International Journal of Heat and Mass Transfer 54 (2011) 5379–5385

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International Journal of Heat and Mass Transfer

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Minichannels with carbon nanotube structured surfaces for cooling applications

S. Shenoy, J.F. Tullius, Y. Bayazitoglu ⇑Department of Mechanical Engineering and Materials Science, Rice University, 6100 Main, Houston, TX 77005, USA

a r t i c l e i n f o

Article history:Received 28 January 2011Received in revised form 30 July 2011Accepted 30 July 2011Available online 6 September 2011

Keywords:MinichannelForced convectionCarbon nanotubesFinsWater

0017-9310/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.ijheatmasstransfer.2011.08.005

⇑ Corresponding author. Tel.: +1 713 348 6291; faxE-mail address: bayaz@rice.edu (Y. Bayazitoglu).

a b s t r a c t

An experimental study was conducted to determine the heat removal ability of multiwall carbon nano-tubes (MWNTs) grown in a silicon minichannel with water as the cooling medium. Two different devicesbased on different MWNT architectures – one fully covered with MWNTs and the other with 6 � 12(rows, columns) of MWNT bundles were tested and compared to a device with no MWNTs. The perfor-mance was evaluated based on a constant heat flux applied to the silicon base versus the correspondingsilicon base temperature. The experiments were performed at two different volumetric flow rates of40 ml/min and 80 ml/min. The experimental results were also compared to computational modelingresults. It was observed that the presence of MWNTs enhanced the heat removal from the silicon base.For the 6 � 12 MWNT bundles device, a silicon base heat flux that was 2.3 times the heat flux to theno MWNTs device can be applied, whereas for the fully covered MWNTs device 1.6 times the heat fluxcan be applied while still keeping the same silicon base temperature.

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1. Introduction

The next generation microchips with high power densitieswould require novel methods of cooling. Minichannels and micro-channels provide an effective way of cooling microchips. The highheat fluxes can be dissipated using forced convection in micro-channels and minichannels [1–5]. Microchannels provide en-hanced heat transfer ability when compared to minichannels dueto having a smaller hydraulic diameter however they come withincreased pumping requirement. In addition the fabrication ofmicrochannels would require novel techniques that are time con-suming and cost intensive. Several methods of altering the micro-channels surfaces including rectangular grooves [6], offset fins [7]and longitudinal fins [8] have been investigated. All of them re-ported an increase in heat transfer due to increased surface area,better flow mixing, and an increased heat transfer coefficient butthey also had the disadvantage of added pressure drop.

The working fluid investigated for liquid cooling has been pre-dominantly water. In addition to the single phase heat transfertechniques mentioned above, flow boiling has also been investi-gated by several authors [9,10]. Two phase flows provide high heattransfer coefficients when compared to single phase flows and aresuited for high heat flux dissipation [11]. Nanofluids with water asthe base fluid with various added nanoparticles offer severaladvantages for cooling. The thermal properties for these fluidscan be tailored to suit the cooling requirements. Experimental

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investigations on convective heat transfer performance have beencarried out using CuO [12,13], Al2O3 [13–15], TiO2 [16] and Cu [17]based nanofluids. Nanofluids come with certain drawbacks likesedimentation, clogging of channels, erosion and increased pres-sure drop [18]. Dielectric fluids as the working fluid have also beeninvestigated by various authors [19–21]. Dielectric fluids with theirlow boiling point and increased wetting properties provide anexcellent way for increased heat transfer however they are plaguedby dry out and reverse flow problems.

Numerous authors have investigated CNTs as thermal interfacematerials in microchannels for cooling. Flow boiling analysis withCNT coating in microchannels and water as the cooling mediumhas been reported by various authors [22,23]. The authors claiman enhancement of critical heat flux owing to the fact that the CNTcoating in microchannels provides numerous nucleation sites. Sin-gle phase cooling using CNTs with water as cooling medium on theother hand was researched by Mo et al. [24]. They applied differentheat rates to the base of the silicon microchannel while holding thepressure drop across the device constant. This was then compared toa silicon microchannel with no CNTs. They observed in the case ofsilicon microchannels with CNT fins, that they could apply 23% high-er input power and still keep the temperature of the transistor lowerthan a silicon microchannel with no CNTs. Jakaboski et al. [25] con-ducted further research on CNTs in silicon microchannels andachieved an increase in heat rate removal. Recently forced convec-tion with water over single walled nanotubes was tested for differ-ent heat fluxes and flow rates [26]. Unlike previously reported, theresults showed that single walled nanotubes result in no increasein heat dissipation but add a thermal resistance. The authors attrib-uted this to the hydrophobic nature of the CNTs.

