Preparation, Characterization, Properties, and Application of Nanofluid

By I. M. Mahbubul

Preparation, Characterization, Properties and Application of Nanofluid begins with an introduction of colloidal systems and their relation to nanofluid. Special emphasis on the preparation of stable nanofluid and the impact of ultrasonication power on nanofluid preparation is also included, as are characterization and stability measurement techniques. Other topics of note in the book include the thermophysical properties of nanofluids as thermal conductivity, viscosity, and density and specific heat, including the figure of merit of properties. In addition, different parameters, like particle type, size, concentration, liquid type and temperature are discussed based on experimental results, along with a variety of other important topics.

The available model and correlations used for nanofluid property calculation are also included.

• Provides readers with tactics on nanofluid preparation methods, including how to improve their stability
• Explores the effect of preparation method and stability on thermophysical and rheological properties of nanofluids
• Assesses the available model and correlations used for nanofluid property calculation

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 20, 2018
• ISBN: 9780128132999

I. M. Mahbubul

Dr. Mohammed Mahbubul Islam is a Research Faculty at King Fahd University of Petroleum and Minerals, Saudi Arabia. He has several years’ hands-on experience working with nanofluid and nanofluid was his concentration in Post-doc, PhD and Masters. He started his career in industry as an R&D engineer. He was research project manager and researcher and established a complete nanofluid research laboratory in University of Malaya before joining King Fahd University. He has published about 50 nanofluid-related publications in peer-reviewed journals.

Book preview

Preparation, Characterization, Properties, and Application of Nanofluid

I.M. Mahbubul

Center of Research Excellence in Renewable Energy (CoRERE), Research Institute, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia

Table of Contents

Cover image

Title page

Copyright

Acknowledgments

1. Introduction to Nanofluid

Abstract

1.1 Introduction

1.2 Colloid

1.3 Nanofluid

1.4 Scope

References

2. Preparation of Nanofluid

Abstract

2.1 Introduction

2.2 One-Step Method

2.3 Two-Step Method

2.4 Comparison of One-Step and Two-Step Methods

References

3. Stability and Dispersion Characterization of Nanofluid

Abstract

3.1 Introduction

3.2 Ultrasonication

3.3 Surfactant

3.4 pH Control

References

4. Thermophysical Properties of Nanofluids

Abstract

4.1 Introduction

4.2 Thermal Conductivity

4.3 Viscosity

4.4 Density

4.5 Specific Heat

4.6 Surface Tension

References

5. Rheological Behavior of Nanofluid

Abstract

5.1 Introduction

5.2 Measurement Method

5.3 Rheological Model

5.4 Rheology Analyses

5.5 Concluding Remarks

References

6. Optical Properties of Nanofluid

Abstract

6.1 Introduction

6.2 Absorption

6.3 Transmittance

6.4 Extinction Coefficient

6.5 Scattering Coefficient

6.6 Concluding Remarks

References

7. Correlation and Theoretical Analysis of Nanofluids

Abstract

7.1 Introduction

7.2 Thermophysical Properties Calculation

7.3 Performance Parameter Calculation

7.4 Figure of Merit Analysis

References

8. Application of Nanofluid

Abstract

8.1 Introduction

8.2 Electronics Cooling

8.3 Solar Collector

8.4 Heat Exchanger

8.5 Engine Cooling

8.6 Refrigerator

8.7 Machining

References

Nomenclature

Abbreviations

Chemical symbols

Greek letters

Dimensionless number

Subscripts

Superscript

Index

Copyright

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Acknowledgments

In the name of Allah, The Beneficent, The Merciful, I would like to express my utmost gratitude and thanks to the almighty Allah (s.w.t.) for the help and guidance that he has given me through all these years. My deepest appreciation is to my family for their blessings and supports.

I would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum and Minerals (KFUPM) for funding the writing of this book through project No. BW161002.

Some experimental works were previously conducted in University of Malaya under the support of the High Impact Research MoE (Ministry of Education Malaysia), grant: UM.C/625/1/HIR/MoE/ENG/40 (D000040-16001).

I wish to thank Dr. Saidur Rahman, Dr. Amalina Binti Muhammad Afifi, Dr. Fahad Abdulaziz Al-Sulaiman, my other colleagues and researchers for their kind support.

