MECHANICAL ENGINEERING SERIES - Enhanced Heat Transfer by Sciencenetwork

MECHANICAL ENGINEERING SERIES – Enhanced Heat Transfer Volume 1

Publisher: Science Network ISBN: 978-0-9869544-5-7 First published in January, 2012 Printed in Canada

A free online edition of this book is available at www.sciencenetwork.ca Additional hard copies can be obtained from reprint@sciencenetwork.ca

MECHANICAL ENGINEERING SERIES

Enhanced Heat Transfer Volume 1

 Science Network Online Open Access Publisher

AUTHORS

Hai-Yan Li Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Bejing 100190, China

Hai-Yan Li obtained her B.E. degree in Thermal Energy and Power Engineering from Tianjin University of Commerce in 2007, and then received her M.E. degree in Refrigeration and Cryogenic Engineering from University of Science and Technology Beijing in 2010. She has been studying for her Ph.D. degree in the Key Laboratory of Cryogenics from Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences since September of the same year. Her research interests are focused on refrigeration and low temperature engineering, waste heat recovery and utilization, as well as new energy and energy efficient technology. She has published several journal papers in the above fields.

Jing Liu Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Bejing 100190, China; Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China

Jing Liu is a professor with the Department of Biomedical Engineering of Tsinghua University (THU) and the Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences (CAS). He simultaneously received his double degrees (B.E. in Power Engineering and Control and B.S. in Physics) in 1992, and Ph.D. in Thermal Science with speciality on bioengineering in 1996, all from THU. He then served as assistant professor there, a postdoctoral research associate at Purdue University, and a senior visiting scholar at Massachusetts Institute of Technology. He has been a professor of CAS since July 1999 and a professor of THU since August 2008. Dr. Liu has authored eight popular books on cutting edge research frontiers in energy and bioengineering areas (among which Micro/Nano Scale Heat Transfer published in 2001 has been reprinted five times), over ten invited book chapters, and over two hundred peer reviewed journal papers and numerious international conference presentations or invited lectures. Dr. Liu authored more than 100 patents. His two latest books Unconventional Energy Technologies and Advanced Technologies for Low-cost Medicine, which again received intense attentions, especially address the ever tough issues facing the current scientific society. Among his numerous contributions, Prof. Liu initiated and systematically developed a group of non-conventional technologies through introducing the room temperature liquid metals and their alloy, used to be neglected before, into modern high-tech areas. This led to many new breakthrough solutions to tackle

the latest and future energy issues such as waste heat harvesting/transport in a wide range of temperatures, electricity generation in either small or large size and powerful thermal management for many advanced electronic or optical chips. Except for his significant contributions to energy issues, Dr. Liu’s work is also fully reflected in the biomedical engineering field. He defined and proposed the concept of nano-cryosurgery which initiates a new “green” tumor therapy, and invented many medical devices especially the hybrid cryosurgical/hyperthermia system for targeted tumor ablation and interventional whole body hyperthermia equipment for treating cancer metastasis. He contributed significantly to the bioheat transfer study through several book publications covering both high- and low-temperature medicine and is a world-renowned expert in the areas. His latest book entitled “Biomedical Engineering on a Mobile Phone: Principle and Application” is dedicated to outline a brand new frontier, which is expected to be a pervasive way to better solve the global health issues. Prof. Liu is a recipient of the National Science Fund for Distinguished Young Scholars of China, National Science and Technology Award for Chinese Young Scientist, Mao Yi-Sheng Science and Technology Awards for Beijing Youth, five highest teaching awards from CAS, etc. His current research interests include green energy technology, thermal management, micro/nano fluidics and system, bioheat transfer, and mobile energy and biomedical technologies.

Maryamalsadat Lajvardi Anwar Gavili Fatemeh Zabihi Taghi Dallali Isfahani Iraj Hadi Jamshid Sabbaghzadeh

National Center for Laser Science and Technology, Tehran, Iran

Table of contents

Chapter 1 - Enhanced Heat Transfer by Room Temperature Liquid Metal Abstract

Introduction

1. Brief on conventional enhanced heat transfer

2. Enhanced heat transfer by liquid metal

2.1. Reducing thermal interface resistance

2.2 Enhancing convective heat transfer

3. Fundamentals in room temperature liquid metal heat transfer

4. Practical issues related to liquid metal heat exchanger

5. Future work

Conclusion

References

Chapter 2 - Experimental and theoretical investigations on the heat transfer of ferrofluid under magnetic field Abstract

Introduction

Preparation of ferrofluids

1.1.

Synthesis

1.2.

Experimental

1.2.1. Material and ferrofluid preparation

1.2.2. Characterization

Thermal conductivity of ferrofluids

2.1.

Experimental setup

2.2.

Effect of magnetic field intensity

2.3.

Effect of temperature

Heat transfer of ferrofluids

3.1.

Theory

3.2.

Experimental setup and calibration

3.3.

Effect of magnetic field intensity

3.4.

Effect of concentration

3.5.

Effect of the magnetic field position and different heat fluxes

3.6.

Heat transfer enhancement

Conclusion

References

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Chapter

1 Enhanced Heat Transfer by Room Temperature Liquid Metal Hai-Yan Li, and Jing Liu Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Bejing 100190, China E-mail :

jliubme@mail.tsinghua.edu.cn

eat transport enhancement plays an ever increasing role in many advanced optoelectronic devices and power systems. The currently available thermal management may encounter technical limit if not innovated enough. Among the many efforts ever made to find new solutions, the liquid metal with melting point around room temperature is emerging as an outstanding heat transfer medium to overcome the â&#x20AC;&#x153;thermal barriersâ&#x20AC;? challenging many conventional ways. This chapter is dedicated to present an overview on the science and art of the enhanced heat transfer enabled by room temperature liquid metal. Particularly, two major directions will be discussed: 1. Reducing heat conduction resistance between two contacting objects using liquid metal as thermal interface material; 2. Enhancing convective heat transport through the flow of liquid metal. The basic features for the enhanced heat transfer by liquid metal will be outlined. Important fundamental and practical issues will be summarized. Some mechanisms behind such heat transfer modality will be interpreted. Future directions worth of pursuing will be suggested.

Introduction It is evident that in the past few decades heat generation in individual power devices, integrated circuits and even complete electronic systems are increasing dramatically, which has prompted the urgent need to improve heat transfer performance. These devise, such as electronic components, computers and base 1

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stations, usually produce high heat flux of 100â&#x20AC;&#x201C;1,000 W/cm2 [1]. Therefore, an effective thermal management is essential to maintain their temperatures under an appropriate value so as to ensure optimum performance and reliability. Over the past few years, significant advances have been made in the enhancement of heat transfer to manage increased heat fluxes. However, most of the currently available thermal management techniques are generally encountering bottleneck with the ever increasing power density of electronic component and the more compact package technology in power systems, leading to the extremely high power in many electronic devices, as presented in Figure 1.

Figure 1 Heat flux and power of electronic products and devices

In general, the enhancement of heat transfer means an increase in heat transfer coefficient [2]. Compared to the traditional air-cooled scheme, the liquid-cooled approach would display much better performance for electronic components, offering significant improvements in circuit density and heat removal capability [3]. Direct or indirect liquid cooling by use of dielectric liquids, including water 2

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and several organic liquids, has long been considered as promising cooling schemes [4]. However, the continuous increase in power for electronic devices has brought about tough challenges for the thermal management of electronic packaging, leading to the necessity for finding new coolants with the higher thermal conductivity, which is expected to result in superior heat transfer coefficients. This chapter will concentrate on illustrating a newly emerging method leading to heat transfer enhancement of electronic, optoelectronic devices and power systems that are of special interest with the fluid-cooled scheme. The room temperature liquid metal with high thermal conductivity is presented to replace dielectric liquids in order to enhance heat transfer and thus better serve for the need of many high-tech areas.

