Ultrasonic atomization: Effect of liquid phase properties
Introduction
The disintegration of liquid sheet or ligament into fine droplets in a gas phase is known as atomization. Basically, atomization is possible by disintegration of either a liquid jet or liquid sheet. The sheet disintegration requires much lower liquid discharge pressure compared to the jets in order to produce droplets of a particular size. Means of controlling the droplet size is desirable in many industrial applications and this will only be achieved by a close study of the mechanism of break up of liquid sheets. In the jet break up model, the drop diameter has been predicted to be 1.89 times the jet diameter based on Rayleigh stability criteria. It has also been shown that the optimum or dominant wavelength of the disturbance that leads to a liquid jet break up into droplets is equal to 4.5 times the jet diameter [1].
The liquid in the form of thin film when allowed to flow on a vibrating surface (frequency >20 kHz) breaks up into fine droplets. This phenomenon is known as ultrasonic atomization. Two major hypotheses have been proposed to explain the mechanism of liquid disintegration during ultrasonic atomization, namely capillary wave hypothesis and cavitation hypothesis. Capillary wave hypothesis is based on Taylor instability criteria [2]. The strong correlation between mean droplet size and capillary wavelength favors the capillary wave theory. Lang [3] measured the surface disturbances by photographing the peaks of the capillary waves on the vibrating surface. Cavitation hypothesis is generally applied to high frequency (16 kHz–2 MHz) and high-energy intensity (W/m2) systems. When the liquid film is sonicated, cavitation bubbles are formed in the liquid film provided the liquid film on the vibrating surface has some minimum thickness. During the implosive collapse of these cavities, especially cavities near the surface of the liquid, high intensity hydraulic shocks are generated. These hydraulic shocks initiate the disintegration of the liquid film and cause direct ejection of the droplets. The effect of cavitation is discussed latter in detail, which gives support to this hypothesis.
Bouguslavskii and Eknadiosyants [4] coupled these two theories and proposed a ‘conjunction theory’. According to this theory, the periodic hydraulic shocks from the cavitation disturbance interacts with the finite amplitude capillary waves and excite them to form droplets. The capillary waves are more random in the conventional atomization process whereas the capillary waves modulated by ultrasonic vibration are more regular in nature (Fig. 1); as a result more uniform droplet size distribution can be observed in the case of ultrasonic atomization. It should also be noted that the cavitation shock (which essentially is a statistically random phenomena) leads to an irregular random disintegration and observed non-uniformity of droplet distribution gives support to this conjuction theory.
Topp and Eisenklam [5] have listed the effect of the different frequencies but there has been no information about the effect of liquid phase viscosity, density and flow rates to the vibrating surface on the droplet size distribution. When majority of the droplets are formed by capillary wave mechanism, the droplet size can be estimated reasonably accurately using Lang’s equation though, the effect of the liquid phase viscosity and the flow rate in modifying this correlation has been highlighted by Rajan and Pandit [6]. A comparison of number mean diameter with capillary wavelength indicated that the drop size is a constant fraction of the capillary wavelength and this fraction turned out to be 0.34 giving the final correlation as;
Further, Rajan and Pandit [6] proposed a correlation by introducing the dimensionless numbers incorporating the physico-chemical properties of the atomizing liquid and the operating parameters of the ultrasound, which is given below:
Rational for choosing the exponents in this correlation are based on the experimental observations reported in the literature where few of the parameters have been experimentally studied. Where as in this work an attempt has been made to include rheological nature of the atomizing liquid (pseudo-plasticity of the atomizing liquid) to offer an universal correlation.
Section snippets
Experimental system and measurement of the droplet size
The schematic diagram of experimental set up is as shown in Fig. 2. The Ultrasonic atomizer of Vibra Cell (USA) is used for the experimentation purpose. The maximum power output of this system was 130 W. The diameter of the circular atomizing tip of the 20 kHz probe is 1.45 cm and that of the 40 kHz frequency horn is 1.5 cm. The liquid is pumped at a constant flow rate using a peristaltic pump through concentric hole (2.5 mm) provided in the horn. Also a provision is made to adjust the power input to
Study of the cavitation phenomenon
In the past literature [3], [4], two theories have been proposed to explain the mechanism of the formation of the droplets during ultrasonic atomization as described before. One of them is the cavitation hypothesis, but there is not much information about the cavitational phenomena while the actual atomization is occurring [3], [7]. From the energy analysis studies (Appendix A) it can also be shown that there is a possibility of cavitational activity in the process of the ultrasonic
Estimation of liquid film thickness
The method for the estimation of the liquid film thickness on the atomizing surface is described for a typical case in Appendix A. (The values of the liquid film thickness on the vibrating surface as a function of the liquid flow rate for 1% aqueous CMC solution have been reported in Table 4.) From Table 4 one can see that for low liquid flow rates the estimated film thickness values are less than the measured (photographically) film thickness values (Appendix A). It may be due to the fact that
Conclusion
The dependence of the droplet size on the rheological nature of the atomizing fluid, liquid flow rate, physico-chemical properties of the liquid and the operating ultrasonic parameters have been studied for the case of ultrasonic atomization. Under the same operating conditions, the droplet size is observed to be lower for pseudo-plastic liquid when compared to the droplet size of the liquid with Newtonian behavior with the viscosity of the former equal to the zero shear-rate viscosity of the
Acknowledgement
The authors acknowledge the funding of the Department of Science and Technology, Govt. of India, New Delhi, India for this research work.
References (11)
- et al.
The mechanism of practical coalescence of liquid drops at liquid–liquid interfaces
J. Colloid. Sci.
(1960) - et al.
Industrial and medical uses of ultrasonic atomizers
Ultrasonics
(1972) - et al.
Correlations to predict droplet size in ultrasonic atomization
Ultrasonics
(2001) Drop sizes of emulsions related to turbulent energy dissipation rates
Chem. Eng. Sci.
(1985)The instability of liquid surfaces when accelerated in a direction perpendicular to their planes
Proc. R. Soc. A
(1950)
Cited by (219)
Acoustothermal dynamic characteristics of water-based copper nanofluids on a vibrating nanosurface
2024, Journal of Molecular LiquidsAn comprehensive review on the spray pyrolysis technique: Historical context, operational factors, classifications, and product applications
2023, Journal of Analytical and Applied Pyrolysis