Experimental Study on Convective Heat Transfer of Aqueous Suspensions of Nano-Diamond Particles

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1 Experimental Study on Convective Heat Traner o Aqueou Supenion o Nano-Diamond Particle Shuichi Torii Department o Mechanical Sytem Engineering, Kumamoto Univerity, Kumamoto, Japan Abtract: Thi paper aim to tudy the convective heat traner behaviour o aqueou upenion o nano-diamond particle lowing through a horizontal tube heated under contant heat lux condition. Conideration i given to the eect o particle concentration and Reynold number on heat traner enhancement. It i ound that (i) igniicant enhancement o heat traner perormance due to upenion o nano-diamond particle in the circular tube low i oberved in comparion with pure water a the woring luid, (ii) the enhancement i inteniied with an increae in the Reynold number and the nano-diamond concentration, and (iii) ubtantial ampliication o heat traner perormance i not attributed purely to the enhancement o thermal conductivity due to upenion o nano-diamond particle. Keyword: Nanoluid, Heat Traner, Nano-diamond 1. INTRODUCTION Crytalline olid have thermal conductivitie higher than luid by one to three order o magnitude. By intuition, one expect that thermal conductivitie o particle-luid mixture are higher than thoe o pure luid, a pointed out by Maxwell [1] more than a century ago. The conventional olid-liquid mixture in which millimeter and/or micrometer-ized particle are added are nown a lurrie. It i nown that lurrie ettle rapidly, clog low channel, erode pipeline and caue evere preure drop and other undeirable problem. Thereore, luid with upended large particle have little practical application in heat traner enhancement. Modern technology mae it poible to produce particle <100 nm in diameter or upending in conventional luid uch a water, engine oil, and ethylene glycol. Thi new cla o luid i reerred to a nanoluid, whoe term i irt named and ued by Choi [2]. Compared with micron-ized particle, nano-particle have much larger relative urace area and a great potential or heat traner enhancement. Baed on thi idea, many tudie were conducted to explore uperior propertie o nanoluid, uch a large urace-area-to-volume ratio, table upenion, and no low paage clogging, which are uitable in heat traner application. Eatman et al. [3] demontrated that oxide nanoparticle, uch a Al 2 O 3 and CuO have excellent diperion propertie in water, oil, and ethylene glycol and orm upenion. Lee et al. [4] meaured thermal conductivity o luid containing Al 2 O 3 and CuO particle o that or the copper oxide/ethylence glycol ytem, thermal conductivity can be enhanced by more than 20% at 4 volume percentage. In particle, they dicloed that the thermal conductivity o nanoluid depend on the thermal conductivitie o both the bae luid and particle. Uing the meaured thermal conductivity and vicoity data, Wang et al. [5] concluded that the increae in preure drop i about the ame a the increae in heat traner or both laminar and turbulent low in a circular tube or all o the luid-particle mixture. Mot o thee tudie are on the eective thermal conductivity under macrocopically tationery condition. There are very ew tudie on the other apect related nanoluid uch a phae change behavior (or example, Da et al. [6] and convective heat traner. Lee and Choi [7] etimated the perormance o microchannel heat exchanger with water, liquid nitrogen and nanoluid a the woring luid and howed the uperiority o a nonoluid-cooled microchannel heat exchanger. Pa and Cho [8] invetigated convective heat traner in the turbulent low regime uing the mixing luid o water-al 2 O 3 and water-tio 2. A or the mechanim o heat traner enhancement o the nanoluid, Xuan and Roetzel [9] ound that the eect o tranport propertie o the nanoluid and thermal diperion are included. Xuan and Li [10] meaured convective heat traner o water-cu nanoluid and ound ubtantial heat traner enhancement. Wen and Ding [11] reported an experimental wor on the convective heat traner o nanoluid made o water and g-al 2 O 3 nanoparticle in the laminar low region. They propoed that the enhancement o convective heat traner i attributed to a non-uniorm ditribution o thermal conductivity and vicoity ield and an attenuation o the thermal boundary layer thicne. The heat traner behavior o aqueou upenion o multi-walled carbon nanotube (CNT) in the laminar tube low i