I appreciate the cooperation of the Elsevier team: the Acquisition Editor (Mr. Simon Holt), Editorial Project Managers (Ms. Leticia Lima, Ms. Anna Valutkevich), Project Manager-Production (Mr. Kamesh Ramajogi), and Copyrights Coordinator (Ms. Sandhya Narayanan).

I would like to extend my thanks to Elsevier and other publishers for granting the necessary permission/license to reuse (reprint/adaptation) the copyrighted items.

1

Introduction to Nanofluid

Abstract

This chapter introduces a general idea about nanofluids. It starts with background information on heat flow, then introduces colloidal systems and nanofluids. It includes brief information about colloidal properties: particle structure (size and shape), particle aggregate, polydispersity, and zeta potential. Then, the basics of nanofluids, heat transfer mechanism of nanofluids, properties of nanofluids, including how other parameters (particle size, concentration, and temperature) affect thermophysical properties and types of nanofluid are introduced.

Keywords

Colloid; particle aggregate; polydispersity; zeta potential; nanofluids; heat transfer mechanism; thermophysical properties

1.1 Introduction

The importance of manipulating and controlling substances at a small scale was highlighted by Richard Feynman (Feynman, 1992) in There’s Plenty of Room at the Bottom (Mahbubul, Elcioglu, Saidur, & Amalina, 2017). In this modern era, customers are looking for high-performance equipment, but in a compact size with less weight. Therefore, the optimization of engineering devices is a major concern of many types of research since this approach affects performance and efficiency (Mahbubul et al., 2014). The performance of heat transfer equipment depends on the following equation:

(1.1)

is the heat transfer coefficient (HTC).

Therefore, heat transfer improvement can be made by increasing (1) heat transfer area, (2) temperature, and (3) HTC (Saidur, Leong, & Mohammad, 2011). Case (1) is usually avoided because increasing the heat transfer area will increase the bulkiness (size and weight) of the equipment. Case (2) needs more input power to increase the temperature and as a result operating costs will be increased. Therefore, technologies have already reached their limit for cases (1) and (2). Tremendous researches are on-going for case (3) by changing different parameters. Now researchers are trying to increase the HTC of liquids by mixing solid particles into these liquids. These types of heterogeneous mixtures are called colloidal systems, which are made up of the dispersed phase and dispersion medium. As the addition of solid particles in a liquid increases the viscosity of the suspension, as a result the pumping power and pressure drop increase, and clogging and blockage of the flow passage can also happen. Therefore, nanosized (10−9 m) solid particles (called nanoparticles and mostly in powder form) are proposed to mix with heat transfer fluids to increase their HTC.

1.2 Colloid

The study of physics and chemistry introduces three states of matter: solid, liquid, and gas, as well as the transformations (melting, sublimation, and evaporation) among them (Everett, 1988). Besides the pure substances, there are solutions, which are the homogeneous/heterogeneous dispersion of two or more similar or different species mixed together on a molecular scale. The system of this kind is called colloids, where one component is finely dispersed in another (Everett, 1988). Table 1-1 shows an example of some typical colloidal systems. Previously, Thomas Graham distinguished substances into two types as crystalloids and colloids based on diffusion characteristics. If a substance can directly diffuse a parchment membrane it is a crystalloid, for example, acids, bases, sugars, and salts. On the other hand, if a substance very slowly diffuses through parchment paper it is a colloid, for example, glue. However, these terminologies have been proved to be inappropriate, as with a change of environmental conditions these states can be changed. Hiemenz and Rajagopalan (1997) define colloid as any particle, which has some linear dimension between 10−9 m (1 nm) and 10−6 m (1 µm) is considered a colloid. Nevertheless, these limits are not rigid, for some special cases (emulsion and some typical slurry) particles of larger size are present. Fig. 1-1 shows some real-life examples of nanometer to millimeter scale substances.

Table 1-1

Source: Adapted from Everett, D.H. (1988). Basic principles of colloid science, Royal Society of Chemistry, London with permission from The Royal Society of Chemistry.

Figure 1-1 Examples of nanometer to millimeter scale substances. Reprinted from Serrano, E., Rus, G., and Garcia-Martinez, J. (2009). Nanotechnology for sustainable energy. Renewable and Sustainable Energy Reviews 13, 2373–2384 (Serrano, Rus, & Garcia-Martinez, 2009), copyright (2009), with permission from Elsevier.

