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

chilled water 152 Chapter 6 CONCLUSIONS 156 REFERENCE 160 APPENDIX A Image Analysis of Ice Formation 170 APPENDIX B Sensitivity Analysis: Duration of Initial Cooling 177 APPENDIX C

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

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

2.3.1 Heat transfer through circular ducts 20 2.3.2 Heat transfer through rectangular channels 24 2.3.3 Heat transfer: industrial heat exchanger 26

Chapter 3 MATHEMATICAL MODEL

3.1 Ice Slurry Generator: Physical Arrangement 31 3.1.1 Mathematical model: ice slurry generator 32 3.1.2 Solution procedures 35

3.2 Ice Formation Analysis 3.2.1 Conception of mushy layer 42 3.2.2 Mathematical analyses 44 3.2.3 Detachment of ice layer 50 3.2.3.1 Droplet moving downward 51 3.2.3.2 Droplet moving upward 53

3.2.4 Mass of ice 55

3.3 Ice Slurry Extraction

3.3.1 Energy balance on cooling Coil 57

3.3.2 Pressure drop analysis 61

3.4 Solution Procedure: Flow Diagram 62

Chapter 4 EXPERIMENTS

4.1 Ice Formation: A Simulation Experiment 4.1.1 The test rig: equipment and accessories 66

4.1.2 Test procedure: ice formation 72

4.1.3 Analysis of experimental data: images 73

4.2 Ice Slurry System: Direct Contact Heat Transfer 77

4.2.1 Description of test rig 79

4.2.2 Experimental procedures 88

4.2.2.1 Charging process 88

4.2.2.2 Discharging process 90

4.3 Uncertainty Analysis 97

Chapter 5 RESULTS AND DISCUSSION

5.1 Ice Slurry Generator 5.1.1 Sensible cooling of water 100 5.1.2 Simulation results for ice slurry generator

5.1.2.1 Effect of coolant temperature 102

5.1.2.2 Effect of other parameters 103

5.1.2.3 Correlation: duration of initial cooling 105 5.1.2.4 Heat transfer between coolant and water 105

5.2 Ice Formation 5.2.1 Comparison of ice formation results 108

5.2.2 Ice formation phenomena 109

5.2.2.1 Nozzle mounted at bottom 110

5.2.2.2 Nozzle mounted at top 115

5.3 Parametric Study: Ice Formation

5.3.1 Effect of droplet diameter 120 5.3.2 Effect of coolant temperature 123

5.4 Ice Slurry Production 5.4.1 Comparison of simulation and experimental values 126

Table of Contents

5.4.2 Parametric analysis: ice production

5.4.2.1 Effect of coolant temperature 129

5.4.2.2 Effect of nozzle diameter 131

5.4.2.3 Effect of number of nozzles 133

5.5 Ice Slurry Energy Extraction 5.5.1 Ice fraction calculation 136

5.5.2 Heat transfer coefficients for ice slurry 140

5.5.2.1 Local heat transfer coefficient 142

5.5.2.3 Average heat transfer coefficient 145

5.5.2.4 Comparison with previous studies 146

5.5.3 Parametric study: cooling capacity of ice slurry 148

5.5.4 Effectiveness of heat exchanger for ice slurry extraction 5.5.4.1 Observation: heat transfer coefficient 150

5.5.4.2 Cooling capacity: ice slurry Vs chilled water 152

Chapter 6 CONCLUSIONS 156

REFERENCE 160

APPENDIX A Image Analysis of Ice Formation 170

APPENDIX B Sensitivity Analysis: Duration of Initial Cooling 177

APPENDIX C Heat Transfer Characteristics of Ice Slurry 181

APPENDIX D Cooling Performance of Heat Exchanger for Ice Slurry and Chilled Water 192

APPENDIX E Calibration and Error Analysis 199

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

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

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, T d =-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

Summary List of Figures

Figure 4.16 Shower spray head and connection on the

base plate of the ice slurry system 82 Figure 4.17 Cold Bath and Chiller used for the ice slurry system 82 Figure 4.18 (a) Centrifugal pump used to pump coolant 83 Figure 4.18 (b) Flow meter to measure coolant flow rate 83 Figure 4.19 Piping system of the ice slurry system 84 Figure 4.20 Extraction and return pipes to utilize ice slurry

in the heat exchanger 84 Figure 4.21 Progressive cavity pump to extract ice slurry 85 Figure 4.22 (a), (b)Fan Tubular Heat Exchanger to utilize ice slurry for

air cooling Electric Heater 86 Figure 4.23 Electric heating system – heater, rheostat, fan 87 Figure 4.24 Flow Chart for experimental procedure showing varying

coolant flow rates in the ice slurry generator 89 Figure 4.25 (a) Pump located before Heat Exchanger 92 Figure 4.25 (b) Pump located after Heat Exchanger 92 Figure 4.26 Flow Chart for experimental procedure showing varying

