HEAT TRANSFER ENHANCEMENT USING NANOFLUIDS IN THE AUTOMOTIVE COOLING SYSTEM ADNAN MOHAMMED HUSSEIN

Transcription

1 HEAT TRANSFER ENHANCEMENT USING NANOFLUIDS IN THE AUTOMOTIVE COOLING SYSTEM ADNAN MOHAMMED HUSSEIN Thesis submitted in fulfillment of the requirements for the award of the degree of Doctor of Philosophy of Mechanical Engineering Faculty of Mechanical Engineering UNIVERSITI MALAYSIA PAHANG DECEMBER 2014

2 vi ABSTRACT The automotive cooling system is a significant part of the car that removes the engine generated heat outside across the radiator. The increasing demand of nanofluids for industrial applications has led many researchers to focus on the subject in the last decade. The limited thermophysical properties and heat transfer fo liquids across the car radiator have resulted in much research to find better coolant fluids. Space constraints are another key issue in the evofofotua applications to remove heat from high heat flux generating surfaces of automobile engines. In order to improve thermophysical properties of the coolant fluid to enhance heat transfer in the automotive cooling system, nanofluids have been utilized as a coolant. This study aims to enhance heat transfer with a slight pressure drop in the automotive cooling system by using multi types of nanoparticles dispersed in various types of basefluids. The appropriate type of nanofluids and the influence of different nanofluids on the heat transfer performance for the car cooling system have been identified. The radiator performance efficiency to reduce the radiator size and weight has been studied. The friction factor and heat transfer enhancement using different types of nanofluids are studied. The TiO 2 and SiO 2 nanopowders suspended in four different base fluids (pure water, EG, 10%EG+90%W and 20%EG+80%W) are prepared experimentally. The thermophysical properties of both nanofluids and base fluids have been measured and validated with the standard and the experimental data available. The experimental test rig setup included a car radiator, collecting tank, pump, rotameter, valves and plastic tubes. The evaluation of the friction factor and heat transfer coefficient by taking readings of the temperature and pressure drop under laminar flow condition were conducted. The volume flowrate was found to be in the range of (1-5LPM) for pure water and (3-12LPM) for other base fluids; while, the inlet temperature and nanofluid volume fraction were in the range of (60-80 o C) and (1-4%) respectively. The CFD analysis for the nanofluids flow inside the flat tube of a car radiator under laminar flow was carried out. A simulation study was conducted by using the finite volume oaotfm to solve the continuity, momentum, and energy equations. The geometry meshing of problem with a description of the boundary conditions was performed by using commercial software to determine the friction factor and heat transfer coefficient. The experimental results showed the friction factor decreased with the increase of the volume flowrate and increased with the increase of nanofluid volume fraction but slightly decreased with the increase of the inlet temperature. The simulation results showed good agreement with the experimental data with deviation not exceeding 4%. The experimental results showed the heat transfer coefficient increased with the increase of the volume flowrate, the nanofluid volume fraction and the inlet temperature. The simulation results showed good agreement with the experimental data with deviation not exceeding 6%. In addition, the SiO 2 nanofluid showed higher values of the friction factor and heat transfer coefficient than TiO 2 nanofluid. The base fluid (20%EG+80%W) gave higher values of the heat transfer coefficient and proper values of friction factor compared to other base fluids. The 4% of SiO 2 nanoparticles suspended in (20%EG+80%W) base fluid was significant augmentation of heat transfer in the automobile radiator. The regression equations among input (Reynolds number, Prandtl number, and nanofluid volume fraction) and response (friction factor and Nusselt number) were found to be correlated. The experimental results were compared with the experimental data available and there were good agreements with a maximum deviation of approximately 5%.