1- Brief on conventional enhanced heat transfer

Aiming to provide a background for the readers to compare and digest the new method, we first briefly review in this section the state of the art of conventional enhanced heat transfer in electronics industry, which can be mainly classified into two categories: enhanced heat conduction and enhanced heat convection. Among the many heat transfer strategies ever developed, a heat pipe or heat pin invented in 60th of last century is a device that can quickly transfer heat from one point to another. Its working fluids generally include water, ethanol, acetone, sodium, or mercury, which are chosen depending on the working conditions of the thermally managed system. At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid in contact with the thermally conductive solid surface turns into vapor by absorbing heat there. The vapor condenses back into liquid at the cold interface, releasing the latent heat. The liquid then returns to the hot interface through either capillary force or gravity action where it evaporates once more and repeats the cycle [5], as shown in Figure 2. Heat pipes are often referred to as the 3

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"superconductors" of heat as they possess an extraordinary heat transfer capacity and rate with almost no heat loss. They have been, and are currently being, studied for a variety of applications [6], including almost the entire spectrum of temperatures encountered in heat transfer areas, such as space crafts, airconditioners, refrigerators, heat exchangers, transistors, capacitors and computer systems, etc.

Figure 2 Principle of the heat pipe thermal cycle (Modified from Ref. [5]) Heat transfer between solid interfaces can be more complicated, where it is desired to have a minimal thermal resistance between the two surfaces in contact [7â&#x20AC;&#x201C;9]. The thermal contact resistance varies considerably depending on the interface geometry [10]. Usually, only a small area would have actual mechanical contact between the two surfaces at the interface due to existence of waviness and roughness, which will have significant impact on the heat conduction across the interface, as there will be gaps filled with low thermal conductivity air. In order to minimise the thermal contact resistance, filler materials are therefore often required to enhance the contact between the mating surfaces. These materials are well known as thermal interface materials (TIM), as shown in Figure 3.

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Solid 1 Solid 2

TIM

Figure 3 TIM filling gaps between solid interfaces

The performance of a thermal interface material depends on its conformability, thermal conductivity, and thickness [11]. Currently available TIMs include thermal greases and gels, elastomeric pads, thermal tapes, phase change materials, thermally conductive adhesives and soft metals [12, 13]. Most TIMs, except pure metals, are composed of a polymer matrix (the most common matrix is silicone) filled with thermally conductive particles [14]. Such materials provide a relatively high thermal conductivity, but none of them is able to attain the theoretical limit of conductivity that the fillers possess, primarily because of the high interfacial thermal resistance between the particles and the polymer matrix in the heterogeneous mixture. As a result, the bulk thermal resistance due to heat conduction within the material becomes a bottleneck as interface thickness increases [15,16]. So far, research on TIMs has focused on improved gels [17], the use of more advanced fillers such as graphite nanoplatelets [18], and the fabrication of TIMs using carbon nanotube arrays (CNTs) [19â&#x20AC;&#x201C;21]. A thermal interface resistance as low as 19.8 mm2KW-1 has been reported [20]. The thermal contact resistance can be reduced by coating the tips of the CNTs with metals [22], or by using thermocompression bonding of CNTs [23]. The heat convection techniques are the most common enhancement ways in various heat transfer application, which can be divided into two categories according to Bergles [24]: namely active and passive techniques. The active techniques require external forces, while the passive ones need no direct application of external power. Table 1 listed most of the currently available active and passive techniques. Overall, their effectiveness is strongly dependent on the mode of heat transfer.

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Table l Active and passive techniques [25-27] Active techniques

Passive techniques

mechanical aids

treated

surfaces

(coatings

and

promoters) surface vibration

rough surfaces

fluid vibration (including ultrasound)

extended surfaces

electrostatic fields

displaced enhancement devices

other electrical methods

swirl flow devices

magnetic fields

surface tension devices

acoustic fields

porous structures

suction or injection

additives (for gases and liquids)

jet impingement

coiled tubes

Rotation

surface catalysis

induced flow instabilities (e.g. pulses)

grooves and rivulets

Currently, the application of active techniques is limited by high power consumption and inconvenient under compact situations, as well as cost, noise, safety or reliability associated with the enhancement device [28]. For passive techniques, their inherent limitation lies in the relatively low thermal conductivity of heat transfer fluids [26]. Therefore, two or more of the above techniques may be utilized simultaneously to produce an enhancement and diminish the limitations shown when the individual techniques are applied separately, i.e. compound enhancement [29], which is called ‘‘third generation’’ heat transfer technology [30].

2- Enhanced heat transfer by liquid metal 2-1- Reducing thermal interface resistance

The effective total thermal interface resistance between two materials is a sum of the resistance due to thermal conductivity of the TIM and contact resistance between TIM and the two contacting surfaces. It is generally expressed as [12]: 6

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Reffective =

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BLT + Rc1 + Rc 2 kTIM A

(1)

where, BLT and kTIM are the bond line thickness and thermal conductivity of the interface material respectively and A is the area; Rc1 and Rc2 are contact resistances of the TIM at the boundary with the two surfaces. The value of Reffective can be minimised by reducing the bond line thickness and employing a TIM with the highest possible thermal conductivity, as well as reducing surface roughness and applying pressure to reduce contact resistance [31]. The surface roughness of contacting surfaces depends on the technical level of the manufacture, which is difficult in the short run to improve. Therefor, the choice of a TIM with high thermal conductivity is crucial. Liquids have low interfacial resistance because they wet a surface forming a continuous contact with a large area. Most liquids, especially convectional heat transfer fluids, such as water, oil and ethylene glycol do not, however, have very high conductivity. Solids, and in particular metals, have very high conductivity but high interfacial resistance [32]. The liquid metal and its alloy with a low melting temperature, potentially offering both low interfacial resistance and high conductivity, can be a very promising thermal interface solution. It can reduce thermal interface resistance of a metal-to-metal joint by a factor of 10 [33]. Thermal performance of such an interface could become more than one order of magnitude greater than many adhesives typically in use [32]. Common candidates for such metals include gallium, gallium-based alloys, mercury and potassiumsodium alloy, etc. In general, alloys comprised of the fewest number of constituents possess the highest thermal conductivities, the composition of which is chosen for a desired melting point. Such metals melt below the operating temperature of the device and remain at liquid state so as to flow into all the surface asperities of the interface. Figure 4 shows the evolution of common TIM materials and the potential for liquid metal alloys (LMAs) to satisfy the continuously shrinking thermal budgets of the next generation of electronics components. 7

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Figure 4 Evolution of common TIM materials (Modified from Ref. [34])

Since liquid metals are liquid at the critical operating condition, low contact pressure is required to force the liquid metals into the interstitial voids. Another advantage lies in its ease in disassembly of the joint after cool down since many liquid metals do not adhere to surfaces unless they are clean and free of oxidation [33]. furthermore, since they contain no organic materials, they do not require curing during application [31]. Liquid metals however have their drawbacks. Drip-out can be a problem particularly in vertically oriented interfaces [32]. Thermal cycling has been shown to lead to creation of voids in the interface. Intermetallic growth, oxidation / corrosion also lead to the gradual decrease in performance and the eventual thermal failure [32]. Corrosion may however be minimised by using a near hermetic seal between the package lid and package substrate. So far, several liquid metal TIM products have been marketed, targeting the CPU applications with their melting points approximately 20째C below the maximum operating temperatures of typical microprocessors: Enerdyne Solutions IndigoTM [35] and Thermax HiFlux [36] are two examples of such products.

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2-2- Enhancing convective heat transfer

The conventional methods to enhance convective heat transfer could increase the cooling rate to some extent. However, the inherently poor thermal properties of traditional heat transfer fluids such as water, ethylene glycol or engine oil greatly limit the cooling performance of the thermal management system. Thus, these conventional cooling techniques are generally not suitable to meet the cooling demand of many high-tech industries [37]. It is well known that metal possess order-of-magnitude larger thermal conductivity than nonmetal, so one might anticipate that liquid metals can be used to do the cooling task. For example gallium has superior thermal features that render it to be excellent for such a purpose [38]. The convective coefficient of liquid metal is much higher than that of water and can be over 1× 105 W/m2· °C due to its high thermal conductivity [39]. The increase in the convective coefficient would consist of: (a) The reduction in the surface size of the heating elements for given ratings, which is beneficial for compact package. (b) The increase in heat emission rates through a given size of heating elements to prevent overheating of the devices. Moreover, the electrically conductive feature of liquid metals allows it to be driven by electrical or magnetic methods. Thus, higher driving efficiency and absolutely silent operation characteristic can be achieved due to no moving parts. Except for mechanical force [40], the electrical or magnetic fields [41, 42], other heat convection enhancement techniques as mentioned in the last section could also be utilized in liquid metal cooling case so as to further produce an performance improvement.