experimentally tudied by Ding et al. [12]. They propoed that enhancement o the convective heat traner i acribed to particle re-arrangement, hear induced thermal conduction enhancement, reduction o thermal boundary layer thicne, and the higher apect ratio o CNT. The purpoe o the preent tudy i to invetigate heat traner characteritic o circular pipe low including nano-diamond particle. Enchae i placed on the eect o the upenion with the particle, i.e., the volume raction o particle and Reynold number on heat traner perormance in the turbulent low. 2.EXPERIMENTAL SETUP and MEASUREMENT METHODS In general, the heat traner coeicient o the thermal Correponding author: S. Torii, torii@mech.umamoto-u.ac.jp 352

2 luid low including nanoluid i aected by the Reynold number, thermal propertie and o on. In thi wor, the vicoity and thermal conductivity o nano-diamond luid are conidered here to tudy the eect o convective heat traner. Nanoparticle upenion are ar more table than upenion o larger particle [13]. One o a ew method o aeing nanoluid tability i to viually inpect luid ample over an extended period o time. Figure 1(a) and (b) depict picture o 0.4% and 1% nano-diamond luid ater 60 day later, repectively. The correponding ph or two nanoluid i 6.62 and 6.35, repectively. One oberve that no concentration gradient appear in both nanoluid. It implie long-term degradation in thermal perormance due to etting inide the cooling ytem reervoir. The vicoity i meaured by uing the Cannon - Fene vicometer. The meaurement are done on nanoluid o dierent nano-diamond concentration. For reerence, a TEM image o the ample i depicted in Fig. 2. The electron micrograph how that the particle are dipered in the luid and ome are in the ormat o agglomerate. Figure 2 TEM image o nanodiamond particle P in P1 P2 P3 Heat inulation material AC olt meter Power upply Data Logger P4 Stainle tube P5 PC P6 P out Cooler alve Tan alve Flow meter Pump Figure 3 Experimental apparatu (a) 0.4 % (b) 1% Figure 1 Nanolud ater 60 day The experimental ytem or meauring the convective heat traner coeicient i illutrated chematically in Fig. 3. It conit o a low loop, a power upply unit, a cooling device, and a low meauring and control unit. The low loop include a pump, a digital lowmeter, a reervoir, a collection tan and a tet ection. A traight eamle tainle tube with 1000 mm length, 4.0 mm inner diameter, and 4.3 mm outer diameter i ued a the tet ection. The whole tet ection i heated with the aid o the joule heating method through a electrode lined to a DC power upply. The power upply i adjutable and had a maximum power upply o 1000W. Six K-type thermocouple (0.01mm in diameter) are mounted on the tet ection at axial poition o 150mm rom the inlet o the tet ection to meaure the wall temperature ditribution, and two urther K type thermocouple are inerted into reervoir and collection tan at the inlet and exit o tet ection to meaure the bul temperature o nanoluid, repectively. The maximum low rate that the pump can deliver i 25 l/min. In the heat traner experiment, the temperature reading rom the 8 thermocouple are recorded by a data logger ytem with a peronal computer. The local heat traner coeicient, h x, i deined a h x q =, (1) ( T T ) wx mx where x repreent axial ditance rom the entrance o the tet ection, q i the heat lux, T wx i the meaured wall temperature, and T mx i the mixed mean temperature, i.e., the luid temperature etimated by the ollowing energy 353

3 balance: Tmx = Tm0 + Q c W p x Here, c p i the heat capacity, T mo i the luid temperature at the inlet. And Q x and W are repectively the heat rate rom the heat wall urace and the average luid velocity over the cro-ection. Note that Eq. (2) i taen a heat lo through the inulation layer into account. The local heat traner coeicient, h x, in Eq. (1) i uually expreed in the orm o the Nuelt number Nu x, a: hx D Nu x =, (3) where D i the tube diameter, and i the luid thermal conductivity. In general, the Nu number i related to the Reynold number, Re, and the Prandtl number, Pr. Thu, h x i arranged in the orm o Re veru Nu x in the ollowing ection, becaue only the nano-diamond nanoluid i employed. Notice that the thermal conductivity o nanoluid i trongly dependent on the nano-particle volume raction. Hamilton and Croer [14] propoed a model or liquid-olid mixture in which the ratio o conductivity o two phae i larger than 100, a: + ( n 1) ( n 