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Review Article

Convective Heat Transfer Enhancement Using Ferrofluid: A ReviewOPEN ACCESS

[+] Author and Article Information
Jaswinder Singh Mehta

Department of Mechanical Engineering,
UIET,
Panjab University,
Sec 25,
Chandigarh 160014, India
e-mail: jsmehta@pu.ac.in

Rajesh Kumar

Department of Mechanical Engineering,
UIET,
Panjab University,
Sec 25,
Chandigarh 160014, India

Harmesh Kumar

Department of Mechanical Engineering,
UIET,
Panjab University,
Sec 25,
Chandigarh 160014, India
e-mail: harmeshkansal@gmail.com

Harry Garg

Optical Devices and Systems,
Central Scientific Instruments Organisation,
Chandigarh 160014, India
e-mail: harry.garg@gmail.com

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received December 23, 2016; final manuscript received June 7, 2017; published online August 29, 2017. Assoc. Editor: Amir Jokar.

J. Thermal Sci. Eng. Appl 10(2), 020801 (Aug 29, 2017) (12 pages) Paper No: TSEA-16-1383; doi: 10.1115/1.4037200 History: Received December 23, 2016; Revised June 07, 2017

Abstract

Ferrofluids, a distinctive class of nanofluid, consists of suspension of magnetic nanoparticles in a nonmagnetic base fluid. Flow and heat transport properties of such a fluid can be manipulated when subjected to external magnetic field and temperature gradient. This unique feature has fascinated researchers across the globe to test its capability as a coolant for miniature electronic devices. The proposed work presents an updated and comprehensive review on ferrofluids with emphasis on heat transfer enhancement of microdevices. Based on the research findings, a number of important variables that have direct bearing on convective heat transport ability of ferrofluid have been recognized. The paper also identifies the key research challenges and opportunities for future research. By critically resolving these challenges, it is anticipated that ferrofluids can make substantial impact as coolant in miniature heat exchangers.

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Introduction

Rapid advancement of technology in the area of miniaturization is moving at a spectacular pace, and ample number of research publications/patents illustrates the interest among research community to explore the potential applications in this field. Decrease in the size and improvement in the computational ability of today's electronic devices have led to the generation of high heat flux, removal of which is utmost desired so that these devices can operate safely and consistently. Due to lesser surface to volume ratios, heat transfer in miniature devices has always remain a daunting task that continue to stimulate researchers over the years. Conventional methods of cooling are insufficient to remove the high heat loads generated by the modern electronic devices. Thus, there is a pressing need for better means for improving the cooling performance of such devices.

Nanofluids, over the last few decades, have shown great potential to be used as new generation coolants for such miniature devices. Several studies [1,2] have reported immense improvement in the cooling performance of heat generating devices by utilizing nanofluids as heat transfer media in comparison to common base fluids. Increase in the thermal conductivity of the fluid due to the presence of nanoparticles such as TiO2, CuO, and Al2O3 in the base fluid [39] lead to the enhancement of heat transfer characteristics. But the limitation of single-phase liquid cooling system using a nanofluid (nonmagnetic) is a poor reliability due to the use of mechanical pumping devices to circulate the fluid besides the additional problems related to vibration, noise, pressure losses in the piping, and leakage of fluid.

Ferrofluid-based cooling systems based on microfluidic technologies can address the issue of dissipation of high heat loads generated by miniature electronic devices. Miniaturized fluidic platforms can allow manipulating small volumes in precise and accurate manner.

Ferrofluids were first developed and classified in 1960s by scientists from NASA research center for rotating shaft seals. These fluids are now being used for a wide variety of applications ranging from sealing in computer hard disk drives, damping, heat transfer enhancement, material separation, and medical applications.

Ferrofluid consists of suspension of single-domain magnetic nanoparticles (such as magnetite, cobalt, and nickle), 10–15 nm in diameter, in a nonmagnetic carrier fluid which are usually coated with a suitable surfactant layer of thickness about 2–3 nm to prevent particles to form clusters. A typical composition of a ferrofluid is approximately 85% carrier fluid (oil, water, glycerin, kerosene, or any clear liquid), 5% magnetic particles (Fe3O4, Fe2O3, iron–nickel oxide, etc.), and 10% surfactant (lauric acid, oleic acid, lecithin, tetra methyl ammonium hydroxide, etc.) by volume as shown in Fig. 1. They are different from magnetorheological (MR) fluid in the sense that ferrofluids have particles in nanorange in comparison to micron size particles (several hundreds of nm up to tenths of μm) of MR fluid. Also ferrofluids maintain their flow-ability under external magnetic fields, while the MR fluid tends to solidify when subjected to strong magnetic field.

Compared to nonmagnetic nanofluids, ferrofluid-driven heat exchangers offer the following advantages:

• (i)Due to the absence of any moving parts to pump/circulate the fluid, the system has high reliability since the fluid is driven by an external magnetic field which can be generated by permanent magnets or electromagnets.
• (ii)Important properties of ferrofluid-like thermal conductivity and viscosity could be tailored to suit specific design requirements.
• (iii)The heat transfer rate and/or flow rate could be varied by changing only the intensity/frequency of magnetic field; thus, the system has self-regulating feature.

