44Science and Engineering of Droplets

2.1.5Rotary Atomization

Rotary atomization is a centrifugal atomization method.[94][109] In a rotary atomization process (Fig. 2.10), a liquid is introduced onto a rotating surface at its center and then spreads out nearly uniformly under the action of centrifugal force. At low liquid flow rates, droplets may form directly near the edge of the surface, whereas at high liquid flow rates, ligaments or sheets may be generated at the edge and eventually disintegrate into droplets. The rotating surface may be a flat disk, vaned disk,[110][111] cup,[112] or slotted wheel, etc. Spinning disk and rotary cup atomizers are the widely used types. An air/gas flow around the periphery of rotating surface is sometimes used to shape the spray and to assist in transporting droplets away from the atomizer. Atomization is carried out usually in a cylindrical or conical chamber to accommodate the umbrellalike, 360° spray pattern created by rotating disk and downward air/ gas streams. A symmetrical feed of liquid can be readily achieved using a cup. A conical-shaped cup is more usable for high flow rates than other shapes. The diameter of the rotating surface ranges from 25 mm to 450 mm. Small disks rotate at high speeds up to 1000 rps whereas large disks rotate at low speeds up to 200 rps. Higher speeds on the order of 2500 rps have been used when a pressurized liquid is fed onto a spinning disk.[88] Lower speeds on the order of 50 rps have been used when a coaxial air jet is used to assist liquid breakup. Atomization capabilities up to 84 kg/min can be achieved.[2] In the spray drying industry, advances have been made in handling high flow rates up to 2400 kg/min at high wheel peripheral speeds, yielding droplets finer than 20 µm.[2]

Rotary atomization processes in spray drying have been studied extensively by many researchers, for example, Kayano and Kamiya,[110] Tanasawa et al.,[111] Hinze and Milborn,[112] Christensen and Steely,[113] and Kitamura and Takahashi.[114] Details of the processes have been described and reviewed by Masters,[2] Dombrowski and Munday,[94] Matsumoto et al.,[109] Christensen and Steely,[113] Eisenklam,[115] and Fraser et al.,[116] among others.

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controlled by varying the liquid flow rate and/or the rotational speed. In addition to these two process parameters, the generation of a uniform sheet thickness and thereby a uniform droplet size requires a large centrifugal force, a vibrationless rotation, a constant liquid flow rate, and a smooth disk/cup surface. Atomization quality for liquids of lower viscosity is usually higher than for those with higher viscosity.

The slippage between the liquid and the disk is a problem associated with flat disk atomizers. It may lead to an ejection velocity of the liquid from the disk edge much lower than the disk peripheral speed. This becomes a main drawback of flat disk atomizers, particularly at high rotational speeds. Therefore, conventional rotary disk atomization can generate a narrow spectrum of droplet sizes only when operating in the ligament mode at low liquid flow rates. In commercial atomizers, straight or curved radial vanes are used to prevent transverse flows of the liquid over the surface and to guide the liquid to the periphery. Thus, no slippage occurs after the liquid flows into the vanes, and the liquid ejection velocity may approach the disk peripheral speed. Another operation problem that seems to occur most often in spinning disk atomization is the production of satellite droplets. However, the satellite droplets can be separated dynamically from the larger primary droplets. This is usually done by using a separate air flow near the disk periphery. The velocity of the air flow is set to such a level that the satellite droplets can be dragged into the flow for collection and/or recirculation while the larger primary droplets can fly across it. Although this arrangement can eliminate the satellite droplets to a certain extent, it might be difficult to design such a secondary flow that would operate properly under variable practical conditions, particularly in agricultural aviation applications.[88]

An attractive feature of rotary atomization is the nearly uniform droplets produced with small disks at high rotational speeds and low liquid flow rates. Therefore, rotary atomization is probably the most generally successful method for producing moderately monodisperse sprays over a wide range of droplet sizes. The mean

Droplet Generation 47

droplet size depends primarily on the disk rotational speed and diameter, the liquid density and surface tension, and to a lesser extent the liquid flow rate and viscosity, as well as the geometry parameters of vanes. For convenience, altering the rotational speed is a commonly used method of varying the droplet size. The average droplet size ranges approximately from 10 to over 200 µm. Droplet sizes from submicrometers up to 3000 µm have been produced for various liquids using spinning disks with different rotational speeds, disk diameters, and liquid flow rates. Geometric standard deviations of 1.05–1.72 have been reported.[88] Another useful feature is the flexibility in operation due to the fact that rotary atomizers allow independent variations of atomization quality, rotational speed, and liquid flow rate. Moreover, rotary atomization processes are extremely versatile and have been successfully applied to the atomization of liquids with a wide range of viscosities. The processes have a broad range of industrial applications such as spray drying and aerial distribution of pesticides, and have been widely used in the chemical processing industry for a century. As more reliable designs and operations are developed, rotary atomization will find increasing applications in spray drying industry.

One of the special rotary atomizers worth mentioning is the windmill type atomizer. In this atomizer, radial cuts are made at the periphery of a disk and the tips of segments are twisted, so that the disk is actually converted into a windmill that can rotate rapidly when exposed to an air flow at aircraft flight speed. The windmill type atomizer has been demonstrated[117] to be an ideal rotary atomizer for generating a narrow spectrum of droplet sizes in the range most suitable for aerial applications of pesticides at relatively high liquid flow rates.

2.1.6Effervescent Atomization

In flashing liquid jet or flashing injection atomization,[118] a high-pressure liquid with dissolved gas flashes, shattering the liquid into small droplets in a fairly regular spray pattern. Flashing even

48 Science and Engineering of Droplets

small quantities of dissolved gas (<15% in mole fraction) can significantly improve atomization. To realize the beneficial effects, however, an expansion chamber is needed upstream of the discharge orifice. The bubble growth rate in dissolved gas systems is usually low, posing a limitation on the practical application of flashing injection atomization with dissolved gas systems.

To overcome the basic problems of flashing injection atomization with dissolved gas systems, aneffervescent atomization method has been developed by Lefebvre et al.[1] In effervescent atomization, air or gas is injected through a plain-orifice nozzle into a bulk liquid within an atomizer at a location upstream of the discharge orifice. Unlike the above-described two-fluid atomization, the gas used in effervescent atomization is not intended to provide kinetic energy for breaking up the liquid stream, nor to be dissolved in the liquid as in the flashing injection atomization, but instead to penetrate into the liquid. The injection velocity is low and the pressure differential between the gas and the liquid is very small. The gas is supplied at nearly the same pressure as that of the liquid, just to allow the gas to penetrate the flowing liquid. The gas then forms bubbles in the liquid, and a two-phase flow condition is produced at the discharge orifice. During discharging, the liquid is squeezed by the gas bubbles into thin ligaments and/or shreds while the gas bubbles explode, an effect similar to that in flashing injection.[118] The ligaments and/or shreds are further shattered into droplets by the explosion of numerous small bubbles while leaving the discharge orifice.

Generally, an increase in the gas amount used may lead to an increase in the number of bubbles and a decrease in droplet size produced. However, atomization quality is very good even at very low injection pressures and low gas flow rates. Good atomization can be achieved at much lower liquid injection pressures than those typically used in pressure atomization. Mean droplet size is comparable to that produced in air-assist atomization for the same gas to liquid ratio. The large holes and passages in effervescent atomizers largely reduce the occurrence of nozzle plugging. This makes the effervescent atomizers ideal for combustion devices burning slurry