3 vii ABSTRAK Sistem penyejukan automotif adalah sebahagian penting dari kereta kerana mengeluarkan penjanaan haba enjin luar di seluruh radiator. Pada dekad yang lalu, permintaan cecair nano semakin meningkat bagi aplikasi perindustrian telah menjadi tumpuan pada ramai penyelidik. Cecair terhad sifat haba dan pemindahan haba di seluruh radiator kereta telah membawa kepada mencari cecair penyejuk yang lebih baik. Kekangan ruang adalah satu lagi isu utama dalam aplikasi industri untuk mengeluarkan haba dari haba yang tinggi permukaan menjana fluks enjin kereta. Dalam usaha untuk memperbaiki sifat haba cecair penyejuk untuk meningkatkan pemindahan haba dalam sistem penyejukan automotif, nanofluids telah digunakan sebagai penyejuk. Jenis sesuai nanofluid dan pengaruh nanofluids berbeza memberi kesan kepada prestasi pemindahan haba untuk sistem penyejukan kereta telah dikenal pasti. Kecekapan prestasi radiator untuk mengurangkan saiz radiator dan berat badan telah dikaji. Tesis ini termasuk kedua-dua kajian eksperimen dan simulasi untuk meningkatkan pemindahan haba, serta dibuat pentanda aras bagi kajian ini menggunakan sistem penyejukan automotif. Pelbagai jenis nanofluids telah dikaji, termasuklah faktor geseran dan pekali pemindahan haba. Nanopowders TiO 2 dan SiO 2 di gabungkan dengan empat jenis cecair asas yang berbeza (iaitu air tulen, EG, 10%EG+90%W dan 20%EG+80 %W) disediakan untuk uji kaji ini. Ciri-ciri kedua-dua termofizikal nanofluids dan cecair asas diukur serta disahkan dengan standard data uji kaji yang ada. Ujian ini termasuk persediaan radiator kereta dan kesan di bawah keadaan operasi pada peningkatan pemindahan haba dianalisis di bawah keadaan aliran lamina. Kadar alir isipadu, adalah dalam lingkungan (1-5LPM) untuk air tulen dan (3-12LPM) untuk cecair asas lain; manakala, suhu masuk dan kepekatan jumlah nanofluid adalah dalam lingkungan (60-80 oc) dan (1-4%) masing-masing. Di samping itu, analisis CFD untuk cecair nano mengalir dalam tiub rata radiator kereta di bawah aliran lamina juga dijalankan. Kajian simulasi dijalankan dengan menggunakan keadah number berangka tidak terhingga bagi menyelesaikan keterusan, momentum dan tenaga persamaan. Proses geometri bersirat dan keadaan sempadan adalah dilakukan dengan menggunakan GAMBIT kemudian menggunakan perisian FLUENT untuk mencari faktor geseran dan pekali pemindahan haba. Keputusan eksperimen menunjukkan faktor geseran berkurang dengan peningkatan kadar alir jumlah dan meningkat dengan pecahan isipadu nanofluid tetapi sedikit berkurangan dengan peningkatan suhu masukan. Tambahan pula, keputusan simulasi menunjukkan keadaan yang baik dengan data uji kaji dengan sisihan tidak melebihi 4%. Keputusan eksperimen menunjukkan pemindahan haba pekali bertambah dengan peningkatan kadar alir isipadu, pecahan jumlah nanofluid dan suhu masukan. Begitu juga, keputusan simulasi menunjukkan keputusan yang baik antara data uji kaji dengan sisihan tidak lebih daripada 6%. Tambahan pula, cecair nano SiO 2 muncul nilai yang tinggi faktor geseran dan pekali pemindahan haba daripada TiO 2 cecair nano. Selain itu, cecair (20%EG+80%W) memberikan nilai yang tinggi pekali pemindahan haba dan nilai-nilai yang betul faktor geseran daripada cecair asas lain. Ia seolah-olah menunjukan bahawa nanopartikel SiO 2 tersebar ke (20%EG+80%W) cecair asas adalah kerana peningkatan yang ketara daripada sifat haba daripada yang lain. Ia juga menunjukan, nanopartikel SiO 2 tersebar ke (20%EG+80%W) cecair asas yang memberi kesan pembesaran yang ketara pemindahan haba dalam radiator kereta. Persamaan regresi antara input (nombor Reynolds, nombor Prandtl dan nanofluid jumlah penumpuan) dan tindak balas (faktor geseran dan nombor Nusselt) telah dijumpai. Keputusan analisis menunjukkan bahawa parameter input penting untuk meningkatkan pemindahan haba dengan sistem penyejukan automotif. Perbandingan antara keputusan eksperimen dan data penyelidik lain turut dijalankan dan terdapat satu keadaan yang baik dengan sisihan maksimum kira-kira 5%.

