niversity of Missouri – Columbia,olumbia, MO 65211
. S. Choirgonne National Laboratory,rgonne, IL 60439
urli Tirumalantel Corporation,illsboro, OR 97124
n experimental investigation of a nanofluid oscillating heat pipeOHP) was conducted to determine the nanofluid effect on theeat transport capability in an OHP. The nanofluid consisted ofPLC grade water and 1.0 vol % diamond nanoparticles of–50 nm. These diamond nanoparticles settle down in the mo-
ionless base fluid. However, the oscillating motion of the OHPuspends the diamond nanoparticles in the working fluid. Experi-ental results show that the heat transport capability of the OHP
ignificantly increased when it was charged with the nanofluid atfilling ratio of 50%. It was found that the heat transport capa-
ility of the OHP depends on the operating temperature. The in-estigated OHP could reach a thermal resistance of 0.03°C/W atheat input of 336 W. The nanofluid OHP investigated here pro-
ides a new approach in designing a highly efficient next genera-ion of heat pipe cooling devices. �DOI: 10.1115/1.2352789�
ntroductionEffective thermal management has become one of the most
erious challenges in many new technologies due to constant de-ands for faster speeds and continuous reduction of device di-ensions. High thermal conductivity of nanofluids produced by
dding only a small amount of nanoparticles into the fluid hasualified nanofluids as a most promising candidate for achievingltra-high-performance cooling �1�. When a small amount �lesshan 1% volume fraction� of copper nanoparticles or carbon nano-ubes were dispersed in ethylene glycol or oil, their thermal con-uctivity could be increased by 40% and 150%, respectively �2,3�.ince Choi’s work �1�, outstanding discoveries and seminalchievements have been reported in the emerging field of nano-uids. Das et al. �4� explored the temperature dependence of the
hermal conductivity of water-based nanofluids containing Al2O3r CuO nanoparticles and reported the discovery of a two- to
Contributed by the Heat Transfer Division of ASME for publication in the JOUR-
AL OF HEAT TRANSFER. Manuscript received March 3, 2006; final manuscript re-
eived May 23, 2006. Review conducted by Ranga Pitchumani.
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four-fold increase in thermal conductivity enhancement for nano-fluids over a temperature range of 21°C to 51°C. Patel et al. �5�have shown a 5–21% increase in thermal conductivity of water atvanishing concentrations ��0.00026 vol % � of monosized goldnanoparticles with citrate stabilization. They suggested that thestrong temperature dependence of thermal conductivity is due tothe motion of nanoparticles and nanoparticle surface chemistry.Even more interesting behavior was discovered by You et al. �6�,Bang and Chang �7�, and Vassallo et al. �8�, who measured thecritical heat flux �CHF� in pool boiling and showed an increase ofCHF. These features make nanofluids strong candidates for thenext generation of coolants.
Oscillating single-phase fluids significantly enhance heat andmass transfer in a channel, and have been employed in a numberof heat transfer devices �9–11�. The oscillating motions generatedby a variable-frequency shaker �10,11� could result in a thermaldiffusivity of up to 17,900 times higher than those without oscil-lations in the capillary tubes, but the use of mechanically drivenshakers may limit its applications to miniature devices. Akachi�12� invented a new device, called oscillating heat pipe �OHP�. Itutilizes the pressure change in volume expansion and contractionduring phase change to excite the oscillation motion of the liquidplugs and vapor bubbles, which results in four unique features thatdo not exist in regular heat pipes: �1� OHP is an “active” coolingdevice, in that it converts intensive heat from the high-power gen-erating device into kinetic energy of fluids in support of the oscil-lating motion; �2� liquid flow does not interfere with the vaporflow in high-heat removal because both phases flow in the samedirection; �3� the thermally driven oscillating flow inside the cap-illary tube will effectively produce some “blank” surfaces thatsignificantly enhance evaporating and condensing heat transfer;and �4� the oscillating motion in the capillary tube significantlyenhances forced convection in addition to the phase-change heattransfer. Because of these features, the OHP has been extensivelyinvestigated in the past five years �13–18�.
Most recently, Ma et al. �19� charged the nanofluids into anOHP and found that nanofluids significantly enhance the heattransport capability in the OHP. When the nanofluid �high-performance liquid chromatography �HPLC� grade water contain-ing 1.0 vol % 5–50 nm of diamond nanoparticles� was charged tothe OHP, the temperature difference between the evaporator andthe condenser can be significantly reduced. For example, when thepower input added on the evaporator is 100 W, the temperaturedifference can be reduced from 42°C to 25°C. In the currentinvestigation, an experimental investigation on an OHP chargedwith nanofluid was conducted to determine the effect of operatingtemperature on the heat transport capability.
