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Continuous ice slurry formation using a functional fluid for ice storage
International Journal of Refrigeration 27 (2004) 73–81 www.elsevier.com/locate/ijrefrig
Continuous ice slurry formation using a functional fluid for ice storage Koji Matsumotoa,*, Yoshiharu Namikib, Masashi Okadac, Tetsuo Kawagoed, Shinji Nakagawae, Chaedong Kangf a
Professor, Chuo University, Department of Precision Mechanics, 1-13-27 Kasuga, Bunkyo-ku, Tokyo, 112-8551 Japan b Mitsubishi Pencil Co. Ltd., 5-23-37 Higashiohi, Shinagawa-ku, Tokyo, 140-8537 Japan c Professor, Aoyama Gakuin University, Department of Mechanical Engineering, 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa, Prefecture 229-8558, Japan d 2-28-3 Katakurachou, Kanagawa-ku, Yokohama, 221-0865 Japan e Lecturer, Toyama Prefectural University, Department of Mechanical System Engineering, 5180 Kurokawa Kosugi-chou, Toyama Prefecture 939-0398, Japan f Assistant Professor, Chonbuk National University, Department of Mechanical Engineering, 664-14, Duckjin-dong, Jeonju 561-756, South Korea Received 6 May 2003; received in revised form 30 June 2003; accepted 30 June 2003
Abstract A functional fluid composed of an oil–water mixture with an additive is transformed into an ice slurry by cooling while stirring. This paper describes a new continuous ice slurry formation method. Experiments were carried out by varying conditions such as the supply time of functional fluid, the stirrer torque, brine temperature and degree of supercooling. As a result, the characteristics of the ice formation and recovery processes were clarified. It was found that the ice particles gradually became uniform in size and spherical, and grew to 3.5 mm in diameter during about 10 h. The factors influencing the size of formed ice particles were discussed because the larger ice particles were expected to melt more rapidly. The ice particle size was found to increase with decreasing degree of supercooling and cooling rate, and with increasing stirrer wing diameter. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Ice slurry; Thermal storage; Two phase mixture; Process; Experimental investigation; Two-phase secondary refrigerant
Ge´ne´ration continue de coulis de glace a` l’aide d’un me´lange huile/eau utilise´ en application accumulation thermique Mots cle´s : Coulis de glace ; Accumulation thermique ; Me´lange diphasique ; Proce´de´ ; Expe´rimentation ; Frigoporteur diphasque
* Corresponding author. Tel.: +81-3-3817-1837; fax: +813-3817-1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected] (S. Nakagawa), [email protected] (C. Kang). 0140-7007/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00102-6
1. Introduction A peak cut and peak shift in electric power demand for the ice storage system can be realized by the use of
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nighttime electric power, instead of the much more costly daytime electric power. This also cuts down on the discharge of CO2 gas by virtue of the characteristics of nighttime electric power. Thus, the spread of the ice storage system use can reduce the strain on the environment. Authors have already studied ice slurry formation by cooling a functional fluid in a small vessel with stirring [1,2]. The functional fluid consisted of 10 vol.% siliconeoil and 90 vol.% water with a small amount of silanecoupler additive. Based on this research, a method of ice slurry formation intended for use in ice storage systems was proposed. This method was much better than conventional methods. A method for continuous ice slurry formation has been investigated by many researchers [3–5]. However, no concrete results have been yielded. Authors have also reported on the continuous formation of ice slurry in a tube-type heat exchanger of [6]. A new apparatus was built, drawing on existing data. The apparatus features a much larger ice formation chamber compared to previous models, because authors take into account the practicalities encountered in ice formation systems. The present paper proposes a new method of continuous ice slurry formation in connection with this new apparatus. The method involved overflowing of the ice slurry out of the cylindrical ice formation chamber by intermittent inflow of functional fluid to this chamber. The overflowed ice slurry is separated into ice particles and functional fluid, and only ice particles are recovered. The amount of recovered ice was monitored while varying the stirring torque and supply time of functional fluid. The total continuous ice formation time varied between 5 and 10 h. Characteristics of the ice formation and recovery processes were discussed. In order to enhance the melting rate of ice particles, experiments were conducted to increase the size of the ice particles by varying the degree of supercooling, cooling rate of functional fluid and wing diameter of stirrer.
cylindrical vessel is termed an ice formation chamber. The ice slurry level in the vessel is raised by intermittent inflow of the functional fluid. The ice slurry subsequently overflows and is discharged from the vessel. Hence, the ice particles formed are removed from the vessel and the stirring power can be decreased. Ice slurry is separated into ice particles and the functional fluid in a recovery compartment. Continuous ice formation is achieved by recycling of only the functional fluid back to the vessel. In order to find the ratio of solid to liquid in the vessel during the ice formation process, the change in the amount of ice formed is measured in terms of the change in the ice slurry stirring torque. 2.2. Experimental apparatus The experimental apparatus is shown in Fig. 1(a). The ice formation part consists of a cylindrical ice formation chamber and a cooling chamber through which cold brine is circulated. The ice formation chamber is made of polyethylene. The inner diameter, height, thickness, and heat transfer area of this chamber are 300 mm, 310 mm, 2.3 mm, and 0.203 m2, respectively. The functional
2. Experiment 2.1. Method of continuous ice formation The functional fluid developed by previous studies [1,2] has excellent characteristics. Hardly any adhesion of ice occurs on the cooling chamber walls when the fluid is frozen. Also, at a very small depression of freezing point, nearly all the water in the functional fluid can be frozen and the ice particles formed by the functional fluid are kept in a dispersed state for a long time. In the present study, continuous formation of ice slurry using this functional fluid was carried out. In order to use data obtained in our previous experiments, we adopted a method that involved stirring in a cylindrical vessel that was directly soaked in cold brine. This
Fig. 1. (a) Experimental apparatus, (b) stirrer wing.
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fluid in this chamber is cooled by circulating cold brine with a magnetic pump. An outlet to discharge ice slurry is located 150 mm above the bottom of the ice formation chamber. The inner diameter and attaching angle of the outlet are 40 mm and 10 , respectively. The stirrer wing is shown in Fig. 1(b).The wing diameter and height of the stirrer used during the ice formation process are 208 and 200 mm, respectively. Its speed is set to 120 rpm. The maximum volume of functional fluid in the ice formation chamber is 10 l without stirring, but when the liquid is stirred at 120 rpm, this value drops to 6.7 l because a part of the liquid overflows through the outlet as a result of the stirring. The 6.7 l is taken to be the ‘‘initial volume of the liquid’’. The functional liquid in the precooling chamber is maintained at about 1.0 C by controlled heater and handy-cooler. The temperature is slightly above the freezing point of the functional fluid at its initial concentration. By the intermittent inflow of functional fluid, the functional liquid level in the ice formation chamber is raised, which is followed by the overflow of ice slurry through the outlet. A bag made of nylon net (mesh34) in the recovery compartment enables the recovery of ice particles by filtering out the ice particles in the ice slurry from the functional fluid. The separated liquid returns to the precooling chamber. The apparatus is set in the cold room maintained at an ambient temperature below 0 C. The temperature of the functional liquid in the ice formation chamber is measured by a platinum resistance thermometer, and the temperatures of the functional liquid in the precooling chamber, the brine, and the cold room environment are measured by type T thermo-couples. The stirring torque of the ice slurry during the ice formation process is measured by a torque meter attached to the shaft of the stirrer. 2.3. Experimental procedure The functional fluid is composed of a 30 l water–silicon oil mixture with 90 vol.% water content and a small amount of silane-coupler additive. The concentration of the silane-coupler, i.e., g-aminopropyltriethoxysilane, is 4 wt.% for the water. The freezing point of the functional fluid at its initial concentration is 1.2 C. The freezing of the functional fluid in the ice formation chamber is initiated by circulating the temperature-controlled cold brine. The degree of supercooling of the functional fluid is controlled by throwing a 1 ml ice particle into the functional fluid contained in the ice formation chamber. Using the relationship between IPF (Ice Packing Factor) and stirring torque obtained through preliminary experiments, the value of IPF in the ice formation chamber can be estimated indirectly by measuring the torque during ice formation. Once the torque reaches a target value because of ice formation, the functional fluid is fed to the ice formation chamber
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at a fixed flow rate of 75 ml/s for a prescribed period of time, accompanied by the simultaneous recovery of ice particles separated from ice slurry. The end of an individual recovery run is marked by the torque reaching the target value again, immediately following which the next recovery run begins. The separation of ice particles from the functional fluid is carried out in a cold room for 60 min. The recovered ice particles are photographed by a digital camera, and their mass is measured. The experiments are repeated at varying target torques and supply times of functional fluid.
