1、4675 Average Modified Stanton Number for Evaluating the Ice-Melting Characteristics of Ice Harvested from a Thermal Storage Tank Akiyoshi Ohira, Ph.D. Michio Yanadori, Ph.D. Yoshitaka Sakano Miyuki Miki ABSTRACT Before constructing ice thermal storage systems for air conditioning in buildings and wo
2、rks, the ice-melting charac- teristics in the storage tankmust be better understood. Accord- ingly, the present study evaluated the ice-melting characteristics of harvested ice from an ice thermal storage system. That is, the effects of the inlet-water release position in the tanks, the inlet-water
3、spraying method, and the water- replacement time (in the case ofthree tanksizes) on the temper- ature of the outlet water from the system were determined. It was consequently found that the average modij?ed Stanton number can be used to evaluate the ice-melting characteristics effectively in an actu
4、al-size tank. INTRODUCTION The electric power consumed by computers and other electric devices in office buildings and works has been increas- ing rapidly over recent years. At the same time, the demand for electricity for running air conditioning during the day has also been increasing significantl
5、y. Electric power companies are thus developing and promoting ice thermal storage systems that can use electric power during the night in order to reduce electric power during the daytime (Saito 2002; Yamada et al. 2002; Teraoka et al. 2002). A conventional ice thermal storage system used for air co
6、nditioning is the ice-on-coil method. However, in Japan, ice thermal storage systems have recently been changing from the ice-on-coil type to the dynamic-type, since the latter type has better ice-melting characteristics and is easier to construct. Figure 1 shows a schematic of a general dynamic-typ
7、e ice thermal storage system, which consists of three parts: an ice generator, ice conveying, and ice melting. This system has three main features. First, the ice thermal storage tank and the ice generator are separate. Second, ice is conveyed by water or air from the ice generator to the tank (Ohir
8、a et al. 1998). Third, the ice making and melting processes are operated at the same time. This means that the outlet water temperature remains low for a longer time. There are two types of dynamic ice thermal storage systems that use an ice-making process. One uses small parti- cles of ice (particl
9、e-ice diameter: 1 mm 0.04 in. or less) (Moriya et al. 1995; Tanino et al. 1995, 1997) and the other uses large ice particles (particle-ice diameter: about 5 mm 0.2 in. or more) or harvested ice (plate-type ice). Knebel (1995) presented a simulation model of an ice-harvesting thermal storage system a
10、nd evaluated a performance of the system. Stewart et al. (1995a, 1995b) experimentally investigated outlet water temperature when the inlet water temperature, the Figure I Schematic of a dynamic-type ice thermal storage system. Akiyoshi Ohira is a researcher and Michio Yanadori is a senior researche
11、r in the Mechanical Engineering Research Laboratory, Hitachi Ltd., Tsuch- iura, Ibaraki, Japan. Yoshitaka Sakano is an engineer in the Refrigerating and Heating Division, Hitachi Industries Co., Ltd., Tsuchiura, Ibaraki, Japan. Miyuki Miki is a senior researcher in the Technical Research Center, Kan
12、sai Electric Power Company, Amagasaki, Hyogo, Japan. 02004 ASHRAE 81 Ice eenmta inlet oioe 500 500 960 i (B) V*=.Z m3 (C) V*=.2 m3 (A) V*=O.l m3 Figure 2 Schematic diagram showing the actual size of a dynamic ice thermal storage apparatus. Figure 3 Types of ice thermal storage tanks. inlet water flo
