Nanofluids fields of application
Generally, nanofluids are used to improve heat transfer and energy efficiency in various thermal systems. These nanofluids could be used in a wide range of industrial applications such as electronics, transport, nuclear… [54-57]. Nanofluids are used as cooling fluids in integrated circuits. For this, several researchers have carried out studies. On the one hand, the two researchers Tsai and Col have fabricated a nanofluid in a water-based microcomputer to cool a central unit. On the other hand, Mal and Cal studied the effect of the nanofluid on the possibilities of heat transport from an oscillation heat pipes. Furthermore, the mixture of ethylene glycol and water are used as coolant in vehicle engines and the addition of nanoparticles in these liquids improves the cooling rate. Nanofluids are also found in the nuclear energy industry due to their performance in cooling nuclear systems. For space applications, the group of researchers has carried out studies to show that the presence of nanoparticles in the coolant in general electronics plays a very important role in space applications.
Heat transfer mechanism associated with cooling systems
It is the transport of energy in matter without displacement of matter. This transport can be carried out by electrons (conductor) or by photons (insulator).
It depends on the thermal conductivity of the material used, assuming that the geometric dimensions of the solid element are adequate. Optimizing conductive transfer therefore means using the most conductive materials possible with certain constraints, however: mechanical / thermal resistance and cost.
Defined as the transport of energy or excess heat by the flow of fluids (gas, fluid) across a surface. It is the most efficient and therefore the most used heat transfer mechanism. Convection can also be subdivided into natural or forced monophasic convection, and into natural or forced biphasic convection. Industrial cooling systems based on forced convection of a two-phase fluid.
Convective exchanges are influenced by three factors:
The coefficient of exchange between the fluid and the wall (or the element to be cooled).
The nature of the fluid used in terms of its thermophysical properties (thermal conductivity, density, specific heat, dynamic viscosity).
The exchange area.
Droplets evaporation-liquid/vapor phase change
Droplet evaporation plays a vital role in various fields of natural science and engineering such as burning liquid-fuels, biological crystal growth and painting. Moreover, the high heat transfer rates associated with evaporation suggests its use in contexts with a variety of thermal applications, such as spray-cooling or in the electronics industry for cooling of integrated circuits with high heat dissipation rates.
The evaporation of a sessile droplet is a fundamental heat and mass transfer process in the nature. When a droplet is deposited on a solid surface, its profile is determined by the equilibrium of gas, liquid and solid phases. Three interfacial tensions (solid/liquid, liquid/vapor and solid/vapor) were described by Young in 1805 , presented in the following equation: Where , , are surface tension od solid /vapor, solid/liquid, liquid/vapor interfaces respectively, s the contact angle which is the droplet angle between liquid/solid and liquid/vapor interfaces. The wettability of solid substrate with droplet depends on the physical properties of liquid and the surface characteristics of substrate, which can classify substrate into The droplet profile evolution can be identified in three different regimes (Fig. I.5): (1) constant contact line regime (CCL), when the wetted area of droplet stays unchanged and the contact angle decreases gradually; (2) constant contact angle regime (CCA), when the contact angle remains constant during the reduction of contact area; (3) mixed regime, when the profile evolution jump from CCL to CCA, vice versa or both contact angle and contact line decrease simultaneously. CCL regime is often observed in the evaporation of sessile droplet on hydrophilic substrate while the evaporation on hydrophobic substrate frequently behaves in CCA regime.
Heat pipes performance using nanofluids
As we know, heat pipes are used extensively in various applications, for achieving high rates of heat transfer utilizing evaporation and condensation processes [76–85]. Heat pipes have been used in space crafts, computers, solar systems, heat and ventilating air conditioning systems and many other applications. Most heat pipe analysis is based on the steady state operation of the device. However, in a number of applications start-up characteristics are quite important. Improper start-up can cause damage to the heat pipe. As such it is important to analyze the start-up process for the heat pipes . Several transient models for the start-up of the heat pipes have been presented in the literature such as the ones by Tournier and El-Genk  and Chang and Colwell . However, these works were all based on a regular cylindrical based geometry.
The thermophysical properties of a liquid, specifically, the thermal conductivity and heat capacity can significantly affect the heat transfer process in the liquids. Both of these properties can be augmented by dispersing the liquid with solid nanoparticles. The new liquid which now has better characteristics in transferring heat is called nanofluid [89, 90]. The other properties of this liquid such as density and viscosity also change as a function of concentration of nanoparticles [75, 76, 90, 91]. So, some researchers has been showed the thermal performance of heat pipe using different types of nanofluid [92, 93]. P. Gunnasegaran et al.  present the effect of Al2O3-water nanofluids on heat transfer in a loop heat pipe. The results showed the positive influence of nanofluid utilizing as a heat pipe working fluid on the system thermal performance. It is found that the thermal resistance of LHP decreases when particle mass concentration of Al2O3-water nanofluid increases. Also, the LHP charged with Al2O3-water nanofluid yields lower wall temperature difference between evaporator and condenser and reaches its steady state faster than LHP charged with pure water.
Furthermore, other types of nanofluids were studied to present their performance in heat pipe systems. K. Alizad et al.  discussed and analyzed the thermal performance of a flat-shaped heat pipes using three different types of nanofluids such as CuO, Al2O3 and TiO2. The results illustrated enhancement in the heat pipe performance while achieving a reduction in thermal resistance for both flat-plate and disk-shaped heat pipes throughout the transient process. Thus, it showed a higher concentration of nanoparticles increases the thermal performance of the two heat pipes shape. The study of the thermal performances of heat pipes using nanofluids, as indicated above, can be carried out by measuring the wall temperature and by finding the thermal resistance of the evaporator. From these measurements, we can obtain and observe the enhancement in heat transfer of heat pipes. In this work, Y.F. Niu et al. , an experimental investigation of thermal performance of miniature heat pipe using SiO2-water nanofluids is presented. The four miniature heat pipes, see Fig. I.15, filled with DI water and SiO2-water nanofluids containing different volume concentrations (0.2%, 0.6% and 1.0%) are experimentally measured on the condition of air and water cooling.