An attempt has been made in this paper to present the updated and comprehensive review of the work done by various researchers on heat transfer enhancement of microdevices using ferrofluid. The Principle of Thermomagnetic Convection of Ferrofluid, Forced Convective Heat Transfer Using Ferrofluid, and Free Convective Heat Transfer Using Ferrofluid sections of the paper will cover fundamental principle of thermomagnetic convection and recent numerical and experimental developments on heat transfer applications of ferrofluid. Based on the review, different important variables affecting the convective heat transfer coefficient of ferrofluid are identified and some future research directions are also suggested.

Principle of Thermomagnetic Convection of Ferrofluid

Ferrofluids consist of millions of tiny magnets in the form of magnetic nanoparticles which are randomly oriented in the carrier fluid. Under the influence of nonuniform magnetic field, atomic moments of magnetic nanoparticles, which were originally randomly oriented, attempt to align along the direction of applied magnetic field. The degree of the resulting magnetization in the fluid depends on the applied magnetic field strength and on the fluid temperature.

The relation between magnetization vector $M$ and magnetic field vector $H$ is given by Display Formula

(1)$M=χmH$

Thus, the magnetic induction $B$ can be written in the terms of magnetization vector as Display Formula

(2)$B=μ0(H+M)$

where the total magnetic susceptibility, $χm$, is a temperature-dependent entity [10] and is expressed as Display Formula

(3)

where $χ0$ denotes differential magnetic susceptibility of the ferrofluid, $β$ is the pyromagnetic coefficient of ferrofluid, and $T0$ is the reference temperature.

At temperatures well below the Curie point, these atomic moments of a ferromagnet are essentially aligned in some favorble direction in the crystal. But as soon as the temperature crosses a certain limit known as Curie temperature (Tc), the ferrofluid loses its magnetization as the moments become randomly oriented. The fluid becomes paramagnetic, and its behavior (degradation in the magnetization of the fluid) with the increase in temperature is shown in Fig. 2.

Thus, when such a fluid is subjected to thermal gradient in the presence of an external magnetic field, due to nonequilibrium magnetization in the fluid, the fluid will experience a magnetic body force, called “Kelvin body force” which results in flow of fluid in the direction of higher temperature section. This effect is similar to natural convection where colder, more dense fluid displaces less dense fluid, and convection currents are setup in the fluid. The principle of thermomagnetic convection is illustrated in Fig. 3.

The following expression is used to determine the Kelvin body force: Display Formula

(4)$FB=μ0(M⋅∇)B$

Here, μ0 represents the magnetic permeability of free space or vacuum, $M$ magnetization, and $B$ magnetic induction.

The Kelvin body force, FB, thus, can be represented as Display Formula

(5)$FB=12μ0χm(1+χm)∇(H⋅H)+μ0χmH(H⋅∇)χm)$

Methods of Synthesis of Magnetic Nanoparticles

In particular, nanoparticles are synthesized by two methods; one is ball milling, where magnetic powder of micron size is mixed with a solvent and a dispersant and ground over a period of approximately 1000 h [11]. The major limitation of ball milling method is difficulty in controlling the particle size. Chemical precipitation is another technique in which procedure starts with a mixture of FeCl2, FeCl3, and water. Addition of NH4OH to the mixture lead to coprecipitation, and then, the system is subjected to different procedures to peptization, magnetic separation, filtration, and finally dilution [12].

Chemical precipitation [13] is the more commonly used approach now-a-days for the synthesis of high quality suspensions of magnetic nanoparticles in a wide range of base fluids.

Heat Transfer Applications of Ferrofluid

The ferrofluid should possess the following important properties to become eligible to be used as a coolant for heat transfer applications [14]:

• higher pyromagnetic coefficient

• higher boiling point

• high saturation magnetization

• lower Curie temperature (in proximity to operating range of heat generating device)

• lower viscosity

This section provides the current developments in the field of heat transfer by using ferrofluids. The ferrofluids circulated by forced circulation (active mode) and natural convection (passive mode) have been covered in the review.

Forced Convective Heat Transfer Using Ferrofluid.

A forced convective heat transfer is the one in which the fluid is forced to flow in the loop by some external means such as pump or fan and in doing so, will carry away the heat at a higher rate than natural convection.

Convective heat transfer coefficient of water-based ferrofluid flowing through a stainless steel tube was compared experimentally for two different configurations, i.e., single in-line and double in-line parallel arrangement of permanent magnets by Asfer et al. [15]. COMSOL multiphysics v4.3 was used to simulate the magnetic field distribution and resulting magnetic force acting on ferrofluid. Ferrofluid synthesized by coprecipitation method and coated with oleic acid was passed through the tube of circular cross section (2 mm inner diameter and 0.3 mm thickness), and NdFeB permanent magnets were used to create magnetic field gradients inside the tube. Experimental measurements of temperature were done using Infrared thermography technique. Heat transfer augmentation was observed for the ferrofluid under the effect of magnetic field as compared to that of no magnetic field. It was also predicted that double in-line parallel arrangement of permanent magnets has resulted in higher value of Nusselt number (Nu) as compared to single-line arrangement for constant mass flow rate of ferrofluid through the tube.

