9ol. No.3 22 Mechanical Engineering Journal

J-STAGE Advance Publication date : 9 March, 2020Paper No.19-00539

© 2020 The Japan Society of Mechanical Engineers[DOI: 10.1299/mej.19-00539]

Vol.7, No.3, 2020Bulletin of the JSME

Mechanical Engineering Journal

1. Introduction

As one of air pollution control equipment, equipment for spraying liquid into aerosol has been put to practical use.

The aerosol particles are removed by contacting liquid in the equipment. The equipment has been used in a filtered venting system in nuclear power plants to remove radioactive aerosol particles and gas, such as Venturi scrubbers (Horiguchi et al., 2016; Tohoku Electric Power Co., 2016). The removal performance was tested by using actual-scale equipment. However, unexpected situations may happen in severe accidents due to large-scale disasters and artificial mistakes at the time of the accident. Since the optimizing of the performance and the design for various accident situations is difficult by performing actual-scale experiments, development of numerical analysis to optimize the performance and the design is important. The numerical analysis has the advantage to evaluate easily the performance against various situation for shorter time. Thus, Japan Atomic Energy Agency started the development of the numerical analysis method for evaluating radioactive aerosol particle removal performance against the above background.

The grasp of the capturing behavior of the aerosol particle on a gas-liquid interface is essential to develop the numerical analysis of the removal performance. The capturing behavior of aerosol particles with a droplet is considered to be the same as capturing behavior in single fiber theory (Hinds, 1985, Tohoku Electric Power Co., 2016). As the behavior in single fiber theory, inertial impaction, interception, diffusion, gravitational settling and electrostatic attraction are considered (Hinds, 1985). The inertial impaction occurs when a particle cannot follow bending flow around the fiber

Observation of aerosol particle capturing behavior near gas-liquid interface

Shinichiro UESAWA* and Hiroyuki YOSHIDA* *Japan Atomic Energy Agency

2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan E-mail: [email protected]

Abstract An aerosol particle capturing by a gas-liquid interface has been applied in many industries. One example of this is the radioactive aerosol removal system in the safety system of the nuclear reactor such as filtered venting system. In this study, to clarify the capturing behavior of aerosol particles by the gas-liquid interface, we developed a direct observation technique of the aerosol particle behavior and have been performing the observation experiment of the aerosol particle capturing behavior near the gas-liquid interface. In the experiment, we observed the aerosol particle capturing behavior on the gas-liquid interface of a single droplet. The capturing behavior of the aerosol particles near the droplet interface was observed by using a high-speed and high-resolution video camera and a fiber light. In this observation, we confirmed that aerosol particles were captured at the gas-liquid interface of the droplet after the velocity of the aerosol particles decreases near the droplet. On the other hand, some aerosol particles closing to the droplet did not reach the interface and were not captured. In comparison between the behavior and Stokes number, particles with higher Stokes number were easy to be captured at the gas-liquid interface. Especially, particles with much higher Stokes number penetrated through the gas-liquid interface and were captured inside the droplet. This capturing behavior was not considered in previous studies because the capturing behavior of a “solid” single fiber applied to the capturing behavior of aerosol particles with a droplet. Thus, this study made clear that Stokes number affects capturing behavior. In addition, we confirmed both soluble and insoluble particles in water also were captured in a droplet in high Stokes number. This result means that particles can be captured in a droplet in high Stokes number even if aerosol particles are insoluble in water.

