International Journal of Heat and Mass Transfer 58 (2013) 348–355

Contents lists available at SciVerse ScienceDirect

International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Heat transfer enhancement of PAO in microchannel heat exchanger usingnano-encapsulated phase change indium particles

W. Wu a, H. Bostanci a, L.C. Chow a,⇑, Y. Hong b, C.M. Wang b, M. Su b, J.P. Kizito c

a Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USAb NanoScience Technology Center, University of Central Florida, Orlando, FL 32816, USAc Department of Mechanical Engineering, North Carolina A&T State University, Greensboro, NC 27411, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 May 2012Received in revised form 8 November 2012Accepted 8 November 2012

Keywords:Microchannel heat transferPhase change nanoparticlesEncapsulation

0017-9310/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.ijheatmasstransfer.2012.11

⇑ Corresponding author. Tel.: +1 407 823 3666; faxE-mail address: louis.chow@ucf.edu (L.C. Chow).

This paper describes a new method to enhance the heat transfer capability of a single phase liquid byadding phase change nanoparticles (nano-PCMs), which absorb thermal energy during solid–liquid phasechanges. Two types of slurries having bare and silica encapsulated indium nano-PCMs have been madeusing colloid method and suspended into poly-a-olefin (PAO) for potential high temperature(150 � 180 �C) applications. The silica shells were devised in an effort to prevent agglomeration of moltenphase change materials. In addition, the silica shells were evaluated for their effect on thermal perfor-mance. Experiments with the microchannel heat exchanger (MC) indicated that the heat transfer coeffi-cient of slurry with 30% bare indium nanoparticle can reach 47,000 W/m2 K at flow rate of 3.5 ml/s(velocity of 0.28 m/s). The magnitude of heat transfer coefficient represents 2 times improvement overthat of single phase PAO, and is also higher than that of single phase water which is at �45,000W/m2 K. A thermal cycling test involving 5000 cycles showed a consistent performance of both typesof slurries, thus negating the need for the encapsulation of In nano-PCMs in PAO.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Phase change materials (PCM) can absorb or release heat energywhen they change phase from solid to liquid, and vice versa. Moresignificantly, the PCM can be encapsulated into small sphericalcontainers of nano sizes and then dispersed into a carrying fluidto enhance the overall thermal properties, especially the fluid heatcapacity. Such slurry can be used to reduce the overall pumpingpower in a coolant loop because of the increased heat capacity ofthe carrying fluid.

Heat transfer fluids (HTFs) have many industrial and civil appli-cations, which include energy storage, heat exchange and electron-ics cooling. Experimental evidence on anomalous improvement inthe thermophysical properties of so-called nanofluids was pro-vided more than a decade ago by Eastman et al. [1]. Many investi-gators followed and produced a large body of literature. Earlierresearch efforts have considered adding high thermal conductivitymaterials, such as silver, copper, alumina, CuO, SiC and carbonnanotubes into HTFs in order to improve their heat transport prop-erties [2]. There has also been a significant amount of work to in-crease the effective specific heat of a liquid by addingmicroencapsulated phase change materials [3–8]. Poly-a-olefin

ll rights reserved..032

: +1 407 823 0208.

(PAO) is a dielectric oil used in cooling of avionics systems. PAOis inexpensive and stable and is used to maintain electroniccomponents and devices at temperatures �160 �C so that siliconcarbide based devices could operate reliably. However, PAO hasinherently poor heat transfer performance due to its low thermalconductivity (0.14 W/m K which is only 23% of that of water).The concept of nanofluids was further developed by incorporatingindium as phase-change nanoparticles into PAO [9] to enhance theheat capacity of PAO. Indium was chosen as the PCM because of itsmelting point of 157 �C. There are several methods to producemicroencapsulated PCM including interface polymerization andcoacervation methods [10]. A method used to produce nanoscaleencapsulated PCMs is called the emulsion polymerization tech-nique [11,12]. The advantages of the resulting nano particles influid mixtures stem from their small sizes and large surface-to-volume ratios, and the ability to disperse uniformly within theliquid. These thermal fluids containing encapsulated nano-PCMshave specific advantages such as high density thermal energystorage, low flow drag and high specific heat capacity. All theseadvantages make nano-PCM slurry a promising material forthermal control, cooling of electronic equipment, and othersystems requiring high heat transfer rates. The intimate contactbetween nanoparticles and fluid reduces the resistance of heattransfer between nanoparticles and fluid, thus allowing rapidexchange of heat energy between phases.