3- Fundamentals in room temperature liquid metal heat transfer

Compared to conventional heat transfer fluids, liquid metals with low melting point are especially desirable for thermal management of high power density devices due to their high thermal conductivity and electrical conductivity, low 9

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vapor pressure, low dissolution in water and extensive temperature range over which they remain in the liquid phase [38]. Thermal conductivities of water (0.607W/mK) and engine oil (0.145W/mK) are about 50 times and 200 times, respectively smaller than that of liquid metals (e.g. 29.4 W/mK for gallium), which accounts for high heat transfer rates of liquid metals. The heat transfer to liquid metals significantly differs from the heat transfer to water. The main reason for such difference is that liquid metals have a very low Prandtl number (Pr), e.g. ratio of the viscosity to thermal diffusivity [43], which is of low order of 10-2 to 10-3 compared to water (Pr = 6.8), due to the very high thermal diffusivity and very low kinematic viscosity of liquid metals. In other words, the contribution to the total heat transfer from the thermal conductivity (compared to the contribution from the convection) is much higher for liquid metal compared to water [43]. Furthermore, high electrical conductivity enables liquid metals to be pumped efficiently with compact magneto-fluid dynamic (MFD) pumps, which are characterized by silent, vibration-free, and low energy consumption rates [44, 45] because of the absence of moving components. Liquid metal can even realize high efficiency self-driving through its thermosyphon effect and will offer better cooling capability than that of water under the same situations [46, 47]. Also, the normal boiling points of liquid metals are generally high, and hence the liquid metal systems can be operated at near atmospheric pressures, and liquid metals can remain in liquid state at higher temperature compared to conventional fluids like water and various organic coolants which makes the design of a compact heat exchanger possible. The key advantages of using liquid metal as the coolant is that they do not require operation at high pressure in order to obtain high temperatures and usually, the melting temperatures are low enough such that they can be used as coolants in thermal devices, such as compact heat exchange systems. As a result, their use usually involves pressure which is small compared to thermodynamic critical pressure of the fluid [48].

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Since conventional fluids have relatively small viscosities, their application as coolants or heat exchange fluids almost always involves turbulent flow. When the flow is turbulent, molecular and turbulent diffusion of heat in the direction of the duct axis are neglected. With the laminar flow of liquid metals, however, the assumption is not justified. Because Peclet numbers (i.e., the product of the Prandtl and duct Reynolds number) are less than unity, convective transport is small relative to molecular diffusion and axial heat conduction may play important role [49, 50]. The important difference between liquid metals and nonmetallic fluids in turbulent flows is that temperature distributions in the latter are relatively insensitive to local changes at the duct walls and thus to boundary conditions and to duct shape. Liquid metal temperature distributions however can be quite sensitive to boundary conditions [48]. Among various kinds of liquid metals, mercury has been used in some earlier versions of fast breeder reactors. It was often adopted as a thermal working fluid in macro scale systems for many years, before its dangers were well understood. The details refer to Ref [51]. Sodium and potassium are also very good cooling fluids, and NaK (sodiumpotassium alloy) has been widely used in fast breeder reactors. Sodium is a normal coolant used in large power stations, and both lead and NaK have been used successfully for smaller generating rigs. However, they are quite reactive to air and water, and are therefore considered fire hazards. Recently, gallium and its alloys have been proven to be perfect working fluids for room-temperature appliances especially computer chip, LED lamp etc. [38, 42, 44, 52]. Gallium (Ga) belongs to Group III of the periodic system and falls in the category of naturally scattered elements. Its content in the earth crust is 15 g/t. Gallium can be extracted from the aluminum-containing mineral, products of leadâ&#x20AC;&#x201C;zinc production, coal combustion products, etc. Yet, the main source of gallium production is provided by alkali solutions in the production of alumina [53]. It has a high specific heat capacity per unit volume, a low vapor pressure at room temperature, a less reactive nature when exposed to oxygen and water, and a 11

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high surface tension which impedes leakage, and these features warrant the future applications of gallium in the thermal management of high heat flux density area. Gallium forms alloys characterized by specific properties with the majority of metals and metalloids. Many of the gallium-based alloys are low-melting; the physical and mechanical properties of some metals are enhanced when seeded with gallium [53]. Gallium-based alloys mainly consist of gallium, indium, and tin, or two of the three types. Gallium and its alloys are used as a replacement for many applications that previously employed toxic liquid mercury or reactive NaK because of their low toxicity and the low reactivity of its component metals. Furthermore, all liquid metals do not behave similarly for forced convection case [54]. Although an amalgam of magnesium and mercury was proved to conform to the same laws as the boiling of nonmetallic liquids [55], the indications are not clear for gallium [51]. Gallium has much higher evaporation point compared to mercury and water (2204.8°C for gallium, 356.65°C for mercury, and 100°C for water). Therefore, it can remain in a single liquid phase within a wide temperature range, which makes it possible to break the restriction of the critical heat flux and thus effectively prevent devices from burning out [51]. Liquid metal with low melting point, as a novel enhanced heat transfer fluid, can also combine the heat transfer enhancement techniques utilized by conventional fluids to further magnify the conductivity. Adding nano particles with superior conductivity to the liquid metal or its alloy is an example [56, 57]. Although the solid nanoparticles with typical length scales of 1–100 nm with high thermal conductivity have been shown to enhance effective thermal conductivity and the convective heat transfer coefficient of the base fluid [58], the enhanced rates are limited by the relatively poor thermal conductivity of conventional base fluids (e.g. water and ethylene glycol) and the low particle concentration allowable for suspension. With an extremely high conductivity however relatively small viscosity, the liquid metal could serve as an idealistic base solution for making highly conductive nano fluid [51, 56] (Table 2). This nano fluid shows a very

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promising future for the thermal management application in super-CPU chip cooling or the situations requesting seriously high heat flux removal. Liquid metals have superior thermal properties compared to conventional heat transfer fluids, which can further intensify the traditional enhancement measures, such as turbulence, vortices, vibration, and heat pipe [52]. The authors believe that, the integration of liquid metal and traditional enhancement measures would further produce a new compound enhancement, which may be called â&#x20AC;&#x153;forth generationâ&#x20AC;&#x2122;â&#x20AC;&#x2122; heat transfer technology. As a promising heat transfer fluid, there still exist some controversial issues about liquid metals, which need further research. For example, an underlying universal assumption in most past experimental efforts had been that the liquid metal systems exhibit a Newtonian flow behavior, which assumes that at any given temperature there exists a linear dependency of the applied shear stress to the shear rate experienced by the liquid and the slope of this dependency would represent the constant shear viscosity [59, 60]. However, recent effort [61] has suggested that certain liquid metal systems show tendencies of non-Newtonian flow behavior. Another instance is about the effect of magnetic field on enhancement of heat transfer. It is generally accepted that the flow of liquid metal is suppressed by the Lorentz force under a magnetic field. However, enhancement of heat transfer rate under a weak magnetic field has also been reported.

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Table 2 Thermal conductivities of various solids and liquids [51, 56] Solids/liquids

Material

Thermal conductivity (W/m K)

Metallic solids

Silver

429

Copper

401

Aluminum

237

Diamond

3300

Carbon nanotubes

3000

Silicon

148

Alumina (Al2O3)

Mercury

8.34a)

Tin

15.08b)

Indium

36.4c)

Sodium

86.9d)

Gallium

29.4m)

Potassium

54.0 n)

Nonmetallic solids

Metallic liquids

Nonmetallic liquids Water

0.613

Ethylene glycol (EG) 0.253 Engine oil (EO)

0.145

Notes: a) 298K; b) 473K; c) 433K; d) 373K; m) 323K; n) at melting point.

