Effect of functionalized MWCNTs water nanofluids on thermal resistance and pressure fluctuation characteristics in oscillating heat pipe

International Communications in Heat and Mass Transfer xxx (2013) xxx–xxx

ICHMT-02845; No of Pages 6

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International Communications in Heat and Mass Transfer

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Effect of functionalized MWCNTs/water nanofluids on thermal resistance and pressurefluctuation characteristics in oscillating heat pipe☆

Md. Riyad Tanshen a,1, B. Munkhbayar a,b,1, Md. J. Nine a, Hanshik Chung c, Hyomin Jeong c,⁎a Department of Energy and Mechanical Engineering, Gyeongsang National University, 445 Inpyeong Dong, Tongyeong, Gyeongnam 650-160, Republic of Koreab School of Chemical Engineering, The University of Adelaide, SA 5005, Australiac Department of Energy and Mechanical Engineering, Institute of Marine Industry, Gyeongsang National University, 445 Inpyeong Dong, Tongyeong, Gyeongnam 650-160, Republic of Korea

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.

E-mail addresses: [email protected] (B. Munk(H. Jeong).

1 Authors are equally contributed: Md. Riyad Tanshen,

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Keywords:MWCNTsOscillating heat pipePressure fluctuationThermal resistance

An influence of multi-walled carbon nanotube (MWCNT) based aqueous nanofluids with different concentrationson the heat transport and the relevant pressure distribution in oscillating heat pipe (OHP) has been investigated.The present paper describes the heat transfer phenomena in terms of thermal resistance, pressure and frequencyof pressure fluctuation in multi-loop oscillating heat pipe (OHP) charged by aqueous nanofluids with MWCNTloadings of 0.05 wt.%, 0.1 wt.%, 0.2 wt.% and 0.3 wt.%. The multi-loop OHP with 3 mm inner diameter has beenconducted in the experiment at 60% filling ratio. Experimental results show that thermal characteristics are signif-icantly inter-related with pressure distribution and strongly depend upon the number of pressure fluctuationswith time. The investigation shows that the 0.2 wt.% MWCNTs based aqueous nanofluids obtain maximumnumber of the fluctuation frequency and low thermal resistance at any evaporator power input. Based on theexperimental results, we discuss the reasons for enhancement and decrement of thermal characteristics of thenanofluids.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The applications of nanofluids are placed inmost of the high efficientheat transfer devices since the innovation of this new heat transfer fluidby Choi in 1995 [1]. Oscillating and pulsating phenomena carry greatrole in the field of heat transfer to control thermal transport intomicro cavity. The self exciting oscillation inside the OHP that is drivenby the fluctuation of pressure waves occurs with greater and rapidheat transfer from one end to another. The pressure fluctuation occursdue to the nucleate boiling by evaporative section and condensationof the working fluid. The OHP can transfer heat in quick response inany orientations. The article describes the vertical orientation of multi-loop OHP.

Recently, many experiments have been studied in the field of theOHP because of its specific features. Extended investigations of theOHP have been investigated since the first OHP developed by Akachiin 1990 [2]. The mechanism that occurs in the OHP is the utilization ofpressure change in volume expansion and contraction during phasechange to excite the oscillatingmotion of liquid plugs and vapor bubblesbetween evaporator and condenser. Comparing to the OHP with otherconventional heat pipes, the unique feature of OHP is that there is no

hbayar), [email protected]

B. Munkhbayar.

ghts reserved.1

Effect of functionalizedMWC://dx.doi.org/10.1016/j.icheat

wick structure to return the condensate to the evaporator and no coun-tercurrent flow between the liquid and the vapor flows because bothoperates in the same direction [3] — (1) The thermally-driven oscillat-ingflow inside the capillary tube effectively produces some free surfacesthat significantly enhance evaporating and condensing heat transfer.(2) The oscillating motion in the capillary tube significantly enhancesthe forced convection in addition to the phase-change heat transfer.These significant characters of OHPmake itself very special heat transferdevice in modern application.

Past works on the OHP can be concluded within several featuressuch as heat transfer characteristics and capability with different fillingratio, flow visualization inside the OHP, effects of length ratio and diam-eter on performance of OHP, nanofluids and other applicable fluids havebeen used as a working fluid for developing OHP performance.

Charoensawan et al. [4], Rittidech et al. [5] and Tong et al. [6] havediscussed the effects of several parameters on thermal performance,such as internal diameter, number of turns, working fluid and incli-nation angle of the device. Wang and Nishio [7] investigated theeffect of length ratio of heating section to cooling section on theultimate heat transport capability of OHP. The influence of gravityon slug flow and influence of number of turns on spatial dynamicpressure affect the OHP performance. Besides the input heat is alsoa strong parameter that affects dynamic instability especially in den-sity wave oscillation [8,9].