13、w rate, and the inlet water distribution were changed. In the present study, a tank size, water volume flow rate, and inlet-water spraying method are changed systematically to examine the outlet water temperature and their relationship. It was consequently found that the average modified Stanton num
14、ber can be used to evaluate the ice-melting characteristics; that is, the outlet-water temperature and the time taken by the ice-packed bed to completely melt for an actual-size tank can be estimated. TYPICAL DYNAMIC ICE THERMAL STORAGE SYSTEM Figure 2 shows the schematic diagram of the actual-size
15、dynamic ice thermal storage apparatus. Two thermal storage tanks-one for ice thermal storage and another for water ther- mal storage-exist underground. Each tank is 6 m (19.7 ft) long, 1 1 m (36.1 ft) wide, and 3.5 m (1 1.5 ft) high. The water level in the tanks is 1.9 m (6.2 ft) from the bottom. Th
16、e water level goes down slightly when, at the end of the ice build cycle, the ice at the bottom of the slide no longer floats and begins to build an ice mountain. But the water volume in each filled tank is regarded as 125 m3 (4414 fi3). A wall divides the two tanks, which are connected by three 1-m
17、2 holes in the wall. The harvested ice generator is located next to the ice thermal stor- age tank, and the ice generator and ice thermal storage tank are connected by a stainless steel slide. Harvested ice is conveyed on this slide by water from the ice generator to the ice thermal storage tank thr
18、ough a hole in the tank. Three spray pipes are located above the water surface in both tanks. The spray pipes of the water storage tank are not used in this experiment. Cold water is pumped from the water thermal storage tank to heat exchangers, where it cools the water that is delivered to the buil
19、dings. Operation of the system consists of three steps. First, the ice generator is turned on, and ice thermal storage tank is filled with harvested ice up to an initial ice-packing factor (IPF,) of 65%. Second, the inlet-water temperature is kept at about 15C (59F). The inlet water is the return wa
20、ter from the heat exchangers after it has picked up the heat from the boilers in this experiment to keep at 15C inlet water. In general, the inlet water is the return water from the heat exchanger after it has picked up the heat from the building. The volume flow rate of the inlet water from the spr
21、ays is 3.5 x m3/s (554.8 gal/ min), and the inlet water is only sprayed into the ice thermal storage tank. Third, the temperatures of the inlet and outlet water, and those inside the tanks, are measured by thennocou- ples connected to a recorder. EXPERIMENTAL APPARATUS AND PROCEDURE Fundamental ice-
22、melting experiments using small-size tanks have been carried out. Figure 3 shows the three sizes of ice thermal storage tanks that were used in the experiments (Ohira and Yanadori 1999). The water volume, V*, in these tanks is 0.1 m3 (3.5 ft3) for tank A, 0.2 m3 (7.1 ft3) for tank B, and 1.2 m3 (42.
23、4 fi3) for tank C. The cross section of tanks A and B is 500 mm (1 9.7 in.) square, and their depths are 500 mm (19.7 in.) and 900 mm (35.4 in.), respectively. Water levels of tanks A, B, and C are controlled constantly at 400 mm (15.7 in.), 810 mm (31.9 in.), and 1360 mm (53.5 in.), respectively. A
24、ll three tanks are covered with a heat insulator. Tanks A and B have one inlet pipe, which is 40 mm (1.6 in.) in inner diam- eter and fixed along the inside of the tank wall. Inlet-water release positionx is changed by changing the length ofthe inlet pipe; that is, x is changed from 10 to 400 mm (0.