Nomenclature

A surface area available for heat transfercp specific heat at constant pressureL length of tubing (norprene, manifold)Dhm hydraulic diameter of manifoldDhn hydraulic diameter of norprene tubingf friction factork thermal conductivity of copper blockkc contraction loss coefficientke expansion loss coefficientDP total pressure dropDPc pressure drop due to contractionDPch pressure drop across minichannelDPe pressure drop due to expansionDPm pressure drop through inlet and outlet manifoldDPn pressured drop through norprene tubingq heat flux

Q heat suppliedDT temperature difference between the thermocouplesTin fluid inlet temperatureTout fluid outlet temperatureTw base temperatureTb bulk fluid temperatureum fluid velocity through manifoldun fluid velocity through norprene tubingv volumetric flow rateDx distance between thermocouples in the copper block

Greek symbolsq density

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As far as we know, there is contradictory study on the enhance-ment of heat transfer using CNTs with water as the working fluid insingle phase. In our research we have presented experimental re-sults ranging from the single phase to the nucleation phase inthe flow boiling regime. A comparison of the experimental resultsto the computationally modeled results was done and observeddifferences are explained. In general we found that the presenceof MWNTs results in enhanced heat removal from the siliconminichannel.

Fig. 1. Steps showing the fabrication of MWNTs on silicon device.

2. Methodology

2.1. Device fabrication

Three different devices were fabricated – one with no MWNTs,one with fully covered MWNTs and one with a 6 � 12 array (6 rowsand 12 columns) of MWNT bundles. Fig. 1 shows the basic steps in-volved in the fabrication of the three devices. 1 mm thick siliconwafer (pre-coated with 500 nm silicon dioxide) was diced into a55 mm � 45 mm rectangular piece. An octagonal hole was lasercut in the center of this piece. The widest and longest part of thechannel is 25 mm and 35 mm, respectively. This wafer piece wasthen bonded onto a 500 lm thick rectangular silicon wafer of sim-ilar dimensions. To form the cover plate for the channel, a 1 mmthick Pyrex wafer was drilled with two holes for inlet and outletof water. The holes are drilled at a distance of 31 mm from eachother. Capillary tubing of 1 mm inner diameter was used then toform the inlet and outlet manifolds. The Pyrex wafer was thenbonded on top of the silicon wafer assembly using epoxy. Thisformed the base version i.e. the device with no MWNTs.

The fully covered MWNT device and the 6 � 12 MWNTs devicewere fabricated along the similar lines as the device with noMWNTs. The MWNTs were grown using chemical vapor depositionat 775 �C with a ferrocene catalyst and a xylene source with a mix-ture of argon and hydrogen as the carrier gas. The SEM image(Fig. 2a) of the MWNTs shows a dense entangled network of tubeswith a broad diameter distribution of 10–100 nm. In the case of thefully covered MWNT device, the 500 lm thick Si wafer had a rect-angular area of 24 mm � 15 mm covered with MWNTs that was500 lm in height in the center of the wafer. The 6 � 12 MWNTbundles were formed by laser cutting a fully covered MWNTs area.Fig. 2b shows an SEM image of a section of these bundles. The bun-dles are staggered in nature with a diameter of 1 mm and a heightof 500 lm.

Raman spectroscopy of the 6 � 12 MWNTs device was done be-fore and after laser cut. It was observed that the characteristic Ra-man spectrum (Fig. 3) of MWNTs remains the same i.e. the MWNTsremained intact. In between the bundles, we observed a peak at512 cm�1 (Fig. 3c), which is the characteristic silicon peak, in addi-tion to the characteristic MWNT peaks. We believe that in betweenthe bundles there were regions where the silicon was exposed inaddition to some residual MWNTs. We would like to point out herethat the as grown MWNTs were hydrophobic in nature but we ob-served that the wetting properties of MWNTs changed towardshydrophilic once the fins were submerged in water over time.