The effect of nonuniform transverse magnetic field on hydrothermal characteristics of water-based ferrofluid containing 4% (by volume) Fe3O4 was examined using single-phase model by Shakiba and Vahedi [16]. The ferrofluid containing iron-oxide nanoparticles was passed through the inner pipe of a horizontal double-pipe heat exchanger with air being considered as cold fluid flowing in counter direction in the outer pipe. Continuity, momentum, and energy equations were modeled and solved using ansys fluent, considering flow to be laminar, steady, and incompressible. The credibility of the model was checked by comparing the numerical results with the results obtained by Kim et al. [17] and Aminfar et al. [18], and a good consistency was reported between the results. A rapid enhancement in Nu was found initially with increase in the intensity of transverse magnetic field but the enhancement percentage becomes almost constant with higher magnetic intensities. Also a higher percentage increase in Nusselt number was observed at lower Reynolds number values. Increase in the intensity of nonuniform transverse magnetic field also supplemented higher change in cold and hot fluid temperatures.

Shahsavar et al. [19] experimentally reported the laminar-forced convective heat transfer behavior of hybrid nanofluid containing tetramethylammonium hydroxide coated Fe3O4 nanoparticles with an average particle size of 10 nm and gum arabic (GA) coated carbon nanotubes (CNTs) for hydro-dynamically fully developed flow in a copper tube (internal and external diameter of 4.8 mm and 6 mm, respectively, with a length of 1.25 m) under the effect of constant and alternating magnetic field. Nusselt number of the nanofluid was observed to increase with an increase of concentration of Fe3O4/CNT nanoparticles in the presence as well as absence of magnetic field. It was also found that heat transfer coefficient increases with Reynolds number in the absence of magnetic field, and an opposite trend was observed in the presence of magnetic field. Another interesting aspect of the experimental result was that the constant magnetic field was found to have a more considerable effect on heat transfer than that of alternating magnetic field. The obtained experimental results of convective heat transfer and Nusselt number were reported to have uncertainties less than 3.9% and 4.9%, respectively.

Forced convection heat transfer in a semi-annulus lid filled with Fe3O4/water ferrofluid was studied under external magnetic field by Sheikholeslami et al. [20] Numerical investigations were carried out to analyze the effect of different parameters such as Reynolds number, nanoparticle volume fraction, magnetic number, and Hartmann number, and a set of governing equations were solved using control volume-based finite element method approach. The results obtained using numerical code were validated by comparing them with the already published work in the literature by Khanafer et al. [21] and Moallemi and Jang [22], and an excellent agreement was found among the results. Thickening of thermal boundary layer and rise in velocity were observed with the increase in nanoparticle volume fraction. The results indicate that the Nusselt number and heat transfer had direct relation with the Reynolds number, nanoparticle volume fraction, and magnetic number and an inverse relationship with the Hartmann number.

Garg et al. [23] carried out numerical investigations to compare the performance of a magnetic cooling system using water and kerosene-based ferrofluid. Different variables such as temperature difference near the heating region across the inlet and outlet, pressure difference across the heating substrate, and heat transfer coefficient were evaluated for both water-based and kerosene-based ferrofluid using COMSOL multiphysics. Smooth and sustainable flow was established for kerosene-based ferrofluid, while pulsating flow with continuous flow variation was found for water-based ferrofluid. However, water-based ferrofluid was found to give higher heat transfer coefficient in comparison to kerosene-based ferrofluid.

Sheikhnejad et al. [24] numerically investigated the steady-state laminar-forced convection ferrofluid flow through a circular axisymmetric horizontal pipe to find the optimum magnetic field distribution for heat transfer study. Ferrofluid was subjected to different magnetic fields in axial direction and constant heat flux conditions. Governing equations were solved numerically using finite volume method and simple algorithm. Augmentation of heat transfer upto 135.7% and increase of pressure drop along the longitudinal axis of the pipe to about 77% were recorded under the influence of external magnetic field. The effect of thermomagnetic number on heat transfer effectiveness was found to be higher as compared to hydromagnetic number that resulted in the decrease in heat transfer especially in the developing zone. Higher intensity of magnetic field combined with lower gradient was stated to be the best option for attaining greater heat transfer.

A comparison was made by Hanafizadeh et al. [25] between three different two-phase models, namely, volume of fluid, mixture, and Eulerian model and four different single-phase models (constant, Maxwell, Brownian, and proposed models) for numerical analysis of forced convective heat transfer of Fe3O4–water nanofluid. Four different correlations for thermal conductivity of nanofluid as proposed by single-phase models were evaluated in low, moderate, and high Reynolds numbers at 1% and 2% volume fractions of nanofluid, and the Maxwell model was found to be in best agreement with the experimental data in fully developed region. The simulation results also pointed an increase in the heat transfer coefficient with the increase in Re and volume fraction of nanoparticles. Numerical simulations performed using multiphase models also revealed that average heat transfer coefficient evaluated by the mixture model have minimum deviation from the experimental results at all Reynolds numbers and nanofluid volume fractions.

