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interferometry for measurements of droplet velocity and size, various forms of Rayleigh, Raman and fluorescence spectroscopies for measurements of temperature and species concentration.[647] The use of the laser-based diagnostic techniques is considered as the state-of- the-art in spray characterization.

In this chapter, the primary measurement techniques for droplet properties are outlined, and their capabilities and limitations are discussed. The approaches to intelligent process control as related to droplet processes are presented along with descriptions of recent developments.

6.1.0MEASUREMENT TECHNIQUES FOR DROPLET SIZE

Many useful measurement techniques for droplet size have been developed. Each of these methods has its advantages and limitations, and none of the methods is fully satisfactory. In addition, no single method/instrument can cover the entire range of droplet sizes or other properties that need to be characterized. The measurement techniques for droplet sizing may be grouped conveniently into four primary categories (Table 6.1): (a) mechanical methods, (b) electrical methods, (c) optical methods, and (d) acoustical method. Mechanical methods are relatively simple and of low cost. Droplets are collected in either liquid or frozen state, followed by microscopic or sieving analysis. Electrical methods involve the detection and analysis of electronic pulses generated by droplets in a measurement volume or on a wire. The electronic signals are then converted into digital data and calibrated to produce information on droplet size distribution. Optical methods have been developed in recent years and are finding an increasing range of applications. Some of the optical methods are capable of simultaneously measuring droplet size and velocity,[648]-[650] as well as velocity and number density.[651] However, many important optical measurement techniques provide only the information of spatial distributions of droplet properties in a spray, whereas the information of temporal distributions

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is generally more desirable. An acoustical method[6] has been evaluated for the measurements of fine droplets. Probe techniques, such as hot wire technique, have a size range from 1 to 600 µm, while imaging methods offer a size range above 5 µm depending on the system used.

Table 6.1. Measurement Techniques for Droplet Size

	Categories	Methods	Size Range
			(µm)
		Collection of droplets on slides or in cells	³~3
	Mechanical	Cascade impactor	³~3
		Molten-wax and frozen-drop techniques
		Mechanical sieving of metal powder
		Pulse counting technique
	Electrical	Charged wire technique
		Hot wire technique	1–600
		Imaging	>~5
		Photography
		Videography
		Holography	5–1000
		Non-imaging	5-3000
		Light-scattering interferometry
	Optical	Phase-Doppler anemometry	0.5-3000
		Light intensity deconvolution technique	0.2-200
		Light scattering technique	10–250
		Malvern particle analyzer	1–500
		Polarization ratio particle sizer
		Intensity ratio method
		Phase optical-microwave method
		Dual-cylindrical wave laser technique
	Acoustical	—	5–30

In some droplet sizing techniques, such as the TSI aerodynamic particle sizer,[652] both droplet shape and density affect the response of the sizer. A higher-density droplet has a smaller drag, leading to a longer flight time through the sensing zone and a larger

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measurement size. On the contrary, a non-spherical droplet has a larger drag than a spherical droplet with the same diameter and density, giving rise to a shorter flight time through the sensing zone and a smaller measurement size. Depending on the surface tension, viscosity, and original size of a droplet, the degree of droplet shape deformation varies, and thus the measurement size may change with all these parameters.

Extensive reviews of droplet size measurement techniques have been given by Chigier,[647][653] Bachalo,[654][655] Hirleman,[656] Jones,[657] and Lefebvre.[1] A comparison of performance of seventeen droplet-sizing instruments from six different types has been made by Dodge.[658]

6.1.1Mechanical Methods

Mechanical methods are fairly simple. The methods may be divided into two major groups. One involves the capture of a sample of droplets on a solid surface or in a cell containing a special liquid. The captured droplets are then observed or photographed by means of a microscope, generating information on droplet size. The other involves freeze-up of droplets into solid particles and subsequent sieving to generate droplet size distribution. The major problem is associated with the extraction and collection of representative spray samples.