Nomenclature

Fbase heat transfer area (m2)c⁄ volumetric concentration (%)cp heat capacity of the working fluid (J/kg K)k thermal conductivity of working fluid (W/m K)hsl latent heat of In (kJ/kg)h heat transfer coefficient for the overall microchannel

area(W/m2 K)MC microchannel heat exchangerPl pressure of working fluid (Pa)Q total power (W)R thermal resistance (K/W)Re Reynolds numberPr Prandtl numberr radius (m)T temperature (K)Tf temperature of inlet fluid (K)t time (s)

Greek/ mass ratio of nanoparticles in the working fluidqs solid density (kg/m3)qv liquid density (kg/m3)s melting time (second)l viscosity (Pa s)

Subscriptsb bulkf carrying fluidh hydraulic diameterm meltingp particles surfacesh shellw wall

W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355 349

If the flow rate and thermal conductivity of liquid remain con-stant, the heat transfer ability is mainly dependent on the heatabsorbing capacity. Our primary objective in developing nano-PCMs is to enhance the thermal capacity of the working fluid byusing the latent heat of melting. Our second objective is to evaluatethe potential agglomeration of the particles when the PCM is in amolten form. To our knowledge, no work has been done on theheat transfer enhancement due to the bare and encapsulated nano-particles of In in microchannel cooling systems. This study partic-ularly investigates the method of adding bare and silicaencapsulated In nano-PCMs into PAO to enhance the heat absorb-ing capacity of the liquid, and to eventually reduce the size of cool-ing systems. Encapsulated nano-PCMs have been made byencapsulating the bare PCM core in silica shells that contains themolten core and prevents coalescence of particles. PAO fluid con-taining bare and encapsulated nano-PCMs at 9% and 30% mass ratioexhibits a much higher heat capacity due to the latent heat of melt-ing. Pressure drop and heat transfer characteristics of this fluid areexperimentally determined, and the size and interface effects ofnano-PCM on melting and solidification are discussed.

2. Synthesis of nanoparticles and experimental setup andprocedure

All chemicals used in this experimental work are obtained fromAldrich without purification. Direct emulsifications of appropriateprecursors are used to prepare metallic nano-PCMs. In the caseof In that has a melting point of 157 �C, emulsification is carriedout by boiling certain amount of In powder (�325 mesh) in PAOat 200 �C with magnetic stirring under protection of nitrogen. Anultrasound sonication could also be used to assist the uniformityand particle size reduction. The nanoparticles are then separatedfrom PAO by centrifuging at 4000 rpm for 10 min, and washed withethanol (90%). Such centrifuging and washing processes are re-peated for three times.

The precursor used to encapsulate nano-PCMs is tetraethoxysi-lane (TEOS, surfactant). Sol–gel method is used to form a thin silicashell around nano-PCMs. After redispersing 50 mg nanoparticlesinto 50 ml of ethanol, 2 ml of NH4OH at the concentration of 28%and 0.2 ml of TEOS are added drop-wisely into the solution. Themixture is then sonicated by a Brason 2510 sonicator at 70 �C for1.5 h to decompose TEOS and make silica shells formed aroundnanoparticles. After finishing encapsulation process, the mixture

is centrifuged to remove the top clear liquid and washed by ethanol.The centrifuging and washing processes are repeated for threetimes to ensure the complete removal of residual TEOS, and theencapsulated nano-PCMs are re-dispersed in PAO at certain ratio.

2.1. Nano encapsulated particles and slurry viscosity properties

A JEOL 1011 TEM (100 kV) and a TECNAI F30 TEM (200 kV)systems were used for imaging the core–shell characteristics ofnano-PCMs. To prepare sample for TEM imaging, an ethanol dropcontaining nanoparticles is placed on a copper TEM grid thatcoated with carbon film. A Perkin Elmer DSC7 device is used tomeasure the thermal physical properties of nano-PCMs. A sampleof about 10 mg is hermetically sealed into an aluminum pan andplaced inside the (differential scanning calorimetry) DSC chamberunder continuously purged nitrogen gas. Dynamic scans areperformed on the samples at the heating rate of 10 �C/min fromroom temperature to a set temperature, and cooling down to theinitial temperature.

Fig. 1(a) and (b) with scale bars show SEM and TEM images ofsilica encapsulated In nanoparticles. Size distribution of silicaencapsulated In nanoparticles is included in Fig. 1c. Fig. 2a showsDSC curves of pure indium nanoparticles (dashed line) and silicaencapsulated indium nanoparticles (solid line), where the meltingand freezing temperatures are at 155 and 135 �C, respectively, forboth of them. The enthalpy of fusion is derived as 19.6 J/g fromthe area of melting peak of silica encapsulated indium nanoparti-cles, which is lower than that of pure indium value (28.5 J/g). Thedifference is due to the presence of silica, whose melting point isover 1600 �C. The mass ratio of indium inside encapsulated nano-particles is determined to be 69% from the ratio of these enthalpies.Fig. 2(b) shows the viscosities of PAO, PAO with bare In nanoparti-cles, and PAO with silica encapsulated In nanoparticles measuredby the custom-made capillary viscometer. Viscosities of PAO (tri-angle), PAO with bare In nanoparticles at 30% particle mass ratio(square), and PAO with silica encapsulated In nanoparticles at 9%particle mass ratio (circle) all decrease as temperature increasesfrom 3 to 45 �C. PAO with bare In nanoparticles has higher viscositythan that of both PAO, and PAO with silica encapsulated In nano-particles. The viscosity of PAO with silica encapsulated In nanopar-ticles at 45 �C (9.49 cP) is close to that of PAO (4.68 cP) thusproviding an advantage of the encapsulated particles over the bareones due to viscosity reduction.

