Applied Thermal Engineering 111 (2017) 1101–1110

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Applied Thermal Engineering

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Research Paper

Experimental analysis of magnetic field effect on the pool boilingheat transfer of a ferrofluid

http://dx.doi.org/10.1016/j.applthermaleng.2016.10.0191359-4311/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: Salimpour@cc.iut.ac.ir (M.R. Salimpour).

Ali Abdollahi a, Mohammad Reza Salimpour a,⇑, Nasrin Etesami b

aDepartment of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, IranbDepartment of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

h i g h l i g h t s

� Pool boiling heat transfer characteristics of a ferrofluid were studied.� Presence of positive magnetic field gradient decreases the boiling heat transfer.� At higher concentrations of nanofluid, the effect of the magnetic field is boosted.

a r t i c l e i n f o

Article history:Received 21 April 2016Revised 6 September 2016Accepted 6 October 2016Available online 6 October 2016

Keywords:Pool boilingHeat transferMagnetic fieldNanofluidExperimental

a b s t r a c t

In this research, an experimental study was conducted to investigate the pool boiling heat transfer ofFe3O4/water nanofluid (ferrofluid) in the atmospheric pressure. This study also investigated the influenceof the magnetic field on the rate of boiling heat transfer of nanofluid. Deionized (DI) water was used toexamine the repeatability, integrity and precision of the experimental apparatus where a well agreementwith the existing correlations was observed. The investigation of various volume concentrations of nano-fluid revealed that boiling heat transfer in high concentrations decreases with an increase of concentra-tion while it rises with the increase of concentration in low concentrations. The boiling heat transfercoefficient at 0.1% volume concentration nanofluid was evaluated as optimal (increasing up to 43%). Inaddition, experimental studies showed that the presence of positive and negative magnetic field gradi-ents decrease and increase the boiling heat transfer, respectively. The findings of this study showed thatat higher concentrations of nanofluid, the effect of the magnetic field on nanoparticles is boosted. Theresults of the experiments indicated that adding nanoparticles would not necessarily increase the boilingheat transfer coefficient. In fact, the surface roughness and the magnetic field gradient on the boiling sur-face were the main factors that could affect the boiling heat transfer coefficient, significantly.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Boiling process is considered as one of the crucial phenomena invarious systems such as air conditioning, electronic devices coolingand so forth. Among the different aspects of the boiling process,increasing the boiling heat transfer has been significant in the fieldof thermal engineering during recent decades. There are fourteenmethods to increase the boiling heat transfer, which can be dividedinto two main categories of active and passive. Passive methods,including extended surfaces, rough surfaces and applying additivesto the fluid require external forces for increasing the heat transferwhile active methods such as applying electrostatic field and vibra-

tion of the surface or fluid do not need a direct application of force[1–3].

Recently, adding nanoparticles to the main fluid is highly con-sidered as a passive method for increasing heat transfer in the boil-ing process. Nanofluid is a colloidal suspension in which solidnanoparticles (smaller than 100 nm) are dispersed within the basefluid. Nanoparticles can be metallic, metallic oxides, nanotubes,and so forth [4–6]. Most of the previous studies [5,7] showed thatadding nanoparticles increases the critical heat flux and convectionheat transfer. Nonetheless, there are not unanimous results regard-ing the influence of adding nanoparticles on boiling heat transfercoefficient. Some researchers [8] have reported no change withinthe boiling heat transfer coefficient while others [9] observed thatthe presence of nanoparticles either increase or decrease heat ratein this process. Table 1 represents a summary of the resultsobtained through literature review regarding boiling heat transfer.

Nomenclature

CP specific heat (J/kg K)Csf coefficient in Rohsenow correlationd distance (mm)g gravity acceleration (m/s2)h heat transfer coefficient (W/m2 K)hfg fluid’s latent heat (J/kg)k thermal conductivity (W/m K)n power in Rohsenow correlationPr Prnadtl numberq00 heat flux (W/m2)Ra average roughness of the surface (lm)T temperature (K)t time (min)U uncertainty

Greek symbolsl dynamic viscosity (N/m2)q density (kg/m3)r surface tension (N/m)f electrical chargeu roughness parameter

Subscriptsl liquids surfacesat saturatedv Vapor

Table 1Literature review on pool boiling heat transfer.

Authors (year) Heater type (Dimension) Nanofluid (Nanoparticle size) Result of boiling heattransfer coefficient(BHTC)

Type of surface analysis(Roughness)

Ref.

You et al. (2003) Copper flat square (1 � 1 cm2) Al2O3-water (not stated) Unchanged Not analyzed [8]Das et al. (2003) Stain Steel (SS) Cylindrical cartridge

(D = 4,6.5 mm)Al2O3-water (38,58.4 nm) Deterioration Roughness measurement

(0.37–0.45 lm)[28,29]

Witharana (2003) Copper Rod (D = 25 mm) Au-water (30 nm)SiO2-water (30 nm)SiO2-EG(30 nm)

EnhancementDeteriorationDeterioration

Not analyzed [30]

Vassolloet et al. (2003) NiCr Wire (D = 0.4 mm) SiO2-water (15,50 nm) Unchanged Nanolayer directlyobserved

[31]

Wen and Ding (2005) SS disk (D = 150 mm) Al2O3-water (167.5 nm) Enhancement Not analyzed [17]Bang and Chang (2005) Rectangular plate (4 � 100 mm2) Al2O3-water (47 nm) Deterioration Roughness measurement

(37.22 nm)[32]

Kim et al. (2006) NiCr Wire (D = 0.2 mm)Ti Wire (D = 0.25 mm)