Collection Techniques. The simplest mechanical method for normal liquid droplets is the slide collection (slide sampling) or impression method. This method was extensively used three decades ago, and has been rarely employed since then. In this method, when the slide is exposed to a spray, droplets impinging on it make impressions. The impressions are then observed and measured usually using a Quantimet image analyzer, although a microscope fitted with a traversing scale may be used for the measurement. The measured data are subsequently converted to actual droplet sizes based on a correction factor proposed by May.[659]

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The slide used in this method is usually made of glass and coated with a material of very fine grain to generate clear impressions of droplets on it. For example, a thin soot layer with a very fine surface can be deposited on a glass slide by burning a kerosenesoaked wick under the glass,[438] and a thin film of magnesium oxide can be produced by burning a magnesium ribbon.[659] The soot layer thickness may be determined from the relation between the droplet Weber number and impression diameter, and thus varies for different liquids.[438] Droplets as small as 3 µm in diameter can be observed and measured using slides with these coatings. In addition, spatial and overall droplet size distributions may be measured with this method by properly manipulating the slide.[459]

The fraction of slide surface to be covered by collected droplets is an important factor influencing overall measurement accuracy and time. If the slide is covered by too many droplets, the measurement error and time will increase due to droplet overlap and tedious counting. If too few droplets are collected, the sample may not be large enough to generate statistically representative data. For the measurement accuracy and ease, a coverage of 0.2% has been found to be sufficient and satisfactory with an upper limit of 1.0%.[1]

Size bias associated with this measurement method can be caused by many phenomena, such as droplet evaporation, selective collection, and correction factor. In a spray of a normal liquid, small droplets have short life time due to evaporation. This effect is especially significant in the measurements of fine sprays. The selective collection may occur in the measurements of the sprays generated with air-blast atomizers. Since small droplets tend to follow streamlines of the air (gas) flow field around the collection slide surface while large droplets impinge onto the slide surface due to large inertia, more large droplets are collected. Thus, the measured droplet sizes tend to be larger than the actual sizes. In addition, the diameter of a deformed (flattened) droplet on the collection surface needs to be corrected to the diameter of the original spherical droplet by multiplying a correction factor. The determination of the value of this correction factor is not trivial because it varies with droplet

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properties (notably surface tension) and velocity as well as coating material properties. It may change from about 0.5 to 0.86 for oil droplets on a clean glass and a magnesium-coated slide, respectively.

To reduce the bias caused by droplet deformation during impingement, droplets can be collected on slides[660] or in cells[661] containing an appropriate immersion liquid in which the droplets do not dissolve but remain stable and suspended during counting and measuring.[317] This is the so-calledliquid immersion sampling technique. In this technique, an appropriate immersion liquid must be selected for each given liquid. For example, Stoddard solvent and white kerosene may be the suitable immersion liquids for the droplets of water and water-alcohol mixtures.[661] For an immersion liquid with a density slightly smaller than that of droplets, the shape of the droplets can remain almost unchanged if the evaporation and disintegration of the droplets are prevented during impinging the immersion liquid. Thus, the droplet sizes can be measured directly without need for correction. However, this method may generate large measurement errors when used for coarse sprays because large droplets tend to split into smaller ones upon impact with the immersion liquid. In addition, droplets may coalesce during collection. To avoid or reduce the breakup and coalescence of droplets, immersion liquids of low viscosity and surface tension may be used to facilitate the penetration of liquid surface by the droplets and to reduce shear stresses acting on the droplets during the penetration. Thus, the droplets may remain stable and suspended near the bottom of the collection cell for some time allowing photographing at high magnification. The magnified images of the droplets can be processed by means of a computer to generate data for droplet size distribution.

Zhang and Ishii[662] used an isokinetic sampling probe and image processing system for droplet size measurements in twophase flows. Droplet samples were extracted from flow field by the sampling probe and collected in an immiscible liquid. Droplet images were grabbed and digitized by the image processing system. Image processing software was developed for droplet counting and sizing, with the capability of distinguishing overlapping droplets.

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Possible measurement bias factors such as droplet deposition in the probe, droplet breakup and coalescence were studied. A simple criterion for minimizing measurement bias was proposed. The system can be used for both water and liquid-metal droplets.

Cascade Impactor method is conceptually a slide collection method. Theoretically, any number of stages may be used to make the desired droplet size groupings. In single-stage impactor method, a slide (i.e., an impaction surface) is placed in a spray below a nozzle. The slide is usually coated with a mixture of carbon and magnesium oxide to retain the droplets impinging on it. A large droplet traveling at high velocity most likely impacts the slide placed in its path due to its large momentum, while a small droplet traveling at low velocity tends to follow air flow around the slide. With increasing air flow velocity, an increased number of small droplets will impact the slide due to increased momentum. In multi-stage (cascade) impactor method, air flow velocity is increased gradually, thus both the velocity and impaction efficiency of droplets increase from slide to slide. For the improvement of impaction efficiency, impactors of large dimensions should be used. The advantages of the cascade impactor method include (a) isokinetic sampling that can overcome discrimination against very large or very small droplets in a spray; (b) easy assessment of the quantity of liquid for each droplet size class collected in each stage of a cascade impactor by gravimetric or chemical methods; (c) satisfactory operation under arduous conditions; (d) capability to withstand the rigors of routine handling; and

(e) ease and simplicity in design and operation relative to optical methods.

