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O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution)
Available online at www.sciencedirect.com
International Journal of Refrigeration 30 (2007) 1300e1308 www.elsevier.com/locate/ijrefrig
Study on formation of high performance ice slurry by W/O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution) Koji Matsumotoa,*, Kazuki Sakaeb, Ken Oikawac, Masashi Okadad,1, Yoshikazu Teraokad, Tetsuo Kawagoee,2 a
Chuo University, Department of Precision Mechanics, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan b Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan c Sumitomo Heavy Industries, Ltd., 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan d Aoyama Gakuin University, Department of Mechanical Engineering, 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan e 2-28-3, Katakurachou, Kanagawa-ku, Yokohama 221-0865, Japan Received 1 February 2007; received in revised form 27 April 2007; accepted 1 May 2007 Available online 8 May 2007
Abstract A W/O type emulsion was developed as a new thermal material for ice storage. The water contents of the emulsions were 70, 80 and 90 vol%, and silicone oil was used. An amino group modified silicone oil with 0.9 vol% was used as a surface-active agent. The freezing points of those emulsions were 0 C. However, due to the emulsion structure, the propagation rate of supercooling dissolution for each emulsion was very slow. Therefore, the propagation rate and maximum supercooling degree were estimated using probability, varying the water content of the emulsion, the method of the ice nucleus charging, and the size and number of ice nuclei. In addition, the influence of various parameters on the propagation rate and maximum supercooling degree was clarified. Ó 2007 Elsevier Ltd and IIR. All rights reserved. Keywords: Cooling; Secondary refrigerant; Ice slurry; Thermal storage; Emulsion; Water; Oil; Thermal property
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected] (Y. Teraoka). 1 Tel.: þ81 42 759 6213; fax: þ81 42 759 6212. 2 Tel./fax: þ81 45 491 7973. 0140-7007/$35.00 Ó 2007 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2007.05.001
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Etude sur la production de coulis de glace tre`s performants a` l’aide d’une e´mulsion eau/huile utilise´e dans l’accumulation thermique de glace: discussion sur les caracte´ristiques de la propagation de la dissolution surrefroidissante Mots cle´s : Re´frige´ration ; Frigoporteur ; Coulis de glace ; Accumulation thermique ; E´mulsion ; Eau ; Huile ; Proprie´te´ thermique
1. Introduction Ice storage systems are superior to many other types of storage systems. For example, ice storage equipment can be smaller because the amount of thermal storage per unit volume is larger than those of other thermal storage systems. In a dynamic ice storage system, since ice slurry used as a thermal storage material has good fluidity, a large amount of cold energy can be transported with less pumping work; the dynamic system can also respond quickly to changes in heat load because the ice particles have a large surface area. Many studies on ice slurries have been carried out [1e7]. Authors have also studied the formation of ice slurries by use of W/O (water in oil) emulsions. This method is expected to be useful because it overcomes certain problems, such as a decrease in latent heat fusion, and ice adhesion to a cooling wall. In one previous report [8], it was clarified that W/O emulsions were promising as a thermal storage material for dynamic ice storage. Moreover, the use of a W/O emulsion allows stable ice slurry formation in a stainless vessel, despite the difficulties previously encountered in achieving this without ice adhesion to the cooling wall. Optimum water and oil contents were also proposed. However, it was confirmed that propagation rate of supercooling dissolution for the W/O emulsions was much lower than that for a general liquid such as water. The effectiveness of statistical methods for estimating the dissolution phenomenon of supercooling was reported by Hozumi et al. [10,11]. In this study, the propagation rate and maximum supercooling degree were discussed based on probability, using the formula ([frequency of result satisfying given condition]/[frequency of all experiments] 100 [%]), varying the water content of the emulsion, the method of ice nucleus charging, and the size and number of ice nuclei.
of the W/O emulsions used were 0 C. Hereinafter, the word ‘‘emulsion’’ represents a W/O emulsion. The experimental apparatus is shown in Fig. 1. A copper plate with 1 mm thick was set on a Peltier module for cooling purposes. Grease with high thermal conductivity was applied to the gap between the plate and the module. The emulsion was then spread on the plate with a thickness and surface area of about 0.3 mm and 50 mm 60 mm, respectively. The emulsion on the plate was kept cool by the module. When the representative temperature of the plate reached the set temperature (8 C), supercooling dissolution of the emulsion began to propagate due to ice nucleus charging. The shape of the ice nucleus was hemispherical and its diameter was about 8 mm. The propagation process of dissolution was observed by CCD, and the propagation rate was obtained using CCD images as follows. Images of a known scale were taken by CCD at 100 magnification before the experiment; propagation of dissolution in a fixed direction was also photographed at 100 magnification, and the propagation distance was obtained by comparison with the known scale. Simultaneously, the time required to reach the distance was measured, and the propagation rate was obtained by dividing the time by the distance (6 mm). The propagation rate of water was also measured using a high-speed camera, and the propagation rates of various emulsions were compared with that of water. Since the
Insulator CCD Copper plate
2. Experiment 2.1. Measurement of propagation rate of supercooling dissolution
Sample Ice nucleus Peltier module
This experiment was carried out in order to clarify the propagation rate for a W/O emulsion. The freezing points
Fig. 1. Experimental apparatus for measurement of propagation rate.
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thermocouple used (Type T) can act as a trigger for dissolution of supercooling, the representative temperature of the plate was determined as follows: a temperature at a fixed point on the plate was measured before the experiment, and the temperature difference between the fixed point and the center of the plate was determined. The representative temperature of the plate was then defined by correcting the temperature at the fixed point based on the temperature difference.
2.2. Measurement of propagation rate in the process of ice slurry formation using probability In order to discuss the propagation rate of supercooling dissolution in the process of ice slurry formation, the probability of propagation within a certain time period was measured. The same method was also used to measure the maximum degree of supercooling. The experimental apparatus is shown in Fig. 2. A PMP (polymethylpentene) vessel with a volume of 2 l and an inner diameter of about 130 mm was used as an ice slurry formation vessel. The emulsion (1.1 l) was placed into the vessel. The surface height of the emulsion was about 86 mm. The emulsion was then cooled by addition of cold brine (5.4 C) with stirring (250 rpm). When it reached the set temperature (0 C), an ice nucleus was added to the emulsion in order to start propagation of dissolution. The temperatures of the emulsion and the ice slurry were measured using a platinum resistance thermometer located at a distance of 15 mm from the wall towards the center of the vessel and 10 mm upward from the bottom. The temperature measured at the above point is the representative temperature of the emulsion. In this way, based on the relationships between the temperature variation measured at the above point and the temperature variations at four different points, measured by Type T thermocouples, it was confirmed that there was synchronism in the propagation process. Synchronism was also observed
Stirrer Thermometer W/O emulsion
between the temperature variation at the above point and the temperature variation of the bulk emulsion surface, measured by a radiation thermometer. The conditions for adding the ice nucleus to the emulsion stirred were as follows: (1) condition 1: a hemispherical ice nucleus of about 8 mm diameter was attached to a stick and placed in the emulsion at a distance of 40 mm beneath the surface of the emulsion and 45 mm from the vessel center towards the vessel wall. It was removed from the emulsion immediately after the start of propagation of dissolution (immediately after the temperature rise of the emulsion); (2) condition 2: a hemispherical ice nucleus of about 8 mm in diameter was dropped into the emulsion at a distance of 45 mm from the vessel center towards the vessel wall; (3) condition 3: 29 hemispherical ice nuclei of about 1 mm in diameter were dispersively dropped into the emulsion in a square region with an area of 30 mm 30 mm. The center of the square region was located at a distance of 45 mm from the vessel center towards the vessel wall. In the case of condition 2, since the ice nucleus remains in the emulsion until the end of the experiment, it continues to affect the propagation of dissolution. In condition 3, ice nuclei are much smaller than those used in conditions 1 and 2. The total volume of the 29 ice nuclei is nearly equal to the volume of the ice nucleus used in conditions 1 and 2; however, the numbers and the total surface area of the ice nuclei are much greater. 2.3. Composition ratio of W/O emulsion The composition ratios of the emulsions used are shown in Table 1, expressed as a volumetric ratio of water and oil. In this experiment, the oil and water used were silicone oil and tap water, respectively. The water contents of the emulsions were 70, 80 and 90 vol%. The viscosity of the silicone oil was about 9038 mPa s (10 cSt) at 25 C [9]. The naming system we use for the emulsions is as follows: an emulsion with a water/oil volumetric ratio of 8:2 is known as (8:2). An amino group modified silicone oil (0.9 vol%) was used as a surface-active agent [8].