0 200 400 600 800 10000

5

10

15

20

Perc

enta

ge (%

)

Diameter (nm)

(a) (b) (c)Fig. 1. SEM (a) and TEM (b) images, and size distribution (c) of silica encapsulated In nanoparticles.

80 100 120 140 160-5

0

5

10

15

20

Temperature (oC)

Hea

t flo

w (

mW

)

Encap In with silica shellNano bare In

0 10 20 30 40 500

10

20

30

40

50

30% In/PAO

9% In@SiO2 /PAO

PAOVi

scos

ity (c

P)

Temperature (oC)

(a) (b)

Fig. 2. DSC curves of bare and encapsulated nano-PCMs (a), and viscosities of PAO, slurry with encapsulated nano-PCMs at 9% mass particle ratio, and slurry with bare nano-PCMs at 30% mass particle ratio at a temperature range of 3–45 �C (b).

Fig. 3. Schematic of experimental setup. 20 mm

10 mm

TC#1

TC#2 Thick film resistor

Copper block 2 mm

TC#3

OutIn

Fig. 4. Details of microchannel heat exchanger-heater assembly.

350 W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355

2.2. Experimental setup

Fig. 3 illustrates a schematic of the experimental flow loop. Itconsists of a microchannel heat exchanger, a pump, a valve, a flowmeter, two mixing sections at inlet and outlet of the heat exchan-ger, and a plate heat exchanger. Working fluid is pumped from the

fluid reservoir using a diaphragm pump. The valve is used for flowrate adjustment in the loop. The flow rate is determined using tworotameters that have been calibrated by measuring the weight ofcollected flow amount. The two mixing sections are used to disturbfluid so that the thermocouples can measure the bulk temperatureat the inlet and outlet of the heat exchanger. The plate heat ex-changer is used to cool down the working fluid after it exits the testsection.

W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355 351

Fig. 4 shows the details of microchannel heat exchanger-heaterassembly. Heat flux is applied to the microchannel heat exchangersurface through a copper plate using an electric heater controlledby a HP 6030A DC power supply. The heater consists of two1-cm2, 5-ohm thick film resistors with BeO substrate made by BarryIndustries Inc. Three thermocouples (Type-T, 36-AWG) are embed-ded halfway in the copper plate and spaced equally across the2 cm2 area as illustrated in Fig. 4. Temperature at the microchannelheat exchanger surface was calculated by extrapolating the aver-age of three thermocouple readings through the known distanceto the surface and assuming steady 1-D conduction through thecopper plate. Other measurements include pressures and temper-atures at inlet and outlet. The microchannel heat exchanger-heaterassembly is thermally isolated by 10 cm thick fiberglass to preventheat loss. The remaining parts of experimental setup such as tub-ing, slurry reservoir and heat exchanger are thermally insulatedas well. Since the calculated heat loss is negligible, heat transferrate was calculated directly from the power provided to the heater.

Fig. 5 shows the general concept and detailed cross-section of aMicro Cooling Concepts (MC2) microchannel heat exchanger. Theoverall size of the heat exchanger is approximately 2 cm � 1 cm �1 cm, with two ports for fluid inlet and outlet on the opposite facesof the heat exchanger surface. A sample microchannel heat ex-changer was partially cut open in an effort to better understandits design features, and the fluid flow paths are depicted in Fig. 5.We note that there are two nominal size microchannel heatexchangers in this study, referred to as 25 lm and 100 lm micro-channels. The measured dimensions for the individual microchan-nels are 25 and 100 lm width (y-direction), respectively, 500 lmheight (z-direction), and 1000 lm length (x-direction). The walland base thicknesses are 50 and 100 lm, respectively. Individualmicrochannels are stacked up as many as 100 layers, and this setof 100 layers is estimated to provide a total surface area of25 cm2 in the first millimeter of construction from the heat ex-changer surface. The dimensions of the inlet and outlet manifoldsare 200 lm width (y-direction), 50 lm height (z-direction), and250 lm length (x-direction).