Kerosene-based Fe2O3 ferrofluid was allowed to flow through a closed-loop pulsating heat pipe in the presence of the magnetic field with an aim to evaluate the critical angle of the pipe at which the maximum heat transfer would be obtained under different heat inputs by Goshayeshi et al. [26]. The effect on thermal efficiency of the system and heat transfer coefficient was examined experimentally by varying the pipe angle in relation to horizontal axis between 0 deg and 90 deg at a filling ratio of 50%. The experimental uncertainties in evaluating thermal resistance and heat transfer coefficient were found to be 3.29% and 4.1%, respectively. Significant decrease in overall thermal resistance and improvement in the thermal performance were observed for Fe2O3 nanofluid-charged oscillating heat pipe (OHP) especially under the magnetic field. Decrease in the boundary layer thickness and increase in the thermal conductivity of the base fluid by the suspended ferronanoparticles under the magnetic field were quoted as reasons behind the enhancement of convective heat transfer. An improvement in the thermal performance of the system was witnessed as the inclination angle of the pipe varied from 0 deg to 75 deg, while the performance deteriorated by increasing the angle from 75 deg to 90 deg, and the best inclination angle thus found to be 75 deg.

Goshayeshi et al. [27] performed an experimental study on Fe2O3–kerosene nanofluid in a copper (OHP) oscillating heat pipe (length 380 mm, inner diameter 1.75 mm, and outer diameter 3 mm) subjected to magnetic field. The temperature distribution, change in thermal resistance, and heat transfer rate of the heat pipe at a liquid-filled ratio of 50% were measured with and without the magnetic field. The experimental error analysis revealed uncertainties of the order of 3.29%, 4.10%, and 3.12% in thermal resistance, heat transfer coefficient, and thermal efficiency, respectively. It was concluded from the experimental observations that when exposed to magnetic field, the overall thermal resistance of the Fe2O3 nanofluid-charged OHP was reduced in comparison to that of the kerosene-charged OHP, which in turn has resulted in an enhancement of about 16% in the heat transfer of the OHP.

Forced convective heat transfer of water-based Fe3O4 ferrofluid in laminar flow regime was investigated numerically for different Reynolds numbers at 100, 600, 1200, and 2000 by Goharkhah and Ashjaee [28]. The fluid flows through a two-dimensional channel of size 0.4 cm (H) × 50 cm (L) and was subjected to an alternating nonuniform magnetic field created by eight identical magnetic dipoles, four being placed above the top surface and four below the bottom surface, and distance between the dipoles was maintained at 0.09 m. Grid independence test was followed by validation of the numerical code by comparing the calculated values of Nu with the results of Ganguly et al. [29] and were found to be in good agreement with the previously published data. Convective heat transfer of the ferrofluid was found to enhance noticeably under the influence of alternating magnetic field compared to zero magnetic field case. A maximum heat transfer enhancement of 13.9% was obtained for a constant Reynolds number of 2000 and a frequency of 20 Hz. Furthermore, heat transfer was found to increase with the Reynolds number and magnetic field intensity.

An experimental examination to study the effect of Reynolds number, ferrofluid concentration, and intensity of magnetic field on convective heat transfer of water-based magnetite (Fe3O4) ferrofluid in a heated tube with 9.8 mm diameter and 2680 mm length was performed by Goharkhah et al. [30]. Four electromagnets were used to generate the magnetic field, and local convective coefficients were measured in the Reynolds number range of 400–1200 for three different volume fractions, φ at 1%, 1.5%, and 2%. The average uncertainty error of 3.2% was reported in the calculation of local convective heat transfer coefficient. Convective heat transfer was found to be an increasing function of Reynolds number, volume fraction, and intensity and frequency of the magnetic field. Significant enhancement in heat transfer up to 18.9% and 31.4% was reported under the influence of constant and alternating magnetic field, respectively. Migration of nanoparticles to the tube surface led to an increase in the local particle concentration which in turn increases the local thermal conductivity under the constant magnetic field, and in case of alternate magnetic field, better flow mixing were stated to be the possible reason for heat transfer augmentation.

Goharkhah et al. [31] synthesized ferrofluid with a mean particle size of 30 nm by conventional coprecipitation process and ultrasonicated it to avoid formation of agglomeration of the nanoparticles. An experimental study was performed on laminar-forced convective heat transfer and pressure drop of water-based magnetite (Fe3O4) nanofluid in a uniformly heated parallel plate channel of an aspect ratio 10. The local convective coefficients were measured at both thermally developing and fully developed regions for different nanofluid concentrations (Φ at 1%, 1.5%, and 2%) in the Reynolds number range of 200–1200. The average uncertainty in the measurement of local convection heat transfer coefficient was calculated to be ± 4.8%. The convective heat transfer experiments show that at Re = 1200 and Φ = 2%, an enhancement of 16% in convective heat transfer was found using the magnetite nanofluid compared to the de-ionized water. Due to the augmentation of fluid viscosity with the addition of nanoparticles, pressure drop in the channel was found be to an increasing function of Reynolds number and the volume fraction of nanoparticles. A parameter called efficiency index (η = hnf/hfPnfPf) was defined to find the effect of change in the convective heat transfer coefficient and change in pressure drop on the overall performance of ferrofluid. Higher values of efficiency index was obtained at lower nanoparticle volume fractions, while values lower than unity was found for lower concentrations indicating that lower volume fractions are better choice for heat transfer augmentation.

