An ice slurry system using direct contact heat transfer for cooling applications

EXPERIMENTS AND ANALYSES: AN ICE SLURRY SYSTEM USING DIRECT CONTACT HEAT TRANSFER FOR COOLING APPLICATIONS MUHAMMAD ARIFEEN WAHED (B.Sc. (Mech. Eng.), B.U.E.T) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgements ACKNOWLEDGEMENTS In the course of this project, much assistance and services have been received form various sources for which the author is indebted. First of all, the author would like to express his deep gratitude to his supervisor Assoc. Professor M.N.A. Hawlader, Department of Mechanical Engineering, National University of Singapore for his sincere guidance, inspiration and valuable suggestions during the course of the study. The author is also thankful to all the staff members of the Thermal Process Laboratory. Finally, the author would like to thank his parents and wife, Salsalina Saberat, for their support and inspiration. i Table of Contents Table of Contents Acknowledgements Table of Contents Summary List of Figures List of Tables Nomenclature Chapter i ii v vii x xi INTRODUCTION 1.1 1.2 1.3 1.4 1.5 Background: Cooling with Ice Slurry Ice Slurry Technology Advantages of Ice Slurry Objectives Scope Chapter LITERATURE REVIEW 2.1 Fundamentals of Ice Formation 2.2 Ice Slurry Production 2.2.1 Static ice production 2.2.2 Dynamic ice Production 2.2.2.1 Phase Change liquids 2.2.2.3 Immiscible liquid 12 16 16 18 2.3 2.3.1 2.3.2 2.3.3 20 24 26 Chapter Ice Slurry Heat Transfer Phenomena Heat transfer through circular ducts Heat transfer through rectangular channels Heat transfer: industrial heat exchanger MATHEMATICAL MODEL 3.1 3.1.1 3.1.2 3.2 3.2.1 3.2.2 3.2.3 3.2.3.1 3.2.3.2 Ice Slurry Generator: Physical Arrangement Mathematical model: ice slurry generator Solution procedures 31 32 35 Ice Formation Analysis Conception of mushy layer Mathematical analyses Detachment of ice layer Droplet moving downward Droplet moving upward 42 44 50 51 53 ii Table of Contents 3.2.4 3.3 3.3.1 3.3.2 3.4 Chapter 4.1 4.1.1 4.1.2 4.1.3 Mass of ice 55 Ice Slurry Extraction Energy balance on cooling Coil Pressure drop analysis Solution Procedure: Flow Diagram 57 61 62 EXPERIMENTS Ice Formation: A Simulation Experiment The test rig: equipment and accessories Test procedure: ice formation Analysis of experimental data: images 66 72 73 4.2 Ice Slurry System: Direct Contact Heat Transfer 4.2.1 Description of test rig 4.2.2 Experimental procedures 4.2.2.1 Charging process 4.2.2.2 Discharging process 77 79 88 88 90 4.3 Chapter Uncertainty Analysis 97 RESULTS AND DISCUSSION 5.1 Ice Slurry Generator 5.1.1 Sensible cooling of water 5.1.2 Simulation results for ice slurry generator 5.1.2.1 Effect of coolant temperature 5.1.2.2 Effect of other parameters 5.1.2.3 Correlation: duration of initial cooling 5.1.2.4 Heat transfer between coolant and water 102 103 105 105 5.2 Ice Formation 5.2.1 Comparison of ice formation results 5.2.2 Ice formation phenomena 5.2.2.1 Nozzle mounted at bottom 5.2.2.2 Nozzle mounted at top 108 109 110 115 5.3 5.3.1 5.3.2 120 123 5.4 5.4.1 Parametric Study: Ice Formation Effect of droplet diameter Effect of coolant temperature 100 Ice Slurry Production Comparison of simulation and experimental values 126 iii Table of Contents 5.4.2 Parametric analysis: ice production 5.4.2.1 Effect of coolant temperature 5.4.2.2 Effect of nozzle diameter 5.4.2.3 Effect of number of nozzles 5.5 Ice Slurry Energy Extraction 5.5.1 Ice fraction calculation 5.5.2 Heat transfer coefficients for ice slurry 5.5.2.1 Local heat transfer coefficient 5.5.2.3 Average heat transfer coefficient 5.5.2.4 Comparison with previous studies 5.5.3 Parametric study: cooling capacity of ice slurry 5.5.4 Effectiveness of heat exchanger for ice slurry extraction 5.5.4.1 Observation: heat transfer coefficient 5.5.4.2 Cooling capacity: ice slurry Vs. chilled water Chapter CONCLUSIONS REFERENCE APPENDIX APPENDIX APPENDIX APPENDIX A B C D APPENDIX E 129 131 133 136 140 142 145 146 148 150 152 156 160 Image Analysis of Ice Formation Sensitivity Analysis: Duration of Initial Cooling Heat Transfer Characteristics of Ice Slurry Cooling Performance of Heat Exchanger for Ice Slurry and Chilled Water Calibration and Error Analysis 170 177 181 192 199 iv Summary Summary The development of an ice slurry system utilizing direct contact heat transfer requires a deeper understanding of the heat transfer process between the water and liquid in the ice slurry generator. In order to fulfill this objective, the present study has been divided into three parts: (i) to study the ice formation and detachment phenomena around the supercooled liquid droplet; (ii) to design and analyze an ice slurry system to provide better understanding of the ice production process for immiscible coolant and water; and (iii) to evaluate the heat transfer characteristics of ice slurry utilized in the heat exchanger for cooling applications. Experiments and