Quasi Two-Dimensional Evaporation and Boiling UnderReduced Pressure Keita Ogawa*, Yuichi Yasumoto†, Mitsuhiro Matsumoto‡and Hidenobu Wakabayashi§ Department of Mechanical Engineering and
Trang 1Quasi Two-Dimensional Evaporation and Boiling Under
Reduced Pressure
Keita Ogawa*, Yuichi Yasumoto†, Mitsuhiro Matsumoto‡and Hidenobu Wakabayashi§ Department of Mechanical Engineering and Science, Kyoto University Kyotodaigaku-Katsura, Nishigyo-ku, Kyoto, 615-8540, Japan
* ogawa.keita.37z@st.kyoto-u.ac.jp
†Yu.Yasumoto@mitsui.com
‡matsumoto@kues.kyoto-u.ac.jp
§wakabayashi.hidenobu.8n@kyoto-u.ac.jp
Received 8 September 2016 Accepted 28 November 2016 Published 5 January 2017
To study the washing mechanism of laminated plates with solvent vapor, we have experimentally
investigated evaporation dynamics of liquid con¯ned between solid plates under reduced pressure.
As the test liquid, we use deionized water and several organic compounds To visualize the °uid
motion in the thin gaps, we adopt glass plates When a test liquid is sandwiched between a normal
(°oat) glass plate and a ground (sand-blasted) one, vertically incident light passes through the
plates without much scattering; once the liquid starts to evaporate, dried rough surface of the
ground glass scatters the light and we can monitor the °ow pattern Based on the transmitted
light intensity, the whole plate area is categorized into three regions; completely wet, completely
dry, and semi-dry one; the last one is supposed to be the state that thin liquid ¯lm spreads on the
plate In the case of water, many tiny spots of semi-dry region appear and expand at the initial
stage, which is probably cavitation of dissolved gas In organic liquid cases, evaporation seems to
start from the edges of the plates At a later stage, the semi-dry region expands with complicated
branching patterns In all cases, occasional rapid motions of liquid were observed, which
corre-spond to two-dimensional °ash boiling We also investigated the in°uence of the control pressure,
the surface roughness, and the plate deformation.
Keywords: Fluid phase change; evaporation; boiling; micro°uidics; visualization.
Nomenclature
D : Gap distance (m)
Ps: Saturated vapor pressure (Pa)
Rz: Ten-point average surface roughness (m)
x l: Gas solubility in mole fraction (−)
: Surface tension (N/m)
: Viscosity (Pa s)
l: Liquid density (kg/m 3 )
1 Introduction
Laminated steel plates are widely used as metal cores in various types of electric transformers These cores are often immersed in insulating mineral oils, and it requires time and cost to wash the oil away when disposed The main target of this study is the cleansing process of such laminated plates Until
Vol 25, No 1 (2017) 1750003 (9 pages)
© World Scienti¯c Publishing Company
DOI: 10.1142/S2010132517500031
Trang 21970s, polychlorinated biphenyls (PCBs) had been
used as an ingredient to insulating oils because of
their chemical stability and excellent ability of
electric insulation Due to the high toxicity, their use
in electric devices is now strictly banned, and ways
of safe and cost-e®ective disposal of such cores are
demanded.1
Recently, a method of vapor washing with organic
solvents has been proposed2; it is experimentally
con¯rmed that, under su±ciently high temperature
(typically 100–200C, depending on the solvents)
and low pressure conditions, the mineral oil between
the core plates is gradually replaced by the solvent
vapor and completely washed away To understand
the washing mechanism and optimize the process, it
is essential to investigate the phase change dynamics
in such narrow gaps in more details
Flows with phase change in various types of
micro channels have been widely studied in thermal
engineering; see Refs.3and 4for recent issues The
general target of these studies is, however, to achieve
a better heat exchanger with larger heat transfer
coe±cient and higher critical heat °ux Their
ex-perimental approaches as well as theoretical
mod-elings are closely related to our goal, although the
situations are di®erent
In order to have better understanding of °uid
behavior in thin gaps, we have tried to visualize the
°uid dynamics using model systems.5 – 7 In this
paper, our recent progress with improved
experi-mental setups is presented and relevant factors are
discussed
2 Experimental Setup
2.1 Model system
To visualize the °uid dynamics in narrow gaps
be-tween plates, we utilize a model system, which is
colorless volatile liquid con¯ned between two glass