The effect of alternating magnetic field on forced convective heat transfer and pressure drop of Fe3O4/water ferrofluid were investigated experimentally at nanoparticles volume concentration at 1%, 1.5%, and 2% and flow rates (Re = 200–1200) by Goharkhah et al. [32]. The ferrofluid was allowed to flow into a channel with uniformly heated top and bottom copper parallel plates. The magnetic field was generated by four electromagnets, and their efficient arrangement and locations along the channel were also obtained by numerical simulations performed using COMSOL. The effect of alternating magnetic field on convective heat transfer was more pronounced as convective heat transfer coefficient was increased by 37.3% in comparison to 24.9% increase under constant field. Better flow mixing and disturbance of thermal boundary layer due to periodic attraction of the cold fluid to the heated surface and releasing it to the bulk flow at disconnection time of the electromagnets were stated to be the possible explanation for augmentation of heat transfer in the case of alternating magnetic field. The average heat transfer coefficient was found to have a direct relation with the frequency of magnetic field and Reynolds number analogous to his earlier experimental findings [31] for ferrofluid flow through a tube. The average uncertainty in the measurement of local convection heat transfer coefficient was estimated to be around ± 3.2%. The pressure drop was also found to increase with the volume fraction of ferrofluid similar to earlier findings [30]; thus, higher concentrations of ferrofluid were not recommended as it may require higher pumping power. It was also observed that in the presence of the alternating magnetic field, there was an insignificant increase in the pressure drop as compared to the augment of heat transfer.

The effects of the volume concentration of magnetic nanoparticles, Reynolds number of the flow, strength, and arrangement of the magnetic field on the forced convective heat transfer in a copper tube with circular section (inner diameter of 4.8 mm, the outer diameter of 6 mm, and the length of 124.5 cm) under constant heat flux were investigated experimentally by Yarahmadi et al. [33]. Water-based ferrofluid with Fe3O4 magnetic nanoparticles having a mean diameter of 25 nm with surfactant coating of citric acid were used for the investigation. A maximum uncertainty error of 4.2% was reported in the experimental data of local convective heat transfer coefficient. Augmentation in the convective heat transfer was observed with the addition of magnetic nanoparticles and with the increase of volume concentration of the nanofluids in the absence of magnetic field. Application of a constant magnetic field led to reduction in the convective heat transfer and increase in viscosity of the ferrofluid. The local convective heat transfer was found to enhance by 19.8% under an oscillatory magnetic field as compared to the case when no magnetic field was applied, and the effect was more significant in lower Reynolds number flows and higher volume concentrations of ferrofluid. Higher maximum heat transfer coefficient enhancement was seen using a longer sequence of magnets along the length of pipe, but a negative trend was observed by increasing the space between the pair of magnets.

The effects of flow rate and nanoparticle volume fraction on heat transfer performance of thermally developing Fe3O4 nanoparticles coated with lauric acid suspended in de-ionized water were investigated experimentally by Kurtoğlu et al. [34]. The uncertainty in the measured value of heat transfer coefficient was found using the propagation of uncertainty method developed by Kline and McClintock [35] and reported to be ± 11.8%. The ferrofluid was run at three different flow rates (at 1 ml/s, 0.62 ml/s, and 0.36 ml/s) through a 2.5-cm long hypodermic stainless steel microtube having an inner diameter of 0.514 mm and an outer diameter of 0.819 mm. The volume fraction of nanoparticles was varied from 0% to 5% with an average particle diameter of 25 nm. The microtube used was subjected to a heat flux of up to 184 W/cm2. An enhancement in the heat transfer coefficient and decrease of around 100% in the maximum surface temperature (using ferrofluid compared to the pure base fluid at heat fluxes > 100 W/cm2) with increasing flow rate were observed as shown in Figs. 4(a), 4(b), 5(a), and 5(b), respectively. The heat transfer coefficient was found to be an increasing function of nanoparticle concentration for all the fluid samples used in the experimental study as shown in Fig. 6.

Forced convective heat transfer of water-based nanofluid containing magnetic nanoparticles of Fe3O4 having a mean diameter of 36 nm for three volume concentrations at 0.6%, 1%, and 2% was numerically investigated by Hosseindibaeebonab et al. [36]. The ferrofluid flows through a copper tube (inner diameter of 9 mm and length of 56 cm) under laminar and uniform heat flux condition. The ferrofluid was subjected to alternating magnetic fields for specified volume concentrations. Under an alternating magnetic field, average heat transfer increase of 27.6 ± 1.22% at a Reynolds number of 80 was observed with increase in the magnetic field frequency and volume concentration. Simulation results were also validated in the study by comparing them with experimental findings of Ghofrani et al. [37], and maximum deviation was reported to be about 10%.

The performance of water-based Mn–Zn ferrite magnetic nanofluid in a counter-flow double-pipe heat exchanger under quadrupole magnetic field was investigated by Bahiraei and Hangi [38]. Heat exchanger was tested for parameters such as concentration, magnetic particles' size, strength of magnetic field, and Reynolds number. The mathematical model for overall convective heat transfer coefficient of the heat exchanger and pressure drop of the nanofluid was constructed in terms of concentration, size of particles, and magnitude of the magnetic field using neural network, and optimization was performed using GA technique with the objective of maximizing overall heat transfer coefficient along with the minimizing pressure drop. Improvement in the heat transfer was observed with the increase of particles' concentration, size, and magnitude of the magnetic field but the same also resulted in greater pressure drop. So, the optimal values of different parameters were obtained for different conditions of relative importance of the objective functions.