When a droplet falling at a certain velocity impacts on an object, it will wet and spread through the surface if the contact angle is less than 90°. If the object is very thin, the droplet may all soak into it and the liquid may reach the other side that can be seen if the droplet contains a tracer dye. In a thick and porous Kromekote® card,[507] a droplet spreads to form a circular stain with simultaneous absorption and penetration into the card. Kromekote® cards have been used in forestry spray trials to collect pesticide droplets for

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assessing target coverage (number of droplets per unit area) and size spectra of the spray cloud reaching the treatment site.

Molten Wax and Frozen-Drop Techniques. Molten wax technique is a mechanical method developed in the early 1940’s.[663] In this method, paraffin wax is heated to an appropriate temperature above its melting point so that its properties approach those of aviation kerosene. The molten wax is then injected into a large pressure chamber, in which the droplets of the molten wax rapidly cool and solidify. The wax particles are subsequently collected and separated into size groups using a sieve. Each size group is weighted to obtain the volume or mass fraction of the corresponding size range, generating directly the cumulative volume or mass distribution and mass median size. Thus, it alleviates large expenditure of time and efforts for sizing and counting of numerous individual droplets. In addition, this technique offers better statistical accuracy due to the huge number of droplets/particles in a sample.

The molten wax method requires that the properties of a simulant are very close to those of the liquid of interest. Thus, the choice of suitable materials is limited. The method also suffers from some practical problems in preheating the wax and errors incurred by changes in physical properties of the wax during cooling after leaving the injector. Since the properties of the wax (notably surface tension and viscosity) critically influence the process of droplet formation, it may not be accurately reproduced due to the changes in these properties with temperature. Therefore, it may be required that the air in the near-nozzle region, where the key process of droplet formation occurs, be heated to the same temperature as that of the molten wax.

To get rid of the drawbacks associated with the molten wax technique, an alternative is to solidify the droplets of the liquid of interest as soon as they are formed after leaving the nozzle. This is the basic concept of the frozen-drop technique, a natural extension of the molten wax technique. The freeze-up and collection of droplets may be carried out in many different ways. For example, liquid droplets from a fuel spray can be collected into a stream of fluid at

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room temperature. The fluid carrying the droplets is then introduced into an alcohol bath at the temperature of dry ice where the droplets freeze into solid particles. The solid particles are subsequently sieved to obtain the size distribution. Water droplets can be collected in a hexane pan,[664] wrapped around with dry ice and kept at –20 °C. The droplets freeze up during falling through the hexane and settle on a shutter. The shutter is then opened, allowing the frozen particles to fall 30 cm and come to rest on a scale pan. The time needed for the traveling and the variation of the scale pan weight with time are measured. The traveling time is dependent on the relative density of the droplets and the carrying liquid, the liquid viscosity, the falling distance, and the droplet diameter. Thus, the droplet size can be calculated on the basis of the measured time data. The entire measurement takes very long time to complete, particularly for small droplets.

Droplets can also be collected and frozen in a liquid nitrogen bath placed below an atomizer.[665] After sufficient droplets are collected, the frozen particles are passed through a series of screens of different sizes to determine the size distribution. The density of the liquid being atomized is required to exceed 1200 kg/m3 to avoid any agglomeration of droplets on the surface of the liquid nitrogen bath. Thus, the method is not applicable to many normal liquids such as water, fuel oils, and kerosene. In a modified version of the frozendrop method, frozen particles are collected in a nitrogen-cooled perspex pot and photographed through a microscope. The final prints are enlarged and analyzed to obtain the droplet size distribution. This method is simple and convenient. However, similarly to the original version, the change in droplet size due to freezing requires a correction factor for the determination of the actual droplet size.

The frozen-drop technique was naturally adopted in measuring molten metal droplet size before any other methods became available. Similarly to the methods for normal liquids, the freeze-up and collection of molten metal droplets may be carried out in many different ways. For example, metal droplets can solidify during flight in gaseous or liquid medium in a spray chamber.[3]-[5] The solidified particles are subsequently sieved to obtain the size distribution.