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O emulsion in ice storage (effective method to propagate supercooling dissolution)
international journal of refrigeration 31 (2008) 832–840
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Formation of high performance ice slurry by W/O emulsion in ice storage (effective method to propagate supercooling dissolution) Koji Matsumotoa,*, Kazuki Sakaeb, Hirofumi Yamauchic, Yoshikazu Teraokad a
Department of Precision Mechanics, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Sony Corporation, 1-7-1 Konan, Minato-ku, Tokyo 108-0075, Japan c Graduate Student of Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan d Aoyama Gakuin University, Department of Mechanical Engineering, Research Associate, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan b
article info
abstract
Article history:
This study focused ice slurry formation in an ice storage system using W/O emulsions with
Received 14 July 2007
70 and 80% water contents. Emulsions consisted of a silicone oil–water mixture with a small
Received in revised form
amount of amino-group-modified silicone oil additive. Ice slurry was formed by cooling the
5 September 2007
emulsion without ice adhesion to the cooling wall, as water in the emulsion did not directly
Accepted 24 October 2007
contact the cooling wall. As the structure of W/O emulsion slowed the propagation rate of
Published online 28 October 2007
supercooling dissolution, voltage and ultrasonic wave were applied to the W/O emulsion to propagate dissolution more quickly and decrease maximum supercooling degree, respec-
Keywords:
tively. Thus, the effects of voltage and ultrasonic wave applications on propagation rate
Ice slurry
were clarified.
Emulsion
ª 2007 Elsevier Ltd and IIR. All rights reserved.
Water Oil Process Enhancement Electric field Ultrasound
Formation de coulis de glace hautement performants a` l’aide d’une e´mulsion d’eau/huile dans l’accumulation de glace (moyen efficace de promouvoir la dissolution lors du surrefroidissement) Mots cle´s : Coulis de glace ; E´mulsion ; Eau ; Huile ; Proce´de´ ; Ame´lioration ; Champ e´lectrique ; Ultrason
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (Y. Teraoka). 0140-7007/$ – see front matter ª 2007 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2007.10.006
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international journal of refrigeration 31 (2008) 832–840
1.
Introduction
Ice storage systems are superior to many other types of thermal storage systems because the amount of thermal storage per unit volume is larger than those of other storage systems, allowing the effective use of energy. Interest in dynamic ice storage systems has developed rapidly. In a dynamic system, ice slurry with good fluidity used as a thermal storage material has many practical characteristics but also some problems, such as a decrease in latent heat fusion due to a reduction in freezing point and ice adhesion to the cooling wall. Thus, many studies on ice slurries have been conducted (Egolf and Kauffeld, 2005; Pronk et al., 2005; Davies, 2005; Oda et al., 2004; Matsumoto et al., 2000; Hirata et al., 2000). The authors of the present paper also have studied the formation of ice slurries using a water-in-oil (W/O) emulsion, which overcomes some of the problems of dynamic ice storage systems. Matsumoto et al. (2006) demonstrated the promise of W/O emulsions for dynamic ice storage. The use of a W/O emulsion allowed stable ice slurry formation in a stainless vessel, despite the difficulties previously encountered with ice adhesion to the cooling wall. Additionally, optimum composition ratios of water and oil also were proposed. But it was confirmed that the propagation rate of supercooling dissolution for W/O emulsions was much slower than that for a liquid such as water. A recent report also has described the factors governing the propagation rate and maximum supercooling degree based on probability (Matsumoto et al., 2007). To dissolve supercooling pure water with very small volume, application of voltage and irradiation of ultrasonic wave have been investigated (Hozumi et al., 1999a,b, 2003a,b). Because the characteristics of W/O emulsions are very different from those of pure water, the results of experiments using pure water cannot be directly applied to W/O emulsions. Thus, the validity of application of voltage and irradiation of ultrasonic wave to W/O emulsions must be discussed. In the present study, in order to effectively improve the propagation rate of supercooling dissolution, ice nucleus was added to a W/O emulsion immediately after application of voltage or ultrasonic wave was irradiated to a W/O emulsion.
tap water; water contents of the emulsions were 70 and 80%. The kinematic viscosity of the silicone oil was 10 mm2/s at 25 C. An amino-group-modified silicone oil (0.9 vol%) was used as a surface-active agent. Hereafter, the word ‘‘emulsion’’ represents a W/O emulsion and ‘‘viscosity’’ represents kinematic viscosity. For example, an emulsion with a water/ oil volumetric ratio of 8:2 is designated (8:2).
2.2. Measurement of propagation rate of supercooling dissolution for ice nucleus charging after voltage application Generally, when voltage is applied to an emulsion, the emulsion is demulsified (Sherma, 1968). During the demulsification process, the dispersed phase separates from the emulsion and condenses (flocculates), followed by coalescence of the condensed dispersed phase. This paper describes a method of partial demulsification to promote the propagation of supercooling dissolution. The apparatus for measurement of the propagation rate of the emulsion cooled by a Peltier module is shown in Fig. 1.The apparatus consists mainly of the Peltier module, copper plate, electrodes and CCD camera. The freezing point of the W/O emulsions used was 0 C. A 1-mm-thick copper plate was used for cooling on the Peltier module. Grease with high thermal conductivity was applied to the gap between the plate and the module. The emulsion with 1 C was spread on the plate at a thickness of 0.5 mm and surface area of 40 40 mm. The emulsion on the plate was cooled by the module immediately after voltage application to the emulsion for 1 s. A pair of electrodes consisted of an aluminum rod with a diameter of 3 mm; the distance between the electrodes was 30 mm. The voltage applied to the samples of 70 and 80% water contents was 10 and 20 V, respectively. When the emulsion temperature reached the set temperature (5 C), supercooling dissolution of the emulsion began to propagate due to addition of one ice nucleus. The ice nucleus was hemispherical in shape with a diameter of about 8 mm. Since the pair of thermocouples (type T) may act as a trigger for dissolution of supercooling, temperature at the center of the back of the plate was measured. Pre-experiments indicated that the temperature of the emulsion on the plate was always
40 mm
2.
Experiment
2.1.
Composition of W/O emulsions
The composition ratios of the W/O emulsions, expressed as a volumetric ratio of water and oil, are shown in Table 1. In this experiment, the oil and water used were silicone oil and
6 mm
Emulsion Ice nucleus
30 mm
4.5 mm
Electrodes
40 mm
Observation region
Table 1 – Composition ratios of W/O emulsions Tap water (ml) Silicone oil (ml) Additive (ml) Ratio of water to oil
880 220 10 8:2
Additive: amino-group-modified silicone oil.
770 330 10 7:3
Copper plate
Peltier module
Fig. 1 – Experimental apparatus for measurement of propagation rate using a Peltier module.
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international journal of refrigeration 31 (2008) 832–840
greater than the back temperature of the plate by about 1 C. Thus, the representative emulsion temperature was defined by correcting the back temperature based on the temperature difference (1 C). The propagation of the dissolution process was observed by CCD at 100-fold magnification in a square region with an area of 6 mm 4.5 mm, as shown in Fig. 1. Simultaneously, the time required to reach a fixed propagation distance (6 mm) was measured. The propagation rate was obtained by dividing the time into the fixed distance. A new emulsion was used for each measurement because formation history affected the state of the emulsion. Because there was a possibility that propagation rate depended on the size of the water droplets (dispersed phase), the size distributions of the water droplets with and without voltage application were measured by CCD at 300-fold magnification at three arbitrary points in the emulsion. The number of water droplets sampled for one condition was about 200. To investigate the propagation rate of supercooling dissolution during the process of ice slurry formation, the probability of propagation within a certain time period was measured. The same method also was used to investigate the maximum degree of supercooling. Since a detailed description of the experimental apparatus and procedure has been reported previously by Matsumoto et al., (2007), only a brief description is provided here. When the emulsion (1.1 L) cooled by addition of cold brine (5.4 C) with stirring (250 rpm) reached the set temperature (0 C), an ice nucleus was added to the emulsion to initiate propagation of dissolution. And then, temperature of the emulsion decreased gradually. After that, the temperature returned to the freezing point (0 C) very slowly after it reached a minimum temperature (maximum supercooling degree). Start of propagation of dissolution is the time immediately after temperature rise of the emulsion and end of propagation of dissolution is just the time when the temperature of the emulsion returns to the freezing point. The ice nucleus was hemispherical in shape with a diameter of about 8 mm. A pair of electrodes was then immediately inserted into the emulsion, as shown in Fig. 2(a) and voltage was applied. The dimensions of the electrodes are shown in Fig. 2(a). The distance between the electrodes is 12 mm. Voltage was applied until the end of propagation of dissolution. The electrodes were removed from the emulsion immediately after end of voltage application. Propagation of dissolution was considered to be completed when bulk temperature of the emulsion returned to 0 C.