3. Results and discussion 3.1. Characteristics of ice formation and ice recovery The experimental conditions are as follows: the brine temperature is 7.0 C; the fixed flow rate of functional fluid is 75 ml/s, the supercooling degree of the functional fluid T is 0.1 C; target torques are 0.100, 0.150 and 0.200 Nm, and supply times are 20, 40, 60 and 80 s. For 80 s, total volume of the functional fluid supplied is 6000 ml, which approximately corresponds to the initial volume of functional fluid in the ice formation chamber (6700 ml). Under these experimental conditions, ice formation runs were carried out continuously for 300 min. In order to find the volume of ice remaining in the ice formation chamber and pipe after a 300 min recovery, functional fluid of a prescribed volume (75 ml/s360 s) was fed to the ice formation chamber and the remaining ice was recovered. The experiment was repeated at least twice at each condition to confirm reproducibility. The variation with time in the stirring torque and ice slurry temperature in the ice formation chamber is shown in Fig. 2(a). The independent variable is the supply time. The fixed target torque is 0.150 Nm. The bold solid line and plain solid line show the temperature and torque, respectively. The horizontal axis represents time after dissolution of supercooling. With ice formation, the stirring torque of ice slurry increases and ice slurry temperature drops. When the torque reaches the target value, the functional fluid is supplied to the ice formation chamber, and the ice slurry is discharged from the outlet of the ice formation chamber. At this time, the torque decreases because the ratio of ice to liquid decreases. Simultaneously, the temperature of the ice slurry in the ice formation chamber increases slightly due to the introduction of functional fluid at 1.0 C from the precooling chamber. After inflow of functional fluid stops, the torque increases and the temperature decreases because of ice formation. This process is repeated for a total period of 300 min after dissolution of supercooling. As seen in Fig. 2(a), the time evolution of the torque and temperature until the torque reaches the target
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value for the first time after dissolution of supercooling appears virtually identical for all supply times; the graphs look different beyond that point. In the case of 80 s, it takes about 60 min to reach the target value again because the ratio of solid to liquid in the ice formation chamber changes greatly due to the large amount of ice slurry discharged. By contrast, in the case of 20 s, it takes about 15 min because the amount of ice slurry discharged is smaller. In other words, the recovery time is quadrupled when the supply time is quadrupled. Similar trends are observed for a target torque of 0.100 and 0.200 Nm. When the supply time is longer, the temperature of the functional fluid in the ice formation chamber is raised by a greater extent because a larger amount of functional fluid in the ice formation
Fig. 2. (a) Variation in stirring torque and ice slurry temperature with time (target torque=0.150 Nm). (b). Relationship between end time of each recovery run and amount of recovered ice.
chamber is replaced with fresh functional fluid from the precooling chamber. The relationship between the end time of each recovery run and the amount of recovered ice corresponding to Fig. 2(a) is shown in Fig. 2(b). From Fig. 2(b), it is found that the amount of recovered ice in each recovery run is nearly stable for all supply times, viz. 20, 40, 60 and 80 s. In the case of 80 s, the average amount of recovered ice in a single recovery run is about 740 g. The corresponding values for the 60, 40 and 20 s are about 600, 400 and 200 g, respectively. The supply time is proportional to the amount of recovered ice below a supply time of 60 s. However, at a supply time of 80 s, the relationship is no longer linear. For longer supply times, the amount of recovered ice in a single recovery run is higher but the number of recovery runs per experiment is lower. At a supply time of 20 s [denoted by the symbol * in Fig. 2(b)], there is little recovered ice at around 270 min, while the value at around 290 min is nearly twice the values of the other recovery runs. The reason is as follows: most of the ice discharged at 270 min stagnates in the outlet because of the low attaching angle of the outlet and a small amount of functional fluid supplied. The stagnant ice is recovered in the next recovery run, i.e. at 290 min. The same trend is observed whenever the supply time is set to 20 s, regardless of the other conditions. Under all conditions, the amount of ice in the first recovery is lower than in subsequent recoveries. The reason is as follows: since the ice slurry level is confirmed to be lower at the start of the first recovery than at the start of ice formation, it is thought that a part of the functional fluid supplied is consumed in raising the level. Hence, amount of ice slurry discharged decreases. The variation with time in the stirring torque and temperature of the ice slurry in the ice formation chamber is shown in Fig. 3(a). The variable in this case is the target torque. The supply time is fixed at 40 s. The time elapsed until the first recovery is about 60, 100 and 120 min for a target torque of 0.100, 0.150 and 0.200 Nm, respectively. The relationship between the end time of each recovery run and the amount of recovered ice corresponding to Fig. 3(a) is shown in Fig. 3(b). Fig. 3(b) indicates that the amount of ice recovered in each recovery run is nearly stable in all cases. The amount of ice recovered at a torque of 0.200 Nm is about twice that recovered at a torque of 0.100 Nm. However, the difference in the amount of recovery ice between the cases of 0.150 and 0.200 Nm is very small. A difference in the target torque implies a difference in the amount of ice formed. Hence, when the amount of functional fluid supplied is constant, the amount of ice in the ice slurry discharged will vary as the target torque is varied. The amount of ice recovered in a single recovery run increases with the target torque. As the
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amount of ice recovered in a single recovery run increases, the number of recovery runs per experiment decreases because the torque takes longer to reach its target value during each recovery run. The average total amount of ice recovered within 300 min (Total I) and the average amount of ice remaining in the ice formation chamber and pipe after 300 min are measured. Total I plus the average amount of remaining ice is termed as Total II. Totals I and II are shown in Fig. 4. Fig. 4 reveals that, for supply times ranging from 20 to 80 s, the values of Totals I and II hardly depend on the supply time. Total I increases with decreasing target torque. This is due to the effects of the total ice recovery time and total amount of functional fluid supplied. For a target torque of 0.100 Nm, the first recovery starts at about 55 min after dissolution of supercooling, while the corresponding figures for 0.150 and 0.200 Nm are about 100 and 120 min, respectively. The time for 100 Nm is
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longer than that for 0.200 Nm by about 65 min. The total amounts of functional fluid supplied to the ice formation chamber for a target torque of 0.100, 0.150 and 0.200 Nm are 44.4, 22.1 and 17.6 l, respectively. The largest amount of ice recovery is observed in the case of 0.100 Nm because the amount of functional fluid supplied is the largest in this case due to the fact that recovery takes the longest amount of time. Differences between Totals I and II values represent the unrecovered ice remaining in the ice formation chamber and pipe during 300 min recovery. If a smaller difference is indicative of better recovery performance, the recovery performance is the best at a target torque of 0.100 Nm. 3.2. Discussions on supply time and target torque For a proper discussion of supply time, it is helpful to define a recovery rate, Rs (g/min): n1 P
Mi =ti
RS
i¼2
n2
ðg=minÞ
ð1Þ
where n: the number of recovery runs, M i (g): amount of recovered ice during the i-th recovery run, and t i (s): time required for the i-th recovery run. The reason why Rs is defined in terms of the amount of recovered ice starting from the second run through to the (n1)-th run is as follows: the first recovery run is excluded because the amount of ice recovered during this run is always lower than during subsequent runs due to the drop in the level of the functional liquid. The n-th run is also excluded because the recovery is stopped as soon as the total recovery time reaches 300 min even if the torque has not reached the target value. Calculations based on Eq. (1) reveal that Rs hardly depends on the supply time at any given target torque. The average Rs values at 0.100, 0.150 and 0.200 Nm are 14.42, 13.22 and 12.54 g/min, respectively.