Colloid science is an interdisciplinary subject; its field of interest overlaps with chemistry, physics, biology, material science, and several other disciplines (Hiemenz & Rajagopalan, 1997). It is the particle dimension—not the chemical composition (organic or inorganic) or physical state (e.g., one or two phases)—i.e., crucial. The last century has seen a renaissance of interest in colloids (Everett, 1988). Therefore, the important properties of colloids have been identified. Some common physical properties of colloids that are studied to evaluate the dispersion characteristics are now discussed.

1.2.1 Particle Structure (Size and Shape)

Physical dimensions, the defining characteristic of colloids, are considered as the most significant feature of colloidal particles. Particle movement depends on its size and shape. Many other properties (e.g., specific surface area, aggregation behavior, and microstructure) are strongly influenced by the particle dimensions. Thermophysical properties of a suspension also depend on particle size and shape (Baheta & Woldeyohannes, 2013; Timofeeva et al., 2010; Timofeeva, Routbort, & Singh, 2009). The easiest particle structure is considered as uniform-size particles with spherical geometry, however, colloidal particles come in all sizes and shapes.

1.2.2 Particle Aggregate

It is a general phenomenon that the smaller particles of a suspension want to join together and makes greater structures known as aggregates. The interparticle force is considered to be the reason behind this aggregation. Particle size distribution is analyzed to check the aggregate size. Fig. 1-2 shows the effective particle diameter also called cluster or aggregate size of particles, which could be several times larger than a single-particle diameter.

Figure 1-2 An example of the cluster or aggregate size of particles. Reprinted from Mahbubul, I.M., Elcioglu, E.B., Saidur, R., and Amalina, M.A. (2017). Optimization of ultrasonication period for better dispersion and stability of TiO2–water nanofluid. Ultrasonics Sonochemistry 37, 360–367, copyright (2017), with permission from Elsevier.

It is noteworthy that two types of aggregates can be seen in a nanofluid. One type of aggregate happens when nanoparticles are agglomerated in dry powder form. These aggregates are unlikely to be broken apart when nanoparticles are suspended in a fluid with high shear or ultrasound. Another type of aggregate happens after the suspension of loose single crystalline nanoparticles (Timofeeva et al., 2009) and a few aggregated nanoparticles (small cluster) form a larger cluster. The later types of clusters need extra care to separate and in some cases need more than one method (ultrasonication and surfactant) to segregate them. Nevertheless, in most cases, such small aggregates are unavoidable.

1.2.3 Polydispersity

When there are different ranges of particle sizes present in any disperse systems this is referred to as polydispersity. The term polydisperse can be understood from its converse term monodisperse. If all the particles of any disperse system are of (approximately) the same size they are monodisperse (Everett, 1988). Polydispersity indexes range from 0 to 1; where very close or equal to 1, indicates extremely broad size distribution, meaning a polydisperse system, but if it is closer to zero this means only one size of particle is present, which denotes a monodisperse system. Fig. 1-3 shows a schematic illustration of the polydispersity index.

Figure 1-3 Pictorial description of the polydispersity index.

1.2.4 Zeta Potential

This is an electrokinetic phenomenon of colloidal systems. Some other colloidal dispersion characteristics are related to the zeta potential (or electrical charge) of the particles. The interparticle energy can be obtained from zeta potential distribution. This interparticle force is related to the stability of a suspension, which is linked to coagulation and flow behavior. The stability of a suspension can be predicted from its absolute zeta potential value. According to Müller (1996), excellent, physical, and limited stability correspond to values of over 60 mV, 30–60 mV, and below 20 mV of zeta potentials, respectively. Fig. 1-4 shows the relationship between absolute zeta potential values with the stability of a suspension. The sedimentation behaviors of colloidal suspensions and flotation behaviors of mineral ores are also related to the zeta potential (Hunter, 1981).

Figure 1-4 Relationship between absolute zeta potential values with the stability of a suspension.