4 viii TABLE OF CONTENTS SUPERVISORS DECLARATION STUDENT S DECLARATION DEDICATION ACKNOWLEDGEMENTS ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS Page iti tv v iu uii uiii ix xiii xiv xviii CHAPTER I INTRODUCTION 1.1 BACKGROUND NANOFLUID Preparation of nanofluid Thermal conductivity of nanofluid Viscosity of nanofluid FRICTION FACTOR AND HEAT TRANSFER AUTOMOTIVE COOLING SYSTEM PROBLEM STATEMENT RESEARCH OBJECTIVE SCOPE OF STUDY 8 CHAPTER II LITERATURE REVIEW 2.1 INTRODUCTION NANOFLUID SYNTHESIS NANOFLUID APPLICATIONS THERMOPHYSICAL PROPERTIES 13

5 ix 2.5 FORCED CONVECTION HEAT TRANSFER Experimental studies of forced convection in a 17 circular tube Numerical studies of forced convection in a 21 circular tube Forced convection in a heat exchanger Forced convection in a car radiator SUMMARY 27 CHAPTER III METHODOLOGY 3.1 EXPERIMENTAL WORK Introduction Nanofluid Preparation Thermophysical Properties Experimental Setup Experimental Procedure Data Collection and Analysis Uncertainty Analysis Regression Analysis NUMERICAL ANALYSIS Introduction Simulation Model Governing Equations Simulation Procedures Assumptions and Boundary Conditions Grid Independence Test DIMENSIONLESS PARAMETERS VALIDATION Friction Factor Heat Transfer Coefficient The overall efficiency Validation with the Experimental Data SUMMARY 67

6 x CHAPTER IV RESULTS AND DISCUSSION 4.1 INTRODUCTION THERMOPHYSICAL PROPERTIES Effect of Nanoparticles Volume Fraction Effect of Temperature FRICTION FACTOR Effect of Nanoparticles Volume Fraction Effect of Inlet Temperature HEAT TRANSFER COEFFICIENT Effect of Nanoparticles Volume Fraction Effect of Inlet Temperature VALIDATION WITH EXPERIMENTAL DATA Thermal Conductivity Validation Viscosity Validation Friction Factor Validation Nusselt Number Validation OUTLET TEMPERATURE TEMPERATURE CONTOUR HEAT TRANSFER HEAT REJECTED HEAT TRANSFER ENHANCEMENT THE COOLING SYSTEM EFFICIENCY REGRESSION ANALYSIS The analysis of variance (ANOVA) Surface Plot SUMMARY 166 CHAPTER V CONCLUSIONS AND RECOMMENDATIONS 5.1 CONCLUSIONS CONTRIBUTIONS RECOMMENDATIONS FOR FUTURE WORK 169

7 xi REFERENCES 170 LIST OF PUBLICATIONS 181 APPENDICES A EXPERIMENTAL DATA MEASURED 183 B UNCERTAINTY ANALYSIS 192 C REGRESSION ANALYSIS 200

8 xii LIST OF TABLES Table No. Title Page 2.1 Thermal conductivity enhancement (k %) results of nanofluids Thermal conductivity models Nanofluid viscosity models The laminar forced convection heat transfer enhancement The turbulent forced convection heat transfer enhancement Thermophysical properties of nanoparticle and base fluids Uncertainty of measured data Thermophysical properties of copper Grid size test for triangular meshes Grid size test for quadratic meshes 61 A.1 Friction factor with flowrate for the nanoparticles in pure water 183 A.2 Friction factor with flowrate for the nanoparticles in pure EG 184 A.3 Friction factor with flowrate for the nanoparticles in 10%EG+90%W 185 A.4 Friction factor with flowrate for the nanoparticles in 20%EG+80%W 186 A.5 Heat transfer coefficient with flowrate for nanoparticles in water 187 A.6 Heat transfer coefficient with flowrate for nanoparticles in EG 188 A.7 Heat transfer coefficient with flowrate for the nanoparticles in 10%EG+90%W 189 A.8 Heat transfer coefficient with flowrate for the nanoparticles in 20%EG+80%W 190 A.9 ph measured before and after tests for SiO 2 nanofluid 191 A.10 ph measured before and after tests for TiO 2 nanofluid 191 B.1 Tube dimensions measurement 193

