Hybrid Nanofluids for Convection Heat Transfer

By Hafiz Muhammad Ali

Hybrid Nanofluids for Convection Heat Transfer discusses how to maximize heat transfer rates with the addition of nanoparticles into conventional heat transfer fluids. The book addresses definitions, preparation techniques, thermophysical properties and heat transfer characteristics with mathematical models, performance-affecting factors, and core applications with implementation challenges of hybrid nanofluids. The work adopts mathematical models and schematic diagrams in review of available experimental methods. It enables readers to create new techniques, resolve existing research problems, and ultimately to implement hybrid nanofluids in convection heat transfer applications.

• Provides key heat transfer performance and thermophysical characteristics of hybrid nanofluids
• Reviews parameter selection and property measurement techniques for thermal performance calibration
• Explores the use of predictive mathematical techniques for experimental properties

• Language: English
• Publisher: Academic Press
• Release date: May 15, 2020
• ISBN: 9780128192818

Book preview

Arabia.

Chapter 1

History and introduction

Mohammad Hemmat Esfe¹, Saeed Esfandeh¹, ² and Mohammad Hassan Kamyab¹,    ¹Department of Mechanical Engineering, Imam Hossein University, College of Engineering, Tehran, Iran,    ²Department of Mechanical Engineering, Jundishapur University of Technology, Dezful, Iran

Abstract

Miniaturizing and downscaling cooling and heating devices has been one of the main concerns of experts and engineers in various industries. Miniaturized sensors, actuators, motors, heat exchangers, pump and heat pumps, and fuel cells are some of the instruments that will help the industries to thrive. Although there are some contradictions between researcher’s ideas about heat transfer mechanisms of nanofluid, the common belief is that nanofluids are intelligently choice to realize the miniaturized and downscaled world. Having impressive improved convective and conductive thermal behaviors of nanofluids introduced them as a candidate for working fluid in miniaturized systems. One of the main barriers in front of moving the nanofluid applications from the laboratory scale to the marketplace and industries is its high cost. In the continuation of nanofluid research, as the second step, the experts tried to use simultaneous use of dissimilar nanoparticles in conventional working fluids known as hybrid nanofluids. They found hybrid nanofluid more effective economically and thermally in comparison to conventional nanofluid. In addition to feasibility of miniaturization of devices and systems using conventional nanofluids, hybrid nanofluids as super thermal and thermophysical improved working fluids than conventional fluids and nanofluids could be applied in desalination systems, solar systems, power plant cycle, etc. to reach more efficient systems.

Keywords

Microscale heat transfer; nanoscale heat transfer; hybrid nanofluid; convection heat transfer; heat transfer enhancement

Chapter Outline

Outline

1.1 History 1

1.1.1 Conventional methods to enhance heat transfer 2

1.1.2 Microscale additives in fluids 2

1.1.3 Nanoscale additives in fluids 3

1.1.4 Nanoscale particles and nanofluids 4

1.2 Introduction 9

1.2.1 Fundamental of conduction 10

1.2.2 Fundamental of convection 11

1.2.3 Fundamental of radiation 11

1.2.4 Fundamental of viscosity 13

1.2.5 Fundamental of density 14

1.2.6 Fundamental of heat capacity 14

1.3 Nanofluid and hybrid nanofluid 15

1.3.1 Unique characteristics of hybrid nanofluid 18

1.3.2 Microscale heat transfer 20

1.3.3 Nanoscale heat transfer 22

1.4 Conclusion 40

Nomenclature 41

References 43

1.1 History

The initial spark of heat transfer science can be attributed to Galileo Galilei [1] with the invention of the Galilean thermometer in the 16th century and Newton [2] by presenting Newton’s cooling law as the first heat transfer formula in the 18th century. Then we can refer to Fourier’s mathematical theory of heat transfer in 1822 [2]. In fact, from the beginning of the 17th century through the middle of the 19th century, many of the basic concepts of heat transfer, such as heat, temperature, thermal energy, specific heat, latent heat, and kinetic energy, were defined, as well as the first and second rules of thermodynamics were introduced. Nevertheless, the history of modern heat transfer science dates back to the 1930s and is still ongoing [3]. The breadth and variety of subsystems of heat transfer science over the years has led to the division of this science into many specialized subdivisions, including the three main subsections of convection, radiation, and conduction heat transfer. Today, heat transfer is one of the important branches of study in many engineering disciplines, including mechanical engineering and chemistry. The application of heat transfer phenomena in various industries, including electronics, marine industries, and power plants, is also clearly visible. For example, in the design of boilers, condensers, evaporators, heat exchangers, and radiators, heat transfer analysis is necessary to calculate their optimized size and determine their type.