plates As the test liquid, puri¯ed water (deionized with ion-exchange resins) and three organic liquids, ethanol (99.5% purity), acetone (99.5%), and hep-tane (99.5%), are used to examine how the volatility di®erence a®ects the dynamics; the basic properties are shown in Table1 To optically observe the °ow dynamics, two types of commercially available transparent glass plates are used, °oat glass plate with smooth surface, and ground (sand-blasted) glass one When liquid exists between the °oat glass plate and the ground one, the system is almost completely transparent; as the evaporation proceeds and the gap is getting dried, it becomes opaque due
to di®used re°ection of incident light, thus the drying process is easily visualized
2.2 Visualization
The experimental setup is schematically shown in Fig.1 The vacuum chamber of cubic shape is made of transparent acrylic resin (20 20 20 cm3Þ, inside
of which square glass plates sandwiching the test liquid are placed with being clamped with metal clips
by their each corner The chamber is depressurized with a rotary vacuum pump (TASCO, 48 L/min) The inside pressure is monitored with a digital di®erential pressure gauge, the uncertainty of which
is 0.1 kPa
All experiments were done at room temperature without any special temperature control In general,
we record the temperature outside the chamber with
a digital thermometer, the uncertainty of which is
0:1 A thermocouple of K type is sometimes uti-lized to directly monitor the temperature of glass plates The latter varies with time due to evaporation during the experiments, but the maximum di®erence from the outside temperature is about 1 K
The area size of the glass plate is 10 10 cm2 with 5 mm thickness The plates are illuminated with a surface emitting LED light from downside
Table 1 Properties of test liquids, data for saturated vapor pressure Ps, viscosity , and surface tension are taken from Ref 8 are gas solubility x lare from Ref.9.
P s(kPa) at 25 C at 25 C 104x lat 25C
Contact angle Contact angle
at 20C at 25C (mPas) (mN/m) Nitrogen Oxygen on °oat glass ð Þ on ground glass ð Þ Water 2.34 3.17 0.890 72.0 0.118 0.229 55.4 14.4
Ethanol 5.88 7.89 1.07 22.0 3.60 5.82 17.9 0
Acetone 24.8 30.8 0.322 23.5 5.42 7.62 6.2 0
Heptane 5.12 6.59 0.370 19.7 13.5 20.8 0 0
Trang 3A photo-sensing unit and a shutter are introduced to
synchronize the digital camera and the pressure
measurement
2.3 Properties of glass surface
According to its fabricating process, the surface of
the °oat glass is very smooth, while the ground one
is rough Surface roughness can a®ect the gap
dis-tance between the plates as well as the wettability
In addition to commercially available ground glass,
we prepared three types of specially sand-blasted
ones with di®erent roughness A surface
measure-ment instrumeasure-ment (Mitutoyo SJ-210) was used to
estimate their surface roughness; an example of the
surface pro¯le is shown in Fig.2 From the obtained
pro¯le data, the 10-point average roughness Rz is
calculated as shown in Table 2, where the label
\regular" indicates the commercially available one
We separately estimated the gap distanceD by a measurement of liquid mass as the di®erence be-tween dry plates and plates ¯lled with ethanol (at
22, mass density8l ¼ 0:788 g/cm3Þ The minimum scale of the mass measurement is 0.01 g, which leads
to 1 m of uncertainty in D The measured D generally agrees withRz as shown in Table2, which suggests that test liquid is con¯ned in a gap of this scale Even when the plates are cramped by metal clips, the value ofD is not changed
By coincidence, this roughness valueRz 40 m
of the \regular" ground glass is of the same order
as that of metal plates widely used in electric transformers
E®ects of surface roughness on °ow in micro-channels were reported in many studies,10 – 12 in which they investigated how nanoscale structures on microchannel walls a®ect the dynamics In our case, microscale roughness supports the gap distance by itself, thus the situation is di®erent
The contact angle is another relevant factor, which is evaluated from the optical image of small droplets on plates For all liquids, the apparent angle is much less on the ground glass than on the
°oat glass, probably due to the microscale structure
of ground glass surface.13The organic liquids spread much more easily than water
3 Results 3.1 Optical image data
Typical examples of the pattern change during evaporation are shown in Fig 3 In each case, the obtained grey-scale images of the glass plate are categorized into three regions with di®erent bright-ness At the initial stages, the gap between the glass plates is (almost) completely ¯lled with liquid, thus most of the incident light can pass the gap without
Fig 1 Experimental setup.
Fig 2 Examples of the surface pro¯le of ground glasses.
Table 2 Surface roughness and estimated gap distance of four types of sand-blasted glass plates The regular one is usually used unless otherwise stated The uncertainty in R zwas
evaluated as a standard deviation of ¯ve measurements on di®erent places.
