PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Agglomeration of aerosol using intense standing wave fieldof cylindrical shape
Rintaro MOTOI 1; Takuya ASAMI 2; Hikaru MIURA3
1,2,3

Nihon UniversityJapan

ABSTRACTUltrasonic dust collectors collect particulate matter 2.5 μm or less in diameter by exploiting particle
agglomeration. Strong aerial ultrasonic waves induce the vibration of fine particles, which causes collisions
between particles resulting in agglomeration that increases the particle diameter. In this research, to improvethe collection efficiency, an aerosol agglomerated with a standing wave field was formed by using a cylinderand a parallel reflector plate for an aerial ultrasonic sound source with a rigid wall. We investigated the effectof the sound source input power on the particle size and total number of particles. Irradiating themicroparticles with ultrasonic waves decreased the total number of microparticles. The particle size
distribution showed that the central diameter was 1.50 μm without ultrasonic irradiation and 2.65 μm when
the input power was 20 W. These results confirmed that ultrasonic irradiation agglomerates fine particles.
Keywords: Ultrasonic agglutination Particle size distribution, Aerial ultrasonic wave

1. INTRODUCTION
Air pollution caused by smoke and soot is increasingly a major environmental problem. Sources ofparticulate matter include factory and car exhaust gases. Dust collectors remove fine particles,
including particulate matter 2.5 μm or less in diameter (PM 2.5), which cause adverse health effects,
from exhaust gases. Ultrasonic dust collectors improve the dust collection efficiency of fine particles
by using ultrasonic waves as a pretreatment to agglomerate fine particles before filtration. In
ultrasonic agglomeration, ultrasonic waves induce large-amplitude vibrations in small fine particles
and small-amplitude vibrations in large fine particles. The particle size of the fine particles isincreased by the hard collisions of the fine particles, which promote agglomeration (1,2).
Conventional ultrasonic wave aggregating apparatus cannot combine the sound field-forming partand the ultrasonic sound source, and the apparatus is large and contains a gap, so the ultrasonic waves
cannot be transmitted efficiently to the fine particles (3,4). To improve dust collection efficiency byusing aerial ultrasonic waves, we have been studying agglomerating PM 2.5 by using a circular flexible
diaphragm with a fixed circumference (5) and apparatus in which the sound field-forming part iscombined with the ultrasonic sound source. We found that the rate of decrease in the concentration offine particles due to ultrasonic agglomeration increases as the input power to the ultrasonic soundsource increases.
In this paper, we focus on fine particles during ultrasonic agglomeration. We investigate changes inthe central diameter of fine particles at different input powers and the rate of decrease in the total
number of fine particles.

2. ULTRASONIC SOUND SOURCE AND AGGLOMERATION CHAMBER
The cylindrical rigid-wall integrated sound source is shown in Figure 1. The sound source consistsof a 28 kHz bolt-clamped Langevin vibration transducer, an exponential horn (amplitude enlargement
ratio: 5.0; diameter of the thick end face: 42 mm; diameter of the thin end face: 8 mm; material:duralumin) for amplitude expansion, a transmission rod for resonance frequency adjustment, and acircumferentially fixed circular flexible diaphragm attached to the tip with bolts. The agglome ration1

csri18033@g.nihon-u.ac.jp
asami.takuya@nihon-u.ac.jp
3miura.hikaru@nihon-u.ac.jp24789

chamber consists of an acrylic cylinder bonded to a circumferentially fixed circular flexible diaphragm(height: 227 mm; inner diameter: 80 mm). Air containing fine particles is introduced via an inlet near
the diaphragm of the agglomeration chamber, and air is discharged from an outlet on the parallel
reflector. Figure 2 shows the vibration displacement of the circumferentially fixed circular flexuralvibration plate measured with a laser Doppler vibrometer. The measurement conditions are a driving
power of 1.0 W and a driving frequency of 28.0 kHz, and the measurement range is 45.5 mm in thecircumferential direction with the center as the origin. The horizontal axis represents the distance fromthe center of the diaphragm to the circumference, and the vertical axis represents the vibration
amplitude. The vibration is higher at the center, decreases toward the circumference, and no vibration
occurs at the fixed end. The maximum amplitude of 3.5 μm occurs about 10 mm from the center. The
amplitude at the center is low, which may be caused by the screw head of the drive point, which limits
the size and vibration amplitude of the drive point. Figure 3 shows the sound pressure distribution in
the agglomeration chamber with a cross section passing through the central axis of the agglomerationchamber. The maximum sound pressure is shown in red and the minimum is shown in blue. A highsound pressure is generated on the center axis of the diaphragm to form a standing wave sound field.
The half-wavelengths of the standing wave in the central axis direction are about 8 mm apart.
Airinlet: Air flow
Units: mm

Bolt-clamped
Langevin
ultrasonic transducer(28 kHz)80

AgglomerationchamberAir
outletVibrating platecombinedwith rigid wall

Figure 1 – Cylindrical rigid wall integrated sound source.

Vibration displacement [μm]5432100102030

4050

Distance from center [mm]Figure 2 – Vibration amplitude of the diaphragm as a function of the distance from the center.

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Vibrating plate combinedwith rigid wall

Agglomerationchamber

Parallelreflector

00.5

1.0

Normalized sound pressure
Figure 3 – Normalized sound pressure distribution in the agglomeration chamber.

