Droplet Generation 49
and residual fuels. The presence of air bubbles is also beneficial to reducing soot formation and exhaust smoke in combustion applications. Moreover, this type of atomizer designs is simple, reliable, and of low cost.
2.1.7Electrostatic Atomization
In electrostatic atomization, an electrical potential is applied between a liquid to be atomized and an electrode placed in the spray at a certain distance from liquid discharge nozzle. As a result of the mutual repulsion of like charges accumulated on the liquid surface, the surface becomes unstable and disrupts when the pressure due to the electrostatic forces exceeds the surface tension forces of the liquid. Droplets will be generated continuously if the electrical potential is maintained above a critical value consistent with liquid flow rate. Both DC and AC systems have been employed to provide high electrical potentials for generating fine droplets. Many configurations of electrode have been developed, such as hypodermic needles, sintered bronze filters, and cones.
A number of investigations have been made to explain the droplet formation mechanisms associated with electrostatic atomization.[119][120] It has been hypothesized that the dispersion of a liquid by electrostatic atomization occurs via the detachment of a single droplet from the capillary tip of the liquid. However, this mechanism has not been proven experimentally. Due to the complex physics involved, a generic theoretical model has not yet been established.
Generally, the droplet size generated in electrostatic atomization is a function of applied electrical potential, electrode size and configuration, liquid flow rate, liquid nozzle diameter, and liquid properties such as surface tension, dielectric constant and electrical conductivity.[121]-[124] When a low electrical potential is applied to a liquid, a stream of relatively uniform droplets will form below the liquid discharge nozzle. As the applied electrical potential is increased, the droplets produced become smaller, and the liquid velocity and droplet production rate both increase, with concomitant
50 Science and Engineering of Droplets
shortening of the distance between adjacent droplets.[121] With further increase in the electrical potential, the stream of droplets turns into a continuous liquid stream or jet issuing from the nozzle. When the electrical potential is increased beyond a certain value, the liquid no longer forms a jet, but instead disintegrates spontaneously into a mist of fine droplets at the nozzle exit.[121] For certain liquids, uniform droplets can be produced by electrostatic atomization. However, monodisperse sprays can not be obtained with a negative potential on the liquid. In some electrostatic atomization experiments of water, sugar solutions, lubricating oil and alcohol, it was observed that there is an upper limit of liquid electrical conductivity, beyond which atomization can not be achieved. Fine droplets can be generated for liquids of relatively low electrical conductivity, such as water, alcohol and dibutyl phthalate. However, some organic liquids of low dielectric constants such as benzene and carbon tetrachloride were found difficult to disperse using electrostatic atomization method.
Electrostatic atomization covers a variety of different processes with respect to electrical potential, liquid flow rate and droplet size. The electrical potential required to disintegrate a liquid depends on the flow rate and electrical properties of the liquid. Generally, however, the liquid flow rates in electrostatic atomization are very low. This drawback has limited its practical applications to electrostatic painting and non-impact printing. The process used in industrial electrostatic paintings involves a liquid flow rate of up to 3 l/min with a single nozzle and an electrical voltage on the order of 100 kV.[88] Painting surfaces are normally used as one electrode. Liquid paint is usually pressurized and fed into the atomizer. Droplet size distributions produced under these conditions are rather broadly dispersed. If a liquid is fed into the atomizer at the atmospheric pressure, the liquid flow rate is usually very low and thus the required electrical voltage is on the order of 10 kV. The average droplet sizes produced may range from 0.1 to 1000 µm with a relatively narrow size distribution.[88] In electrostatic atomization for manufacturing printed circuits in the electronic industry, liquid materials (usually molten metals) are atomized and deposited onto
Droplet Generation 51
circuit boards normally under vacuum conditions. The liquid flow rate is extremely low (up to 6.7 × 10-5 l/min) and, correspondingly, the applied electrical voltage is typically on the order of 10 kV. The droplet sizes produced may range from 300 to 600 µm.[88] Significant work on electrostatic atomization has also been done for oil burner applications. In some designs, such as the spray triode invented by Kelly,[123] the electrostatic atomizer appears to have potential capability of handling the high flow rates used in common combustion devices. Detailed descriptions of experiment, theory and industrial applications of electrostatic atomization have been given by Okuda and Kelly.[125]
It should be noted that, to produce a reasonably monodisperse spray, the liquid flow rate should be maintained at an extremely low level, and thus the scaling up of such devices may pose some difficulties. It is also rather difficult to assess the liquid flow rate that can be achieved due to few quantitative studies and lack of comprehensive understanding of the underlying principles. Another drawback of the electrostatic atomization technique is that both the production and properties of droplets are significantly dependent on the electrical properties of the liquid, limiting the type of liquids that can be successfully atomized.