Regular Fine Medium Rough
R z (m) 39.13 0.8 25.51 1.1 55.89 1.0 81.09 4.6
D (m) 30.1 21.3 47.7 62.7
Trang 4scattering, which corresponds to the brightest area.
At the later stages, the gap becomes empty and the
light is randomly scattered on the ground glass plate
surface, which should be the darkest area In
be-tween, we have found the third area, half-dark
re-gion An example of enlarged image where all
three regions exist is shown in Fig 4 The
bound-aries between wet and semi-dry are clear, but one
between semi-dry and dry is obscure Considering that all liquids well spread on ground glass plates (Table1), we suppose that this half-dark region is a
\partially dried" state, in which thin liquid ¯lm remains on the glass surface, with light scattering being partially suppressed
Thickness of liquid ¯lm in microchannels has been investigated in several papers,14 , 15in which the
(a) Water at 23C
(b) Ethanol at 23C
(c) Acetone at 23C
(d) Heptane at 20C Fig 3 Examples of obtained sequential image during the evaporation process; (a) water, (b) ethanol, (c) acetone, and (d) heptane.
Trang 5thickness is correlated to °ow properties (e.g., the
capillary number and the Weber number) The
sit-uation is di®erent in our case, however, because the
°ow speed largely varies with time and the surface
roughness is comparable with the gap distance, although the apparent thickness ( 10 m from Fig.4) seems to be the same order
3.2 Comparison among liquids
We found clear di®erence in the way of evaporation between water and organic liquids In the case of water, as seen in Fig 3(a), many of half-dark spots appear almost simultaneously at the initial stage, from which partial dry area expands These spots are supposed to be bubbles caused by dissolved gas
In contrast to the water case, evaporation of ethanol starts from the plate edges; at a later stage, semi-dry region expands with complicated branch-ing patterns (Figs 3(b) and 5) Typical thickness (width) of the branches is 0.3–2.0 mm, which is much larger than the gap distance of 40 m The advancing speed of branches is typically 1–10 mm/s (Fig.5) Similar branching patterns are observed in acetone and heptane cases (Figs 3(c)and 3(d))
To see the e®ect of dissolved gas on the evapo-ration pattern, we did a similar experiment with water which was degassed for several minutes under reduced pressure just before the use As shown in Fig.6, the number of spots largely decreased and a
62 s
63 s
64 s
Fig 5 Development of branching patterns in the ethanol case.
Enlarged image (40 40 mm) are shown.
120 s 196 s
(a) Water
120 s 200 s
(b) Degassed water Fig 6 The pattern di®erence between intact water and degassed water In degassed water case, the number of bubble spots is much smaller.
Fig 4 Enlarged image which shows the three regions;
completely wet (the brightest region), completely dry (the
darkest), and semi-dry one (half-dark); ethanol sandwiched
between a ground glass plate and a cover glass.
Trang 6branching pattern similar to organic liquids appears.
Thus, the dissolved gas (air) seems to cause the
spots in water However, organic liquids can dissolve
much more air than water, as shown in Table1 We
did experiments with degassed and aerated ethanol,
and found no di®erence in branching pattern Thus,
the pattern di®erence between water and organic
liquids may be attributed to the wettability
di®er-ence The bubble spots are hard to emerge on high
wettability surface
The dry-up time essentially depends on the
sat-urated vapor pressure Ps At 23C, water takes
about 1200 s to complete the evaporation, while
ethanol takes 170 s, and acetone 40 s At a lower temperature of 19C, ethanol takes 230 s, slightly longer than heptane ( 170 s) although the saturated vapor pressurePsof ethanol is higher than that of heptane Thus other factors, such as surface wettability and viscousity, also a®ect the evapora-tion speed
The uncertainty of dry-up time is about 5 s in an ethanol case, which is obtained from ¯ve experi-ments at the same temperature
3.3 Evaporation dynamics
Each pixel of the obtained grey-scale images is cat-egorized into three classes according to its intensity Although a surface emitting light source is used, it is apparent that the brightness is not very uniform due
to the optical aberration Also, the camera tends to keep the overall intensity constant, which leads to arti¯cal change of brightness We made a correction for intensity data by use of an image of the light source without glass plates as a reference for the spatial uniformity and the time variations Figure7
shows an example of the intensity histogram for a corrected image From these histograms, we can set two thresholds of constant values and categorize the image into three parts, which leads to pseudo-color (blue, green and red) images, as shown in Fig.8
In our image data, 10 cm correponds to 1028 pixels; based on this, we evaluate the area of each region An example of area change with time is
Fig 7 Examples of light intensity histogram for a grey-scale
image of ethanol at 23C, from which we can set two thresholds
indicated by dotted lines (color online).