TiO2, Al2O3-water Deterioration Scanning electronmicroscope (SEM)

[33]

Kim et al. (2007) SS Wire grade 316 (D = 0.381 mm) Al2O3-water (110–210 nm),SiO2-water (20–40 nm),ZrO2-water (110–250 nm)

Deterioration SEM, Contact angle,profilometer

[7]

Chophkar (2007) Plane Copper plate (D = 60.5 mm) ZrO2-water (20–25 nm) Enhanced at lowconcentrationDeteriorated at highconcentration

Shows the SurfaceRoughness profile

[9]

Coursey & Kim (2008) Cu and Cuo plates, glass, gold coatedplates

Al2O3-water (45 nm)Al2O3-ethanol (45 nm)

EnhancementUnchanged

Contact angle [34]

Narayan et al. (2007) Vertical tubular surfaces Al2O3-water (47,150 nm) Enhancement Image processing called‘‘machine vision”(48,49,524 nm)

[24]

Liu and Liao (2008) Vertical copper bar (D = 80 mm,H = 120 mm)

CuO (50 nm) and SiO2 (35 nm)nanoparticle in water andalcohol

Deteriorated Roughness Measurement,Atomic Force Microscope(AFM)

[35]

Golubovic et al. (2009) NiCr Wire (D = 0.64 mm) Al2O3-water (22.6,46 nm)Bi2O3-water (38 nm)

Not Reported SEM, Contact angles,element analysis

[36]

Taylor and Phelon (2009) Wire heater (D = 0.255 mm) Al2O3-water (20 nm) Enhancement SEM [5]Soltani et al. (2009) SS cartridge heater (D = 25 mm

H = 76 mm)Ƴ-Al2O3 (20–30 nm)-CMC(carbon methyl cellulose)

Enhancement Not analyzed [37]

Liu et al. (2010) Cu cube(40 � 40 mm2) Carbon nanotube (CNT)-water(15 nm)

Enhancement Transmission ElectronMicroscopy(TEM)

[38]

Kwark et al. (2010) Square Cu plate (10 � 10 mm2) Al2O3-waterCuO-waterDiamond-water

Deteriorated SEM [19]

Suriyawong andWongwises (2010)

Cu and Al Circle plates TiO2-water (21 nm) Enhancement Not analyzed [39]

Kathirawan et al. (2011) SS plate heater (30 mm2) Multi walled CNT (MWCNT)-water (5–8,10–15 nm)

Enhancement Not analyzed [40]

Shikhbahae et al. (2012) Ni wire Fe3O4-water, ethylon glycol(EG) (50 nm)

EnhancementUnder electric field

SEM, Contact angle [41]

Hedge et al. (2012) Ni wire (D = 0.19 mm) Al2O3-water (10-80 nm) Not studiedCHF Enhancement

SEM [42]

Sharma et al. (2013) Vertically flat SS316 Sheet (0.914 mmthickness, 34 mm length and 5 mmwidth)

ZNO-water (<90 nm) Not studied SEM, Contact angles [43]

Raveshi et al. (2013) Circular Cu plate (D = 40 mm) Al2O3-water, EG (20-30 nm) Enhancement Timer roughness Tester [3]Umesh and Raja (2015) Flanging SS plates and enhanced

surfacesCuo-pentane Enhancement Not analyzed [44]

1102 A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110

20 25 30 35 40 45 50 55 60 65 70

Inte

nsit

y (a

.u.)

2 theta (degree)

220

311

511

440

Fig. 1. XRD pattern diagram for synthesized Fe3O4 nanoparticles sample.

A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110 1103

As shown in Table 1, the reported experimental results are in con-flict due to the complexity of boiling mechanism and the depen-dence of results to various factors including the type, size andconcentration of nanoparticles; the kind, characteristics, roughnessand geometry of the heater surface. While most of researchershave investigated Al2O3-water nanofluid, there are limited studiesin which SiO2-water, ZrO2-water, Bi2O3-water, TiO2-water, CuO-water, CuO-pentane and Fe3O4-water, ethylon glycol wereexamined.

In the recent decades, the effect of a magnetic field on the boil-ing heat transfer has been analyzed by numerous researchers, mostof which have performed their experiments on mercury boiling[10–13]. Applying a magnetic field is considered as an active boil-ing heat transfer method, which can be combined with the passivemethod of adding nanoparticles to the base fluid for obtaining asynergic result. Only a few studies have analyzed the simultaneousinfluence of these two methods. Liu et al. [14] studied the boilingheat transfer of iron oxide/water nanofluid on a vertical heaterbar made of Stainless Steel (outer diameter of 10 mm and a lengthof 180 mm) in the presence of a non-uniform magnetic field. Theyobserved an increase of pool boiling heat transfer and attributedthis increase to the effect of non-uniform magnetic field on thereduction of departure diameter.

Various factors such as nanofluid concentration, average size ofnanoparticles, average roughness of boiling surfaces and magneticfield gradient are significant parameters affecting the rate of poolboiling heat transfer of the nanofluid. In fact, these factors haveeffects on the density of nucleation sites, bubble releasing ratefrom the surface, the size of the bubbles on the boiling surface,thermal resistance of the surface, and surface tension. Althoughextensive researchers focus on this problem, the effects of concen-tration and average size of nanoparticles on fundamental parame-ters of boiling heat transfer of Fe3O4/DI water on a flat surface havenot been studied, yet. Moreover, the influence of a magnetic fieldand magnetic gradient on a magnetic nanofluid has been barelyanalyzed. In this study, an experimental setup is prepared to inves-tigate boiling heat transfer of Fe3O4/DI water nanofluid at variousvolume concentrations. In addition, this work reveals the influenceof the magnetic field on heat transfer of the surface of a cylindricalCopper block in atmospheric pressure. Moreover, a parametricstudy is performed to investigate the effects of the nanofluid con-centration and the magnetic field magnitude gradient on the fun-damental parameters of the nanofluid boiling.