1) ( ) =, (4) + ( n 1) + ( ) where i the thermal conductivity o the dicontinuou particle, i the thermal conductivity o the luid, i the volume raction o particle, and n i the empirical hape actor. and n are deined by and (2) = (5) + 3 = ψ n (6) repectively. Here ψ i the phericity deined a the ration o the urace area o a phere with a volume equal to that o the particle to urace area o the particle. ψ=1 i aumed in the preent tudy, becaue the nanoparticle are een lie a phere, a een in Fig. 2. Three volumetric concentration o nano-diamond luid, 0.1%, 0.4% and 1% are teted in the preent tudy. The Reynold number i ranged rom 3000 to An uncertainty analyi [15] yield the ollowing reult: the uncertainty in nanoluid lowrate i etimated to be ±1.5%, the uncertainty in the phyical propertie i le than 1%, and the uncertainty in the temperature meaurement i etimated to be ±1.5%. The thermocouple were calibrated in a thermotat water bath and the accuracy wa ound to be within 0.1 K. 3. RESULTS AND DISCUSSION 3.1. icoity o nanoluid The vicoity o nano-dimanod luid wa meaured under variou condition. Figure 4 how reult or dierent concentration. For comparion, the correponding vicoitie or dierent concentration are theoretically etimated uing the Batchelor Model. The vicoity o nanoluid increae with increaing nano-dimanond concentration. Such behavior i alo oberved by Kinloch et al [16] or highly concentrated aqueou upenion o multiwalled carbon nanotube. The reult have an important implication to nano-diamond luid lowing through the tubular geometry ued in thi wor. In other word, the vicoity o nano-diamond luid i higher than the theoretical value over the wide range o volume raction. Uing the meaured value, the Reynold number i determined in the ollowing. icoity ratio η n / η Batchelor Experimental value olume raction Figure 4 Experimental and theoretical reult or dierent volume raction 3.2. Convective heat traner coeicient Having etablihed conidence in the experimental ytem, ytematic experiment were perormed at dierent low condition (Reynold number), and dierent nano-diamond concentration. Figure 5 how the eect o nano-diamond concentration on the local heat traner coeicient at variou axial ditance rom the entrance o the tet ection at Re=6000. Here, local heat traner coeicient i divided by the heat traner coeicient or the thermally and hydrodynamically ully-developed region in the pure luid pipe low. It i oberved that (i) the preence o nano-diamond particle increae the convective heat traner coeicient igniicantly, and the increae i more coniderable at high nano-diamond concentration, and (ii) at a given nano-diamond concentration, the heat traner coeicient decreae with axial ditance becaue o the entrance region eect. A imilar trend but with le igniicant enhancement wa obtained at dierent lower Reynold number (not hown) and wa alo oberved by Xuan and Li [10] in the turbulent low regime and Wen and Ding 354

4 [11] at the entrance region in the laminar glow regime. It i oberved in Fig. 5 that the local heat traner coeicient approache the contant value along the axial direction, that i the thermally ully-developed region appear in the downtream region. In the ollowing ection, conideration i given to eect o Reynold number and nano-diamond concentration on heat traner perormance in the thermally ully-developed region, i.e., at x/d=180. Notice that a comparion o nanoluid with pure luid indicate that the enhancement o the local heat traner coeicient i much more dramatic than that purely due to the enhancement o eective thermal conductivity. For reerence, an increae in the thermal conductivity i depicted in Fig. 6 in the orm o volume raction veru dimenionle thermal conductivity with dierence nano-particle material a a parameter. Here, each thermal conductivity i normalized by one o the pure luid. It i een that the eective thermal conductivity increae with increaing nano-particle concentration and maximum enhancement i about 16% at 5 % o volume raction. Figure 7 depict the enhancement o the heat traner coeicient with reerence to pure luid. It can be een that the heat traner enhancement increae with increaing Reynold number. Thi trend become larger with an increae in the nano-diamond concentration. Figure 7 illutrate the eect o the Reynold number on the heat traner coeicient at x/d=180. The meaured value are ummarized in Fig. 7 in the orm o Re veru Nu with the nano-diamond