Fig. 3. (a) Variation in stirring torque and ice slurry temperature with time (supply time=40 s). (b) Relationship between end time of each recovery run and amount of recovered ice.
Fig. 4. Relationship between total amount of recovered ice and supply time.
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On the basis of these Rs values, the total amount of ice recovered over a 10 h period, during which nighttime electric power is utilized, is estimated. The results are shown in Fig. 5. The horizontal dashed line in Fig. 5 marks the mass of the initial volume of functional fluid in the ice formation chamber prior to ice formation, i.e., 5.80 kg. It is found that, for all target torques, the amount of recovered ice for 10 h is larger than the mass of initial volume of functional fluid, especially, the amount in the case of 0.100 Nm is 1.34 times as large as the mass of initial volume. Next, in order to be able to discuss the amount of ice recovered in a single run of functional fluid supply, a recovery ratio, Rr, is defined: n1 P
Rr
Mi =ðFr StÞ
i¼2
n2
ð2Þ
where n: the number of recovery runs, Mi (g): the amount of recovered ice during the i-th recovery run, Fr (g/s): the flow rate of functional fluid supplied and St (s): the supply time of functional fluid. In the same manner as Rs, Rr is defined in terms of the amount of recovered ice from the second through to
(n1)-th run. The Rr calculations for each target torque are shown in Fig. 6. The values shown represent the average Rr values. For any given supply time, Rr increases with increase of the target torque because the amount of ice formed increases with increase of the target torque. For all target torques, Rr is nearly independent of the supply time until a supply time of 60 s. At a supply time of 80 s, Rr decreases. This can be explained as follows. The ice content in the ice formation chamber with recovery is lower for longer supply times. Thus, the ice content in the ice slurry discharged per unit time gradually decreases. The same trend is not observed, however, for a supply time of 20 s and a target torque of 0.100 Nm. The reason for this needs to be further investigated. While the above discussion is a good starting point, it is believed that determination of the optimal supply time and target torque will require further investigation. 3.3. Ten-hour continuous ice slurry formation The results of a 10 h continuous ice slurry formation experiment, employing nighttime electric power, are shown in Fig. 7(a) and (b). Fig. 7(a) shows the variation of stirring torque and ice slurry temperature in the ice formation chamber over 10 h, while Fig. 7(b) shows the
Fig. 5. Prediction of total amount of recovered ice over a 10 h period.
Fig. 6. Relationship between recovery ratio and supply time.
Fig. 7. (a) Variation in stirring torque and ice slurry temperature over a 10 h period. (b) Relationship between end time of each recovered run and amount of recovered ice.
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amount of recovered ice at the end time of each recovery run for the same experiment as in Fig. 7(a). Fig. 8 shows the mean diameter of the recovered ice particles as a function of end time of each recovery run. Fig. 8 does not directly correspond to Fig. 7(a) and (b) because the values shown in Fig. 8 are average values. The experimental conditions are as follows: the brine temperature is 7.0 C; the supercooling degree of the functional fluid T is 0.1 C; the target torque is 0.200 Nm; the fixed flow rate of functional fluid is 75 ml/s, and supply time is 40 s. Growth of ice particles is observed. The mean diameter of ice particles D is defined as the average of the maximum and minimum lengths of ice particles and the values of D are calculated based on digital camera images. Fig. 7(b) shows that there is virtually no change in the amount of recovered ice in going from the second to the fourth recovery run, however, the amount gradually increases after the fourth run. Fig. 8 reveals that the D value obtained by the present method is larger than that obtained in conventional methods; D is as large as 2.7 mm even during the early phase of ice formation process. D hardly changes until 240 min, however, it increases with time after 300 min. The values of D at 240 and 605 min are 2.7 and 3.5 mm, respectively. While the amount of the ice formed is the same, the stirring torque is higher in the case of smaller diameters and larger numbers of ice particles. As can be seen in Fig. 7(a), the time required to reach the subsequent recovery run increases with each run because the stirring torque decreases due to the growth of ice particles. As a result, the time it takes the torque to reach its target value increases with each run. This, in turn, leads to an increase in the amount of recovered ice because the amount of ice formed in the ice formation chamber is already higher. The increase in the amount of ice formation is observed to cause temperature variations [Fig. 7(a)]. Fig. 7(a) also indicates that the lowest temperature attained by the functional fluid during each recovery run decreases gradually with each recovery run
[see temperature maxima and minima of each recovery run in Fig. 7(a)]. This is attributable to the freezing point depression caused by the freezing-induced rejection of solute. And then, the freezing point depression increases with each recovery run because the drop in temperature due to the increase in the amount of ice formation caused by the growth of ice particles. Photographs of ice particles recovered during the first recovery run [115 min] and at the end of the experiment [605 min] are shown in Fig. 9(a) and (b), respectively. These ice particles are larger than those obtained through conventional methods, and the size increases with recovery. Moreover, the ice particles, originally shaped like circular plates, attain a spherical structure, and they gradually become more uniform in size.
Fig. 8. Mean diameter of recovered ice particles as a function of end time of each recovery run over a 10 h period.
Fig. 9. Photographs of recovered ice particles (a) (t=115 min), (b) (t=605 min).
3.3.1. Mechanisms for the coarsening and size-based ordering of ice particles The reason why ice particles coarsen is as follows: the cooling rate decreases dramatically by using large amounts of functional fluid. In previous experiments, we used 450 ml of the functional fluid [2]. At that time, the cooling rate of the functional fluid from the start of cooling until just before dissolution of supercooling was 75 C/h. In the present experiment, 6.7 l of functional fluid is used and the cooling rate is 0.169 C/h. The average sizes of the ice particles in our previous and current experiments are measured to be 0.5–1 mm [at the end of the experiment] and 2.7 mm [at an earlier
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stage of recovery], respectively. Moreover, Fig. 8 shows that ice particles coarsen with recovery, as mentioned above. This is because they are discharged by overflow of the ice slurry. Ice particles in the ice formation chamber are observed to be relatively small and uneven in the early stages of ice formation, during which they are all discharged by the inflow of functional fluid regardless of their size because they are all relatively small. Thus, the size distribution of discharged ice particles is fairly uneven at this time. As ice formation proceeds, the ice particles coarsen and the size unevenness is enhanced. Since the discharge of larger and heavier ice particles is more difficult, they tend to remain in the ice formation chamber, rather than being discharged. There is a cut-off size above which an ice particle will not be discharged. This constraint gradually promotes size uniformity among discharged particles, as well as increasing the ratio of larger size particles in the ice formation chamber. Hence, as mentioned above, since the stirring torque decreases, the ice slurry temperature drops to a greater extent by the time the torque reaches the target value. At this time, ice formation is mainly driven by coarsening the ice particles, which means that, during the next recovery, the size of discharged ice particles is larger. It is thought that the size of recovered ice particles increases gradually after about 300 min. The mean diameter of ice particles D reaches 3.5 mm after 605 min. Moreover, the particles become more uniform in size and more spherical as a result of overflow and continuous stirring, respectively. The discussion above shows that a stable, 10-h-long continuous ice slurry formation run, which utilizes nighttime electric power is feasible. The present system enables the recovery of ice particles that are more uniform in size and more spherical. These particles are expected to have remarkably higher melting rates by virtue of their size.