Experimental System and ProcedureFor the vapor and liquid regions to remain separate, the diam-
eter of the tube for an OHP must be small enough for capillaryforces to dominate over gravitational forces. This diameter can beapproximately calculated by the Bond number, i.e., Bo=D2g��l
−�v� /�. The maximum diameter allowed to create distinct slugs isfound by applying a critical Bond number determined experimen-tally and is defined as Dmax=�Bocrit� /g��l−�v�. Shafii et al. �20�use a critical Bond number of 1.84 while Khandekar et al. �16� usethe value of 2 to determine the minimum tube diameter. In thecurrent investigation, a Bond number of 1.84 and the properties ofwater are used to determine the inside diameter of the tube. Con-sidering commercial availability, Alloy 122 copper tubing with aninside diameter of 1.65 mm and an outer diameter of 3.18 mmwas used for the OHP investigated herein. The heat pipe, asshown in Fig. 1, has 12 turns traversing three sections: evaporator;adiabatic; and condenser section. The evaporator has dimensions
of 59.9 mm�301.5 mm where a uniform heat flux was added on
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ne side and the condenser has the same dimension where theooling plate was attached. The distance between the evaporatornd the condenser was 101.6 mm.
Nanofluids used in this study were HPLC grade water contain-ng diamond nanoparticles. The nanoparticles with size rangingrom 5 nm to 50 nm were fabricated by the 20 kW rf plasma withhigh frequency of 13.56 MHz. The nanofluid was produced by
dding nanoparticles directly into HPLC grade water. Most of theanoparticles in the motionless water settle down as shown in Fig.. TEM investigation shown in Fig. 3 indicated that only the nano-articles with size �10 nm can be suspended in the motionlessater. In the current investigation, the volume ratio of nanopar-
icles charged to the heat pipe was 1.0% of the base fluid. Once
he OHP was built, it was placed on a scale and the charging tube
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was connected to a vacuum pump. The air was removed and theworking fluid was then introduced into the OHP. Once the correctvolume percent was achieved, as indicated by the scale, the charg-ing tube was sealed. Previous experiments �21,22� have shownthat the OHP with a working fluid of water can operate at a fillingratio �liquid volume/total volume� of 50%. As a result, a fillingratio of 50% was used for the current study.
Figure 4 illustrates the experimental setup used to test the heatpipe. In order to reduce the contact thermal resistance from theheater to the evaporating section, a copper plate with dimensions59.9 mm�301.5 mm�6.6 mm was used. The semicirculargrooves machined on the plate created a good fitting with tubingto further reduce the contact thermal resistance. The condenserwas fashioned in the same manner. The OMEGATHERM “201”thermal paste was used between the tubing and the copper platesto reduce contact resistance. The paste was also used to adhere theheater and water cooled blocks onto the evaporator and condenserplates, respectively. The heat input to the evaporator was providedby a strip heater. Two water cooled aluminum blocks were placedon the condenser plate. The temperature controlled water was sup-plied by a Julabo F34 circulator. Type T thermocouples wereplaced on the outside surface of OHP in the locations shown in
Fig. 2 The sedimentation of untreated diamond nanoparticlesat settling times of „a… 0 min, „b… 1 min, „c… 2 min, „d… 3 min, „e…4 min, „f… 5 min, and „g… 6 min
Fig. 3 Transmission electron microscopy image of diamondnanoparticles collected from suspension phases in a motion-
less nanofluid
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ig. 1�b�, which could also be used to detect the movement ofapor plugs and liquid slugs by the temperature variation. A NICXI-1000 data acquisition system and personal computer pro-ided an uncertainty of 0.1°C of the temperature measurementere used to record the temperatures. Power was supplied by ataco 3PN501B voltage regulator and the voltage was measuredy a Fluke 45 dual display multimeter. The entire OHP was sur-ounded by insulation material to ensure heat transfer occurs onlyn the evaporator and condenser regions. At the highest power of36 W, the temperature on the outer surface of insulation materialas 28°C. The heat loss through the insulation to the atmosphereas calculated to be less than 3.0% of the overall heat load. TheHP was tested vertically, i.e., the evaporator on the bottom and
ondenser on the top.Prior to the start of the experiment, the system was allowed to
quilibrate and reach steady state such that the temperature of theooling media and the heat pipe were constant at 0±0.5°C overhe sample time, which was controlled by the cooling bath. Whenhe desired steady-state condition has been obtained, the inputower was increased in small increments. The test indicated that aime of approximately 60 min was necessary to reach steady statet low power levels and 10 min at high power levels. To obtainhe data for each successive power level, the power was incre-
ented and the OHP was allowed to reach steady state until theaximum heat power of 336 W which could be provided by the
eater. During the tests, the thermal power and the temperatureata were simultaneously recorded using the data acquisition sys-em controlled by a personal computer.
esults and DiscussionUsing the experimental setup and procedures described above,
he effect of operating temperature and power input on the heatransport capability in the OHP was studied. Due to the heaterower limit, tests were conducted by varying the heat load be-ween 0 W and 336 W. The operating temperature controlled byhe condenser water supply was between 10°C and 70°C. For allests, the heat pipe was charged with a filling ratio of 50%. Inrder to demonstrate the nanofluid effect, the same heat pipeharged only with HPLC grade water was tested as well.