2.3. Measurement of propagation rate of supercooling dissolution during ultrasonic wave irradiation The experiments to discuss the effect of irradiation of ultrasonic wave on propagation rate were carried out. Irradiation of ultrasonic wave to the emulsion often caused a phase transition of the emulsion (Matsumoto et al., 2006). However, the emulsion could be returned to its original state by stirring if partial phase transition occurred. Thus, efforts were made to minimize phase transition. Optimal irradiation conditions are discussed. Stirring was stopped after the emulsion was cooled to 1 C. The emulsion was exposed to ultrasonic wave immediately after stirring was stopped. The emulsion temperature at irradiation was 1 C, because of the rise in temperature of the
emulsion due to irradiation. Irradiation was directed at the center of the vessel, as shown in Fig. 2(b), and irradiation time was 1 s. Stirring began again immediately after irradiation was completed. The frequency and amplitude of the ultrasonic wave were 20 kHz and 1–40 mm, respectively. The intensity of the ultrasonic wave was proportional to its amplitude.
3.
Results and discussion
3.1. Propagation rate of supercooling dissolution with ice nucleus charging after voltage application Propagation rates were measured using the apparatus as shown in Fig. 1. Dissolution of supercooling of the emulsion was propagated radially from ice nucleus toward the right side of the copper plate as shown in Fig. 1. Results are shown in Fig. 3. The result for each composition ratio was an average value of 10 measurements. From comparison of the cases of voltage application with that of no application, propagation rate increased by about 30% at each composition ratio. Average water droplet size for (7:3) with and without voltage application was 66.36 and 62.86 mm, respectively, a relatively small difference. But the existence of larger volume water droplets with a droplet size in the range of 200–500 mm was confirmed during voltage application, although this did not occur without voltage application. Thus, droplet size distributions were rearranged on a volume ratio basis, with volume ratio defined as (total volume of water droplets existing in a certain size range)/(total volume of all water droplets) 100(%). Applied voltage was set to 10 V. An example of the distribution rearranged for (7:3) is shown in Figs. 4 and 5, which correspond to the cases of no voltage and voltage application, respectively. Figs. 4 and 5 show that, for the case of voltage application, the volume ratio of larger water droplets was dramatically greater compared to no voltage. A previous study reported that larger ice particles formed by dissolution were effective for increasing propagation rate (Matsumoto et al., 2007). This result is in agreement with the data presented in Figs. 4 and 5. Since larger water droplets due to voltage application lead to formation of larger ice particles, propagation rate could be increased by voltage application as shown in Fig. 3. For (8:2), a similar tendency was obtained. Since applied voltages were different, the case of (7:3) could not be compared directly with (8:2). However, Fig. 3 indicates that propagation rate for (8:2) had a tendency to decrease compared with (7:3). One reason for the decrease in propagation rate could be the multiplier effect of viscosity and oil thickness of the emulsion, causing a decrease in propagation rate.
3.2. Propagation rate of supercooling dissolution with ice nucleus charging after voltage application during ice formation Since propagation rate could be increased by voltage application, as shown in Fig. 3, the method of ice nucleus charging after voltage application to improve propagation rate in the ice slurry was examined. In pre-experiments, an applied voltage was discussed, varying the voltage in the range of 0–250 V, and it was determined to be 50 V.
international journal of refrigeration 31 (2008) 832–840
12 mm
a
835
Insulation coating
200 mm
3 mm
4 mm 10 mm
Electrodes
Electrodes
160 mm
Emulsion Electrodes
15 mm
Stirrer rod In the emulsion Thermometer Ice formation vessel
b
Ultrasonic generator
Emulsion
Ice formation vessel Fig. 2 – Setting position and shape of electrodes.
Thirty experiments were conducted at each composition ratio. The propagation rate and maximum supercooling degree were examined based on probability and by using the formula ([frequency of result satisfying given condition]/[frequency of all experiments] 100(%)), in the same manner as previously reported (Matsumoto et al., 2007). To estimate propagation rate, the times from addition of the ice nucleus
to the start of propagation of dissolution and from the start of propagation to the end were measured. For comparison, the results of ice nucleus charging without voltage application are shown. Measurement results for the time period from addition of the ice nucleus to the start of propagation are shown in Figs. 6 and 9, which represent the composition ratios of (7:3) and (8:2), respectively. Results for the time period from the
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international journal of refrigeration 31 (2008) 832–840
15
40
Volume ratio [ ]
Propagation rate [ m/s]
50
30
20
10
10
5
Applied voltage = 10 or 20V Applied voltage = 0V
0
7:3
0
8:2
0
100
200
300
400
500
Water droplet size [ m]
Fig. 3 – Relationship between propagation rate and composition ratio.
Fig. 5 – Distribution of water droplet sizes with applied voltage for (7:3).
start of propagation to the end are shown in Figs. 7 and 10, which represent (7:3) and (8:2), respectively. The results for maximum supercooling are shown in Figs. 8 and 11, which correspond to (7:3) and (8:2), respectively. The average value for each result is shown as ‘‘A.V.’’ and is used to estimate the result. In the horizontal axes, for example, ‘‘0–5’’ means 0 t < 5 min. The case of (7:3) was discussed. Fig. 6 shows that the average time period from addition of the ice nucleus to the start of propagation for ice nucleus charging after voltage application was shorter by 1/3 than for ice nucleus charging without voltage application. This was caused by the coalescence of water droplets due to voltage application, as mentioned above. Similarly, Fig. 7 shows that the average time from the start of propagation to the end for ice nucleus charging after voltage application was about 1/4 shorter than that for ice nucleus charging without voltage. This reason for this is the same as for the results shown in Fig. 6 (i.e., since larger water droplets formed by voltage application became larger ice particles due to dissolution of supercooling, those larger ice particles propagate dissolution effectively). From Figs. 6 and 7, upon voltage application, both time periods from addition of the ice nucleus
to the start of propagation and from the start of propagation to the end were within 5 min. Fig. 8 shows that the average maximum supercooling degree for ice nucleus charging after voltage application could be decreased significantly compared to ice nucleus charging without voltage application. As mentioned in a previous report (Matsumoto et al., 2007), since the emulsion cooling rates were the same for a fixed composition ratio, the maximum supercooling degree was approximately proportional to the time period from addition of the ice nucleus to the start of propagation. Therefore, since the time period from addition of the ice nucleus to the start of propagation decreased significantly for ice nucleus charging after voltage application, as shown in Fig. 6, the average maximum supercooling degree also decreased remarkably. For cases of voltage application, the maximum supercooling degrees were within 0.5 C. To estimate propagation rate, the total time from addition of the ice nucleus to the end of propagation was examined. As shown in Figs. 6 and 7, the average total time for (7:3) in the case of ice nucleus charging after voltage application was over 70% shorter than that for ice nucleus charging without
15
100 Ice nucleus + Voltage (A.V. 3 min) Ice nucleus (A.V. 9 min)
10
)
5
Probability (
Volume ratio [
]
80
60
40
20
0
0
100
200
300
400
500
Water droplet size [ m] Fig. 4 – Distribution of water droplet sizes without applied voltage for (7:3).