4- Practical issues related to liquid metal heat exchanger

Liquid metal heat exchangers are not new in some very special situations. Liquid metals have been used as coolants in nuclear reactors for many decades due to their ideal heat transport properties [62]. The potential use of liquid metals as working fluids in the power plants for space application also led to quite intensive research related to the vaporization of liquid metals such as mercury [48]. But most of the former trials are focused on either high melting point alloy such as Na, K or mercury which is however highly toxic. For most newly emerging industrial fields such as high performance computer, data center, LED lighting, 14

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heat recovery and removal in advanced power and energy systems, implementation of liquid metal heat transfer appears rather appealing and innovative in either theoretical or technical concepts and opens tremendous new opportunities to tackling the ever tough thermal management issues raised in such areas. It is some what a surprise to notice that, investigations on the room temperature liquid metal such as gallium or its alloy and innovating their roles in many daily life appliances as outlined above reserved to be silent until recent years [38]. The unique features of such metal fluids, especially facing the high flux heat transfer, are compelling enough to guarantee their usage as the coolants for the heat exchanger in many important fields, especially electronic and power engineering [51]. As the complexity and power level in the electronic devices continues to grow, improved technologies are intensely required to effectively remove the heat generated [57]. In order to manage this escalated need for heat dissipation, electronic packages have significantly improved thermally and heat sinking devices have grown larger and more efficient. However, improvements in interface thermal resistance failed to keep pace with innovations in other packaging components [16]. The demand for commercial packaging solutions which further reduce interface thermal resistance has been identified by both the International Technology Roadmap for Semiconductors [63] and National Electronics Manufacturing Initiative [64] as one of the key factors in the continuing push by the electronics industry for system miniaturisation and performance gains. Herein, one key long-standing issue to attain a good thermal contact is to find a thermal interface material with high thermal conductivity. Present TIMs typically have relatively lower thermal conductivity, approximately 3 â&#x20AC;&#x201C; 7 W/mK for very specialized materials [16]. Liquid metals, by contrast, with a much higher thermal conductivity (e.g. 29.4 W/mK for gallium), may be regard as an excellent solution. They provide good thermal contact between the two surfaces (such as the surface of a heat sink and the surface of a printed circuit board) and much higher thermal conductivity. In this respect, IBM sets an 15

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example in photovoltaic technology [65]. A very thin layer of a liquid metal made of a gallium and indium compound is applied between a chip and a cooling block. Such layer, called thermal interface layer, fills the gap between the two contacting interfaces. The layer reduces thermal resistance in the area, thereby resulting in further maintenance of the chip at lower temperatures. Furthermore, Ma and Liu proposed the concept of nano liquid-metal fluid, which may lead to the development of a further conductive paste with the highest conductivity as a liquid in narure [56]. It should be noted that liquid metals are electrically conductive. Therefore they can be used for conducting heat and/or electricity between non-metallic and metallic surfaces. For the case which needs to avoid electricity conduction, they also can be used subject to an appropriate insulating treatment. Although liquid metal cooling has been proposed for many decades, and proven to be an effective heat dissipation method, practical liquid metal heat exchangers or heat sinks are rarely applied in commercial application up to now. This may partially be attributed to the relatively high price of liquid metals and large amount of coolant charge in conventional system. To solve this issue, Deng and Liu proposed a novel solution, i.e. hybrid liquid metal–water cooling system, which utilize a liquid metal ‘‘heat spreader’’ in front of the water cooling module to fabricate a two stage compound cooling system. In this way, the system not only has high convective heat transfer coefficient in the cold plate, but also reduces the cost to be acceptable [1]. Besides water, other coolants such as ethylene glycol can also be combined with liquid metals to constitute a hybrid system. The other obstacle blocking large scale commercialization and utilization of liquid metal heat exchangers is the strong lack of the fundamental design theory and the economic feasibility [51, 52], which are of great concern to guide the device, design, and determine the commercial prospect of liquid metal cooling products. As demonstration, the authors’ group had filled the vacuum by design and implementation of a few practical liquid metal cooling devices for heat dissipation of high performance CPUs, part of which are shown in Figure 5. 16

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Figure 5 Two working samples developed in the authorsâ&#x20AC;&#x2122; lab for liquid metal chip cooling

Although the liquid metal heat exchanger cannot be regarded as the best solution to replace all the conventional heat exchangers in the market, it does opens a highly promising option for many advanced industrial areas [51]. In the extremely high heat flux applications, such as large power lasers, high profile chips and high brightness LED, the liquid metal heat exchangers are expected to show more apparent advantages and, thus, are more practical.

5- Future work

Although the research related to the utilization of liquid metal as heat transfer media has begun since nearly half a century ago, technological interests derives mainly from the potential use of liquid metals as working fluids in the nuclear reactors. Within an extremely long period of time, researches basically are limited to those specific liquid like sodium, potassium or their alloy, and mercury, which have either high melting temperature or are just toxic and dangeous. Extensively applying the room temperature liquid metals especially gallium and its alloy as the new coolant and heat transfer medium for the thermal management of computer chip have been long overlooked until proposed by Liu and his colleague 17

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in 2002 [38]. With tremendous efforts and continuous researches made by this group and more scientists over the world [39, 66-72], it is now very clear that, the heat transfer enhancement by liquid metals, as a newly emerging technology in the electronic and other high heat flux fields, provides many new scientific and technical opportunities worth of pursuing in the coming time. Firstly, the experimental data available for thermal physical properties and flow behavior of liquid metals with melt point of room temperature are rather limited and insufficient to guide the practical application. Current theories and models mainly relate to liquid metals with high melting temperature (e.g. sodium and potassium) or the toxic mercury with very low melting point, which may not be applied to room temperature liquid metals. Therefore, further measurements on basic properties of such liquid metals, and more theoretical and experimental investigations along this direction are highly needed in order to clearly understand and accurately predict their hydrodynamic and thermal characteristics. Secondly, driving performance of liquid metal is very different from the conventional coolants due to high electrical conductivity. Besides the mechanical pumps which are most frequently used in water-cooling heat exchanger systems, MHD pumps, peristaltic and electro-wetting pumps can also be used. More mechanisms and new ways to drive liquid metals are worth of exploring. Thirdly, electrical conductivity, on the other hand, limits the application of liquid metals for TIM. At the occasion where electrical contact of two surfaces is forbidden, liquid metals can not be directly filled between them as TIM. Therefore, development of insulated liquid metals is of great importance. Finally, the matching of liquid metals and existing heat transfer enhancement techniques, developed in view of the characteristics of water, still remains unproved. Thus, theoretical and experimental researches are needed to comprehensively understand the enhancement effect when applying current enhancement techniques to liquid metals, from which, the universal laws will be drawn to evolve more new heat transfer enhancement methods specific to applications. 18

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Conclusion

This chapter has presented an overview of enhanced heat transfer technology. Liquid metals exhibit outstanding heat transfer characteristics compared to the conventional fluids, which accelerate the energy exchange process. Therefore, they are very suitable and practical to deal with high heat flux requirements of optoelectronic devices and power systems. It is expected that a new thermal science frontier is emerging and a lot of new basic applications will be brought about with the development of liquid metal enhanced heat transfer technology, many of which deserve further research. Consequently, a great many obvious economic and social benefits will be achieved if liquid metal enhanced heat transfer technology can be perfectly and broadly utilized.

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2 Experimental and theoretical investigations on the heat transfer of ferrofluid under magnetic field Maryamalsadat Lajvardi, Anwar Gavili, Fatemeh Zabihi, Taghi Dallali Isfahani, Iraj Hadi, Jamshid Sabbaghzadeh National Center for Laser Science and Technology, Tehran, Iran E-mail :

m_lajvardi20012001@yahoo.com

n this research, the enhancement of heat transfer coefficient of ferrofluids under magnetic field was investigated in laminar flow. Magnetic field was created by Helmholtz coils. The intensity of magnetic field in these coils could be adjusted by the electric current. In order to prepare a stable and uniform ferrofluid, magnetic Fe3O4 nanoparticles were hydrothermally synthesized and dispersed in water. The thermal conductivity of the ferrofluid was experimentally investigated under magnetic field. The saturation time and the reversibility of the thermal conductivity were also examined after turning off the magnetic field. Based on the obtained results, the heat transfer coefficient of ferrofluid could be enhanced up to 60 % using a 5.0 vol % of Fe3O4 nanoparticles with an average diameter of 10 nm under 120 mT magnetic fields.