Table 1 Composition ratios of W/O emulsions
Brine
Constant-temperature bath
Fig. 2. Experimental apparatus for measurement of propagation rate and maximum supercooling degree with stirring.
Tap water (ml) Silicone oil (ml) Additive (ml) Ratio of water to oil
990 110 10 9:1
880 220 10 8:2
770 330 10 7:3
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Table 2 Apparent viscosity of W/O emulsions [8] (0 C, rotation speed: 5 rpm)
Fig. 3. W/O emulsion (8:2).
2.4. Structure and viscosity of W/O emulsion A photograph of the (8:2) emulsion, as a representative example, and an enlarged photograph showing its structure are shown in Figs. 3 and 4, respectively. It can be seen that the emulsion was cloudy and had a high viscosity, and that the dispersed phase consisted of many spherical water droplets with a mean diameter of several tens of micrometer, each coated with an oil layer. The viscosity of the emulsion was measured using a rotational viscometer with a rotation speed of 5 rpm. The results of these measurements are shown in Table 2. Since all of the emulsions were nonNewtonian fluids, the viscosity was expressed as an apparent viscosity; hereinafter, the word ‘‘viscosity’’ represents apparent viscosity. The size of the dispersed phase had a tendency to decrease with increasing water content, and the viscosity of the emulsion increased as the size of the dispersed phase decreased. In this way, the viscosity of the emulsions increased, as shown in Table 2. 2.5. Characteristics of propagation of supercooling dissolution in W/O emulsion
Composition ratio
Apparent viscosity (Pa s)
7:3 8:2 9:1
10.1 23.0 72.1
was found that the time required to propagate supercooling dissolution of the emulsion was much greater than that required for a general liquid such as water.
3. Results and discussion 3.1. Propagation rate of supercooling dissolution Propagation rates for various composition ratios are shown in Fig. 6. Error bars represent the range of measurement values. For each composition ratio, the average value from about 20 measurements is shown as a symbol. In the following discussion, the result of Fig. 6 was estimated by those average values. It was found that for all composition ratios, the propagation rate was about 1/1800 that of water. Moreover, the propagation rate decreased slightly with increasing water content. Generally, it is thought that the propagation rate has a tendency to increase with increasing water content because this results in a decrease in the thickness of the oil layer; meanwhile, there is a contrasting tendency for the propagation rate to decrease with increasing water content, because this causes an increase in the viscosity of the emulsion, as shown in Table 2. Since the effects of oil layer thickness and viscosity are opposed to each other, it is thought that difference between these effects results in the overall outcome, as shown in Fig. 6.
An example of temperature variation in the emulsion during the process of ice slurry formation is shown in Fig. 5. It 2
Temperature (°C)
1
Oil (Continuous Phase)
A: Addition time of ice nucleus B: Start of propagation of dissolution C C: End of propagation of dissolution
0 -1
A
B
-2 -3
Water (Dispersed Phase) 100. 000 μm Fig. 4. Enlarged photograph of W/O emulsion (8:2).
-4 2000
4000
6000
8000
10000
Time (s) Fig. 5. Example of temperature variation of W/O emulsion.
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Dissolution rate ( m/sec)
100 80
Water 1.1×105(μm/sec)
80
60
60
40
40
20
20
7:3
8:2
9:1
Water
Ratio of water to oil
3.2. Propagation rate of supercooling dissolution during the process of ice slurry formation The temperature variation of the emulsion was measured during formation of an ice slurry with stirring. In order to estimate the propagation rate, the time periods from addition of the ice nucleus to the start of propagation of dissolution (A to B in Fig. 5) and from the start of propagation to the end (B to C in Fig. 5) were discussed based on probability. The maximum degree of supercooling (point B in Fig. 5), which was also discussed on this basis, was set as the point immediately before the temperature of the emulsion began to rise. For each composition ratio, about 30 measurements were carried out for each of conditions 1, 2, and 3 as described in Section 2.2. In the pre-experimental stage, it was confirmed that the stirring wing, platinum resistance thermometer and thermocouple had no effect on the propagation of dissolution. The measurement results for the time period from addition of the ice nucleus to the start of propagation are shown in Figs. 7, 9, and 11, which represent emulsions with 100 Condition1 Condition2 Condition3
60
40
20
0
0∼10
10∼20
20∼30
0
0~10
10~20
20~30
30~
Time (min)
Fig. 6. Propagation rate of supercooling dissolution.
80
Condition1 Condition2 Condition3
30∼
Time (min) Fig. 7. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (7:3).
Fig. 8. Probability distribution of time taken from the start of propagation to the end (7:3).