Saha et al. [10] conducted flow visualization for closed-loop PHPmade from Teflon tube of 2 mm internal diameter and partially filledwith R142b. The PHP consisted of 10 meandering turns and it is

NTs/water nanofluids on thermal resistance and pressure fluctuation...,masstransfer.2013.08.011

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Nomenclature

Bo Bond numberEö Etövös numberD Tube diameterg Acceleration of gravity [m/s2]I Input current [A]N Total number of DataPm Mean pressure [Pa]ffiffiffiffiffip2

qRMS value of pressure or pressure fluctuation [Pa]

Q Heat load [W]R Thermal resistance [°C/W]t Time [s]Te Average OHP wall temperature in evaporator [°C]Tc Average OHP wall temperature in condenser [°C]wt Weight concentration [%]V Input voltage [Volts]ρ Density[Kg/m3]σ Surface tension [N/m]

Subscriptc Condenser sectioncri Critical valuee Evaporator sectioni Innerliq Liquidvap Vapor

2 M.R. Tanshen et al. / International Communications in Heat and Mass Transfer xxx (2013) xxx–xxx

400 mm from the evaporator to condenser. The evaporator was heatedby a hot bath and the condenser was cooled by a cold bath. It was con-cluded that the highest thermal performance for the PHP is achievedwhen the FR is from 0.5 to 0.6.

In 1998, Chandratilleke et al. [11] developed the cryogenic loop heatpipes. The development of cry cooler cooled superconducting magnetapplications, where heat transport distance is large, and the heatconduction by a copper block will be constrained by its cross sectiontransport capacity.Mo et al. [12,13] shows that the heat transport capac-ity of loop heat pipe with liquid nitrogen as working fluid is very low(26 W) when it operates in horizontal direction and its lowest thermalresistance reaches 1.3 K/W, which is too high for the most of cryogenicheat transport system.

Recently, it was found that the heat transport capability can beincreased when nanoparticles [14] or microparticles were addedinto the base fluid in an OHP. The thermally-excited oscillatingmotion in the OHP can make the particles suspended in the basefluid. Although the nanoparticles added on the base fluid cannotlargely increase the thermal conductivity [13], the oscillating mo-tion of the particles in the fluids may have additional contributionto the heat transfer enhancement in addition to the thermal conduc-tivity. Ma et al. [15] charged the nanofluids into OHP and found thatthe nanofluids significantly enhance the heat transport capability ofOHP. The investigations show that OHP charged with diamondnanofluids can reach a thermal resistance of 0.03 °C/W at powerinput of 336 W. In 2010, Qu et al. [16] also conducted an investiga-tion of the effect of spherical 56 nm Al2O3 particles on the heattransport capability in anOHP and found that theAl2O3 particles can en-hance heat transfer and an optimal mass fraction exists; although theseinvestigations have demonstrated that the particles can enhance heattransfer in the OHP. It is not well understood whether there exists anoptimum particle size for a given type of the particles. Maezawa et al.[17] reported that the appearance and movement of bubbles areaffected by surface tension and buoyancy in the channel. The relation

Please cite this article as:M.R. Tanshen, et al., Effect of functionalizedMWCInt. Commun. Heat Mass Transf. (2013), http://dx.doi.org/10.1016/j.icheat

of surface tension and buoyancy could be explained by the followingdimensionless formula:

Bo ¼ Di

g ρliq−ρvap

� �σ

24

350:5

¼ffiffiffiffiffiffiffiEo:::

p

When Eö = 4, the bubble will get seized on both sides of the wall butnot moving statically, and the liquid forms liquid slug flow, is calculatedinstead of getting the critical pipe diameter Dcri of OHP. The formula is:

Di bD□ cri≤2�ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

σ

ρlic−ρVap

� �g

vuut :

The tube inner diameter should be small enough to ensure the flowoscillations, i.e. smaller than a critical value. Lately, Nine et al. [18] hasinvestigated a stable hybrid nanofluid combining Al2O3 nanoparticleswith ground and non-ground type of the MWCNTs into water. Theyshowed that a small inclusion of ground MWCNTs into Al2O3/waternanofluids can influence the thermal characteristics in a large scalewhere the stability of nanofluids does not deteriorate. The article con-centrates on the pressure characteristics inside the OHP charged withdifferent concentrations of the nanofluids. The present work explainsthe inside phenomena of the OHP in terms of pressure where previousworks focused on some other necessary parameters. Pressure distribu-tion and repeatedfluctuation into theOHPusing different concentrationsof the nanofluids have been investigated under various evaporatorpower input. It is well known that the heat transfer by oscillating heatpipe occurs for quick fluctuation of pressure intensity. However, someworks over pressure distribution inside the OHP can be hardly found.