Recently, Sato et al.[126] proposed a new method for the production of uniformly sized insulating liquid droplets (such as kerosene or plastic monomer) in immiscible liquid media (distilled water) by means of an applied convergent electric field generated using AC or pulsed voltage. Video imaging and photographing of the disintegration of the liquid column showed that kerosene droplets with an essentially uniform diameter ranging from 100 to 250 µm were produced synchronously with the applied AC frequency using a nozzle of 100 µm in diameter. The droplet size can be controlled by varying the AC frequency, nozzle diameter, liquid flow rate, and velocity ratio between the oil-phase and co-flowing water. With increasing flow rate of the co-flowing liquid and increasing synchronous frequency, droplet size decreases due to the elongation of the liquid jet. The disintegration mechanism is most likely the forced
52 Science and Engineering of Droplets
oscillation of the liquid jet stimulated by each cycle change of the applied voltage.
2.1.8Vibration Atomization
The breakup of a liquid into droplets can be brought about by periodical vibrations of the liquid. For example, droplets can be generated by periodically vibrating a liquid jet or liquid reservoir. The simplest form of vibration atomization is the vibrating capillary droplet generation that was originally developed to study the collision and coalescence of small water droplets. This simple droplet generator consists of a hypodermic needle vibrating at its resonant frequency and can generate a stream of uniform droplets. The size and frequency of droplet generation are dependent on the needle diameter, resonant frequency, liquid flow rate, and amplitude of oscillation of the needle tip.
The technique of periodical vibrations for droplet generation is based on the instability of a liquid jet emerging from a capillary tube or an orifice. If a liquid jet is emitted from an orifice under pressure, the jet is by nature unstable and will eventually disintegrate into droplets by the action of internal and/or external forces depending on the orifice diameter as well as flow conditions and properties of the liquid and surrounding medium. The disintegration of a liquid jet into fairly uniform droplets may be achieved by applying periodical vibrations of proper amplitude and frequency to the jet. Vibrations may be generated by using (a) a piezoelectric transducer, (b) an acoustic vibrator, or (c) a direct mechanical means. Accordingly, vibration atomization may be loosely divided into three categories:
(a) ultrasonic atomization, (b) acoustic atomization, and (c) mechanical vibration atomization. Each of the three categories differs not only in design but also in droplet size range that can be generated.
In ultrasonic atomization, a liquid droplet is produced when powerful high frequency sound waves are focused onto the liquid. A liquid may be present in large volume and contained in a reser-
Droplet Generation 53
voir. A certain form of concave reflector such as a curved acoustic transducer made of barium titanate may be placed at the bottom of the bulk liquid to generate high frequency sound waves that propagate upwards into the bulk liquid. The intensive waves may cause the formation and subsequent collapse of cavities. The liquid over the acoustic transducer may also form a wavy layer. If the strength of the waves is sufficiently high to overcome the surface tension forces of the liquid, droplets will eject from the ripple crests on the top surface of the bulk liquid. Alternatively, a liquid may be introduced onto a rapidly vibrating solid surface over which it spreads into a thin film while capillary waves form in the film. When the amplitude of the vibrating surface increases to a level at which the wave crests in the film become unstable and collapse, the liquid film breaks up and ejects away from the surface into a mist of droplets.[127] The resultant droplet size is dependent on the ripple wavelength that in turn is determined by the vibration frequency.
The disintegration of a liquid jet/stream emerging from an orifice can be precisely controlled by periodically vibrating the orifice. The periodical vibrations can be achieved by implanting the orifice into a disk made of an electrostrictive material such as a piezoelectric crystal and applying electrical signals to the piezoelectric crystal. This is the design concept of ultrasonic atomization with an electrostrictive disk type generator.