Table of contents :
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
RESEARCH OBJECTIVES AND THESIS OUTLINE
CHAPTER .I LITERATURE REVIEW
I.1 NANOFLUIDS OVERVIEW
I.2.1 Thermal conductivity
I.2.2 Dynamic viscosity
I.2.3 Density and specific heat
I.3 TYPES OF NANOPARTICLES
I.4 NANOFLUID PREPARATION
I.5 INFLUENCE OF NANOFLUIDS ON THERMOPHYSICAL PROPERTIES
I.5.1 Effect of the volume concentration of nanoparticles
I.5.2 Effect of the type of nanoparticles
I.5.3 Effect of temperature
I.6 ADVANTAGES AND DISADVANTAGES OF NANOFLUIDS
I.6.1 Advantages of nanofluids [47-49]
I.6.2 Disadvantages of nanofluids [50-53]
I.7 NANOFLUIDS FIELDS OF APPLICATION
I.7.1 Heat transfer mechanism associated with cooling systems
a- Conductive transfer
b- Convective transfer
c- Droplets evaporation-liquid/vapor phase change
I.7.2 Main cooling systems used in power electronics
1- Active cooling systems
1.1 Active direct cooling
1.2 Active indirect cooling
2- Passive cooling systems
2.1 Passive direct cooling
2.2 Passive indirect cooling
I.7.3 Passive phase change cooling systems – Heat pipes
1.7.4 Definition and operating principle
1.7.5 Types of heat pipes
I.8 HEAT PIPES PERFORMANCE USING NANOFLUIDS
CHAPTER .II NANOFLUIDS SYNTHESIS AND THERMOPHYSICAL PROPERTIES: ACOUSTIC AND THERMOGRAPHY METHODS
II. 1 INTRODUCTION
II. 2 INFRARED AND ACOUSTIC METHOD TO TRACK THE DYNAMIC DEPOSITION OF COPPER OXIDE
II.2.1 Methodology and experimental setup
II.2.1.1 Infrared and optical measurements
II.2.1.2 Acoustic measurements
II.2.2 Experimental results
II.2.2.1 Calibration of the acoustic measurement method: Water as a reference liquid
II.2.2.2 Optical, infrared and acoustic measurements of CuO-water droplet at ambient temperature
II.2.2.3 Effect of substrate temperature on nanoparticles deposition
II. 3 DYNAMIC VISCOSITY MEASUREMENTS OF GOLD NANOFLUIDS USING HIGH FREQUENCY ACOUSTIC METHOD (≈1GHZ)
II.3.1 Experimental setup
II.3.2 Results and discussion
II.3.2.1 Calibration of the acoustic measurements method: Water as a reference liquid
II.3.2.2 Values of dynamic viscosity of gold nanofluid during the evaporation of 4%Cv Au-water droplet
II.3.2.3 Viscosity measurements as function of gold nanoparticles concentrations
II. 4 CONCLUSION
CHAPTER .III EXPERIMENTAL STUDIES ON EVAPORATION KINETICS OF GOLD NANOFLUIDS DROPLETS
III.2 METHODOLOGY AND EXPERIMENTAL SETUP
III.2.1 Infrared and optical measurements
III.2.2 Acoustic measurements
III.3 EXPERIMENTAL RESULTS
III.3.1 Infrared and optical investigations
III.3.1.1 Au nanofluid (1% Cv; 0.1mM PBS, reactant free)-Water mixture, Particle size effects
III.3.1.2 Au nanofluid (1% Cv; Citrate Capped-PBS)-Water mixture, Citrate effects
III.3.2 Acoustic investigations: Gold nanoparticles stability during droplet evaporation at ambient
CHAPTER .IV THERMAL PERFORMANCES OF TWO-PHASE HEAT TRANSFER AND MICRO HEAT EXCHANGERS DEVICES USING SELF-REWETTING NANOFLUIDS
IV.2 THERMAL PERFORMANCE OF TWO-PHASE HEAT TRANSFER DEVICES
IV.2.1 Experimental setup
IV.2.1.1 Working fluids:
IV.2.1.3 Error analysis
IV.2.1.3 Porous media properties
IV.2.1.4 Thermal resistance:
IV.2.2 Experimental results and discussion
IV.2.2.1 Heat transfer characteristics
IV.2.2.2 Vapor pocket dynamics
IV.3 THERMAL PERFORMANCES OF A MICRO HEAT EXCHANGERS USING NANOFLUID AND SELFREWETTING FLUID
IV.3.1 Experimental method
IV.3.1.1 Polydimethylsiloxane (PDMS) – based microchannel device, microfabrication and experimental facility.
IV.3.1.2 Microfabrication of Polydimethylsiloxane (PDMS) microchannel
IV.3.1.3 Surface emissivity measurements of ITO coated glass and PDMS for infrared thermography
IV.3.1.4 Working fluids used in the experiment
IV.3.2 Data reduction
IV.3.3 Results and discussion
IV.3.3.1 System calibration: Water as a reference
4.2 Gold nanofluid and Butanol self-rewetting liquid as a working fluids
CHAPTER .V GENERAL CONCLUSION AND PERSPECTIVES
V.1 CONCLUSION AND PERSPECTIVES