1.1.1 Conventional methods to enhance heat transfer

In recent years, many studies have been conducted on methods to increase the heat transfer rate in equipment used in various industries, which can be divided into two general active and inactive methods. Among the active methods, it is possible to increase the thermal areas [4–6], the application of electric current or magnetic field [7–9] and fluid injection or suction [10–13], and on the other hand inactive methods, special geometry flat plates and additive or enriched fluids are used to achieve more heat exchange. According to the information in Table 1.1 on the thermal conductivity of different fluids, conventional heat transfer fluids such as water, ethylene glycol, and engine oils have poor thermal properties compared to metals and even metal oxides. This has led many scholars to think about how to improve the thermal properties of conventional fluids in heat transfer.

Table 1.1

Akoh [14] introduced the idea of a very fine magnetic particle dispersal in conventional heat transfer fluids, for the first time. Subsequently many studies have been conducted on the behavior and estimation of the thermal conductivity coefficient of the fluids with the solid particles dispersed therein, among which the theoretical and classical models of Maxwell [15] and Hamilton and Crosser [16] are the pioneers to estimate the thermal conductivity of solid–liquid mixtures. Indisputably the researchers in their theoretical models have not referred to the problems caused by the sedimentation of these solid particles. The erosion of the system and the increase in power required for pumping as well as increasing energy consumption were other unannounced points in the Akoh, Maxwell, and Hamilton–Crosser researches.

1.1.2 Microscale additives in fluids

As stated, one of the ways to improve the heat transfer is to add particles with better thermal properties to fluids such as water and ethylene glycol. Ahuja [17] measured the thermal and viscosity conductivity of 50 and 100 µm of polyester spherical particles dispersed in sodium chloride and glycerin. Based on the results, the thermal conductivity of the fluid was increased three times when compared with the base fluid. Choi and Tran [18], in the American National Argon Laboratory, introduced new fluids for industrial applications. Masuda et al. [19] used ultrafine aluminum oxide, silicon oxide, and titanium oxide to enhance the thermal properties of host fluids and to calculate their thermal conductivity and viscosity. They also observed the conglomeration of microparticles in the base fluid. These fluids with particles in millimeters or micrometers were never welcomed and commercially used, due to their low stability, rapid settling, obstruction and trapping of the flow path, rapid erosion of the pipe wall and equipments, and the sharp increase in pressure drop across the fluid flow.

1.1.3 Nanoscale additives in fluids

According to the difficulties with the use of microparticles in fluids, researchers have sought to find a way to solve these problems. Choi et al. [20] first dispersed solid particles with nanosize (between 1 and 100 nm) in the fluids, and the resulting suspension was called nanofluid. According to their studies, nanosized particles formed more stable suspensions than suspensions containing microparticles, so that their low sedimentation rate would minimize the problem of obstruction of fluids paths. This research and its results can be considered as the starting point for using nanoparticles to improve the thermophysical properties of fluids. Masuda et al. [19] and Pak and Cho [21] are other pioneering scientists in this field.

Metal nanoparticles (aluminum, silver, copper, nickel, etc.), metal oxides (aluminum oxide, copper oxides, iron oxide, silicon oxide, titanium oxide, etc.), or polymers such as graphene, carbon nanotubes, and the like are nanoparticles that could be used as nanoscale additives in fluids. Water, ethylene glycol, propylene glycol, oil, etc. could also be used as the base fluid. In general, the advantages of adding nanoparticles to a fluid compared with fluids containing microparticles and ordinary fluids are as follows:

• increasing the effective surface (Fig. 1.1) and fluid heat capacity;

• increasing the effective thermal conductivity of the fluid; and

• lowering the possibility of obstruction caused by the presence of nanoparticles in the fluid compared to microparticles.

Figure 1.1 An image of the surface-enhanced effect of nanostructured materials.

1.1.4 Nanoscale particles and nanofluids

In terms of structure, materials generally have three dimensions: length, width, and height. If at least one of these dimensions be in the scale of nanotechnology (1–100 nm), that material is called nanostructured. The reason of attraction for nanotechnology is that nanoscale materials have completely different properties than macroscale materials. All of the physical, chemical, and biological properties of the macroscopic scale may vary substantially in nanoscale. These properties are conductivity of heat and electricity, magnetic properties, optical properties, physical strength of materials, reactivity, and reaction speed [22,23].