1.3 Nanofluid

A nanofluid is a combination of nanoparticles in a base fluid to enhance the thermal performance of the fluid. A nanofluid is a colloidal dispersion of a two-phase system where nanoparticles are in solid phase and the base fluid is in the liquid phase. It is a special kind of heat transfer fluid, which has higher thermal conductivity than traditional host fluids (e.g., glycols, water, engine oil, and so on). Nanoparticles can be metals (e.g., copper, nickel, aluminum, etc.), oxides (e.g., alumina, titania, copper oxide, silica, iron oxide, etc.), and other elements (e.g., carbon nanotubes, graphene, silicon carbide, calcium carbonate, titanium nanotubes, etc.) (Mahbubul, Saidur, & Amalina, 2013). Because of the small size and large specific surface areas of nanoparticles, nanofluids possess better heat transfer properties like high thermal conductivity, less clogging in flow passages, long-term stability, and homogeneity (Chandrasekar, Suresh, & Chandra Bose, 2010).

The National Argonne Laboratory was the pioneering organization, which for first introduced the term nanofluids and demonstrated that the use of nanoparticles augments the heat transfer performances of liquids in 1995 (Choi & Eastman, 1995). However, just a couple of years before introducing the term nanofluids, Masuda, Ebata, Teramae, and Hishinuma (1993) investigated the thermophysical properties of some oxide (Al2O3, SiO2, and TiO2) nanoparticles dispersed in water and published a paper in 1993 written in Japanese. Although they (Masuda et al., 1993) used nanometer-sized particles, however, they termed them as ultra-fine particles.

Since then a tremendous amount of research has been on-going about thermal conductivity, viscosity, density, specific heat, different modes of heat transfer, pressure drop, pumping power, different properties of nanofluids (e.g., fundamental, thermal, physical, optical, magnetic, etc.), etc. The most widely used heat transfer fluids, such as water, oil, glycols, and refrigerants, have lower heat transfer properties; however, their huge utilization in the field of heat-transfer, automotive, electronics, and industrial processes requires the reprocessing of these heat transfer fluids, making it important to improve their thermal performance (Mahbubul, Saidur, & Amalina, 2012). Fig. 1-5 shows the increasing trend in research and publications about nanofluids. The data in Fig. 1-5 were collected from the web of science database with the search option nanofluid OR nanofluids on the topic.

Figure 1-5 Publications with nanofluid or nanofluids in the topic of the web of science database until 2017.

1.3.1 Mechanism of Nanofluids

One of the most important things about nanoparticles is the large ratio of surface area to volume. The surface area of about one gram of some types of nanomaterials can be equal to a football field. This means that if one gram of nanoparticles is spread side by side of each particle, then they will take up the area equivalent to a football field. Therefore, this large surface area of particles makes it easy for them to be soluble in liquid, which also increases the thermal conductivity of the suspension (as nanoparticles are chosen with high thermal conductivity).

Fig. 1-6 shows the thermal conductivities of some heat transfer fluids (at 300K) and solid materials (metals and metal oxides). It can be seen from Fig. 1-6 that the thermal conductivity of metallic particles has a much higher value than the thermal conductivity of the fluids (Mahbubul, 2015). Therefore, it is desirable that their mixture will have a higher thermal conductivity than the thermal conductivity of traditional fluids (Murshed, Leong, & Yang, 2008).

Figure 1-6 Thermal conductivities of heat transfer fluids (at 300K) and solid materials (metals and metal oxides).