composition ratios of (7:3), (8:2) and (9:1), respectively. The results for the time period from the start of propagation to the end are shown in Figs. 8 and 10, which correspond to (7:3) and (8:2), respectively. For (9:1), the experiment was stopped because ice adhesion to the cooling wall occurred immediately after the start of propagation. In Fig. 7, the condition 1 was compared with the condition 2, where the methods of ice nucleus charging were different. In this composition ratio (7:3), the emulsion showed the lowest viscosity, as shown in Table 2. From Fig. 7, it can be seen that the time from addition of the ice nucleus to the start of propagation for condition 1 was shorter than that of condition 2 (remember that the size and number of ice nuclei are the same); the time required until the start of propagation for condition 1 was almost always less than 10 min. The reason for this phenomenon is as follows. In condition 1, in which the ice nucleus was pulled from the emulsion but was fixed at one point in the emulsion until the start of propagation, the relative velocity between the fixed ice nucleus and the emulsion was greater than that between the dropped ice nucleus and the emulsion, as in condition 2. Therefore, in condition 1, the fixed ice nucleus came in contact with more water droplets (dispersed phase), which resulted in a shorter time until the start of propagation. In conditions 2 and 3 (Fig. 7), the method of charging with the ice nucleus was the same, but the sizes and numbers of the ice nuclei were different. When an ice nucleus is dropped, it must thrust through the oil layer (continuous phase) and continue to come in contact with water droplets (dispersed phase) for a certain time in order to initiate propagation. If the size of the ice nucleus is smaller, it thrusts through the oil layer with more difficulty. Additionally, the thickness of the oil layer increases with decreasing water content, but the viscosity of the emulsion decreases with decreasing water content. Thus, the effects of oil layer thickness and viscosity have opposing effects on the ability of the ice nucleus to thrust through the oil layer and start propagation. The ice nuclei in condition 3 were much smaller than that used in
K. Matsumoto et al. / International Journal of Refrigeration 30 (2007) 1300e1308
condition 2; therefore, in spite of the lowest viscosity, the ability of these ice nuclei to come in contact with many water droplets decreased due to the increased difficulty of pushing through the oil layer. Thus, in condition 3, the effect of the number of ice nuclei did not prove advantageous in terms of initiating propagation. Meanwhile, the larger size of the ice nucleus in condition 2 was not advantageous for the start of propagation because of the thickest oil layer. Therefore, the difference between the results for conditions 2 and 3 was very small. Fig. 9 shows the results of ice nucleus addition in the (8:2) emulsion. For conditions 1 and 2, the methods of ice nucleus charging were different, but the size and number of ice nuclei were the same. In this case, it was found that the time from addition of ice nucleus to the start of propagation in condition 1 was greater than that of condition 2. The viscosity of this emulsion (8:2) is greater than that of the emulsion (7:3) shown in Figs. 7 and 8. In condition 1, the ice nucleus attached to the stick was fixed at one point in the emulsion, and the flow of the emulsion in the vicinity of the ice nucleus was slower and sometimes stagnant due to the large viscosity; thus, the relative velocity between the ice nucleus and the emulsion was decreased in the vicinity of the ice nucleus, and the number of water droplets with which the ice nucleus could come in contact decreased. In condition 2, however, the ice nucleus remained in the emulsion and continued to be effective in initiating propagation. Therefore, it was thought that the relationship between conditions 1 and 2 in this case (8:2) was the reverse of the previous relationship (shown in Fig. 7). Next, condition 2 was compared with condition 3 in Fig. 9. In this case, the method of ice nucleus charging was the same but the sizes and numbers of ice nuclei were different. The thickness of the oil layer in (8:2) was less than that of (7:3), but the viscosity of the (8:2) emulsion was greater than that of (7:3). Since, in condition 3, the sizes of the ice nuclei were so small, they were hardly able to thrust through the oil layer due to the difference between the effects of oil layer thickness and viscosity. Therefore, 100 Condition1 Condition2 Condition3
80
60
40
20
0
0∼10
10∼20
20∼30
30∼
Time (min) Fig. 9. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (8:2).
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for condition 3, the number of ice nuclei did not confer an advantage in terms of initiating propagation. Meanwhile, in condition 2, the ice nucleus was larger, so there was a slight advantage in initiating propagation. It can be seen in Fig. 9 that the time taken for the start of propagation in condition 2 was slightly less than that in condition 3. Next, condition 1 was compared with condition 3; here, both the methods of ice nucleus charging and the sizes and numbers of ice nuclei were different, and it was found that the time taken until the start of propagation in condition 3 was shorter than that of condition 1, because, as mentioned above, in condition 1 the flow of the emulsion in the vicinity of the fixed ice nucleus was slow and sometimes stagnant. Fig. 11 shows the time taken from the addition of the ice nuclei to the start of propagation for the (9:1) emulsion. For conditions 1 and 2, the methods of ice nucleus charging were different, and the size and number of ice nuclei were the same. In the case of (9:1), since the viscosity of the emulsion was significantly increased compared to other emulsions, the flow of the emulsion due to stirring was much slower. Therefore, there was little difference between the two methods of ice nucleus charging in terms of the time taken until the start of propagation. The comparison of condition 2 with condition 3 in Fig. 11, with the same methods of ice nucleus charging but different sizes and numbers of ice nuclei, showed that the time taken was shorter in condition 3. In this case, the ability of a smaller ice nucleus to thrust through the oil layer increased, because the advantage of the thinnest oil layer outweighed the disadvantage of the largest viscosity. Thus, the increased number of ice nuclei became a slight advantage. And it was found that the time taken until the start of propagation in condition 3 was shorter than that of condition 2. Comparison of Fig. 7 with Figs. 9 and 11 showed that for the same condition of ice nucleus charging, the effect of increasing viscosity (due to increasing water content) was dominant factor in the time required from addition of the ice nucleus to the start of propagation. For each condition, the tendency of the time taken is as follows: (7:3) < (8:2) < (9:1). We now discuss the time taken from the start of propagation to the end. It can be seen in Fig. 8 that in the case of (7:3), the time from the start of propagation to the end for condition 2 was slightly less than that for condition 1. The reason for this phenomenon is that the methods of ice nucleus charging were different, while the size and number of ice nuclei were the same. As mentioned in the discussion of Fig. 7, in condition 1, the fixed ice nucleus came in contact with more water droplets compared to condition 2, because the relative velocity between the ice nucleus and the emulsion was greater. Therefore, in condition 1, the time from addition of the ice nucleus to the start of propagation was shorter. However, if more supercooled water droplets were converted to ice particles by the fixed ice nucleus in condition 1, it is likely, based on the size of the water droplets shown in Fig. 4, that the size of each ice particle formed
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by dissolution was very small. In this case [(7:3)], in addition, the oil layers with which adjacent water droplets were covered were particularly thick. As mentioned above, the size of ice particles formed under condition 1 was so small that the number of particles formed had hardly any effect on propagation. Thus, in condition 1, propagation of dissolution hardly occurred, and the time from the start of propagation to the end was relatively long. Meanwhile, in condition 2, the ice nucleus continued to be effective in causing propagation of dissolution because it was not removed from the emulsion. Therefore, the time from the start of propagation to the end was shorter than that of condition 1. Next, the time taken from the start to the end of propagation under condition 1 was compared with that of condition 3 (Fig. 8). In this case, the methods of ice nucleus charging and the sizes and numbers of ice nuclei were different. As mentioned above, under condition 1, the fixed ice nucleus caused dissolution of supercooling in more water droplets; however, it is thought that the ice particles formed by this dissolution were very small, and thus the effect on propagation was not significant. Meanwhile, in condition 3, the effects of both first dissolution (due to the addition of 29 smaller ice nuclei) and propagation (due to both the addition of 29 smaller ice nuclei and the smaller size of the ice particles formed) were also small. However, in condition 1, the fixed ice nucleus could come in contact with a larger number of supercooled water droplets, which meant that the possibility of converting them to ice particles was increased compared to condition 3, because of the ice nucleus with larger size in condition 1. Moreover, in condition 1, the larger ice nucleus was effective in causing propagation of dissolution until it was removed from the emulsion. Due to the above factors, the time from the start of propagation to the end was shorter in condition 1 than in condition 3. In the case of the (8:2) emulsion (Fig. 10), it was found that the difference between conditions 2 and 3 in terms of time from the start of propagation to the end was very small. Since, in condition 3, the sizes of both the ice nuclei added and the ice particles formed were very small, the ice nucleus and the resulting ice particles were relatively unable to thrust through the oil layers covering adjacent water droplets. Therefore, the effects of propagation of dissolution were weak. Meanwhile, even the larger ice nucleus added in condition 2 did not act effectively for propagation of dissolution compared with the (7:3) emulsion, because of the thickness of the oil layer and the viscosity of the emulsion. Due to these factors, the difference in the time taken from the start of propagation to the end in conditions 2 and 3 was small. Moreover, the time taken in condition 1 was less than that for condition 2, because of the mixed effects of the sizes of the ice nuclei and the ice particles formed, the thickness of the oil layer, and the viscosity. Figs. 8 and 10 show that since the main factor in the time taken from the start of propagation to the end was the viscosity of the emulsion, the time taken increases in the order (7:3) < (8:2) for each condition.
