Int. J. Plasma Environ. Sci. Technol. 16 (2022) e03003 (8pp)

Regular Paper

DOI: 10.34343/ijpest.2022.16.e03003

Characteristics of high-speed mist generated bycondensation of water vapor in pressurized air
Yunchen Xiao1,2, Siwei Liu1, Tomoki Nakajima1, Takehiko Sato1
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Institute of Fluid Science, Tohoku University, Japan
Graduate School of Engineering, Tohoku University, Japan* Corresponding author: takehiko.sato.d7@tohoku.ac.jp (Takehiko Sato)
Received: 28 July 2022
Revised: 27 October 2022
Accepted: 31 October 2022
Published online: 4 November 2022

Abstract
The application of droplets has been extensively discussed and explored for decades. The cleaning effect, which is basedon the typical application of droplets, requires small droplets and high droplet velocities to improve cleaning efficiency.
In this study, a high-speed mist-generating device was developed based on the condensation of water vapor in pressurized
air. It can stably produce high-speed mist with nanoscale droplets. Several experiments were conducted to determine mist
characteristics. The cleaning effect was validated by cleaning process indicators, which proved that the mist can clean thesurface within 10 s. The temperature and pressure distributions were measured. Then, the mist velocity in the range of
35–49 mm along the x-direction was estimated to be 52–65 m s−1 at 35 mm and 40–51 m s−1 at 49 mm. The mist at thenozzle exit was determined to be a supersonic flow based on the Schlieren visualization. Moreover, electric current was
detected on the aluminum plate, which was set downstream. The current decreases with an increase in the distance fromthe nozzle exit and increases with an increase in the inlet gas pressure.
Keywords: Supersonic flow, pressure, temperature, cleaning, charge generation.

1. Introduction
Oral health is increasingly being focused owing to the increase in the public awareness of personal oral hygiene[1, 2]. Plaque is the primary cause of dental diseases (e.g., gingivitis and periodontal diseases); therefore,
plaque removal is of great importance and has been widely studied [3]. As per reports, tooth brushing, which
has been commonly applied to remove plaque for decades [4–6], cannot reach the interdental areas between
adjacent teeth, and the teeth can be damaged owing to overbrushing [3,7]. Several novel cleaning methods
have been developed and evaluated in recent years [8, 9]. Among these cleaning methods, the use of highspeed microdroplets, a micro-size droplet with high velocity, is assumed to be a potential, effective way ofremoving dental plaque [10, 11]. In our previous study, the removal mechanism of microdroplets from artificial
dental plaque was experimentally evaluated [12]. Plaque was removed by applying normal stress instead ofshear stress. However, as the size of the droplets is relatively large in the test, it may cause collateral damage
to the surface, in addition to removing the dental plaque. Previous results proved that the cleaning efficiency
with regard to the application of droplets is more related to velocity than the diameter, and the efficiency ishigh for small droplets with respect to their volume [13]. In addition, charges were detected in the mist flow,which was expected to change the characteristics of the target object (e.g., electric potentials, surface
conditions, dielectric properties, and others). Therefore, it is reasonable to predict a significant cleaning effectof the high-speed mist and expect to improve cleaning efficiency using small droplets.
In this study, a high-speed mist generation device was developed based on the condensation of water vapor
in pressurized air. In contrast to our previous study, the condensation of water vapor in pressurized air generates
high-speed mist of smaller size and higher velocity, which is assumed to improve the cleaning effect. The
characteristics of high-speed mist were investigated based on the following aspects: (1) cleaning efficiency,(2) temperature distribution, (3) pressure and velocity distribution, (4) the visualization of the dynamics of the1