analyses were carried out to study the ice formation mechanism between two immiscible liquids, water and coolant, FC-84, by direct contact heat transfer. This process involves the investigation of the physical phenomenon of ice formation around the supercooled liquid droplet and subsequent detachment from the droplet surface under different operating conditions- upward and downward propagation of liquid droplet in the water column. The experimental findings of ice formation show a good agreement with analytical results. The analysis of this ice formation process is then further extended for different parametric conditions such as droplet diameter, liquid initial temperature and the injection velocity of coolant. The analyses show that these parameters have significant effect on the growth of ice layer. This ice generation knowledge is then applied to the ice production analysis of the ice slurry generator which utilizes immiscible coolant, FC-84 and water for ice production. To analyze the ice slurry generation process of an ice slurry generator, a mathematical model has been developed to simulate the cooling of water and subsequent ice production v Summary in the system. Experiments are performed for both initial sensible cooling and ice generation processes to validate the proposed model. This model is then utilized to analyze the effect of different parameters such as initial coolant temperature, nozzle diameter, number of nozzles, etc on the ice production of the ice slurry generator. The analyses show that the ice generation process in the ice slurry generator can be improved significantly by increasing the number of nozzles, decreasing the nozzle diameter, and decreasing the initial coolant temperature in the system. Nozzle position inside the ice slurry generator also plays an important role - more ice slurry is produced when the nozzle is placed at the bottom than at the top. These analyses provide better understanding of the ice slurry generator utilizing direct contact heat transfer of immiscible liquids. For cooling applications, heat transfer characteristics of ice slurry in a compact heat exchanger have been discussed. To evaluate the thermodynamic and hydraulic behavior of ice slurry for different ice fractions, corresponding experimental investigations have been carried out for different design cooling loads and flow rates. The ice slurry heat transfer correlation obtained from these investigations is then utilized to evaluate the cooling performance of the heat exchanger. The analyses show that cooling performance of the heat exchanger increases significantly when ice slurry is used instead of chilled water at 7°C. The analyses, therefore, assist in understanding the physical phenomena of ice formation by direct contact heat transfer, the operational behavior of the ice slurry generator based on this process and the utilization of ice slurry for space cooling applications. vi List of Figures Summary Figure No. Title Page No Figure 3.1 Schematic diagram of the Ice Slurry Generator 32 Figure 3.2 Water temperature in the ice slurry generator 43 Figure 3.3 (a) Physical phenomena of ice formation 45 Figure 3.3 (b) Schematic of liquid droplet-ice-mushy-water layers. 45 Figure 3.3 (c) Cross-section of liquid droplet -ice-mushy-water layers. 45 Figure 3.4 Schematic diagram of initial ice formation process during an infinitesimal duration 46 Figure 3.5 Ice particles accumulated over ice slurry generator 51 Figure 3.6 (a) Forces on a downward moving liquid droplet in fluid 51 Figure 3.6 (b) Forces on a upward moving liquid droplet in fluid 53 Figure 3.7 Cross section of a tube in the heat exchanger 57 Figure 3.8 Schematic diagram for energy balance inside the tube of the heat exchanger 58 Figure 3.9 Flow diagram of simulation model 64 Figure 4.1 Schematic diagram of ice formation analysis 66 Figure 4.2 Experimental setup of ice formation analysis 67 Figure 4.3 Digital CCD camera, QImaging Retiga 2000R 68 Figure 4.4 (a) Digital zoom module, 70 XL 69 Figure 4.4 (b) Fiber optic illuminator 69 Figure 4.5 Profile of the image grabbing software (Screen Shot) 70 Figure 4.6 Profile of the image analysis software (Screen Shot) 70 Figure 4.7 Image of measurement scale for calibration purposes 71 Figure 4.8 Metallic balls for ice formation analysis 73 Figure 4.9(a) Methodology of image analysis 73 ◦ Figure 4.9(b) Ice formation phenomena (D= 50 mm, Td=-10 C, iron ball) 74 Figure 4.10 Profile of the ice formation analysis (Screen Shot) 75 Figure 4.11 Experimental setup of ice slurry system – ice slurry generator and ice slurry extractor 77 Figure 4.12 Schematic diagrams of ice slurry system – ice slurry generator and ice slurry extractor 78 Figure 4.13 Glass column test section of ice slurry system 79 Figure 4.14 Cad drawing, Acrylic Base of ice slurry system 80 Figure 4.15 (a) Flange connections on Acrylic Base, top view 81 Figure 4.15 (b) Flange connections on