Cooling Loads 94 Figure 4.27 Flow Chart for experimental procedure showing varying

ice slurry extraction rate 95 Figure 5.1 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 Figure 5.2 Variation of ice formation time for coolant temperatures 102 Figure 5.3 Heat transfer coefficient of the ice slurry generator for

different coolant flow rates ( 8 lit/min, 10 lit/min, 12 lit/min) 106 Figure 5.4(a) Ice formation phenomena (D= 40 mm, T d =-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 Figure 5.5 Velocity distribution of a droplet injected from a nozzle mounted

at bottom (V=0.50m/s, Dd= 4mm, Td= -10°C) 111 Figure 5.6 Effect of droplet diameter on the distance traveled by the

upward propagating liquid droplet for injection velocity 114 Figure 5.7 Velocity distribution of a droplet injected from a nozzle

mounted at top (V=0.15m/s, D d = 4mm, T d = -10ºC) 116 Figure 5.8 Effect of droplet diameter on the distance traveled by the

downward propagating liquid droplet for injection velocity 119 Figure 5.9 Effect of droplet diameter (D d = 4mm, 6mm, 8mm and 10mm)

on the growth of ice layer on droplet surface (T d = -10ºC and V=0.15m/s) 121

Figure No Title Page No Figure 5.9 Effect of droplet diameter (D d = 4mm, 6mm, 8mm and 10mm) on

growth of ice layer on droplet surface (T d = -10ºC and V=0.15m/s) 121 Figure 5.10 Effect of droplet diameter (D d = 4mm, 6mm, 8mm and 10mm)

on the growth of mushy layer on droplet surface

(T d = -10ºC and V=0.15m/s) 122 Figure 5.11 Effect of initial liquid droplet temperature (T d = -5ºC, -10ºC and -15ºC)

on the growth of ice layer on droplet surface

(D d = 10mm and V=0.15m/s) 123 Figure 5.12 Effect of initial liquid droplet temperature (T d = -5ºC, -10ºC and -15ºC)

on the growth of mushy layer on droplet surface

(D d = 10mm and V=0.15m/s) 124 Figure 5.13 Comparison of experimental and simulation results of the

generated ice slurry for ice slurry generators with nozzle

positioned at bottom 126 Figure 5.14 Comparison of experimental and simulation results of the

generated ice slurry for ice slurry generators with nozzle top 128

Figure 5.15 Effect of ice production for different coolant temperatures

(T d = -5ºC, -10ºC and -15ºC) in the ice slurry generator

(Water height =1m, Cylinder Dia.= 0.3m, N d =1mm and N z =50 ) for different coolant flow rates( 8 lpm, 10 lpm and 12 lpm 129 Figure 5.16 Effect of ice production for different nozzle diameters

(N d = 0.8mm, 1mm and 1.2mm) in the ice slurry generator

(Water height =1m, Cylinder Dia.= 0.3m, T d = -10ºC and Nz =50 ) for different coolant flow rates ( 8 lpm, 10 lpm and 12 lpm) 131 Figure 5.17 Effect of ice production for different number of nozzles

(N z = 20, 40 and 60) in the ice slurry generator

(Water height =1m, Cylinder Dia.= 0.3m, T d = -10ºC and N d =1mm ) for different coolant flow rates ( 8 lpm, 10 lpm and 12 lpm) 134 Figure 5.18 Temperature profile of the outside surface temperature

of the heat exchanger at different locations (∆L=30 cm) 137 Figure 5.19 Ice fraction for different flow rates of ice slurry 139 Figure 5.20 Local heat transfer coefficient of ice slurry for different

ice fraction, ice slurry extraction rate 5 lpm 143 Figure 5.21 Average heat transfer coefficient for different ice fraction

(2%, 3%, 4% and 5%) 144 Figure 5.22 Accuracy of heat transfer correlation 145 Figure 5.23 Comparison of avg Nu number for ice slurry flow through pipe 147 Figure 5.24 Thermal performance of ice slurry for different ice fraction

( 2% ~ 5%), Room Temperature, T a = 20ºC 149 Figure 5.25 Dependency of the heat transfer coefficients- air and

ice slurry on the overall heat transfer coefficient 151 Figure 5.26 Comparative analysis of the cooling performance between ice

slurry and chilled water for different flow rates

(5lpm, 8lpm and 10lpm) 154

Summary List of Tables

Table 3.1 Thermo-physical properties of FC-84 32

Table 4.1 Ice layer thickness, Experimentally measured (D= 50 mm, T d =-10◦C, Iron ball) 76

Table 4.2 Heat exchanger geometric configuration 86

Table 4.3 Operating parameters for discharging experiments 93

Table 4.4 Uncertainty of the equipments 98

Table 5.1 Coefficients for different parameters to estimate the duration

of sensible cooling by direct contact heat transfer 104

Table 5.2 Ice layer thickness, Experimentally measured (D= 40 mm, T d =-10◦C, Iron ball) 108

Table 5.3 Residence time of droplet for bottom mounted nozzle 112

Table 5.4 Residence time of droplet for top mounted nozzle 116

Table 5.4 Cooling capacity of ice slurry for different designed cooling loads 152

Table 5.6 Cooling capacity of chilled water (7°C) for different designed cooling loads 153

CHAPTER 1 INTRODUCTION

With the invention of the vapor compression refrigeration process in 1748 by Michael Faraday, dependency on the cooling system expands widely ranging from the food processing to the medical applications and from the air-conditioning to the cooling of beverages on a sunny day At the early stage, flammable and toxic ammonia was used in these cooling systems [1] It was then replaced by different refrigerants, chlorofluorocarbons (CFC) and hydro-chlorofluorocarbons (HCFC) The 1987 Montreal Protocol, 1997 Kyoto Protocol and the United Nations Convention for Climate Change attributed refrigerants as key players for both the depletion of ozone layer and the potential of global warming A very efficient solution regarding this environmental impact is to combine a secondary cooling system with a primary refrigerant system, which is confined in a protected area to reduce the refrigerant losses During the last few years, both industrial and commercial organization began to install compact chiller units along with the secondary coolant loop instead of large charges of primary refrigerant (CFC and HCFC) cooling system In this secondary cooling system, chilled water is used

as coolant, which is cooled by the primary refrigerants (Ammonia and Propane) To develop more energy efficient and cost effective thermal cooling system, research has been focused on the development of phase change materials (PCM) to substitute the conventional chilled water For refrigeration and air-conditioning, coolants must have the desired abilities such as higher thermal conductivity, higher heat transfer coefficient, pumping ability, higher heat storage capacity, stabilized temperature, etc For these