Keywords : Aerosol, Gas-liquid interface, Droplet, Inertial impaction, Scrubbing

Received: 29 October 2019; Revised: 10 December 2019; Accepted: 2 March 2020

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2© 2020 The Japan Society of Mechanical Engineers

Uesawa and Yoshida, Mechanical Engineering Journal, Vol.7, No.3 (2020)

[DOI: 10.1299/mej.19-00539]

due to the inertia of the particle. The dimensionless number characterizing the effect of the inertial impaction is Stokes number,

𝑆𝑡𝑘 . (1)

In this paper, droplet radius dd / 2 [m] is used for the characteristic length L [m] of the flow. As shown the equation, the dominant capturing behavior of a denser and larger particle in higher gas velocity is the inertial impaction. The interception occurs when a particle moving along a fiber surface reaches within the radius of the particle from the fiber surface. The diffusion occurs due to a particle moving on a streamline near a fiber contacts the surface of the fiber by Brownian motion. The diffusion is the dominant capturing behavior for a nanoparticle. Thus, the capturing behavior by contacting liquid in the filtered venting system such as Venturi scrubbers is considered that the dominant behavior are the inertial impaction and the interception because the radioactive aerosol particles in steam contact the gas-liquid interface at high velocity. However, the capturing behavior at the gas-liquid interface may be different from the behavior in single fiber theory, such as penetration through the gas-liquid interface. In addition, the wettability and solubility of the particles in water may affect the capturing behavior. The evaluation test of the aerosol particle removal performance with a single droplet was performed (Ardon-Dryer et al., 2015). However, nobody has observed the behavior of aerosol particles near the gas-liquid interface, since the particle is a micro scale particle and moves at high speed. Thus, the behavior has not been clarified yet.

Japan Atomic Energy Agency developed the direct observation technique of the aerosol particle behavior near the gas-liquid interface as a part of the development of the numerical analysis method for evaluating radioactive aerosol particle removal performance. In this paper, we report the direct observation technique of aerosol particle behavior near the gas-liquid interface, and the observation result of the capturing behavior at the gas-liquid interface of micro scale aerosol particles.

2. Direct observation technique of aerosol particle near gas-liquid interface

To observe micro scale aerosol particles, a high magnification lens like a microscope is required. However, since the

depth of field becomes shallower by using higher magnification lenses, it is technically difficult to directly observe the gas-liquid interface moving at high speed and fine aerosol particles simultaneously. In our technique, the experiment was performed under the condition simulating the Stokes number by fixing the droplet to the tip of the syringe needle and applying air containing aerosol particles to the stationary droplet.

As shown in Fig. 1, the experimental apparatus is composed of a syringe, an aerosol generator, a tube, a high-speed video camera (FASTCAM SA-X, Photron) equipped with a microscope lens (INFINITUBETMZ, Edmund Optics) and fiber lights (SLA-100A, Sigma Koki and CAVILUX Smart, Cavitar). The fiber light generates less heat and does not

Fig. 1 Schematic of the experimental apparatus (left) and Snapshot of the experimental apparatus around a droplet.

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[DOI: 10.1299/mej.19-00539]

affect subjects. In the experiment, a droplet was formed at the tip of the injection needle, and after the droplet came to rest at the tip, air including particles were blown on the droplet from the aerosol generator. The droplet diameter was from 0.7 mm to 1.4 mm. The droplet water is commercial distilled water (049-16787, FUJIFILM Wako Pure Chemical Industries, Ltd.) not to change the surface tension in each experiment. Besides, to decrease the effect of electrostatic attraction, a static eliminator (SJ-F010, KEYENCE) was used.

Several lenses were used for the observed particle size to photograph fine aerosol particles. For observation of particles with a particle size of 10 m order or more, an infinitely corrected objectives (M-PLAN APO 5X, Mitutoyo) with a magnification of 5 times was used. The depth of field is 14 m and the spatial resolution is 4.2 m/pixel in this measurement system. An infinite corrected objective (M-PLAN APO SL 20X, Mitutoyo) with a magnification of 20 times was used for observation of particles in the order of 1 m to 10 m. The depth of field is 3.5 m and the spatial resolution is 1.1 m/pixel in this measurement system. By using the high-speed video camera equipped with these objective lenses and the backlight, it is possible to photograph the behavior of fine particles. Simultaneous measurement of particle size, shape and velocity of each aerosol particle can be expected in the technique although it is difficult to measure by a method using scattered light from laser light.