Acrylic Base, bottom view 81 vii List ofSummary Figures Figure No. Title Page No Figure 4.16 Shower spray head and connection on the base plate of the ice slurry system Figure 4.17 Cold Bath and Chiller used for the ice slurry system Figure 4.18 (a) Centrifugal pump used to pump coolant Figure 4.18 (b) Flow meter to measure coolant flow rate Figure 4.19 Piping system of the ice slurry system Figure 4.20 Extraction and return pipes to utilize ice slurry in the heat exchanger Figure 4.21 Progressive cavity pump to extract ice slurry Figure 4.22 (a), (b)Fan Tubular Heat Exchanger to utilize ice slurry for air cooling Electric Heater Figure 4.23 Electric heating system – heater, rheostat, fan. Figure 4.24 Flow Chart for experimental procedure showing varying coolant flow rates in the ice slurry generator Figure 4.25 (a) Pump located before Heat Exchanger Figure 4.25 (b) Pump located after Heat Exchanger Figure 4.26 Flow Chart for experimental procedure showing varying Cooling Loads Figure 4.27 Figure 5.1 Figure 5.2 Figure 5.3 82 82 83 83 84 84 85 86 87 89 92 92 94 Flow Chart for experimental procedure showing varying ice slurry extraction rate 95 Comparison of experimented and simulated results of temperature histograms during sensible cooling for different coolant flow rates (8 lit/min, 10 lit/min and 12 lit/min) 101 Variation of ice formation time for coolant temperatures 102 Heat transfer coefficient of the ice slurry generator for different coolant flow rates ( lit/min, 10 lit/min, 12 lit/min) 106 Figure 5.4(a) Ice formation phenomena (D= 40 mm, Td=-10◦C, Iron ball) 107 Figure 5.4(b) Comparison of experimental and simulation results for the ice formation process for different sizes (Dia. = 50 mm, 40mm, 30mm) metal balls. 109 Velocity distribution of a droplet injected from a nozzle mounted at bottom (V=0.50m/s, Dd= 4mm, Td= -10°C) 111 Figure 5.5 Figure 5.6 Figure 5.7 Figure 5.8 Figure 5.9 Effect of droplet diameter on the distance traveled by the upward propagating liquid droplet for injection velocity 114 Velocity distribution of a droplet injected from a nozzle mounted at top (V=0.15m/s, Dd= 4mm, Td= -10ºC) 116 Effect of droplet diameter on the distance traveled by the downward propagating liquid droplet for injection velocity 119 Effect of droplet diameter (Dd = 4mm, 6mm, 8mm and 10mm) on the growth of ice layer on droplet surface (Td= -10ºC and V=0.15m/s) 121 viii List of Figures Summary Figure No. Figure 5.9 Figure 5.10 Figure 5.11 Figure 5.12 Figure 5.13 Figure 5.14 Figure 5.15 Figure 5.16 Figure 5.17 Figure 5.18 Figure 5.19 Figure 5.20 Figure 5.21 Figure 5.22 Figure 5.23 Figure 5.24 Figure 5.25 Figure 5.26 Title Page No Effect of droplet diameter (Dd = 4mm, 6mm, 8mm and 10mm) on growth of ice layer on droplet surface (Td= -10ºC and V=0.15m/s) 121 Effect of droplet diameter (Dd = 4mm, 6mm, 8mm and 10mm) on the growth of mushy layer on droplet surface (Td= -10ºC and V=0.15m/s) 122 Effect of initial liquid droplet temperature (Td = -5ºC, -10ºC and -15ºC) on the growth of ice layer on droplet surface (Dd = 10mm and V=0.15m/s) 123 Effect of initial liquid droplet temperature (Td = -5ºC, -10ºC and -15ºC) on the growth of mushy layer on droplet surface (Dd = 10mm and V=0.15m/s) 124 Comparison of experimental and simulation results of the generated ice slurry for ice slurry generators with nozzle positioned at bottom. 126 Comparison of experimental and simulation results of the generated ice slurry for ice slurry generators with nozzle top 128 Effect of ice production for different coolant temperatures (Td = -5ºC, -10ºC and -15ºC) in the ice slurry generator (Water height =1m, Cylinder Dia.= 0.3m, Nd =1mm and Nz =50 ) for different coolant flow rates( lpm, 10 lpm and 12 lpm 129 Effect of ice production for different nozzle diameters (Nd = 0.8mm, 1mm and 1.2mm) in the ice slurry generator (Water height =1m, Cylinder Dia.= 0.3m, Td = -10ºC and Nz =50 ) for different coolant flow rates ( lpm, 10 lpm and 12 lpm) 131 Effect of ice production for different number of nozzles (Nz = 20, 40 and 60) in the ice slurry generator (Water height =1m, Cylinder Dia.= 0.3m, Td = -10ºC and Nd =1mm ) for different coolant flow rates ( lpm, 10 lpm and 12 lpm) 134 Temperature profile of the outside surface temperature of the heat exchanger at different locations (∆L=30 cm) 137 Ice fraction for different flow rates of ice slurry 139 Local heat transfer coefficient of ice slurry for different ice fraction, ice slurry extraction rate lpm. 143 Average heat transfer coefficient for different ice fraction (2%, 3%, 4% and 5%) 144 Accuracy of heat transfer correlation 145 Comparison of avg. Nu. number for ice slurry flow through pipe 147 Thermal performance of ice slurry for different ice fraction ( 2% ~ 5%), Room Temperature, Ta = 20ºC 149 Dependency of the heat transfer coefficients- air and ice slurry on the overall heat transfer coefficient. 