9 xiii LIST OF FIGURES Figure No. Title Page 1.1 The automotive cooling system Schematic of the experimental setup of laminar flow convective heat transfer of nanofluids inside circular tube with constant surface temperature. Test rig schematic of experimental study of turbulent nanofluids flow forced convection heat transfer in a double pipe and plate heat exchangers. Test rig schematic of forced convection heat transfer to reduce circulating water in an automobile radiator under turbulent flow The cooling loop setup of automobile cooling system Base fluid and nanopowders used in the experiment Stirrers apparatus used in this experimental work The heat capacity and thermal conductivity measuring apparatus Brookfield DV-I prime viscometer Viscosity calibration for pure water ph meter apparatus The experimental test rig setup Car radiator used in the present experimental work Thermocouples connecting at the inlet section Measurement devices and calibration process Calibration data of thermocouple Pump, flow meter, tubes and valves Manometer tube Plastic container, electrical heater, and voltage regular Flow meter calibration. 43

10 xiv 3.16 Flat tube configuration Meshing by GAMBIT Grid size meshing FLUENT simulation procedures The velocity profile and temperature gradient along the tube length Nusselt number and friction factor along the tube length Velocity at the mid plane of tube Grid independence test at different Reynolds numbers The effect of the nanofluid volume fraction on the density The effect of the volume fraction on the specific heat capacity The effect of the volume fraction on the thermal conductivity The effect of the nanofluid volume fraction on the viscosity The density of nanoparticles in water at different temperature The density of nanoparticles in EG at different temperature The density of nanoparticles in 10%EG+90%W The density of nanoparticles in 20%EG+80%W Nanofluid heat capacity at different temperature for water Nanofluid heat capacity at different temperature for EG Nanofluid heat capacity at different temperature for 10%EG+90%W. Nanofluid heat capacity at different temperature for 20%EG+80%W. The effect of temperature on thermal conductivity of nanoparticles suspended in water. The effect of temperature on thermal conductivity of nanoparticles suspended in EG. The effect of temperature on thermal conductivity of nanoparticles suspended in 10%EG+90%W. The effect of temperature on thermal conductivity of nanoparticles suspended in 20%EG+80%W The effect of temperature on viscosity of nanoparticles in water

11 xv 4.18 The effect of temperature on viscosity of nanoparticles in EG The effect of temperature on thermal conductivity of nanoparticles suspended in (10%EG+90%W) and (20%EG+80%W). The effect of the nanofluid volume fraction on the friction factor for nanofluid with water as a base fluid. The effect of the nanofluid volume fraction on the friction factor for nanofluid with EG as a base fluid. The effect of the nanofluid volume fraction on the friction factor for nanofluid with 10%EG as a base fluid The effect of the inlet temperature on the friction factor Thermal conductivity validation Comparison of thermal conductivity of base fluid with the standard Viscosity validation Comparison of viscosity of base fluid with the standard Validation of friction factor Validation of Nusselt number The outlet temperature at different flowrate for nanoparticles suspended in water as a base fluid. The outlet temperature at different flowrates for nanoparticles suspended in EG as a base fluid. The outlet temperature at different flowrates for nanoparticles suspended in 10% EG+90% W, as a base fluid. The outlet temperature at different flowrates for nanoparticles suspended in 20% EG+80% W, as a base fluid Temperatures contour for pure water inside flat tube The coolant heat transfer at different flowrates for nanoparticles suspended in water as a base fluid. The coolant heat transfer at different flowrates for nanoparticles suspended in EG as a base fluid The coolant heat transfer at different flowrates for nanoparticles 140