Chapter 1 Introduction

purposes, different types of phase change slurries are investigated such as carbon dioxide slurry, shape-stabilized slurry, microencapsulated slurry and ice slurry [2,3,4] Among these slurries, ice slurry is a promising technology, as it involves a simple process of conversion water into ice, obtaining very high density of enthalpy and the wide range of cooling applications from food industries to district cooling

1.1 Background: Cooling With Ice Slurry

Although coordinated and focused research activities on ice slurries were undertaken a few decades ago [5], the technique has been used in various applications in different countries from the ancient Roman times [6] In China and other Far East countries, ice slurry is used for the cooling of cargo railway cars Similar techniques are used in Germany for cooling the catering vessels in the passenger trains [7] In Japan, ice slurry

is used in large air conditioning installations, such as the CAPCOM building, Herbis Osaka Building, Kyoto Station Building, etc, having total floor area of more than 15,000

m2 [8] It is pumped to the various air handling coils directly which would save fan energy and duct size Similar technology for air-conditioning system is used in commercial buildings (Techno-Mart 21, largest commercial building in Korea), institutional buildings (Stuart C Siegel Center, Virginia, USA, Middlesex University,

UK, etc), airport (Zurich-Kloten Airport) and many more [9, 10]

Another 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

ice slurry plants This technology is now also used in onshore fish processing plants Wang et al [13] reported that the performance of ice slurry is better than the traditionally used flake ice for the preservation of quality fish

Ice slurry is also used for other commercial applications such as meat processing, brewery, dairy processing, rapid cooling of vegetables, and retail food storage in different countries Paul [14], in Germany, describes one such meat processing plant 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 (treatment of cardiac arrest, sports injuries etc), fire fighting, artificial snow production etc

In spite of all these current applications of ice slurry systems, further research and development are needed particularly on the generation of ice slurry in an efficient, reliable and economic way for broad range of applications

1.2 Ice Slurry Technology

With the invention of mechanical refrigeration, it is possible to produce ice in different forms such as, blocks, cubes, flake, etc The simplicity of freezing water, high latent heat

of ice and a higher degree of stratification between water and ice due to the different densities make the application of ice slurry a promising technology for the future

Technically, ice slurry is a mixture of small (typically 0.1 to 1 mm in diameter) ice particles with a carrier fluid, which is generally water This two-phase liquid stores

on the ice formation mechanisms, these slurry generators are classified into two categories: homogeneous nucleation and heterogeneous nucleation

The first patent on the ice slurry generator based on the heterogeneous nucleation was filed at 1976 [17] In this method, a cylinder or a plate attached to the evaporator of a refrigerating unit is used to cool a mixture of water and brine It would then form a layer

of ice on the cooled surface from where ice is removed by means of mechanical scraper Later, based on this concept new techniques like fluidized bed technology and improved design utilizing the drag force of the fluid for hydro-scraping of ice have been developed

In homogeneous nucleation, cold refrigerant, either expandable gas or single phase immiscible liquid, is dispersed into the secondary fluid, generally water Due to the direct contact heat transfer between two fluids, ice crystals are spontaneously dispersed in the water Since, no additional heat exchanger is used in this process, the method appears to

be more efficient In 1999, Coldeco of France [18] owned a patent for commercial manufacturing of ice slurry generator based on direct contact evaporation However, no unit is yet commercialized and researches have still been continuing for the development

of direct contact ice slurry generator

1.3 Advantages of Ice Slurry

In this system, heat exchange between two immiscible fluids (Primary fluid, FC-84 and

secondary fluid, water) occurs The primary immiscible fluid, which is heavier than water, is cooled below freezing temperature of water by a conventional chiller The fluid

is then passed through a nozzle which delivers the cooled refrigerant into the water in the form of droplets Ice forms when the water comes in contact with the cooled fluid droplet As ice is lighter than water, ice moves to the top of the tank and the heavier immiscible fluid moves to bottom of the tank for recirculation The advantages of the system are as follows,

Energy Efficiency:

Unlike static ice systems, where ice forms at the heat transfer surface, ice slurry produced

in an ice slurry generator initially adheres to the droplet surface and, subsequently detached due to buoyancy forces, resulting in higher convective and conductive heat transfer and, hence, higher energy efficiency Moreover, defrosting is not required to harvest the ice for storage into tanks, resulting in higher energy efficiency

Simplified Tank Design

Ice slurry can be pumped into storage tanks, reducing the need for extra structural support required for ice harvesters located above the storage tanks

Storage Flexibility

Ice slurry can be stored in tanks of any shape As an example, the height of an ice storage tank can be increased, resulting in a reduction of the tank footprint which leads to valuable floor space savings This is difficult to achieve in static and other dynamic ice storage systems

Chapter 1 Introduction

Application Flexibility

Ice packing factor of the ice slurry can be varied and it would support flexible demand load for air conditioning applications

Space Saving Design

Since there are no moving parts involved in this type of system, it would be compact that saves space in the refrigeration equipment room

Ease of Variation

Depending on the conditions, various expansions and modifications are possible in this system Such as, facility of installing different types of nozzles, installation of storage at a suitable places etc