100
Condition1 Condition2 Condition3
80
60
40
20
0
0∼10
10∼20
20∼30
30∼40
40∼50
50∼
Time (min) Fig. 10. Probability distribution of time taken from the start of propagation to the end (8:2).
In order to estimate the propagation rate based on the total time from addition of the ice nucleus to the end of propagation, the values shown in Table 3 (obtained from Figs. 7e10) were used. The results shown in Table 3 are average values. From Table 3, the following rough tendencies were obtained. In terms of propagation rate, in the case of the (7:3) emulsion, the time taken in condition 1 was almost equal to that of condition 2, and the time taken in condition 2 was shorter than that of condition 3; in the case of the (8:2) emulsion, the time taken in condition 2 was slightly shorter than that taken in condition 3, and the time taken in condition 1 was shorter than that taken in condition 3. Based on the above results, the use of condition 1 or condition 2 is likely to be effective in increasing the propagation rate in an emulsion with a fixed composition ratio. The effect of the size of the ice nucleus on propagation rate is greater than that of the number of ice nuclei: a larger ice nucleus is more effective in increasing the propagation rate than large numbers of smaller ice nuclei. It is also concluded that in order to increase the propagation rate, the formation of ice particles with a larger size is effective. Based on Figs. 7e10, for each condition of adding the ice nucleus, the propagation rate increases in the order (7:3) < (8:2). This tendency was similar to that shown in Fig. 6. Since the method of ice nucleus charging, the size and number of ice nuclei, the size and number of ice particles formed, the thickness of the oil layer and the viscosity of the emulsion were all found to have an effect on the Table 3 Average values of total time taken from addition of the ice nucleus to the end of propagation (average propagation rate)
7:3 8:2
Condition 1 (min)
Condition 2 (min)
Condition 3 (min)
25.3 45.6
25.7 48.1
33.5 49.1
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100
100 Condition1 Condition2 Condition3
80
80
Condition1 Condition2 Condition3
60
60 40
40 20
20
0
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
2.5∼
Maximum supercooling degree (°C)
0
0∼10
10∼20
20∼30
30∼
Fig. 13. Maximum supercooling degree (8:2).
Time (min) Fig. 11. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (9:1).
propagation rate, it was concluded that mechanism of propagation is very complicated. The maximum supercooling degrees in emulsions of various composition ratios [(7:3), (8:2) and (9:1)] are shown in Figs. 12e14, respectively. Based on Fig. 12, it was found that the maximum supercooling degree in the (7:3) emulsion was smaller in condition 1 than in condition 2, and the difference between conditions 2 and 3 was very small; this tendency agreed well with that shown in Fig. 7. Similarly, from Figs. 13 and 14, for the (8:2) emulsion, the maximum degree of supercooling in condition 2 was equal to that of condition 3, and less than that of condition 1; for the (9:1) emulsion, the maximum degree of supercooling in condition 3 was less than that of condition 1, while the difference between conditions 1 and 2 was very small. These tendencies agreed approximately with those shown in Figs. 9 and 11, respectively. In these experiments, if the composition ratios of the emulsions are the same, the cooling rates of the emulsions are the same under all three conditions, because the brine temperature is constant. The above fact was confirmed from the measurement results of temperature variation.
Therefore, for a fixed composition ratio, the maximum supercooling degree was approximately proportional to time taken from addition of the ice nucleus to the start of propagation. Comparison of Fig. 12 with Figs. 13 and 14 shows that the relationships between the maximum supercooling degrees for the three conditions of ice nucleus charging were as follows: for condition 1 (7:3) < (9:1) < (8:2); for condition 2 (7:3) ¼ (8:2) ¼ (9:1); and for condition 3 (7:3) was slightly larger than (8:2), and (8:2) > (9:1). In this paper, we clarified the influence of the method of ice nucleus charging, the size and number of ice nuclei added on the propagation rate and the maximum supercooling degree; however, the influence of the surface area of ice nucleus is unknown, and this will be the subject of further investigation in the future. 4. Conclusions (1) It was clarified that the method of ice nucleus addition, the sizes and numbers of ice nuclei added and ice particles formed, the thickness of the oil layer, and the viscosity of the emulsion affected the propagation rate and the maximum supercooling degree of W/O emulsion. 100
100
Condition1 Condition2 Condition3
80
60
40
40
20
20 0 0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
2.5∼
Condition1 Condition2 Condition3
80
60
0
0∼0.5
0∼0.5
0.5∼1.0
1.0∼1.5
1.5∼2.0
Maximum supercooling degree (°C)
Maximum supercooling degree (°C)
Fig. 12. Maximum supercooling degree (7:3).
Fig. 14. Maximum supercooling degree (9:1).
2.0∼2.5
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(2) In order to increase the propagation rate of a W/O emulsion of a fixed composition ratio, it is effective to add a larger ice nucleus and form larger ice particles. (3) The propagation rate of a W/O emulsions had a tendency to decrease with increasing water content of the emulsion. (4) It was clarified that the maximum supercooling degree of a W/O emulsion of fixed composition ratio is approximately proportional to the time taken from addition of the ice nucleus to the start of propagation. Acknowledgements This study was financially supported by Chuo University as one of the 2005 Research Projects for Promotion of Advanced Research at Graduate School and by 2007 Chuo University Grant for Special Research. The authors wish to thank K. Nishiyama, H. Sekine, Y. Mitamura, M. Doi, and H. Yamauchi, who are graduates of Chuo University, for their collaboration.
References [1] P.W. Egolf, M. Kauffeld, From physical properties of ice slurries to industrial ice slurry applications, Int. J. Refrigeration 28 (1) (2005) 4e12. [2] P. Pronk, T.M. Hansen, C.A. Infante Ferreira, G.J. Wittkamp, Time-dependent behavior of different ice slurries during ice storage, Int. J. Refrigeration 28 (1) (2005) 27e36.