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flow near the nozzle, and (5) the electrical property of the mist droplet. The results validated the cleaning effectof the high-speed mist.
2. Experimental setup
The mist-generating device is shown in Fig.1 (a). It comprised a heater (AS ONE Corporation, EHP-170N)
and stainless steel water vessel with an inlet for pressurized gas and an outlet for mist. A thermocouple(represented as T in Fig. 1), heater, and relay (not shown here) were used to construct the PID feedback control
system. This system helps stabilize the temperature of the water vessel, so that sufficient energy for evaporation,
but not excess energy for explosive boiling, can be applied to the vessel. The total amount of water was 200
mL in one test, and it was heated to continuously evaporate; the temperature was fixed to the boiling pointtemperature at a specific saturated vapor pressure. As a reference, in this study, an inlet pressure of 5 atm andwater temperature of 152.3 °C were considered as the standard conditions, and it can stably generate highspeed mist for ~35 min.
The mixture of water vapor and pressurized gas flowed through a nozzle (Spraying Systems Co., HB-1/8VV-SS-15-01) (Fig. 1 (d)). During this process, the water vapor condenses owing to the rapid decrease intemperature and finally forms a large number of small droplets. This type of mixture is defined as high-speed
mist in this study. To stabilize the nozzle temperature, the outlet pipe and nozzle were continuously heatedwith a wire heater at 30 W (Tokyo Technological Labo, CRX-1).
The Schlieren visualization system was constructed downstream of the mist (Fig. 1 (b)). It comprised axenon lamp (KATOKOKEN CO, LTD, LS-300), two convex lenses, a knife, and a camera (Nikon, D4). The
visualization was carefully implemented. The xenon lamp was at the focus of the first convex lens (f = 300mm), which transferred the point light into parallel light. The nozzle was placed in this parallel light region,and the mist flowed perpendicular to the light. The parallel light was focused on one knife by the second
convex lens (f = 200 mm), and a part of the light was separated by the knife to generate Schlieren images.
The electrical current detection system is shown in Fig. 1 (c). An aluminum plate (50×45×2 mm) was placedin front of the nozzle at distance d. An electrometer (KEITHLEY 6517A) was used to measure the electric
current from the plate to the ground. The current was measured and recorded at different distances, d, and inlet
gas pressures, p.

Fig. 1. Schematic of the mist-generating device and test platform. (a) High-speed mist-generating
device. P denotes the pressure gauge, T denotes the temperature meter. (b) Schlieren visualization
system. (c) Electric current detection system. (d) Primary size of the nozzle and the definitions of
three axes in this study. Note: the Schlieren visualization system (b) and the Electric current
detection system (c) are not applied at the same time, and all results in the following sections wereacquired by using the two systems individually.

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3. Results and discussion
3.1 Cleaning efficiency of high-speed mist
The cleaning process indicator (gke-GmbH, gke Clean-Record® cleaning process indicator W-CPI-Y) wasused to verify the cleaning effect of the high-speed mist. The index to be evaluated is whether the organic
material coated on the indicator (yellow parts in Fig. 2) can be removed by the impact of the mist. The indicator
was fixed at a specified position, and a stepping motor stage (SIGMAKOKI CO., LTD, SGSP26-100) with asingle-axis stage controller (SIGMAKOKI CO., LTD, GSC-01) was used to control the exposure time of theindicator toward the mist. For each condition, the indicator was exposed to mist three times. The images of theindicators with different statuses after exposure to the mist are shown in Fig. 2. If the impact region of theindicator turned from yellow to white, it indicated a strong cleaning effect. Therefore, it was denoted as the“effectively cleaned” status and marked as “+” in the results. Otherwise, if the region is "not effectively cleaned,
it was marked as “‒” and regarded as the “no cleaning effect” status.

Fig. 2. Pictures of the indicators with different statuses. (a) No cleaning effect status. In this case,there is no region of effectively cleaned. This status will be marked as – in Table 1 and Table 2. (b)Effectively cleaned status. In this case, the impact region of the indicator turned to be white fromyellow, indicating a strong cleaning effect. This status will be marked as + in Table 1 and Table 2.

The cleaning effects of the mist at different inlet gas pressures and distances are summarized in Tables 1
and 2. The material on the indicator was completely removed in less than 10 s at a distance of 1 cm from thenozzle when the gas pressure was greater than 4 atm. However, the mist flow did not exhibit any cleaning
effect at 2 and 3 atm for an exposure time of 10 s. Even for an exposure time of 60 s, the mist did not exhibit
a strong cleaning effect at p = 2 or 3 atm. It is assumed that the cleaning mechanism is related to the collision
process, which requires a detailed microscopic observation. Further studies are required to address this issue.
Table 1. Indicator status at different pressure and different position with an exposure time of 10 s.
Position
10 mm
20 mm
30 mm
Pressure
2 atm
–––
–––
–––
–––
–––
3 atm
–––
4 atm
5 atm

+++
+++

–––
+++

–––
–––

Table 2. Indicator status at different pressure and different position with an exposure time of 60 s.
Position
10 mm
20 mm
30 mm
Pressure
2 atm
–––
–––
–––
–––
–––
3 atm
–––
4 atm
5 atm