Fig. 5. Design concept and cross-sectional vie

2.3. Test conditions and procedure

Prior to each experiment, the microchannel heat exchanger wassoaked and cleaned with acetone to remove any contamination. Allexperiments were performed at one atmosphere pressure. A totalof 300 ml slurry is pumped between a hot source and a cold sink,and adequate temperature controls are employed. The slurry isheated in the reservoir by using an immersion heater, and the slur-ry temperature at the microchannel heat exchanger inlet is main-tained at low end of the melting profile. After the heat exchange inthe test chamber, slurry temperature is lowered to room tempera-ture which is 19 �C to assure re-solidification of nano-PCMs. Oncethe steady-state flow and temperature conditions were attained,the local mean temperature of the heater, and inlet and outlet tem-perature of the slurry were calculated by the arithmetic mean oftemperature readings. The heat transfer coefficient was then ob-tained by h = Q/Fbase(Tw–Tf).

2.4. Uncertainty analysis

The temperature measurement precision is maintained to bewithin ±0.18 �C and the thermocouples are positioned within±10 lm of the center line. A Keithley 2700 data acquisition systemwas used with a digital voltmeter having a sensitivity of ±1 lV, asix-figure scale and an accuracy of 0.01% of the reading. The errorin power calculation from the voltage and current measurementcan be ignored in the experiments. The error in pressure differencewas less than 5%. Thermal balance between supplied power to theheater and amount absorbed by the working fluid was less than 5%.The error in heat transfer coefficient calculation based on the tem-perature and heat flux measurements is estimated to be less than10%. The relative error of the flow rate measurement was esti-mated to be within ±3%. Depending on the temperature of the hea-ter, a heat loss corresponding to 5%–10% of the electrical-powerinput was estimated. The heat flux at the surface of the copperplate was obtained from the measured electrical-power afteraccounting for the heat loss. We repeated our experiment five

w of MC2 microchannel heat exchanger.

6

352 W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355

times and during these measurements the average heat loss wasfound to be ±5.5%, representing a reasonable degree of accuracy.

0 0.5 1 1.5 2 2.5 3 3.5 40

1

2

3

4

5

Flow rate (ml/s)

h ×

10-4

(W/m

2 .K)

100 µm MC PAO100 µm MC WaterData from MC2 (extrapolated for water)

Fig. 7. Comparison of heat transfer coefficients of 100 lm microchannel with waterand PAO. The inlet temperature was 19 �C and the heat flux input was varied from10 W/cm2 to 50 W/cm2.

Table 1Thermophysical properties of water, PAO and In.

Density(kg m�3)

Specific heat(J kg�1 K�1)

Thermalconductivity(W m�1 K�1)

Viscosity(mPa s)293 K

Water 997 4180 0.61 1.01PAO[16] 784 2242 0.14 4.45In [17] 7300 233 81.6 –In PAO

9%bare

853 2142 0.157 15.1

In PAO9%encap

870 2001 0.154 14.9

In PAO30%

1083 1701 0.189 24.7

3. Results and discussion

Performance of the current system was mainly evaluated interms of pressure drop and heat transfer measurements. It shouldbe noted that the particle mass ratio in slurries solely reflects theamount of PCM material (In), and therefore slurries with encapsu-lated nano-PCMs involved higher number of particles when com-pared to slurries with bare nano-PCMs, to compensate the massof shell material (silica).

Pressure gauges at the inlet and outlet of the test chamber wereused to evaluate the pressure drop throughout the tests. Additionalperformance characterization experiments were also conducted todetermine the pressure drop versus flow rate, and the thermalresistance versus flow rate. Pressure versus flow rate data werelimited at 5 ml/s because of the maximum available pumpingpower. Fig. 6 shows the pressure drop across the microchannelheat exchanger, measured at flow rates of 0–8 ml/s in 25 and100 lm microchannels with bare and encapsulated nano-PCMshaving 9% and 30% particle mass ratio. The pressure drop resultswere all obtained at a room temperature of 19 �C at inlet with noheat added to the system (no melting occurred). When data fromPAO are compared, the pressure drop of 25 lm microchannel ishigher than that of 100 lm microchannel as can be expected. Forslurries with the same particle mass ratios (u = 0.09 and 0.30),encapsulation helped to reduce the pressure drop. However, the ef-fect of channel size on pressure drop is more pronounced than theeffect of encapsulation as evidenced by the consistently higher le-vel of pressure drops with the 25 lm microchannel. For the samenano-PCM slurry and microchannel size, pressure drop increaseswith the particle mass ratio, which can be explained by the in-crease in the slurry viscosity. Present experimental results indi-cated that slurry with bare nano-PCM at 30% particle mass ratioyields a sharp pressure-drop increase of 700% when compared toPAO. Therefore, slurries with higher particulate loads are expectedto make the slurry too viscous and should be avoided.