1.4 Objectives

The objectives of this study are the following:

i Developing a simulation model for the ice slurry generator to evaluate the ice production phenomenon The model would serve as a design tool for the performance analysis of the system and assist to analyze similar type of system for different sizes and conditions

ii Developing a mathematical model to simulate the growth of ice layer around the supercooled liquid droplet by direct contact heat transfer This model will be extended for two different cases,

a Nozzle located at the top of the tank and supercooled liquid droplet moving downwards in an ice generator;

b Nozzle located at the bottom and supercooled liquid droplet moving upwards and downwards in an ice generator

iii Conducting experiments to measure the growth of ice layer around the supercooled liquid droplet in contact with water and compare the experimental values with the simulated 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 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 has been designed and fabricated for cooling applications which would lead to an implementation of this type of new system for future usage The system simulation model should help other researchers and engineers on further exploration of such system for different conditions Therefore, the findings of this project would inspire of the researchers to develop an efficient and cost effective ice slurry system for commercial applications

1.5 Scope

An introduction to the ice slurry system is included in Chapter 1, which provides a general pre-view of the ice slurry technology for cooling applications Chapter 2 contains the detailed literature review of ice formation phenomena, different methodology of ice

Chapter 1 Introduction

slurry production, heat transfer phenomena during melting of ice slurry in heat exchanger for cooling applications Analytical models are discussed in Chapter 3 These include the physical phenomenon of ice formation around the super cooled liquid droplet, system simulation of ice slurry generator and ice slurry extractor for air cooling Experimental procedures together with the description of different equipments are presented in chapter

4 Design and fabrication processes of the ice slurry system – ice slurry generator and ice slurry extractor, are also discussed Chapter 5 presents detailed analyses of results and discussion obtained from the experiments and analyses Chapter 6 includes the conclusion drawn from the analytical and experimental analyses of the ice slurry system

CHAPTER 2 LITERATURE REVIEW

In recent years, energy markets are experiencing high demand and limited supplies, resulting in volatile and soaring prices In response to higher energy costs, recent research has been focused on the technologies of lower cost and higher energy-efficiency

With this perspective, the ice slurry can be considered as a better alternative to the conventional chilled water system, as it has the advantage of higher cooling capacity, about five times [19], due to the latent heat of fusion of melting ice Recently, attention has been given to advance this ice slurry technology for successful implementation for different applications- air conditioning, industrial and commercial refrigeration

To implement this technology, there are several issues that need to be resolved These are the ice generation phenomenon, effective method of ice slurry production and the utilization of the produced 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 during cooling applications

2.1 Fundamentals of Ice Formation

Phase change of water to ice occurs under isothermal condition by releasing the latent heat of fusion This ice formation phenomena starts from a small nucleus, upon which water molecules of the surrounding liquid phase are integrated for ice crystal growth

Chapter 2 Literature Review

In nucleation, nuclei formation is initiated by the foreign substrate [20] associated with the energy The proposed theory for ice nucleation is based on the simplified thermodynamics consideration However, it fails to include an important factor, the interaction and the structure of the first monolayer of the liquid in the intimate contact with the foreign surface, which influences both the surface free energy and the volume free energy directly Further research had been continued to overcome the limitations of this nucleation model One of the proposed cases is to consider a secondary nucleation [21], where the nucleation is induced on the seed crystal and it occurs at the surface of a previously existing crystal Chen et al [22] experimentally investigated the nucleation probability of supercooled water inside cylindrical capsules They concluded that lower coolant temperature, larger size of capsule and additional mass of different nucleates agents would enhance the ice nucleation probability

Based on the heterogeneous nucleation process, a refrigerator machine was developed It contains a special evaporator with a double cylinder or plate in which a part of the water/brine mixture is cooled at the wall The ice crystals produced were then scraped away mechanically from the surface To understand the phenomena of ice crystallization

on the heat exchanger surface, researchers have made significant efforts to model the process coupled with heat and mass transfer

Myers et al [23] proposed a one-dimensional model for ice growth when the supercooled fluid is impinged on a cooled surface In this model, the ice thickness can be determined

by combining the mass balance with the phase change Stefan problem, though a number

of assumptions were made during this analysis This model helps to understand the physical process of ice formation Naterer [24] presented a model to portray the

formation processes of ice, transition and the combined ice condition; when the phase change heat transfer occurred due to the incoming supercooled liquid droplets on heated curved surfaces This heat transfer model can correctly approach the simultaneously growth of unfrozen water and ice layers under appropriate thermal conditions

Stewart et al [25] studied the ice crystal growth in a falling film flowing down the surface of cooled plate To approximate the ice growth rate, a numerical model was suggested for supercooled liquid film, considering ice particles as equivalent heat sources They also discussed the effect of the parameters that control the enhancement of heat transfer However, this model only considered the laminar flow condition Ismail et

al [26] extended the previous model of ice growth for laminar falling film flowing down

a cooled vertical plate by including the effect of axial diffusion on the vertical plate Their work showed a rigorous analysis of the ice formation on the vertical plate because

of the in-depth analyses on the effects of the parameters that control the enhancement of the heat transfer for ice generation Hirata et al [27, 28] investigated the rate of ice formation on a cooled horizontal solid surface and its removal phenomena In their study, ethylene glycol solution was used to produce freezing while the cooled plates of different materials such as polyvinyl chloride, acrylic resin, silicon resin and copper plates were used They reported that the ice formation and detachment from the plate were related to the heat flux In their study, they assumed an uniform heat flux at the interface between ice layer and plate surface; but this assumption may not be true after the ice layer formation on the plate surface because of the lower conduction coefficient of ice While there is much on the ice formation over the plate surface under various conditions, ice formation phenomenon on immiscible super cooled liquid surface has not yet been

Chapter 2 Literature Review

investigated In the present study ice formation, as well as the detachment of the ice layer from the moving immiscible liquid droplet surface, is analyzed to provide better understanding of the physical phenomena