Fig. 2. (a) DLS diagram and (b) TEM image in 0.1% volume concentrationsynthesized nanofluid.

2. Test procedure

2.1. Nanofluid preparation

The applied nanofluid within this research has been synthesizedaccording to the introduced approach by Berger et al. [15]. Thechemical equation of the nanofluid synthesis is as below:

FeCl2 þ 2FeCl3 þ 8NH3 þ 4H2O ! Fe3O4 þ 8NH4Cl ð1ÞPrimarily, 1 mL of a FeCl2 solution (2 M solution of FeCl2 in 2 M

HCl) has been mixed with 4 mL of FeCl3 (1 M solution of FeCl3 in2 M HCl). Afterward, 50 mL of 0.7 M ammonia solution has beenadded with a syringe pump with the rate of 375 mL/h. It shouldbe mentioned that the required materials have been purchasedfrom Merck.

In the meanwhile of adding ammonia solution, the solution hasbecome darker and magnetic particles have formed. By centrifug-ing the solution, highly dark sediment remained at the bottom ofthe test tube. Finally, 8 mL of a 25% tetra-methyl-ammoniumhydroxide solution has been added to the sediment to providethe solid-in-fluid suspension after stirring. Putting the final solu-

tion within a stirrer for 30 min results in the separation of exces-sive ammonia from the solution.

Fig. 1 demonstrates the diagram of X-ray Diffraction (XRD) pat-tern corresponding to the synthesized Fe3O4 nanoparticles withoutsurfactant. The XRD pattern indicates that the breaks of peaks havebeen indexed in (2 2 0), (311), (2 2 2), (400), (422), (511), and(440), which holds an excellent concordance with the pattern ofFe3O4. In addition, this pattern indicates a high purity for thisproduct.

Fig. 2 illustrates the results of Dynamic Light Scattering (DLS)and Transmission Electron Microscopy (TEM) tests. DLS –for

1104 A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110

showing the size distribution of magnetic nanoparticles- hasshown an average diameter of 25 nm for nanoparticles for a 0.1%volume concentration nanofluid.

Fig. 2b illustrates TEM, which represents the size of nanoparti-cles. The TEM result was taken in solid state of nanoparticles (Com-pany Philiphs and Model EM-208). The figure shows an averagediameter of 25 nm for nanoparticles which has a well agreementwith DLS test results.

The zeta potential (f) measures the electrical charge on the sur-face of particles and implies the physical stability of colloidal sys-tems. Based on previous studies [16], suspensions of nanoparticleswith f absolute value greater than 30 mV are thoroughly stabilized.In the present study, for the 0.1% volume concentration nanofluidwith pH of 7–8, f holds the value of �44.1 mV implying highly sta-bilized status of the nanofluid.

2.2. Testing instrument

The purpose of this study is to investigate the characteristics ofFe3O4/water nanofluid pool boiling on a flat circular plate (upperbase surface of cylindrical block). The general set-up of boiling testrig consists of four main parts including main boiling reservoir,cylindrical copper block, cooling system and power control system.Fig. 3a and b illustrates schematic set-up and the actual image ofthe boiling test rig, respectively.

The main boiling reservoir consists of the cylindrical Pyrex ves-sel, insulation system and holding frame. As shown in Fig. 3b, thecylindrical Pyrex vessel is with the height of 250 mm, outer diam-eter of 55 mm and glass thickness of 5 mm. At insulation system,

Fig. 3. (a) General set-up and (b) actual image.

the distal section of the glass was glued to a Teflon cylinder. Theused insulating Teflon cylinder is with the thickness of 90 mmand diameter of 300 mm. The Teflon material is Polytetrafluo-roethylene (PTFE), and its thermal conductivity (0.29 W/m K) wasmeasured using KD2 Pro instrument with the resolution of0.01 W/m K.

The copper block is a high-grade copper cylinder with the diam-eter and the length of 45 mm and 100 mm, respectively. The ther-mal conductivity of high-grade copper is 401W/m K. Since thethermal conductivity of the copper is several times higher thanthat of PTFE, radial heat transfer is negligible. In the center of thisTeflon cylinder, a hole with outer diameter of 45 mm was tappedfor locating the copper block. In the copper, three elements withdiameters of 9 mm and length of 50 mm made of steel werelocated in the bottom part, each having a power of 800 W. The heatenergy is transferred to the surface of the copper block throughconductive heat transfer. Fig. 4 depicts the geometry of copperblock and the location of attached heaters. The boiling heat transfersurface is located on the top surface of the copper cylinder, whichis in contact with the fluid. As is observed in Figs. 4 and 6, the cop-per block has three holes in the body and three holes on its lowersurface. The three holes within the body were the location of PT-100 Resistance Thermometer Detector (RTD) for measuring thetemperature. The holes were 22.5 mm deep, up to the middle ofthe copper cylinder, having diameters of 3 mm. They were locatedat distances of 7 mm, 19 mm, and 30 mm from the top surface ofthe copper (uncertainty of 0.1 mm) and a relative angle of 120�with each other. Furthermore, in order to evaluate the one-dimensional status of the heat transfer, the temperature measure-ment was carried out in a 30 mm distance from the surface in var-ious angles. The depth of these holes was different in differentangles. The acceptable difference of measured temperatures in thiscase was less than 0.1 K. Three holes were embedded into thelower surface of the copper cylinder for three heating elements.These holes had similar diameters of 9 mm and a depth of50 mm with a relative angle of 120�.