12 xvi 4.38 suspended in 10%EG+90%W, as a base fluid. The coolant heat transfer for nanoparticles suspended to 20%EG+80%W, as a base fluid The heat rejected for nanoparticles suspended in water The heat rejected for nanoparticles suspended in EG The heat rejected for nanoparticles suspended in 10%EG+90%W. The heat rejected for nanoparticles suspended to 20%EG+80%W The effect of volume fraction on the heat transfer enhancement The effect of the inlet temperature on the heat enhancement The effect of volume fraction on the efficiency The effect of the inlet temperature on the efficiency The experimental and prediction data regression Surface plot of Nusselt number and of both Reynolds Number and the volume fraction. Surface plot of Nusselt number and of both Reynolds Number and Prandtl number. Surface plot of friction factor and of both Reynolds Number and the volume fraction. Surface plot of friction factor and of both Reynolds Number and Prandtl number

13 xvii LIST OF ABBREVIATIONS Symbol A B C d D E f F f s g h I k L LPM m n N p P ph Q R S T u V m σ σ Meaning Area Bias error Specific heat capacity Minor dimension of radiator tube Major dimension of radiator tube Voltage Friction factor Force Skin friction coefficient Gravity Heat transfer coefficient Current Thermal conductivity Length Liter per minute Mass Number of radiator tubes Dimensionless number Pressure Power Acidity scale Heat transfer rate Precision error Specific gravity Temperature Velocity Volume flowrate Mass flowrate Average standard deviation Standard deviation Volume

14 xviii η CFD EG max min rpm Subscripts eff f p nf in out c cross s b rej x y z av Thermal diffusivity Volume fraction of solid phase Weight ratio of solid phase Viscosity Density Efficiency Computational Fluid Dynamics Ethylene glycol Maximum Minimum Revolution per minute Effective Liquid phase Solid phase Nanofluid phase Inlet Outlet Coolant Cross section Surface Bulk Rejected x-axis y-axis z-axis Average Dimensionless Number Nu Nusselt number Pe Peclet number Pr Prandtl number Re Reynolds number

15 1 CHAPTER I INTRODUCTION 1.1 BACKGROUND The general definition of convection is the energy transfer between the surface and fluid due to the temperature difference and this energy transfer by either forced (external, internal flow) or natural convection (Kays et al., 1993). Forced convection is a mechanism, or type of transport in which fluid motion is generated by an external source such as a fan, a pump, a suction device, etc. It is considered as one of the main methods of heat transfer as significant amounts of heat energy can be transported very efficiently, and this mechanism is found to be very common in many engineering fields, including air conditioning, central heating, steam turbines, and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchanges, flow over a flat plate, and pipe flow at a different temperature from the stream (Incropera et al., 2011). The increasing demand for more efficient heat transfer fluids in many applications has led to enhance heat transfer to meet the cooling challenge necessary such as the photonics, transportation, electronics and energy supply industries (Das et al., 2007). Conventional fluids nowadays are inherently poor heat transfer fluids and with the increasing demand of industries micro-sized heat generating systems, are unable to provide adequate heat transfer. One of the possible solutions to this limited capability can be achieved by integrating the high heat transfer capability of solid metals into a flowing heat transfer fluid.