[3] K. Matsumoto, M. Okada, T. Kawagoe, C. Kan, Ice storage system with watereoil mixture (formation of suspension with high IPF), Int. J. Refrigeration 23 (5) (2000) 336e344. [4] T. Hirata, K. Nagasaka, M. Ishikawa, 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) (2000) 333e339. [5] T. Hirata, M. Kato, K. Nagasaka, M. Ishikawa, Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface (part II: ice removal condition), Int. J. Heat Mass Transfer 43 (5) (2000) 757e765. [6] K. Matsumoto, Y. Shiokawa, M. Okada, T. Kawagoe, C. Kan, Ice storage system with watereoil mixture (discussion about influence of additive on ice formation process), Int. J. Refrigeration (2002) 11e18. [7] Y. Oda, M. Okada, S. Nakagawa, K. Matsumoto, T. Kawagoe, Continuous ice formation in a tube by using watereoil emulsion for dynamic-type ice-making cold thermal energy storage, Int. J. Refrigeration 27 (4) (2004) 353e359. [8] K. Matsumoto, K. Oikawa, M. Okada, Y. Teraoka, T. Kawagoe, 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) (2006) 1010e1019. [9] Handbook of Heat Transfer, fourth ed., JSME, 1986, p. 331 (in Japanese). [10] T. Hozumi, A. Saito, S. Okawa, K. Watanabe, Effects of electrode materials on freezing of supercooled water in electric freeze control, Int. J. Refrigeration 26 (5) (2003) 537e542. [11] T. Hozumi, A. Saito, S. Okawa, Y. Eshita, Effects of shapes of electrodes on freezing of supercooled water in electric freeze control, Int. J. Refrigeration 28 (3) (2003) 389e395.
International Journal of Refrigeration 30 (2007) 1300e1308 www.elsevier.com/locate/ijrefrig
Study on formation of high performance ice slurry by W/O emulsion in ice storage (discussion on characteristics of propagation of supercooling dissolution) Koji Matsumotoa,*, Kazuki Sakaeb, Ken Oikawac, Masashi Okadad,1, Yoshikazu Teraokad, Tetsuo Kawagoee,2 a
Chuo University, Department of Precision Mechanics, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan b Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan c Sumitomo Heavy Industries, Ltd., 5-9-11 Kitashinagawa, Shinagawa-ku, Tokyo 141-8686, Japan d Aoyama Gakuin University, Department of Mechanical Engineering, 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa 229-8558, Japan e 2-28-3, Katakurachou, Kanagawa-ku, Yokohama 221-0865, Japan Received 1 February 2007; received in revised form 27 April 2007; accepted 1 May 2007 Available online 8 May 2007
Abstract A W/O type emulsion was developed as a new thermal material for ice storage. The water contents of the emulsions were 70, 80 and 90 vol%, and silicone oil was used. An amino group modified silicone oil with 0.9 vol% was used as a surface-active agent. The freezing points of those emulsions were 0 C. However, due to the emulsion structure, the propagation rate of supercooling dissolution for each emulsion was very slow. Therefore, the propagation rate and maximum supercooling degree were estimated using probability, varying the water content of the emulsion, the method of the ice nucleus charging, and the size and number of ice nuclei. In addition, the influence of various parameters on the propagation rate and maximum supercooling degree was clarified. Ó 2007 Elsevier Ltd and IIR. All rights reserved. Keywords: Cooling; Secondary refrigerant; Ice slurry; Thermal storage; Emulsion; Water; Oil; Thermal property
* Corresponding author. Tel.: þ81 3 3817 1837; fax: þ81 3 3817 1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected] (Y. Teraoka). 1 Tel.: þ81 42 759 6213; fax: þ81 42 759 6212. 2 Tel./fax: þ81 45 491 7973. 0140-7007/$35.00 Ó 2007 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2007.05.001
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Etude sur la production de coulis de glace tre`s performants a` l’aide d’une e´mulsion eau/huile utilise´e dans l’accumulation thermique de glace: discussion sur les caracte´ristiques de la propagation de la dissolution surrefroidissante Mots cle´s : Re´frige´ration ; Frigoporteur ; Coulis de glace ; Accumulation thermique ; E´mulsion ; Eau ; Huile ; Proprie´te´ thermique
1. Introduction Ice storage systems are superior to many other types of storage systems. For example, ice storage equipment can be smaller because the amount of thermal storage per unit volume is larger than those of other thermal storage systems. In a dynamic ice storage system, since ice slurry used as a thermal storage material has good fluidity, a large amount of cold energy can be transported with less pumping work; the dynamic system can also respond quickly to changes in heat load because the ice particles have a large surface area. Many studies on ice slurries have been carried out [1e7]. Authors have also studied the formation of ice slurries by use of W/O (water in oil) emulsions. This method is expected to be useful because it overcomes certain problems, such as a decrease in latent heat fusion, and ice adhesion to a cooling wall. In one previous report [8], it was clarified that W/O emulsions were promising as a thermal storage material for dynamic ice storage. Moreover, the use of a W/O emulsion allows stable ice slurry formation in a stainless vessel, despite the difficulties previously encountered in achieving this without ice adhesion to the cooling wall. Optimum water and oil contents were also proposed. However, it was confirmed that propagation rate of supercooling dissolution for the W/O emulsions was much lower than that for a general liquid such as water. The effectiveness of statistical methods for estimating the dissolution phenomenon of supercooling was reported by Hozumi et al. [10,11]. In this study, the propagation rate and maximum supercooling degree were discussed based on probability, using the formula ([frequency of result satisfying given condition]/[frequency of all experiments] 100 [%]), varying the water content of the emulsion, the method of ice nucleus charging, and the size and number of ice nuclei.
of the W/O emulsions used were 0 C. Hereinafter, the word ‘‘emulsion’’ represents a W/O emulsion. The experimental apparatus is shown in Fig. 1. A copper plate with 1 mm thick was set on a Peltier module for cooling purposes. Grease with high thermal conductivity was applied to the gap between the plate and the module. The emulsion was then spread on the plate with a thickness and surface area of about 0.3 mm and 50 mm 60 mm, respectively. The emulsion on the plate was kept cool by the module. When the representative temperature of the plate reached the set temperature (8 C), supercooling dissolution of the emulsion began to propagate due to ice nucleus charging. The shape of the ice nucleus was hemispherical and its diameter was about 8 mm. The propagation process of dissolution was observed by CCD, and the propagation rate was obtained using CCD images as follows. Images of a known scale were taken by CCD at 100 magnification before the experiment; propagation of dissolution in a fixed direction was also photographed at 100 magnification, and the propagation distance was obtained by comparison with the known scale. Simultaneously, the time required to reach the distance was measured, and the propagation rate was obtained by dividing the time by the distance (6 mm). The propagation rate of water was also measured using a high-speed camera, and the propagation rates of various emulsions were compared with that of water. Since the
Insulator CCD Copper plate
2. Experiment 2.1. Measurement of propagation rate of supercooling dissolution
Sample Ice nucleus Peltier module
This experiment was carried out in order to clarify the propagation rate for a W/O emulsion. The freezing points
Fig. 1. Experimental apparatus for measurement of propagation rate.
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thermocouple used (Type T) can act as a trigger for dissolution of supercooling, the representative temperature of the plate was determined as follows: a temperature at a fixed point on the plate was measured before the experiment, and the temperature difference between the fixed point and the center of the plate was determined. The representative temperature of the plate was then defined by correcting the temperature at the fixed point based on the temperature difference.