Evaluation of microchannel heat transfer performance wasstarted by filling the system with water and performing a calibra-

0 2 4 6 8 100

1

2

3

4

5

6

7

Flow rate (ml/s)

Pres

sure

dro

p ×

10-2

(kPa

)

100 µm MC PAO25 µm MC PAO100 µm MC 9% Encap In100 µm MC 9% Bare In25 µm MC 9% Encap In25 µm MC 9% Bare In100 µm MC 30% Bare In25 µm MC 30% Bare In

Fig. 6. Pressure drop of 25 and 100 lm microchannels with PAO, and slurrieshaving bare and encapsulated nano-PCMs at 9% and 30% particle mass ratios (noheating involved).

tion experiment. Fig. 7 shows the heat transfer coefficients ob-tained from 100 lm microchannel with water and PAO. In thesetests, inlet temperature was maintained at 19 �C while flow ratewas ranged between 0.7 and 3.5 ml/s, and heat flux input was var-ied from 10 W/cm2 to 50 W/cm2. The solid line in this figure repre-sents the extrapolated range of manufacturer’s data for water in aneffort to estimate the performance at low flow rates. At the highestflow rate, the heat transfer coefficient for water is �45000 W/m2 K,and it reasonably matches the manufacturer’s data. The heat trans-fer coefficient for PAO is �22000 W/m2 K which is about half of thecorresponding water data. As summarized in Table 1, water pos-sesses superior thermophysical properties compared to PAO interms of heat transfer performance (for instance, PAO has athermal conductivity 4x smaller than that of water at roomtemperature).

Fig. 8 shows the heat transfer coefficients of 25 and 100 lmmicrochannels with slurries featuring bare and encapsulatednano-PCMs at 9% and 30% particle mass ratios. All of these resultswere obtained when the inlet temperature is controlled at 19 �Cand the outlet temperature was set to be less than 50 �C, a temper-ature much lower than the melting point of nano-PCMs (157 �C).For PAO, the heat transfer coefficients of 25 lm microchannel arehigher than that of 100 lm microchannel. For slurries with thesame particle mass ratios (u = 0.09 and 0.30), bare and

0.5 1 1.5 2 2.5 3 3.5 41.6

1.8

2

2.2

2.4

2.6

2.8

Flow rate (ml/s)

h ×

10-4

(W/m

2 .K)

100 µm MC PAO25 µm MC PAO100 µm MC 9% Encap In100 µm MC 9% Bare In25 µm MC 9% Encap In25 µm MC 9% Bare In100 µm MC 30% Bare In25 µm MC 30% Bare In

Fig. 8. Heat transfer coefficients of 25–100 lm microchannels with slurries havingbare and encapsulated nano-PCMs at 9% and 30% particle mass ratios when the inlettemperatures are set below 50 �C (no melting occurs). The heat flux input wasvaried from 10 W/cm2 to 50 W/cm2.

130 135 140 145 150 155 160 165 1702

2.5

3

3.5

4

4.5

5

MC Inlet temperature (oC)

h ×

10-4

(W/m

2 .K)

100 µm MC PAO25 µm MC PAO100 µm MC 9% Encap In100 µm MC 9% Bare In25 µm MC 9% Encap In25 µm MC 9% Bare In100 µm MC 30% Bare In25 µm MC 30% Bare In

Fig. 9. Heat transfer coefficients of 25–100 lm microchannels with PAO, andslurries having bare and encapsulated nano-PCMs at 9% and 30% particle massratios and 3.5 ml/s flow rate. The heat flux input was fixed at 50 W/cm2.

130 135 140 145 150 155 160 165 17015

20

25

30

35

40

MC Inlet temperature (oC)

Tw

- T

f (o C

)

100 µm MC PAO25 µm MC PAO100 µm MC 9% Encap In100 µm MC 9% Bare In25 µm MC 9% Encap In25 µm MC 9% Bare In100 µm MC 30% Bare In25 µm MC 30% Bare In

Fig. 10. Comparison of temperature difference between wall and outlet using 25and 100 lm microchannels with PAO, and slurries having bare and encapsulatednano-PCMs at 9% and 30% particle mass ratios and 3.5 ml/s flow rate. The heat fluxinput was fixed at 50 W/cm2.

W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355 353

encapsulated nano-PCMs resulted in nearly the same heat transfercoefficients. Similar to the observation with the pressure dropexperiments, encapsulation has less effect on the heat transfercoefficient enhancement compared to the channel size. For thesame kind of nano-PCM and microchannel size, the heat transfercoefficients increase with the particle mass ratio. For the slurrywith 30% bare nano-PCM, heat transfer coefficient at flow rate of3.5 ml/s can reach 28000 W/m2 K which is about 30% higher thanthat of PAO. This can be attributed to thermal conductivityenhancement. The bulk thermal conductivity of a slurry can be cal-culated according to Maxwell’s formula [13]:

kb

kf¼

2þ kp

kfþ 2c� kp

kf� 1

� �2þ kp

kf� c� kp

kf� 1

� � ð1Þ

where the kf is thermal conductivity of fluid, c⁄ is the volumetricconcentration and kp is thermal conductivity of the PCM particles,which is obtained by [10]