16 2 In the past, attempts have been made to add micro-sized metal particles into conventional liquids, which ended in significant results as well as large disadvantages. Flow characteristics such as viscosity will change, which leads to the need for higher pumping power in order to add the micro particles. There is also the concern of agglomeration over time as well as corrosion of the system that can result in high maintenance demands in both. The concept of nanofluids refers to a new kind of heat transfer fluid formed by dispersing nano-scaled metallic or nonmetallic particles in base fluids (water, ethylene glycol, and oil). Energy transport of the nanofluid is affected by the properties and dimension of nanoparticles as well as a solid volume fraction. Some experimental investigations have revealed that the nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids and have great potential for heat transfer enhancement. The addition of nano-sized particles is very appropriate to augment heat transfer compared with the addition of millimeter or micrometer sized particles to liquids with little effect on pressure drop. A possible effective method for heat transfer enhancement is to include high thermal conductivity particles in the liquid. Some general examples of applications that can benefit from this technology include home heating and cooling appliances, automotive radiator systems, power plant cooling systems, computer processing cooling equipment and other examples involving the transfer of heat from one medium to another. The use of high conductivity heat transfer materials will benefit fully the available energy of a system will reduce the environmental footprint of companies as well as their operating costs. It is believed that the most important reasons to enhance heat transfer of the nanofluids may be due to intensification of a turbulence eddy, repression or interruption of the boundary layer as well as nanoparticles suspension. Taking advantage of the nanoparticles in the liquid causes the particles to stay in the solution for a long time. Another feature is that these particles have larger surface area for thermal conductivity compared to ordinary liquids. From an engineering point of view, forced convection utilizing liquid coolants in laminar or turbulent flow regimes are always a key heat transfer solution (Manglik, 2003).

17 3 The better convective heat transfer performance means higher values of heat transfer coefficient. There are a number of techniques to enhance heat transfer like modified heat transfer surface roughness, fins (extended surfaces), injection, and so on. However, these techniques have been been known to cause higher-pressure drop and hence lift pumping power requirement. The convective thermal performance creates barriers in designing small heat rejecting devices due to the low thermal conductivity and high viscosity of conventional heat transfer fluids such as water, ethylene glycol, oil, and ammonia. Therefore, an innovative coolant with improved heat transfer properties is required. The solid particles, usually exhibit higher thermal conductivity than liquids, and one approach to enhance thermal conductivity of liquids is by using suspensions, which entail the dispersion of particles into base fluids. The effective thermal conductivity of the suspension increases with the increas of Reynolds number and nanofluid volume fraction. Because of the shortageof available technology in those years the particles size was large (in micro- scale). Therefore, this size results in two disadvantages: the first is the inadequate stability, and other is the larger particles easily causing erosion to flow loop components. 1.2 NANOFLUID A Nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol, and oil (Das et al., 2007) Preparation of Nanofluid The use of additives is another technical application to enhance heat transfer performance of liquids. The metallic or nonmetallic particles suspension changes the transport properties and heat transfer characteristics of the base fluid (Li & Xuan, 2002). An effective method to improve thermophysical properties of fluids is to suspend small solid particles in the fluids. Recent reports on nanofluids have included particles

18 4 clustering as a possible mechanism for the abnormal augmentation of thermal conductivity when nanoparticles are suspended in the liquids (Masuda et al., 1993). Nanofluid samples are prepared by dispersing pre-weighed quantities of dry particles in a base fluid. The ph of each aqueous mixture has to be measured to ensure that no changes occur in the nanofluid volume fraction. In order to break up any particle aggregates, the mixtures were subjected to ultrasonic mixing for a short time (Nagar et al., 2012) Thermal Conductivity of Nanofluid With the rapid development in the industrial techniques and nanotechnology, the production of nanoparticles has been made possible. Nanofluids are dispersed nanoparticles in heat transfer liquids, which were visualized by Choi (1995). First impression of the effectiveness of nanofluid has been observed to be greater than expected of thermal conductivity with small nanoparticle volume fraction (You et al., 2003). Since then, investigators have shown interest in using nanoparticles for heat transfer enhancement and thermal performance (Lee et al., 1999). As a result of the large number of ongoing investigations, many review papers have been published on nanofluids with most of them focusing on observed and predicted augmentation in thermal conductivity (Xie et al., 2011 and Nguyen et al., 2007). In an attempt to reduce the experimental artifacts produced by the traditional hot-wire approach to the measurement of thermal conductivity of nanofluids, some researchers have adopted optical measurement techniques, but did not observe any significant improvement in the thermal conductivity (Putnam et al., 2006; Venerus et al., 2006 and Rusconi et al., 2006) Viscosity of Nanofluid The nanofluid convective heat transfer performance depends on the relationship between the increment of both thermal conductivity and viscosity (Prasher et al., 2006). As such, it is critical to study nanofluid viscosity when a system has adopted fluid flow.