2.2 Ice Slurry Production

In an ice slurry system, production of ice slurry is the key concern Much research has been conducted on different methods of ice slurry generation during the last few decades

An overview of these known methods is given below in this section

2.2.1 Static ice production

Egolf and Kauffeld [29] described the scraped-surface type ice slurry generator, which is now commercially available It is a shell and tube heat exchanger which has a rotor or blade assembly housed inside the tube, where the ice slurry is generated Coolant stream evaporates on the outer shell side of the heat exchanger This type of generator is capable

of producing a mixture of small ice crystals and water from a binary super-cooled solution As it is an experimental prototype, the capital cost of this type of scraped-surface ice slurry generator is relatively high [30] In this system, the rate of ice crystallization depends on log mean temperature differences between the refrigerant and ice slurry streams inside the tube, as recommended by Russell et al [31] However, continuous accumulation of ice layers on the ice slurry generator would reduce the heat transfer rates and immediately affect the ice slurry temperature In addition, due to the mechanical abrasions, rotating scrappers, brushes and orbital rods would wear out over time and need to be replaced at a given time interval Zakeri [32,33] proposed an ice slurry generator employing a vacuum freeze process In this system, an evaporator cooled

by a conventional refrigerant freezes the water necessary to remove the heat of fusion

during the ice generation process in the liquid A small displacement volume vacuum pump is placed to maintain the triple point pressure in the ice slurry generator In this type of system, water as refrigerant is limited to temperature between the freezing point temperature of water, 0ºC and -4ºC due to the increase in water volume to be compressed

at lower temperatures Because of this verity, vacuum ice slurry generator fails to be considered for regular applications Also, the design of this type of system would need careful attention to avoid pressure drops as these harm the low-pressure system much more than is the conventional refrigeration system

Tanino and Kozawa [34-36], Kozawa et al [37,38] and Mito et al [39] proposed ice generator systems using the supercooled water method This type of system consists of a chiller, an ice slurry storage tank, a supercooler, a releaser (for releasing the supercooled water), a heat exchanger and a pump The water from the ice slurry storage tank is sent

to the supercooler, where the temperature is reduced and cooled to -2ºC through brine solution This supercooled water is then passed through a diffuser, where it changes phase to become ice slurry with very fine ice particles This ice slurry is then pumped to the storage tank where it is separated by the density difference between ice and water Kirby and Nelson [40], Lasvignotes et al [41], Nagato [42] also proposed different supercooled ice slurry generators The differences among these systems lie in the use of the type of heat exchangers for producing supercooled water, and the methods of supercooling and release of their supercooled energy These types of systems are commercially available in Japan for large-scale on-site-type ice storage systems for air conditioning applications Meanwhile, due to the constraint of higher capacity, the commercial viability of such supercooled ice slurry systems cannot be accounted for

Chapter 2 Literature Review

medium and small scale air conditioning systems Meewise and Ferriera [43, 44] recommended a new method of producing ice slurry utilizing the fluidized bed system Primary refrigerant is evaporated on the shell side while ice is formed on or near the inside surface of the tubes mounted in the shell of the heat exchanger The particles, such

as steel or glass, are fluidized by the upward flowing ice slurry This prevents the

build-up of ice layer on the heat exchanging surfaces and enhances the heat transfer rate Multiple fluidized beds can be arranged in a shell and tube heat exchanger with longer tubes, which would be more efficient for larger scale applications However, in this type

of system, the achievable ice fraction in the slurry is limited due to the minimum fluid velocity, which would impact the particles on the heat exchanging wall to prevent build-

up of ice layer Also, for limited space application, this type of system is not applicable due to the inherent vertical layout of the fluidized beds Davies [45, 46] developed an ice slurry system that built-up ice on the chilled surfaces and removed periodically by means

of recuperative heat exchange other than scraping In this system, a pair of heat exchangers is integrated in evaporator section of the direct-expansion refrigerators to super cool fluid with icing and de-icing on the heat exchanging surfaces During de-icing, the direction of refrigerant flow between two identical heat exchangers can be reversed at optimal frequency The evaporator head, therefore, has twice the heat transfer surface than a scraped surface generator of similar capacity and it has the advantage of operating without any moving parts This would make the system cheaper to construct, operate and maintain than existing systems However, during de-icing, a portion of generated ice is melted due to the heat transfer, possibly reducing the quality of ice generation from the system

A prototype of the hydro-scraped ice slurry generator [47] was developed by the Technical Centre of Dinan (Pole Cristal) in France In this system, ice slurry is produced with the help of carrier fluids mixing with high concentrations of additives on the heat exchanger (shell and tube or plate heat exchanger) surface in the evaporating section of a standard refrigeration loop with the control of evaporation temperature The ice crystals formed on the evaporator surface are then removed by the flowing refrigerant itself This type of system is still in the research stage, and no additional information has yet been published Another type of ice slurry generator with the special INCs (ice nucleation coatings) was developed by the Danish Technology Institute together with a Danish refrigeration company [48] It is a shell-and-tube heat exchanger with the evaporating refrigerant on the shell side and ice slurry formed inside the coated tubes The coating is composed of nanoparticles synthesized from organic modified silicone alkoxides These nanoparticles were thermally cross-linked to the pre-treated surface of the aluminum half pipes and formed a smooth and very thin nanostructure hybrid layer by inorganic-organic network Due to the selection of the reactive functional group of the organosilane, the nano structured layer contains both the hydrophilic segments as well as the hydrophobic anti-adhesive segments Thus INCs are capable of both ice nucleation and ice repletion simultaneously However, lacking knowledge on the mechanical properties and chemical stability of the coatings limits this type of ice slurry generator for commercial applications

