Open Journal of Fluid Dynamics, 2013, 3, 48-54 Published Online July 2013 (
Experimental Study on Latent Heat Storage
Characteristics of W/O Emulsion by Ultrasonic
Wave Impression
Shin-ichi Morita1, Yasutaka Hayamizu1, Akihiko Horibe2, Naoto Haruki2,
Hideo Inaba3, Issei Higashi4
1Department of Mechanical Engineering, Yonago National College of Technology, Yonago, Japan
2Graduate School of Natural Science and Technology, Okayama University, Okayama, Japan
3Shujitsu University, Okayama, Japan
4Edit Inc., Fukuoka, Japan
Received May 24, 2013; revised June 1, 2013; accepted June 8, 2013
Copyright © 2013 Shin-ichi Morita et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The flowable latent heat storage material like Oil/Water type emulsion, microencapsulated latent heat material-water
mixture or ice slurry, etc., is enabled to transport the latent heat in a pipe. Supercooling phenomenon of the dispersed
latent heat storage material in continuous phase is obstructed by a latent heat storage. The latent heat storage rates of
dispersed waterdrops in W/O (Water/Oil) emulsion are investigated experimentally in this study. The waterdrops in
emulsion have the diameter within 3 - 25 μm, the averaged diameter of waterdrops is 7.3 μm and the standard deviation
is 2.9 μm. Supercooling release of waterdrops in emulsion is examined by short time impressing of the ultrasonic. The
direct contact heat exchange method is chosen as the phase change rate evaluation of waterdrops in W/O emulsion. The
supercooled temperature is set as parameters of this study. The previous obtained experimental result, as the condition
without impressing ultrasonic wave, showed that the 35 K or more degree from melting point brings 100% latent heat
storage rate of W/O emulsion. It is clarified that it is possible to reduce 20 K of supercooling degree by impressing the
Keywords: Heat Storage; Latent Heat; W/O Emulsion; Direct Contact Heat Exchange; Ultrasonic
1. Introduction
Recently, much attention has been paid to research on the
latent heat storage system. Using of ice heat storage sys-
tem brings an equalization of electric power demand,
because it will be solved that the electric-power-demand-
concentration on day-time of summer by the air condi-
tioning. The flowable latent heat-storage system can
scale down the pipe size due to the augmentation of the
transported heat. Slurry type latent heat-storage (phase
change material) system is one of the available cooling
systems, which can take the place of the sensible (water)
heat-storage system. Phase change emulsion [1] and
phase change microcapsule slurry [2], clathrate hydrate
slurries [3] etc. are mentioned as a typical slurry type
latent heat storage substance. It is known that a super-
cooling phenomenon disturbs a latent heat-storage [4,5].
A supercooling rate increases with reduction of disper-
sion droplet diameter except for the microencapsulated
latent heat storage substance [6]. Some studies of super-
cooling by bulk water have been reported. The pure wa-
ter layers were cooled from upper side by five kinds of
copper cooling surfaces (electrolytic polished, nickel-
plated, buffed, porous, gold-plated copper disks) under
various cooling rate (0.05 - 0.5 K/s), and the degrees of
supercooling at the appearance of dendritic ice were
measured by A. Saito, et al. [7]. As a result, the porous
surface showed the lowest degree of supercooling. S.
Okawa, et al. [8] investigated the effect of the electric
field on freezing of supercooled water, experimentally. It
was found that supercooled water freezes instantly by
applying the electric charge at the voltage less than 100
VDC. The experimental results indicated that ultrasonic
vibration strongly promotes the freezing of supercooled
water, for both pure water and tap water by T. Inada, et
al. [9]. Furthermore, they found that ultrasonic vibration
opyright © 2013 SciRes. OJFD
is also effective for making ice slurry. H. Inaba, et al. [10]
reported the supercooling degree for the test tap water
sample that was set in test tubes. The supercooling de-
gree was decreased with an increase in the mass ratio of
ice nucleating substance. However, it was clarified that
the supercooling degree for each test sample increased by
repeating the process of freezing and melting. T. Ho-
zumi, et al. [11] studied the mechanism of the freezing of
the supercooled water under the effect of the ultrasonic
wave. It was shown that the impression of the ultrasonic
wave (0.28 W/cm2) brought the generation of ice nuclea-
tion on every experimental condition. All of these inves-
tigations were carried out by using the bulk water.