151 Comparative analysis of the cooling performance between ice slurry and chilled water for different flow rates (5lpm, 8lpm and 10lpm) 154 ix Chapter Results and Discussion There are a few possible reasons for this phenomenon. One possible reason is that, at the entrance, the availability of ice particles would be higher near the pipe wall. This result in higher heat transfer rate between the bulk fluids and the wall and, thereby, increases the melting rate of ice slurry. Another possible reason is that there would be more turbulence of the propagating ice slurry at the entrance region than downstream inside the pipe. This would result in the increase of the convective heat transfer between bulk fluid and the inner surface of the heat exchanger tube at the upstream of the pipe. Since, the bulk temperature of the ice slurry is always º C the local heat transfer coefficient would be influenced by the fraction of ice slurry. For the increase of ice slurry fraction, the local heat transfer coefficient would increase. Figure 5.21 Average heat transfer coefficient for different ice fraction (2%, 3%, 4% and 5%) 144 Chapter Results and Discussion 5.5.2.3 Average heat transfer coefficient As described in Equation (5.7), integration of local heat transfer coefficients along the axial position of the heat exchanger tube gives the average values of the Nusselt number between these points. Dependency on the thermodynamics and hydrodynamics properties of ice slurry has been shown in Figure 5.21, where the Reynolds number, Re of ice slurry varies between 7000 and 15000, for ice fractions ~ 2%, 3%, 4% and 5%. Based on the curves, as shown in Figure 5.21, which illustrates the relation between the (Nu/Pr0.4) and Reynolds number (Re), it is possible to Figure out the constant values of C and m as a function of ice fraction. Relative analyses of these constant values are presented in the Appendix [C]. Calculated Nusselt Number 160 120 80 40 0 30 60 90 120 150 Measured Nusselt Number Figure 5.22 Accuracy of heat transfer correlation From the regression analyses of the experimental findings, the following correlation, equation (5.9), for the ice slurry heat transfer has been established. 145 Chapter Nu IS = C IS Re IS Results and Discussion m IS PrIS 0.4 (5.9) where, C IS = 0.003 − 0.365φi + 11.5φi − 100φi , and m IS = 0.33 + 77.803φi − 2533φi + 24167φi Where, This correlation is valid for Reynolds number, 7300 ≤ Re IS ≤ 15000 and ice fraction, ≤ φ i ≤ 5% . Figure 5.22 shows the accuracy of the empirical correlation developed from the experimental results. From the Figure 5.22, it is observed that experimental values of the ice slurry heat transfer lie between 7% and -7% of the predicted values obtained form the ice slurry empirical Nusselt number correlation. The ice slurry heat transfer correlation, therefore, can predict the values within ± 7% of the actual ones for given range of ice fractions. 5.5.2.4 Comparison with previous studies The heat transfer characteristics of ice slurry have been investigated by a number of researchers during the last few years and the results obtained from their investigations were diverge due to the absence of specific relations. Several empirical relations are proposed for the calculation of both local and average heat transfer coefficients based on the parameters affecting the heat transfer rate of ice slurry. It is found that the significant parameters are ice fraction, slurry viscosity, flow velocity, Reynolds number and Prandle number of flowing fluid, tube thermal property and tube geometry. Figure 5.23 shows the comparative results of the present study with those of almost similar studies by other researchers. The comparison may not be considered absolute, or the other studies may have different conditions and different parameters such as flow rate 146 Chapter Results and Discussion of ice slurry, heating flux, fraction of ice slurry. However, from the investigation, it is possible to understand the effect of the parameters on the heat transfer of the ice slurry. Figure 5.23 Comparison of average Nusselt number for ice slurry flow through pipe Figure 5.23 reveals that there is a similarity in the pattern of the average heat transfer results for different ice fractions reported by Egolf et al. [79] with the present investigations. In their studies, they proposed a correlation based on the experiments with a m long pipe of 15.74 mm inside diameter for ice fractions in the range of 4% to 33% and flow velocity 5-12 m/s. This ice slurry heat transfer correlation is based on the Reynolds numbers and ice fraction. Compared with their works, it is observed that the present investigations were carried out with ice fractions in the range of 2% to 9%. This would decline in the average Nusselt numbers of ice slurry. However, for both studies, it is found that with the increase of ice fraction, the heat transfer coefficient increases for ice slurry. The correlation of ice slurry heat transfer