasing pressure, the liquid emerges from the orifice as a thin,

30 Science and Engineering of Droplets

distorted-pencil-shaped stream. A liquid cone may form at the orifice, but may contract into a closed bubble by surface tension forces. With further increasing pressure, the bubble opens into a hollow tulip-shape terminating in a ragged edge where the liquid disintegrates into large droplets. At high pressures, the curved surface of the tulip-shape straightens and a conical liquid sheet forms. As the sheet extends and its thickness diminishes, it becomes unstable and breaks up into ligaments that subsequently disintegrate into droplets in a well-defined hollow-cone spray. For low-viscosity liquids, the lowest injection pressure at which atomization can be achieved is about 100 kPa.

Solid-cone spray atomizers have been applied to many industrial processes such as gas scrubbing, gas washing, coke quenching, fire protection, chemical processing, foam breaking, dust suppression, gravel washing, vegetable cleaning, spray cooling, and drenching operations. The Delavan nozzle is a typical example of the solid-cone spray atomizers.

Various hollow-cone simplex atomizers (Fig. 2.1) have been developed for combustion applications, differing from each other mainly in the way that swirl is imparted to the issuing liquid jet. In these atomizers, swirl chambers may have conical slots, helical slots (or vanes), or tangential slots (or drilled holes). Using thin, removable swirl plates to cut or stamp the swirl chamber entry ports leads to economies of the atomization systems if spray uniformity is not a primary concern. Large simplex atomizers have found applications in utility boilers and industrial furnaces. Oil flow rates can be as high as 67 kg/min.

Solid-cone spray atomizers usually generate relatively coarse droplets. In addition, the droplets in the center of the spray cone are larger than those in the periphery. In contrast, hollow-cone spray atomizers produce finer droplets, and the radial liquid distribution is also preferred for many industrial applications, particularly for combustion applications. However, in a simplex atomizer, the liquid flow rate varies as the square root of the injection pressure. To double the flow rate, a fourfold increase in the injection pressure is

32 Science and Engineering of Droplets

pressure is attained, the valve opens and the liquid flows through both the primary and secondary swirl ports simultaneously. Duplex atomizers exhibit superior performance to simplex atomizers, particularly at low flow rates. However, duplex atomizers tend to produce smaller spray cone angles in the combined flow range than in the primary flow range. In some designs, this problem is resolved by using a smaller tangent circle for the primary swirl ports than the secondary ports, giving rise to a constant spray cone angle. Duplex atomizers have been used in gas turbines for the atomization of fuels.

Dual-orifice atomizers have the same advantage as duplex atomizers, i.e., better atomization at low liquid flow rates. A dualorifice atomizer (Fig. 2.3) comprises two simplex nozzles with the primary nozzle mounted inside the secondary nozzle concentrically. During operation, the primary spray does not interfere with either the secondary orifice or the secondary spray within the orifice. At low flow rates, a liquid flows entirely through the primary nozzle. Since the pressure required to force the liquid through the small ports in the primary swirl chamber is high, the atomization quality is good. With increasing flow rate, the injection pressure increases and eventually reaches a level at which the liquid is admitted to the secondary nozzle. Starting from this point and over a range of higher flow rates, the atomization quality is relatively poor due to the low secondary pressure. With further increase in the flow rate, the secondary pressure increases correspondingly, improving the atomization quality. In practice, one can employ a primary spray cone angle slightly wider than that of the secondary spray cone so that the two sprays coalesce and share energy within a short distance from the atomizer. However, this may alleviate the problem only to some extent. Therefore, the dual-orifice atomizer must be designed in such a way that the pressure for starting the secondary flow does not coincide with an engine operation point at which high combustion efficiency and low pollutant emission are primary requirements. Dual-orifice atomizers have been widely used in many types of aircraft and industrial engines.

A spill-return atomizer (Fig. 2.4) is essentially a simplex atomizer. The difference between the two types of atomizers is that the rear wall of the swirl chamber is solid in a simplex, while in a

-

- ~

~~

-~

.- c.

~

Primary Secondary

Droplet Generation 33

~

O

u

.J:

~

~

..= u

rn

~

0

t)

";j

8 u

..= t) 00

34 Science and Engineering of Droplets

spill-return atomizer it contains a passage through which a liquid can be spilled away from the atomizer. The liquid is supplied to the swirl chamber always at the maximum pressure and flow rate. At the maximum operation capacity, a valve located in the spill line is closed, allowing the liquid to entirely emerge from the nozzle to form a well-atomized spray. Otherwise, the liquid will be diverted away from the swirl chamber by opening the valve, so that less liquid is left to pass through the nozzle. Satisfactory atomization may be achieved for flow rates down to 1% of the maximum value.[92] The atomization quality is high over a wide range of flow rates at a constant pressure and generally tends to improve as the flow rate is decreased. An even wider range of flow rates can be readily achieved if the pressure is also varied over a range. This is the most useful feature of spill-return atomizers. In addition, a spill-return atomizer is free from the blockage by contaminants in the liquid because the flow passages are designed to allow large flows during operation. A drawback associated with spill-return atomizers is the large variation in spray cone angle with change in flow rate. Lowering flow rate widens the spray cone angle due to the reduced axial velocity component and nearly unchanged tangential velocity component. The spray cone angle at a minimum flow rate may be 50° wider than at a maximum flow rate. Moreover, metering the flow rate is more complex than with other types of atomizers, and a large-capacity pump is required to handle the large recirculating flows. Therefore, spill-return atomizers are mainly used in large industrial furnaces, and the interest in the applications to gas turbines has declined. However, due to the excellent atomizing capability and freedom from blockage, spill-return atomizers are still an attractive alternative for spraying various fuels currently being considered for gas turbine applications, particularly those with high viscosity and aromaticity which may form gum deposition during operation.

The simplex atomizer has many other variants.[1] Designs are aimed at achieving good atomization over a wide range of flow rates by varying the effective flow area without the need for excessive hydraulic pressures. In practice, pressure-swirl atomizers used in gas