The freezing of waterdrop in W/O emulsion brings
volume expansion. T. Inada et al. [12] measured the
amount of freezes of the waterdrop in a W/O emulsion
by microscope observation on the cooling rate of 0.067
K/sec. The counting of freeze numbers of the fine water-
drops is one of methods of measuring the degree of su-
percooling in W/O emulsion. However, this method does
not measure the amount of latent heat storages itself. H.
Inaba and S. Morita [13] showed the cold heat-release
characteristics of emulsion by air-emulsion direct contact
heat exchange method. By the same method, S. Aoyama
and H. Inaba [14] described that the direct contact heat
exchange characteristic between ice (averaged diameter
3.10 mm) and hot air. It can be said that the direct con-
tact heat exchange method is the effective method for
measuring the amount of latent heat storage. This study
shows the experimental results of the latent heat-storage
rate (supercooling rate of dispersed waterdrops) of the
water in oil (W/O) emulsion by direct contact heat ex-
change method.
P. Schalbart et al. [15] investigated the low-energy
emulsification method of Oil-in-Water (O/W) emul-
sions that have a narrow droplet size distribution in the
range of 200 - 250 nm. These nano-emulsions of tet-
radecane in water showed stability against sedimentation
and creaming for more than 6 months and low viscosity
(2 - 4 times than that of water). The mass percentages of
the nano-emulsion are 20% tetradecane, 6% surfactant
and 74% water. The waterdrops of this research are ex-
isted in 3 - 25 μm in diameter, and the average diameter
is 7.3 μm and the standard deviation is 2.9 μm. It is
thought that the stabilization of a nano-emulsion exceeds
a microemulsion. In this research, it is adopted from the
reason that micro-emulsion is easy to produce and close
to utilization. It was found that the emplacement of the
phase-change materials (PCMs) has no effect on the heat
transfer inside the emulsion. Test emulsion set as an ex-
pected temperature was supplied to the heat storage tank,
and it cooled with the low cooling rate. Authors’ previ-
ous research [16], as the condition without impressing
ultrasonic, showed that the 35 K or more degrees from
melting point brings 100% latent heat storage rate of
W/O emulsion. The supercooled temperature and the
cooling rate are set as parameters of this study. The
evaluation is performed by comparison between the re-
sults of this study and the past one.
2. Experimental Apparatus and Procedure
2.1. Test Emulsion
Figure 1 shows the external appearance and the micro-
graph (×400) of test emulsion. The emulsion has fluidity,
even if the dispersed water particles are the solid phase.
Test W/O emulsion consists of tap water as dispersed
phase, silicon oil as continuous phase and surfactant. The
silicone oil of continuous phase is used TSF451-10
(Momentive Performance Materials Holdings Inc.). The
emulsification is carried out by nonionic surfactant (DKS
NL-Dash403: Daiichi kogyo seiyaku CO. Ltd.). The
mass composition ratio of test emulsion is tap water 5%,
silicon oil 94% and surfactant 1%. The specific heat of
the emulsion used for data evaluation is calculated using
the additive property law. The base data of water and
silicon oil is used publication data [17,18]. The specific
heat of surfactant is used the actual value that measured
by water calorimeter. For example, the specific heat of an
emulsion is 1.90 kJ/(kg·K) at 298 K. The diameter of
distributed waterdrops (N = 300) in test emulsion exists d
= 3 - 25 μm, the average diameter of dm = 7.3 μm and
standard deviation is S = 2.9 μm. The stable time of test
emulsion is at least 210 minutes, so that the cooling of
experiment is carried out within the stable time.
2.2. Experimental Apparatus
Figure 2 shows the schematic diagram of experimental
apparatus for measuring latent heat storage rate of emul-
sion by direct contact heat exchange method. Experi-
mental apparatus consists of the test section and the hot
air supply line. The air supplied from a compressor is
sent to the test section through a surge tank, an air-dryer,
a heater, and a flow meter. Dried hot air is sent to the
Figure 1. Appearance and micrograph of emulsion.
Copyright © 2013 SciRes. OJFD
Copyright © 2013 SciRes. OJFD
Section view A-A'
Ultrasonic transduce
Test sectionComp.
Pressure gauge
Surge tank
Bubbling deviceAir dryer
Thermo couple
Air bubbles
Acrylic resin
TestW/O Emulsion
Test section
Ultrasonic transducer
Figure 2. Experimental apparatus.
Table 1. Experimental conditions.
bubbling device that set in the bottom of the test section.