Introduction Ultrahigh-performance cooling plays an important role in the development of energy-efficient heat transfer fluids which are required in many industrial and commercial applications. Convectional heat transfer fluids such as water, oil and ethylene glycol have low thermal conductivities. Using these fluids limit the improvement of many engineering equipment such as heat exchangers and electronic devices. To overcome this limitation, great attempts have been

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accomplished by many researches in order to substitute them by heat transfer fluids with higher thermal conductivities and heat transfer properties. An innovative way was presented by Choi [1] for enhancing the thermal conductivity of fluids by adding nanomaterials to the basefluids. These new classes of heat transfer fluids containing nanoparticles in the size range under 100 nm which are uniformly and stably suspended in a basefluid are called nanofluids. Ferrofluids consist of a suspension of mono domain magnetic nanoparticles in a nonmagnetic polar/non-polar carrier fluid. The diameter of the magnetic nanoparticles should typically be under 15 nm coated with adsorbed surfactant layers to keep a stable suspension state[2]. In the absence of magnetic field, the particles have random orientation while the fluid has no net magnetization. However, under ordinary magnetic field strengths the dipole moments align with the externally applied magnetic field. A number of researches have shown that ferrofluids have a good potential for coolant applications under controlled magnetic field. This chapter consists of three sections. Section 1 focused on the methods of synthesizing and dispersing Fe3O4 ferrofluids in basefluid. Section 2 discusses on the enhancement of the thermal conductivity of ferrofluids compared to pure fluids under magnetic field. In this section the thermal conductivity behavior of ferrofluids under different magnetic field intensities and temperature variations have been investigated. Finally in section 3 represents the convective heat transfer of ferrofluids is presented. In this section the effect of different concentration, magnetic field intensities and position of magnetic field on the heat transfer coefficient have been investigated and mechanism of it enhancement explained.

1- Preparation of ferrofluids

Ferrofluids consist of colloidal single-domain ferromagnetic nanoparticles dispersed in non-magnetic polar or non polar liquid carriers. The ferromagnetic nanoparticles should be under 15 nm in diameter resulting in large surface to volume ratio and therefore high surface energies. However these nanoparticles tend to aggregate so as to minimize the surface energy. In addition to high surface 28

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energy, the dipole-dipole interactions in magnetic nanoparticles have a destabilizing effect on the colloid dispersion. In general, electrostatic or steric repulsion can be used to disperse the nanoparticles and keep them in a stable colloidal state. Electrostatic stability is enhanced with the appropriate decision of the pH solution. At a pH range far from the iso-electric point, the attractive forces are hindered by the similar charges on the surface of the particles. Capping agents such as surfactants and polymers can be chemically or physically coated particles forming single or double layers [3-5]. Created repulsive forces balance the magnetic and the van der Waals attractive forces. Thus by steric repulsion, the magnetic nanoparticles can be stabilized in liquid carriers while capping agents control the growth of the particles in nano scale size. Capping agents containing functional groups such as carboxylic acids, phosphates and sulfates can be bind to the surface of the magnetic particles [3-6]. A number of suitable capping agents for coating are: citric acid [7], oleic acid[4], poly (meth acrylic acid) [8], dimercaptosuccinic acid [9], octyl phosphonic acid and dodecyl sulfonic acid [10].

1-1- Synthesis

The first report of ferrofluid was obtained by grinding micron sized magnetic particles for 500-1000 hours in a surfactant solution until the size of the particles reached the desired nanometer range [11]. However many other methods have been introduced which are more efficient results in the synthesis particles with a narrow size distribution. A well known example is the synthesis of magnetite nanoparticles by the addition of a stochimetric mixture of Fe (II) and Fe (III) salts to an alkali solution [12]:

2Fe3+ + Fe2+ + 8OH- â&#x2021;&#x2019; Fe3O4 + H 2 O

(1)

The first water based ferrofluid was synthesized in 1938 by Elmor via mechanochemical procedure [13]. Rosensweig et al developed the synthesis of oil based ferrofluids stabilized by surfactants in 1987 [14]. Later, other metallic magnetic particles such as Fe, Ni, Co were synthesized by metal vapor deposition [15], sonolysis [16], reduction of metal salts [17] and thermal deposition of carbonyl 29

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compounds [18]. So far a large variety of magnetic colloids are developed, e.g. core-shell particles consisting of magnetic cores and silica shell [19], colloidal alloys [20]and magnetic rods [21].

1-2- Experimental 1-2-1 Material and ferrofluid preparation 40 mmol FeCl3.6H2O as iron source and 20 mmol tri-sodium-citrate dehydrates as capping agent was dissolved in 60 ml water at 70Â°C. Then 130 mmol hydrazine hydrates (80%) were injected to the solution with a rate of 3 ml/min. The mixture was then transferred to a Teflon lined stainless steel autoclave and heated at 160Â°C for 5 h in an oven. The final product was washed with distilled water and ethanol and dried. Tri-sodium-citrate molecules have three functional groups of carboxylate. Carbonyl groups are known to interact strongly with the metal oxide surfaces [6]. According to Nakamotoâ&#x20AC;&#x2122;s report [22], three types of interactions between the carboxylate group and the surface of metal oxides are: bridging bi-dentate structure (figure 1a), bridging mono-dentate structure (figure 1b) and bi-dentate quelate structure (figure 1c).

Therefore, the presence of chemical surfactants results in size control as well as creating hydrogen bonding with water molecules in preparation process. Two important factors; particles size and hydrogen bonding affect the stability and homogeneity of ferrofluids. The prepared ferrofluid with and without surfactant have a black color. As shown in figure 2 (a), the sample without surfactant losses 30

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itâ&#x20AC;&#x2122;s the uniform dispersion under external magnetic field separating the magnetic nanoparticles from the basefluid. In the sample with surfactant the homogeneity of the ferrofluid is kept and nanoparticles remain dispersed under the external magnetic field.

Figure 1: Aqueous dispersion of Fe3O4 nanoparticles; (a) without surfactant, (b) with surfactant; in presence of a magnetic field gradient.

1-2-2- Characterization The X-ray diffraction pattern of the synthesized Fe3O4 nanoparticles is shown in figure 3(a). The pattern shows different reflection planes indexed as (220), (311), (400), (511) and (440), which are in a good agreement with the standard Fe3O4 XRD pattern. Figure 3 (b) shows the SEM image while the TEM images are shown in figures 3 (c) and (d). As can be seen in figures 3 (c) and (d)

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311

45 40

In ten sity

35 30

440

220

511 400

20 15 10 5 0 10

2Î&#x2DC;

(a)

(c)

(b)

(d)

Figure 2: (a) X-ray Diffraction of synthesized Fe3O4 particles, (b) FESEM and (c, d) TEM image of Fe3O4 nanoparticles.

there are a number of agglomerated particles due to the large specific surface area and high surface energy (darker areas in the micrograph). According to these images the average diameters of the nanoparticles are 10 nm.

2- Thermal conductivity of ferrofluids Convectional heat transfer fluids such as water, oil and ethylene glycol have low thermal conductivities. The performance of many engineering equipment such as heat exchangers and electronic devices are limited by using these fluids. A great deal of work has been done by many researches to prepare heat transfer fluids with higher thermal conductivities. An innovative way for enhancing the thermal conductivity of fluids is to add nanomaterial to the nanofluids as presented by 32

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Choi [1]. These nanofluid can be used for energy transportation is affected by the type and properties of the nanoparticles. Compared with base fluids, a number of recent experiments on nanofluids have indicated dramatic improvements in the effective static thermal conductivity of these fluids [1, 23-27]. Among the different research carried on nanofluids containing metallic, nonmetallic and CNTs nanoparticles, many of them have focused on the thermal conductivities of ferrofluidâ&#x20AC;&#x2122;s [28-34]. Abareshi et al [28] and Zhu et al [33] experimentally measured the effect of Fe3O4 magnetic nanoparticles on the thermal conductivity of nanofluids without applying any magnetic field. Philip et al [29] experimentally measured the thermal conductivity of the magnetic nanoparticles (Fe3O4) coated by Oleic acid suspended in hexadecane under magnetic field. Hong et al [24] and Wensel et al [32] experimentally measured the thermal conductivity of single wall carbon nanotubes coated by Fe2O3 nanoparticles suspended in water. Wright et al [34] studied on the thermal conductivity behavior of single wall carbon nanotubes coated by Ni nanoparticles suspended in water. In this section we experimentally measured the thermal conductivity of Fe3O4 magnetic nanoparticles suspended in DI water. The magnetic field was provided by two Helmholtz coils with its inner space filled with a magnetic iron core. The enhancement of the thermal conductivity and its behavior under magnetic field and temperature variation was very interesting. The results showed that nanoparticles have a good potential for coolant applications under controlled magnetic field.