2.2. Experimental setup

The experimental setup used to test the devices can be dividedinto two parts – flow loop and the test fixture. Fig. 4 shows the flowloop for water. De-ionized water is pumped through the setupusing a peristaltic pump. The peristaltic pump consists of a pumphead and a drive to control the flow rate. A rotameter flow meter

Fig. 2. SEM images (a) MWNTs; (b) MWNTs bundles as cylindrical columns.

Fig. 3. Raman spectra of the MWNTs. (a) Before laser cutting. (b) Bundles. (c) In between the bundles. The peak at 512 cm�1 shows that there is exposed silicon in addition toresidual MWNTs.

Fig. 4. Flow loop. Fig. 5. Test fixture.

S. Shenoy et al. / International Journal of Heat and Mass Transfer 54 (2011) 5379–5385 5381

is used to measure the volumetric flow rate of the water. The waterthen passes through the device holder that is used to hold the threedevices to be tested and is finally collected in a beaker. A togglevalve is provided to take care of unintended pressure build up inthe flow loop due to obstructions.

The test fixture that holds the devices to be tested is shown inFig. 5. The device is held in the device holder made of fiberglass.The fiberglass has a 55 mm � 45 mm rectangular recess and inthe middle of the recess is a 25 mm � 15 mm rectangular slot thathouses a copper block. A ceramic heater that is controlled by a var-iac heats the copper block that in turn heats the back surface of thesilicon of the devices. Insulation is used around the copper block tominimize heat loss. A thin layer of thermal interface material is

applied to achieve good thermal contact between the copper blockand the silicon surface. In addition, mechanical clamping was usedto achieve a good thermal contact. Three holes drilled into the cop-per block are used to house 30 gauge type T thermocouples. Thethermocouples were anchored to the copper block using a thermalepoxy. The thermocouples are used to measure the heat flux ap-plied to the devices using Fourier’s law

q ¼ kDTDx

ð1Þ

where Dx is the distance between the thermocouples, DT is the dif-ference between the thermocouple temperatures and k is the ther-mal conductivity of the copper block. The top most thermocouple

Fig. 6. Computational model dimensions. (a) Minichannel consisting of a silicon wafer with a hexagonal channel with a Pyrex coverplate on top. (b) A copper block abuttingthe minichannel bottom surface that heats the silicon base.

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that is just below the silicon bottom surface is assumed to give thesilicon surface temperature. The heater comes with an inbuilt ther-mocouple that provides a method to monitor the heater tempera-ture. The data from the thermocouples is recorded using aMeasurement Computing data acquisition Unit. Pressure dropacross the device was measured using a differential pressure trans-ducer. Since this is an open loop system, only one port of the trans-ducer was connected to the inlet of the device. Since the outlet ofthe water was at atmospheric pressure, the pressure transducerprovides the differential pressure.

2.3. Data reduction and measurement uncertainty

Uncertainties in measured quantities are 6% for the rotameter,0.1 �C for the thermocouples and 7% for the pressure transducer.The heat loss was determined using computational modeling forthe no MWNTs devices at different flow rates. The heat loss valuesobtained were then used to adjust the measured heat flux thus giv-ing the heat flux applied to the base for the three different devices.The uncertainty associated with the heat flux applied to the basewas calculated using the Kline and McClintock method [27]. Theheat flux had an uncertainty range of 3.6–17%, the uncertaintiesbeing higher at lower heat fluxes.

3. Computational modeling

The experimental setup is modeled using computation fluiddynamics software ANSYS CFX. This program meshes its geometrybased on the finite volume method. In this technique, the region offocus is divided into small sub-regions known as control volumes.The equations are solved iteratively for each control volume. Anapproximation of the values solved by the equations can be ob-tained for each control volume. When combining the control vol-ume, values can display the behavior of the whole region as anentity. The more accurate of the solution is proportional to the sizeand shape of the control volume and the size of the final residuals.