The generated air bubble is directly heat exchanged by
contact with a test sample. The inlet air temperature is
fixed to 303 K. The heat quantity is calculated by a dif-
ference between inlet and outlet air temperature. All tem-
perature measurement is performed by K-type thermo
couples which have diameter 0.18 mm. The test section
is made of transparent acrylic resin, and it has an inner
diameter 100 mm, a height 550 mm and 10 mm in thick-
ness. The 3 units of ultrasonic transducer (Alex Corpora-
tion, ultrasonic oscillator NMS150-3FP, the oscillating
frequency 28, 50 or 80 kHz, max. output 150 W) are at-
tached to the acrylics cylinder pipe. The whole of test
section is stored in a low-temperature-controlled room,
and is made into the structure which prevents the influ-
ence by the temperature change by freezer on-off by
covering with thermal insulation. Table 1 indicates the
experimental conditions of this study.
Frequency of ultrasonic f 28, 50, 80 kHz
Air mass flow Ga 0.06465 kg/min
Air pressure P 0.1 MPa
Inlet air temperature Tain 303 K
Initial temp. of emulsion Tei 283 ± 0.5 K
Temp. of freezer Tf 233 K
Cooling rate uc 0.0015 - 0.0045 K/sec
Height of emulsion Z 173 mm
Degree of supercooling ΔT 5 - 20 K
Adding time of ultrasonic τu 20 sec
6) Measure the data and calculate the quantity of heat.
The experiment of this research is conducted by the
following procedures. 2.3. Quantity of Heat Storage Calculation
1) Set 233 K in a temperature control room; Figure 3 shows the time history of test emulsion tem-
perature on experiment. The variation of temperature is
shown from the emulsion set time into test section. The
temperature of an emulsion decreases with time and
reaches the melting point of water 273 K in about 800
seconds. The ultrasonic is impressed 20 seconds when
the emulsion temperature reaches to 268 K. Test emul-
sion is continuously cooled till reach to the supercooling
2) Set sample into the test section;
3) Cool the sample below to the melting point of the
latent heat storage material;
4) Impress the ultrasonic wave at 268 K (20 sec, 75%
of 150 W × 3 units) and keep cooling;
5) Heat exchange by the temperature controlled dry air
at the test section;
02000 4000 6000 800010000
Time τ[sec]
=5 [mass%]
=3600 [sec]
= 20 [sec]
f = 28 [kHz]
ΔT =10 [K]
=0.0029 [K/sec]
add ultrasonic
air bubbling start
latent heat stora
Latent heat release
Figure 3. Time history of Te.
degree 10 K (263 K) in about 4800 seconds. Test emul-
sion’s mass used for the experiment is me = 1.5 kg and
the temperature controlled room temperature is set Tf =
233 K. The cooling rate uc of this research is about uc =
0.0015 - 0.0045 K/sec. After the completion of cooling, a
bypass line is changed and the dried air (303 K) is flowed
to air bubbling device in test section. The temperature of
test emulsion rises by jet of an air bubble. The constant
temperature near the melting point of the water resulting
from latent heat release is observed in about 8200 -
10,000 seconds.
The amount of latent heat storages is measured with
the measurement data at the time of heat release. The
exchanged heat quantity of air is calculated by Equation
(1), in which the mass flow rate of air Ga, specific heat
Cpa and the temperature difference of inlet and outlet air
(Tain Taout). The sensible heat of test emulsion is derived
by Equation (2), in which specific heat of emulsion Cpe,
mass of emulsion me and temperature difference of emul-
sion by time step (Ti Ti 1). The quantity of latent heat
storage of emulsion is calculated by Equation (3). The
amount of theoretical latent heat of dispersed water in
emulsion is expressed by Equation (4). All condition of
this experiment is carried out by C = 5 mass% and me =
1.5 kg, so that the theoretical latent heat of test emulsion
is calculated Qlt = 25 kJ by using latent heat of ice Qm =
333.7 kJ/kg. Equation (5) is defined as the latent heat
storage rate of emulsion.
totapaain aout
QQ Q (3)
lte m
100QC mQ (4)
 (5)
The measurement accuracy of the experiment was au-
thorized in advance by using the silicone oil as the test
sample. The accuracy approval experiment of the direct
heat exchange method was conducted 6 times by carry-
ing out 10 K temperature rise of the silicone oil. These
pre-experimental results showed the calorimetric meas-
urement accuracy ±1.7% of this research of a sensible
heat base. It is checked by previous test that the amounts
of sample heating by ultrasonic impression (20 seconds),
is less than 3% (0.75 kJ) of the latent heat of emulsion.