1kpdp

¼ 1kmcdmc

þ dp � dmc

kmsdpdmcð2Þ

where kmc is the thermal conductivity of the PCM core material, kms

is the thermal conductivity of the shell material, dp is particle diam-eter and dmc is the diameter of the PCM core. With kmc = 81.6W/m K, kms = 1.4 W/m K, dp = 120 nm (averaged) and dmc = 100 nm,the slurries with bare nano-PCM at 9% particle mass ratio, encapsu-lated nano-PCM at 9% particle mass ratio, and bare nano-PCM at30% particle mass ratio have thermal conductivity of 0.157, 0.154and 0.189 W/m K, respectively, compared to PAO’s thermal conduc-tivity of 0.14 W/m K. This level of thermal conductivity enhance-ment due to the nano-PCMs is listed in Table 1. Although thereare several recent correlations such as [14] which may be able toprovide a better estimate of the thermal conductivity of the nano-PCM slurries, it is still not clear which correlation can yield the bestprediction. We acknowledge that the traditional conductivity mod-els, such as the Maxwell equation, may not be the best choice topredict thermal conductivity of nano-PCM slurry. However, this isactually not very important to this study since our main goal is todemonstrate how phase change in the nanoparticles increases heattransfer.

Fig. 9 includes additional heat transfer coefficients of 25 and100 lm microchannels with slurries having bare and encapsulatednano-PCMs at 9% and 30% particle mass ratios. In order to comparethe heat transfer performance under the same condition, flow rateand heat flux were set at 3.5 ml/s and 50 W/cm2, respectively,while the fluid inlet temperature was varied between 130 and168 �C. These results were recorded when the inlet temperaturespanned the phase change range, therefore nano-PCMs underwentmelting. For PAO, the heat transfer coefficients of 25 lm micro-channel is higher than that of 100 lm microchannel as before.The effect of encapsulation on heat transfer enhancement is negli-gible, while the effect of channel size is more obvious at a givenparticle mass ratio. Data indicate that as inlet temperature

0 1000 2000 3000 4000 5000 600092

93

94

95

96

97

98

99

100

Cycle number

Perc

enta

ge c

hang

e of

hea

t tra

nsfe

r per

form

ance

(%)

Bare 30%Encap 9%Bare 9%

Fig. 11. 5000 thermal cycling, slurries with bare and encapsulated nano-PCMsdemonstrated very consistent thermal performance.

Fig. 12. Nano-PCM model used for melting process calculation.

354 W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355

increases, heat transfer coefficient of PAO increases throughout thetest range. For slurries, heat transfer coefficient sharply increasesat inlet temperatures starting 146 �C when nano-PCMs begin tomelt, and reaches a maximum level at 152 �C when full phasechange is utilized. At higher inlet temperatures, heat transfer coef-ficient starts to decrease due to the nano-PCMs’ reduced capacityto absorb heat. The heat transfer coefficient of slurry with barenano-PCM at 30% particle mass ratio, at the inlet temperature of152 �C and flow rate of 3.5 ml/s (flow velocity of 0.28 m/s), canreach 47000 W/m2 K. At comparable flow conditions this heattransfer coefficient is about 2 times higher than that of PAO, andeven slightly higher than that of water at �45000 W/m2 K.

Fig. 10 shows temperature difference between the wall and theoutlet of 25 and 100 lm microchannels with slurries having bareand encapsulated nano-PCMs at 9% and 30% particle mass ratio.The PAO has no downward peak in the absence of phase change,but slurries with nano-PCMs have peaks where the peak area isproportional to the particle mass ratio, indicating the melting ofIn between 145 and 158 �C. The lowest temperature difference of18 �C between the wall and the outlet is achieved by the slurrywith bare nano-PCM at 30% particle mass ratio when the inlet tem-perature is 152 �C. At this condition, phase change process seemsto be fully utilized.

Fig. 11 illustrates the change of heat transfer coefficient during5000 thermal cycling, where each cycle includes melting and solid-ification processes. Data from the slurries with bare and encapsu-lated nano-PCMs demonstrated very consistent thermalperformance. Both bare and encapsulated nano-PCM slurries at-tained 97% of their initial heat transfer performance. These resultstherefore imply that encapsulation, in an effort to prevent coales-cence of particles during phase change, is not needed for the slurryfeaturing PAO and In nano-PCMs. The main reason in the explana-tion of the comparable performance of bare nano-PCMs is thoughtto be the existence of oxide shells around the particles. Oxidationof In is unavoidable during the synthesis process, and can providea helpful protective shell for the core material avoiding its leakagefor a long time. Furthermore, two other possible mechanismsmight be helping to bare nano-PCMs. First, the added surfactantduring the synthesis of particles might help resist coalescence ofmolten In nanoparticles and ensure the stability of colloidal sus-pension. We have observed the thermal stability of the slurry bythermal cycling nanoparticles using in situ TEM coupled with aheating stage. Second, bare In nanoparticles have residual charges

which generate strong repulsive electrical force to avoid theagglomeration of molten nanoparticles.