Terekhov, Kalinina, and Lemanov (2010) mention two main principles of thermal conductivity in solids and fluids: free electrons (electron heat conductivity) and atom oscillations (photon or lattice thermal conductivity). Keblinski, Phillpot, Choi, and Eastman (2002) and Eastman, Phillpot, Choi, and Keblinski (2004) explain four possible mechanisms of the enhanced thermal conductivity of nanofluids. These are Brownian motion, liquid layering at the liquid/particle interface, ballistic phonon transport, and clustering of nanoparticles. However, they showed that the effect of Brownian motion is not significant for nanofluids. Moreover, they highlighted that there could be some other mechanisms for the enhanced thermal conductivity of nanofluids. Chandrasekar and Suresh (2009) published a review with detailed discussion on the heat transport mechanism in nanofluids. They compiled the following mechanisms: Brownian motion, nanolayer, nanoclusters, thermophoresis, and diffusive/ballistic nature of heat transport. Moreover, they added other parameters that enhance the thermal conductivity of nanofluids, which are specific surface area of nanoparticles, dispersion of nanoparticles, and pH of nanofluid. Iacobazzi, Milanese, Colangelo, Lomascolo, and de Risi (2016) investigated the effects of interfacial layering, Brownian motion, clustering, ballistic phonon motion, thermal boundary resistance, and mass difference scattering specifically for Al2O3 nanofluid. The researchers explained that the impact of the interfacial layer on the increase of nanofluid thermal conductivity was not very pronounced. The ballistic phonon motion might be effective on nanofluid thermal conductivity for nanoparticles smaller than 35 nm. The Brownian motion effect was more pronounced for suspensions of nanoparticles in comparison with those that were microsized. Mass difference scattering also caused strong variations in thermal conductivity. A schematic illustration of some mechanisms is shown in Fig. 1-7 (Li, Zhou, Tung, Schneider, & Xi, 2009). Some possible interfacial effects in nanoparticle suspensions are shown in Fig. 1-8 (Timofeeva, 2011).

Figure 1-7 Schematic diagrams of several possible mechanisms. (A) Enhancement of k due to the formation of the highly conductive layer–liquid structure at the liquid/particle interface; (B) ballistic and diffusive phonon transport in a solid particle; (C) enhancement of k due to increased effective φ of highly conducting clusters. Reprinted from Li, Y., Zhou, J.E., Tung, S., Schneider, E., and Xi, S. (2009). A review on development of nanofluid preparation and characterization. Powder Technology 196, 89–101, copyright (2009), with permission from Elsevier.

Figure 1-8 Interfacial effects in nanoparticle suspensions. Reprinted from Timofeeva, E.V. (2011). Nanofluids for heat transfer–potential and engineering strategies. In: Dr. Amimul Ahsan (Ed.), Two Phase Flow, Phase Change and Numerical Modeling, InTech.

1.3.2 Properties of Nanofluids

For the application of nanofluids in engineering and medicine, the basic knowledge of different phenomena is necessary (Banerjee, 2013). The properties of nanofluids also depend on their preparation and application. Since nanofluids are colloidal suspensions, the dispersion behavior of nanofluids can be analyzed from their colloidal properties. Therefore, stability and dispersion properties are characterized by means of sedimentation/photo-capturing method, microstructure (particle size, shape, aggregation, and polydispersity) by transmission electron microscopes (TEMs); cryogenic electron microscopy (Cryo-TEM) by which dispersion is directly monitored; and particle size dispersion, zeta potential, and concentration by UV-visible spectroscopy.

Thermophysical (including rheological) and photothermal properties are characterized for thermal engineering applications. Thermal conductivity, rheology (viscosity and flow behavior), density, specific heat, surface tension, and latent heat are some of the most common thermophysical properties. Thermophysical properties are calculated to determine the performance parameter, for example, HTC, pressure drop, and energy efficiency of a thermal system. Among the thermophysical properties, thermal conductivity is considered as the most important property of any fluid for heat transfer applications. Thermal conductivity is directly related to HTC which is related to the performance of any system. Viscosity is a significant parameter for all heat transfer applications related to fluids (Nguyen et al., 2007). Viscosity is an important transport phenomenon for the design of a chemical process. The performances of heat exchangers are measured by HTC, which is also influenced by viscosity as well as distillation calculation and other heat transfer performances are influenced by viscosity (Smith, Wilding, Oscarson, & Rowley, 2003). The stability of a suspension is related to the density of its particles. Density is needed to calculate the required weight and space (volume) required for a system to operate with nanofluids. It is also necessary for consumer products during packaging and for bottling. The most important influence of viscosity and density is designing a piping system, as pressure drop and pumping power are dependent on these properties of a fluid. Fig. 1-9 shows how different parameters (base fluid type, additives, nanoparticle type, concentration, size, and shape) of nanofluids affect the thermophysical properties of nanofluids. It is better to determine these properties by using precise equipment; however, an approximation can be done by mathematical modeling and correlations.

Figure 1-9 Complexity and multivariability of nanoparticle suspensions. Reprinted from Timofeeva, E.V. (2011). Nanofluids for heat transfer–potential and engineering strategies. In: Dr. Amimul Ahsan (Ed.), Two Phase Flow, Phase Change and Numerical Modeling, InTech.

