Operational Limits of Vibrating Orifice Aerosol Generator James B. Wedding and James J. Stukel' University of Illinois at Urbana-Champaign, Urbana, 1 1 1 . 61801
The relationship between the aperture diameter and the operating frequency for monodisperse vibrating orifice aerosol generators is discussed. The previously reported range 3.5 5 (X/D,) 5 7.0 for monodisperse aerosol generation is not applicable for all generator aperture diameters. Results are presented denoting the range of AID, values for which monodisperse aerosols are generated with varying generator aperture sizes.
noted that increasing the wavelength by decreasing the frequency or increasing the jet velocity can lead to operating difficulties because the range of the disturbance wavelength may be exceeded which causes the aerosol to become nonuniform with apparently little effect resulting from the mechanical disturbance. Empirical results for disturbance wavelength aerosol generation such as those given by Schneider and Hendricks (1964) in terms of D, are
x
Aerosol generation work was conducted recently by Berglund and Liu (1973) using the vibrating orifice principle for liquid jet disintegration and the solvent evaporation technique. Basically, the device feeds liquid a t a known rate through apertures held in place in a stainless steel cup by a Teflon O-ring. The cup is bonded to a piezoelectric transducer which, when excited by the correct ultrasonic frequency, f, causes the cylindrical liquid jet to become unstable and disintegrate into droplets that are injected into a turbulent jet of air to prevent coagulation. The device has great flexibility, if operated within certain limitations, in its ability to generate high-quality aerosols over a wide range of sizes by varying both the aperture size and the nonvolatile solute concentration in a volatile solvent for a particular aperture. The working of the device is described in full by Berglund and Liu (1973). In this note, the subject of the aforementioned limitations will be addressed. Experiments using a prototype of the described aerosol generation system were carried out which verified the performance of the aerosol generator and the amazing monodisperse nature of the particles produced. By manually sizing several hundred particles using standard light microscopy techniques and utilizing standard statistical calculation methods, we found that the particle size relative standard deviation was k0.012. Care, however, must be exercised in applying the working aerosol generator parameters to achieve these results. In particular, there exists a specific range of disturbance frequencies that cause the jet to break up into a controlled, quality aerosol. In the paragraphs that follow, these frequency bounds will be discussed. Droplets cannot be made arbitrarily small by making disturbance wavelengths, A, smaller and smaller by increasing the frequency modulation or by decreasing the velocity of the jet. The reason for this is that the liquid cylinder remains stable with respect to displacements from equilibrium for wavelengths less than sD,. Thus, one established limitation not to be exceeded is the maximum frequency or minimum, A, of
XD,
(1) where D , is the jet diameter (which does not equal DA, Xm,, =
the aperture diameter). This result was predicted early by Rayleigh (1945) and later verified by Strom (1969) and Berglund (1972). On the other hand, an upper limit on A, say A,,, or a general relation for A which, when applied yields a range of h in which only monodisperse aerosols are generated. has not been developed. This note will report on an operating range for h in which the aerosol is monodisperse for various aperture diameters. It should be ~~
1
To whom correspondence should be addressed
456
Environmental Science 8, Technology
3.5 5 - I 7.0
D,
helpful in establishing an approximate operating frequency range for generating monodisperse aerosols but the relation is not exact. The exact values must be determined experimentally as X is a function of both aperture size and flow rate. The initial procedure to determine the exact operating range for X is to apply Equation 1 and use the fact that Q (flow rate) and f are related to X by the relationship
A, X X = -Q
(3)
f
and noting that the diameter calculated is not DA (aperture diameter) but a larger value, D, (jet diameter). Thus, by the time disturbance causes the jet to break up, the jet has spread out and the diameter is no longer DA. (The disturbance acts only over an aperture thickness of 4 mils.) The diameter, D,, then, is a diameter that can be used to compare results predicted by theory (Rayleigh, 1945) and is calculated utilizing the fact that the maximum frequency occurs at the minimum wavelength given by Equation 1 which was found to be agreeably identically
o ~5 XIDj53.95, Q.0.191 cc/min, o T < A/ Dj 5 4.64, Q=0.214 cc/min, A
v o o
DA=21.5p =22.1,u r 5 X / D j 57.21, Q.0.0764 cclmin, DA=10.5p T S A / D j 5 6.62, Q.0.0623 cclmin, DA=8.3p .rr< X / D j 5 5.99, Q.0.0308 cc/min, DA=3.0p .rrs X/Dj 5 7.78, Q=0.0158cc/min, Dp=3.0p .rr 5 X/Dj 5 3.99, Q.0.191 cclmin, Dp'19.5~ Dp
Volatile Solvent: Ethanol Non-volatile Solute: D.O.P. 10 9
8 -
0
A
7
V
6
-
0
F5
x
4 lT
2 1 C
0
2
4
6
8
10 12 14 16 18 20 22 24 26 Dp,
microns
Variation of disturbance wavelength and jet diameter with orifice diameter Figure 1.