2. Experimental setup

2.1. Experimental apparatus and procedure

The installation of multi-loop OHP is shown in Fig. 1 that consists of100 mm evaporating section powered by a plate type heater and con-densing section connected with a constant cooling bath where themid-dle section between evaporator and condenser is thermally insulated.The heat pipe is made by copper with an inner diameter of 3 mm anda total length of 6 m. The evaporative section is made by two aluminumplates which grooved the inner part precisely to set OHP like sandwichtype and both parts are perfectly attached with a flat plate type of elec-tric heater. The evaporative section covers a vertical length of 100 mmOHP where the working input power is maintained in the range of50 W to 400 W. Similarly, 140 mm of upper loop section was insertedthrough the sealed condensing tank where the isothermal cooling unitis used to keep the condenser under 18 °C. The middle of this OHPwith 200 mm adiabatic section is covered by 2 cm thick glass wool forthe perfect thermal insulation. T-type thermocouple was soldered tothe outer wall of the OHP in both condensing and evaporative sectionsto measure the wall temperature of OHP.

After running the system, it takes time approximately 15 to 20 minto get thermally stable. It occurs due to the electric heating that is notdirectly wrapped on the OHP. One piezoresistive absolute pressure sen-sor (Model-Kistler 4045A5 Kistler Instruments (Pte) Ltd. Singapore) isset with another small pass tube below the condensing section of theOHP to take the pressure characteristics inside the tube. The sensor isperfectly sealed and tested several times. Data acquisition rate was100 data points/s and the duration of data was approximately 10 min.To find out the frequency distribution, FFT analysis is worked with10,240 data. For the convenience of understanding, a part of real exper-imental setup has been shown just at the right side of Fig. 1.



Fig. 1. A schematic diagram of experimental setup (left) and a skeleton structure (right).

3M.R. Tanshen et al. / International Communications in Heat and Mass Transfer xxx (2013) xxx–xxx

2.2. Preparation and characterizations of nanofluids

MWCNTs with ~20 nm diameter, ~5 μm length, greater than 95%purity, less than 3% impurities and a specific surface area of 40–300 m2/g (purchased from Carbon Nanomaterial Technology Co., Ltd,South Korea) used in this workwere synthesized by chemical vapor de-position (CVD). It has been known that MWCNTs have a hydrophobicsurface, which is prone to aggregation and precipitation in water inthe absence of a dispersant/surfactant, as described in our previousworks [19,20]. For better dispersion,MWCNTs were polarized by chem-ical treatment. A simple method for purifying MWCNTs by using nitricacid (HNO3) and sulfuric acid (H2SO4) was employed. Purification wasperformed by ultrasonication 1510E-DTH (Branson Ultrasonic Corpora-tion 41, Danbury, CT 06813, USA) for 5 h to remove the impurities andamorphous carbon and to improve exterior activity. Calorimetry wasperformed to measure the output power and frequency of the appliedultrasonic vibration; these were determined to be 63 W and 42 kHz,respectively. After ultrasonication, anhydrous ethanol was used toneutralize the acids. Previous investigation revealed that hydrophilicfunctional groups such as C\O\C, C_O, O\H and \COOH mightbe introduced onto the surface of purified CNT or Graphenestructure [21–23]. The acidic mixture of MWCNTs containing car-boxyl radicals was diluted by adding distilled water. This dilutionand filtration processes conducted using a vacuum filter was repeat-ed until a pH of 7 was reached. The moisture was removed by placingthe MWCNTs in a vacuum oven for 12 h at 120 °C. The structure, sizeand purity of raw and purified CNTs were confirmed by transmissionelectron microscopy (TEM) (JEM-2100 F, JEOL; Tokyo, Japan), asshown in Fig. 2. In the process of acidification, the non-crystalline