Some other applications of nanofluids are in microelectronics, fuel cells, pharmaceutical processes, hybrid engines [24], internal combustion engines in cooling and heat cycles, chillers, heat exchangers, and lowering temperature of boilers exhaust gas from chimney [25]. Nanofluids increase the thermal conductivity and heat transfer coefficient relative to the base fluid [26]. It should be noted that in addition to thermal behavior, knowing the rheological behavior of the nanofluids in deciding whether they are practically suitable for convective heat transfer is very important [27].

Nanostructured materials are divided into different groups based on the number of nanosized dimensions that is known as free dimension. The free dimension refers to dimensions of a material with larger size than nanosize. Accordingly the material is divided into four parts nanoparticles, nanowires, thin films, and bulk nanomaterials.

The energy structures (alignment or band) of materials are length, width, and height directions. In other words, every three-dimensional object has three distinct energy structures along its three dimensions, the resultant of them defines total energy structure of material. Dimensions of nanoscale materials that are in nanoscale have quantum confinement. The quantum confinement means that the bands of energy become discrete due to the size limitations of the nanoscale, and more limitations led to more energy levels. Therefore one of the main differences between different types of nanostructured materials is the number of continuous energy bands and discrete energy levels in three dimensions, which results in a large variation in their properties.

In fact, nanomaterials are divided into four categories of nanodimensional: zero-dimensional, one-dimensional, two-dimensional, three-dimensional, and bulky three-dimensional nanomaterials in terms of their nanosized dimensions, which can be produced by top-down or bottom-up method. Top-down and bottom-up methods (Fig. 1.2) are used to build all of the mentioned nanostructures and are not related to a specific group of nanostructures.

Figure 1.2 Top-down and bottom-up production methods to make different types of nanostructures.

Generally by changing the size of nanoparticles in the range of 1–100 nm, the surface-to-volume ratio and the energy balance distance will change. These two variables are the cause of many changes in properties and features. In other words, by controlling the size of the nanoparticles, they can control nanoparticles properties, which is very important.

1.1.4.1 Properties

Properties and features of nanoparticles generally depend on their material and size. Certainly it is not possible to check all of these properties. As a solution, all the properties and features in nanoparticles can be explained by two factors that are increasing the surface-to-volume ratio and discretization of energy levels. Some of these properties are summarized briefly.

1.1.4.1.1 Optical properties

In general, when the light hits an atom, it may be absorbed, reflected, or crossed. The excitation mechanism of electrons is different in atoms, in normal materials, and in nanoparticles, which is shown in Fig. 1.3.

Figure 1.3 Electrons excitation, from the left to right, in atoms, in normal materials, and in nanoparticles, respectively. Nanoparticles act like atoms because they have discrete energy levels and are known as artificial atoms.

According to Fig. 1.3, the absorption of light in ordinary materials with continuous energy bands also occurs, and electrons are transferred from the valence band to the conduction band (however, here the thermal energy can also excite the electrons toward the conductive band). In the right side of Fig. 1.3, the absorption of light by nanoparticles is also shown. As shown in the figure, nanoparticles, such as atoms, have discrete energy levels. Hence, nanoparticles are called artificial atoms as well. Moreover, nanoparticles below 10 nm are called quantum dots.

By changing the size of the nanoparticles, the distance between the energy levels changes. Smaller size of nanoparticles cause increase in distance between the energy levels and vice versa. This makes it possible to adjusting the gap between energy levels by changing the size of the nanoparticles to absorb certain waves at a specific frequency. For example, it is possible to adjust the dimensions of the specific nanoparticles to absorb waves of infrared, ultraviolet, radio, and so on. This feature in the military and electronics industries has a great application.

Different colors of the nanoparticles in Fig. 1.4 show different distance between their energy levels. In Fig. 1.4, the color of the nanosized gold and silver in different sizes is shown with image of their electron microscopy.

Figure 1.4 The effect of nanoparticles sizes on their color.

In nature three elements of iron, nickel, cobalt, and the combination of other elements with these three elements have magnetic properties, and other elements or compounds alone have no magnetic properties. In the world, magnets and magnetic materials are widely in use from simple applications such as window lifters, car wipers, printers, scanners, electrical appliances in kitchens, and speakers to extremely complex applications such as generator motors, and so on. Only certain compounds can have magnetic properties and it counts as a limitation.