All these types of ice slurry generators are static types, which produced ice bonds on the cooling surface and form a layer on it These systems reduce the coefficient of performance (COP) of the air conditioning system as the ice layer on a cooling surface

Chapter 2 Literature Review

increases thermal 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 generators, are not established technologies as are static systems, and unsolved technical issues need to be investigated The following sections discuss the recent works on these types of systems

2.2.2 Dynamic ice production

The ice slurry system that utilizes direct contact heat transfer between immiscible refrigerant (one primary and other secondary) has attracted attention of researchers during recent years The most important characteristic of this process is that a large quantity of ice can be produced continuously and quickly from a relatively small ice generator [50] In this process, two possible methods may be used: one where refrigerant

is evaporated to supersaturate the water leaving ice particles; or where, super cooled immiscible refrigerant is used to produce ice slurry For the first method, higher pressure

is required in the evaporator, but for the second method atmospheric conditions are sufficient The ice slurry system based on these methods is discussed in the following section

2.2.2.1 Phase Change liquids

At present, several research groups have been working on ice slurry generators based on direct contact heat transfer Kondel et al [51] were among the earliest researchers who proposed an industrial type ice slurry generator for Chicago Bridge & Iron Company,

USA In their proposed system, a primary refrigerant evaporates directly in water for the production of ice slurry for secondary cooling systems Wobst and Vollmer [52], who worked for ILK, Dresden Germany, also designed a similar type of ice generation system

In their experimental installation, immiscible primary refrigerant (R600a) was expanded and then injected into a water tank to evaporate During evaporation of the primary refrigerant, water would form dispersed ice particles From the investigation, it was found that the heat transfer rate per unit volume in the direct contact tank is approximately 1000W/m3K, which is quite large This implies that though a large size ice generator tank would be required for this process, the need for additional tanks for storage would be eliminated; and this would reduce the cost of the ice slurry installation

Kiatsiriroat et al [53] studied an ice thermal storage using an injection of R12 refrigerant into the water to exchange heat directly They found that the coefficient of performance was about 3.4-3.6, which is higher than that of the conventional system Kiatsiriorat et al [54] also studied the heat transfer characteristics of a direct contact evaporator using R12 and R22 Refrigerants (R12 or R22) injected at temperature lower than 0°C take heat directly from the water in a storage tank and then gets evaporated A correlation that relates the dimensionless parameters, such as Stanton number, Stephan number, Prandtl number and pressure ratio, was also developed They found higher heat transfer coefficients with the direct contact heat transfer technique Asaoka et al [55, 56] proposed a new method of ice slurry production by using ethanol solution In this process, a vacuum pump was used to evaporate the ethanol solution and to produce ice slurry subsequently The performance of the system depends on the concentration of ethanol solution and the production rate of ice slurry

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Several research groups have suggested utilizing the thermo-physical properties of the phase 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 Morita [58] They also proposed

a new method for producing ice particles, composed of water and tetradecane, by injecting the tetradecane oil into a cold ethylene glycol water solution [59-61] Bo et al [62] used the same procedure while using paraffin wax (Rubitherm RT5) as the PCM for ice generation

2.2.2.3 Immiscible liquids

In this type of ice slurry system, the immiscible liquid, secondary refrigerant, is cooled by the evaporator of the refrigeration cycle and then the super cooled liquid is dispersed in the slurry generator tank for ice formation Very few research groups have investigated this process for ice slurry generation

Fukusako et al [63] proposed an alternative approach for generating ice slurry by direct contact heat exchanger The generator with heavy, non-miscible liquid used to exchange heat from the primary refrigeration cycle and then sprays the cool liquid into the ice slurry feed water using the injection nozzles The supercooled liquid forms ice which moves upward due to the bouyance force and the liquid sinks to the bottom of the tank from where it returns to the pump and the evaporator Matsumoto et al [64] and Ure [65] described similar type of system However, they have not mentioned the details of the

sample fluids except that they are organic compounds, which would meet the requirements of immiscible, higher density and lower freezing point than those for water Matsumoto et al [66] studied a new method of continuous ice formation, where a water-oil emulsion was cooled with stirring in a vessel and changed into ice, oil and water suspension They found that ice-oil and water suspension (sluish ice), which has good fluidity, can be formed without adhering to the cooling surface They extended their studies [67,68] to improve the continuous ice generation process Higher supercooling dissolution rate of water-oil mixture slowed down the ice formation process To address this problem, they proposed a new method of applying voltage and ultrasonic wave to the water-oil emulsion during ice generation process This would result in a higher rate of dissolution of the mixture and reduce the degree of supercooling Thongwik et al [69] proposed carbon dioxide and a mixture of compressor oil and a surfactant as secondary coolants to produce ice by direct contact heat transfer with water From their investigations, they suggested that liquid carbon dioxide was a better choice than other fluids

Noma et al [70] developed an ice storage system that can produce ice by direct contact heat transfer between water and coolant, a fluorocarbon They investigated this direct contact heat transfer method for continuous ice generation in the system They discussed the similarities between the ice formation process in the current system and that of frazil ice formation in nature Though, this study presents a new concept of ice slurry generation by immiscible liquids, no further studies were reported to reveal the detailed analysis of this type of system

Chapter 2 Literature Review

Wijeysundera et al [71] proposed Fluroinert (FC-84) as a primary working fluid to produce ice slurry by direct contact heat transfer They developed a model to evaluate the heat transfer coefficient between water and coolant droplet during water cooling and ice generation Some investigations have been carried out to understand the 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 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 a particular condition, the 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 particles and water which can be pumped But, due to the possible influence of the concentration of the ice slurry [72] and the agglomeration of ice crystals [73, 74], it would behave as non-Newtonian fluid