0
0∼5
5∼10
10∼15
15∼20
Time (min) Fig. 6 – Probability distribution of time taken from addition of ice nucleus to start of propagation for (7:3).
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international journal of refrigeration 31 (2008) 832–840
100
100 Ice nucleus + Voltage (A.V. 4 min) Ice nucleus (A.V. 16.3 min)
Probability (
) Probability (
Ice nucleus (A.V. 10.1 min)
80
)
80
Ice nucleus + Voltage (A.V. 3.5 min)
60
40
40
20
20
0
60
0 0∼5
5∼10
10∼15
15∼20
20∼
0∼5
Fig. 7 – Probability distribution of time taken from start of propagation to end for (7:3).
voltage application. Thus, during the ice slurry formation process, adding the ice nucleus after voltage application was very effective to improve propagation rate. Figs. 9–11 show the results for (8:2). The average times from addition of the ice nucleus to the start of propagation, from the start of propagation to the end, and the average maximum supercooling degree decreased significantly compared to only ice nucleus charging. Moreover, since the average total time decreased by about 85% in this composition ratio, voltage application was also effective to improve propagation rate. The propagation rate (average total time) for (7:3) (Figs. 6 and 7) was compared with that for (8:2) (Figs. 9 and 10). For ice nucleus charging after voltage application, the difference in propagation rates for (7:3) and (8:2) was very small. This may be because viscosity of (7:3) emulsion is nearly equal to that of (8:2). For both composition ratios shown in this paper, the decrease in viscosity of the emulsions was caused by an increase in water droplet size due to voltage application. Thus, since the difference in viscosity between (7:3) and (8:2) became much smaller, the difference in the propagation rates between both composition ratios was also very small.
15∼20
20∼25
3.3. Propagation rate of supercooling dissolution after ultrasonic wave irradiation during ice formation The propagation rate and maximum supercooling degree were examined based on probability. To estimate propagation rate, the same procedure described in Section 3.2 was used. Irradiation time was set to 1 s. Ultrasonic wave irradiation was initiated when the temperature of the emulsion reached 1 C. Ultrasonic wave irradiation was done at a frequency of 20 kHz; the intensity of the ultrasonic wave was proportional to its amplitude. The values for amplitude were 20 (condition a), 28 (condition b), and 38 mm (condition c), respectively. For comparison, the results for ice nucleus charging after voltage application are also shown. The results for the time period from ultrasonic wave irradiation to the start of propagation are shown in Figs. 12 and 15, which represent (7:3) and (8:2), respectively. Results for the time period from the start of propagation to the end are shown in Figs. 13 and 16, which correspond to (7:3) and (8:2), respectively. Results for maximum supercooling degree are shown in Figs. 14 and 17, which correspond to (7:3) and (8:2), respectively. The case of (7:3) was discussed. Fig. 12 shows that the average time from ultrasonic wave irradiation to the start of
100 Ice nucleus + Voltage (A.V. 0.4 ºC) Ice nucleus (A.V. 1.4 ºC)
Ice nucleus + Voltage (A.V. 3.8 min) Ice nucleus (A.V. 38 min)
80
Probability ( )
80
Probability ( )
10∼15
Fig. 9 – Probability distribution of time taken from addition of ice nucleus to start of propagation for (8:2).
100
60
40
20
0
5∼10
Time (min)
Time (min)
60
40
20
0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
2.5∼3.0
Maximum supercooling degree (ºC) Fig. 8 – Probability distribution of maximum supercooling degree for (7:3).
0
0∼5
5∼10
20∼30
30∼40
40∼50
50∼60
Time (min) Fig. 10 – Probability distribution of time taken from start of propagation to end for (8:2).
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international journal of refrigeration 31 (2008) 832–840
100
100 Ice nucleus + Voltage (A.V. 0.4 ºC) Ice nucleus (A.V. 1.3 ºC)
80
Probability (
Probability (
)
)
80
60
40
60
40
20 20 0 0
0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
0∼10
10∼20
2.5∼3.0
propagation was shortest under condition c. This may be due to cavitation caused by the ultrasonic wave. Since intensity of cavitation is proportional to intensity of ultrasonic wave, intensity of cavitation is proportional to the amplitude of the ultrasonic wave. The intensity of cavitation corresponding to the total impulsive force due to cavitation increases with increase in amplitude of the ultrasonic wave. Thus, the average time for condition c was shortest. However, for conditions a and b, the inequality between the intensities of cavitation (magnitudes of amplitude) was reversed compared with that between the average times. This is due to heat generation caused by ultrasonic wave irradiation. For condition c, heat generation also occurred, but the effect of improvement of propagation rate due to irradiation overcame that of heat generation because of its larger intensity. Thus, for condition c, the average time was shortest. Fig. 13 shows that for the average time from the start of propagation to the end, condition b > condition a > condition c. Average time under condition c is shortest mainly due to partial phase transition of the emulsion after ultrasonic
30∼40
40∼50
50∼
Time (min)
Maximum supercooling degree (ºC) Fig. 11 – Probability distribution of maximum supercooling degree for (8:2).
20∼30
Condition a (A.V. 24.9 min)
Condition c (A.V. 6.0 min)
Condition b (A.V. 29.2 min)
Voltage + Ice nucleus (A.V. 4 min)
Fig. 13 – Probability distribution of time taken from start of propagation to end for (7:3).
wave irradiation. When a portion of the emulsion becomes an O/W type due to phase transition, the average time decreases because water droplets partially form a continuous phase. The degree of phase transition depends on the intensity of cavitation (magnitude of amplitude). The partially transformed emulsion could be returned to its original condition (W/O emulsion) by stirring. Under conditions a and b, the inequality between average times was reversed compared to the intensities of cavitation (magnitudes of amplitude) because of heat generation. Moreover, under the three conditions, distributions of the times tended to be scattered compared with ice nucleus addition after voltage application because of heat generation. Fig. 14 shows that the average maximum supercooling degree decreased with an increase in the amplitude of the ultrasonic wave. As mentioned above, when cooling rates of the emulsions are the same under a fixed composition ratio, the maximum supercooling degree is approximately proportional to the time until the start of propagation. However, under
100 100
80
Probability ( )
Probability ( )
80
60
40
20
60
40
20 0
0∼2
2∼4
4∼6
6∼8
Time (min) Condition a (A.V. 5.9 min)
Condition c (A.V. 1.2 min)
Condition b (A.V. 6.5 min)
Voltage + Ice nucleus (A.V. 3 min)
0
0~0.5
0.5~1.0
1.0~1.2
1.2~1.4
1.4~1.6
1.6~
Maximum supercooling degree (ºC) Condition a (A.V. 1.5 ºC)
Condition c (A.V. 1.1 ºC)
Condition b (A.V. 1.3 ºC)
Fig. 12 – Probability distribution of time taken from irradiation of ultrasonic wave or application of voltage to start of propagation for (7:3).
Fig. 14 – Probability distribution of maximum supercooling degree for (7:3).