A cooling system was applied for keeping a constant fluid levelwithin the boiling reservoir. As is revealed from Fig. 3a, this systemincludes a glass condenser with the length of 150 mm, which waslocated on the Pyrex glass. The ending tube of the condenser was acone (the diameter changing from 30 mm to 60 mm). Therefore,using a Teflon band the area between the glass reservoir and thecondenser was sealed, appropriately. Having pumped the coolantwater of condenser from the cooling container, it enters the con-denser after passing through the Rotameter. The coolant watercondenses the vapors and returns it to the boiling reservoir. Theexiting water from the condenser will also go back to the coolingcontainer. Since the vapor side of the condenser is open to theatmosphere, the pressure of boiling reservoir is equal to atmo-spheric pressure.

In order to obtain different points of the boiling curve, a powercontrol system was invoked, Fig. 3. This system consisted of a volt-age controller (auto-trans) to change the voltage and provide var-ious input powers, an ammeter for measuring the current, avoltmeter for measuring the voltage, and a conductor to controlthe direction of the input current to the elements.

In all test runs, the boiling surface was polished with a N320sand paper and cleaned thereafter with Acetone and DI water toprovide similar conditions for surfaces. As shown in Fig. 5, theaverage roughness of the boiling surface was Ra = 0.48 lm whichwas measured using a roughness tester of model SJ210. The stan-dard uncertainty of surface roughness measurement was approxi-mately 10%. In order to avoid heat loss, the lower surface wasthoroughly covered with rock wool insulation.

In order to obtain the temperatures of different points of thecopper block, saturation temperature of the fluid, and the temper-

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

0.0

Rou

ghne

ss (

µm)

2.0 4.0 6.0 8.distanc

0 10.0 e (mm)

12.0 14.0 16.0

Fig. 5. Profile of surface roughness – Ra = 480 nm.

Fig. 4. Geometry of cylindrical copper block (scale: mm).

Table 2Uncertainty of the measured parameters.

Measured quantity Uncertainty

Distance 0.1 mmTemperature 0.1 KVoltage 1VAmperage 0.1 ARoughness 0.05 lmThermal conductivity 0.01 W/m K

A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110 1105

ature of entering and exiting cooling water within the condenser,the PT-100 RTD’s were used. These RTD’swere calibrated for thespecified temperature range of the present study with a precisionof 0.1 K. Atonics screens (Model TC4Y) were used to show thetemperature.

2.3. Measurement

In order to obtain the boiling curve, it is required to register theboiling surface temperature and saturation temperature of thefluid to calculate the transferred heat flux to the fluid. Table 2 rep-resents the uncertainty of all the measured parameters. Readingthe boiling surface temperature with a direct attachment of ther-mocouple on the surface produces an extra nucleation site, fol-lowed by significant occurring error in the final results. Most ofthe researchers have conducted a direct temperature measurementand only a few have calculated the surface temperature throughextrapolation [3]. As is shown in Fig. 6, two PT-100 RTD’s werepositioned at different distances from the boiling surface. In fact,the boiling surface temperature in the present study was obtained

indirectly through extrapolation. The third RTD (T3) was used tocontrol the power (through thermostat) for preventing any damageto the system.

In each experiment, 200 mL of nanofluid with a specific concen-tration was charged into the boiling main reservoir. Then, thenanofluid was warmed up for different powers to become stable,and temperatures were recorded 10 min after temperature stabi-lization (dTdt ¼ 0:01 K

min).

Fig. 6. Positioning of PT-100 RTD.

Fig. 7. Boiling heat transfer coefficient as a function of heat flux between DI watercompared with Rohsenow correlation.

1106 A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110

As mentioned formerly, the copper block is made of high-gradecopper and a PTFE Teflon layer was put to its surrounding. Sincethe thermal conduction of copper is three times higher than PTFE,radial heat loss can be neglected. The transferred heat flux to theboiling surface can be assumed as one-dimensional, and its valuecan be calculated as below:

q00 ¼ kT2 � T1

d2 � d1ð2Þ

Moreover, the temperature of the boiling surface can be calcu-lated using extrapolation and Eq. (2). Hence, the differencebetween the boiling surface temperature and saturation tempera-ture is obtained by Eq. (3):

DTS ¼ TS � Tsat ¼ T1 � q00d1

k

� �� Tsat ð3Þ

Additionally, the amount of heat transfer coefficient from heatflux and the wall superheat temperature can be calculated throughEq. (4):

h ¼ q00

DTSð4Þ

3. Results and discussion

3.1. Water boiling curve

Fig. 7 compares experimental boiling curves of DI water withthe results of the Rohsenow correlation. The Rohsenow correlationcan be expressed as follows [6,17]:

DTS ¼ hfg

Cp;lCsf

q00

llhfg

rgðql � qvÞ

� �0:5" #1

3

prn ð5Þ

where hfg is the fluid’s latent heat, r is surface tension, and ql andqv are fluid and vapor densities, respectively. In addition, Pr isPrandtl number, Csf and n of polished copper surface are constantsequal to 0.013 and 1, respectively.

Uncertainty analysis was performed according to the methodproposed by Moffat [18] as in Eqs. (6)–(8).