The particles used in this experiment were polystyrene latex (PSL), cesium iodide (CsI) and titanium dioxide (TiO2). The PSL particle was used as particles with a known particle size because of their excellent uniformity of particle size and sphericity. CsI is the main fission product compound at severe accidents of light water reactors (Girault et al., 2013). TiO2 was used because the comparison with CsI which is soluble in water and TiO2 which is insoluble. We used two type generators as aerosol generation methods. A dry type aerosol generator (PB 100, LaVision) generates aerosol by winding up the CsI and TiO2 powder enclosed in the generator with compressive air. A wet type aerosol generator (ATM-226, Topas) generates aerosol by spraying the CsI solution and PSL particle suspension. A sprayed microdroplet evaporates immediately in the atmosphere and a CsI particle crystallized. Comparing with the dry type generator which scatters powder directly, the wet type generator can generate finer aerosol particles. In this paper, the diameter of particles generated with the dry type generator was from 6 m to 33 m. The diameter of particles with the wet type generator was from 4 m to 10 m. In the experiment with the dry type generator, the gas flow rate and the gas velocity were from 1.7 L/min to 11.2 L/min and from 2.3 m/s to 14.8 m/s, respectively. In the experiment with the wet type generator, the gas flow rate and the gas velocity were from 1.6 L/min to 6.6 L/min and from 0.9 m/s to 3.9 m/s, respectively. The Stokes number was 0.1 [-] – 1000 [-] in the experiment because the Stokes number in the spray and the Venturi scrubber are 0.1 [-] – 100 [-] and 1 [-] – 1000 [-], respectively. Even in this experiment using several m to several tens of m particles, particles near 0.5 µm, which is difficult to remove by gravity settling, pool scrubbing and so on, can be evaluated by similarity law of the Stokes number. In the high gas velocity conditions, the gas-liquid interface vibrated. However, the vibration velocity of the interface was lower relative to the moving velocity of the particles. Thus, the effect on the capturing behavior of the particles is considered to be small and ignored in this paper.

3. Experimental results and discussions 3.1 Particle behavior around gas-liquid interface

In order to understand the behavior of aerosol particles around gas-liquid interface, we observed the behavior of PSL particles, which have a known particle diameter and high sphericity, around the droplet at first. The aerosol was generated by using the wet type aerosol generator. The record speed and exposure time were 100,000 fps and 0.1 s, respectively. In this observation, the infinitely corrected objectives with the magnification of 20 times was used. Figure 2 is a composite image of continuous images of behavior of particles around a part of the upper surface of the droplet. The black region shows the droplet. The gas-liquid interface is the boundary of the black and the gray region. The aerosol particles came from the top of the image. The particles on the center line, pointed out with white arrows, deposited on the gas-liquid interface. The particle diameter was 4 m. The diameter was obtained from the particle image when the particle was shown in the black dot which is the most in-focus. The time next to the arrows shows the observation time of the particle. The velocity of the particle was approximately 2.2 m/s and almost constant by 200 m from the interface. However, the particle velocity decreased with approaching the gas-liquid interface. After that, the particle moved along the interface and deposited on the interface finally. On the other hand, other particles did not deposit on the interface. Although those particles moved along the interface, those did not reach the interface. Thus, even if the particles flow in the projection area of the droplet, not all particles are captured.

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[DOI: 10.1299/mej.19-00539]

3.2 Deposition on gas-liquid interface PSL particles behavior around gas-liquid interface were observed in this measurement technique. However, PSL

particle is too light to evaluate the behavior of radioactive aerosol particles because particle behavior is strongly influenced by particle mass density as shown eq. (1). In this study, we used CsI particles in addition to PSL particles. CsI is the main fission product compound at severe accidents of light water reactors (Girault et al., 2013). The observation of aerosol particle behavior was performed by using the wet type aerosol generator. The record speed and exposure time were 40000 fps and 2.5 s, respectively. In this observation, the infinitely corrected objectives with the magnification of 20 times was used. The observation area was the upper of the droplet as with Fig. 2.