2-1- Experimental setup The experimental setup used for measuring the thermal conductivity of the ferrofluid is schematically shown in Figure 4. As shown, two Helmholtz coils are considered to provide a magnetic field. Each coil contains 4000 turns of wire that can be a carrier of maximum 10 Ampere current. The inner space of the coils is filled with a magnetic iron core, creating a uniform magnetic field between the coils. The coils are connected to a 6 KW (maximum current 40 Ampere and 33

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maximum voltage 150 Volt) DC power supply with controllable current and voltage. A Gauss meter (Magnet-Physik, FH51) with a transverse probe was used to measure the intensity of the magnetic field. 8

2 5

Figure 3: Schematic diagram of the experimental setup used for measuring the thermal conductivity of the ferrofluid, 1 & 2 - Helmholtz coils, 3 - DC power supply, 4 – stainless steel box containing the ferrofluid sample, 5 –test sample of the ferrofluid (1.0 cm diameter, 10.0 cm height) , 6 – water bath output, 7 – water bath input, 8 – KD2 probe instrument.

Applying electrical current on the coils creates heat which affects the thermal conductivity measurements. For this reason the ferrofluid sample was held in a stainless steel box connected to a water bath to eliminate the heat and therefore the temperature of the sample was kept constant. The stainless steel box had no noticeable effect on the magnetic field distribution and stabilized temperature was achieved by isolating the box with pipe isolating foam tape model TP-PN07 with 0 a thermal conductivity of 0.034 ( W / M K ). The water bath instrument was

Brookfield TC-502 with a programmable controller preparing constant temperatures with an error of 0.01 0K. This insures that the variation in thermal conductivity is not a consequence variation of various temperatures. A KD2 Pro 34

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thermal property analyzer instrument from Decagon Devices Inc was used for measuring the thermal conductivities by applying the hot wire method. The time between each measurement was 15 minutes and the output data from this instrument was with an accuracy of at least R2 > 0.9998.

2-2- Effect of magnetic field intensity Before applying any magnetic field on the samples their thermal conductivities were measured two times. After applying the magnetic field, the thermal conductivity was measured for a long time under the magnetic field. At last the magnetic field was removed and the thermal conductivity measurement was continued. Several magnetic field intensities were tested; the results for three of them are mentioned here. The mean thermal conductivity value measured by the 0 KD2 Pro for water at 25 (0C) was 0.598 ( W / M K ). Results of two different

magnetic field intensities are shown in figure 5.

1.1 30 mT magnetic field intensity 50 mT magnetic field intensity

Thermal conductivity ( W/M K)

1.0

0.9

0.8

0.7

Turning on MF Turning off MF

0.6

100

200

300

400

Time ( min)

Figure 4: The thermal conductivity of the ferrofluid versus time under two different magnetic field intensities of 30 mT (circle points) and 50 mT (triangle points) at 25 (0C).

As shown in this figure before applying any magnetic field, the thermal conductivity of the sample was approximately equal to the thermal conductivity of DI water. Magnetic field increases the thermal conductivity of the ferrofluid until 35

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it reaches a maximum value and then after a slight decrease a constant value is maintained. Turning off the magnetic field decreases the thermal conductivity. This decrease does not happen immediately. The decrease in the thermal conductivity of the ferrofluid is like a decay function of time and was not obtained before applying the magnetic field and the ultrasonification of the sample after turning off the magnetic field. Lower magnetic field intensities have lower maximum values and their maximum value is reached longer. However the thermal conductivity increased more than 50% under a 30 mT magnetic field as shown in figure 5 while the maximum increase under a 50 mT magnetic field was 65.5%. However by increasing in the intensity of the magnetic field results in shorter time for reach maximum value and higher maximum value. As shown in the figure 6 for the magnetic field intensity of 100 mT, increasing the intensity of the magnetic field changes the behavior of the thermal conductivity of the ferrofluid. In comparison with low magnetic field intensities the time to reach a maximum value is shorter but its value is higher.

2.0

Thermal conductivity ( W/m K)

1.8 1.6 1.4 1.2 1.0

Turning off MF

0.8

Turning on MF

0.6 0

100

200

300

400

Time ( min)

Figure 5: The variation of the thermal conductivity of the ferrofluid under a magnetic field intensity 100 mT at 25 (0C).

After reaching a maximum value in the presence of the high intensity magnetic field, the thermal conductivity of the ferrofluid decreases sharply without 36

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maintaining a constant value. The decreasing continues with turning off the magnetic field. The maximum value shows over 200% increase in the thermal conductivity of the ferrofluid. In all intensities, the thermal conductivity of the ferrofluid does not return to its starting value even after turning off the magnetic field. The thermal conductivity of some of the samples was measured 24 hours 0 after turning off the magnetic field. In all cases it remained near 0.7 ( W / M K )

until the sample was ultrasonificated. The reason of this effect is that, turning off the magnetic field causes the Fe3O4 molecules in each nanoparticle to return to the state before turning on the magnetic field while the chain like structure of the nanoparticles remained in the state before turning off the magnetic field. To completely understand the variations of the maximum thermal conductivity value of the ferrofluids versus the intensities of the magnetic fields are shown in figure 7. As shown in this figure, increasing the intensities of the magnetic field increases the maximum value of the thermal conductivity nonlinearly. Due to the long time of each test, the generated heat in the coils and the difficulties concerning with its removal, applying an intensive magnetic field on the ferrofluid samples is not feasible.

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Maximum thermal conductivity ( W/m K )

1.8

1.6

1.4

1.2

1.0

0.8

0.6 0

100

M agnetic field intensity ( mT ) Figure 6: Maximum value of the thermal conductivity of the ferrofluids versus the magnetic field intensities at 25 (0C).

2-3- Effect of temperature In the next experiment we investigated the effect of the temperature variation on the thermal conductivity of the ferrofluids. As shown in figure 8, we applied a magnetic field intensity of 50 mT at 20 0C. After the thermal conductivity reached a maximum value without turning off the magnetic field, the temperature of the sample was increased in 5 째C steps through controlling the water bath temperature. In each temperature two measurements were done, indicating an intensive decrease in the thermal conductivity of the ferrofluid.

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Thermal conductivity ( W/m K)

1.0

0.8

Turning on 0.6 T = 25 0CT = 30 0CT = 35 0C

T = 20 0C

100

200

300

Time ( min)

Figure 7: The thermal conductivity Variation of the ferrofluid with temperature for the magnetic field intensity of 50 mT and initial

In non-magnetic nanofluids with increasing the temperature, the thermal conductivity increased while in ferrofluids a decrease in the thermal conductivity is observed. From the statistical mechanics of paramagnetic material we know that with an increase in the temperature, magnetization decreases as shown by the Brillouin equation for quantum systems or the Langevin equation for classical systems [35]. The Langevin equation is used for describing the magnetization behavior of classical systems such as ferrofluids [2]. In these systems, the ratio of the magnetization and magnetize saturation for a ferrofluid with a solid volume fraction of φ is calculated according to following relation:

M / M s = M / φ M d = Coth(α ) − 1/(α ) ≡ L(α )

(2)

3 M M K In this equation α = (π / 6)( M d Bd / K BT ) , where s , d , B , d , B and T are the

permeability of free space, magnetize saturation, domain magnetization, Boltzman constant, the nanoparticle diameter, magnetic field intensity and temperature respectively. The Langevin function diagram of our sample is shown in figure 9 according to the sample characteristics and the magnetic field properties as follows: 39

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B = 50 ( mT ), M d = 15.9 ( kA / m), d ≈ 10 (nm), K B = 1.38 × 10−23 (J/ o K) ⇒ α = 30.2/T(o K)

(3)

As shown in figure 9, temperature variations are in the range of 20-35 °C. The behavior of the Langevin equation for the magnetic field intensity of 50 mT is linear due to the small value of α ( α = 0.102895 at T = 20 °C) which results in

L(α ) ≈ α / 3 . Also the variation of magnetization is very low compared to the thermal conductivity of the ferrofluid.