Time-dependent hydrothermal behavior of water-based ferrofluid flowing in a helical channel subjected to nonuniform transverse magnetic field was investigated numerically using the control volume technique by Aminfar et al. [39]. The two-phase mixture model was used to solve the governing equations. The flow was assumed to be viscous, laminar, and incompressible, and further the effect of magnetic field on the viscosity and thermal conductivity of the ferrofluid was assumed to be negligible. The validity of the model was determined by comparing it with the previously published experimental work using water–Al2O3 nanofluid in the absence of magnetic field. Also, a good agreement was found between the results obtained for Nusselt number executing the present code and experimental results of Kim et al. [17] in a horizontal tube. Under the influence of nonuniform magnetic field, increase in the flow rate and velocity gradient in the vicinity of the inner wall of the channel was observed which contributed to the augmentation of heat transfer. An increase of 16% in the average Nusselt number was reported at Mn = 1.4 × 106 as shown in Fig. 7. However, total friction factor along the channel length was found to increase upon application of magnetic field as depicted in Fig. 8.

Free Convective Heat Transfer Using Ferrofluid.

In natural or free convection, fluid is moved by natural means such as the buoyancy effect, i.e., convection current will setup, and colder fluid will displace the hotter fluid without the use of pump or fan.

Comparison of heat transfer performance between semicircular and triangular notched cavity was numerically investigated by Rabbi et al. [40]. The two-dimensional square enclosure in both cases was filled with Fe3O4–water ferrofluid, subjected to external uniform magnetic field. Ferrofluid was modeled as Newtonian and incompressible single-phase fluid, and the flow was considered to be steady, two-dimensional, and laminar. The reliability of simulation code was tested by comparing the flow and thermal field by plotting streamlines and isothermal contours with the results of Oztop et al. [41], and a good agreement was established with the previous study. It was reported that in notch-induced convection, natural convection was proved to be the more favorable mode of heat transfer compared to forced convection. Augmentation of Hartmann number lead to stronger magnetic field, thus result in creation of Lorentz force in flow field and decrease of Nusselt number. Richardson number and nanoparticle volume fraction have positive impact on the overall heat transfer rate. Higher convection effect was found in semicircular notched cavity compared to triangular notched cavity on account of availability of more area.

A feasibility study of enhancement of cooling performance of a natural convection-based passive vertical flow loop by thermomagnetically pumped kerosene-based ferrofluid under static magnetic field was demonstrated by Aursand et al. [42]. Enhancement factor (Qeff/Qref) was used to evaluate the improvement in cooling performance of the loop. The value of enhancement factor was found to be quite high in single-phase regime at higher field strengths. The same trend was observed too for two-phase boiling system but on a lesser scale. Also, ferrofluid-driven heat transfer allows reduction of the heat sink size by up to 75% while retaining the original performance. However, the model used did not include magnetoviscous effect that might affect the value of enhancement factor claimed in the study, and the assumption of fully developed flow does not hold good as Graetz number was found to have higher values than that required for fully developed flow, thus underestimating the values of Nusselt number.

A one-dimensional multiphase model to predict the effects of applied heat and magnetic field on thermomagnetically pumped kerosene-based ferrofluid in a pipe flow was proposed by Aursand et al. [43]. The model includes the effects of heat transfer, friction, gravity, and magnetic field on the fluid as well as the dependence of the ferrofluid magnetization on applied magnetic field and temperature. The performance of the thermomagnetic pump was measured in terms of pressure difference achieved due to the magnetic field effect generated by symmetrically placed solenoid on both sides of the cylindrical pipe. Simulation results showed that the magnetization model was successful in reproducing the performance of thermomagnetic pump. Results were validated and found to be in agreement with the experimental data presented in Iwamoto et al. [44]. However, the correct assessment of performance of thermomagnetic pump was found to be highly dependent on correlations used for modeling heat transfer coefficient.

Jafari et al. [45] examined the effect of gravitational-induced sedimentation on heat transport and thermomagnetic convection in a kerosene-based ferrofluid using computational fluid dynamics. A disk with a height and diameter 3.5 mm and 75 mm, respectively, was used to observe the behavior and heat transfer characteristic of the fluid. The thermal efficiency of the system was measured in terms of dimensionless parameters such as gravitational (Rag) and magnetic Rayleigh numbers (Ram), and the heat transfer by thermomagnetic convection was found to be more efficient in comparison to pure natural convection. It was also concluded that there is very less possibility for the formation of agglomerates if the ratio of magnetic energy/gravitational energy be maintained above unity.

The effects of magnetohydrodynamics and ferrohydrodynamics on the flow and heat transfer characteristics of Fe3O4/water ferrofluid were investigated numerically for different governing parameters such as the Rayleigh number, nanoparticle volume fraction, magnetic number, and Hartmann number using control volume-based finite element method by Sheikholeslami and Rashidi [46]. The study was well validated by comparing the output of the code with the previous work of Khanafer et al. [21] reported in the literature for natural convection in an enclosure filled with Cu–water nanofluid. Heat transfer coefficient was found to be an increasing function of Magnetic number, Rayleigh number, nanoparticle volume fraction and decreasing function of Hartmann number.