Figure 3 is a composite image of continuous images of deposition on the gas-liquid interface. Two CsI particles generated with the wet type generator flew to the gas-liquid interface and deposited on the interface. The size of both particles was approximately 2 m. The average velocity of the left particle in the figure between 0 s and 10 s was 0.09 m/s. The velocity of the right-side particle was 0.13 m/s. Each particle diverted to the left and right, respectively. At 700 s, both particles reached the gas-liquid interface. In the image at the next frame time, those particles were not confirmed. The result means that the particles deposited at the droplet interface were captured inside the droplet without remaining

Fig. 2 Composite image of continuous images of behavior of particles around a part of the upper surface of the droplet

Fig. 3 Composite image of continuous images of the deposition on the gas-liquid interface

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[DOI: 10.1299/mej.19-00539]

at the gas-liquid interface. Figure 4 are continuous images of the deposition on the gas-liquid interface after moving to slide on the gas-liquid

interface. The CsI particle was generated with the wet type generator. The particle size was approximately 3 m and the average velocity between 0 s and 0.250 ms was 0.07 m/s. The particle approached the gas-liquid interface and reached the interface at 1.250 ms. However, the behavior of the particle after that was different from the behavior shown in Fig. 3. The particle moved on the interface in the clockwise direction until 4.0 ms. The moving velocity was slower compared to the flying velocity, the moving velocity was approximately 0.05 m/s. At 4.025 ms, the particle cannot be confirmed in the image. From the result, it is considered that the particle was captured in the liquid through the gas-liquid interface between 4.0 ms and 4.025 ms. The behavior of the particle moving on the gas-liquid interface has not been considered in the previous behavior. We must construct and add a new model to the capturing behavior.

3.3 Moving near gas-liquid interface

Some PSL particles that did not reach the droplet were observed as shown in Fig. 2. Particles that did not reach the

droplet were observed in the case of CsI aerosol. Figure 5 is a composite image of continuous images of the particle moving in the vicinity along the gas-liquid interface. In this observation, the infinitely corrected objectives with a

Fig. 4 Continuous images of deposition on the interface after moving to slide on the droplet interface

Fig. 5 Composite image of continuous images of the particle moving in the vicinity along the gas-liquid interface

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magnification of 5 times was used. The record speed and the exposure time were 40,000 fps and 25 s, respectively. The CsI aerosol particles were generated by using the wet type generator. The particle size was approximately 3 m. The average velocity between 0 s and 100 s was 0.86 m/s. The particle approached the gas-liquid interface with decreasing the velocity. After that, the particles moved near the gas-liquid interface. After 500 s, although the particle kept moving along the interface, it did not contact the interface. Therefore, it was confirmed that the particle was not captured in the droplet even if it approached the gas-liquid interface.

3.4 Penetration through gas-liquid interface

Figure 6 is a composite image of continuous images of particle penetrating through the gas-liquid interface. The

black pointed at the arrow is the CsI particle, and those all black points are the same particle. The time showed with the arrow is the time at each position of the particle. 0 s is the time of coming into the observation area. In this observation, the infinitely corrected objectives with a magnification of 5 times was used. The record speed and the exposure time were 100,000 fps and 0.37 s, respectively. CsI aerosol particles were generated by using the dry type generator. The obtained observation result was wider in field of view than in Fig. 2, 3 and 4. A whole of the upper surface of the droplet was observed.

The observed particle size was approximately 25 m. The velocity at 0 s in the figure was approximately 13 m/s. The CsI particle that flew towards the droplet reached the gas-liquid interface after 40 μs. After that, the position of the particle in the vicinity of the gas-liquid interface could not be confirmed due to the difference in the refractive index of gas and liquid. However, the particle was confirmed inside the liquid at 180 s. Before the particle was observed, other particles were not confirmed inside the droplet, the particle confirmed inside the liquid was on the straight line of the penetrating particle through the gas-liquid interface. In addition, the other flying particles were not confirmed. For the above reasons, it was considered that the particle penetrated the liquid through the gas-liquid interface. After that, the particle continued to exist inside the liquid without moving. Therefore, it was confirmed that the CsI particle that flew to the droplet was captured inside the droplet. This capturing behavior was not considered in previous studies because the capturing behavior in single fiber theory applied to the capturing behavior of aerosol particles with a droplet. Furthermore, comparing the solid fiber filter, captured particles inside the droplet is hard to diffuse from the gas-liquid interface. Thus, the capturing behavior is the important behavior for the removal performance of aerosol particles with liquid.