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international journal of refrigeration 31 (2008) 832–840
100
Probability (
)
80
60
40
20
0
0∼5
5∼10
10∼15
20∼
Time (min) Condition a (A.V. 6.5 min)
Condition c (A.V. 3.0 min)
Condition b (A.V. 5.9 min)
Voltage + Ice nucleus (A.V. 3.5 min)
Fig. 15 – Probability distribution of time taken from irradiation of ultrasonic wave or application of voltage to start of propagation for (8:2).
from the start of propagation to the end was much larger by about eight times for ultrasonic wave irradiation compared to that for ice nucleus charging after voltage application under all three conditions. The average total times under the three conditions became much larger than that for ice nucleus charging after voltage application by about five times. For this composition ratio, ultrasonic wave irradiation is not effective for improving propagation rate compared to condition c in (7:3). The improvement of propagation rate and reduction in maximum supercooling degree were hypothesized to be caused by cavitation due to ultrasonic wave irradiation. Propagation rate should increase significantly if cavitation occurs at about the same time in each water droplet, but actually, propagation rate is relatively slower. The reason for this remains to be investigated. Propagation rate for (7:3) was compared with that for (8:2). For the average time until the start of propagation shown in Figs. 12 and 15, under conditions a and b, the difference between (7:3) and (8:2) was very small; under condition c 100
80
Probability ( )
conditions a and b, the inequality between the average times until start of propagation shown in Fig. 12 was reversed compared to that between the average maximum supercooling degrees because of heat generation. The results for ultrasonic wave irradiation were compared with those for ice nucleus charging after voltage application. As shown in Fig. 12, only under condition c the average time until the start of propagation was shorter than that for ice nucleus charging after voltage application by less than 1/2. However, it must be taken into account that ultrasonic wave irradiation occurs when the emulsion temperature reaches 1 C, meaning the supercooling degree of the bulk emulsion rises. Moreover, Fig. 13 shows that the time from the start of propagation to the end under all three conditions was larger than that for ice nucleus charging after voltage application. For the maximum supercooling degree, since the temperature of ultrasonic wave irradiation was 1 C, but that of ice nucleus charging was 0 C, the two conditions could not be compared directly. To estimate propagation rate, the total time from ultrasonic wave irradiation to the end of propagation was investigated. Figs. 12 and 13 show that the average total time under condition c was shorter than those under conditions a and b by about 1/4–1/5. The difference in average total times between conditions a and b was small. The results for ultrasonic wave irradiation were compared with those of ice nucleus charging after voltage application. For the average total time, only condition c was nearly equal to the case of ice nucleus charging after voltage application. However, the trends for the two cases were very different. Figs. 12 and 13, showing results under condition c, indicate that the average time from ultrasonic wave irradiation to the start of propagation was much shorter, but the time from the start of propagation to the end was relatively larger. However, for ice nucleus charging after voltage application, both average times were nearly the same. Thus, only condition c is effective for improving propagation rate. Next, the case of (8:2) was examined. Fig. 15 shows the average time from irradiation of ultrasonic wave to the start of propagation decreased with an increase in the amplitude of the ultrasonic wave. This correlation was different from that of (7:3) as shown in Fig. 12. This is due to the larger viscosity of (8:2) emulsion, which resulted in minimal cavitation because of an increase in ultrasonic wave decay, in compared to that in (7:3). Thus, the difference in cavitation intensities between the three conditions decreased. Fig. 16 shows that, for the average time from start of propagation to end, the differences among three conditions were very small because the effect of higher viscosity overcame that of cavitation intensity. Figs. 15 and 16 show that, for the average total time, a little difference existed among the three conditions, and the case of condition c was slightly shorter than the other conditions. Fig. 17 shows that average maximum supercooling degree decreased with an increase in the amplitude of the ultrasonic wave. This trend was similar to that shown in Fig. 15. The results for ultrasonic wave irradiation were compared with those for ice nucleus charging after voltage application. Fig. 15 shows that the average time from ultrasonic wave irradiation to the start of propagation was slightly shorter than that for ice nucleus charging after voltage application under condition c. However, Fig. 16 shows that the average time
60
40
20
0
0∼10
10∼20
20∼30
30∼40
40∼50
50∼
Time (min) Condition a (A.V. 31.1 min)
Condition c (A.V. 30.8 min)
Condition b (A.V. 31.2 min)
Voltage + Ice nucleus (A.V. 3.8 min)
Fig. 16 – Probability distribution of time taken from the start of propagation to the end for (8:2).
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international journal of refrigeration 31 (2008) 832–840
(4) Propagation rate for (7:3) was faster than that for (8:2) in both methods of ice nucleus charging after voltage application and ultrasonic wave irradiation. (5) The difference in maximum supercooling degree for (7:3) and (8:2) was very small in both methods. (6) Addition of ice nucleus to a W/O emulsion after voltage application was more effective for increasing propagation rate of dissolution compared with only ice nucleus charging and ultrasonic wave irradiation.
100
Probability (
)
80
60
40
20
0
0∼0.5
0.5∼1.0
1.0∼1.2
1.2∼1.4
1.4∼1.6
1.6∼
Maximum supercooling degree (ºC) Condition a (A.V. 1.5 ºC)
Acknowledgments
Condition c (A.V. 1.1 ºC)
Condition b (A.V. 1.3 ºC)
This study was supported by a 2007 Chuo University Grant for Special Research.
Fig. 17 – Probability distribution of maximum supercooling degree for (8:2).
references (7:3) < (8:2). Figs. 13 and 16 show that, for the average time from the start of propagation to the end, (7:3) < (8:2) under all three conditions. Especially, under condition c, the average time in (7:3) was much shorter. Moreover, since for the average total time, (7:3) < (8:2) under all three conditions, the propagation rate for (7:3) was faster than that for (8:2) under all three conditions. Especially, under condition c, the increase in propagation rate for (7:3) was significant compared with (8:2), due to viscosity of the emulsion. Since the emulsion of (7:3) has lower viscosity, irradiation of ultrasonic wave was effective to propagate dissolution. Figs. 14 and 17 show that the differences in maximum supercooling degrees between (7:3) and (8:2) were very small under all three conditions. These results indicate that addition of ice nucleus after voltage application is more effective for improving propagation rate compared with ultrasonic wave irradiation because formation of larger ice particles and a decrease in viscosity of the emulsion due to voltage application act to improve propagation rate effectively. For this step, dissolution of supercooling and propagation of dissolution could not be realized by applying voltage without ice nucleus charging. For all cases where only voltage was applied to the emulsion, demulsification of the emulsions occurred and the emulsions could not be returned to their original state by stirring. Therefore, further investigation of dissolution of supercooling and propagation of dissolution occurring only during voltage application must be done further.
4.
Conclusions
(1) Addition of ice nucleus to W/O emulsion after voltage application was effective for increasing propagation rate of dissolution and reducing the maximum supercooling degree. (2) Propagation rate of dissolution could be approximately increased by increasing irradiation intensity of the ultrasonic wave. (3) Maximum supercooling degree could be decreased by increasing irradiation intensity of the ultrasonic wave.