0.0344

0.0340

M / φ Μd

0.0336

0.0332

0.0328

0.0324 20

Temperature ( K)

Figure 8: The magnetization value of the Langevin equation versus temperature of the ferrofluid under a magnetic field intensity of 50 mT.

The thermal conductivity decreases with increasing the temperature as an exponential decay and its value drops under a normal value without any magnetic field. In another experiment the initial temperature of 10 °C was used under low magnetic field intensity until the thermal conductivity of the ferrofluid reached a maximum value and then the temperature was varied. The ferrofluids thermal conductivity behavior was the same as the previous results. Therefore to constraint space these results are not show here.

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3- Heat transfer of ferrofluids

Besides the significant thermal conductivity behavior of ferrofluids under magnetic fields, they have a remarkable potential for heat transfer applications due to their controllable thermomagnetic convection. The thermomagnetic convection property can be controlled by varying the ferrofluid properties, the magnetic field strength and the temperature distribution. A number of theoretical and experimental researches have been presented on the heat transfer and fluid flow of ferrofluids in the presence of uniform and non uniform magnetic field. Krakov and Nikiforov [36] studied the influence of the relative orientation of the magnetic nanoparticles on the thermomagnetic natural convection of ferrofluids in a square cavity under temperature gradient and uniform magnetic field. Yamaguchi et al [37] experimental worked on the natural convection in a square enclosure and heat transfer enhancement observed by increasing the magnetic Rayleigh number. The numerical simulation done by Gavili et al [38] showed good agreement with Yamaguchiâ&#x20AC;&#x2122;s result. Kikura et al [39] and Swada et al [40] carried out experimental investigations in a cubical enclosure and concentric horizontal annuli under the influence of a varying magnetic field. In their investigations the permanent magnet was placed on the different sides of the enclosure and then the effect of the magnetic field gradient on the ferrofluid heat transfer was studied. Combined natural and magnetic convective heat transfer of ferrofluids in a cubic enclosure was numerically simulated by Synder et al [41] and their results showed good agreement with results of Kikura and Swada [39, 40]. Zablockis et al [42] numerically investigated the thermomagnetic convection generated by a non-uniform constant magnetic field of a solenoid in a heated cylinder. However, in all of these investigations natural thermomagnetic convection was studied. In this section, the forced thermomagnetic convection and heat transfer performance of ferrofluids will be discussed. Our objective is to investigate on the enhancement of the heat transfer of ferrofluids in the presence of a magnetic field. The results show that the heat transfer of ferrofluids increases remarkably by

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applying a magnetic field. The work may open a route to engineer the next generation of high-efficiency heat transfer nanofluids.

3-1- Theory

According to our experimental setup, it is reasonable to assume a constant heat flux boundary condition on the tube wall, since the resistive heating method is used in this work. The heat flux of the wall can be calculated as the equation (4):

q′′ = P / Ah = ( I ⋅ V ) /(π Di L)

(4)

Where P, I, V, Di and L are the power of the power supply, the measured current, the supplied voltage, the tube inner diameter and the tube heating length respectively. The local convective heat transfer coefficient is defined as the equation (5):

h( x) = q′′ /(Tw ( x) − T ff ( x)) where the local wall temperature ( the thermocouples (

Tw,o

(5)

Tw,i

) is extrapolated temperature recorded from

) obtained at the outer wall of the tube as equation (6):

Tw,i ( x) = Tw,o ( x) − ( P / 2π LKt ) ln(r0 / ri )

(6)

Assuming that the local fluid temperature follows a linear profile along the channel length

can be calculated from the energy conservation equation as

follow:

T f ( x) = T f ,in ( x) + ( q"π Dx / ρ f C p f Q) In this equation

ρ f C pf ,

(7)

and Q are density, heat capacity and volumetric flow rate

of the ferrofluid respectively. The x represents the axial distance from the entrance of the test section.

3-2- Experimental setup and calibration

An experimental setup was prepared built to study on the heat transfer features of ferrofluids in a tube under the effect of magnetic fields. As schematically shown in figure 10, the experimental system consists of a loop, heat unit, cooling part, 42

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measuring and control unit. The loop included a peristaltic pump and a test section. A straight tube with 400 mm length, 8 mm inner diameter, and 20 mm outer diameter was used as the test section. Eight thermocouples are connected at different places of the test section to measure the wall temperature. Another thermocouple is also located at the end of the cooling section to check the fluid temperature of the end of the loop. The chiller with 1.6 KW cooling capacity is used to keep a constant temperature of the ferrofluid at the inlet of the test section. To apply a constant-heat flux, the test section is heated electrically by a DC power supplier whit a maximum power of 300 W. Cooling Section

Pump

Test Section

Reservoir Tank

Data DataAcquisition Acquisition

DC Power Supply

Figure 9: Schematic of the experimental setup

In order to minimize the heat loss from the test section, the whole test section was thermally isolated. A data acquisition system recorded the temperature of the thermocouples. Since the production of a magnetic field with a magnitude up to 120 mT along the test section was complicated, four separated coils coupled with each other with adjustable distance were used. The DC power supply and Gauss meter were the same as mentioned in section 2. The four coils were placed above and below the test section, where changing the current varied the magnetic field intensity. The direction of the magnetic field is perpendicular to the ferrofluid flow direction. The first coupled coils were placed at the entrance of the test section while the distance of the second set is about 5cm from the first set.

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Before conducting systematic experiments on ferrofluids in the presence of a magnetic field, the reliability and accuracy of the experimental setup was validated using distilled water as the working fluid. The results are shown in figure 11, together with the prediction of the following well-known Shah equation for laminar flow under constant heat flux boundary conditions [43]. 1/ 3 1.953 ( Re Pr( D / x) ) Nu =   4.364 + .0722 Re Pr( D / x)

( Re Pr( D / x) ) ≥ 33.3 ( Re Pr( D / x) ) 〈33.3

(8)

Figure 10: Initial test with distilled water for the validation of the setup

3-3- Effect of magnetic field intensity

The effect of different magnetic field intensities on the ferrofluid temperature profile is shown in figure 12. In this section the loaded heat over the test section was 65 W for the different magnetic fields. The results indicated that the temperature profile of the distilled water and the ferrofluid overlapped each other through the length of the test section in the absence of a magnetic field. However this profile was remarkably decreased by applying magnetic field with different order for fluids containing magnetic nanoparticles. The maximum temperature drop of 5.5 ºC at the entrance of the test section was due to the effect of the

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magnetic field with the maximum intensity of 120 mT. This temperature drop is 4 ยบC at the end of test section where no magnetic field is applied. 39 37 35

Temperat ure

33 31 29 27 25 23 21 19 0

DIFerrofluid, water Ferrofluid, B=80 B=0 Ferrofluid, B=100 mT Ferrofluid, B=120 mT mT 7 8 9

Thermocouples Number

Figure 11: Magnetic field effect on the temperature profile of the 5% vol ferrofluid.

Figure 13 depicts the variation of local heat transfer coefficient versus the axial distance from the entrance of the test tube. The coefficients were measured in the thermal developing region.

DI-water Ferrofluid, Ferrofluid, B=80 Ferrofluid, B=100 Ferrofluid, B=120

Figure 12: Axial profile of local heat transfer coefficient for different magnetic field (5% vol)

As shown, three different magnetic fields were applied on a 5 % volume fraction ferrofluid of Fe3O4. In this figure the following trends can be observed: (a) - heat transfer coefficient (h) decreases with increasing the axial distance from the entrance of the test tube, (b) - distilled water and water based fluids containing magnetic nanoparticles in the absence of a magnetic field have the same heat 45

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transfer trend through the length of the test section, (c) - the heat transfer coefficient of the ferrofluids in the presence of magnetic field significantly increases, (d) - Applying stronger magnetic fields results in intensive heat transfer.