2.2. Measurement of propagation rate in the process of ice slurry formation using probability In order to discuss the propagation rate of supercooling dissolution in the process of ice slurry formation, the probability of propagation within a certain time period was measured. The same method was also used to measure the maximum degree of supercooling. The experimental apparatus is shown in Fig. 2. A PMP (polymethylpentene) vessel with a volume of 2 l and an inner diameter of about 130 mm was used as an ice slurry formation vessel. The emulsion (1.1 l) was placed into the vessel. The surface height of the emulsion was about 86 mm. The emulsion was then cooled by addition of cold brine (5.4 C) with stirring (250 rpm). When it reached the set temperature (0 C), an ice nucleus was added to the emulsion in order to start propagation of dissolution. The temperatures of the emulsion and the ice slurry were measured using a platinum resistance thermometer located at a distance of 15 mm from the wall towards the center of the vessel and 10 mm upward from the bottom. The temperature measured at the above point is the representative temperature of the emulsion. In this way, based on the relationships between the temperature variation measured at the above point and the temperature variations at four different points, measured by Type T thermocouples, it was confirmed that there was synchronism in the propagation process. Synchronism was also observed
Stirrer Thermometer W/O emulsion
between the temperature variation at the above point and the temperature variation of the bulk emulsion surface, measured by a radiation thermometer. The conditions for adding the ice nucleus to the emulsion stirred were as follows: (1) condition 1: a hemispherical ice nucleus of about 8 mm diameter was attached to a stick and placed in the emulsion at a distance of 40 mm beneath the surface of the emulsion and 45 mm from the vessel center towards the vessel wall. It was removed from the emulsion immediately after the start of propagation of dissolution (immediately after the temperature rise of the emulsion); (2) condition 2: a hemispherical ice nucleus of about 8 mm in diameter was dropped into the emulsion at a distance of 45 mm from the vessel center towards the vessel wall; (3) condition 3: 29 hemispherical ice nuclei of about 1 mm in diameter were dispersively dropped into the emulsion in a square region with an area of 30 mm 30 mm. The center of the square region was located at a distance of 45 mm from the vessel center towards the vessel wall. In the case of condition 2, since the ice nucleus remains in the emulsion until the end of the experiment, it continues to affect the propagation of dissolution. In condition 3, ice nuclei are much smaller than those used in conditions 1 and 2. The total volume of the 29 ice nuclei is nearly equal to the volume of the ice nucleus used in conditions 1 and 2; however, the numbers and the total surface area of the ice nuclei are much greater. 2.3. Composition ratio of W/O emulsion The composition ratios of the emulsions used are shown in Table 1, expressed as a volumetric ratio of water and oil. In this experiment, the oil and water used were silicone oil and tap water, respectively. The water contents of the emulsions were 70, 80 and 90 vol%. The viscosity of the silicone oil was about 9038 mPa s (10 cSt) at 25 C [9]. The naming system we use for the emulsions is as follows: an emulsion with a water/oil volumetric ratio of 8:2 is known as (8:2). An amino group modified silicone oil (0.9 vol%) was used as a surface-active agent [8].
Table 1 Composition ratios of W/O emulsions
Brine
Constant-temperature bath
Fig. 2. Experimental apparatus for measurement of propagation rate and maximum supercooling degree with stirring.
Tap water (ml) Silicone oil (ml) Additive (ml) Ratio of water to oil
990 110 10 9:1
880 220 10 8:2
770 330 10 7:3
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Table 2 Apparent viscosity of W/O emulsions [8] (0 C, rotation speed: 5 rpm)
Fig. 3. W/O emulsion (8:2).
2.4. Structure and viscosity of W/O emulsion A photograph of the (8:2) emulsion, as a representative example, and an enlarged photograph showing its structure are shown in Figs. 3 and 4, respectively. It can be seen that the emulsion was cloudy and had a high viscosity, and that the dispersed phase consisted of many spherical water droplets with a mean diameter of several tens of micrometer, each coated with an oil layer. The viscosity of the emulsion was measured using a rotational viscometer with a rotation speed of 5 rpm. The results of these measurements are shown in Table 2. Since all of the emulsions were nonNewtonian fluids, the viscosity was expressed as an apparent viscosity; hereinafter, the word ‘‘viscosity’’ represents apparent viscosity. The size of the dispersed phase had a tendency to decrease with increasing water content, and the viscosity of the emulsion increased as the size of the dispersed phase decreased. In this way, the viscosity of the emulsions increased, as shown in Table 2. 2.5. Characteristics of propagation of supercooling dissolution in W/O emulsion
Composition ratio
Apparent viscosity (Pa s)
7:3 8:2 9:1
10.1 23.0 72.1
was found that the time required to propagate supercooling dissolution of the emulsion was much greater than that required for a general liquid such as water.
3. Results and discussion 3.1. Propagation rate of supercooling dissolution Propagation rates for various composition ratios are shown in Fig. 6. Error bars represent the range of measurement values. For each composition ratio, the average value from about 20 measurements is shown as a symbol. In the following discussion, the result of Fig. 6 was estimated by those average values. It was found that for all composition ratios, the propagation rate was about 1/1800 that of water. Moreover, the propagation rate decreased slightly with increasing water content. Generally, it is thought that the propagation rate has a tendency to increase with increasing water content because this results in a decrease in the thickness of the oil layer; meanwhile, there is a contrasting tendency for the propagation rate to decrease with increasing water content, because this causes an increase in the viscosity of the emulsion, as shown in Table 2. Since the effects of oil layer thickness and viscosity are opposed to each other, it is thought that difference between these effects results in the overall outcome, as shown in Fig. 6.
An example of temperature variation in the emulsion during the process of ice slurry formation is shown in Fig. 5. It 2
Temperature (°C)
1
Oil (Continuous Phase)
A: Addition time of ice nucleus B: Start of propagation of dissolution C C: End of propagation of dissolution
0 -1
A
B
-2 -3
Water (Dispersed Phase) 100. 000 μm Fig. 4. Enlarged photograph of W/O emulsion (8:2).
-4 2000
4000
6000
8000
10000
Time (s) Fig. 5. Example of temperature variation of W/O emulsion.
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Dissolution rate ( m/sec)
100 80
Water 1.1×105(μm/sec)
80
60
60
40
40
20
20
7:3
8:2
9:1
Water
Ratio of water to oil
3.2. Propagation rate of supercooling dissolution during the process of ice slurry formation The temperature variation of the emulsion was measured during formation of an ice slurry with stirring. In order to estimate the propagation rate, the time periods from addition of the ice nucleus to the start of propagation of dissolution (A to B in Fig. 5) and from the start of propagation to the end (B to C in Fig. 5) were discussed based on probability. The maximum degree of supercooling (point B in Fig. 5), which was also discussed on this basis, was set as the point immediately before the temperature of the emulsion began to rise. For each composition ratio, about 30 measurements were carried out for each of conditions 1, 2, and 3 as described in Section 2.2. In the pre-experimental stage, it was confirmed that the stirring wing, platinum resistance thermometer and thermocouple had no effect on the propagation of dissolution. The measurement results for the time period from addition of the ice nucleus to the start of propagation are shown in Figs. 7, 9, and 11, which represent emulsions with 100 Condition1 Condition2 Condition3
60
40
20
0
0∼10
10∼20
20∼30
0
0~10
10~20
20~30
30~
Time (min)
Fig. 6. Propagation rate of supercooling dissolution.
80
Condition1 Condition2 Condition3
30∼
Time (min) Fig. 7. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (7:3).