Davies, T.W., 2005. Slurry ice as heat transfer fluid with a larger number of application domains. Int. J. Heat Mass Transfer 28 (1), 108–114. Egolf, P.W., Kauffeld, M., 2005. From physical properties of ice slurries to industrial ice slurry applications. Int. J. Refrigeration 28 (1), 4–12. Hirata, T., Nagasaka, K., Ishikawa, M., 2000. Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface (part I: ice removal phenomena). Int. J. Heat Mass Transfer 43 (3), 333–339. Hozumi, T., Saito, A., Okawa, S., Matsui, R., 1999a. Experimental study on the effect of the ultrasonic wave on the freezing of the supercooled water. Trans. Jpn. Soc. Refrig. Air Condition. Eng. 16 (2), 141–146. in Japanese. Hozumi, T., Saito, A., Oka, S., 1999b. Study on the mechanism of the freezing of the supercooled water under the effect of the ultrasonic wave. Trans. Jpn. Soc. Refrig. Air Condition. Eng. 16 (2), 147–156. in Japanese. Hozumi, T., Saito, A., Okawa, S., Watanabe, K., 2003a. Effects of electrode materials on freezing of supercooled water in electric freeze control. Int. J. Refrigeration 26 (5), 537–542. Hozumi, T., Saito, A., Okawa, S., Eshita, Y., 2003b. Effects of shapes of electrodes on freezing of supercooled water in electric freeze control. Int. J. Refrigeration 28 (3), 389–395. Matsumoto, K., Okada, M., Kawagoe, T., Kan, C., 2000. Ice storage system with water–oil mixture (formation of suspension with high IPF). Int. J. Refrigeration 23 (5), 336–344. Matsumoto, K., Oikawa, K., Okada, M., Teraoka, Y., Kawagoe, T., 2006. Study on high performance ice slurry formed by cooling emulsion in ice storage (discussion on adaptability of emulsion to thermal storage material). Int. J. Refrigeration 29 (6), 1010–1019. Matsumoto, K., Sakae, K., Oikawa, K., Okada, M., Teraoka, Y., Kawagoe, T., 2007. Study on formation of high performance ice slurry by W/O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution). Int. J. Refrigeration 30 (8), 1300–1308. Oda, Y., Okada, M., Nakagawa, S., Matsumoto, K., Kawagoe, T., 2004. Continuous ice formation in a tube by using water–oil emulsion for dynamic-type ice-making cold thermal energy storage. Int. J. Refrigeration 27 (4), 353–359. Pronk, P., Hansen, T.M., Infante Ferreira, C.A., Wittkamp, G.J., 2005. Time-dependent behavior of different ice slurries during ice storage. Int. J. Refrigeration 28 (1), 27–36. Sherma, P., 1968. Emulsion Science. Academic Press, pp. 67–72.
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Formation of high performance ice slurry by W/O emulsion in ice storage (effective method to propagate supercooling dissolution) Koji Matsumotoa,*, Kazuki Sakaeb, Hirofumi Yamauchic, Yoshikazu Teraokad a
Department of Precision Mechanics, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Sony Corporation, 1-7-1 Konan, Minato-ku, Tokyo 108-0075, Japan c Graduate Student of Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan d Aoyama Gakuin University, Department of Mechanical Engineering, Research Associate, 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan b
article info
abstract
Article history:
This study focused ice slurry formation in an ice storage system using W/O emulsions with
Received 14 July 2007
70 and 80% water contents. Emulsions consisted of a silicone oil–water mixture with a small
Received in revised form
amount of amino-group-modified silicone oil additive. Ice slurry was formed by cooling the
5 September 2007
emulsion without ice adhesion to the cooling wall, as water in the emulsion did not directly
Accepted 24 October 2007
contact the cooling wall. As the structure of W/O emulsion slowed the propagation rate of
Published online 28 October 2007
supercooling dissolution, voltage and ultrasonic wave were applied to the W/O emulsion to propagate dissolution more quickly and decrease maximum supercooling degree, respec-
Keywords:
tively. Thus, the effects of voltage and ultrasonic wave applications on propagation rate
Ice slurry
were clarified.
Emulsion
ª 2007 Elsevier Ltd and IIR. All rights reserved.
Water Oil Process Enhancement Electric field Ultrasound
Formation de coulis de glace hautement performants a` l’aide d’une e´mulsion d’eau/huile dans l’accumulation de glace (moyen efficace de promouvoir la dissolution lors du surrefroidissement) Mots cle´s : Coulis de glace ; E´mulsion ; Eau ; Huile ; Proce´de´ ; Ame´lioration ; Champ e´lectrique ; Ultrason
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (Y. Teraoka). 0140-7007/$ – see front matter ª 2007 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2007.10.006
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1.
Introduction
Ice storage systems are superior to many other types of thermal storage systems because the amount of thermal storage per unit volume is larger than those of other storage systems, allowing the effective use of energy. Interest in dynamic ice storage systems has developed rapidly. In a dynamic system, ice slurry with good fluidity used as a thermal storage material has many practical characteristics but also some problems, such as a decrease in latent heat fusion due to a reduction in freezing point and ice adhesion to the cooling wall. Thus, many studies on ice slurries have been conducted (Egolf and Kauffeld, 2005; Pronk et al., 2005; Davies, 2005; Oda et al., 2004; Matsumoto et al., 2000; Hirata et al., 2000). The authors of the present paper also have studied the formation of ice slurries using a water-in-oil (W/O) emulsion, which overcomes some of the problems of dynamic ice storage systems. Matsumoto et al. (2006) demonstrated the promise of W/O emulsions for dynamic ice storage. The use of a W/O emulsion allowed stable ice slurry formation in a stainless vessel, despite the difficulties previously encountered with ice adhesion to the cooling wall. Additionally, optimum composition ratios of water and oil also were proposed. But it was confirmed that the propagation rate of supercooling dissolution for W/O emulsions was much slower than that for a liquid such as water. A recent report also has described the factors governing the propagation rate and maximum supercooling degree based on probability (Matsumoto et al., 2007). To dissolve supercooling pure water with very small volume, application of voltage and irradiation of ultrasonic wave have been investigated (Hozumi et al., 1999a,b, 2003a,b). Because the characteristics of W/O emulsions are very different from those of pure water, the results of experiments using pure water cannot be directly applied to W/O emulsions. Thus, the validity of application of voltage and irradiation of ultrasonic wave to W/O emulsions must be discussed. In the present study, in order to effectively improve the propagation rate of supercooling dissolution, ice nucleus was added to a W/O emulsion immediately after application of voltage or ultrasonic wave was irradiated to a W/O emulsion.
tap water; water contents of the emulsions were 70 and 80%. The kinematic viscosity of the silicone oil was 10 mm2/s at 25 C. An amino-group-modified silicone oil (0.9 vol%) was used as a surface-active agent. Hereafter, the word ‘‘emulsion’’ represents a W/O emulsion and ‘‘viscosity’’ represents kinematic viscosity. For example, an emulsion with a water/ oil volumetric ratio of 8:2 is designated (8:2).
2.2. Measurement of propagation rate of supercooling dissolution for ice nucleus charging after voltage application Generally, when voltage is applied to an emulsion, the emulsion is demulsified (Sherma, 1968). During the demulsification process, the dispersed phase separates from the emulsion and condenses (flocculates), followed by coalescence of the condensed dispersed phase. This paper describes a method of partial demulsification to promote the propagation of supercooling dissolution. The apparatus for measurement of the propagation rate of the emulsion cooled by a Peltier module is shown in Fig. 1.The apparatus consists mainly of the Peltier module, copper plate, electrodes and CCD camera. The freezing point of the W/O emulsions used was 0 C. A 1-mm-thick copper plate was used for cooling on the Peltier module. Grease with high thermal conductivity was applied to the gap between the plate and the module. The emulsion with 1 C was spread on the plate at a thickness of 0.5 mm and surface area of 40 40 mm. The emulsion on the plate was cooled by the module immediately after voltage application to the emulsion for 1 s. A pair of electrodes consisted of an aluminum rod with a diameter of 3 mm; the distance between the electrodes was 30 mm. The voltage applied to the samples of 70 and 80% water contents was 10 and 20 V, respectively. When the emulsion temperature reached the set temperature (5 C), supercooling dissolution of the emulsion began to propagate due to addition of one ice nucleus. The ice nucleus was hemispherical in shape with a diameter of about 8 mm. Since the pair of thermocouples (type T) may act as a trigger for dissolution of supercooling, temperature at the center of the back of the plate was measured. Pre-experiments indicated that the temperature of the emulsion on the plate was always
40 mm
2.
Experiment
2.1.
Composition of W/O emulsions
The composition ratios of the W/O emulsions, expressed as a volumetric ratio of water and oil, are shown in Table 1. In this experiment, the oil and water used were silicone oil and
6 mm
Emulsion Ice nucleus
30 mm
4.5 mm
Electrodes
40 mm
Observation region
Table 1 – Composition ratios of W/O emulsions Tap water (ml) Silicone oil (ml) Additive (ml) Ratio of water to oil
880 220 10 8:2
Additive: amino-group-modified silicone oil.