An ultrasonic atomizer (Fig. 2.11) typically consists of a pair of piezoelectric transducer elements sandwiched between a pair of titanium horn sections. The horn, i.e., velocity transformer, is used to increase the amplitude of the vibrations at the atomizing surface. The common contact plane of the two piezoelectric transducer disks constitutes one of two electrical input terminals for high frequency electrical signals. The metal horn forms the other terminal. The disks will either expand or contract simultaneously, depending on the polarity of the input signals. The cyclic expansion and contraction generate a propagation of traveling pressure waves longitudinally outward in both directions along the nozzle axis, at the same fre-
Droplet Generation 55
The droplet size generated by an ultrasonic atomizer depends primarily on the signal frequency and liquid flow rate. Ultrasonic atomizers may be of low-frequency (between 20 kHz and 100 kHz) or high-frequency (up to 3 MHz). More power can be supplied at the lower frequencies. The orifice diameter normally ranges from 3 to 20 µm.[88] The horn diameter is usually designed to be less than one quarter of the wavelength of vibration waves. This restricts the liquid flow rate. High-frequency ultrasonic atomizers are characterized by low throughput and fine droplet sizes, and are frequently used as nebulizers for inhalation in medical applications. The droplet sizes produced in the frequency range of 10 to 1000 kHz are approximately 3–50 µm.[88] Generally, the droplet sizes in ultrasonic atomization may range from submicrometers up to a few hundred micrometers.
Originally, ultrasonic atomizers were developed for use in small boilers for domestic heating and the emphasis of applications has been in the combustion area since then. Over the past decade, however, ultrasonic atomization has been extensively developed and applied to a variety of industrial areas such as humidification, medication, pharmaceutical coatings, semiconductor processing, spray drying, and vaporization of volatile anaesthetic agents.[19] One of the most useful features of ultrasonic atomization is its low spray rate due primarily to the low amplitudes of vibrations that ultrasonic transducers generate.[88] For example, a combination of frequency of 700 kHz and droplet size of 10 µm yields a flow rate of 2.2 × 10-5 l/min, while a combination of 10 kHz and 200 µm results in a flow rate of 2.5 × 10-3 l/min.[88] Thus, the droplets generated can be readily entrained and conveyed in a moving stream as a uniform mist. This feature makes ultrasonic atomization particularly suitable for applications such as coating, humidification, moisturizing, and spray drying. In addition, ultrasonic atomizers are capable of generating very fine droplets (1–5 µm)[129] at extremely low flow rates (< 0.1 l/min at a frequency of 55 kHz). This feature is highly desired in some pharmaceutical and lubrication processes. However, this also
56 Science and Engineering of Droplets
makes it difficult to successfully handle the high flow rates prevailing in most engine and furnace applications.
One of the approaches for increasing the liquid flow rate is to increase both the vibration frequency and liquid flow velocity, while keeping the wavelength according to Rayleigh’s linear theory on the instability of a liquid jet,[37] as will be discussed in the following chapter. Another possible solution is to combine the effect of ultrasonic atomization with that of whistle atomization.[88] In this approach, the amplitude of signals produced initially by an ultrasonic transducer is further amplified by means of the resonant effects created by the hollow space of horn shape. While this design allows the capacity to be increased substantially, it will restrict the operating frequency of the transducer to one value. Since the droplet size generated by ultrasonic atomization is dependent on the transducer frequency, this arrangement is limited to producing one droplet size. Thus, a series of different sets are necessary to cover a required size range. Other designs may be used to increase the liquid flow rate. For example, many atomizers may be installed in parallel. This arrangement, however, may cause mutual interference of vibrations produced by individual transducers, leading to polydisperse droplets. One way to resolve the problem is to install each transducer in a separate chamber. Alternatively, multiple tubes/nozzles can be mounted in an atomization system and vibrated at a common frequency, rather than installing many individual units. It may also be feasible to design a large perforated plate mounted on a shallow liquid reservoir whose wall or bottom can be vibrated.