Fig. 8. Probability distribution of time taken from the start of propagation to the end (7:3).
composition ratios of (7:3), (8:2) and (9:1), respectively. The results for the time period from the start of propagation to the end are shown in Figs. 8 and 10, which correspond to (7:3) and (8:2), respectively. For (9:1), the experiment was stopped because ice adhesion to the cooling wall occurred immediately after the start of propagation. In Fig. 7, the condition 1 was compared with the condition 2, where the methods of ice nucleus charging were different. In this composition ratio (7:3), the emulsion showed the lowest viscosity, as shown in Table 2. From Fig. 7, it can be seen that the time from addition of the ice nucleus to the start of propagation for condition 1 was shorter than that of condition 2 (remember that the size and number of ice nuclei are the same); the time required until the start of propagation for condition 1 was almost always less than 10 min. The reason for this phenomenon is as follows. In condition 1, in which the ice nucleus was pulled from the emulsion but was fixed at one point in the emulsion until the start of propagation, the relative velocity between the fixed ice nucleus and the emulsion was greater than that between the dropped ice nucleus and the emulsion, as in condition 2. Therefore, in condition 1, the fixed ice nucleus came in contact with more water droplets (dispersed phase), which resulted in a shorter time until the start of propagation. In conditions 2 and 3 (Fig. 7), the method of charging with the ice nucleus was the same, but the sizes and numbers of the ice nuclei were different. When an ice nucleus is dropped, it must thrust through the oil layer (continuous phase) and continue to come in contact with water droplets (dispersed phase) for a certain time in order to initiate propagation. If the size of the ice nucleus is smaller, it thrusts through the oil layer with more difficulty. Additionally, the thickness of the oil layer increases with decreasing water content, but the viscosity of the emulsion decreases with decreasing water content. Thus, the effects of oil layer thickness and viscosity have opposing effects on the ability of the ice nucleus to thrust through the oil layer and start propagation. The ice nuclei in condition 3 were much smaller than that used in
K. Matsumoto et al. / International Journal of Refrigeration 30 (2007) 1300e1308
condition 2; therefore, in spite of the lowest viscosity, the ability of these ice nuclei to come in contact with many water droplets decreased due to the increased difficulty of pushing through the oil layer. Thus, in condition 3, the effect of the number of ice nuclei did not prove advantageous in terms of initiating propagation. Meanwhile, the larger size of the ice nucleus in condition 2 was not advantageous for the start of propagation because of the thickest oil layer. Therefore, the difference between the results for conditions 2 and 3 was very small. Fig. 9 shows the results of ice nucleus addition in the (8:2) emulsion. For conditions 1 and 2, the methods of ice nucleus charging were different, but the size and number of ice nuclei were the same. In this case, it was found that the time from addition of ice nucleus to the start of propagation in condition 1 was greater than that of condition 2. The viscosity of this emulsion (8:2) is greater than that of the emulsion (7:3) shown in Figs. 7 and 8. In condition 1, the ice nucleus attached to the stick was fixed at one point in the emulsion, and the flow of the emulsion in the vicinity of the ice nucleus was slower and sometimes stagnant due to the large viscosity; thus, the relative velocity between the ice nucleus and the emulsion was decreased in the vicinity of the ice nucleus, and the number of water droplets with which the ice nucleus could come in contact decreased. In condition 2, however, the ice nucleus remained in the emulsion and continued to be effective in initiating propagation. Therefore, it was thought that the relationship between conditions 1 and 2 in this case (8:2) was the reverse of the previous relationship (shown in Fig. 7). Next, condition 2 was compared with condition 3 in Fig. 9. In this case, the method of ice nucleus charging was the same but the sizes and numbers of ice nuclei were different. The thickness of the oil layer in (8:2) was less than that of (7:3), but the viscosity of the (8:2) emulsion was greater than that of (7:3). Since, in condition 3, the sizes of the ice nuclei were so small, they were hardly able to thrust through the oil layer due to the difference between the effects of oil layer thickness and viscosity. Therefore, 100 Condition1 Condition2 Condition3
80
60
40
20
0
0∼10
10∼20
20∼30
30∼
Time (min) Fig. 9. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (8:2).
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for condition 3, the number of ice nuclei did not confer an advantage in terms of initiating propagation. Meanwhile, in condition 2, the ice nucleus was larger, so there was a slight advantage in initiating propagation. It can be seen in Fig. 9 that the time taken for the start of propagation in condition 2 was slightly less than that in condition 3. Next, condition 1 was compared with condition 3; here, both the methods of ice nucleus charging and the sizes and numbers of ice nuclei were different, and it was found that the time taken until the start of propagation in condition 3 was shorter than that of condition 1, because, as mentioned above, in condition 1 the flow of the emulsion in the vicinity of the fixed ice nucleus was slow and sometimes stagnant. Fig. 11 shows the time taken from the addition of the ice nuclei to the start of propagation for the (9:1) emulsion. For conditions 1 and 2, the methods of ice nucleus charging were different, and the size and number of ice nuclei were the same. In the case of (9:1), since the viscosity of the emulsion was significantly increased compared to other emulsions, the flow of the emulsion due to stirring was much slower. Therefore, there was little difference between the two methods of ice nucleus charging in terms of the time taken until the start of propagation. The comparison of condition 2 with condition 3 in Fig. 11, with the same methods of ice nucleus charging but different sizes and numbers of ice nuclei, showed that the time taken was shorter in condition 3. In this case, the ability of a smaller ice nucleus to thrust through the oil layer increased, because the advantage of the thinnest oil layer outweighed the disadvantage of the largest viscosity. Thus, the increased number of ice nuclei became a slight advantage. And it was found that the time taken until the start of propagation in condition 3 was shorter than that of condition 2. Comparison of Fig. 7 with Figs. 9 and 11 showed that for the same condition of ice nucleus charging, the effect of increasing viscosity (due to increasing water content) was dominant factor in the time required from addition of the ice nucleus to the start of propagation. For each condition, the tendency of the time taken is as follows: (7:3) < (8:2) < (9:1). We now discuss the time taken from the start of propagation to the end. It can be seen in Fig. 8 that in the case of (7:3), the time from the start of propagation to the end for condition 2 was slightly less than that for condition 1. The reason for this phenomenon is that the methods of ice nucleus charging were different, while the size and number of ice nuclei were the same. As mentioned in the discussion of Fig. 7, in condition 1, the fixed ice nucleus came in contact with more water droplets compared to condition 2, because the relative velocity between the ice nucleus and the emulsion was greater. Therefore, in condition 1, the time from addition of the ice nucleus to the start of propagation was shorter. However, if more supercooled water droplets were converted to ice particles by the fixed ice nucleus in condition 1, it is likely, based on the size of the water droplets shown in Fig. 4, that the size of each ice particle formed
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by dissolution was very small. In this case [(7:3)], in addition, the oil layers with which adjacent water droplets were covered were particularly thick. As mentioned above, the size of ice particles formed under condition 1 was so small that the number of particles formed had hardly any effect on propagation. Thus, in condition 1, propagation of dissolution hardly occurred, and the time from the start of propagation to the end was relatively long. Meanwhile, in condition 2, the ice nucleus continued to be effective in causing propagation of dissolution because it was not removed from the emulsion. Therefore, the time from the start of propagation to the end was shorter than that of condition 1. Next, the time taken from the start to the end of propagation under condition 1 was compared with that of condition 3 (Fig. 8). In this case, the methods of ice nucleus charging and the sizes and numbers of ice nuclei were different. As mentioned above, under condition 1, the fixed ice nucleus caused dissolution of supercooling in more water droplets; however, it is thought that the ice particles formed by this dissolution were very small, and thus the effect on propagation was not significant. Meanwhile, in condition 3, the effects of both first dissolution (due to the addition of 29 smaller ice nuclei) and propagation (due to both the addition of 29 smaller ice nuclei and the smaller size of the ice particles formed) were also small. However, in condition 1, the fixed ice nucleus could come in contact with a larger number of supercooled water droplets, which meant that the possibility of converting them to ice particles was increased compared to condition 3, because of the ice nucleus with larger size in condition 1. Moreover, in condition 1, the larger ice nucleus was effective in causing propagation of dissolution until it was removed from the emulsion. Due to the above factors, the time from the start of propagation to the end was shorter in condition 1 than in condition 3. In the case of the (8:2) emulsion (Fig. 10), it was found that the difference between conditions 2 and 3 in terms of time from the start of propagation to the end was very small. Since, in condition 3, the sizes of both the ice nuclei added and the ice particles formed were very small, the ice nucleus and the resulting ice particles were relatively unable to thrust through the oil layers covering adjacent water droplets. Therefore, the effects of propagation of dissolution were weak. Meanwhile, even the larger ice nucleus added in condition 2 did not act effectively for propagation of dissolution compared with the (7:3) emulsion, because of the thickness of the oil layer and the viscosity of the emulsion. Due to these factors, the difference in the time taken from the start of propagation to the end in conditions 2 and 3 was small. Moreover, the time taken in condition 1 was less than that for condition 2, because of the mixed effects of the sizes of the ice nuclei and the ice particles formed, the thickness of the oil layer, and the viscosity. Figs. 8 and 10 show that since the main factor in the time taken from the start of propagation to the end was the viscosity of the emulsion, the time taken increases in the order (7:3) < (8:2) for each condition.