proposed by Chirstensen and Kauffeld [76] was 147 Chapter Results and Discussion applied to the current experimental investigations and it was found that the similar pattern of heat transfer values for different ice fractions ( 2% ~ 5%). However, the heat transfer values from this correlation deviated by about 10% to 20%. These deviations may be attributed to the thermal properties of the carrier fluid, which was 10% ethanol concentration for the correlation studies while pure water for the present investigations. Similar phenomena were also noticed when Kawami [86] correlation was compared with the results of present investigation. Applying the correlation, proposed by Stamatiou and Kawaji [92] for turbulent flow of ice slurry, in the current investigative conditions, it is found that the heat transfer coefficients are consistent with the present work (within the range of 20%). On the other hand, with the current investigative conditions, the correlation by Kondel et al. [78] suggested that the decreasing characteristics of heat transfer for the increase of the ice fraction. This phenomenon is consistent with his hypothesis, although it conflicts with the present work. The possible explanation is that the proposed correlation is for much higher Reynolds number, which may attribute to such discrepancy with the present study. From these comparisons of the average Nusselt number of ice slurry, it can be postulated that the present correlation developed for the current studies is consistent with those of the previous researchers. 5.5.3 Parametric study: cooling capacity of ice slurry Ice slurry extracted from the ice slurry generator, is utilized in the heat exchanger for cooling application. To analyze the cooling performance of the extracted ice slurry for 148 Chapter Results and Discussion different parameters such as ice fraction and atmospheric condition along the heat exchanger, a detailed study has been carried out in the following section. Ice fraction Figure 5.24 shows the effect of ice fraction ( 2% to 5%) on the cooling capacity of ice slurry along the heat exchanger. For this analysis, different ice slurry flow rates (5 lpm, lpm and 10 lpm) were considered; while the air flow rate at Ta = 20ºC in the heat exchanger was kept constant. 3.0 , kW Cooling capacity 2.5 10 lpm 2.0 1.5 lpm 1.0 0.5 lpm 0.0 /L ,x e nc sta i D 5% 4% 3% Ice 2% Frac tion 10 Figure 5.24. Thermal performance of ice slurry for different ice fraction ( 2% ~ 5%), Room Temperature, Ta = 20ºC From Figure 5.24, it is observed that there is a decrease in the cooling capacity of ice slurry along the heat exchanger for a particular ice fraction. At the inlet of the heat 149 Chapter Results and Discussion exchanger, the fraction of ice slurry is maximum and it decreases continuously when passed through the heated heat exchanger to cool the air by the melting of ice in the slurry. From the figure, it is also observed that the cooling capacity of higher fraction (from 2% to 5%) ice slurry is increased by as high as about 2.5 times when the slurry flow rate was maintained at 10 liter/min in the heat exchanger. However, the melting rate of ice in the slurry would be similar irrespective of ice fraction due to latent heat of fusion at 0ºC. The effect of the ice slurry flow rate through the heat exchanger is also observed from Figure 5.24. Higher ice slurry flow rate attributes higher cooling performance in the heat exchanger. With the increase of the slurry flow rate from liter/min to 10 liter/min, the cooling performance of the heat exchanger is increased by about to times for different ice fraction in the range of 2% to 5%. One possible explanation is that with the increase of the ice slurry flow rate, the available mass of ice would be increased in the heat exchanger. This would result in more latent heat of fusion of ice slurry and, subsequently, increase the cooling performance of the heat exchanger. This performance could be further enhanced when the fraction of ice in the slurry is increased, as may be observed in Figure 5.24. 5.5.4 Effectiveness of heat exchanger for ice slurry extraction 5.5.4.1 Observation: heat transfer coefficient To evaluate the thermal performance of the heat exchanger for different conditions, a series of experiments were conducted to cool the heated air by the different fractions of ice slurry along the heat exchanger. In this analysis, the effect of the heat transfer rate of 150 Chapter Results and Discussion these two fluids- ice slurry and air, on the overall heat transfer rate of the heat exchanger has been investigated. 