Fig. 10. Relationship between mean diameter of ice particles and degree of supercooling.
3.4. Factors influencing ice particle size Besides the factors discussed above that constitute an inherent part of the present system, there are additional factors influencing ice particle size, such as degree of supercooling, cooling rate and wing diameter of stirrer. The experiments conducted to determine the effects of these factors for the following conditions. Fixed flow rate of functional fluid: 75 ml/s; target torque: 0.150 Nm: supply time: 40 s. 3.4.1. Influence of degree of supercooling, T The experimental conditions are as follows: the brine temperature is 7.0 C, the wing diameter of stirrer isf208 mm and the stirrer speed is 120 rpm. Experiments are carried out in which the supercooling degree T is varied. Ice particles are reported to coarsen at T40.2 C [7]. Thus, the value of T is successively set to 0.1, 0.4 and 0.8 C. Ice particles are recovered just after the ice slurry temperature reaches about 1.5 C in the first recovery; their mean diameter, D, is calculated and their shape is examined. The relationship between T and D is shown in Fig. 10. As is seen in Fig. 10, D=2.8 mm at T=0.1 C and D=1.2 mm at T=0.8 C. Clearly, D decreases with increase of T. D at T=0.1 C is above twice as large as at T=0.8 C. At T=0.1 and 0.4 C, the ice particles were shaped like circular plates, while at T=0.8 C, they are square-shaped. 3.4.2. Influence of cooling rate, Cs Cooling rate, Cs [ C/h], is defined as the temperature variation of the functional fluid per hour from the start of cooling until just before dissolution of supercooling. The experimental conditions are as follows: T=0.1 C; the wing diameter of stirrer isf208 mm, and the stirrer speed is 120 rpm. Cs is changed by varying the brine
Fig. 11. Relationship between mean diameter of ice particles and cooling rate of functional fluid.
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temperature. Cs is equal to 0.103, 0.169 and 0.284 ( C/h) at the brine temperatures 4.0, 7.0 and 10.0 C, respectively. Ice particles are recovered just after the ice slurry temperature reaches about 1.5 C in the first recovery; their mean diameter D is calculated, and their shape is examined. The relationship between Cs and D is shown in Fig. 11. As seen in Fig. 11, D=3.0, 2.8 and 1.8 mm at Cs=0.103,0.169, and 0.284 ( C/h), respectively. Clearly, D decreases with increase of the absolute value of Cs. In all cases, the ice particles are observed to have a circular plate-like structure.
3.4.3. Influence of wing diameter of stirrer The wing diameters of f208 mm and f148 mm are used. In both cases, the stirrer height is 200 mm. The experimental conditions used are as follows: T=0.1 C; the brine temperature is 7.0 C and the stirrer speed is 120 rpm. Ice particles are recovered just after the temperature of ice slurry reaches about 1.5 C in the first recovery; their mean diameter D is calculated, and their shape is examined. D is determined to be 2.2 and 2.8 mm for the wing diameters of f148 mm and f208 mm, respectively. Clearly, D increases with increase of the wing diameter. In the present method, the initial volume of functional fluid before ice formation varies with wing diameter because the amount of liquid that overflows from the outlet by stirring increases with increase of the wing diameter. The initial volume is 8.8 and 6.7 l for a wing diameter of f148 mm and f208, respectively. That is, for a fixed brine temperature, Cs is different in the two cases because the initial volume is different. This means that varying the wing diameter also varies Cs. The value of Cs is 0.108 and 0.169 ( C/h) for a wing diameter off148 mm and f208 mm, respectively. In the previous section [Section 3.4.2], we established that D increases with decreasing absolute value of Cs. However, the value of D [2.8 mm] at f208 mm [jCsj=0.169 ( C/h)] is larger than the value [2.2 mm] at f148 mm [jCsj=0.108 ( C/h)]. The trend predicted by the cooling rate clearly contradicts the relationship presented at the end of Section 3.4.2. This leads us to the conclusion that the influence of stirrer wing diameter on the ice particle size is the predominant factor. However, this would have to be confirmed using two kinds of wing diameters. Setting the value of Cs at f208 mm equal to the value obtained at f148 mm is expected to yield ice particles larger than D=2.8 mm at f208 mm. Finally, the present method enables the formation of ice particles with a size of 3.5 mm, which is larger than the size achievable through conventional methods. Fabricating larger ice particles requires control over the cooling rate of the functional fluid, particularly, its degree of supercooling.
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4. Conclusion (1) A new method of continuous ice slurry formation for ice storage was proposed. The resultant characteristics of ice formation and ice recovery were identified. (2) The present method enabled stable continuous ice slurry formation over a 10 h period utilizing nighttime electric power. (3) The present method served to order the size of ice particles. The size of recovered ice particles became uniform in size and spherical thanks to the continuous stirring. (4) The mean diameter of recovered ice particles reached 2.2–3.5 mm, which is larger than that achievable through conventional techniques. (5) he mean diameter of recovered ice particles was found to increase with decreasing degree of supercooling, decreasing absolute value of the cooling rate, and increasing wing diameter. The influence of the degree of supercooling was especially remarkable.
Acknowledgements The authors wish to thank Mr. Namiki and Mr. Tamaki, graduate students at Chuo University, for their collaboration. This study was financially supported Chuo University Grant for Special Research 2002-2003. References [1] Matsumoto K, Okada M, Kawagoe T, Kan C. Ice storage system with water–oil mixture (formation of suspension with high IPF). Int J Refrigeration 2000;23(5):336–44. [2] Matsumoto K, Shiokawa Y, Okada M, Kawagoe T, Kan C. Ice storage system with water–oil mixture (discussion about influence of additive on ice formation process). Int J Refrigeration 2002;25(1):11–18. [3] Moriya M, Tanino M, Kikuchi S, Hayashi T, Okonogi T, Kozawa Y. An ice storage system using supercooled water (1st Report; stable control of supercooling water and ice making) [in Japanese]. Trans JSRAE 1995;12(3):253–62. [4] Hirata T, Nagasaka K, Ishikawa M. Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface, part I: ice removal phenomena. Int J Heat Mass Transfer 2000;43(3):333–9. [5] Kawabe S, Fukusako H, Yamada M, Yanagita K. Continuous production characteristics of slush-ice by use of a horizontal oscillating cooled wall [in Japanese]. Trans JSRAE 1998;15(3):193–201. [6] Chibana K, Kang C, Okada M, Matsumoto K, Kawagoe T. Continuous formation of slurry ice by cooling water–oil emulsion in a tube. Int J Refrigeration 2002;25(2):259–66. [7] Wakisaka M, Shirai Y. Freeze concentration and its recent development [in Japanese]. Trans JSRAE 2001; 18(4):365–75.