Absorption, transmittance, emission, extinction, and scattering are examples of optical properties of nanofluids, which are mostly related to the solar thermal application. Similar to thermophysical properties, determination of the optical properties using precise equipment is better, however approximation can be done using mathematical modeling and correlations.

1.3.3 Types of Nanofluid

The types of nanofluids depend on the types of nanomaterials and base fluids.

1.3.3.1 Types of Nanoparticles

Nanoparticles are defined based on their size. A nanoparticle has at least one dimension of the particle within nanometer (10−9 m) size (Mahbubul et al., 2014). Nanomaterials can be nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheets, or droplets (Yu & Xie, 2012). Further, nanoparticles are classified based on their root. Some common and mostly used nanoparticle groups are metallic, metallic oxides, and carbon-based compounds.

1.3.3.2 Types of Base Fluids

Water is the most widely used heat transfer fluid. Therefore, in nanofluid study, it is again the number one base fluid. Different types of glycols: ethylene (mono, di, tri), propylene, etc. are mixed with water in most cases as antifreeze agents to increase the freezing as well as boiling point of water mixtures. Different types of oils are also used as heat transfer fluids, especially for high heat transfer applications like in a concentrated solar power (CSP) system. Refrigerants are also promising for use as the base fluid for nanoparticles. Although there are some researchers such as Bi, Guo, Liu, and Wu (2011), Bi, Shi, and Zhang (2008), Mahbubul (2015), Peng, Ding, Jiang, Hu, and Gao (2009), and Ding, Peng, Jiang, and Gao (2009) have discussed nanoparticles with refrigerants termed nanorefrigerants; real-life applications are still in the laboratory research scale. Phase change materials (PCMs) are also a promising medium to disperse nanoparticles (Kibria, Anisur, Mahfuz, Saidur, & Metselaar, 2015). Usually the mixture of PCMs and nanomaterials is called nanoenhanced PCM (NEPCM).

1.3.3.3 Hybrid Nanofluid

Hybrid nanofluids are mixtures of two or more different types of nanoparticles to achieve better properties. Where in classical nanofluids the mixture is in two phases (base fluid and nanoparticles), a hybrid nanofluid is the mixing of three phases (base fluid and two types of nanoparticles).

1.4 Scope

Chapter 8 will cover the detailed applications of nanofluids. Here, some related engineering application areas are listed (but not limited to) engine cooling, heating and cooling of buildings, cooling of electronics, solar water heating, chillers, refrigeration, thermal storage, drilling, lubrication, coolant in machining, fuel cell, and many others (Saidur et al., 2011).

Nanofluids have potential in several major biomedical applications, such as cell labeling/cell separation, tissue repair, drug delivery, magnetic resonance imaging, hyperthermia, magnetofection, cancer therapeutics, cryoconservation, nanocryosurgery, sensing-imaging, and many others (Gupta & Gupta, 2005).

Therefore, high-volume, low-cost, well-dispersed, industrial-scale nanofluid production has a great opportunity for contributing to nanotechnology (Das, Choi, Yu, & Pradeep, 2007).

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2

Preparation of Nanofluid

Abstract

This chapter introduces the importance of nanofluid preparation on stability and dispersion of nanoparticles. One-step and two-step nanofluid preparation methods are discussed broadly, with examples and comparative analyses. Under the one-step (single-step) method, different physical and chemical methods used by researchers are compiled and presented with a schematic illustration and chemical reaction schemes. The quality of some nanofluids prepared by the one-step method is included in terms of dispersion and stability. A basic two-step (double step) method is exemplified by a simple diagram. Several two-step methods (ultrasonication, high-pressure homogenizer, mechanical stirrer, shaker) used by researchers are tabulated, discussed, and compared in terms of nanofluid quantity and quality. Two types of ultrasonication process are discussed with illustrations. The effect of bulk heat generation by ultrasonic horn is also described, with an example of evaporation effect and heat generation analyses. Finally, the dispersion stability of nanofluids prepared by one-step and two-step methods is compared.