Fig 8 Example of a pseudo-color image for ethanol at 23C;
(blue) region ¯lled with liquid, (green) partially dried, (red)
dried region (color online).
Fig 9 Change of (a) the chamber pressure and (b) the area of three regions; case of ethanol at 23C, with no active pressure control.
Trang 7shown in Fig 9 for the ethanol case Evaporation
becomes faster when the chamber pressure reaches
the saturated vapor pressure at 70 s The area
increase of evaporating surface (dry–semidry
boundary) due to the complex pattern may also
contribute to the rate change
3.4 Flash boiling
As shown in Fig 10, abrupt and rapid motion of
liquid are occasionally observed during the
evapo-ration, which seems to be °ash boiling in a thin gap;
similar phenomena in conventional microchannels
were reported.16The velocity of front propagation is
40–100 mm/s, much faster than that of the
blanch-ing pattern development This phenomenon should
play an important role in the process of oil retrieve
from transformer cores during the vapor cleansing;
quantitative analysis on the condition-dependent frequency is under way
4 Discussion: Dominant Factors
As the phenomena are complex, there should be many parameters relevant to the evaporation dy-namics, among which we investigate four factors
4.1 Pressure
Using a vacuum controller, we study how the chamber pressure a®ects the evaporation rate Shown in Fig.11is an example for ethanol at 23C, which has the saturated vapor pressurePs of about 7.0 kPa During the initial 70 s, the pressure change is almost the same for all cases, and similar branching patterns develop After Ps is reached, the expansion of the completely dry area starts The dry-up time monotonically shortens with the chamber pressure decrease
4.2 Temperature
Temperature change during the evaporation is negligible in general because the glass plates have much larger heat capacity than the sandwiched liquid
We expect that plates of higher temperature (such as the systems used in vapor washing) lead to faster evaporation, and have con¯rmed it by pre-liminary experiments Reform of the chamber to precisely control the plate temperature is under way
4.3 Surface roughness
Surface roughness can a®ect the gap distance be-tween the plates as well as the wettability We prepared three types of specially sand-blasted glass plates with di®erent roughness, in addition to those used in the previous section Table 2 shows the measured Rz, according to which, we label them
\¯ne", \medium", and \rough"; the label \regular" indicates the commercially available ground glass
As shown in Fig.12, liquid evaporates more rapidly from the gap with rougher surface This seems rea-sonable since liquid can move and evaporate in larger gaps
During the experiments, we sometimes noticed highly asymmetric patterns as shown in Fig 13
Fig 10 Example of two-dimensional °ash boiling phenomena;
ethanol case The three images were taken every 0.2 s The
rapidly changed area is indicated by circles.
Trang 8This indicates that some inhomogeneity of
rough-ness exists on the sand-blasted surface This type of
surface undulation can a®ect the evaporation speed;
further investigation will be done
4.4 Plate warping
We compared the dry-up time of ethanol sandwiched
between °oat and regular ground glass plates
for three di®erent plate sizes The results at 19C
are 150 s for 5 5 cm, 230 s for 10 10 cm,
and 165 s for 15 15 cm plates; the plate thick-ness is all 5 mm Although the quantity of contained liquid increases with the plate size, the dry-up time becomes shorter for the largest plate, a possible reason for which is plate warping Since the plates are clamped at their four corners, the gap for larger plates can be widened during the evaporation
To evaluate how the plate deformation a®ects the evaporation, we did the experiment with plates of di®erent thicknesses, 5 and 10 mm; the area is all
10 10 cm The plates are clamped by metal slips and bolts with a constant force, by use of a torque driver The dry-up time of ethanol sandwiched be-tween a °oat glass plate and a regular sand-blast one
at 14C is 240 s for the plates with 5 mm thick-ness, while 350 s for the 10 mm one This result is understood by the fact that thinner glass plates can warp more easily, and evaporation from the gap is accelerated
5 Conclusion
In order to investigate the washing mechanism of laminated plates with vapor, we have done a series
of visualization experiments of liquid phase change
in narrow gaps under reduced pressure by utilizing ground (sand-blasted) glass plates with several test liquids As the evaporation proceeds, three regions appear; completely wet, completely dry, and the semi-dry one in between Complicated branching
Fig 11 Change of (a) the chamber pressure and (b) the area
of wet region for various control pressures; ethanol at 23C.
Note that a time lag of 70 s exists before the chamber pressure
reaches the set pressure.
Fig 12 Area change of wet region for various surface
rough-ness; ethanol case at 14C.
Fig 13 Examples of a pseudo-color image during the evapo-ration process with a glass plate of medium roughness for eth-anol at 14C; (blue) region ¯lled with liquid, (green) partially dried, (red) dried region (color online).