19 5 The increase of fluid viscosity requires the addition of more particles, regardless of whether, the particles are rotating or non-rotating in the flow field (Hiemenz, 1977). 1.3 FRICTION FACTOR AND HEAT TRANSFER In the last ten years, there has been more attention paid to enhance the convective heat transfer performance of nanofluid, due to its increasing acceptance for practical applications. However, the nanofluid convective heat transfer coefficient (h) or Nusselt number (Nu) is incompatible throughout the literature. Experimental studies of the friction factor and nanofluid heat transfer enhancement with the flow velocity and nanofluid volume fraction inside heated tube under laminar flow condition have been introduced by Li & Xuan (2002) and Xuan & Li (2003). 1.4 AUTOMOTIVE COOLING SYSTEM Most internal combustion engines have used fluid coolant run through a heat exchanger (radiator) cooled by air. The water is often used directly for the engine cooling, but there can be obstacles such as sedimentation that may clog the coolant s passage, or salt may be chemically damaging to the engine. Most engine liquids for cooling use a mixture of water and chemicals such as antifreeze and rust inhibitors. The industry term for the antifreeze mixture is engine coolant. Some antifreeze uses no water at all, but instead uses a liquid with different properties, such as propylene glycol or ethylene glycol. The main consideration is that an engine fails even if just one part overheats. Therefore, it is essential that the cooling system keep all parts at appropriately low temperatures. The engine cooling system shown in Figure 1.1 is able to vary the size of its passageways through the engine block so that coolant flow may be adapted to the needs of each part. A typical four-cylinder engine cruising along the highway at around 50 miles per hour, will produce 4000 controlled explosions per minute inside the engine as the spark plugs ignite the fuel in each cylinder to propel the vehicle down the

20 6 road. Obviously, these explosions produce an enormous amount of heat that, if not controlled, will destroy an engine in a matter of minutes. Controlling these high temperatures is the job of the cooling system. Figure 1.1: The automotive cooling system. Source: Yadav (2011) The radiator is made usually of flattened aluminum or brass tubes with many fins on the external surface of the tubes. These fins are capable of transferring heat from tubes into the air stream to be carried away from the vehicle. Each of the two ends of the radiator is covered by a tank, and in most cars, the tubes run vertically to the tank on the top and bottom. On the back of the radiator on the side closest to the engine, there are one or two electric fans inside a housing that is designed to direct the air flow going through the radiator while the vehicle is moving slowly or stops while the engine is running. The engine temperature will increase if this fan stops working. The electric fan is controlled by the vehicle's computer. A temperature sensor monitors engine temperature and sends this information to the computer, which determines if the fan should be turned on and actuates the fan relay if additional air flow through the radiator is necessary. There was also a water pump, which is a simple device that keeps the fluid moving as long as the

21 7 engine is running. It is usually set up on the front of the engine and operates whenever the engine is working. 1.5 PROBLEM STATEMENT The conventional fluids (water and EG) have been used as a coolant in the automotive cooling system. However, the limited thermophysical properties of these fluids limit heat transfer across the car radiator. The increasing demand for energy and better performance has led to the investigation of other methods. Space constraints are another key issue in the automotive cooling system. Sometimes overheating occurs in the engine because the radiator is not functioning up to the standard expectations. The motivations for using nanofluids as a coolant in the automotive cooling system are as follows: (i) (ii) (iii) Helps engineers optimize the design of a car radiator. Reduces the circulating water of an automobile radiator. Decreases the fan radiator operating time, size and cost. 1.6 RESEARCH OBJECTIVES The main objectives of this study on the use of nanofluids in the automobile cooling systems are: (i) To improve thermophysical properties of the coolant fluid for enhancing heat transfer in the automotive cooling system. (ii) To identify the influence of different nanofluids on the heat transfer enhancement. (iii) To identify an appropriate type of nanofluid to be utilized in the car radiator. (iv) To investigate ways to enhance the performance efficiency of the automotive cooling system.