Keywords

Nanofluids; preparation methods; physical methods; chemical methods; ultrasonic vibration; stability; dispersion; high-pressure homogenizer; single step; two step

2.1 Introduction

According to Mahbubul, Elcioglu, Saidur, and Amalina (2017), in order to achieve a true nano behavior and to have higher thermal conductivity, stable properties, microchannel cooling without nanoparticle clogging, reduced erosion probability, and reduction of pumping power—in comparison to suspensions of larger particles, nanoparticles should be monodispersed (not agglomerated) within the suspensions (Das, Choi, & Patel, 2006). The stability of nanofluids is of critical importance in this respect (Mahbubul et al., 2017). The performance and capability of nanofluids depend on their dispersion properties, which are related to the preparation method. For practical application of nanofluids, it is necessary that the nanoparticles be uniformly dispersed in fluids to make a stable suspension (Lee et al., 2008). If the nanofluids are not stable, clogging, aggregation, and sedimentation can happen, reducing the performance of suspensions by decreasing thermal conductivity and increasing viscosity. According to Everett (1988), It is a fundamental principle of thermodynamics that, if a system is kept at a constant temperature, it will tend to change spontaneously in a direction which will lower its free energy. This is exemplified by the simple mechanical case of a weight that falls under the influence of gravity.

Ghadimi, Saidur, and Metselaar (2011) discussed a different aspect of sedimentation with the help of Stokes law (Hiemenz & Rajagopalan, 1997): in a stationary state, the sedimentation velocity of small spherical particles in a liquid follows Eq. (2.1).

(2.1)

is the acceleration of gravity. This equation reveals a balance of the gravity, buoyancy force, and viscous drag that are acting on the suspended nanoparticles. According to , no sedimentation will take place because of the Brownian motion of nanoparticles (diffusion). However, smaller nanoparticles have a higher surface energy, increasing the possibility of nanoparticle aggregation. Thus, stable nanofluid preparation strongly links up with applying smaller nanoparticles as well as to prevent the aggregation process concurrently (Wu, Zhu, Wang, & Liua, 2009) (this paragraph is adapted from Ghadimi et al. (2011), copyright (2011), with permission from Elsevier).

Preparation of nanofluids is not simply the mixture of solid particles into base fluids. Generally, two techniques have been using to prepare nanofluids: (1) a one-step (single step) method and (2) a two-step (double-step) method.

2.2 One-Step Method

When both the manufacturing of nanoparticles as well as the mixture of nanofluid is done in a joint process this is called a one-step method (Eastman, Choi, Li, Yu, & Thompson, 2001; Zhu, Lin, & Yin, 2004). Some commonly used techniques for the one-step method of nanofluid preparation include: direct evaporation method, also called vacuum evaporation onto a running oil substrate (VEROS) technique (Akoh, Tsukasaki, Yatsuya, & Tasaki, 1978; Wagener, Murty, & Günther, 1996), physical vapor deposition (PVD) technique (Eastman et al., 2001), and liquid chemical method (Zhu et al., 2004). The one-step method has both merits and demerits. The most important advantages are the enhanced stability, uniform dispersion, and minimized agglomeration because drying, storage, and transportation of nanoparticles are avoided in this process. The drawbacks of this method are the limited quantity of production due to the slow production process, only low-pressure fluids can be synthesized by this process, and a low concentration of nanoparticles is dispersed in most cases (Mohammed, Al-aswadi, Shuaib, & Saidur, 2011).

2.2.1 Physical Method

Some examples of nanofluid preparation by using one-step physical method are described here.

Chang, Jwo, Fan, and Pai (2007) prepared TiO2 nanoparticles and dispersed in water by a one-step physical method. They used ultrasonic-aided submerged arc nanoparticle synthesis system. A schematic of the synthesis system is reprinted in Fig. 2-1. It can be seen in Fig. 2-1 that the setup consists of a few systems (ultrasonic, heating, temperature, and pressure control). Titanium bulk (rod) was used to produce nanoparticles, and is submerged in dielectric liquid (deionized water) in the vacuum chamber. Electrical energy was used as the heat source to generate enough arc with temperatures of 6000–12,000°C, where titanium is melted and vaporized as well as deionized water is also vaporized. A high vapor pressure is created by inertia force of the surrounding deionized water due to the narrow effect and help quick removal of the vaporized metal. Then, the vaporized metal is condensed in the deionized water within the vacuum