Fig. 2. TEM morphology of the (a) raw and (b) purified MWCNTs. Inset photo


impurities, such as soot, and metal, such as iron oxide used as cata-lysts, were removed and the tube caps were opened behaviors ob-served by using X-ray diffraction (XRD) pattern and Raman spectrain Kim et al. [22]. After purification, such similar characteristic ofthe MWCNTs was observed by using TEM micrograph in this study.The HRTEM image shows that the raw MWCNTs contained largeamounts of impurities, such as catalyst particles, amorphous carbon,carbon particles and multishell carbon nanocapsules (black arrowsin Fig. 2(a)). The catalyst particles are evidently embedded in thetips or the tube cores of MWCNTs. The outer walls of the rawMWCNTs were coated with a layer of amorphous carbon, whichcan be clearly observed in Fig. 2(a). The purified MWCNTs wereclean and possessed no carbon particles, as shown in Fig. 2(b). Inter-estingly, the structures of the amorphous carbon and carbon parti-cles of the raw MWCNTs were completely eliminated, and the tipsof the nanotubes opened (white arrows in Fig. 2(b)) after purifica-tion. The morphology and tubular structure of MWCNTs were stillobserved in Fig. 2(b), suggesting that the structural integrity ofMWCNTs is not deteriorated. It means that the structure of theMWCNTs remained unchanged after purification. This purificationmechanism is that the surface modifications feature functionalgroups or modifier molecules that are attached to CNTs by covalentbonds, either directly or with functional groups, such as \COOH,\COH, and \OH groups, on the sidewalls and termini of the CNTs.

To compare the dispersion and thermal characteristics of thenanofluids on the OHP, the raw and purified structures of MWCNTswith various concentrations (0.05–0.3 wt.%) were ultrasonically dis-persed into the aqueous solution for 1 h. The ice water was repeatedlyadded into ultrasonic bath during ultrasonication in order to prevent

shows the dispersion state of the nanofluids regarding to the structures.




rising of the temperature of the suspension. The stabilities of nanofluidswere observed 3 days after ultrasonication. The experiments were car-ried out with distilled water and no surfactant was used in this study.

The inset photo of Fig. 2(a) and (b) shows the dispersion state of rawand purifiedMWCNTs, respectively. The inset of Fig. 2(a) shows the dis-persion states of raw MWCNTs particles in aqueous solution, in whichit can be seen that the large amounts of particles aggregated on the sur-face of the water and along the wall of the bottle instead of beingdispersed, despite the application of ultrasonication energy. This aggre-gation of the raw MWCNTs is known to be due to a lack of hydrophilicgroups in their structure, prohibiting interaction with the polar solvent.Contrary to Fig. 2(b) it shows the best dispersibility of nanofluids con-taining purified structure of theMWCNTs. The rest of purifiedMWCNTsremained as a colloidal (well dispersed) solution for several days, with-out appreciable change from their dispersion state after ultrasonication.

3. Data reduction

Several thousands of raw data signals were received in the form ofvoltage that has been converted into pressure and analyzed to find themean and RMS value in following equations:

Mean Pressure; Pm ¼ ∑Nt¼1 p tð ÞN

RMS value; Prms ¼ffiffiffiffiffip2

q¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∑N

t¼1 pm−p tð Þ½ �2N

s; where N ¼ 10240:

The overall thermal resistance (R) has been calculated for constantwall temperature by following equation, as shown below:

R ¼ Te−Tc

Q;

where R is the thermal resistance and Te, Tc, and Q are the average walltemperature in evaporator, average OHPwall temperature in condenserand power input supplied to evaporative section, respectively.

4. Result and discussion

The heat input in evaporator, filling ratio of working fluid as well asthe shape, diameter, and angle of installation have great influences onthe performance of OHP. These aforementioned parameters are interre-lated with inside pressure fluctuation for the best performance of OHP.The functionalized MWCNT based aqueous nanofluids are employed in

Fig. 3. Thermal resistance with different concentrations of the nanofluids at 60% fillingratio.


this experiment varyingMWCNT loadings (0.05 wt.%, 0.1 wt.%, 0.2 wt.%and 0.3 wt.%). It has been found that a low concentration of MWCNTs isfitted for the experiment due to the MWCNT higher volumetric ratio.Optimal concentration by calculating thermal resistance varying differ-ent MWCNTs weight fraction is shown in Fig. 3.

It can be observed from Fig. 3 that the thermal resistance decreaseswith raising an input power with the addition of MWCNTs into wateralso decrease thermal resistance effectively. The concentration of0.2 wt.% MWCNTs obtains best performance among all of the differentconcentrations used in the experiment. The decline of thermal resis-tance starts with the inclusion of 0.05 wt.% MWCNTs that continuesuntil 0.2 wt.%, but, the thermal resistance sharply increases at 0.3 wt.%of MWCNTs. The aqueous solution of 0.2 wt.% MWCNTs is optimal forthe system to transport heat efficiently due to a suitable change in vis-cosity and conductivity. This change in viscosity with non-uniformparticles with higher thermal conductivity of working fluid containsmore energy to form vapor slugs into the fluid. Therefore, when thebubbles explode, it releases greater energy near to the condensingpart and the fluid plug that carries the energy smoothly also gets facilityfromMWCNT shear thinningbehavior to carry theheat fromevaporatorto condenser. On the other hand, the fluid containing 0.3 wt.%MWCNTs

Fig. 4. A frequency distribution of the nanofluids with different concentrations at 60% fill-ing ratio and 200 W evaporative temperatures.



