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AN EXPERIMENTAL CLASSIFICATION OF THE ENCAPSULATED WATER SUPERCOOLING
 

AN EXPERIMENTAL CLASSIFICATION OF THE ENCAPSULATED WATER SUPERCOOLING

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Juan J. Milón G.1, Sergio L. Braga2

Juan J. Milón G.1, Sergio L. Braga2

1 Universidad Católica San Pablo. Arequipa, Perú.
2 Pontificia Universidade Católica do Rio de Janeiro, Rio de Janeiro, Brasil. 184s

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    AN EXPERIMENTAL CLASSIFICATION OF THE ENCAPSULATED WATER SUPERCOOLING AN EXPERIMENTAL CLASSIFICATION OF THE ENCAPSULATED WATER SUPERCOOLING Document Transcript

    • AN EXPERIMENTAL CLASSIFICATION OF THE ENCAPSULATED WATER SUPERCOOLING Juan J. Milón G.1, Sergio L. Braga2 1 Universidad Católica San Pablo. Arequipa, Perú. 2 Pontificia Universidade Católica do Rio de Janeiro, Rio de Janeiro, Brasil. 184s ABSTRACT An experimental apparatus was developed to investigate the supercooling phenomenon in water inside cylindrical capsules used for cold storage processes. The coolant, is a water- alcohol mixture (50% vol.), controlled by a constant-temperature bath. Temperatures varying within a one-second time are measured inside and outside the capsule. A cylinder with an internal diameter 45 mm, thickness 1.5 mm and length 170 mm was made in aluminum material. The water supercooling period and the nucleation temperature were investigated for different coolant temperatures. The results indicate that different curves are a characteristic, in cylindrical capsules, in the cooling process of water with supercooling presence and they can represent a coherent classification of the phenomenon. KEYWORDS: dendritic ice, nucleation, phase change, supercooled water, thermal storage. INTRODUCTION Water is widely used in thermal storage systems as phase-change material (PCM) due to its following advantages; high value in latent heat, stable chemical properties, low cost and easy acquisition, no environmental pollution concerns, and good compatibility with the materials used in air-conditioning equipment. However, there are also a few disadvantages in using water as PCM. The most serious problem encountered is the supercooling phenomenon, which occurs during the thermal storage cooling system. While the water is cooled in an enclosed container, freezing does not occur at its regular freezing point (0°C) at atmospheric pressure. Instead, it is normally cooled below 0°C before ice nucleation happens. Supercooled water refers to a state of metastable liquid even though the water temperature is below its freezing temperature. The metastable state will end when ice nucleation occurs and the thin plate-like crystal of dendritic ice grows into the region of supercooled water. During the dendritic ice growth process, latent heat is released from the dendritic ice and absorbed by the supercooled water. At the end of this initial growth process, normally the water temperature returns to its freezing point (0°C). If the metastable state exists and remains during the thermal storage process, thermal energy can only be stored in the form of sensible heat. In this case, the storage capacity is strongly reduced. There are several studies about supercooling water process. Chen et al. [1], studied a numerical and an experimental methods to analyze the influence of nucleation agents in the water solidification process inside cylindrical copper capsules of different sizes. Yoon et al. [2], studied the freezing phenomenon of saturated water within the supercooled region in a horizontal circular cylinder using the holographic real time interferometry technique. Milón and Braga [3], studied the supercooling phenomenon in spherical capsules of different diameters and determined the parameters that influence the phenomenon appearance. Okawa et al [4], studied a freezing of supercooled water on a metallic surface. Gilpin [5] studied the dendritic ice formed in a water pipe during the freezing process. He showed that growth of dendritic ice can cause blockage of the water pipe. The extent of dendritic growth is largely determined by
    • the temperature distribution existing in the pipe at the time of ice nucleation. Milón and Braga [6], studied the dendritic ice formation in the supercooled water enclosed in cylindrical capsules and mentioned the particularitities of water under these conditions. A classification of the characteristic curves presented statistically in the different experiments with water supercooling is not mentioned in the reference, for this reason; the present work is a guide for future studies. EXPERIMENTAL MODEL The experimental model, shown schematically in Figure 1, consists in a test section (a), an observing system (b), a cooling system, which includes a CTB (Constant Temperature Bath) (c), and a measurement and data acquisition system (d). ACQUISITION SYSTEM (d) CTB UPPER (c) RESERVOIR (c) TEST SECTION (a) CTB-IC (c) LOWER AIR DIGITAL RESERVOIR COMPRESSOR CAMERA (c) (c) (b) Figure 1. Experimental Model. Test section. This section contains the capsule to be analyzed. The box acrylic walls are 10 mm thick, externally covered with a 25 mm thick insulation. A cross section of the test section is shown in the Figure 2. The diffuser is intended to homogenize the coolant temperature (TC) in the test section. The temperature control is carried out by a constant temperature bath which receives the signals from a temperature sensor RTD type PT-100. K type thermocouples were used for the circulating fluid temperatures registration. Two flanges are installed in the vertical walls to facilitate the capsule installation. An overflow tank, working at atmospheric pressure, was installed to compensate the volume variation during the PCM-phase-change