Continuous ice slurry formation using a functional fluid for ice storage Koji Matsumotoa,*, Yoshiharu Namikib, Masashi Okadac, Tetsuo Kawagoed, Shinji Nakagawae, Chaedong Kangf a
Professor, Chuo University, Department of Precision Mechanics, 1-13-27 Kasuga, Bunkyo-ku, Tokyo, 112-8551 Japan b Mitsubishi Pencil Co. Ltd., 5-23-37 Higashiohi, Shinagawa-ku, Tokyo, 140-8537 Japan c Professor, Aoyama Gakuin University, Department of Mechanical Engineering, 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa, Prefecture 229-8558, Japan d 2-28-3 Katakurachou, Kanagawa-ku, Yokohama, 221-0865 Japan e Lecturer, Toyama Prefectural University, Department of Mechanical System Engineering, 5180 Kurokawa Kosugi-chou, Toyama Prefecture 939-0398, Japan f Assistant Professor, Chonbuk National University, Department of Mechanical Engineering, 664-14, Duckjin-dong, Jeonju 561-756, South Korea Received 6 May 2003; received in revised form 30 June 2003; accepted 30 June 2003
Abstract A functional fluid composed of an oil–water mixture with an additive is transformed into an ice slurry by cooling while stirring. This paper describes a new continuous ice slurry formation method. Experiments were carried out by varying conditions such as the supply time of functional fluid, the stirrer torque, brine temperature and degree of supercooling. As a result, the characteristics of the ice formation and recovery processes were clarified. It was found that the ice particles gradually became uniform in size and spherical, and grew to 3.5 mm in diameter during about 10 h. The factors influencing the size of formed ice particles were discussed because the larger ice particles were expected to melt more rapidly. The ice particle size was found to increase with decreasing degree of supercooling and cooling rate, and with increasing stirrer wing diameter. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Ice slurry; Thermal storage; Two phase mixture; Process; Experimental investigation; Two-phase secondary refrigerant
Ge´ne´ration continue de coulis de glace a` l’aide d’un me´lange huile/eau utilise´ en application accumulation thermique Mots cle´s : Coulis de glace ; Accumulation thermique ; Me´lange diphasique ; Proce´de´ ; Expe´rimentation ; Frigoporteur diphasque
* Corresponding author. Tel.: +81-3-3817-1837; fax: +813-3817-1820. E-mail addresses: [email protected] (K. Matsumoto), [email protected] (M. Okada), [email protected] (S. Nakagawa), [email protected] (C. Kang). 0140-7007/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00102-6
1. Introduction A peak cut and peak shift in electric power demand for the ice storage system can be realized by the use of
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nighttime electric power, instead of the much more costly daytime electric power. This also cuts down on the discharge of CO2 gas by virtue of the characteristics of nighttime electric power. Thus, the spread of the ice storage system use can reduce the strain on the environment. Authors have already studied ice slurry formation by cooling a functional fluid in a small vessel with stirring [1,2]. The functional fluid consisted of 10 vol.% siliconeoil and 90 vol.% water with a small amount of silanecoupler additive. Based on this research, a method of ice slurry formation intended for use in ice storage systems was proposed. This method was much better than conventional methods. A method for continuous ice slurry formation has been investigated by many researchers [3–5]. However, no concrete results have been yielded. Authors have also reported on the continuous formation of ice slurry in a tube-type heat exchanger of [6]. A new apparatus was built, drawing on existing data. The apparatus features a much larger ice formation chamber compared to previous models, because authors take into account the practicalities encountered in ice formation systems. The present paper proposes a new method of continuous ice slurry formation in connection with this new apparatus. The method involved overflowing of the ice slurry out of the cylindrical ice formation chamber by intermittent inflow of functional fluid to this chamber. The overflowed ice slurry is separated into ice particles and functional fluid, and only ice particles are recovered. The amount of recovered ice was monitored while varying the stirring torque and supply time of functional fluid. The total continuous ice formation time varied between 5 and 10 h. Characteristics of the ice formation and recovery processes were discussed. In order to enhance the melting rate of ice particles, experiments were conducted to increase the size of the ice particles by varying the degree of supercooling, cooling rate of functional fluid and wing diameter of stirrer.
cylindrical vessel is termed an ice formation chamber. The ice slurry level in the vessel is raised by intermittent inflow of the functional fluid. The ice slurry subsequently overflows and is discharged from the vessel. Hence, the ice particles formed are removed from the vessel and the stirring power can be decreased. Ice slurry is separated into ice particles and the functional fluid in a recovery compartment. Continuous ice formation is achieved by recycling of only the functional fluid back to the vessel. In order to find the ratio of solid to liquid in the vessel during the ice formation process, the change in the amount of ice formed is measured in terms of the change in the ice slurry stirring torque. 2.2. Experimental apparatus The experimental apparatus is shown in Fig. 1(a). The ice formation part consists of a cylindrical ice formation chamber and a cooling chamber through which cold brine is circulated. The ice formation chamber is made of polyethylene. The inner diameter, height, thickness, and heat transfer area of this chamber are 300 mm, 310 mm, 2.3 mm, and 0.203 m2, respectively. The functional
2. Experiment 2.1. Method of continuous ice formation The functional fluid developed by previous studies [1,2] has excellent characteristics. Hardly any adhesion of ice occurs on the cooling chamber walls when the fluid is frozen. Also, at a very small depression of freezing point, nearly all the water in the functional fluid can be frozen and the ice particles formed by the functional fluid are kept in a dispersed state for a long time. In the present study, continuous formation of ice slurry using this functional fluid was carried out. In order to use data obtained in our previous experiments, we adopted a method that involved stirring in a cylindrical vessel that was directly soaked in cold brine. This
Fig. 1. (a) Experimental apparatus, (b) stirrer wing.
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fluid in this chamber is cooled by circulating cold brine with a magnetic pump. An outlet to discharge ice slurry is located 150 mm above the bottom of the ice formation chamber. The inner diameter and attaching angle of the outlet are 40 mm and 10 , respectively. The stirrer wing is shown in Fig. 1(b).The wing diameter and height of the stirrer used during the ice formation process are 208 and 200 mm, respectively. Its speed is set to 120 rpm. The maximum volume of functional fluid in the ice formation chamber is 10 l without stirring, but when the liquid is stirred at 120 rpm, this value drops to 6.7 l because a part of the liquid overflows through the outlet as a result of the stirring. The 6.7 l is taken to be the ‘‘initial volume of the liquid’’. The functional liquid in the precooling chamber is maintained at about 1.0 C by controlled heater and handy-cooler. The temperature is slightly above the freezing point of the functional fluid at its initial concentration. By the intermittent inflow of functional fluid, the functional liquid level in the ice formation chamber is raised, which is followed by the overflow of ice slurry through the outlet. A bag made of nylon net (mesh34) in the recovery compartment enables the recovery of ice particles by filtering out the ice particles in the ice slurry from the functional fluid. The separated liquid returns to the precooling chamber. The apparatus is set in the cold room maintained at an ambient temperature below 0 C. The temperature of the functional liquid in the ice formation chamber is measured by a platinum resistance thermometer, and the temperatures of the functional liquid in the precooling chamber, the brine, and the cold room environment are measured by type T thermo-couples. The stirring torque of the ice slurry during the ice formation process is measured by a torque meter attached to the shaft of the stirrer. 2.3. Experimental procedure The functional fluid is composed of a 30 l water–silicon oil mixture with 90 vol.% water content and a small amount of silane-coupler additive. The concentration of the silane-coupler, i.e., g-aminopropyltriethoxysilane, is 4 wt.% for the water. The freezing point of the functional fluid at its initial concentration is 1.2 C. The freezing of the functional fluid in the ice formation chamber is initiated by circulating the temperature-controlled cold brine. The degree of supercooling of the functional fluid is controlled by throwing a 1 ml ice particle into the functional fluid contained in the ice formation chamber. Using the relationship between IPF (Ice Packing Factor) and stirring torque obtained through preliminary experiments, the value of IPF in the ice formation chamber can be estimated indirectly by measuring the torque during ice formation. Once the torque reaches a target value because of ice formation, the functional fluid is fed to the ice formation chamber
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at a fixed flow rate of 75 ml/s for a prescribed period of time, accompanied by the simultaneous recovery of ice particles separated from ice slurry. The end of an individual recovery run is marked by the torque reaching the target value again, immediately following which the next recovery run begins. The separation of ice particles from the functional fluid is carried out in a cold room for 60 min. The recovered ice particles are photographed by a digital camera, and their mass is measured. The experiments are repeated at varying target torques and supply times of functional fluid.