However, investigating the rheology of ice slurry, Ayel et al [75] proposed that ice slurries with 0 to 0.15 of ice fraction have the Newtonian fluid characteristics

Christensen and Kauffeld [76] investigated the heat transfer characteristics of ice slurry flowing through a cylindrical pipe In their experiment, a constant heat flux (varied in the range of 6 kW/m2 to 14 kW/m2) was provided to the inner pipe by condensing refrigerant R134a between two concentric pipes The tube was 26.9 mm OD, 21.6 mm ID and 1 meter length During the experiment, ice slurry (10% mass ) was produced with an aqueous ethanol solution The heat transfer coefficient was determined by considering the temperature difference between the refrigerant R134a and ice slurry They found that the average heat transfer coefficient of the ice slurry increases with the increase of ice mass fraction and Reynolds number; the measured heat transfer coefficient increased from

3000 kW/m2 for an ice fraction of 5% to 7000 kW/m2 for a mass fraction of 25% Jensen

et al [77] proposed a similar experiment with ammonia, NH3 instead of R134a in the outer pipe The proposed correlation for ice slurry heat transfer, which was valid for constant heat flux (1.6 kW/m2 to13 kW/m2) in the pipe (12 < Dh< 20 mm) when ice fraction is in the range of 5% to 30%

Kondel et al [78] experimentally measured heat transfer rate in an ice slurry flow with a constant heat flux imposed by two electrical heaters (30 kW heating power) on a stainless tube of 24 mm inner diameter and 459.6 cm length Flow velocity of ice slurry through the pipe was varied between 2.8 m/s and 5 m/s During each experimental operation, at different downstream positions, the local heat transfer coefficients of ice slurry were determined by evaluating the temperature difference between the pipe wall and the bulk temperature of ice slurry which was kept constant at bulk temperature of 0°C For each

Chapter 2 Literature Review

cross section, the inner tube wall temperature was calculated using the energy conservation equation and the measured outer temperature It was found that with the increase of the ice fraction (% by mass) between 4% and 12%, average Nusselt number

of ice slurry decreased for Reynolds number in the range of 5000~6000

Egolf et al [79] performed heat transfer experiments on 2m long pipe with 15.74 mm internal diameter During the experiment, the constant heat flux was varied from 28 kW/m2 to 114 kW/m2 and ice slurry flow velocity between 5 m/s and 12 m/s; glycol was used to produce ice slurry for different ice fractions 4%~33% by mass Comparing the experimental results with the heat transfer coefficient calculated by Dittus-Boelter equation, Snoek and Bellamy [80] proposed an empirical correlation for ice slurry It postulated that the ice slurry heat transfer coefficient increases with increasing ice mass fraction

Guilpart et al.[81, 82] worked with a copper tube test section 30 mm inner diameter and 0.5 meter length An aqueous ethanol solution was used to produce ice slurry in the experimental device and constant heat flux (0-12 kW/m2) was applied by heater across the tube The heat transfer coefficient was determined by evaluating the temperature difference between the wall and the bulk fluid temperature measured at different equidistant positions of the pipe A Graetz-Nusselt type equation was proposed from their experimental investigations to calculate the heat transfer for laminar slurry flow Sari et

al [83] performed experiments to determine the heat transfer rates in ice slurry flowing through an aluminum tube in a heat exchanger The cylindrical heater was equipped with four electrical heating elements to provide constant heat flux of 14.1 kW/m2 to the outer wall of the pipe The heated section had a length of 1 meter and 23 mm inner diameter

During the experiment, the ice slurry was produced using an aqueous Talin solution with

an initial Talin concentration of 10% (by mass) Experimental data on wall surface temperatures, fluid temperatures, mass flows, densities and inlet and outlet pressures were recorded A perturbation analysis was then performed to numerically determine the heat transfer rate in an ice slurry flow However, the basic requirement for the validity of this perturbation model could only be approximately fulfilled, as the experimental investigation was limited to small heat fluxes

Bedecarrats et al.[84] studied the heat transfer behavior of ice slurry with an aqueous ethanol solution of initially 10% (by mass) ethanol The outer side of the heat exchanger pipe was heated with hot water, where the hot water temperature was controlled at the location of the pipe entrance The experimental results were based on a modified Wilson method [85], which was used to determine the values of the heat transfer coefficient from the determination of the overall thermal resistance of the heat exchanger Overall resistance was determined by measuring the heat flow and the logarithmic temperature difference between the fluid at the inlet and the outlet The author postulated that an increase in ice fraction would increase the heat transfer coefficient, while mass flow rate would be unaltered; and an increase in the mass flow rate also lead to an increase of the heat transfer coefficient, when ice concentration is maintained constant

Each of these research groups suggested an empirical or semi-empirical correlation for heat transfer characteristics of ice slurry through pipe flow Basic differences of these relations are the concentration of ice slurry solution, heat transfer phenomena of ice slurry and the applied cooling load However, all these correlations suggest that ice slurry

Chapter 2 Literature Review

has high heat transfer coefficient (in the range of 103 ~ 104 W/m2 K) and depends on the fraction of ice in the slurry

2.3.2 Heat transfer through rectangular channels

The thermal hydraulic behavior of ice slurries in laminar and turbulent flows through non-circular channels is of special interest due to the wide application of such geometries

in compact heat exchanger The following sections present the recent researches on the fundamental aspects of the fluid mechanics and heat transfer of ice slurries in rectangular channels