The ultrasonic atomizers made by the Delavan company and used in combustion, spray etching, and spray drying may operate at a frequency of 50 kHz, and generate droplets of 30 to 60 µm. Larger ultrasonic atomization devices made by Lechler GmBH, Germany, have a length of 14 cm and can generate water droplets of 30 µm at a flow rate of about 1 kg/min. The capability of various ultrasonic atomizers to atomize various liquids has been evaluated by Berger,[128] and the performance of a variety of ultrasonic atomizers has been examined by many investigators.[128]-[137] Designs of new piezoelectric
Droplet Generation 57
droplet generators and devices[138]-[141] have been developed and evaluated, including those for liquid metal atomization.[142]-[144]
Instead of using electrical signals to an electrostrictive orifice in ultrasonic atomization, periodical vibrations can also be imparted onto a tube in the form of acoustic waves using audio signals. This is the design concept of acoustic atomization with vibrating tube using an audio speaker. In acoustic atomization, an oscillator generates signals that are amplified and transmitted to a speaker. A liquid jet is introduced through a tube and vibrations are transmitted to the tube via a metal rod that is connected to the speaker diaphragm. The liquid jet is then disintegrated into droplets of uniform sizes. The diameter of the capillary tube ranges typically from 50 to 1500 µm,[88] much larger than the orifice diameter used in ultrasonic atomization. The resultant droplet diameter lies between 100 and 3000 µm. The signal frequency is in the range of 0.3 to 30 kHz. Exceptionally good uniformity and predictability of droplet sizes have been reported by investigators.
In mechanical vibration atomization, periodical vibrations are applied to a liquid jet by using some mechanical means. For example, a fine whisker can be used to dip periodically into a liquid tube above a liquid reservoir.[88] The whisker is connected to a flat spring of silicon iron that is vibrated by an electromagnetic field. An AC current of 50 Hz is used to create the magnetic field. The whisker has a round shaped tip of 0.015 mm in diameter. When the whisker emerges above the liquid surface, a liquid thread is formed between the whisker tip and the liquid surface. As the whisker moves up further, this thread separates from both the whisker tip and the liquid surface under the action of gravitational and surface tension forces and/or by blowing with a traverse air flow, forming a droplet. The droplet is subsequently blown away by the air flow. The droplet size produced is dependent on the immersion depth of the whisker as well as the liquid surface tension and viscosity. In an alternative approach, a periodically rotating needle is employed that breaks up the liquid jet emerging from a capillary tube.
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Disintegration of liquids can also be achieved by applying periodical virations to a liquid reservoir rather than to an orifice or tube. The reservoir wall or bottom may be made of piezoelectric crystals so that a pressurized liquid contained in the reservoir can be squirted out through an orifice or multiple orifices into droplets. This type of technique may be considered to be similar to those of vibrating tube/orifice in that individual droplets are periodically produced by externally controlled disturbances, although there are distinct differences between them in configuration and design.
There may be many other types of periodical vibrations for liquid disintegration or dispersion in various applications. The features, operation parameters, performance and applications of various experimental and commercially available ultrasonic and acoustic atomizers have been described and compared by Topp and Eisenklam.[129]
Vibration atomization techniques can generate droplets of a wide range of sizes (approximately 1–1000 µm). Droplets smaller than 50 µm can be best produced using an electrostrictive transducer such as piezoelectric crystals. Periodic acoustic signals generated by sound-speaker type vibrations are well suited for producing droplets larger than 50 µm. These two distinctively separate ranges are due primarily to the difference in the vibration frequencies used in the two techniques.
Droplets produced using periodic vibrations generally exhibit excellent monodispersity whether the vibrations are generated by a piezoelectric crystal, a sound speaker, or some sort of mechanical means. It has been reported that in atomization using an ear- phone-like vibrator, only 0.2–1.5% of the droplets produced have sizes different from the rest.[88] The converted standard deviation on a weight basis is 0.0008–0.0022; that is equivalent to a geometric standard deviation of 1.001–1.002. Geometric standard deviations of 1.01 and 1.005–1.08 have been achieved in atomization processes using a piezoelectric transducer and a whisker, respectively.[88] The droplets generated by the vibrating tube method at low flow rates appear to be more monodisperse than those produced by ultrasonic atomization techniques.