Uq00

q00 ¼ UT2�T1

T2 � T1

� �2

þ Ud2�d1

d2 � d1

� �2" #1=2

ð6Þ

UDTs

DTs¼ UT1�Tsat

T1 � Tsat

� �2

þ Ud1

d1

� �2

þ Uq00

q00

� �2" #1=2

ð7Þ

Uh

h¼ Uq00

q00

� �2

þ UDTs

DTs

� �2" #1=2

ð8Þ

The uncertainties of the temperature and distance measure-ment are 0.1 K and 0.1 mm, respectively. The results indicate thatthe maximum uncertainties in the heat flux calculation and boilingheat transfer coefficient are 4% and 5.8%, respectively. Based uponFig. 7, experimental results hold a good agreement with those ofRohsenow correlation. On the other hand, regarding the signifi-cance of repeatability, the experiments were replicated in differentdays with similar conditions. The results show that the main char-acteristics of the boiling heat transfer were remained unchangedthrough the replications.

3.2. Boiling curve for Fe3O4-DI water nanofluid

As mentioned in the case of DI water experimental tests, theseexperiments were replicated three times with similar conditionsfor all of the concentrations. Fig. 8 represents boiling curve ofexperimental test runs for Fe3O4-DI water nanofluid with 0.1% vol-ume concentration and their repeatability in three replications inthree different days. As is clearly observed, the uncertainty formost of the heat fluxes is nearly 5%, and the characteristics of boil-ing heat transfer also remained the same for different tests ofnanofluid.

It is also seen that the nanofluid, compared with DI water,retains a lower temperature difference for a similar heat flux.Hence, it maintains a higher heat transfer coefficient for any heatflux, compared with DI water. According to Fig. 8, in the case ofhigher heat fluxes (nuclear boiling), there is a growing increaseof boiling heat transfer coefficient. Hence, the nucleation sites

Fig. 10. Boiling heat transfer coefficient as a function of heat flux for differentvolume concentrations of the nanofluid.Fig. 8. Boiling heat transfer coefficient as a function of nanofluid heat flux with 0.1%

V concentration compared with DI water.

A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110 1107

and the separation of bubbles increase in the nuclear boilingregion. Therefore, the maximum boiling heat transfer coefficientof nanofluid increased more than 43% in comparison with that ofDI water.

3.3. Effect of volume concentration on nanofluid boiling curve

Fig. 9 illustrates the heat flux rate versus wall superheat for var-ious nanofluid concentrations. Since the heat flux rate changes theboiling heat transfer coefficient, the variation of the boiling heattransfer coefficient is depicted in Fig. 10. Fig. 9 clearly shows thatthe rate of heat flux significantly increases when the concentrationof the nanoparticle is raised from zero to 0.1%. By further increaseof the nanoparticles concentrations, the rate of the heat flux decli-nes and reaches less than the heat rate of the fluid without

Fig. 9. Heat flux as a function of wall superheat for different volume concentrationsof the nanofluid.

nanoparticles. Hence, there is an optimum point for nanofluid vol-ume concentration in which heat transfer coefficient is maximized.This phenomenon has also been reported by other researchers[3,8,19–21]. Based on Refs. [22,23], fluid properties such as thermalconductivity and viscosity may have significant effects on the boil-ing performance of nanofluids. Hence, thermal conductivity andviscosity of nanofluid were measured using KD2 Pro instrumentand Ubbelhode Viscometer, respectively; and it was seen that forthe nanofluid concentration range of the present study (0.01–0.2% V), the thermal conductivity and viscosity variations are 3–5.4% and 2.6–4.2%, respectively. Therefore, this slight change inthermal conductivity and viscosity do not affect boiling perfor-mance, remarkably. Thus, the main mechanism that could haveinfluenced the boiling heat transfer coefficient is roughness of sur-face. This matter conforms to the findings of Refs. [3,24].

Most of the researchers have described the precipitation of par-ticles on the surface as a main reason for observed empiricalresults [3,6,24]. In these studies, the parameter of u was consid-ered as the ratio of surface roughness to the average diameter ofthe particles. According to the published results of theseresearches, the heat flux increases in low concentrations anddecreases in high concentrations in the cases where a roughnessparameter is greater than one. However, there is a reduction inthe heat flux [24] when roughness parameter is smaller than orequal to unity. In these experiments, all of the surfaces were pol-ished with N320 sand paper to obtain roughness equals 480 nm.According to Fig. 2, the average diameter of nanoparticles is nearly25 nm. Since the surface roughness is greater than the averagenanoparticles size, smaller particles will settle in the nucleationsites and form new nucleation sites. Thus, there are an increasednumber of nucleation sites for the case of low concentrations.

On the other hand, Figs. 9 and 10 indicate that increasing theconcentration of nanoparticles from 0.1% to 0.4% lowers the heattransfer. According to Fig. 2, the size distribution of nanoparticlesgrew up to 50 nm; thus, a higher number of particles lie in thedimensional range of nucleation sites when the partial fractionincreases. Since adhesive force will also increase by raising thediameter of particles, larger particles adhere to nucleation sitesand consequently, nucleation sites deactivate and the heat transferdecreases dramatically [3,6,24]. Additionally, Aghayari et al. [25]showed that increasing the volume concentration of nanofluid

Fig. 12. Testing device with permanent magnets.

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raises the fluid viscosity and restrains the separation of boilingbubbles. Therefore, the heat transfer of fluid decreases.

Fig. 10 confirms that the boiling heat transfer coefficientincreases with the rise of heat flux in the concentrations up to0.1%. But, it decreases for the higher concentrations of the nano-fluid. Fig. 11 clearly shows that the boiling heat transfer coefficientis a function of volume concentration of the nanoparticles in thenanofluid. As mentioned before, raising volume concentration upto 0.1% increases heat transfer coefficient as much as 43%, whilefurther growing of volume concentration reduces the heat transfercoefficient. Therefore, there is an optimum value for nanofluid vol-ume concentration equal 0.1% V in which heat transfer coefficientis maximal.