770 330 10 7:3
Copper plate
Peltier module
Fig. 1 – Experimental apparatus for measurement of propagation rate using a Peltier module.
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international journal of refrigeration 31 (2008) 832–840
greater than the back temperature of the plate by about 1 C. Thus, the representative emulsion temperature was defined by correcting the back temperature based on the temperature difference (1 C). The propagation of the dissolution process was observed by CCD at 100-fold magnification in a square region with an area of 6 mm 4.5 mm, as shown in Fig. 1. Simultaneously, the time required to reach a fixed propagation distance (6 mm) was measured. The propagation rate was obtained by dividing the time into the fixed distance. A new emulsion was used for each measurement because formation history affected the state of the emulsion. Because there was a possibility that propagation rate depended on the size of the water droplets (dispersed phase), the size distributions of the water droplets with and without voltage application were measured by CCD at 300-fold magnification at three arbitrary points in the emulsion. The number of water droplets sampled for one condition was about 200. To investigate the propagation rate of supercooling dissolution during the process of ice slurry formation, the probability of propagation within a certain time period was measured. The same method also was used to investigate the maximum degree of supercooling. Since a detailed description of the experimental apparatus and procedure has been reported previously by Matsumoto et al., (2007), only a brief description is provided here. When the emulsion (1.1 L) cooled by addition of cold brine (5.4 C) with stirring (250 rpm) reached the set temperature (0 C), an ice nucleus was added to the emulsion to initiate propagation of dissolution. And then, temperature of the emulsion decreased gradually. After that, the temperature returned to the freezing point (0 C) very slowly after it reached a minimum temperature (maximum supercooling degree). Start of propagation of dissolution is the time immediately after temperature rise of the emulsion and end of propagation of dissolution is just the time when the temperature of the emulsion returns to the freezing point. The ice nucleus was hemispherical in shape with a diameter of about 8 mm. A pair of electrodes was then immediately inserted into the emulsion, as shown in Fig. 2(a) and voltage was applied. The dimensions of the electrodes are shown in Fig. 2(a). The distance between the electrodes is 12 mm. Voltage was applied until the end of propagation of dissolution. The electrodes were removed from the emulsion immediately after end of voltage application. Propagation of dissolution was considered to be completed when bulk temperature of the emulsion returned to 0 C.
2.3. Measurement of propagation rate of supercooling dissolution during ultrasonic wave irradiation The experiments to discuss the effect of irradiation of ultrasonic wave on propagation rate were carried out. Irradiation of ultrasonic wave to the emulsion often caused a phase transition of the emulsion (Matsumoto et al., 2006). However, the emulsion could be returned to its original state by stirring if partial phase transition occurred. Thus, efforts were made to minimize phase transition. Optimal irradiation conditions are discussed. Stirring was stopped after the emulsion was cooled to 1 C. The emulsion was exposed to ultrasonic wave immediately after stirring was stopped. The emulsion temperature at irradiation was 1 C, because of the rise in temperature of the
emulsion due to irradiation. Irradiation was directed at the center of the vessel, as shown in Fig. 2(b), and irradiation time was 1 s. Stirring began again immediately after irradiation was completed. The frequency and amplitude of the ultrasonic wave were 20 kHz and 1–40 mm, respectively. The intensity of the ultrasonic wave was proportional to its amplitude.
3.
Results and discussion
3.1. Propagation rate of supercooling dissolution with ice nucleus charging after voltage application Propagation rates were measured using the apparatus as shown in Fig. 1. Dissolution of supercooling of the emulsion was propagated radially from ice nucleus toward the right side of the copper plate as shown in Fig. 1. Results are shown in Fig. 3. The result for each composition ratio was an average value of 10 measurements. From comparison of the cases of voltage application with that of no application, propagation rate increased by about 30% at each composition ratio. Average water droplet size for (7:3) with and without voltage application was 66.36 and 62.86 mm, respectively, a relatively small difference. But the existence of larger volume water droplets with a droplet size in the range of 200–500 mm was confirmed during voltage application, although this did not occur without voltage application. Thus, droplet size distributions were rearranged on a volume ratio basis, with volume ratio defined as (total volume of water droplets existing in a certain size range)/(total volume of all water droplets) 100(%). Applied voltage was set to 10 V. An example of the distribution rearranged for (7:3) is shown in Figs. 4 and 5, which correspond to the cases of no voltage and voltage application, respectively. Figs. 4 and 5 show that, for the case of voltage application, the volume ratio of larger water droplets was dramatically greater compared to no voltage. A previous study reported that larger ice particles formed by dissolution were effective for increasing propagation rate (Matsumoto et al., 2007). This result is in agreement with the data presented in Figs. 4 and 5. Since larger water droplets due to voltage application lead to formation of larger ice particles, propagation rate could be increased by voltage application as shown in Fig. 3. For (8:2), a similar tendency was obtained. Since applied voltages were different, the case of (7:3) could not be compared directly with (8:2). However, Fig. 3 indicates that propagation rate for (8:2) had a tendency to decrease compared with (7:3). One reason for the decrease in propagation rate could be the multiplier effect of viscosity and oil thickness of the emulsion, causing a decrease in propagation rate.
3.2. Propagation rate of supercooling dissolution with ice nucleus charging after voltage application during ice formation Since propagation rate could be increased by voltage application, as shown in Fig. 3, the method of ice nucleus charging after voltage application to improve propagation rate in the ice slurry was examined. In pre-experiments, an applied voltage was discussed, varying the voltage in the range of 0–250 V, and it was determined to be 50 V.
international journal of refrigeration 31 (2008) 832–840
12 mm
a
835
Insulation coating
200 mm
3 mm
4 mm 10 mm
Electrodes
Electrodes
160 mm
Emulsion Electrodes
15 mm
Stirrer rod In the emulsion Thermometer Ice formation vessel
b
Ultrasonic generator
Emulsion
Ice formation vessel Fig. 2 – Setting position and shape of electrodes.
Thirty experiments were conducted at each composition ratio. The propagation rate and maximum supercooling degree were examined based on probability and by using the formula ([frequency of result satisfying given condition]/[frequency of all experiments] 100(%)), in the same manner as previously reported (Matsumoto et al., 2007). To estimate propagation rate, the times from addition of the ice nucleus
to the start of propagation of dissolution and from the start of propagation to the end were measured. For comparison, the results of ice nucleus charging without voltage application are shown. Measurement results for the time period from addition of the ice nucleus to the start of propagation are shown in Figs. 6 and 9, which represent the composition ratios of (7:3) and (8:2), respectively. Results for the time period from the
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15
40
Volume ratio [ ]
Propagation rate [ m/s]
50
30
20
10
10
5
Applied voltage = 10 or 20V Applied voltage = 0V
0
7:3
0
8:2
0
100
200
300
400
500
Water droplet size [ m]
Fig. 3 – Relationship between propagation rate and composition ratio.