9000 .K 63 m / W ,t ne ici f eo cr ef sn art ta eH Air 62 (Ice slurry) Heat transfer coefficient, W/m2.K 64 6000 Overall Heat transfer 61 60 3000 Ice slurry 59 58 0 12 15 18 Cooling load, kW/m2 Figure 5.25 Dependency of the heat transfer coefficients- air and ice slurry on the overall heat transfer coefficient. Figure 5.25 shows the dependency of the heat transfer coefficients- air and ice slurry on the overall heat transfer coefficient for different cooling loads ( kW/m2 to 16.5 kW/m2). From Figure 5.25, it was observed that the ice slurry heat transfer rate is in the range of 2500 W/m2 to 7500 W/m2; while the air heat transfer rate is in the range 58 W/m2 to 63 W/m2. It has been observed that the heat transfer rate of the ice slurry is much higher than that of the air side. Thus from this analysis, it can be postulated that the overall heat transfer coefficient of the heat exchanger depends on the air side heat transfer rate during cooling application. 151 Chapter Results and Discussion 5.5.4.2 Cooling Capacity: ice slurry Vs. chilled water To compare the cooling performance of the heat exchanger between ice slurry and chilled water, a series of experiments were conducted for the similar operating conditions as mentioned in Section 4. Corresponding experimental results are presented in Appendix [D]. Table 5.5 shows the cooling performance of the heat exchanger for different designed cooling loads when the extraction rate of ice slurry is varied from 5liter/min to 10 liter/min. Table 5.5 Cooling capacity of ice slurry for different designed cooling loads Extraction rate, lpm 10 Designed Cooling Load, KW Ice Fraction Ice slurry temp at inlet, ºC Ice slurry temp at outlet, ºC Air inlet temp., ºC Air outlet temp., ºC Heat transfer, KW 0.5 1% 5.7 25.1 20 0.31 2% 6.4 25.1 20.5 0.36 1.5 3% 5.8 25.1 21.1 0.35 4% 5.3 25.2 21.6 0.35 2.5 5% 6.0 25.2 22.0 0.43 0.5 1.50% 7.2 25.1 20.1 0.26 2.10% 6.2 25.2 20.7 0.24 1.5 3.50% 6.3 25.3 21.3 0.25 4.50% 6.2 25.4 21.8 0.27 2.5 6.00% 6.3 25.5 22.3 0.30 0.5 2% 8.3 25.1 20.3 0.19 3.50% 8.3 25.3 20.9 0.21 1.5 5.20% 8.3 25.4 21.4 0.23 7% 8.3 25.5 22.0 0.23 2.5 9% 8.6 25.5 22.4 0.26 152 Chapter Results and Discussion Similarly, Table 5.6 shows the cooling performance of the heat exchanger for different designed cooling loads when the extraction rate of chilled water is varied from 5liter/min to 10 liter/min. Here, it is to be noted that the chilled water temperature at the inlet of the heat exchanger was in the range of 6.5ºC to 7.0ºC, which is the standardized temperature or commercial district cooling applications. Table 5.6 Cooling capacity of chilled water (7°C) for different designed cooling loads Extraction rate, lpm 10 Designed Cooling Load, KW 0.5 1.5 2.5 0.5 1.5 2.5 0.5 1.5 2.5 Chilled water temp at inlet, ºC 6.6 6.7 6.6 6.8 6.4 6.8 6.5 6.6 6.7 6.9 6.7 6.7 6.8 6.8 6.9 Chilled water temp at outlet, ºC Air inlet temp., ºC Air outlet temp., ºC 11.5 11.8 11.9 12.1 12 12.2 12.3 12.8 12.8 13.3 12.8 13.9 14.15 14.3 14.6 25.4 25.4 25.3 25.3 25.3 25.3 25.3 25.4 25.4 25.6 25.3 25.3 25.4 25.6 25.5 20.9 21.3 21.5 21.7 21.8 21.3 21.6 21.8 22.0 23.0 21.6 21.7 22.0 22.4 22.6 Heat transfer, KW 0.24 0.26 0.27 0.27 0.28 0.22 0.23 0.25 0.25 0.25 0.16 0.19 0.20 0.20 0.21 Figure 5.26 shows the comparative analysis of the heat transfer rate between ice slurry and chilled water. It has been observed that the heat transfer rate had been increased by about 1.2 to 1.6 times when ice slurry was used in the heat exchanger instead of the chilled water for the defined design cooing loads. Thus, for the similar cooling loads, surface area of the heat exchanger can be reduced significantly during ice slurry utilization instead of chilled water. This would reduce the heat exchanger size which 153 Chapter Results and Discussion would subsequently lower the installation as well as the maintenance cost of the cooling system. From the Figure 5.26, it is also observed that, for ice slurry, both flow rate and ice fraction affect the cooling performance of the heat exchanger. When the coolant flow rate increases from liter/min to 10 liter/min, the cooling performance of the heat exchanger has been enhanced. Deviation of these results can be attributed to the ice fractions in the slurry. 10 lpm 1.8 1.6 lpm lpm (UA)is/(UA)cw 1.4 1.2 0.8 0.6 0.4 0.2 0.5 1.5 2.5 Cooling Load, kW Figure 5.26 Comparative analysis of the cooling performance between ice slurry and chilled water for different flow rates (5lpm, 8lpm and 10lpm) From Table 5.5, it has been observed that the ice fraction in liter/min extraction was comparatively higher than the other extracted flow rates. As a result, higher ice fractions would offset the higher coolant flow rate of ice slurry from liter/min to liter/min, while higher coolant flow rate would compensate the higher ice fractions of the slurry. From the analysis, it can be postulated that compared to chilled water higher flow rate ice 154 Chapter Results and Discussion slurry with increased ice fraction can enhance the cooling performance of the heat exchanger significantly. Summary This chapter discusses the results obtained from the experimental and analytical investigations of the ice slurry system. Analyses of the ice slurry generator, ice formation phenomena by direct contact heat transfer and utilization of ice slurry for cooling application would provide an in-depth knowledge of such systems. 