3.4. Effect of magnetic field on boiling curve of the nanofluid

Since the nanoparticles are magnetic materials, the characteris-tics of nanofluid are highly affected by the magnetic field. Fig. 12illustrates the testing device for the examination of the nanofluidwithin a magnetic field of two permanent magnets. The magnetsare made of ceramic in the form of a cube with dimensions of150 � 100 � 25 mm. The distance between two magnets is 60 mm.

Fig. 13 illustrates the schematic of magnetic field contourwithin the distance between two magnets. As is evident from thefigure, the magnetic field is uniform between the two magnets.According to Figs. 12 and 13, the magnitude gradient of the mag-netic field increases from bottom to the top of the vessel. Thus,the gradient of magnetic field is positive and normal to the boilingsurface.

Fig. 14 compares the boiling heat transfer coefficients of thebase fluid and Fe3O4-DI water nanofluid (with 0.1% volume concen-tration) with and without the presence of a magnetic field. Accord-ing to the asymmetric form of water molecules, the hightemperature water (96 �C) and the high intensity magnetic fieldbetween two permanents (500–600 Gauss) the percentage ofIonization water is high. Thus, the magnetic field can affect them[26]. As is seen from this figure, the presence of magnetic fieldreduces heat transfer for both the base fluid and the nanofluid.Moreover, it is noteworthy that heat transfer reduction of thenanofluid is higher than that of the base fluid. In fact, this occursdue to the simultaneous effect of a magnetic field on both the base

Fig. 13. Contour of magnetic field magnitude between two magnets.

Fig. 11. Increase of boiling heat transfer coefficient of the nanofluid compared withwater, as a function of nanofluid’s volume concentration.

fluid and the nanoparticles. As mentioned above, the gradient ofmagnetic field magnitude is positive. The magnetic force towardthe boiling surface caused that bubbles to be pulled horizontally.Based upon previous experimental study [27], the bubbles areelongated from the location with stronger field to the location withweaker field. Hence, the diameter of bubbles will be larger andthey will be elongated mostly in the horizontal direction. Whenthe bubbles become larger, their separation from the surface ismore difficult and the heat transfer in the presence of magnetic

Fig. 16. Boiling heat transfer coefficient as a function of heat flux for two differentvolume concentrations of the nanofluid in the presence of positive magnetic fieldgradient.

Fig. 14. Boiling heat transfer coefficient as a function of heat flux for 0.1% volumeconcentration nanofluid and base fluid in the presence of positive magnetic fieldgradient.

Fig. 15. Boiling heat transfer coefficient as a function of heat flux for 0.1% volumeconcentration nanofluid and base fluid in the presence of negative magnetic fieldgradient.

A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110 1109

field will decrease, consequently. Additionally, the variation of heatflux in Fig. 14 shows that the boiling heat transfer coefficient offluid in a specific concentration significantly reduces in the pres-ence of the magnetic field with high heat fluxes. In fact, these phe-nomena occurred in the nuclear boiling region (higher fluxes)where the influence of the magnetic field on the separation of bub-bles is high. However, the magnetic field does not retain a signifi-cant importance in low heat fluxes (free convection region). In fact,the influence of the magnetic field on the nanoparticles can beattributed to their rearrangement in the direction of the field.Moreover, Fig. 14 shows that the boiling heat transfer coefficientof base fluid is reduced nearly 11% in the presence of the magneticfield while this reduction is approximately 23% for a 0.1% volumeconcentration nanofluid. The discrepancy between boiling heattransfer coefficients can be due to the presence of nanoparticleswithin the base fluid. As mentioned before, the arrangement ofnanoparticles in the magnetic field direction on the surface hindersthe separation of the bubbles from the surface, resulting in lowerheat transfers.

In order to increase the heat transfer, the magnetic field gradi-ent should be decreased from the boiling surface in the upwarddirection. This means that the magnetic field should get weakeras it gets farther from the boiling surface. In this case, the bubblesare elongated from the location with stronger field to the locationwith weaker field, the diameter of bubbles will be larger, and theywill be elongated mostly in the vertical direction. When the bub-bles become smaller, their separation from the surface is easierand the heat transfer in the presence of magnetic field willincrease, consequently. It is worthwhile to note that a high magni-tude of magnetic field fluctuates the boiling heat transfer substan-tially, whether increasing or decreasing.

Fig. 15 illustrates the increment of the boiling heat transfercoefficient in the presence of negative magnetic field. This figureshows that the boiling heat transfer coefficient of base fluid isincreased nearly 6% in the presence of negative magnetic fieldwhile this increase is approximately 13% for a 0.1% volume concen-tration nanofluid. On the other hand, the positive magnetic fieldgradient reduces boiling heat transfer coefficient approximately23% for a 0.1% volume concentration nanofluid.

Fig. 16 compares the boiling heat transfer coefficients of nano-fluid with two different volume concentrations of 0.1% and 0.25%

in the presence of the magnetic field. It is observed that increasingthe concentration of the nanofluid restrains the reduction of boil-ing heat transfer coefficient in the presence of magnetic field. Rais-ing the volume concentration from 0.1% to 0.25% increases thepartial fraction of the nanoparticles, and hence the influence ofthe magnetic field on the nanoparticles is higher than its effecton the base fluid. Therefore, in high concentrations, the heat trans-fer reduction percentage decreases due to the overshadowingeffect of magnetic field on nanoparticles compared with the basefluid. In order to enhance the heat transfer rate in high concentra-tions of the nanofluid, a stronger magnetic field should beprovided.