45 KHZ
70 KHZ
82 KHZ-103 K
120 KHZ
(Monodisperse Range) Variation of particle size distribution with vibrating orifice frequency range 21.5 p. 0 = 0,191 ccjmin. D, = 23.27 p , monodisperse Operating range 82 kHz 5 f 5 103 kHz with
Figure 2. DOP dissolved in ethanol: D A =
= 8.45 w. Particles.collected on oilphobic glass slide
R . This same methodology was repeated for the upper limit on A, A,, using Equation 3 and the value just calculated for D,. This procedure leads to a general relationship which includes all the operating parameters necessary to determine the disturbance wavelength which produces monodisperse aerosol when applied to a cylindrical liquid jet
This result has been proved experimentally by applying the above procedure utilizing Equation 4 to various sized apertures and flow rates. Equation 4 has been left in expanded form for clarity purposes and for comparison with earlier published reports. The results are recorded graphically in Figure 1 to reveal the fact that AID, is indeed variable and that any one empirical relation such as Equation 2 is not an exact expression. Equation 4 exactly gives X uniquely for a range of aperture sizes and flow rates. It is seen in the plot that Equation 2 holds reasonably well for apertures in the 10-p size but that the operation envelope decreases to a small size range at the nominal aperture size of 20 p and increases a t the smaller aperture size of 3 p . Thus, one is rather limited by the freauencv one mav a n d v and still have monodisuerse aero-
particle diameter
tion 2 is shown in Figure 2. It is seen that aerosols generated with A/Dj values ranging between the empirical limits of 3.5-7.0 corresponding to an f range of 92-46 kHz results in poor aerosol quality (note photographs a t 45 kHz and 70 kHz). Applying Equation 4 and the described procedure gives an optimum operating range of 103-82 kHz with Dj equal to 23.27 p for an aperture diameter of 21.5 p at a flow rate of 0.191 cc/min. As can he seen from Figure 2, this operating range results in a monodisperse aerosol, A sample at 120 kHz is included as an example of an excessive disturbance frequency and an aerosol of a polydisperse nature. It is felt that the information given in Figures I and 2 will he useful for those individuals attempting to generate monodisperse aerosol utilizing the disintegrated jet of liquid technique.
Literature Cited Berglund, R. N., PhD Thesis, University of Minnesota, 1972. Berglund, R. N., Liu, B. Y. H., Enuiron. Sci. Tecknol., 7,
147
11 9721
~
Rayleigh, J. W. S., “Theory of Sound,” Vol. 2, p 131, Dover Publishing, New York, N.Y., 1945. Sehneider. J . M.. Hendricks. C. D.. Re”. Sei Instrum.. 35. 1349 (1964).
Strom, L., ibid., 40, 718 (1969)
Volume 8 , Number
5. May 1974
457