The effect of unsteady magnetohydrodynamic convection in a semicircular-shaped enclosure filled with Cobalt–kerosene ferrofluid subjected to external magnetic field was analyzed using numerical and statistical technique by Rahman et al. [47]. The Galerkin-weighted residuals method of finite element method was used for the numerical simulation. The reliability of the numerical code was checked by comparing the isotherm and streamline produced by the present code with previously published work of Ghasemi et al. [48], and the results were found to be in good agreement. Ferrofluid was modeled as Newtonian and incompressible fluid, and it was concluded that increasing Ra results in better average heat transfer coefficient irrespective of the magnetic field strength. Increase in local Nusselt number was also observed for higher volume fraction of solid nanoparticle, while Hartmann number had negative effect on average heat transfer coefficient.

Shahsavar et al. [49] experimentally determined the effect of ultrasonication time on thermal conductivity of the water-based magnetic fluid containing homogeneous mixture of Fe3O4 coated with tetramethylammonium hydroxide and GA carbon nanotubes. The experiments were conducted to gauge the effect of parameters such as magnetic nanoparticles mass concentration, CNT mass concentration, and temperature on thermal conductivity of the fluid. It was found that the thermal conductivity of the studied nanofluid is higher than the ferrofluid with the same Fe3O4 mass fraction and is totally dependent on the temperature and Fe3O4 and CNT concentration. An enhancement of 13% and 15.59% in thermal conductivity at 0.494% FF + 0.105% CNT and augmentation of 26.07% and 34.26% was observed at 2.428% FF + 1.535% CNT with 5-min sonication time in the temperature range of 25–55 °C compared to the base fluid. An increase in the thermal conductivity of the fluid was observed first, and after attaining the maximum value at an optimum sonication time, the value started to decrease.

The influence of magnetic field strength and nanoparticle concentration on fluid velocity, temperature, flow drag, pressure drop, and heat transfer rate of water-based magnetite (Fe3O4) nanofluid prepared by the coprecipitation and solgel method were calculated by Lo and Weng [50]. The fluid was passed through an isothermally heated horizontal microtube having an internal diameter of 285 μm and a length of 32.23 mm under the electromagnetic field induced by an external induction device. A finite difference method called marching implicit procedure was used for the numerical analysis of thermal flow fields. The numerical results were compared with experimental findings under different field strengths and were found to conform to experimental data. The results reveal that the clogging effect increased with nanoparticle concentration and lead to greater pressure drop and decrease in average heat transfer rate. Furthermore, with increase in the strength of external magnetic field, higher pressure drop and higher average flow drag were observed resulting in insignificant rise in the average heat transfer rate.

Seo et al. [51] numerically investigated the flow and thermal characteristics of water-based ferrofluid in a rectangular microchannel. The influence of rectangular and triangular blocks on the channel wall under external magnetic field generated by permanent magnets was envisaged using COMSOL multiphysics. Numerical analysis predicted enhancement in heat transfer due to the generation of high-velocity secondary flows when the fluid was subjected to perpendicular-type magnet. Also, higher improvement in heat transfer performance was observed for rectangular block than for triangular block as the former results in the generation of widely distributed secondary flow. However, code validation was not reported in the specified study.

Selimefendigil et al. [52] examined the effects of Rayleigh number, heater location, strength, and location of magnetic dipole on heat transfer of a ferrofluid in a partially heated square cavity under the influence of a magnetic dipole using a general purpose finite element code, by COMSOL. Overall a good agreement was reported between the numerical findings of the code for Nu and velocity profile with the results of Oztop and Abu-Nada [53]. The temperature gradient was applied between left and right vertical wall of the cavity, while the remaining walls were maintained at constant temperature. An increase in the average heat transfer coefficient was observed with the increase of Rayleigh number, while a reverse trend was followed with magnetic dipole strength. Decreasing the horizontal distance between magnetic dipole sources resulted in increase of average heat transfer. Also minimum heat transfer was achieved when the magnetic dipole source was positioned at the middle of vertical wall.

Sheikholeslami and Ganji [54] assessed the effect of different parameters such as the Rayleigh number, Hartmann number, Nanoparticle concentration, and Magnetic number on flow and heat transfer characteristics of water-based magnetite ferrofluid in a semi-annulus enclosure with sinusoidal hot wall subjected to external magnetic field. Control volume-based finite element method was used to solve the governing equations. The code was tested for mesh independence followed by its validation. The results obtained using the current code was compared with the work of Khanafer et al. [21] for natural convection, and an excellent agreement was reported between the present study and previous work. An enhancement in the heat transfer was observed with the Rayleigh number and nanoparticle volume fraction. For low Rayleigh number, Nusselt number increases with augment of Hartmann number, while a decrement in the value was recorded with increase of Magnetic number at high Rayleigh number.

Table 1 summarizes other important experimental/numerical investigations (2011–2014) on heat transfer enhancement using different types of ferrofluid, mentioning details about model used, test equipment, parameters studied, and important remarks/findings of the work.

Conclusion and Future Directions

A comprehensive review of heat transfer enhancement using ferrofluid supported by experimental/numerical investigations is done in this paper.