Fig. 6 Composite image of continuous images of particle penetration through the gas-liquid interface

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3.5 Relation of particle behavior and Stokes number It has long been known that it is easy to capture particles with higher Stokes number (Ueoka, 1957). The Stokes

number is a dimensionless number and relates to the followability of aerosol particles to flow. In this study, in order to clarify the cause of the difference of the capturing behavior, the relation of the particle behavior and the Stokes number was discussed. The results are shown in Fig. 7. The vertical axis shows the Stokes number. The horizontal axis shows the position of the particle relative to the projection area of the droplet in the flow direction when particles flow into the observation area, as shown in Fig. 8. 0.0 [-] and 1.0 [-] mean the center of the droplet and the droplet radius, respectively. CsI density (4500 kg/m3), TiO2 density (4230 kg/m3), the viscosity of air, the droplet diameter, the observed particle diameter and the measured particle velocity at more 200 m away from the droplet were used as physical values in the Stokes number. In general, the velocity in the Stokes number is flow velocity well away from an obstacle. In this paper, 200 m was considered far away from the obstacle because the particle began to be was affected by the obstacle from approximately 100 m from the obstacle. Figures 7 and 9 are the result obtained from particles with diameter of 4 m or more.

The particle behavior was different for the Stokes number and the approaching position near the gas-liquid interface. In the higher Stokes number of approximately 100-1000 [-], the particle penetration through the gas-liquid interface was confirmed. In this region, the particle diameter and the velocity were large and higher, respectively. Thus, the inertial force was larger. Particles which had larger inertial force can penetrate through the gas-liquid interface.

In the Stokes number of approximately 30-100 [-], the penetration through the gas-liquid interface or the deposition on the gas-liquid interface were confirmed. In this region, the penetration through the gas-liquid interface was not judged because particles cannot be observed near the interface in the droplet with refraction of the droplet. However, we confirmed that all captured particles did not reach the center of the droplet. Particles cannot reach the center because the inertial force was weak due to the finer particle and the slower velocity.

Fig. 7 Relation of particle behavior and Stokes number

Fig. 8 Position of the particle relative to the projection area of the droplet in the flow direction when particles flow into the

observation area

0.1

1

10

100

1000

Stk

[-]

1.00.80.60.40.20.0x [-]

Penetration through gas-liquid interfacePenetration through gas-liquid interface or deposition on gas-liquid interface(cannot judge)

Deposition on gas-liquid interface

Moving near gas-liquid nterface

Ueoka (1957)

0.0 1.0x-1.0

DropletParticle

Observation area

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In the Stokes number of approximately 0.4-30 [-], the deposition on the gas-liquid interface and the moving near the interface were observed. However, the cause of the difference between the deposition and the moving is not still clear because particles with higher inertial force cannot reach the gas-liquid interface and particles with the lower inertial force can reach the interface in the range of this Stokes number. In addition to the Stokes number, the approaching position x [-] may affect the capturing behavior. The inertial force perpendicular to the gas-liquid interface was different at each approaching position because the droplet had a curved surface. Figure 7 supports this suggestion. Even if particles have same Stokes number, particles near the center of the droplet was easy to be captured in comparison with particles out of the center.

The broken line in Fig. 7 shows the theoretical collision efficiency ([removal area of particles in the flow by collision] / [droplet diameter]) with respect to the Stokes number assuming that the potential flow is upstream from the droplet (Ueoka, 1957). The collision efficiency can be changed to [radius of the removal area by collision] / [droplet radius] as shown in Fig. 7. The region where x [-] in the figure is smaller than the broken line is the region where fine particles are considered to be removed. Comparing the previous research with the results of this experiment, although all points were in the region where x [-] is smaller than the broken line, some particles moved along the interface and are not captured. Thus, we confirmed that the particles which were theoretically predicted to be captured were not actually captured.