3. CONFIGURATION OF THE AGGLOMERATION DEVICE
Figure 4 shows a block diagram of the apparatus for measuring aerosol agglomeration. It consists ofan aerosol generation chamber, an air pump, a flow meter for measuring the flow rate, and anagglomeration chamber, all connected with transparent tubing (inside diameter: 8 mm). The air flows
from the blower pump through the flow meter, aerosol generating chamber, and agglomerationchamber (blue arrows). Lit incense sticks consisting of charcoal and aromatic wood were used as anaerosol generator because they mainly produce particles 1.0 μm size, which are PM 2.5.

Particlesampling point
Particleemissionchamber

Air flowmeter 1

Agglome rationchamber

Air outlet

䠖 Air flow

Pump

Air inlet

Ultrasonic source

Figure 4 – Block diagram of the agglomeration device.

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4. PARTICLE SIZE DISTRIBUTION OF AGGLOMERATED FINE PARTICLES
To investigate the agglomeration effect of ultrasonic irradiation on fine particles, the particle size
was measured as the sound source input power was changed. Three lit incense sticks were placed in theagglomeration chamber, air was fed into the agglomeration chamber (0 s), and the experiment wasconducted for 750 s. The air flow rate was kept constant at 1.0 L/min, the input power to the soundsource was 20 W, and supertone irradiation was not performed. Ultrasonic irradiation was applied tothe particles for 150 s from 750 s to 600 s when the smoke concentration was stable. After theexperiment was complete, the pump was stopped, and particles were collected by attaching a syringe tothe air outlet of the agglomeration chamber and withdrawing 200 mL of air by aspiration. The
collected particles were deposited in a glass dish over 1 h. The fine particles were photographed (20image in total) using a microscope (magnification factor of 2500), and the particle size was determinedusing image processing software (ImageJ).
The median diameter (d 50) was measured to evaluate the particle size distribution. The median
diameter is the 50th percentile particle diameter, that is, the particle diameter that splits the particle size
distribution with half of the diameters larger and half of them smaller. Similarly, d 10 is the 10thpercentile particle diameter and d90 is the 90th percentile particle diameter.
4.1 Particle Size Distribution of Fine Particles
Figure 5 shows the particle distribution without ultrasonic irradiation. The horizontal axis indicatesthe particle size, and the vertical axis indicates the frequency distribution ( left axis, black bars) andcumulative distribution (right axis, red line). d10 was 0.9 μm, d50 was 1.6 μm, and d90 was 2.9 μm.
There were almost no particles with a diameter of 4.0 μm or more.
Figure 6 shows the particle size distribution at an input power of 20 W. The vertical and horizontal
axes are the same as in Figure 5. d10 , d50, and d90 were 1.1, 2.65, and 6.7 μm, respectively. These results
demonstrated that ultrasonic irradiation aggregated the fine particles.
The particle size distributions were also measured at input powers of 5 and 10 W (Figure 7). Thehorizontal axis shows the input power, and the vertical axis shows the particle size obtained from thecumulative distribution. At input powers from 0 to 10 W, d 50 and d90 increased as the input powerincreased, and d 90 was about 2.5 times larger at 10 W compared that at 0 W. These results show that theratio of large particles formed by agglomeration increased at an input power of about 10 W.

Frequency [%]8ᴺFrequency
ؑ : Cumulativedistribution

65042

Cumulative distribution [%]10010000 1 2 3 4 5 6 7 8 9 10Particle size [μm]Figure 5 – Particle size distribution without ultrasonic irradiation.

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100Frequency [%]8ᴺFrequency
ؑ : Cumulativedistribution

650
42Cumulative distribution [%]10000 1 2 3 4 5 6 7 8 9 10Particle size [μm]Figure 6 – Particle size distribution after ultrasonic irradiation (20 W).

87Particle size [μm]6- : d90- : d50- : d105432
100510152025

Input power [W]Figure 7 – Relationship between input power and cumulative particle size.

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4.2 Number of ParticlesThe decrease in the number of fine particles as a proportion of the total number of particles wasexamined. Table 1 shows the total number of particles at each input power. Without ultrasonicirradiation, the total number of fine particles was 5355, whereas at an input power of 20 W, the total
number of particles decreased to 1995.
Figure 8 shows the rate of decrease in the number of particles at each input power, where thenumber of particles without ultrasonic irradiation was 100%. The horizontal axis indicates the input
power and the vertical axis indicates the rate of decrease. The number of particles decreased as theinput power increased compared with that at 0 W, and the number of particles at 20 W was about 30%of that at 0 W. Thus, the total number of fine particles was reduced by ultrasonic irradiation.

Table 1 – Number of particles at each input power.
Input power[W]

Number of particles

0510205355

29442471

1995

Particle reduction rate [%]1008060
402000510152025

Input power [W]Figure 8 – Particle number reduction rate as a function of input power.

5. CONCLUSIONS
The agglomeration of fine particles using aerial ultrasonic waves was investigated, and the particle
size distributions for various input powers were measured. Irradiating fine particles with ultrasonicwaves increased d50 and d90 compared with no ultrasonic irradiation, owing to the agglomeration ofsmall particles. Increasing the input power decreased the total particle number and increased theagglomeration effect.

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