Kawanami et al.[86-88] performed experiment with a horizontal duct of 1 meter length and 60 mm width; and different wall heights: 20, 40 and 60 mm The top and bottom walls were electrically heated to guarantee a constant heat fluxes between 2 kW/m2 and

8 kW/m2 An initial 20% ethylene-glycol aqueous solution and 20% ice concentration was chosen for the experimental investigations From the analyses, the author observed that there was a significant difference in the heat transfer coefficients at the top and bottom walls of the channels; the values were higher at the top wall due to the close contact of the ice particles and melting in that region For different flow velocities, 0.2 m/s and 0.7 m/s, of ice slurry in the channel at a given flux and similar height, larger heat transfer coefficients were determined at the top of the channel However, increasing the velocity reduced the difference between the mean heat transfer coefficients at the top and the bottom walls The variation in channel height showed that the mean heat transfer coefficient was higher at the top channel wall, but both heat transfer coefficients decreased with the increasing channel height The fact is that a decreasing channel height would increase the local velocity of the ice slurry near the upper wall of the channel,

which would lead to stronger mixing and, therefore, increase the heat transfer rate in the region

Kitanovski et al.[89] studied heat transfer to an ice slurry of a 10% propylene-glycol solution This slurry flowed through a rectangular, horizontal aluminum heat exchanger

of 1 meter length and 23 mm hydraulic diameter Mass flow meters were installed at the inlet and outlet of the heat exchanger and thermocouples were placed along the discrete cross-section of the heat exchanger During the experiment, heat fluxes in the range between 1.8 kW/m2 and 7.2 kW/m2 were applied across the heat exchanger It was noticed that at the outlet, fluid temperature was above 0°C, while the corresponding density measurements clearly showed a significant ice fraction From these observations, the author hypothesized the overheating of the carrier fluid during heat transfer The author also postulated that in isothermal flows of ice slurry, thermal diffusivity due to the turbulence characteristics decreased as a function of the location This observation was made by numerical investigation by altering the diffusivity and applying curve fitting to the measured and calculated temperature profiles Stamatiou et al [90,91] and Stamatiou and Kawaji [92] also studied thermal and flow behavior of ice slurry in rectangular channel with a constant heat flux The experiment was conducted for the vertically, upward adiabatic ice slurry flow through a rectangular channel (0.305-m X 0.61-m X 0.025-m ) with hot film anemometer for heating Experiments were performed for ice fractions in the range between 6 and 16% by volume and mean velocities up to 0.15 m/s From the experimental analyses, authors observed that the velocity distribution across the channel displayed flat profiles for average ice fractions as low as 2% ( by volume) and

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the ice fraction distribution was slight peaking near the adiabatic walls for average ice fraction less than 8%; while, at higher ice fractions, the distributions became flatter

To visualize the melting phenomena of ice slurry, Sari and Guilpart [93, 94] performed experiments with a camera using the shadowgraph method The experiment was conducted in a transparent tube of 100 mm inner diameter and 69 mm length Three different heat fluxes- 8, 16 and 24 kW/m2, generated by electric heaters, were applied across the tube The ice slurry was produced with an ethylene-glycol solution, where ice particles were about 0.5 mm in diameter Because of the lower density of ice, ice particles rise to the top of the cylindrical tube Due to the thermal contact of these ice particles, ice slurry began to melt In the initial stage of the melting process, the heat transport is dominated by thermal conduction After a mixing process between the water- produced

by melting of some ice crystals- and the ethylene glycol aqueous solution, a stable ice particle distribution is formed in the melting region This would eventually cause free convection, which would lead to the formation of a number of stratified layers due to the diffusion between the ice particle containing region and the heated wall Analyzing the ice slurry melting pictures by shadowgraph method, the author postulated that the number

of stratified layers increased with an increasing initial concentration of the aqueous binary solution as well as with decreasing heat flux at the wall of the tube

However, these facts were pertinent for no flow conditions of ice slurry through the pipe,

as the melting experiment was conducted for stagnant flow condition

2.3.3 Heat transfer: industrial heat exchanger

Ice slurry used in indirect cooling applications to exchange heat with other fluids, heat exchangers are a very common choice in current technology Heat transfer characteristics

of ice slurry in laminar and turbulent flows through small channels of the compact heat exchangers are of special interest due to the wider condition of applications Nonetheless, very few investigations, experimental and numerical, have been carried out to focus on the utilization of ice slurry for compact heat exchangers The following sections present the research performed on the heat transfer aspects of ice slurry in the compact heat exchangers

Stamatiou and Kawaji [95] performed a dimensional analysis to derive a heat transfer correlation for turbulent ice slurry flow in the compact heat exchangers A multiple regression analysis was performed to evaluate the effect of ice-crystals on convective heat transfer in dimensionless variables- average Nusselt number, Reynolds number, Prandtl number, average ice fractions and ratio of viscosities at bulk carrier fluid and the wall temperature The correlation was developed for narrow channel with shorter entry length, and it attributed higher heat transfer coefficients for ice slurry The author also proposed heat transfer correlations from the best-fit of experimental data when the ice slurry flowed through a large channel, in terms of local Nusselt number, Graetz number, average ice fractions and ratio of viscosities at bulk carrier fluid and the wall temperature

A similar heat transfer correlation was proposed by Ben-Lakhder et al [96] for ice flow through tube without the viscosity ratio But, there were discrepancy between these two proposed correlations due to the different definition of Graetz number Ionescu et al [97] also worked on the thermal behavior of ice slurry when the flow condition was laminar and transitional Based on the experimental investigations, they proposed correlations giving the local and global Nusselt numbers, which depends on the Graetz or Reynolds number and on the particle mass fraction Meanwhile, Lee et al [98] performed a