1110 A. Abdollahi et al. / Applied Thermal Engineering 111 (2017) 1101–1110

4. Conclusion

In this paper, an experimental set-up is prepared to investigatethe boiling mechanism of nanofluid and evaluate boiling heattransfer of Fe3O4/DI water nanofluid in the presence of the mag-netic field. The applied nanofluid for this study was synthesizedthrough a one-step approach. The results indicate that increasingthe concentration of the nanofluid up to 0.1% increases the boilingheat transfer coefficient while after this value, a reverse trend isobserved. Therefore, there is an optimum value for the concentra-tion of nanoparticels that increase the boiling heat transfer coeffi-cient more than 43%. In addition to the properties of the nanofluid,the surface characteristics have significant effect on boiling heattransfer.

Moreover, the influence of magnetic field on magnetic nano-fluid is investigated. Our findings show that the boiling heat trans-fer coefficient decreases in the presence of the positive magneticfield gradient and increases in the presence of the negative mag-netic field gradient. The diminution or increment of boiling heattransfer within the nanofluid is due to the simultaneous effect ofmagnetic field on both the base fluid and the nanoparticles. Inlow concentrations of nanoparticles, the effect of magnetic fieldon the base fluid is higher than that on nanoparticles. As the con-centration of nanoparticles is increased in the nanofluid, the influ-ence of magnetic field on nanoparticles intensifies. Therefore, inhigh concentrations, the heat transfer reduction percentage isreduced due to the overcoming effect of magnetic field on thenanoparticles than that on the base fluid. According to obtainedresults, it is concluded that the addition of nanoparticles or theapplication of a magnetic field are not the only crucial factorsaffecting the boiling heat transfer coefficient. Indeed, various sig-nificant factors such as the surface roughness, average nanoparti-cles size and magnetic field gradient affect the boiling heattransfer coefficient.

References

[1] A.E. Bergles, Enhancement of pool boiling, Int. J. Refrig 20 (1997) 545–551.[2] L. Cheng, D. Mewes, A. Luke, Boiling phenomena with surfactants and

polymeric additives: a state-of-the-art review, Int. J. Heat Mass Transf. 50(2007) 2744–2771.

[3] M.R. Raveshi, A. Keshavarz, M.S. Mojarrad, S. Amiri, Experimental investigationof pool boiling heat transfer enhancement of alumina–water–ethylene glycolnanofluids, Exp. Thermal Fluid Sci. 44 (2013) 805–814.

[4] S. Chol, Enhancing thermal conductivity of fluids with nanoparticles, ASME-Publ.-Fed 231 (1995) 99–106.

[5] R.A. Taylor, P.E. Phelan, Pool boiling of nanofluids: comprehensive review ofexisting data and limited new data, Int. J. Heat Mass Transf. 52 (2009) 5339–5347.

[6] Z. Shahmoradi, N. Etesami, M.N. Esfahany, Pool boiling characteristics ofnanofluid on flat plate based on heater surface analysis, Int. Commun. HeatMass Transf. 47 (2013) 113–120.

[7] S. Kim, I.C. Bang, J. Buongiorno, L. Hu, Surface wettability change during poolboiling of nanofluids and its effect on critical heat flux, Int. J. Heat Mass Transf.50 (2007) 4105–4116.

[8] S. You, J. Kim, K. Kim, Effect of nanoparticles on critical heat flux of water inpool boiling heat transfer, Appl. Phys. Lett. 83 (2003) 3374–3376.

[9] M. Chopkar, A. Das, I. Manna, P. Das, Pool boiling heat transfer characteristicsof ZrO2-water nanofluids from a flat surface in a pool, Heat Mass Transf. 44(2008) 999–1004.

[10] P.S. Lykoudis, Bubble growth in the presence of a magnetic field, Int. J. HeatMass Transf. 19 (1976) 1357–1362.

[11] L.Y. Wagner, P.S. Lykoudis, Mercury pool boiling under the influence of ahorizontal magnetic field, Int. J. Heat Mass Transf. 24 (1981) 635–643.

[12] M. Takahashi, A. Inoue, T. Kaneko, Pool boiling heat transfer of mercury in thepresence of a strong magnetic field, Exp. Thermal Fluid Sci. 8 (1994) 67–78.

[13] M.A.L. de Bertodano, S. Leonardi, P.S. Lykoudis, Nucleate pool boiling ofmercury in the presence of a magnetic field, Int. J. Heat Mass Transf. 41 (1998)3491–3500.

[14] L. Junhong, G. Jianming, L. Zhiwei, L. Hui, Experiments and mechanism analysisof pool boiling heat transfer enhancement with water-based magnetic fluid,Heat Mass Transf. 41 (2004) 170–175.

[15] P. Berger, N.B. Adelman, K.J. Beckman, D.J. Campbell, A.B. Ellis, G.C. Lisensky,Preparation and properties of an aqueous ferrofluid, J. Chem. Educ. 76 (1999)943.

[16] M. Abareshi, E.K. Goharshadi, S.M. Zebarjad, H.K. Fadafan, A. Youssefi,Fabrication, characterization and measurement of thermal conductivity ofFe3O4 nanofluids, J. Magn. Magn. Mater. 322 (2010) 3895–3901.

[17] D. Wen, Y. Ding, Experimental investigation into the pool boiling heat transferof aqueous based c-alumina nanofluids, J. Nanopart. Res. 7 (2005) 265–274.