100
Condition1 Condition2 Condition3
80
60
40
20
0
0∼10
10∼20
20∼30
30∼40
40∼50
50∼
Time (min) Fig. 10. Probability distribution of time taken from the start of propagation to the end (8:2).
In order to estimate the propagation rate based on the total time from addition of the ice nucleus to the end of propagation, the values shown in Table 3 (obtained from Figs. 7e10) were used. The results shown in Table 3 are average values. From Table 3, the following rough tendencies were obtained. In terms of propagation rate, in the case of the (7:3) emulsion, the time taken in condition 1 was almost equal to that of condition 2, and the time taken in condition 2 was shorter than that of condition 3; in the case of the (8:2) emulsion, the time taken in condition 2 was slightly shorter than that taken in condition 3, and the time taken in condition 1 was shorter than that taken in condition 3. Based on the above results, the use of condition 1 or condition 2 is likely to be effective in increasing the propagation rate in an emulsion with a fixed composition ratio. The effect of the size of the ice nucleus on propagation rate is greater than that of the number of ice nuclei: a larger ice nucleus is more effective in increasing the propagation rate than large numbers of smaller ice nuclei. It is also concluded that in order to increase the propagation rate, the formation of ice particles with a larger size is effective. Based on Figs. 7e10, for each condition of adding the ice nucleus, the propagation rate increases in the order (7:3) < (8:2). This tendency was similar to that shown in Fig. 6. Since the method of ice nucleus charging, the size and number of ice nuclei, the size and number of ice particles formed, the thickness of the oil layer and the viscosity of the emulsion were all found to have an effect on the Table 3 Average values of total time taken from addition of the ice nucleus to the end of propagation (average propagation rate)
7:3 8:2
Condition 1 (min)
Condition 2 (min)
Condition 3 (min)
25.3 45.6
25.7 48.1
33.5 49.1
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100
100 Condition1 Condition2 Condition3
80
80
Condition1 Condition2 Condition3
60
60 40
40 20
20
0
0.5∼1.0
1.0∼1.5
1.5∼2.0
2.0∼2.5
2.5∼
Maximum supercooling degree (°C)
0
0∼10
10∼20
20∼30
30∼
Fig. 13. Maximum supercooling degree (8:2).
Time (min) Fig. 11. Probability distribution of time taken from addition of the ice nucleus to the start of propagation (9:1).
propagation rate, it was concluded that mechanism of propagation is very complicated. The maximum supercooling degrees in emulsions of various composition ratios [(7:3), (8:2) and (9:1)] are shown in Figs. 12e14, respectively. Based on Fig. 12, it was found that the maximum supercooling degree in the (7:3) emulsion was smaller in condition 1 than in condition 2, and the difference between conditions 2 and 3 was very small; this tendency agreed well with that shown in Fig. 7. Similarly, from Figs. 13 and 14, for the (8:2) emulsion, the maximum degree of supercooling in condition 2 was equal to that of condition 3, and less than that of condition 1; for the (9:1) emulsion, the maximum degree of supercooling in condition 3 was less than that of condition 1, while the difference between conditions 1 and 2 was very small. These tendencies agreed approximately with those shown in Figs. 9 and 11, respectively. In these experiments, if the composition ratios of the emulsions are the same, the cooling rates of the emulsions are the same under all three conditions, because the brine temperature is constant. The above fact was confirmed from the measurement results of temperature variation.
Therefore, for a fixed composition ratio, the maximum supercooling degree was approximately proportional to time taken from addition of the ice nucleus to the start of propagation. Comparison of Fig. 12 with Figs. 13 and 14 shows that the relationships between the maximum supercooling degrees for the three conditions of ice nucleus charging were as follows: for condition 1 (7:3) < (9:1) < (8:2); for condition 2 (7:3) ¼ (8:2) ¼ (9:1); and for condition 3 (7:3) was slightly larger than (8:2), and (8:2) > (9:1). In this paper, we clarified the influence of the method of ice nucleus charging, the size and number of ice nuclei added on the propagation rate and the maximum supercooling degree; however, the influence of the surface area of ice nucleus is unknown, and this will be the subject of further investigation in the future. 4. Conclusions (1) It was clarified that the method of ice nucleus addition, the sizes and numbers of ice nuclei added and ice particles formed, the thickness of the oil layer, and the viscosity of the emulsion affected the propagation rate and the maximum supercooling degree of W/O emulsion. 100
100
Condition1 Condition2 Condition3
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40
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Maximum supercooling degree (°C)
Fig. 12. Maximum supercooling degree (7:3).
Fig. 14. Maximum supercooling degree (9:1).
2.0∼2.5
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(2) In order to increase the propagation rate of a W/O emulsion of a fixed composition ratio, it is effective to add a larger ice nucleus and form larger ice particles. (3) The propagation rate of a W/O emulsions had a tendency to decrease with increasing water content of the emulsion. (4) It was clarified that the maximum supercooling degree of a W/O emulsion of fixed composition ratio is approximately proportional to the time taken from addition of the ice nucleus to the start of propagation. Acknowledgements This study was financially supported by Chuo University as one of the 2005 Research Projects for Promotion of Advanced Research at Graduate School and by 2007 Chuo University Grant for Special Research. The authors wish to thank K. Nishiyama, H. Sekine, Y. Mitamura, M. Doi, and H. Yamauchi, who are graduates of Chuo University, for their collaboration.
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