    • process. To define the volume of the PCM, a sliding disk (movable in the axial direction) is used. K type thermocouples of 0,076 mm diameter, covered with Teflon, are also shown. RTD PT-100 COOLANT SECONDARY DIFFUSER THERMOCOUPLES CAPSULE OVERFLOW TANK FIXED DISK PCM SLIDING DISK COOLANT PRIMARY THERMOCOUPLES DIFFUSER Figure 2. Test section. Cooling system The cooling system is schematically shown in Figure 1. It is composed by two constant temperature baths (CTB), two reservoirs (upper and lower), and the coolant. The initial temperature for the tests is set by the CTB-IC while the other CTB is set at the test temperature. The temperature control system is of the PID type (Proportional Integral Derivative) and it is able to maintain the temperature within a range of ±0.05 °C with a refrigeration power of 800 W at 0 ºC and of 1000 W of electric power heating. The coolant is an alcohol-water solution (50% in volume). Measuring and Data Logging System The Measuring and Data Logging system includes the data acquisition system and a personal computer (PC). The acquisition equipment, which communicates with the PC by the RS232 communication port, receives, processes and transmits the temperature signals to the PC, for storage and posterior analysis. EXPERIMENTAL PROCEDURE The initial water temperature inside the capsule (25.0 ºC) is set using the constant temperature bath for the initial condition (CTB-IC). The other CTB, controls the coolant temperature in the upper reservoir. The coolant of the upper reservoir passes through the test section setting the test temperature, absorbing the initial thermal loads and immediately after getting to the lower reservoir. The coolant of the CTB is addressed toward the test section until the conclusion of the test. After each test, a pump impels the coolant toward the upper reservoir for a new test. This procedure is carried out to maintain constant the test temperature through each test. The length of each test varies from 30 to 60 minutes, depending on the test conditions. The data is acquired every second. Uncertainties are presented in Table 1.
    • Table 1: Uncertainties of measurements Parameter Uncertainty Temperature 0.1 ºC Time 0.01 s Length 0.01 mm RESULTS AND DISCUSSION Displayed Curves with an occurrence probability of more than 50% For the classification shown below, it is necessary to define a thermocouple position (Fig 3). The displayed curves represent most of the observed curves (a probability of more than 50%). thermocouple capsule position PCM Figure 3. Temperature sensor position. In the Fig. 4, the freezing process without supercooling is observed. This type of curve, with stable change phase, normally happens in capsules of high thermal conductive and low coolant temperatures. It is observed that the process begins with a PCM temperature with Ti. It immediately descends releasing liquid sensible heat (a) down to the temperature of phase change and later releases latent heat (b), finally, the last particle of PCM is ice (region around the thermocouple) and releases solid sensible heat (e). In this process supercooling does not happen Ti Temperature, ºC a, b d e Thermocouple position Liquid water a dendritic ice Tdi ice Tf d e Tc Time, s Figure 4. Freezing process without supercooling, encapsulated water
    • In fig 5, the PCM is cooled releasing sensible heat (a), down to the density inversion temperature (Tdi) and goes through the freezing temperature (Tf), immediately after, it keeps lowering in metastable liquid state (b) below Tf. The metastable state will end when ice nucleation occurs (T=Tn) and thin-plaque-like crystals of dendritic ice grows in the water supercooled region (c). During the dendritic ice growth process, latent heat is released from the dendritic ice and absorbed by the supercooled water. At the end of this dendritic growth process, usually, the water temperature returns to its freezing point (0°C) and phase change starts (d) later, solid sensible heat is released (e). This curve type appears commonly for different capsule materials with coolant temperatures, relatively low. Ti a, b c d e Temperature, ºC Thermocouple position Liquid water a dendritic ice Tdi ice Tf d b c Tn e Tc Time, s Figure 5. Supercooling and freezing process, encapsulated water In Fig 6, is observed a supercooling process and instantaneous freezing, after the PCM is supercooled, and when nucleation occurs (c), the energy level is so low in that area that the PCM reaches Tf , and the phase change occurs quickly and immediately after it releases solid sensible heat (e). This happens with high-thermal-conductivity capsules and coolant at low temperatures. In Fig 7, the hypercooling with an instantaneous phase change process can be observed. Unlike the previous curve, when nucleation happens, energy is such, that the phase change occurs at temperatures below Tf (d) and it immediately releases sensible heat (e).