To represent the minichannel, the model consists of a rectangu-lar silicon slab under a rectangular glass slab as shown in Fig. 6a.The silicon has dimensions of 45 mm � 55 mm � 2.15 mm(length �width � height). The silicon has a 1 mm deep octagonalgroove with the widest and longest part of the channel being25 mm and 35 mm, respectively. To enclose the channel, a glassplate of dimensions 45 mm � 55 mm � 1 mm is placed on the sil-icon. Two holes, 1 mm in diameter, are placed 12 mm away from

the glass slab edge length on either side to create an inlet and out-let for the fluid to flow. A copper block is added to the bottom sur-face of the silicon to match the geometry of the experimental work.It has dimensions 25 mm � 15 mm � 35 mm. The geometry used isshown in Fig. 6b.

A constant heat flux ranging from 5 to 20 W/cm2 is applied tothe bottom surface of the copper block. The maximum value ofheat flux used was curtailed by the fact that the software is moreaccurate in modeling the single-phase flows than two phase flows.Therefore the experiments show results for higher heat fluxes thanmodeling. Water is used as the working fluid in this heat exchangerwith volumetric flow rates ranging from 40 to 80 ml/min. A no-slipboundary condition and no interfacial resistance were applied toeach of the interfaces. Initial inlet temperature and outlet staticpressure values were set for all simulations to be approximately25 �C and 0 Pa, respectively. To monitor the thermal properties,the outer walls of the silicon and glass slabs and the copper blockare assumed to be adiabatic. Similar to the experiment, the threedevice configurations – no MWNTs device, fully covered MWNTsdevice and 6 � 12 MWNT bundles devices are simulated. In thecase of devices with MWNTs, even though in reality the MWNTsact as a nanostructured porous medium, the software limits us toconsider them as a solid entity. A MWNT thermal conductivity of400 W/m K [28,29] was used in all simulations.

4. Results

4.1. Pressure drop analysis

Table 1 shows the values of the measured pressure drop and thepredicted measure drop for single phase. The measured pressuredrop measures not only the pressure drop across the channel butalso includes the pressure drop across the norprene tubing, the in-let and outlet manifolds and the pressure drop due to contractionand expansion as the water enters through different tubing sizes.Therefore the predicted pressure drop was determined by

DP ¼ DPm þ DPn þ DPc þ DPe þ DPch ð2Þ

where DPm and DPn are the pressure drops across the manifolds andthe norprene tubing. They are expressed as

DPm ¼ fL

Dhm

q2

u2m ð3Þ

and

Table 1Single phase experimental and predicted pressure drops for the three devices.

Flow rate (ml/min) Device Pressure drop (kPa)

Predicted Measured

40 No MWNTs 2.76 2.66Fully covered MWNTs 2.85 2.866 � 12 MWNT bundles 2.78 2.76

80 No MWNTs 7.53 7.9Fully covered MWNTs 7.75 9.06 � 12 MWNT bundles 7.56 8.05

Table 2Heat flux data denoting the transition point from single phase to boiling.

Device Flow rate (ml/min) Heat flux (W/cm2)

6 � 12 MWNT bundles 80 19.7940 18.93

Fully covered MWNTs 80 18.5740 17.25

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DPn ¼ fL

Dhn

q2

u2n ð4Þ

where um and un are the fluid velocities in the manifold and nor-prene tubing and q is the density of water.

DPc is the pressure drop due to contraction as the water flowsfrom the larger norprene tubing into the inlet manifold tubingand DPe is the pressure drop due to expansion as the water flowsfrom the outlet manifold into the norprene tubing. They are ex-pressed as [6]

DPc ¼q2ðu2

m � u2nÞ þ

kcqu2m

2ð5Þ

and

DPe ¼q2ðu2

n � u2mÞ þ

kequ2n

2ð6Þ

The loss coefficients due to contraction (kc) and expansion (ke) aretaken as unity. DPch is the pressure drop across the minichanneland is determined through computational modeling. We observethat the predicted pressure drop values and the measured pressuredrop values are in good agreement with each other. The differencein two values was found to be within the measurement uncertaintycalculated for the pressure transducer. For both flow rates, the fullycovered MWNTs device caused higher pressure drops when com-pared to the no MWNTs and 6 � 12 MWNT bundles devices.