Fig. 5. RMS value of pressure into OHP at different concentrations of the nanofluids.

5M.R. Tanshen et al. / International Communications in Heat and Mass Transfer xxx (2013) xxx–xxx

depicts higher thermal resistance compared to the other concentra-tions. It is mainly attributed to the physical characteristics of MWCNTs.The higher volumetric ratio of MWCNTs and the surface characteristicsprovide highly viscous fluid that does not allow the fluid to move freelywith the same power supplied in evaporative section. The fluid contain-ing 0.3 wt.% MWCNTs is unable to generate sufficient vapor plugs inthe middle of highly viscous working fluid. Surface tension is anotherreason why the fluid cannot be capable to transport heat as quick asother fluids.

Fig. 4 is more comprehensive to understand the influence of differ-ent concentrations of the nanofluids transferring energy through OHP.The repeated fluctuation of pressure inside OHP facilitates thesystem to generate bubbles that provide the flow non-uniform oruniform and irregular or regular. The uniformity and velocity of thefluctuation can be estimated by frequency analysis from the ac-quired raw data of pressure. The frequency distribution of the insidepressure at 60% filling ratio of 0.2 wt.% MWCNT based aqueousnanofluids shows that the higher frequency near to the condensingpart is approximately 43 Hz achieved, whereas, DI water shows ap-proximately 20 Hz. A higher frequency of pressure means rapid re-sponse to evaporative load for energy transportation. Fig. 4 showsthe number of pressure frequency increases with increasing the con-centration, but, after it decreases at 0.3 wt concentration. 0.2 wt.%MWCNT based aqueous nanofluid is identified as an optimal concen-tration by acquisition pressure distribution into OHP.

Fig. 6.Mean value of pressure into OHP at different concentrations of the nanofluids.


Higher magnitude of pressure implies the higher intensity of pres-sure. As shown in Fig. 5, 0.2 wt.% MWCNT based aqueous nanofluidshows higher magnitude of pressure intensity which is associated tothe vapor pressure inside OHP which gets higher for the concentrationof 0.2 wt.%. A more bubble generation creates greater vapors thatoccur in high pressure amplitude. It will begin to condense and heatwill be released when the vapor bubbles reach the condenser quickly.As the vapor changes phase, the vapor pressure decreases, and theliquid flows back toward the condenser end. Fig. 6 describes meanpressure for different concentrations of MWCNTs/water solution at dif-ferent evaporative power input. Similarly, the vapor pressure from thebubbles using different concentrations is responsible for different valuesof mean pressure. The pressure gradually increases with increasingevaporative load. Therefore, the amplitude from themean value of pres-sure is another significant parameter to understand an intensity andload capacity of OHP.

5. Conclusion

An experimental investigation is carried out to measure the thermalresistance and fluctuation of pressure inside oscillating heat pipeemploying aqueous solution of MWCNTs varying different concentra-tions. In oscillating heat pipe, the heat transfer occurs due to repeatedpressure fluctuation and a greater repetition of pressure fluctuationmeans higher heat transfer.

• Acid treatment on raw MWCNTs provides it amorphous free thatenhances the dispersibility of the MWCNTs.

• The inclusion of functionalized MWCNTs increases thermal transpor-tation from evaporative section to condensing section. A lowest ther-mal resistance has been achieved by 0.2 wt.% MWCNT based aqueousnanofluids.

• 0.2 wt.% MWCNT based aqueous nanofluids which provide higherfrequency of the pressure and mean pressure inside OHP are alsoachieved. Thermal characteristics are significantly inter-related withpressure distribution and strongly depend upon the number of pres-sure fluctuations with time.

Acknowledgment

This research was supported by Basic Science Program through theNational Research Foundation of Korea (NRF) funded by the Ministryof Education, Science and Technology (2012-0004544) and the FutureLeading Project through the Small andMediumBusinessAdministration(No. S2044441).

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Effect of functionalized MWCNTs water nanofluids on thermal resistance and pressure fluctuation characteristics in oscillating heat pipe