25、4 to 15.7 in.) for tank A and from 10 to 810 mm (0.4 to 3 1.9 in.) for tank B. Their single outlet pipes are fixed horizontally near the surface of the water. These experiments used 30-mm2 (1.2411.) ice cubes because their ice-packing factor is higher than other kinds of ice, and the permeability of
26、 water in an ice-packed bed is high; i.e., IPF, was about 65% (Yandori et al. 1996). IPF, is defined as (Gi,), / (GicelO+ G,). Outlet water was delivered to another tank, where it was raised to 15C (59F) within *0.5“C and used as inlet water. About 15C (59F) inlet water was supplied from the water t
27、ank. The water volume in tank C is 1.2 m3 (42.4 fi3); that is, 960 mm (37.8 in.) square in cross section and 1480 mm (58.3 in.) high. Tank C is covered with a heat insulator. Nine sprays 82 ASH RAE Transactions: Research are fixed above the water surface, and inlet water volume flow rate is controll
28、ed by valves in order to spray the water uniformly. This spray method for tank C is different from that for tanks A and B. This is because the cross-sectional area of tank C is larger than that of tanks A and B, and the single inlet pipe cannot spray water uniformly. The single outlet pipe, which ha
29、s an inner diameter of 40 mm (1.6 in.), is fixed hori- zontally near the bottom of the tank. This experiment used harvested ice. The initial ice packing factor, IPFo, was about 65%. The inlet water was kept at about 15C and controlled within t2 / +represents the ratio of time when Tour remains under
30、2“C (35.6“F) and is almost the same value that is defined as (QiA / ice + QA 3.3 x lo4 (5.2 gaymin) (t* = 63 min) 5.0 x lo4 (7.9 gaymin) (t* = 42 min) -p- V,=1.7 X 1W m3/s (th1 26 min) -0- V,=3.3 X 10mVs (%3 min) O 200 400 800 1200 210 262 0.80 130 155 0.84 When t*reached251 minutes, (Vw= 8.3 x lo5
31、m3/s 1.3 gal/min), To, increased to 2C (35.6“F) until 900 minutes. Harvested ice was completely melted at 1094 minutes. When the volume flow rate was 1.7 x m3/s (2.7 gal/min) (t* = 126min)or3.3 x 104m3/s(5.2gal/min)(t*=63 min),the To, change was similar to that of 8.3 x lo- m3/s (1.3 gaymin) (t* = 2
32、5 1 min). In both conditions, To, remains stable under about 2C until 390 minutes at a flow rate of 1.7 x 1 O4 m3/s (2.7 gal/ min) (t*= 126min)and210minutesat3.3 x 104m3/s(5.2gaV min) (t* = 63 min). When the water replacement time t* is shortest, i.e., 42 minutes ( Vw= 5.0 x loe4 m3/s 7.9 gal/min),
33、To, increased up to 2C until about 130 minutes. The harvested ice was completely melted at 155 minutes. It is clear that however short t* was, To, remained under 2C for a long time. Noteworthy is that average t2/+is about 0.82 for tank C. About 82% of the initial amount of ice was melted when To, re
34、mained stable under about 2C. Ice-melting characteristics of tank C differ slightly from those of tanks A and B (Ohira and Yanadori 1999). This is because the outlet pipe is fixed horizontally near the bottom of the tank. Water at O“C, which initially exists around the ice, is mixed with the water u
35、nder the ice-packed bed. It is influenced by natural convection in the water under the ice-packed bed so it is difficult to reach O“C, and the outlet water temperature increases. However, by this spraying method, the outlet water can be kept under 2C for a long time, and almost the same experimental
36、 results are changed in tanks A and B. ACTUAL-SIZE TANK PERFORMANCE Figure 5 shows the change in outlet water temperature with time in an actual-size ice thermal storage tank (V* = 125 m3 4414 ft3) used for the air conditioning of factories and office buildings. The initial IPF, is 65%. The inlet wa
37、ter was kept at about 15C (59F) and was only sprayed in the ice ther- mal storage tank. The water volume flow rate Vw is 3.5 x lo- m3/s (554.8 gal/min), and water-replacement time t*, which is defined as the water volume of the ice-thermal-storage tank. is t (min) 60 min. It is clear that To, remain
