Droplet Generation 59
In vibration atomization, each individual droplet is produced one at a time by means of a periodic disturbance. Therefore, the resultant droplet size is not greatly dependent on the liquid properties. For a given liquid, the droplet size is practically determined only by liquid flow rate and vibration frequency.
Some experiments revealed that monodisperse sprays can be generated in the wavelength range of 3.5d0 to 7d0,[88] where d0 is the liquid jet diameter. There always exists a certain minimum threshold frequency below which the droplets generated are not uniform. This threshold frequency was found to be 0.7 and 0.4 times the optimum frequency for high and low viscosity liquids, respectively. The amplitude of disturbances was found to have very little effect on the monodispersity of droplets. The optimum wavelength range is very narrow while uniform droplets can be generated over amplitudes of several orders of magnitude. However, high amplitudes may be necessary for highly viscous liquids. Since the amplitude of pulses can be readily varied for a standard pulse generator, it is an operating condition rather than design criterion.
2.1.9Whistle Atomization
Whistle atomization is also known as acoustic or ultrasonic gas atomization. In a whistle atomizer,[88][131] a liquid jet is disintegrated into droplets by directing high-pressure gas on it, as schematically depicted in Fig. 2.12. The focusing gas flows create strong sound waves inside the nozzle, and hence it is called whistle or stemcavity atomizer. This type of atomizer operates typically at a sound frequency of about 10 kHz and may generate droplets of about 50 µm at flow rates up to 75 l/min. In the Hartmann-whistle acoustic atomizer, a converging-diverging nozzle is used to generate a supersonic gas jet. Acoustic oscillations in shock wave pattern are produced when the jet impacts an open-ended chamber. This type of atomizer operates at an ultrasonic frequency (>20 kHz) and can
Droplet Generation 61
spray angle of larger than 90° which is much larger than those in twofluid atomization.[5] Liquid supply pressure is strongly coupled with the gas pressure. An increase in the gas pressure from 0.1 to 0.4 MPa, for example, requires an increase in the liquid supply pressure from 0.1 to 0.17 MPa for a constant water flow rate of 0.17 kg/min.[5]
Precise mechanisms of atomization in these whistle-type devices are still ambiguous, and the behavior and role of the shock waves are less clear. Plausibly, the shock waves may have certain beneficial effects on atomization. However, systematic studies[131] indicated that the sound field is not a critical factor in this type of atomization process. The degree of atomization is much more sensitive to gas pressure than resonant frequency.[115] These seem to imply that the liquid breakup is caused mainly by the aerodynamic forces of the gas, similarly to the interaction mechanisms prevailing in two-fluid atomization. It was also suspected that all the whistle atomizers operate simply on a principle similar to that in air-assist type of atomizers,[129] and the whistle atomizers can be considered to extend the range of normal internal-mixing twin-fluid atomizers without resorting to very small orifices.[5] In addition, it was found that the degree of atomization is almost independent of liquid viscosity.[115] This is a promising feature for the atomization of heavy fuel oils.
2.1.10 Vaporization-Condensation Technique
Vaporization-condensation technique involves atomization of a liquid, vaporization of the atomized droplets, and subsequent condensation of the vapor.[88] The original droplets are normally generated by pressure atomization. Therefore, the spray produced initially is typically polydisperse. The spray is then vaporized by heating it to above the boiling point of the liquid, usually using a combustor or an electrical heater. The vapor is subsequently mixed with a stream of hot air containing a regulated number of condensation nuclei. The mixture of the air and vapor passes through a section in which it is slowly cooled, becoming supersaturated and condensing uniformly upon the nuclei to form uniform droplets.
62 Science and Engineering of Droplets
Vaporization-condensation technique has been applied to spraying certain insecticide chemicals. A pulse jet or a reciprocating engine has been used as a heating source. Liquid flow rates are up to 40 gallons per hour. The technique can also be used to produce a cloud of smoke for military applications. The desired droplet size is in the same range as the light wavelength for obscuration, i.e., 0.4 to 0.8 µm. Generally, the droplets produced by vaporization-condensa- tion technique are in the size range of aerosols, i.e., smaller than 1
µm.[88]
When a vapor jet emerges into ambient air, a large amount of air is entrained into the jet due to drag and turbulent mixing. In the vapor-air mixing region, the vapor condenses on the nuclei that proceed to grow into droplets. As the distance from the vapor nozzle increases, the droplets encounter more air and the vapor pressure decreases until the growth rates of the droplets become negligible. The mean droplet size produced is primarily determined by the condensation rates that in turn are controlled by the heat content and velocity of the vapor, the liquid-air ratio, and the number of nuclei. The latter is generally known to be an important factor governing the droplet size in the condensation process. Since the droplet formation is based on the vapor condensation on nuclei, the droplet size is somewhat difficult to control. The process parameters that need to be carefully controlled include the number of nuclei, vapor temperature and pressure, and flow rates of vapor and air.
The droplet size distribution produced by vaporization-con- densation technique is strongly dependent on the chemical composition and properties of the liquid. If well controlled on a small scale, vaporization-condensation technique can produce moderately monodisperse sprays with geometric standard deviations ranging from about 1.2 to 1.8.[88]
In an evaluation of various techniques for droplet generation,[88] periodic vibration of liquid jet, spinning disk and ultrasonic atomization techniques have been rated as the most appropriate methods for producing monodisperse sprays. These techniques were found to be very effective and appeared promising for refinement,