Fig. 5 – Distribution of water droplet sizes with applied voltage for (7:3).
start of propagation to the end are shown in Figs. 7 and 10, which represent (7:3) and (8:2), respectively. The results for maximum supercooling are shown in Figs. 8 and 11, which correspond to (7:3) and (8:2), respectively. The average value for each result is shown as ‘‘A.V.’’ and is used to estimate the result. In the horizontal axes, for example, ‘‘0–5’’ means 0 t < 5 min. The case of (7:3) was discussed. Fig. 6 shows that the average time period from addition of the ice nucleus to the start of propagation for ice nucleus charging after voltage application was shorter by 1/3 than for ice nucleus charging without voltage application. This was caused by the coalescence of water droplets due to voltage application, as mentioned above. Similarly, Fig. 7 shows that the average time from the start of propagation to the end for ice nucleus charging after voltage application was about 1/4 shorter than that for ice nucleus charging without voltage. This reason for this is the same as for the results shown in Fig. 6 (i.e., since larger water droplets formed by voltage application became larger ice particles due to dissolution of supercooling, those larger ice particles propagate dissolution effectively). From Figs. 6 and 7, upon voltage application, both time periods from addition of the ice nucleus
to the start of propagation and from the start of propagation to the end were within 5 min. Fig. 8 shows that the average maximum supercooling degree for ice nucleus charging after voltage application could be decreased significantly compared to ice nucleus charging without voltage application. As mentioned in a previous report (Matsumoto et al., 2007), since the emulsion cooling rates were the same for a fixed composition ratio, the maximum supercooling degree was approximately proportional to the time period from addition of the ice nucleus to the start of propagation. Therefore, since the time period from addition of the ice nucleus to the start of propagation decreased significantly for ice nucleus charging after voltage application, as shown in Fig. 6, the average maximum supercooling degree also decreased remarkably. For cases of voltage application, the maximum supercooling degrees were within 0.5 C. To estimate propagation rate, the total time from addition of the ice nucleus to the end of propagation was examined. As shown in Figs. 6 and 7, the average total time for (7:3) in the case of ice nucleus charging after voltage application was over 70% shorter than that for ice nucleus charging without
15
100 Ice nucleus + Voltage (A.V. 3 min) Ice nucleus (A.V. 9 min)
10
)
5
Probability (
Volume ratio [
]
80
60
40
20
0
0
100
200
300
400
500
Water droplet size [ m] Fig. 4 – Distribution of water droplet sizes without applied voltage for (7:3).
0
0∼5
5∼10
10∼15
15∼20
Time (min) Fig. 6 – Probability distribution of time taken from addition of ice nucleus to start of propagation for (7:3).
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100
100 Ice nucleus + Voltage (A.V. 4 min) Ice nucleus (A.V. 16.3 min)
Probability (
) Probability (
Ice nucleus (A.V. 10.1 min)
80
)
80
Ice nucleus + Voltage (A.V. 3.5 min)
60
40
40
20
20
0
60
0 0∼5
5∼10
10∼15
15∼20
20∼
0∼5
Fig. 7 – Probability distribution of time taken from start of propagation to end for (7:3).
voltage application. Thus, during the ice slurry formation process, adding the ice nucleus after voltage application was very effective to improve propagation rate. Figs. 9–11 show the results for (8:2). The average times from addition of the ice nucleus to the start of propagation, from the start of propagation to the end, and the average maximum supercooling degree decreased significantly compared to only ice nucleus charging. Moreover, since the average total time decreased by about 85% in this composition ratio, voltage application was also effective to improve propagation rate. The propagation rate (average total time) for (7:3) (Figs. 6 and 7) was compared with that for (8:2) (Figs. 9 and 10). For ice nucleus charging after voltage application, the difference in propagation rates for (7:3) and (8:2) was very small. This may be because viscosity of (7:3) emulsion is nearly equal to that of (8:2). For both composition ratios shown in this paper, the decrease in viscosity of the emulsions was caused by an increase in water droplet size due to voltage application. Thus, since the difference in viscosity between (7:3) and (8:2) became much smaller, the difference in the propagation rates between both composition ratios was also very small.
15∼20
20∼25
3.3. Propagation rate of supercooling dissolution after ultrasonic wave irradiation during ice formation The propagation rate and maximum supercooling degree were examined based on probability. To estimate propagation rate, the same procedure described in Section 3.2 was used. Irradiation time was set to 1 s. Ultrasonic wave irradiation was initiated when the temperature of the emulsion reached 1 C. Ultrasonic wave irradiation was done at a frequency of 20 kHz; the intensity of the ultrasonic wave was proportional to its amplitude. The values for amplitude were 20 (condition a), 28 (condition b), and 38 mm (condition c), respectively. For comparison, the results for ice nucleus charging after voltage application are also shown. The results for the time period from ultrasonic wave irradiation to the start of propagation are shown in Figs. 12 and 15, which represent (7:3) and (8:2), respectively. Results for the time period from the start of propagation to the end are shown in Figs. 13 and 16, which correspond to (7:3) and (8:2), respectively. Results for maximum supercooling degree are shown in Figs. 14 and 17, which correspond to (7:3) and (8:2), respectively. The case of (7:3) was discussed. Fig. 12 shows that the average time from ultrasonic wave irradiation to the start of
100 Ice nucleus + Voltage (A.V. 0.4 ºC) Ice nucleus (A.V. 1.4 ºC)
Ice nucleus + Voltage (A.V. 3.8 min) Ice nucleus (A.V. 38 min)
80
Probability ( )
80
Probability ( )
10∼15
Fig. 9 – Probability distribution of time taken from addition of ice nucleus to start of propagation for (8:2).
100
60
40
20
0
5∼10
Time (min)
Time (min)
60
40
20
0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
2.5∼3.0
Maximum supercooling degree (ºC) Fig. 8 – Probability distribution of maximum supercooling degree for (7:3).
0
0∼5
5∼10
20∼30
30∼40
40∼50
50∼60
Time (min) Fig. 10 – Probability distribution of time taken from start of propagation to end for (8:2).
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international journal of refrigeration 31 (2008) 832–840
100
100 Ice nucleus + Voltage (A.V. 0.4 ºC) Ice nucleus (A.V. 1.3 ºC)
80
Probability (
Probability (
)
)
80
60
40
60
40
20 20 0 0
0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
0∼10
10∼20
2.5∼3.0
propagation was shortest under condition c. This may be due to cavitation caused by the ultrasonic wave. Since intensity of cavitation is proportional to intensity of ultrasonic wave, intensity of cavitation is proportional to the amplitude of the ultrasonic wave. The intensity of cavitation corresponding to the total impulsive force due to cavitation increases with increase in amplitude of the ultrasonic wave. Thus, the average time for condition c was shortest. However, for conditions a and b, the inequality between the intensities of cavitation (magnitudes of amplitude) was reversed compared with that between the average times. This is due to heat generation caused by ultrasonic wave irradiation. For condition c, heat generation also occurred, but the effect of improvement of propagation rate due to irradiation overcame that of heat generation because of its larger intensity. Thus, for condition c, the average time was shortest. Fig. 13 shows that for the average time from the start of propagation to the end, condition b > condition a > condition c. Average time under condition c is shortest mainly due to partial phase transition of the emulsion after ultrasonic
30∼40
40∼50
50∼
Time (min)
Maximum supercooling degree (ºC) Fig. 11 – Probability distribution of maximum supercooling degree for (8:2).
20∼30
Condition a (A.V. 24.9 min)
Condition c (A.V. 6.0 min)
Condition b (A.V. 29.2 min)
Voltage + Ice nucleus (A.V. 4 min)
Fig. 13 – Probability distribution of time taken from start of propagation to end for (7:3).
wave irradiation. When a portion of the emulsion becomes an O/W type due to phase transition, the average time decreases because water droplets partially form a continuous phase. The degree of phase transition depends on the intensity of cavitation (magnitude of amplitude). The partially transformed emulsion could be returned to its original condition (W/O emulsion) by stirring. Under conditions a and b, the inequality between average times was reversed compared to the intensities of cavitation (magnitudes of amplitude) because of heat generation. Moreover, under the three conditions, distributions of the times tended to be scattered compared with ice nucleus addition after voltage application because of heat generation. Fig. 14 shows that the average maximum supercooling degree decreased with an increase in the amplitude of the ultrasonic wave. As mentioned above, when cooling rates of the emulsions are the same under a fixed composition ratio, the maximum supercooling degree is approximately proportional to the time until the start of propagation. However, under
100 100
80
Probability ( )
Probability ( )
80
60
40
20
60
40
20 0
0∼2
2∼4
4∼6
6∼8
Time (min) Condition a (A.V. 5.9 min)
Condition c (A.V. 1.2 min)
Condition b (A.V. 6.5 min)
Voltage + Ice nucleus (A.V. 3 min)
0
0~0.5
0.5~1.0
1.0~1.2
1.2~1.4
1.4~1.6
1.6~
Maximum supercooling degree (ºC) Condition a (A.V. 1.5 ºC)
Condition c (A.V. 1.1 ºC)
Condition b (A.V. 1.3 ºC)
Fig. 12 – Probability distribution of time taken from irradiation of ultrasonic wave or application of voltage to start of propagation for (7:3).