38、ed at about 0C (32F) for about 10 min and stayed stable under 2C (35.6“F) until 120 min. The outlet water temperature in the achal-size tank increased a little like that of experimental tank C when the Figure 4 Tank c outlet Water temperature versus time (v* = 1.2 m). Table 1. Comparison of Experime
39、ntal Results of Tank C I I l I 1.7 x lo4 (2.7 gaymin) 390 415 (t* = 126 min) ASH RAE Transactions: Research 83 J 15 - n Y O 60 120 180 240 300 360 t (min) Figure 5 Outlet water temperature versus time (actual-size tank: V* = 125 m3). volume flow rate was 3.3 x m3/s (5.2 gal/min) (t* = 63 min); t Yan
40、adori and Ohira 2000). It is found that the ice size, which is from about 3 mm (= 0.1 in.) ice particle to about 150 mm (5.9 in.) square crushed harvested ice in the tank, has good ice-melting characteristics in these studies. And the following evaluation of ice-melting can be applied to these ice s
41、izes. Heat transfer coefficient K is defined as where To is ice-melting surface temperature, that is, 0C. Thus, K is defined as where the amount of heat release q is given by 4 = PwVwcp,(Tjn - o,) . (3) Modified Stanton number St is defined as St = - KV* - -xt*(= K p = NTU) . (4) The amount of accum
42、ulated heat release, Q, is defined Pw vwcp w p wcp w P w vwcpw as ejW = PwVwCpw(Tin - ,)dt. (5) It consists of the amount of accumulated latent heat, Qice, and the amount of accumulated sensible heat, Q, for water at 0C. It follows that Qice is given by the difference between Q, and Q, If the averag
43、e water temperature in the tank equals the outlet water temperature, Qice is given by Qjce = Qjw - Qw = jPwVwcpw(T, - Toat)“ (6) -pwv* ( I- “ x zcpw(Tout- O) . (ice)o However, when the tank is filled with harvested ice at high density, the outlet water temperature can be kept at 0C for a long time b
44、ecause the ice-melting contact area in the ice- packed bed is large. If the outlet water temperature can be kept at about 0C in the case of large &e, Qw will be negligible. Then Qice can be approximated by Qice = jPwwc,w(T,-To,)dr- wV*cpw(T,-O) . (7) The amount of melted ice, G, is given by (8) G. =
45、 Qjce/L. ice The ice-packing factor, IPF, during ice melting is defined by using Gice as If the ice-packing factor, IPFL, is used, Gice is defined as IPF, Gice = A x 1.6) Figure 7 shows the relationship between t* and St, (over 1.6) with changing V*. St, is defined as st, = -JsIPF,) 1 IPFo When V* i
46、s O. 1 m3 or 0.2 m3, the inlet water release position x is fixed at i O mm. When the inlet water is sprayed from the water surface, that is, V* is 1.2 m3 or 125 m3, St, is about 1.6. In addition, when x is fixed near the bottom of the 5 4 3 CZ 2 1 O 5 V,=1.7 x104 m3/s Vb1.2 m3 iPF0=65% Tm+15C 0 Vw=3
47、.3 x lo4 m3/s 1 Harvested ice O V,=&3 x 10“mVs E 65 60 50 40 30 20 10 O JPF, (%) Figure 6 ModiJed Stanton number versus IPF,. tank, St, is about 1.8. Therefore, if the size of the tank or the method of inlet water spraying is changed, or if the ice packed bed is long, St, will be in the range of 1.6
48、 to 1.8, and ice-melt- ing characteristics will be constant. Figure 8 shows the relationship between t* and the time taken for the ice-packed bed to completely melt 9 Here, $-is proportional to t* for the four sizes of tank (V* = O. 1,0.2,1.2, and 125m3). This is because St, for these tanks is almos
49、t the same value regardless of the inlet water spraying method (see Figure 7). If the method of inlet water spraying or the inlet water release position, x, is selected appropriately, St, will be almost the same value for all four sizes of tank and $-can be estimated by Equation 14. tf = 4.3 x t* (14) Small Average Modified Stanton Number (Sf, 1.6) Figure 9 shows the relationship between t* and $-for tank B (V* = 0.2 m3) when the inlet water release position changes- x changes from 8 i O to 1 O mm. According
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