Figure 9 shows the Stokes number and the approaching position of captured CsI and TiO2 particles. The result shows the effect of the solubility of particles in water to particle capturing. CsI is highly soluble in water. The solubility of CsI in water is 848 g/L. TiO2 is insoluble in water. In the Stokes number 100 – 1000, the CsI and TiO2 particles can be captured in the droplet. The result means that even if a particle is insoluble in water, the particle can be captured in droplet in high Stokes number. In lower Stokes number 1 – 10, the higher soluble CsI particles were captured. However, the insoluble TiO2 particles were not captured. Thus, the result indicated that the particle solubility in water affects the particle capturing with the droplet in lower Stokes number.

4. Conclusions

The direct observation technique of micro scale aerosol particle behavior was developed to clarify the aerosol particle

capturing behavior near the gas-liquid interface. As the capturing behavior on the gas-liquid interface, the deposition on the gas-liquid interface, the deposition on the interface after moving to slide on the interface and the particle penetration through the gas-liquid interface were confirmed. In particular, the penetration through the gas-liquid interface and the moving on the interface were not considered in the previous capturing behavior because the capturing behavior of a solid fiber filter was applied to the previous capturing behavior of droplets. By comparing each particle behavior for Stokes number, the penetration through the gas-liquid interface was observed with the higher Stokes number. In lower Stokes number, the deposition on the gas-liquid interface and the moving near the gas-liquid interface were observed. Thus, inertial force affects capturing behavior. In addition, we confirmed insoluble TiO2 particles in water were captured in the

Fig. 9 Relation of CsI and TiO2 particles behavior and Stokes number

0.1

1

10

100

1000

Stk

[-]

1.00.80.60.40.20.0x [-]

CsITiO2

CsITiO2

Capturing

No capturing

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droplet in high Stokes number. Thus, even if aerosol particles are insoluble in water, particles can be captured in the droplet in high Stokes number. On the other hand, in lower Stokes number, the insoluble particles are difficult to be captured in the droplet in the comparison with the soluble particles in water. In the future, we are going to perform experiments for finer particles to discuss other capturing mechanism than inertial impaction. In addition, we are going to investigate the effect of the wettability and the electrical charge of the particles to the capturing behavior because it may affect the capturing behavior.

Nomenclature

d diameter [m] L characteristic length [m] Stk Stokes number [-] U flow velocity [m/s] Greek letters density [kg/m3] viscosity [Pa s] Subscripts d droplet p particle

References Ardon-Dryer, K., et al., Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets

on a single-droplet basis, Atmospheric Chemistry and Physics, Vol.15 (2015), pp.9159-9171. Girault, N., et al., Insights into iodine behaviour and speciation in the Phebus primary circuit, Annals of Nuclear Energy,

Vol.61 (2013), pp.143-156. Hinds, W. C., Aerosol Technology (1985), Inoue Shoin, inc., Tokyo, pp.168-172 (in Japanese). Horiguchi, N., et al., Numerical simulation of two-phase flow behavior in Venturi scrubber by interface tracking method,

Nuclear Engineering and Design, Vol.310 (2016), pp.580-586. Tohoku Electric Power Co., Inc., Pressure relief devices in the reactor containment vessel in Onagawa nuclear power

plant unit 2 (the filtered vent system in the reactor containment vessel) (2016), pp.45-75 (in Japanese) (https://www.nsr.go.jp/data/000161017.pdf September 19, 2017).

Ueoka, Y., Study on the Venturi scrubber (1st report) Analysis of the collection efficiency at throat, Transactions of the Japan Society of Mechanical Engineers, Vol.23 (1957), pp.309-313 (in Japanese).

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9ol. No.3 22 Mechanical Engineering Journal