One of the most interesting and highly functional properties in nanoscale dimension is that many of the materials that do not have magnetic properties in their normal size can have magnetic properties below a certain size in terms of nanotechnology. Aluminum oxide nanoparticles, gold, etc. are some notable examples. This will remove the above limitation, and considering the vast application of magnetic materials, new materials with improved properties can be produced. For instance, the magnetic properties of some nanoparticles are used in medicine and pharmaceutical applications as well. The reason for the creation of magnetic properties in materials that do not have magnetic properties in the ordinary dimension is the high increase in the surface and the creation of broken bonds on the surface in the nanoscale. When a bond is established, two electrons in one orbital are placed in opposite directions. This arrangement leads to the neutralization of the magnetic fields around them. The electrons are charged particles, so with moving of charged particles, magnetic field will generate around them (Fig. 1.5). Furthermore the atoms in the electrons have two kinds of spin moves (around themselves) and orbits moves (around the nucleus) that cause magnetic field. The magnetic field is a vector quantity, and the direction is also of great importance.

Figure 1.5 A microscopic schematic of generating magnetic properties in nanoparticles.

However, the broken or incomplete bond means that there is a single electron in orbital and there is no second electron to neutralize its magnetic field. On the nanoscale, since the percentage of the atoms on the surface and the broken bonds is very high, most materials can have magnetic properties.

1.1.4.1.2 Antibacterial properties

Some nanoparticles such as silver and gold have antimicrobial or antibacterial properties, which mean that germs cannot grow on them. These particles are commonly used in cosmetics, hygiene, textiles, and so on. Its uses include the creation of hands-free cleansing gels, use in soaps and shampoos, use in clothing, use in manufacture of antimicrobial clothes and medical equipment, etc. Some nanoparticles, such as zinc oxide or titanium oxide, have photocatalytic properties. These nanoparticles are semiconductor and have an energy gap. These materials are usually used to purify water and pollutants. When the light is exposed to these particles, these electrons are excited and move toward conduction band. There, the electron is transferred to the pollutant and it is destroyed. The term photocatalyst is said to materials that activate their catalytic properties with light exposure.

1.1.4.1.3 Catalytic properties

The catalyst is a material that changes the chemical reaction (increase or decrease) but does not participate in the chemical reaction. A factor that influences the quality and performance of catalysts is a variable called a specific area. The larger the catalyst area, the better its catalytic properties. The specific area of a catalyst is obtained using Eq. (1.1):

(1.1)

This quantity is usually reported in units of m²/g and its value for commercial catalysts is between 100 and 400 m²/g. The 100 m²/g means that 1 g of this material is 100 m². Nanoparticles can also be used as catalysts due to their high surface. However, the properties of the catalyst, as well as magnetic properties, occur in certain dimensions. In other words, nanoparticles typically have the properties of catalysts, with a specific surface area between 100 and 400 m²/g. Therefore among the nanoparticles with equal volume, nanoparticles of higher surface exhibit better catalytic properties.

An example of nanoparticles acting as a catalyst is shown in Fig. 1.6, with different materials placed on their surface and chemical reactions are carried out.

Figure 1.6 Nanoparticles as catalysts and chemical reactions on them.

1.2 Introduction

In recent years, many studies have also been carried out on nanofluids in a variety of contexts such as thermal conductivity [28–30], forced convection heat transfer [31–34], natural convection heat transfer [35–37], combined convection [38], boiling heat transfer [39], heat exchangers [40], flat plat solar collectors [41], car radiators [42], sliding mechanisms such as bearings [43], electricity [44], and cooling electronic devices [45]. Studies have also been carried out on various properties of nanofluids such as viscosity [46–48]. Considering the significant properties of nanoparticles, nanofluids can be considered as one of the most suitable and effective choices in heat transfer. Small particles in the nanofluid reduce corrosion, pressure drop, etc. and make the fluid more stable in comparison to micro or larger particles. The high ability of nanoparticles to influence the thermophysical properties of fluids is one of the most important reasons for researchers and craftsmen to use these materials in various industries and applications. Growth of thermal conductivity, the high heat transfer in a phase, and the high critical thermal flux, can be considered as the main reasons for the researchers to welcome the use of