+++
+++

–+–
–++

3

–––
–––

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3.2 Distributions of total temperature and pressure
The total temperature and pressure distributions of the mist are considered to be important parameters if thedeveloped device can be used for practical applications (e.g., dental cleaning requires a moderate temperature
for the patient). A dramatic decrease in temperature and pressure is expected after the mist leaves the nozzle,
particularly for safety reasons. The spatial distributions of the total temperature and pressure of the high-speed
mist were measured and are discussed in this section. A thermocouple was fixed at the 3-axis moveable stage,
and the total temperatures at different positions were measured. As shown in Fig. 1 (d), the x, y, and z directions
denote the downstream, horizontal, and vertical directions. The total temperature distribution is shown in Fig.
3. The mist near the nozzle remained at a high temperature, and its temperature rapidly decreased with anincrease in the distance along the x direction owing to the strong heat transfer. The temperature stabilized in
the range of 35–49 mm along the x direction. The temperature distributions along the y and z directions wererelatively symmetric. However, the thermocouple setup inevitably affects the flow characteristics, thus
affecting the temperature measurement. In future studies, we will consider this case and improve our results
using non-invasive methods.

Fig. 3. The total temperature distribution along three directions with an inlet gas pressure of 5 atm. (a)The distribution along the x-direction; (b) The distribution along the y-direction; (c) The distribution
along the z-direction.

The pressure distribution of the mist flow with an inlet gas pressure of 5 atm was measured using a pitot
tube (LK-00, OKANO WORKS, Ltd.) and flow meter (FV-21, OKANO WORKS, Ltd.). Owing to theperformance limitation of the flow meter, pressures in the range of 0–2 kPa can be detected. Therefore, thepressure at each downstream position (3.5, 3.7, 3.9, 4.1, 4.3, 4.5, 4.7, and 4.9 cm) was measured repetitively.
The pressure value at each point was measured three times and averaged to determine the pressure variation
with respect to the position. The distributions of dynamic pressure, static pressure, and total pressure are shownin Fig. 4. The dynamic pressure (Fig. 4 (a)) and total pressure (Fig. 4 (b)) were directly measured using thepitot tube, and the static pressure (Fig. 4 (c)) was acquired by subtracting the former results.
The droplets in the experiment of this study could not be observed using the high-speed camera, as evident
in our previous study [12], indicating much smaller droplet sizes. It was difficult to directly estimate the droplet
velocity using optical images. However, the droplets in the mist can be accelerated or decelerated by thesurrounding gas and can eventually reach a stable velocity. The droplet velocity mainly depended on the mist
velocity. Therefore, the gas flow velocity, which is regarded as an index of droplet velocity in this study, canbe calculated based on the pressure distribution of the mist [14, 15].
The mist density should not be regarded as constant owing to its compressibility upon pressure or
expandability based on heat. Therefore, the mist density affected by temperature and pressure is defined byequation (1).
𝜌

𝜌

.

(1)

.

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where ρ0 denotes the density of the fluid at 20 °C, θ denotes the gas temperature, and ps denotes the staticpressure, which is expressed in kPa. Based on the static pressure distribution shown in Fig. 4 (c), we canconsider that compressibility is negligible. Thus, density is the function of the temperature distribution.
Then, the mist velocity can be roughly estimated using the dynamic pressure value and Equation (2).
𝑣

2

𝑝 /𝜌

(2)

where pd denotes the dynamic pressure.
Based on the temperature distribution shown in Fig. 3, the temperature reaches a relatively stable value inthe range of 35–49 mm along the x direction, maintained at ~40 °C. The density of water vapor is 0.7122 kg
m−3 at 40 °C, and the density of dry air is 1.128 kg m−3 at 40 °C. The velocities of vapor and air can be estimatedusing the dynamic pressure distribution illustrated in Fig. 4 (a) and its density at 40 °C, and the calculated
results are illustrated in Fig. 4 (d). It is assumed that the high-speed mist is a mixture of dry air and water vapor,and the mist density should be interposed between the two values. Therefore, it is reasonable to assume thatthe mist velocity in the range of 35–49 mm along the x direction can be predicted to be between the velocitylines of the vapor and air, as shown in Fig. 4 (d).

Fig. 4. The pressure distribution and velocity distribution of the mist from 35 to 49 mm at p = 5 atm. (a) Dynamic
pressure of the flow as a function of distance; (b) Total pressure of the flow as a function of distance; (c) Staticpressure calculated as a function of distance; (d) Velocity of the flow estimated from dynamic pressure.