Further enhancement in heat transfer performance of nano-PCM slurries would be achieved by higher latent heat, higher phasechange rate, and lower super cooling features. In the meantime, thepresented experimental data can offer helpful design criteria forthermal management systems with PCMs.

3.1. Discussion on melting time of nanoparticles

Heat absorption of particles is determined by the particle sizeand material properties. Heat transfer from the fluid to the parti-cles is controlled by the difference between the fluid temperatureand the surface temperature of the particles. A nano-PCM modelillustrated in Fig. 12 can be helpful in the calculation of meltingprocess. Considering a phase change temperature of Tm, andneglecting the sensible heat capacity, the heat absorbed at theinterface must be conducted through the liquid to the solid andis described by [15]:

q��¼ 4pklðTs � TmÞ

1=r � 1=rpð3Þ

where Ts and Tm are the surface temperature and the melting pointof nanoparticles, respectively; rp is the radius of the nanoparticlesbefore melting, and kl is the thermal conductivity of nanoparticles.In addition, neglecting the sensible heat capacity, the heat absorbedat the interface must be conducted through the liquid to the solids,which means

q��¼ m

�hsl ¼ ðql � 4pr2 � dr

dsÞhsl ð4Þ

where ql is the density of nanoparticles, and hsl is the latent heat offusion of the nanoparticles. Combining Eq. (3) and (4), and integrat-ing gives:

4pklðTs � TmÞð1r � 1

rpÞ

¼ qlhsl4pr2 � drds

� �ð5Þ

where s is the melting time when the solid radius is r. The meltingtime is dependent on size and difference between the surface tem-perature of the nanoparticle and the melting temperature of thenanoparticle material:

ðTs � TmÞ � sqlhsl

kl

� � ¼ r2p

13

rrp

� �3

� 12

rrp

� �2

þ 16

" #ð6Þ

In the case of silica encapsulated nanoparticles, silica shell has alower thermal conductivity (1.3 W/m K) than that of metallic mate-rial, and Eq. (5) is modified to include the contribution of the silicashell:

W. Wu et al. / International Journal of Heat and Mass Transfer 58 (2013) 348–355 355

ðTs � TmÞRIn þ RSiO2

¼ 4pqInhslr2 � drds

� �ð7Þ

where RIn ¼ 14pkIn

1r � 1

rIn

� �;RSiO2 ¼ 1

4pkSiO2

1rIn� 1

rSiO2

� �.

Integrating Eq. (7) gives

s � ðTs � TmÞ ¼ qInhsl13

1kInrIn

þ 1kSiO2 rSiO2

� 1kSiO2 rIn

� �r3

�

�12� r2

kInþ 1

6� r

2In

kIn� 1

3� r3

In

kSiO2 rSiO2

þ r2In

3kSiO2

�ð8Þ

For encapsulated nano-PCMs, kIn, kSiO2 , rIn, rSiO2 , hsl, qIn are 81.8 W/m K, 1.3 W/m K, 100 nm, 120 nm, 28.52 J/g, and 7.3 g/cm3, respec-tively. As r goes to 0 nm, the Eq. (8) becomes:

sðTs � TmÞ ¼ 0:92� 10�7 s � K ð9Þ

The melting time s is 0.92 ls when Ts � Tm = 0.1 K. For bare nano-PCMs on the other hand, the melting time using Eq. (6) is 0.44 ls,which is 2x faster than that of encapsulated ones calculated withEq. (8). At the flow rate of 3.5 ml/s, the resident time of nanoparti-cles passing through a 1000 lm microchannel length is 4 ms. There-fore, there is enough time for In nanoparticles to melt.

4. Conclusion

This paper describes a new method to enhance the heat transferproperty of a single phase liquid by adding bare or encapsulatednano-size phase change materials (nano-PCMs), which will absorbthermal energy during solid–liquid phase changes. Silica encapsu-lated In nanoparticles have been made using colloid method andsuspended into poly-a-olefin (PAO) for potential high temperatureapplications (150 to �180 �C). Performance of the system wasmainly evaluated in terms of pressure drop and heat transfer mea-surements, and the conclusions can be summarized as follows:

j For the same microchannel size and other operational condi-tions (particle mass ratio, flow rate, inlet temperature and heatinput):s Slurry with encapsulated nano-PCMs encounters a lower

pressure drop compared to that of bare nano-PCMs.s Slurries with bare and encapsulated nano-PCMs provide

nearly the same heat transfer performance.j The slurry with bare nano-PCM at 30% particle mass ratio, at the

inlet temperature of 152 �C and flow rate of 3.5 ml/s (flowvelocity of 0.28 m/s), can reach 47,000 W/m2 K heat transfercoefficient. At comparable flow conditions this heat transfercoefficient is about 2 times higher than that of PAO, and evenslightly higher than that of water at �45,000 W/m2 K.