3. Results and discussion 3.1. Characteristics of ice formation and ice recovery The experimental conditions are as follows: the brine temperature is 7.0 C; the fixed flow rate of functional fluid is 75 ml/s, the supercooling degree of the functional fluid T is 0.1 C; target torques are 0.100, 0.150 and 0.200 Nm, and supply times are 20, 40, 60 and 80 s. For 80 s, total volume of the functional fluid supplied is 6000 ml, which approximately corresponds to the initial volume of functional fluid in the ice formation chamber (6700 ml). Under these experimental conditions, ice formation runs were carried out continuously for 300 min. In order to find the volume of ice remaining in the ice formation chamber and pipe after a 300 min recovery, functional fluid of a prescribed volume (75 ml/s360 s) was fed to the ice formation chamber and the remaining ice was recovered. The experiment was repeated at least twice at each condition to confirm reproducibility. The variation with time in the stirring torque and ice slurry temperature in the ice formation chamber is shown in Fig. 2(a). The independent variable is the supply time. The fixed target torque is 0.150 Nm. The bold solid line and plain solid line show the temperature and torque, respectively. The horizontal axis represents time after dissolution of supercooling. With ice formation, the stirring torque of ice slurry increases and ice slurry temperature drops. When the torque reaches the target value, the functional fluid is supplied to the ice formation chamber, and the ice slurry is discharged from the outlet of the ice formation chamber. At this time, the torque decreases because the ratio of ice to liquid decreases. Simultaneously, the temperature of the ice slurry in the ice formation chamber increases slightly due to the introduction of functional fluid at 1.0 C from the precooling chamber. After inflow of functional fluid stops, the torque increases and the temperature decreases because of ice formation. This process is repeated for a total period of 300 min after dissolution of supercooling. As seen in Fig. 2(a), the time evolution of the torque and temperature until the torque reaches the target
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value for the first time after dissolution of supercooling appears virtually identical for all supply times; the graphs look different beyond that point. In the case of 80 s, it takes about 60 min to reach the target value again because the ratio of solid to liquid in the ice formation chamber changes greatly due to the large amount of ice slurry discharged. By contrast, in the case of 20 s, it takes about 15 min because the amount of ice slurry discharged is smaller. In other words, the recovery time is quadrupled when the supply time is quadrupled. Similar trends are observed for a target torque of 0.100 and 0.200 Nm. When the supply time is longer, the temperature of the functional fluid in the ice formation chamber is raised by a greater extent because a larger amount of functional fluid in the ice formation
Fig. 2. (a) Variation in stirring torque and ice slurry temperature with time (target torque=0.150 Nm). (b). Relationship between end time of each recovery run and amount of recovered ice.
chamber is replaced with fresh functional fluid from the precooling chamber. The relationship between the end time of each recovery run and the amount of recovered ice corresponding to Fig. 2(a) is shown in Fig. 2(b). From Fig. 2(b), it is found that the amount of recovered ice in each recovery run is nearly stable for all supply times, viz. 20, 40, 60 and 80 s. In the case of 80 s, the average amount of recovered ice in a single recovery run is about 740 g. The corresponding values for the 60, 40 and 20 s are about 600, 400 and 200 g, respectively. The supply time is proportional to the amount of recovered ice below a supply time of 60 s. However, at a supply time of 80 s, the relationship is no longer linear. For longer supply times, the amount of recovered ice in a single recovery run is higher but the number of recovery runs per experiment is lower. At a supply time of 20 s [denoted by the symbol * in Fig. 2(b)], there is little recovered ice at around 270 min, while the value at around 290 min is nearly twice the values of the other recovery runs. The reason is as follows: most of the ice discharged at 270 min stagnates in the outlet because of the low attaching angle of the outlet and a small amount of functional fluid supplied. The stagnant ice is recovered in the next recovery run, i.e. at 290 min. The same trend is observed whenever the supply time is set to 20 s, regardless of the other conditions. Under all conditions, the amount of ice in the first recovery is lower than in subsequent recoveries. The reason is as follows: since the ice slurry level is confirmed to be lower at the start of the first recovery than at the start of ice formation, it is thought that a part of the functional fluid supplied is consumed in raising the level. Hence, amount of ice slurry discharged decreases. The variation with time in the stirring torque and temperature of the ice slurry in the ice formation chamber is shown in Fig. 3(a). The variable in this case is the target torque. The supply time is fixed at 40 s. The time elapsed until the first recovery is about 60, 100 and 120 min for a target torque of 0.100, 0.150 and 0.200 Nm, respectively. The relationship between the end time of each recovery run and the amount of recovered ice corresponding to Fig. 3(a) is shown in Fig. 3(b). Fig. 3(b) indicates that the amount of ice recovered in each recovery run is nearly stable in all cases. The amount of ice recovered at a torque of 0.200 Nm is about twice that recovered at a torque of 0.100 Nm. However, the difference in the amount of recovery ice between the cases of 0.150 and 0.200 Nm is very small. A difference in the target torque implies a difference in the amount of ice formed. Hence, when the amount of functional fluid supplied is constant, the amount of ice in the ice slurry discharged will vary as the target torque is varied. The amount of ice recovered in a single recovery run increases with the target torque. As the
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amount of ice recovered in a single recovery run increases, the number of recovery runs per experiment decreases because the torque takes longer to reach its target value during each recovery run. The average total amount of ice recovered within 300 min (Total I) and the average amount of ice remaining in the ice formation chamber and pipe after 300 min are measured. Total I plus the average amount of remaining ice is termed as Total II. Totals I and II are shown in Fig. 4. Fig. 4 reveals that, for supply times ranging from 20 to 80 s, the values of Totals I and II hardly depend on the supply time. Total I increases with decreasing target torque. This is due to the effects of the total ice recovery time and total amount of functional fluid supplied. For a target torque of 0.100 Nm, the first recovery starts at about 55 min after dissolution of supercooling, while the corresponding figures for 0.150 and 0.200 Nm are about 100 and 120 min, respectively. The time for 100 Nm is
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longer than that for 0.200 Nm by about 65 min. The total amounts of functional fluid supplied to the ice formation chamber for a target torque of 0.100, 0.150 and 0.200 Nm are 44.4, 22.1 and 17.6 l, respectively. The largest amount of ice recovery is observed in the case of 0.100 Nm because the amount of functional fluid supplied is the largest in this case due to the fact that recovery takes the longest amount of time. Differences between Totals I and II values represent the unrecovered ice remaining in the ice formation chamber and pipe during 300 min recovery. If a smaller difference is indicative of better recovery performance, the recovery performance is the best at a target torque of 0.100 Nm. 3.2. Discussions on supply time and target torque For a proper discussion of supply time, it is helpful to define a recovery rate, Rs (g/min): n1 P
Mi =ti
RS
i¼2
n2
ðg=minÞ
ð1Þ
where n: the number of recovery runs, M i (g): amount of recovered ice during the i-th recovery run, and t i (s): time required for the i-th recovery run. The reason why Rs is defined in terms of the amount of recovered ice starting from the second run through to the (n1)-th run is as follows: the first recovery run is excluded because the amount of ice recovered during this run is always lower than during subsequent runs due to the drop in the level of the functional liquid. The n-th run is also excluded because the recovery is stopped as soon as the total recovery time reaches 300 min even if the torque has not reached the target value. Calculations based on Eq. (1) reveal that Rs hardly depends on the supply time at any given target torque. The average Rs values at 0.100, 0.150 and 0.200 Nm are 14.42, 13.22 and 12.54 g/min, respectively.