[18] R.J. Moffat, Describing the uncertainties in experimental results, Exp. ThermalFluid Sci. 1 (1988) 3–17.

[19] S.M. Kwark, R. Kumar, G. Moreno, J. Yoo, S.M. You, Pool boiling characteristicsof low concentration nanofluids, Int. J. Heat Mass Transf. 53 (2010) 972–981.

[20] M. Kole, T. Dey, Investigations on the pool boiling heat transfer and criticalheat flux of ZnO-ethylene glycol nanofluids, Appl. Therm. Eng. 37 (2012) 112–119.

[21] M. Kole, T. Dey, Thermophysical and pool boiling characteristics of ZnO-ethylene glycol nanofluids, Int. J. Therm. Sci. 62 (2012) 61–70.

[22] L. Cheng, L. Liu, Boiling and two-phase flow phenomena of refrigerant-basednanofluids: fundamentals, applications and challenges, Int. J. Refrig 36 (2013)421–446.

[23] L. Cheng, F. Bandarra, P. Enio, J.R. Thome, Nanofluid two-phase flow andthermal physics: a new research frontier of nanotechnology and its challenges,J. Nanosci. Nanotechnol. 8 (2008) 3315–3332.

[24] G.P. Narayan, K. Anoop, S.K. Das, Mechanism of enhancement/deterioration ofboiling heat transfer using stable nanoparticle suspensions over vertical tubes,J. Appl. Phys. 102 (2007) 074317.

[25] R. Aghayari, M. Kerdegari, A.K. Shahin Khosravi, H. Maddah, A.H. Khalaj,Synthesis and thermo-physical properties of Fe3O4 nanofluid (2015).

[26] D.A. Skoog, Fundamentals of Analytical Chemistry, Grupo Editorial Norma,2004.

[27] J. Ishimoto, M. Okubo, S. Kamiyama, M. Higashitani, Bubble behavior inmagnetic fluid under a nonuniform magnetic field, JSME Int. J. Ser. B 38 (1995)382–387.

[28] S.K. Das, N. Putra, W. Roetzel, Pool boiling characteristics of nano-fluids, Int. J.Heat Mass Transf. 46 (2003) 851–862.

[29] S.K. Das, N. Putra, W. Roetzel, Pool boiling of nano-fluids on horizontal narrowtubes, Int. J. Multiph. Flow 29 (2003) 1237–1247.

[30] S. Witharana, Boiling of refrigerants on enhanced surfaces and boiling ofnanofluids, Royal Institute of Technology, Stockholm, Sweden, 2003.

[31] P. Vassallo, R. Kumar, S. D’Amico, Pool boiling heat transfer experiments insilica–water nano-fluids, Int. J. Heat Mass Transf. 47 (2004) 407–411.

[32] I.C. Bang, S.H. Chang, Boiling heat transfer performance and phenomena ofAl2O3–water nano-fluids from a plain surface in a pool, Int. J. Heat Mass Transf.48 (2005) 2407–2419.

[33] H. Kim, J. Kim, M.H. Kim, Effect of nanoparticles on CHF enhancement in poolboiling of nano-fluids, Int. J. Heat Mass Transf. 49 (2006) 5070–5074.

[34] J.S. Coursey, J. Kim, Nanofluid boiling: the effect of surface wettability, Int. J.Heat Fluid Flow 29 (2008) 1577–1585.

[35] Z.-H. Liu, L. Liao, Sorption and agglutination phenomenon of nanofluids on aplain heating surface during pool boiling, Int. J. Heat Mass Transf. 51 (2008)2593–2602.

[36] M.N. Golubovic, H.M. Hettiarachchi, W. Worek, W. Minkowycz, Nanofluids andcritical heat flux, experimental and analytical study, Appl. Therm. Eng. 29(2009) 1281–1288.

[37] S. Soltani, S.G. Etemad, J. Thibault, Pool boiling heat transfer performance ofNewtonian nanofluids, Heat Mass Transf. 45 (2009) 1555–1560.

[38] Z.-H. Liu, X.-F. Yang, J.-G. Xiong, Boiling characteristics of carbon nanotubesuspensions under sub-atmospheric pressures, Int. J. Therm. Sci. 49 (2010)1156–1164.

[39] A. Suriyawong, S. Wongwises, Nucleate pool boiling heat transfercharacteristics of TiO2–water nanofluids at very low concentrations, Exp.Thermal Fluid Sci. 34 (2010) 992–999.

[40] R. Kathiravan, R. Kumar, A. Gupta, R. Chandra, P. Jain, Pool boilingcharacteristics of multiwalled carbon nanotube (CNT) based nanofluids overa flat plate heater, Int. J. Heat Mass Transf. 54 (2011) 1289–1296.

[41] M. Sheikhbahai, M.N. Esfahany, N. Etesami, Experimental investigation of poolboiling of Fe3O4/ethylene glycol–water nanofluid in electric field, Int. J. Therm.Sci. 62 (2012) 149–153.

[42] R.N. Hegde, S.S. Rao, R. Reddy, Flow visualization and study of critical heat fluxenhancement in pool boiling with Al2O3-water nanofluids, Int. J. Therm. Sci. 16(2012) 445–453.

[43] V.I. Sharma, J. Buongiorno, T.J. McKrell, L.W. Hu, Experimental investigation oftransient critical heat flux of water-based zinc–oxide nanofluids, Int. J. HeatMass Transf. 61 (2013) 425–431.

[44] V. Umesh, B. Raja, A study on nucleate boiling heat transfer characteristics ofpentane and CuO-pentane nanofluid on smooth and milled surfaces, Exp.Thermal Fluid Sci. 64 (2015) 23–29.