4.2. Heat transfer analysis

The experimental and modeling results of the heat flux appliedto the base versus the silicon base temperature for the three differ-ent devices are shown in Fig. 7. The CFD modeling results for the noMWNTs device are omitted since they have been used to calculatethe heat loss values. The experimental results are an average plot-ted over 2 different runs for 40 ml/min and 80 ml/min volumetricflow rates. The plot shows the experimental results for both the

Fig. 7. Experimental and modeling results of heat flux applied to the base a

single phase and boiling regimes. Since the modeling softwarewas limited to single phase flows, the modeling results do not in-clude the boiling regime. As clearly seen from the graphs, the de-vices with MWNTs perform much better than the device with noMWNTs in both regimes. It is seen that higher volumetric flowrates for each device results in higher heat transfer which meansthat a higher heat flux can be applied to the base while still keepingthe silicon base temperature at a certain value. Table 2 providesthe minimum heat fluxes for the different devices beyond whichvisible boiling starts to occur. For the 6 � 12 MWNT bundles deviceand the fully covered MWNTs device, surface area is much higherthan the device with no MWNTs therefore more heat is removed.The system takes longer to reach the saturation temperature pro-viding a higher heat flux before nucleation. No visible boilingwas observed in the case of no MWNTs device.

Fig. 8 shows the measured and predicted water temperaturerise using the energy balance equation

qvcpðTout � TinÞ ¼ Q ð7Þ

We have only considered heat flux ranges that keep the water insingle phase. For both 40 ml/min and 80 ml/min, the measuredwater temperature rise and the predicted water temperature riseare reasonably close proving the validity of our heat loss calcula-tions. In the single phase regime, the devices with MWNTs performmuch better. For a volumetric flow of 40 ml/min, one can apply only6 W/cm2 using no MWNTs device compared to 10 W/cm2 usingfully covered MWNTs device and 14 W/cm2 using 6 � 12 MWNTbundles device while keeping the silicon base temperature at70 �C. Using 80 ml/min flow rate and the same base temperatureone can apply higher heat fluxes of 8 W/cm2, 15 W/cm2, 18 W/cm2 using no MWNTs device, fully covered MWNTs device and6 � 12 MWNT bundles device, respectively. For the fully coveredMWNTs device, the decrease in hydraulic diameter, as seen fromthe increase in pressure drop, is the major factor for heat transferenhancement. In the case of 6 � 12 MWNT bundles device, theenhancement could be due to a number of factors; predominantlyit is due to the increase in surface area due to the bundles and alsodue to the decrease in hydraulic diameter as evident from the slightincrease in the pressure drop. In addition to these factors, at higher

t different silicon base temperatures for: (a) 40 ml/min; (b) 80 ml/min.

Fig. 8. Measured water temperature rise and the predicted water temperature rise using energy balance for: (a) 40 ml/min; (b) 80 ml/min.

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temperatures there is significant wetting of the nanotubes by water[30] that might lead to increased heat transfer and is explained indetail later in the paper.

The difference between the modeling results and the experi-mental results for fully covered MWNTs device for both 40 ml/min and 80 ml/min is significant as evident through Fig. 7. Themeasured pressure drop and the predicted pressure drop are inagreement therefore we cannot attribute this to the difference inhydraulic diameters. There need to be additional factors that causethe significant difference in the modeling and experimental results.One of the main reasons could be that the model considers the fullycovered MWNTs area as a solid block, whereas in reality the fullycovered MWNTs area has numerous hydrophilic MWNTs inter-twined and entangled with nanoscale pores in between themallowing water to penetrate through the MWNTs. To illustrate fur-ther, the heat transfer coefficient obtained through modeling for40 ml/min and 80 ml/min are 1649 W/m2 K and 2327 W/m2 K.Using the heat transfer coefficients we can find the correspondingsurface area available for heat transfer in experiments using New-ton’s law of cooling,