155 Chapter Conclusions CHAPTER CONCLUSIONS The objective of this research was to analyze the ice slurry system for cooling applications. The results of these analyses are summarized below. In this system, ice formation mechanism was developed based on the concept of direct contact heat transfer between two immiscible liquids and ice accumulation by the dynamic method utilizing the body force, buoyancy and drag forces of these liquids. The following conclusions can be drawn from experimental and simulation studies: A mathematical model coupled with heat transfer and force balance was developed to analyze the growth of ice layer around the supercooled liquid droplet and to investigate the ice production in the ice slurry generator. Experimental investigations by image processing technique and measurement of ice layer formation were performed. The findings of ice layer growth from visual investigations have shown similar results obtained from the simulation model for ice layer growth. The model for the growth of the ice layer was then further extended to investigate the physical phenomena of ice formation for different nozzle locations—nozzle at bottom and nozzle at top. From the analyses, it was found that the bottom positioned nozzle performed better due to the significant role of the injected velocity of liquid droplet on ice formation. The analyses also showed that the growth of the ice layer increases with the increase of the liquid droplet diameter and with the decrease of the initial liquid temperature. 156 Chapter Conclusions In the ice slurry generator, the ice production procedure includes both initial sensible cooling and subsequent fusion cooling. The experimental results for initial sensible cooling and subsequent fusion cooling of ice formation for different nozzle locations—nozzle at bottom and nozzle at top, were in good agreement with values obtained from the simulation model developed for ice slurry generator. For estimating the duration of initial sensible cooling in the ice slurry generator, a correlation was proposed for different parameters such as coolant temperature, nozzle diameter, number of nozzles and geometric condition of the cylinder. The validated model of the ice slurry generator was used to analyze the effect of different parameters such as initial coolant temperature, nozzle diameter and number of nozzle on the ice slurry production. From the analyses, it was found that the ice generation process in the ice slurry generator can be improved significantly by increasing the number of nozzles, decreasing the nozzle diameter, and lowering the initial coolant temperature in the system. It was also found from the analyses that, of these parameters, lower initial coolant temperature attributes to a higher ice production in the ice slurry generator. Ice production rate can be increased as high as 50% by decreasing the initial coolant temperature from -5˚C to -15˚C. However, due to higher energy consumption for decreasing coolant temperature, it would not be economically viable. To investigate the heat transfer characteristics of ice slurry for cooling applications, ice fractions passing through the heat exchanger were varied from 2% to 9% for different operating conditions such as ice slurry flow rate and cooling load. 157 Chapter Conclusions Based on the experimental results, an empirical correlation of heat transfer for turbulent flow ice slurry in the heat exchanger has been proposed. Form the analysis, it was found that the mass fraction of ice plays a significant role in the cooling performance of the heat exchanger. For air cooling by an ice slurry, the overall heat transfer coefficient of the heat exchanger depends on the air side heat transfer rate during cooling application. The cooling performance of the air cooled heat exchanger has been increased by about 1.1 to 1.6 times when the ice slurry was used in the heat changer instead of the chilled water at ºC for the defined design cooling loads. This would reduce the heat exchanger size which would subsequently lower the installation as well as the maintenance cost of the cooling system. 158 Chapter Conclusions Recommendations The following recommendations are made for future extension of the work. In the present study, ice formation phenomenon has been investigated by digital CCD camera. However, better visualization technique with improved hardware can be adapted to the ice formation analysis. This may provide more information regarding the physical phenomena of ice generation around the supercooled liquid droplet. In the current study of direct contact heat transfer method, the immiscible, higher density coolant had been suggested by Wijeysundera et al. [71]. However, efforts can be focused on the exploration of the similar properties (immiscibility, higher density and lower freezing point) coolant, which is more cost effective. The proposed ice slurry system can be installed in an industrial scale in a building, where conventional chilled water system has already been utilized and compare the relative operating costs. This would provide the commercial viability of the ice slurry system. 