j Based on 5000 thermal cycling, slurries with bare and encapsu-lated nano-PCMs demonstrated very consistent thermal perfor-mance indicating that coalescence of In particles is not an issue,and thus encapsulation of particles may not be needed. Abilityof avoiding the coalescence of molten In particles is believedto be due to the naturally developing oxide shell outside the

particles, and possibly two other mechanisms, namely, the sur-factant used during the synthesis of particles, and residualcharges which generate strong repulsive electrical force.

The introduced method would greatly benefit thermal manage-ment applications that dictate the use of certain working fluids.The slurries featuring nano-PCMs would enhance thermal proper-ties of the working fluids and eventually improve the system effi-ciency and size/weight specifications.

Acknowledgments

This work was supported by National Science Foundation (NSF)through Grant CBET No. 0828466, and Air Force Research Labora-tory (AFRL) through Universal Technology Corporation. Materialcharacterization was performed at the Materials CharacterizationFacility at the University of Central Florida.

References

[1] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thompson, Anomalously increasedeffective thermal conductivities of ethylene glycol-based nanofluidscontaining copper nanoparticles, Appl. Phys. Lett. 78 (2001) 718.

[2] L.Q. Wang, X.H. Wei, Nanofluids: synthesis, heat conduction, and extension, J.Heat Transfer 131 (2009) 033102.

[3] R.L. Zeng, X. Wang, B.J. Chen, Y.P. Zhang, J.L. Niu, H.F. Di, Heat transfercharacteristics of microencapsulated phase change material slurry in laminarflow under co heat flux, Appl. Energy 86 (12) (2009) 2661–2670.

[4] B.J. Chen, X. Wang, R.L. Zeng, Y.P. Zhang, X.C. Wang, J.L. Niu, Y. Li, H.F. Di, X.Wang, An experimental study of convective heat transfer withmicroencapsulated phase change material suspension: laminar flow in acircular tube under constant heat flux, Exp. Therm. Fluid Sci. 32 (2008) 1638–1646.

[5] X.C. Wang, X. Wang, J.L. Niu, Y. Li, B.J. Chen, R.L. Zeng, Q.W. Song, Y.P. Zhang,Flow and heat transfer behaviors of phase change material slurries in ahorizontal circular tube, Int. J. Heat Mass Transfer 50 (2007) 2480–2491.

[6] Y. Yamagishi, H. Takeuchi, A.T. Pyatenko, N. Kayukawa, Characteristics ofMPCM slurry as a heat transfer fluid, AIChE J. 45 (1999) 696–707.

[7] X. Hu, Y. Zhang, Novel insight and numerical analysis of convective heattransfer enhancement with microencapsulated phase change material slurries:laminar flow in a circular tube with constant heat flux, Int. J. Heat MassTransfer 45 (2002) 3163–3172.

[8] M. Goel, S.K. Roy, S. Sengupta, Laminar forced convection heat transfer inmicrocapsulated phase change material suspensions, Int. J. Heat Mass Transfer37 (1994) 593–604.

[9] Z.H. Han, F.Y. Cao, B. Yang, Synthesis and thermal characterization of phase-changeable indium/polyalphaolefin nanofluids, Appl. Phys. Lett. 92 (2008)243104.

[10] E. Choi, Y.I. Cho, H.G. Lorsch, Forced convection heat transfer with phase-change-material slurries: turbulent flow in a circular tube, Int. J. Heat MassTransfer 37 (1994) 207.

[11] Y.T. Fang, S.Y. Kuang, X.N. Gao, Z.G. Zhang, Preparation and characterization ofnovel nanoencapsulated phase change materials, Energy Convers. Manage. 49(2008) 3704–3707.

[12] F. Tiarks, K. Landfester, M. Antonietti, Preparation of polymeric nanocapsulesby miniemulsion polymerization, Langmuir 17 (2001) 908.

[13] J.C. Maxwell, A Treatise on Electricity and Magnetism, third ed., Dover, NewYork, 1954. 440.

[14] C.W. Nan, R. Birringer, D.R. Clarke, H. Gleiter, Effective thermal conductivity ofparticulate composites with interfacial thermal resistance, J. Appl. Phys. 81(10) (1997) 6692–6699.

[15] F.P. Incropera, D.P. Dewitt, Introduction to Heat Transfer, sixth ed., Wiley,2006.

[16] PAO Data Sheet, www.Matweb.com, 2010.[17] The Society of Thermophysical Properties, Thermophysical Properties,

Handbook, Youkendo, Tokyo, 1994.