Figure 5 shows comparisons of a nanofluid effect on the heatransport capability in an OHP with the one charged only withPLC grade water. As shown, when the OHP was charged withanofluids, the thermal resistance significantly reduced. Obvi-usly, this large reduction in thermal resistance is due to the nano-articles added in the base fluid. However, the detailed informa-ion of the nanoparticle effect on the thermal conductivity ofanofluid or/and convection coefficient is not known. When theanofluids investigated here were motionless, most of the nano-articles settled down as shown in Fig. 2. For a functional OHP,he thermal energy added on the evaporator produces strong os-illating motions, which suspends the nanoparticles in the base
Fig. 4 Experimental setup
uid. When the oscillating motion is generated in an OHP, heat is
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transferred from the evaporator to the condenser through phase-change heat transfer in the evaporator and condenser, and forcedconvection through the oscillating motions of vapor bubbles andliquid plugs. The oscillating motion excited by the thermal energyin the OHP enhances the forced convection heat transfer, andmore importantly it keeps the nanoparticles suspended in the fluid.For both OHPs either charged with pure water or nanofluid, thereexists a startup heat input. When the heat input was less than thisrequired startup, no oscillating motions were observed and thetemperature difference between the evaporator and condenser in-creased linearly as the power input increases. Once the oscillatingmotion started in the OHP, the further increase of power input didnot significantly increase the temperature difference. Before theheat input reached the startup, there were no oscillating motionssimilar to the situation shown in Fig. 2, where most of the nano-particles settled down. At this situation, the nanoparticles mightstay in the evaporating section for a vertical position, i.e., theevaporator on the bottom, and this might be the reason that theheat resistance for the nanofluid OHP is the same as the one withpure water before the heat pipe started to function. Once the os-cillating motions started and the nanoparticles were well mixed,the thermal resistance reduced significantly. The nanoparticle sus-pension excited by the oscillation motion of vapor bubbles andliquid plugs is the primary reason enhancing the heat transportcapability in a nanofluid OHP. When the nanoparticle size is re-duced, the thermal conductivity of the nanofluid increases �1–3�.However, the nanoparticles may agglomerate, settle, or coalesceto the walls with long-term operation of the nanofluid OHP. Pre-liminary long-term testing of the prototype device shows that theheat transfer performance remains the same over a period of atleast six months, although further research is warranted to estab-lish the longer-term reliability.
Figure 6 shows the operating temperature Top effect on the ther-mal resistance occurring in the OHP. As shown, when the operat-ing temperature increased, the thermal resistance significantly de-creased. When the operating temperature was at 70°C, thethermal resistance occurring in this OHP can reach 0.03°C/W ata total heat input of 336 W. If the heat transfer rate from theevaporator to the condenser is expressed as Q=UA�Te−Tc�= �Te
−Tc� /R, where Q is the heat transfer rate, U is the overall heattransfer coefficient, A is the total cross-sectional area of the OHP,Te is the average temperature on the evaporator, and Tc is theaverage temperature on the condenser, the overall heat transfercoefficient �U� increases from 8.13 W/ °C at 20°C to 27.8 W/ °Cat 70°C, which increased 3.4 times when only the operating tem-perature increased from 20°C to 70°C. When the operating tem-
Fig. 5 Thermal resistance comparison between a watercharged OHP and a nanofluid charged OHP „filling ratio=50%,vertical, Top=20°C…
perature increases, higher thermal conductivity, lower viscosity of
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anofluid, and stronger oscillating motion might be the primaryactors enhancing the heat transport capability in a nanofluid os-illating heat pipe. Further investigation of these aspects is neededor a conclusive determination, and will be addressed in a futureork.
onclusionsAn experimental investigation of nanofluid oscillating heat pipe
OHP� was conducted to determine the nanofluid effect on theeat transport capability in an OHP. The nanofluid consisting ofPLC grade water and 1.0 vol % diamond nanoparticles of–50 nm can significantly increase the heat transport capability inn OHP. Due to the thermally excited oscillating motion occurringn the OHP, the diamond nanoparticles can be suspended in thease fluid, which can increase the heat transport capability ofanofluid. The operating temperature can significantly affect theeat transport capability in the investigated OHP and when theperating temperature increases, the heat transport capability in-reases. The investigated OHP charged with nanofluids can reachthermal resistance of 0.03°C/W at a power input of 336 W.
cknowledgmentThis work was supported by the National Science Foundation
nder Contract No. CTS-0507913 to the University of Missouri –olumbia, Intel Corporation, and the U.S. Department of Energy,ffice of FreedomCar and Vehicle Technologies and the Office ofasic Energy Science under Contract No. W-31-109-Eng-38 torgonne National Laboratory.
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