3.3 Visualization of the mist flow
The supersonic flow of the high-speed mist at the nozzle exit was observed using the Schlieren method (Fig.
5). The characteristics and patterns of the flow are influenced by the experimental layout (e.g., nozzle design
and pressure). In our case, the pressure at the nozzle exit (5 atm, as illustrated in Fig. 5) was much larger than
the ambient pressure; thus, an underexpanded supersonic flow was formed upon leaving the nozzle exit (Fig.
5 (a)). The Schlieren images of the high-speed mist at the nozzle exit are shown in Figs. 5 (b) and (c). Fig. 5(b) shows the gas flow before heating, and Fig. 5 (c) shows the mist flow after heating. Although the Schlieren
images are not clear, it is still possible to recognize the Mach dish in the flow, as shown in Figs. 5 (b) and (c).

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It is reasonable to conclude that the gas and mist leaving the nozzle exit are supersonic. However, the patternsof the gas and mist flows are slightly different in terms of horizontal length and vertical expansion. The mist
flow is shorter in the supersonic region, indicating a faster decrease in velocity compared to the gas flow. The
velocity decrease may have been caused by the condensation and cooling of the mist flow. As the supersonicflow expands, the temperature of the supersonic flow decreases; thus, water vapor condenses into water
droplets. The water vapor space was filled with the surrounding air. The introduction of the surrounding air
caused a decrease in the average velocity of the mist flow. Consequently, the length of the supersonic region
decreased for the mist flow.

Fig. 5. Supersonic flow of the high-speed mist at the nozzle exit. (a) Schematic pattern of the underexpanded
supersonic flow. (b) Schlieren image of the gas flow with p = 5 atm at the nozzle exit. (c) Schlieren image of themist flow with p = 5 atm at the nozzle exit.

Fig. 6. (a) Mist current as a function of distance at p = 2, 3, 4 and 5 atm; (b) Mist current as a function of pressure
at x = 2 mm.

3.4 Mist current
Charges were detected in the high-speed mist during the experiment. Charged droplets are expected tosignificantly affect their applications, especially for cleaning effect. For example, charge separation or
accumulation may help clean the selected particles. Moreover, the droplets may change the electric potentialof the treated object, which may affect surface conditions and dielectric properties [16]. The current flow of
the mist was measured to understand the charge distribution and generation mechanism.
The mist is regarded as a stable flow, and its electrical characteristics are homogeneous, such that the electric
current is stable during one record. Five samples were recorded in 2 s to determine the average. The average
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values were defined as the current under the measurement condition. The current as a function of distance withdifferent inlet gas pressures and the current as a function of pressure at a distance of 2 mm are shown in Figs.
6 (a) and (b), respectively. The current value gradually reduces to zero with an increase in distance andincreases with an increase in pressure.
6. Conclusion
In this study, a novel high-speed mist-generating device was developed based on the principle of thecondensation of water vapor in pressurized air. The cleaning effect of the mist was validated by cleaning
process indicators, and it was found that the material on the indicator was completely removed in less than 10
s at a distance of 1 cm from the nozzle when the gas pressure was greater than 4 atm. An inlet gas pressure of
5 atm and water temperature of 153 °C were recommended for this study. Thus, the distributions of thetemperature and pressure were measured in the range of 35–49 mm along the x direction. Based on theexperimental results, the mist velocity distribution was estimated to be 52–65 m s−1 at 35 mm and 40–51 m s−1
at 49 mm. The Schlieren method was used to visualize the mist flow at the nozzle exit. In this study, it was
found that the gas and mist flows were underexpanded supersonic flows. The electric current on the metal platenear the supersonic region was measured. The current decreased with an increase in the distance from thenozzle exit and increased with an increase in the inlet gas pressure. This phenomenon might be caused bycharge separation in the mist flow or owing to friction with or impact on the plate. Further analyses are required
to clarify the generation mechanism of the current and influence of the droplet size and velocity.
Acknowledgment
This study was partly supported by the Priority Collaborative Research Project of the Institute of Fluid Science, Tohoku
University (Project Code: J19J002, J20J001, J21J001). We would like to thank Prof. M. Watanabe (Hokkaido University),Prof. H. Yui (Tokyo University of Science), Prof. T. Yano (Osaka University), Assoc. Prof. K. Kobayashi (Hokkaido
University), Prof. S. Fujimura (Tohoku Medical and Pharmaceutical University), Prof. Y. Iga (Tohoku University), Assoc.
Prof. T. Sanada (Shizuoka University), and the Chief Research Officer K. Urashima (National Institute of Science andTechnology Policy, MEXT) for the academic support.

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