Fig. 3. (a) Variation in stirring torque and ice slurry temperature with time (supply time=40 s). (b) Relationship between end time of each recovery run and amount of recovered ice.
Fig. 4. Relationship between total amount of recovered ice and supply time.
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On the basis of these Rs values, the total amount of ice recovered over a 10 h period, during which nighttime electric power is utilized, is estimated. The results are shown in Fig. 5. The horizontal dashed line in Fig. 5 marks the mass of the initial volume of functional fluid in the ice formation chamber prior to ice formation, i.e., 5.80 kg. It is found that, for all target torques, the amount of recovered ice for 10 h is larger than the mass of initial volume of functional fluid, especially, the amount in the case of 0.100 Nm is 1.34 times as large as the mass of initial volume. Next, in order to be able to discuss the amount of ice recovered in a single run of functional fluid supply, a recovery ratio, Rr, is defined: n1 P
Rr
Mi =ðFr StÞ
i¼2
n2
ð2Þ
where n: the number of recovery runs, Mi (g): the amount of recovered ice during the i-th recovery run, Fr (g/s): the flow rate of functional fluid supplied and St (s): the supply time of functional fluid. In the same manner as Rs, Rr is defined in terms of the amount of recovered ice from the second through to
(n1)-th run. The Rr calculations for each target torque are shown in Fig. 6. The values shown represent the average Rr values. For any given supply time, Rr increases with increase of the target torque because the amount of ice formed increases with increase of the target torque. For all target torques, Rr is nearly independent of the supply time until a supply time of 60 s. At a supply time of 80 s, Rr decreases. This can be explained as follows. The ice content in the ice formation chamber with recovery is lower for longer supply times. Thus, the ice content in the ice slurry discharged per unit time gradually decreases. The same trend is not observed, however, for a supply time of 20 s and a target torque of 0.100 Nm. The reason for this needs to be further investigated. While the above discussion is a good starting point, it is believed that determination of the optimal supply time and target torque will require further investigation. 3.3. Ten-hour continuous ice slurry formation The results of a 10 h continuous ice slurry formation experiment, employing nighttime electric power, are shown in Fig. 7(a) and (b). Fig. 7(a) shows the variation of stirring torque and ice slurry temperature in the ice formation chamber over 10 h, while Fig. 7(b) shows the
Fig. 5. Prediction of total amount of recovered ice over a 10 h period.
Fig. 6. Relationship between recovery ratio and supply time.
Fig. 7. (a) Variation in stirring torque and ice slurry temperature over a 10 h period. (b) Relationship between end time of each recovered run and amount of recovered ice.
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amount of recovered ice at the end time of each recovery run for the same experiment as in Fig. 7(a). Fig. 8 shows the mean diameter of the recovered ice particles as a function of end time of each recovery run. Fig. 8 does not directly correspond to Fig. 7(a) and (b) because the values shown in Fig. 8 are average values. The experimental conditions are as follows: the brine temperature is 7.0 C; the supercooling degree of the functional fluid T is 0.1 C; the target torque is 0.200 Nm; the fixed flow rate of functional fluid is 75 ml/s, and supply time is 40 s. Growth of ice particles is observed. The mean diameter of ice particles D is defined as the average of the maximum and minimum lengths of ice particles and the values of D are calculated based on digital camera images. Fig. 7(b) shows that there is virtually no change in the amount of recovered ice in going from the second to the fourth recovery run, however, the amount gradually increases after the fourth run. Fig. 8 reveals that the D value obtained by the present method is larger than that obtained in conventional methods; D is as large as 2.7 mm even during the early phase of ice formation process. D hardly changes until 240 min, however, it increases with time after 300 min. The values of D at 240 and 605 min are 2.7 and 3.5 mm, respectively. While the amount of the ice formed is the same, the stirring torque is higher in the case of smaller diameters and larger numbers of ice particles. As can be seen in Fig. 7(a), the time required to reach the subsequent recovery run increases with each run because the stirring torque decreases due to the growth of ice particles. As a result, the time it takes the torque to reach its target value increases with each run. This, in turn, leads to an increase in the amount of recovered ice because the amount of ice formed in the ice formation chamber is already higher. The increase in the amount of ice formation is observed to cause temperature variations [Fig. 7(a)]. Fig. 7(a) also indicates that the lowest temperature attained by the functional fluid during each recovery run decreases gradually with each recovery run
[see temperature maxima and minima of each recovery run in Fig. 7(a)]. This is attributable to the freezing point depression caused by the freezing-induced rejection of solute. And then, the freezing point depression increases with each recovery run because the drop in temperature due to the increase in the amount of ice formation caused by the growth of ice particles. Photographs of ice particles recovered during the first recovery run [115 min] and at the end of the experiment [605 min] are shown in Fig. 9(a) and (b), respectively. These ice particles are larger than those obtained through conventional methods, and the size increases with recovery. Moreover, the ice particles, originally shaped like circular plates, attain a spherical structure, and they gradually become more uniform in size.
Fig. 8. Mean diameter of recovered ice particles as a function of end time of each recovery run over a 10 h period.
Fig. 9. Photographs of recovered ice particles (a) (t=115 min), (b) (t=605 min).
3.3.1. Mechanisms for the coarsening and size-based ordering of ice particles The reason why ice particles coarsen is as follows: the cooling rate decreases dramatically by using large amounts of functional fluid. In previous experiments, we used 450 ml of the functional fluid [2]. At that time, the cooling rate of the functional fluid from the start of cooling until just before dissolution of supercooling was 75 C/h. In the present experiment, 6.7 l of functional fluid is used and the cooling rate is 0.169 C/h. The average sizes of the ice particles in our previous and current experiments are measured to be 0.5–1 mm [at the end of the experiment] and 2.7 mm [at an earlier
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stage of recovery], respectively. Moreover, Fig. 8 shows that ice particles coarsen with recovery, as mentioned above. This is because they are discharged by overflow of the ice slurry. Ice particles in the ice formation chamber are observed to be relatively small and uneven in the early stages of ice formation, during which they are all discharged by the inflow of functional fluid regardless of their size because they are all relatively small. Thus, the size distribution of discharged ice particles is fairly uneven at this time. As ice formation proceeds, the ice particles coarsen and the size unevenness is enhanced. Since the discharge of larger and heavier ice particles is more difficult, they tend to remain in the ice formation chamber, rather than being discharged. There is a cut-off size above which an ice particle will not be discharged. This constraint gradually promotes size uniformity among discharged particles, as well as increasing the ratio of larger size particles in the ice formation chamber. Hence, as mentioned above, since the stirring torque decreases, the ice slurry temperature drops to a greater extent by the time the torque reaches the target value. At this time, ice formation is mainly driven by coarsening the ice particles, which means that, during the next recovery, the size of discharged ice particles is larger. It is thought that the size of recovered ice particles increases gradually after about 300 min. The mean diameter of ice particles D reaches 3.5 mm after 605 min. Moreover, the particles become more uniform in size and more spherical as a result of overflow and continuous stirring, respectively. The discussion above shows that a stable, 10-h-long continuous ice slurry formation run, which utilizes nighttime electric power is feasible. The present system enables the recovery of ice particles that are more uniform in size and more spherical. These particles are expected to have remarkably higher melting rates by virtue of their size.