1. (1)Based on the review, it is evident that magnetic nanoparticles under the effect of magnetic field greatly enhance the convective heat transport ability of ferrofluid in comparison to that of no magnetic field. The result holds good for both free and forced convective heat transfer modes.
2. (2)Different important variables that have direct bearing on convective heat transfer coefficient of ferrofluid have been identified and shown in Fig. 9Fig. 9

Diagram showing important variables that have significant effect on convective heat transfer coefficient of ferrofluid

.
3. (3)With increase in strength of the external magnetic field, heat transfer tend to increase as noted in both modes of convective heat transfer, i.e., free and forced convection, but larger pressure drop penalty is also encountered at higher values of the field, consequently suppressing the benefit of enhancement in the heat transfer coefficient.
4. (4)Contradictory results exist in the literature regarding the superiority of nature of magnetic field on heat transfer enhancement. Some studies pointed out that the effect of constant magnetic field on heat transfer is more pronounced in comparison to alternate magnetic field, however, others opined that alternate magnetic field results in higher value of convective heat transfer coefficient. This aspect required to be addressed properly so that an assertive conclusion can be drawn.
5. (5)Since ferrofluids consist of suspended magnetic nanoparticles in the base fluid, long-term stability need to be ensured as stability would limit the reliability of the cooling systems that run on ferrofluid. Measurement of zeta potential of the fluid (test for stability) has not been quoted in majority of the experimental work reported in this review.
6. (6)Heat transfer coefficient was found to increase with Re, but higher Re also leads to larger pressure drop. So, an optimum balance must be established between Nu and pressure drop while designing such a system for different values of Re.
7. (7)Lower values of Reynolds number result in higher percentage enhancement in convective heat transfer coefficient as reported in both free and forced convective heat transfer.
8. (8)Enhancement in heat transfer is an increasing function of nanoparticles volume fraction as claimed in the literature, but it has also resulted in higher pressure drop due to the augmentation of fluid viscosity. Both these trends are contradictory to each other and precaution must be exercised while selecting the volume fraction of magnetic nanoparticles.
9. (9)There was a noticeable increase in the heat transfer coefficient with intensity and frequency of alternate magnetic field. Increased flow mixing and disturbance of thermal boundary layer were stated to be the possible reasons behind heat transfer augmentation.
10. (10)Nanoparticles size has inverse relation with convective heat transfer coefficient. Large size promotes clogging and sedimentation of nanoparticles which adversely affect the stability of the fluid.
11. (11)Very limited work has been done using ferrofluid as a heat transfer media in microchannels. Mostly, researchers have restricted themselves to flow between parallel plates or fluid flow through a pipe. So, future course of action should focus on potential applications of ferrofluid for the cooling of microscale devices using microchannels.
12. (12)Effect of important parameters such as channel shape/size and nanoparticle shape/geometry on heat transfer application of ferrofluid has not been explored yet or is scarce.
13. (13)The effect of orientation of magnetic field on the thermal performance of cooling devices utilizing ferrofluid as a coolant has been studied to a lesser extent. More emphasis should be given to critically enhance the cooling ability of ferrofluid under varying conditions of magnetic force.
14. (14)As ferrofluid is a two-phase mixture consisting of liquid base fluid and nanosized magnetic particles, a more realistic approach for analyzing such fluids could be developed by the well-proven mixture theory. So far, researchers have tried single-phase and two-phase mixture model approaches for analysis but with varying results. So, a well-established theory should be developed for accurate prediction of flow and thermal behaviors of ferrofluids.
15. (15)There is great mismatch between the results obtained using single-phase and two-phase modeling approach as well as between numerical and theoretical predictions which should be looked upon. Reliable correlations and accurate model need to be developed that can minimize the gap/error between numerical predictions and experimental observations.
16. (16)The effect of boundary layer thickness on heat transfer in microscale devices need to be accounted for.
17. Ferrofluids, thus have a promising potential for heat transfer applications, particularly for mini/microscale devices which generate high heat flux. A numerical analysis should be followed by the experimental research to widespread the use of ferrofluids for such applications.

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References

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Figures

Fig. 1

Components of ferrofluid

Fig. 2

Effect of temperature on magnetization

Fig. 3

Thermomagnetic convection principle

Fig. 4

(a) Enhancement of heat transfer coefficient measured as ratio of h/hpure water for different heat fluxes at flow rate, Q = 0.36 ml/s [34] and (b) enhancement of heat transfer coefficient measured as ratio of h/hpure water for different heat fluxes at flow rate, Q = 1 ml/s [34]

Fig. 5

(a) Increase in surface temperature with respect to ambient temperature for different heat fluxes at flow rate, Q = 0.36 ml/s [34] and (b) increase in surface temperature with respect to ambient temperature for different heat fluxes at flow rate, Q = 1 ml/s [34]

Fig. 6

Percentage enhancement of heat transfer as a function of dilution amount at different flow rates, Q [34]

Fig. 7

Effect of applying nonuniform transverse magnetic field with different intensities on the average Nusselt number [39]

Fig. 8

Effect of applying nonuniform transverse magnetic field on the friction factor along the channel length [39]

Tables

Table 1 Summary of experimental/numerical investigations on heat transfer enhancement using ferrofluid (2011–2014)

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