159 [...]... direct contact heat transfer between two immiscible liquids to provide a deeper understanding of the variables that would persuade cooling and ice production of the ice slurry system 2.3 Ice Slurry Heat Transfer Phenomena The higher cooling capacity of ice slurry enhances the heat exchanger performance, which affects the sizing and costing of the system However, to design an optimum heat exchanger for. .. applications and investigate the viability of the ice slurry to utilize as coolant for space cooling applications This study focuses on both analytical and experimental work on ice formation and ice slurry system An analysis of ice formation may lead to a better understanding of the physical phenomena of ice layer growth between two immiscible liquids In terms of system development, an ice slurry system. .. results for the validation of the mathematical model developed for the above mentioned cases iv Developing an ice slurry system, based on the concept of direct contact heat transfer between two immiscible liquids An extraction system will then be incorporated with the slurry generator to evaluate the performance of the system v Analyzing the heat transfer characteristics of ice slurry for cooling applications. .. heat transfer phenomena of the ice slurry system by direct contact heat transfer between water and immiscible coolant However, a detailed analysis of the system to understand the variables that affect the heat transfer process during initial cooling and subsequent ice production have not yet been done The present study is to evaluate the ice generation process in the ice slurry generator utilizing direct. .. detailed heat transfer characteristics of ice slurry need to be explored During the last few years, several research papers were published regarding the rheological behavior and the heat transfer characteristics of ice slurry 2.3.1 Heat transfer through circular ducts The fluidity of ice slurry is an important factor in utilizing it directly in the heat exchanger Generally, ice slurry is a mixture of ice. .. that install ice slurry system for cooling and air-conditioning requirements of the 3800 m2 site Gladis [15] describes an ice slurry plant for processing of 90,000 kg of cheese daily located in Hanford, California, USA Other future applications of ice slurries [16] are ice pigging (frequent and efficient internal cleaning of the inside components of pipes, ducts and heat exchangers), medical applications. .. ice slurry for cooling applications In this chapter, a review of the published literature that addresses these issues of the ice slurry technology is presented The chapter comprises the following sections: the fundamentals of ice formation, the conventional static ice generation methods, the dynamic ice generation method- direct contact heat transfer and the investigation of the ice slurry heat transfer. .. resistance between the cooling surface and the solid-liquid interface Therefore, further studies are needed for different methods of ice slurry generation to improve the performance of the system Dynamic types, however, offer superior efficiency characteristics [49], because ice is produced by direct contact heat transfer and is removed naturally by buoyancy forces But, dynamic systems, like direct contact. .. change material to produce ice by direct contact of heat transfer with water Mori et al [57] proposed a method for manufacturing crystal ice from polymer gel, a PCM (phase change material) In this method, ice is generated in the water absorbing polymer gel by direct heat exchange Another type of PCM, tetradecane particles, for ice production in a double-tube heat exchanger was proposed by Inaba and... important sector for cooling needs by ice slurry is the mining industry Ophir and Koren [11] describe one such slurry plant at the Western Deep Level Gold Mine in South Africa Fishermen in Chili, Netherlands and Iceland utilize ice slurry for direct chilling of fishes and other catches [12] They produce ice slurry from the sea water on board in their small 2 Chapter 1 Introduction ice slurry plants . understanding of the ice slurry generator utilizing direct contact heat transfer of immiscible liquids. For cooling applications, heat transfer characteristics of ice slurry in a compact heat. Experimental setup of ice slurry system – ice slurry generator and ice slurry extractor 77 Figure 4.12 Schematic diagrams of ice slurry system – ice slurry generator and ice slurry extractor. EXPERIMENTS AND ANALYSES: AN ICE SLURRY SYSTEM USING DIRECT CONTACT HEAT TRANSFER FOR COOLING APPLICATIONS MUHAMMAD