Fig. 10. Relationship between mean diameter of ice particles and degree of supercooling.
3.4. Factors influencing ice particle size Besides the factors discussed above that constitute an inherent part of the present system, there are additional factors influencing ice particle size, such as degree of supercooling, cooling rate and wing diameter of stirrer. The experiments conducted to determine the effects of these factors for the following conditions. Fixed flow rate of functional fluid: 75 ml/s; target torque: 0.150 Nm: supply time: 40 s. 3.4.1. Influence of degree of supercooling, T The experimental conditions are as follows: the brine temperature is 7.0 C, the wing diameter of stirrer isf208 mm and the stirrer speed is 120 rpm. Experiments are carried out in which the supercooling degree T is varied. Ice particles are reported to coarsen at T40.2 C [7]. Thus, the value of T is successively set to 0.1, 0.4 and 0.8 C. Ice particles are recovered just after the ice slurry temperature reaches about 1.5 C in the first recovery; their mean diameter, D, is calculated and their shape is examined. The relationship between T and D is shown in Fig. 10. As is seen in Fig. 10, D=2.8 mm at T=0.1 C and D=1.2 mm at T=0.8 C. Clearly, D decreases with increase of T. D at T=0.1 C is above twice as large as at T=0.8 C. At T=0.1 and 0.4 C, the ice particles were shaped like circular plates, while at T=0.8 C, they are square-shaped. 3.4.2. Influence of cooling rate, Cs Cooling rate, Cs [ C/h], is defined as the temperature variation of the functional fluid per hour from the start of cooling until just before dissolution of supercooling. The experimental conditions are as follows: T=0.1 C; the wing diameter of stirrer isf208 mm, and the stirrer speed is 120 rpm. Cs is changed by varying the brine
Fig. 11. Relationship between mean diameter of ice particles and cooling rate of functional fluid.
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temperature. Cs is equal to 0.103, 0.169 and 0.284 ( C/h) at the brine temperatures 4.0, 7.0 and 10.0 C, respectively. Ice particles are recovered just after the ice slurry temperature reaches about 1.5 C in the first recovery; their mean diameter D is calculated, and their shape is examined. The relationship between Cs and D is shown in Fig. 11. As seen in Fig. 11, D=3.0, 2.8 and 1.8 mm at Cs=0.103,0.169, and 0.284 ( C/h), respectively. Clearly, D decreases with increase of the absolute value of Cs. In all cases, the ice particles are observed to have a circular plate-like structure.
3.4.3. Influence of wing diameter of stirrer The wing diameters of f208 mm and f148 mm are used. In both cases, the stirrer height is 200 mm. The experimental conditions used are as follows: T=0.1 C; the brine temperature is 7.0 C and the stirrer speed is 120 rpm. Ice particles are recovered just after the temperature of ice slurry reaches about 1.5 C in the first recovery; their mean diameter D is calculated, and their shape is examined. D is determined to be 2.2 and 2.8 mm for the wing diameters of f148 mm and f208 mm, respectively. Clearly, D increases with increase of the wing diameter. In the present method, the initial volume of functional fluid before ice formation varies with wing diameter because the amount of liquid that overflows from the outlet by stirring increases with increase of the wing diameter. The initial volume is 8.8 and 6.7 l for a wing diameter of f148 mm and f208, respectively. That is, for a fixed brine temperature, Cs is different in the two cases because the initial volume is different. This means that varying the wing diameter also varies Cs. The value of Cs is 0.108 and 0.169 ( C/h) for a wing diameter off148 mm and f208 mm, respectively. In the previous section [Section 3.4.2], we established that D increases with decreasing absolute value of Cs. However, the value of D [2.8 mm] at f208 mm [jCsj=0.169 ( C/h)] is larger than the value [2.2 mm] at f148 mm [jCsj=0.108 ( C/h)]. The trend predicted by the cooling rate clearly contradicts the relationship presented at the end of Section 3.4.2. This leads us to the conclusion that the influence of stirrer wing diameter on the ice particle size is the predominant factor. However, this would have to be confirmed using two kinds of wing diameters. Setting the value of Cs at f208 mm equal to the value obtained at f148 mm is expected to yield ice particles larger than D=2.8 mm at f208 mm. Finally, the present method enables the formation of ice particles with a size of 3.5 mm, which is larger than the size achievable through conventional methods. Fabricating larger ice particles requires control over the cooling rate of the functional fluid, particularly, its degree of supercooling.
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4. Conclusion (1) A new method of continuous ice slurry formation for ice storage was proposed. The resultant characteristics of ice formation and ice recovery were identified. (2) The present method enabled stable continuous ice slurry formation over a 10 h period utilizing nighttime electric power. (3) The present method served to order the size of ice particles. The size of recovered ice particles became uniform in size and spherical thanks to the continuous stirring. (4) The mean diameter of recovered ice particles reached 2.2–3.5 mm, which is larger than that achievable through conventional techniques. (5) he mean diameter of recovered ice particles was found to increase with decreasing degree of supercooling, decreasing absolute value of the cooling rate, and increasing wing diameter. The influence of the degree of supercooling was especially remarkable.
Acknowledgements The authors wish to thank Mr. Namiki and Mr. Tamaki, graduate students at Chuo University, for their collaboration. This study was financially supported Chuo University Grant for Special Research 2002-2003. References [1] Matsumoto K, Okada M, Kawagoe T, Kan C. Ice storage system with water–oil mixture (formation of suspension with high IPF). Int J Refrigeration 2000;23(5):336–44. [2] Matsumoto K, Shiokawa Y, Okada M, Kawagoe T, Kan C. Ice storage system with water–oil mixture (discussion about influence of additive on ice formation process). Int J Refrigeration 2002;25(1):11–18. [3] Moriya M, Tanino M, Kikuchi S, Hayashi T, Okonogi T, Kozawa Y. An ice storage system using supercooled water (1st Report; stable control of supercooling water and ice making) [in Japanese]. Trans JSRAE 1995;12(3):253–62. [4] Hirata T, Nagasaka K, Ishikawa M. Crystal ice formation of solution and its removal phenomena at cooled horizontal solid surface, part I: ice removal phenomena. Int J Heat Mass Transfer 2000;43(3):333–9. [5] Kawabe S, Fukusako H, Yamada M, Yanagita K. Continuous production characteristics of slush-ice by use of a horizontal oscillating cooled wall [in Japanese]. Trans JSRAE 1998;15(3):193–201. [6] Chibana K, Kang C, Okada M, Matsumoto K, Kawagoe T. Continuous formation of slurry ice by cooling water–oil emulsion in a tube. Int J Refrigeration 2002;25(2):259–66. [7] Wakisaka M, Shirai Y. Freeze concentration and its recent development [in Japanese]. Trans JSRAE 2001; 18(4):365–75.