    • Ti Temperature, ºC a, b c e Thermocouple position a Liquid water dendritic ice Tdi ice Tf b c e Tn Tc Time, s Figure 6. Supercooling and Instantaneous freezing process, encapsulated water. Ti Temperature, ºC a, b c d e Thermocouple position Liquid water a dendritic ice Tdi d ice Tf b Tn c e Tc Time, s Figure 7. Hypercooling and instantaneous freezing process, encapsulated water In fig 8, the permanent supercooling and non-freezing process is observed. This occurs when the metastable liquid state (b) remains for an undetermined time and the PCM only releases liquid sensible heat. This curve is common in high and low thermal conductivity capsules with high coolant temperatures (near Tf)
    • Ti Temperature, ºC a, b Thermocouple position Liquid water a Tdi Tf b Tc Time, s Figure 8. Permanent supercooling and non-freezing process, encapsulated water. Special curves observed sporadically The curves described in the Figs. 9 and 10 are special cases which appeared rarely in the tests, but due to their particular characteristics, are here described. Fig. 9, shows the curve for double supercooling, when the PCM presents an apparent supercooling (a'), the PCM, returns to Tf, after a brief period of time. This probably occurs by convective-cells generation that produces a thermal equilibrium in the region. Then the PCM presents a higher degree of supercooling (b) and nucleation occurs (b'), and dendritic ice appears (c). Finally, the phase change process continues (d) and the sensible heat is released (e). Temperature, ºC Ti a, b c d e Thermocouple position Liquid water a Tdi dendritic ice ice Tf d Tn a’ b c b’ e Tc Time, s Figure. 9. Special freezing process, double supercooling, encapsulated water.
    • In fig. 10 another particular case is observed. When the PCM is supercooled (b), nucleation occurs and the PCM temperature returns above Tf. Later in a considerable amount of time it, returns to Tf and quickly occurs the phase change (d) finally, sensible heat is released (e). Temperature, ºC Ti a, b c d e a Thermocouple position Liquid water Tdi dendritic ice ice Tf d e Tn b c Tc -10 0 200 300 400 500 600 Time, s Figure 10. Special freezing process, supercooling and nucleation above Tf, encapsulated water CONCLUSIONS In regard to the tests performed under controlled conditions, a classification of encapsulated water cooling processes was made. Five characteristic curves appeared (with a probability of more than 50% in 20 tests under the same experimental conditions). Two curves appeared seldom in the tests. These were shown and analyzed NOMENCLATURE Ti initial temperature, [ºC] Tdi inversion density temperature Tf freezing temperature, [ºC] Tn nucleation temperature TC coolant temperature, [ºC] PCM phase change material CTB constant temperature bath CTB-IC constant temperature bath to initial condition of the PCM ACKNOWLEDGEMENTS This paper was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq of Brazil. The authors also wish to thank the Agreement Pontificia Universidade Católica do Rio de Janeiro and Universidad Católica San Pablo for motivating this research work.
    • REFERENCES [1] Chen, S. L., Lee, T. Z., 1998, A study of Supercooling Phenomenon and Freezing Probability of Water Inside Horizontal Cylinders, International Journal of Heat and Mass Transfer 41: (4-5, 3), 769-783. [2] Yoon, J.J., Moon, C.G., E.Kim, Son, Y. S., Kim, J.D., Kato, T, 2001, Experimental Study on Freezing of Water with Supercooled Region in a Horizontal Cylinder, Applied Thermal Engineering 21Ç 657-668. [3] Milón, J.J., Braga, S.L., 2003, Supercooling Water in Cylindrical capsules, Fifteenth Symposium on Thermophysical Properties, National Institute of Standards and Technology, Colorado, USA. [4] Okawa, S., Saito, A., Minami, R., 2001,The Solidification Phenomenon of the Supercooled Water Containing Solid Particles. International Journal of Refrigeration 24 (2001), 108–117. [5] Gilpin, R.R., 1977, The Effect of Cooling Rate on the Formation of Dendritic Ice in a Pipe whith no Main Flow. ASME Journal of Heat Transfer 99: 419-424 [6] Milón, J.J., Braga, S.L., 2004, Dendritic Ice Growth in Supercooled Water Inside Cylindrical Capsule, International Refrigeration and Air Conditioning Conference at Purdue, Colorado, USA. Clima 2005 The authors affirm that the paper has not been published elsewhere and, if the paper is accepted, will not be published elsewhere prior to Clima2005. If the paper is accepted, the authors will grant to Clima20005 a nonexclusive license to publish the paper in any fashion.