Trang 9pattern of the semi-dry area is observed in the case
of organic solvents, while tiny nucleate bubbles
ap-pear in the water case due to dissolved air Rapid
motions of liquid similar to °ash boiling are
occa-sionally observed, which should be important in
modeling the evaporation dynamics
The dry-up time depends on many factors, such
as the surface temperature, the chamber pressure,
the vapor pressure, the wettability, the viscosity,
the dissolved gas, the surface roughness, and the gap
distance, among which the e®ects of surface
rough-ness were discussed in some details Note that the
microscale roughness has a simlar order to the gap
distance in our system while e®ects of much smaller
scale roughness are usually investigated in typical
microchannels.10 – 12 The wettability is another
rele-vant factor for phase change in microchannels.17
However, we have not done experiments with
sur-face modi¯cation to change the wettability, and the
discussion about the wettability e®ects remains
in-direct, i.e., comparison among di®erent test liquids
Our ¯nal goal is to improve the vapor washing
process For that purpose, quantitative modeling of
quasi two-dimensional °uid phase change is required
with taking account of various factors
Acknowledgment
We are grateful to Dr Eiichi Kato and his colleagues
at Central Research Laboratory, NEOS Company
Ltd., for stimulating discussion and encouragement
We also thank Fuji Manufacturing for providing
sand-blasted plates of various surface roughness A
part of this work is ¯nancially supported by JSPS
KAKENHI (No 15K05826)
References
1 Web page of US Environmental Protection Agency,
https://www.epa.gov/pcbs.
2 Y Kuroiwa, Disposal plant of electric transformers
(in Japanese), Indus Machinery 7 (2010) 57 –61.
3 S G Kandlikar, Fundamental issues related to °ow
boiling in minichannels and microchannels, Exp.
Therm Fluid Sci 26 (2002) 389 –407.
4 S G Kandlikar, History, advances, and challenges
in liquid °ow and °ow boiling heat transfer in microchannels: A critical review, J Heat Transf.
134 (2012) 034001-1 –15.
5 Y Yasumoto, Y Okura and M Matsumoto, Quasi two-dimensional boiling under reduced pressure, Proc 25th Int Symp Transport Phenomena, Krabi, Thailand (2014), p 134.
6 Y Yasumoto, Y Okura, K Ogawa and M Matsu-moto, Quasi two-dimensional evaporation and boiling under reduced pressure, Proc 5th Int Symp Micro Nano Tech., Calgary, Canada (2015), pp S8 –115.
7 K Ogawa, Y Yasumoto and M Matsumoto, Quasi two-dimensional boiling under reduced pressure, Proc 1st Paci¯c-Rim Thermal Eng Conf., Hawaii, U.S.A (2016), p 14530.
8 D R Lide (ed.), CRC Handbook of Chemistry and Physics, 72nd (CRC Press, Boston, 1992).
9 IUPAC Solubility Data Series, Vols 7 and 10 (1982).
10 M Ojha, A Chatterjee, G Dalakos, P C Wayner
Jr and J L Plawsky, Role of solid surface structure
on evaporative phase change from a completely wetting corner meniscus, Phys Fluids 22 (2010) 052101-1 –15.
11 G Zhou and S.-C Yao, E®ect of surface roughness
on laminar liquid °ow in micro-channels, App Therm Eng 31 (2011) 228 –234.
12 J.-J Zhao, Y.-Y Duan, X.-D Wang and B.-X Wang, E®ect of nanostructured roughness on evap-orating thin ¯lms in microchannels for Wenzel and Cassie-Baxter states, J Heat Transf 135 (2013) 041502-1 –9.
13 B Bhushan and Y Jung, Micro- and nanoscale characterization of hydrophobic and hydrophilic leaf surfaces, Nanotechnology 17 (2006) 2758 –2772.
14 K Moriyama and A Inoue, Thickness of the liquid
¯lm formed by a growing bubble in a narrow gap between two horizontal plates, J Heat Transf 118 (1996) 132 –139.
15 Y Zhang, Y Utaka and Y Kashiwabara, Formation mechanism and microlayer in microchannel boiling system, J Heat Transf 132 (2010) 122430-1 –7.
16 G Hetsroni, A Mosyak, E Pogrebnyak and Z Segal, Explosive boiling of water in parallel micro-channels, Int J Multiphase Flow 31 (2005) 371 –392.
17 C Choi, J.-S Shin, D.-I Yu and M.-H Kim, Flow boiling behaviors in hydrophilic and hydrophobic microchannels, Exp Therm Fluid Sci 35 (2011)
816 –824.