Fig. 14 – Probability distribution of maximum supercooling degree for (7:3).
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international journal of refrigeration 31 (2008) 832–840
100
Probability (
)
80
60
40
20
0
0∼5
5∼10
10∼15
20∼
Time (min) Condition a (A.V. 6.5 min)
Condition c (A.V. 3.0 min)
Condition b (A.V. 5.9 min)
Voltage + Ice nucleus (A.V. 3.5 min)
Fig. 15 – Probability distribution of time taken from irradiation of ultrasonic wave or application of voltage to start of propagation for (8:2).
from the start of propagation to the end was much larger by about eight times for ultrasonic wave irradiation compared to that for ice nucleus charging after voltage application under all three conditions. The average total times under the three conditions became much larger than that for ice nucleus charging after voltage application by about five times. For this composition ratio, ultrasonic wave irradiation is not effective for improving propagation rate compared to condition c in (7:3). The improvement of propagation rate and reduction in maximum supercooling degree were hypothesized to be caused by cavitation due to ultrasonic wave irradiation. Propagation rate should increase significantly if cavitation occurs at about the same time in each water droplet, but actually, propagation rate is relatively slower. The reason for this remains to be investigated. Propagation rate for (7:3) was compared with that for (8:2). For the average time until the start of propagation shown in Figs. 12 and 15, under conditions a and b, the difference between (7:3) and (8:2) was very small; under condition c 100
80
Probability ( )
conditions a and b, the inequality between the average times until start of propagation shown in Fig. 12 was reversed compared to that between the average maximum supercooling degrees because of heat generation. The results for ultrasonic wave irradiation were compared with those for ice nucleus charging after voltage application. As shown in Fig. 12, only under condition c the average time until the start of propagation was shorter than that for ice nucleus charging after voltage application by less than 1/2. However, it must be taken into account that ultrasonic wave irradiation occurs when the emulsion temperature reaches 1 C, meaning the supercooling degree of the bulk emulsion rises. Moreover, Fig. 13 shows that the time from the start of propagation to the end under all three conditions was larger than that for ice nucleus charging after voltage application. For the maximum supercooling degree, since the temperature of ultrasonic wave irradiation was 1 C, but that of ice nucleus charging was 0 C, the two conditions could not be compared directly. To estimate propagation rate, the total time from ultrasonic wave irradiation to the end of propagation was investigated. Figs. 12 and 13 show that the average total time under condition c was shorter than those under conditions a and b by about 1/4–1/5. The difference in average total times between conditions a and b was small. The results for ultrasonic wave irradiation were compared with those of ice nucleus charging after voltage application. For the average total time, only condition c was nearly equal to the case of ice nucleus charging after voltage application. However, the trends for the two cases were very different. Figs. 12 and 13, showing results under condition c, indicate that the average time from ultrasonic wave irradiation to the start of propagation was much shorter, but the time from the start of propagation to the end was relatively larger. However, for ice nucleus charging after voltage application, both average times were nearly the same. Thus, only condition c is effective for improving propagation rate. Next, the case of (8:2) was examined. Fig. 15 shows the average time from irradiation of ultrasonic wave to the start of propagation decreased with an increase in the amplitude of the ultrasonic wave. This correlation was different from that of (7:3) as shown in Fig. 12. This is due to the larger viscosity of (8:2) emulsion, which resulted in minimal cavitation because of an increase in ultrasonic wave decay, in compared to that in (7:3). Thus, the difference in cavitation intensities between the three conditions decreased. Fig. 16 shows that, for the average time from start of propagation to end, the differences among three conditions were very small because the effect of higher viscosity overcame that of cavitation intensity. Figs. 15 and 16 show that, for the average total time, a little difference existed among the three conditions, and the case of condition c was slightly shorter than the other conditions. Fig. 17 shows that average maximum supercooling degree decreased with an increase in the amplitude of the ultrasonic wave. This trend was similar to that shown in Fig. 15. The results for ultrasonic wave irradiation were compared with those for ice nucleus charging after voltage application. Fig. 15 shows that the average time from ultrasonic wave irradiation to the start of propagation was slightly shorter than that for ice nucleus charging after voltage application under condition c. However, Fig. 16 shows that the average time
60
40
20
0
0∼10
10∼20
20∼30
30∼40
40∼50
50∼
Time (min) Condition a (A.V. 31.1 min)
Condition c (A.V. 30.8 min)
Condition b (A.V. 31.2 min)
Voltage + Ice nucleus (A.V. 3.8 min)
Fig. 16 – Probability distribution of time taken from the start of propagation to the end for (8:2).
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international journal of refrigeration 31 (2008) 832–840
(4) Propagation rate for (7:3) was faster than that for (8:2) in both methods of ice nucleus charging after voltage application and ultrasonic wave irradiation. (5) The difference in maximum supercooling degree for (7:3) and (8:2) was very small in both methods. (6) Addition of ice nucleus to a W/O emulsion after voltage application was more effective for increasing propagation rate of dissolution compared with only ice nucleus charging and ultrasonic wave irradiation.
100
Probability (
)
80
60
40
20
0
0∼0.5
0.5∼1.0
1.0∼1.2
1.2∼1.4
1.4∼1.6
1.6∼
Maximum supercooling degree (ºC) Condition a (A.V. 1.5 ºC)
Acknowledgments
Condition c (A.V. 1.1 ºC)
Condition b (A.V. 1.3 ºC)
This study was supported by a 2007 Chuo University Grant for Special Research.
Fig. 17 – Probability distribution of maximum supercooling degree for (8:2).
references (7:3) < (8:2). Figs. 13 and 16 show that, for the average time from the start of propagation to the end, (7:3) < (8:2) under all three conditions. Especially, under condition c, the average time in (7:3) was much shorter. Moreover, since for the average total time, (7:3) < (8:2) under all three conditions, the propagation rate for (7:3) was faster than that for (8:2) under all three conditions. Especially, under condition c, the increase in propagation rate for (7:3) was significant compared with (8:2), due to viscosity of the emulsion. Since the emulsion of (7:3) has lower viscosity, irradiation of ultrasonic wave was effective to propagate dissolution. Figs. 14 and 17 show that the differences in maximum supercooling degrees between (7:3) and (8:2) were very small under all three conditions. These results indicate that addition of ice nucleus after voltage application is more effective for improving propagation rate compared with ultrasonic wave irradiation because formation of larger ice particles and a decrease in viscosity of the emulsion due to voltage application act to improve propagation rate effectively. For this step, dissolution of supercooling and propagation of dissolution could not be realized by applying voltage without ice nucleus charging. For all cases where only voltage was applied to the emulsion, demulsification of the emulsions occurred and the emulsions could not be returned to their original state by stirring. Therefore, further investigation of dissolution of supercooling and propagation of dissolution occurring only during voltage application must be done further.
4.
Conclusions
(1) Addition of ice nucleus to W/O emulsion after voltage application was effective for increasing propagation rate of dissolution and reducing the maximum supercooling degree. (2) Propagation rate of dissolution could be approximately increased by increasing irradiation intensity of the ultrasonic wave. (3) Maximum supercooling degree could be decreased by increasing irradiation intensity of the ultrasonic wave.
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