A ¼ QhðTw � TbÞ

ð8Þ

where Tw is the base temperature, Tb is the bulk fluid temperature, Qis heat supplied to the base and h is the heat transfer coefficient. For40 ml/min using an experimental heat flux of 13.9510 W/cm2 and abulk fluid temperature of 32.17 �C, the surface area obtained was36% higher than the modeling surface area and for 80 ml/min usingan experimental heat flux of 15.32 W/cm2 and a bulk fluid temper-ature of 27.21 �C, the surface area obtained was 32% higher than themodeling surface area. This ties in well with the amount of waterabsorbed by the carbon nanotubes versus the fluid temperature re-ported in [30]. It is also clear that as the fluid temperature rises,there is increased wetting of the MWNTs which could lead to in-creased heat transfer. In the case of 6 � 12 MWNT bundles devicethe region occupied by MWNTs is much less than the fully coveredMWNTs device thus any increase in wetting would not cause a sub-stantial increase in heat transfer. Therefore, the modeling resultsand the experimental results for both 40 ml/min and 80 ml/minare in reasonable agreement.

5. Conclusions and future work

An experimental study was conducted to determine the heat re-moval ability of MWNTs grown in a silicon minichannel with water

as the cooling medium. It was observed that the presence ofMWNTs resulted in enhanced heat removal from the silicon base.In the single phase regime, using a fully covered MWNTs device1.6 times the heat flux to the silicon base compared to a no MWNTsdevice can be applied while still maintaining the same silicon basetemperature. This increase had a drawback of the fully coveredMWNTs device increasing the pressure drop by 7% for 40 ml/minand 14% for 80 ml/min. When using the 6 � 12 MWNT bundles de-vice 2.3 times the silicon base heat flux compared to a no MWNTsdevice can be applied while maintaining the same silicon basetemperature. The corresponding increase in pressure drop ob-served was 1.8% for 40 ml/min and 3.7% for 80 ml/min. It was alsoobserved that at higher heat fluxes there is increased wetting ofMWNTs by water resulting in enhanced heat removal. The increasein heat transfer may also be contributed to the motion of the indi-vidual nanotube. The MWNTs are intertwined within the struc-tures with one end fixed to the silicon surface and the other sidefree. The unbounded end may act like a nanoscale cantilever beam,resonating with heat and enhancing heat removal because of theBrownian motion effect similar to the phenomena in nanofluids.

The computational modeling results differed from the experi-mental results with the experimental results showing higher heatremoval than predicted by the model. This was true especially inthe case of the full grown MWNTs device. One of the reasons weattribute this is due to the fact that the computational modelingconsidered the MWNTs as a solid medium whereas in reality theMWNTs are a nanostructured porous medium. Therefore any fu-ture computational modeling of MWNTs should take their porosityinto account. The optimization of the micro-fin size, shape, andspacing also has an effect on the removal of heat from the surface.Many parameters affect the heat transfer performance of fins espe-cially its geometry, fluid flow rate, and material properties. A pre-liminary study was done to determine the optimum fin shapewhere heat transfer performance was evaluated for cylindrical,pencil and square shapes. Cylindrical and pencil shaped finsshowed similar results and were better than square shaped fins.However, this analysis was conducted by making too manyassumptions. A further more detailed study needs to be conductedto optimize the fin parameters.

As an extension of our experimental work, we intend to evalu-ate the heat performance of dielectric fluids in a similar scenario.We believe that these fluids would perform better than waterdue to their increased wettability of MWNTs. The low boiling pointof dielectric fluids however entails that we consider two-phaseflows while modeling them. In addition, we intend to quantifythe penetration of different fluids into MWNTs at different

S. Shenoy et al. / International Journal of Heat and Mass Transfer 54 (2011) 5379–5385 5385

temperatures in order to determine optimal flow characteristicsand also the design of devices with MWNTs.

Future work may also consider solving analytically this systemusing the Lattice Boltzmann Method. Unlike CFD software such asANSYS CFX, the Lattice Boltzmann Method is based on microscopicmodels and mesoscopic kinetic equations. This method can includefiner details needed at the micro- or meso-scales such as the sur-face tension effects and the porosity of MWNTs.

Acknowledgements

This work was partially supported by LANCER directed researchfunds from Lockheed Martin POTT0715421 and Alliances for Grad-uate Education and the Professoriate (AGEP) program through theNSF Grant HRD-0450363. We would like to thank Dr. R. Vajtai andProfessor P.M. Ajayan for the MWNT samples and discussion, andProfessor Y.K. Joshi for his valuable input.

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