22 8 1.7 SCOPE OF THE STUDY (i) (ii) (iii) (iv) The scope of this work includes the following: A study of the effects of nanofluid volume fraction on the friction factor and Nusselt number by experimental work in the laminar condition at Reynolds number range from 250 to The experimental work includes test rig setup and connecting all devices for taking readings of temperature and pressure drop. Additionally, the nanofluids preparation processes and measurement of thermophysical properties are conducted. Computational Fluid Dynamics (CFD) simulation studies of Newtonian fluid flow in a flattened tube provide an understanding of the fundamental flow behavior. Furthermore, the friction factor and heat transfer enhancement in the automotive cooling system are evaluated using finite volume technical. The CFD analysis of friction factor and Nusselt number will be validated with the experimental data. The heat transfer enhancement and performance efficiency are evaluated to determine the optimum nanofluid type as a coolant in the automotive cooling system. Develop regression correlations of the friction factor and Nusselt number with the input parameters (Reynolds number, Prandtl number and the nanofluid volume fraction) in the laminar flow regimes.

23 9 CHAPTER II LITERATURE REVIEW 2.1 INTRODUCTION The new class of convection heat transfer includes metallic or non-metallic nanoparticles of typical size less than 100 nm dispersed in the base fluids (water, ethylene glycol and oil). The poor liquid thermophysical properties have led to the use of solid particles as an additive suspended to enhance thermophysical properties and improve the heat transfer characteristics of liquids, the key idea being to improve the thermal conductivity. Since solid particles have a larger thermal conductivity than liquids; solid particles suspended in the liquid will improve the thermal conductivity of liquids. Recent advances in material technology have made it possible to produce innovative heat transfer fluids by introducing nanometer-sized particles into the base fluids, which could change the transport and thermophysical properties of the liquids. Nanofluids are represented as solid-liquid composite materials consisting of solid nanoparticles with sizes not larger than 100 nm suspended in a liquid (Chon et al., 2005). Nanofluids have attracted significant interest recently due to reports of enhancement of thermophysical properties and many industrial applications (Baharanchi, 2013 and Balla et al., 2012). 2.2 NANOFLUID SYNTHESIS METHODS A nanofluid may be synthesized by simply mixing nanoparticles dispersed in a liquid. In fact, the processes of synthesis are more involved. Metal oxide, carbon nanotube, nitride, carbide and other nanoparticles may be readily purchased from the

24 10 market. These nanoparticles can normally be handled outside the boxes and other sealed containers during the preparation of the nanofluid. In order to break up the agglomerated particles and form a well-dispersed nanoparticle suspension, a stabilizing agent is added to avoid re-agglomeration of the nanoparticles. In addition, the stabilizing agent affects the thermophysical properties, for instance, by altering the optical properties, or viscosity of the nanoparticle solutions. Furthermore, the applicability of the nanofluid to the system hinges on the ability of real-life products to retain their small size character and thus ensure proper dispersal. Due to the high surface reactivity of metal, it reacts with the environment so nanofluids cannot be synthesized from pure solid metal nanoparticles. A single-step method for the synthesis of nanofluid with limited agglomeration has been reported by Das et al. (2007). Over the years, a few processes have been developed for direct nanofluids synthesis. The metal salt solutions are reduced with a stabilizing agent, and the small particles form as sediment (Schmid, 2008). There are also a number of plasma synthesis approaches: Metal oxide, nitride or carbide nanoparticles may be synthesized by condensation/evaporation with the nitrogen, oxygen, or a light hydrocarbon. The main advantages of the plasma synthesis processes are the possibility to synthesize complex chemical compositions and structures, in-situ stabilization, and scalability (Vollath, 2007). The nanoparticles in a host fluid interact strongly through van der Waals interactions because of the large surface area of the nanofluid, and incessant solid-solid collisions due to Brownian effect. Brownian motion is the random motion of particles suspended in a fluid resulting from their collision with the quick atoms or molecules in the gas or liquid. The term "Brownian motion" can also refer to the mathematical model used to describe such random movements, which is often referred to as a particle theory (Choi, 1995).