3-4- Effect of concentration

In order to investigate on the effect of the ferrofluid concentration on the temperature profile, we repeated the above experiment for a vol 2.5 % Fe3O4 and different magnetic fields. It could be seen that reducing the ferrofluid concentration to half result in a decreased rate of temperature drop. Figure 14 shows that the maximum temperature drop is about 4 ยบC at the entrance of the test section which decreases to 2.5 ยบC at the end of the test section. 39 37 35 33 31

Temp e

29 27 Ferrofluid, B=0

Ferrofluid, B=80 mT

Ferrofluid, B=100 Ferrofluid, B=120 mT

21 19 0

Thermocouples Number

Figure 13: Magnetic field effect on the temperature profile 2.5 % vol ferrofluid

3-5- Effect of the magnetic field position and different heat fluxes

In another experiment, we shifted the position of the coils in order to investigate on the effect of the position of the coils on the temperature profile for a 5 % vol Fe3O4 ferrofluid. In this test four thermocouples 3, 4, 6, and 7 were directly under the magnetic field and the distance between the coils were 10 cm. Figure 15 shows the trend of the temperature variation along the test section for the new 46

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position of the coils. The comparison of this figure with figure 12 shows that as the coils get closer to each other, the magnetic field has more effect on decreasing the temperature along the test section. When the coils get far from each other, the temperature drops locally under the magnetic fields while the temperature out of the magnetic field increased. 38 36 34

Temper ature

32 30 28 26 24

Ferrofluid, B=0 Ferrofluid, B=100 mT

22 20 18 0

Thermocouples Number

Figure 14: The effect of magnetic field position on the ferrofluid temperature profile (5 % vol)

Different heat fluxes have been loaded on the test section. Figure 16 indicates the effect of the magnetic field on the heat transfer of ferrofluids for two different heat fluxes. According to this figure, the temperature profiles of the ferrofluid under a heat power of 70 W in the absence of a magnetic field and the ferrofluid under a heat power of 80 W in the presence of 100 mT magnetic fields could cover each other with high accuracy. The temperature profile for the heat load of 80 W on the test section in the absence of a magnetic field could be considered as a voucher profile. This figure could significantly highlight the cooling ability of ferrofluids in the presence of a magnetic field.

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55 52 49 46 Temperature

43 40 37 34 B=0, Q=80W

31 28

B=100 mT, Q=80W

B=0, Q=70W

22 19 0

Thermocouples number

Figure 15: Magnetic field effect on the temperature profile for different heating load

3-6- Heat transfer enhancement

Many research groups have focused on determining the mechanism which enhances the heat transfer of nanofluid relative to its basefluid due to the presence of nanoparticles. However the mechanism and the rate of enhancement are still controversial. One possibility for the increase of h could be due to the enhanced thermal conductivity of the nanofluid. Sommers and Yerkes [44] suggested that the observed increase in heat transfer was a result of the enhanced thermophysical properties of the Al2O3/ propanol nanofluid and not mechanisms such as: the Brownian motion-induced nano-convection, liquid layering, and other interfacial effects. However, another possibility presented by Ahuja [45] for micron-sized particles was that particle rotation could cause convective heat transfer augmentation. Wen and Ding suggested that the enhancement might be due to particle migration within the flow field, especially near the entrance [46]. In this experiment, there are two probable explanations based on ferrofluid thermophysical properties for the heat transfer increment in the presence of a magnetic field.

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One possibility could be caused by the increase of the ferrofluid’s thermal conductivity. In a magnetic suspension, each magnetite particle is a single domain super paramagnetic with a magnetic moment [2]. The interparticle dipole-dipole U (ij ) interaction d between the magnetic particles is as equation (9) [47]:

U d (ij ) = − 3 ( mi ⋅ rij )( m j ⋅ rij ) / rij5 − ( mi ⋅ m j ) / rij3  ,

rij = ri − rj

(9)

r The dipolar interaction energy depends on the distance ( ij ) between the i th and m the j th particles and the mutual orientation of their magnetic moments, mi and j .

L (U ( ij ) ) k BT  The coupling constant (  d ) defines the effective attraction between two ferromagnetic particles where

(U ( ij ) ) , k T , k d

and T are the

magnetic dipolar energy, thermal energy, Boltzman constant and temperature respectively. In the absence of magnetic field, the magnetic moments are oriented in random direction and the nanoparticles are influenced by the Brownian motion as the thermal energy exceeds the magnetic dipolar attraction (L<1). By applying a magnetic field to the ferrofluid, the magnetic dipolar interaction energy becomes strong enough to dominate the thermal energy so that the magnetic moments of the nanoparticles start aligning in the direction of the magnetic field. As the magnetic field increases, the particles start forming doublets, triplets and short chains along the direction of the magnetic field. The lengths of the chains increase with increasing the magnetic field [29]. Based on Philip’s theory [30], due to the linear chain-like structures of the magnetic nanoparticles, the percolation theory [48] could support the abnormal enhancement in the thermal conductivity of the ferrofluid. In the section under magnetic field the heat transfer of the ferrofluid with laminar flow increases due to the enhanced thermal conductivity. The percent of the magnetic particles which have tendency to align in the direction of the magnetic field significantly increase with the increase of the intensity of magnetic field and considerable enhancement in the heat transfer of the ferrofluid could be observed due to the higher magnetic field as can be observed in figure 13. Also as shown in figure 14 when the volume fraction of magnetic 49

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nanoparticles decreases less particles are aligned. It is predicted in this case that the thermal conductivity of the ferrofluid decreases and the amount of the temperature drop decreases with respect to figure 12. Another possibility for the temperature drops of the ferrofluid due to the heat capacity of the ferrofluid increment under magnetic field effect. Recently some research has been focused on the heat capacity measurement of ferrofluids [49, 50]. Korolov et al [49] observed a 30 % increases in the specific heat capacity of colloidal magnetic nanoparticles in comparison with microparticles of magnetite. He studied the effect of magnetic field intensity on the colloidal solution of magnetic nanoparticles. Based on our experimental results it is obvious that loading heat increases the molecular movement and causes a temperature increment. As we have mentioned before, magnetic nanoparticles of the particles and water molecules are bonded to each other with hydrogen bonding and this bonding causes molecular movement between nanoparticles and water molecules. Due to the applied magnetic field on the ferrofluid, the movements of the nanoparticles is restricted and as these particles are bonded to the base fluid through hydrogen bonding the molecular movement of water molecules also decreases. For this reason at constant heat load, less temperature enhancement is observed with respect to the temperature increment in the absence of a magnetic field. By this theory, the heat capacity enhancement of the ferrofluid in under the effect of magnetic field could be predicted. It is necessary to mention that our experimental data has been obtained for sample (b) in figure 2 which shows no phase separation under magnetic field due to the hydrogen bonding between the nanoparticles and the water molecules. However, the temperature drop of ferrofluids under magnetic field could not be obtained for sample (a) in figure 2. As we have discussed in this section the increase in the heat transfer of ferrofluids under magnetic field occurred due to the changes of the thermophysical properties such as the thermal conductivity or the heat capacity of the ferrofluids under magnetic field [2].

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Conclusion

In this chapter, the preparation and synthesize of ferrofluid are presented. A stable and uniform dispersed of 10 nm Fe3O4 magnetic nanoparticles in water was prepared successfully. The thermal conductivity of this ferrofluid in the presence of magnetic field has been investigated. The results show a drastic enhancement in thermal conductivity under conventional magnetic field intensity. The practical use of ferrofluid in the presence of magnetic field for heat transfer enhancement has been demonstrated. For this purpose an experimental setup is designed to show the performance of ferrofluids as a new class of coolants and their heat removal ability. The convective heat transfer performance of the ferrofluids in a heated copper tube was experimentally investigated for laminar flows. Base on the obtained results in the laminar flow regime, no enhancement in the convective heat transfer of ferrofluids can be achieved in the absence of magnetic field. By increasing the magnetic field intensity and concentration of magnetic nanoparticles, a remarkable enhancement of heat transfer coefficient could be observed. It should be emphasized that enhancement of heat transfer coefficient under magnetic field could be attributed to the thermophysical properties of ferrofluids including thermal conductivity, specific heat capacity and viscosity.

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