3. Experimental Results and Discussions
Latent Heat Storage Rate of Test W/O Emulsion
Figure 4 indicates the variation of the quantity of latent
heat storage Ql of test W/O emulsion with the supercool-
ing degree ΔTsc on each cooling rate uc. The quantity of
latent heat storage Ql of test W/O emulsion increases
with increasing of the supercooling degree ΔTsc. The
solid line in a figure shows the averaged value by the
experimental data of this study. The maximum line and
minimum line by an experimental value are shown in the
figure with the dashed line. It seems that the correlation
with a cooling rate is not seen as for the amount of latent
heat storages. A loose increase is observed until the
amount of latent heat storages reaches the supercooling
degree of about ΔTsc = 3 K. In supercooling degree 3 - 12
K, the rate of increase of the amount of latent heat stor-
ages is large.
Figure 5 shows the relationship between the number
of freezed waterdrops in test emulsion, that converted
from the amount of latent heat storage, and the super-
cooling degree. The number of freeze waterdrops is in-
creased with increasing of the supercooling degree.
Table 2 is indicated the achievements of passed re-
searches about bulk supercooling water. As for the re-
search on bulk water, the influence about the type of wa-
ter, the heat transfer surface, the volume, the cooling rate
has been considered and the method of supercooling re-
Figure 6 shows the variation of probability of freezing
φ with the supercooling degree ΔTsc. Probability of freez-
ing φ is calculated by probability distribution function for
the purpose of performing comparison with the data of
other researchers about supercooling release of bulk wa-
ter. The broken and chain lines in a figure show the re-
sults of the bulk water of (a) Okawa et al.: Electricity (b)
Inada et al.: Ultrasonic and (c) Inaba et al.: Ice nucleat-
ing substance. The data of (b) Inada et al. is the result of
tap water by using the ultrasonic impressing. The data of
(c) Inaba et al. shows the result of the same heat transfer
surface and a cooling rate. The W/O emulsion’s prob-
ability of freezing φ reaches to about 100% in the super-
cooling degree ΔTsc of more than 15 K. As compared with
the same conditions of bulk water, it is understood that
the supercooling degree of a W/O emulsion is large and
it is difficult to carry out a latent heat storage.
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579 111315171921
0510 15 20 25 30 35
=0.0015-0.002 [K/sec]
=0.002-0.0025 [K/sec]
=0.0025-0.003 [K/sec]
=0.003-0.0035 [K/sec]
=0.0035-0.004 [K/sec]
=0.004-0.0045 [K/sec]
=0.0045-0.005 [K/sec]
Max data
Min data
Averaged data
Figure 4. Variation of Ql with ΔTsc. Figure 5. Relationship between N and ΔTsc.
Table 2. Achievement of passed researches on bulk water.
Authors Ref. Water type Heat transfer surface Vol. cm3ΔTsc KUc K/sec Supercooling release method
(a) Okawa et al. [8] Pure water Acrylic/Water 0.005 4 - 6 0.0037 Electricity
(b) Inada et al. [9] Tap water Copper/Water 1.3 1.5 - 4.50.039 - 0.049Ultrasonic
(c) Inaba et al. [10] Distilled water Oil/Water 1.0 2 - 4 0.005 - 0.008Ice formation
(d) Oil/Water 1.0 3 - 7 - Piston
(e) Oil/Water/Oil 0.05 4 - 10 - Piston
(f) Polypropylene/Water1.0 2 - 9 0.0017 Ultrasonic
Hozumi et al. [11] Ultra pure water
Glass/Water 100 3 - 5 - Waterdrop
Figure 6. Relationship between φ and ΔTsc.
Figure 7 indicates the relationship between φ and ΔTsc
on each experimental condition of impressing or without
impressing the ultrasonic. It is clarified that it is possible
to reduce 20 K of supercooling degree by impressing the
ultrasonic wave.
release of W/O emulsion by ultrasonic impression, and
the following conclusions were obtained:
1) It is suggested that the W/O emulsion’s probability
of freezing by using ultrasonic reaches to about 100% in
the supercooling degree of more than 15 K.
2) It is clarified that as compared with the same condi-
tions of bulk water, the supercooling degree of a W/O
emsion is large.
4. Conclusions
It was performed by experimental study of supercooling
Figure 7. Variation of φ with ΔTsc.
It is revealed that ultrasonic impression is the method
of supercooling release of W/O emulsion.
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