concentration, a the parameter. For comparion, the ollowing well-nown Gnielini correlation equation [17] under the contant heat lux boundary condition i uperimpoed in Fig. 7 a a olid line Heat traner coeicient ratio X/D Figure 5 Streamwie variation o heat traner coeicient o nanoluid or dierent volume raction ( 8)( Re 1000) Pr Nu = (7) Pr 2 1 ( ) Here, i the riction actor a: [ 1.82 log ( ) 1. ] 2 = Re (8) Thermal conductivity ratio n / Diamond Al 2 O 3 CuO olume raction Figure 6 Comparion o theoretical reult or dierent nano-particle material Nu Water Re Figure 7 Eect o nano-particle concentration on Nuelt number Next i to invetigate the mechanim o heat traner enhancement. The heat traner coeicient, h, i a macrocopic parameter decribing heat traner when a luid low acro a olid urace o dierent temperature. The boundary layer increae with axial ditance until ully developed ater which the boundary layer thicne and hence the convective heat traner coeicient i contant. Thi theory ugget that both an increae in the thermal conductivity,, and/or a decreae in the thermal boundary layer thicne caue an ampliication o the convective heat traner coeicient. The maximum enhancement o the thermal conductivity under the condition o the convective heat traner experiment in thi wor doe not exceed 16% or 5.0 % nano-dimamond luid, a een in Fig. 6. Meanwhile, Fig. 7 how that the enhancement o the convective heat traner coei- 355

5 cient i much greater than that due to the increae in the thermal conductivity, particularly at high nona-diamond concentration and high Reynold number. One may thereore imply attribute the large enhancement purely to a decreae in the thermal boundary layer thicne. No doubt, the reduction in the thermal boundary layer thicne could be an important actor, but urther enhancement on the thermal conduction under dynamic condition could be another important actor. 4. SUMMARY Experimental tudy ha been perormed to invetigate the heat traner behaviour o aqueou upenion o nano-diamond particle. Experimental and theoretical method are employed to obtain the eective thermal conductivity, vicoity and convective heat traner coeicient. Conideration i given to the eect o particle concentration and Reynold number on heat traner enhancement. The eective reult are ummarized a ollow: (1). Signiicant enhancement o heat traner perormance due to upenion o nano-diamond particle in the circular tube low i oberved in comparion with pure water a the woring luid. (2). The enhancement depend on the Reynold number and the nano-diamond concentration. In other word, the maximum heat traner enhancement in the thermally and hydrodynamically ully-developed low region tae place with an increae in the Reynold number and nano-diamond concentration. (3). Subtantial ampliication o heat traner perormance i not attributed purely to the enhancement o thermal conductivity due to upenion o nano-diamond particle. The above dicuion i motly rom the macrocopic point o view. Microcopically, particle migration and re-arrangement due to non-uniorm hear rate over the pipe cro-ection could alo be a reaon or the oberved large heat traner enhancement. Thu urther wor i needed to dicloe mechanim o heat traner enhancement. NOMENCLATURE Cp peciic heat D pipe diameter riction actor, Eq. (8) hx local heat traner coeicient, Eq. (1) thermal conductivity, Eq. (4) thermal conductivity o the luid thermal conductivity o the dicontinuou particle n empirical hape actor in Eq. (4) Nu Nuelt number, Eq. (7) Nux local Nuelt number, Eq. (3) Pr Prandtl number Qx heating rate Q heat lux Re Reynold number Tmo inlet luid temperature Tmx mixed mean temperature, (2) volume raction o particle W mean velocity o luid x coordinate, m thermal diuivity, m2/ vicoity, Kg/m inematic vicoity, m2/ denity, g/m3 Subcript luid mx mean olid or urace wx axial REFERENCES 1. J.C. Maxwell, A Treatie on Electricity and Magnetim, 2nd Edition, ol. 1, p. 435, Clarendon Pre, Oxord, U. K. (1881). 2. U.S. Choi, Enhancing thermal conductivity o luid with nanoparticle, Development and Application o Non-Newtonian Fluid (edited by D. A. Springer and H. P. Wang), ASME FED - ol. 231/MD - ol. 66, 6-12 (1995). 3. J.A. Eatman, U.S. Choi, S. Li, L.J. Thompon, S. Lee, Enhanced thermal conductivity through the development o nano-luid, Proceeding o the Sympoium on Nanophae and Nanocompoite Material II, ol. 457, Material Reearch Society, Boton, 3-11 (1997). 4. S.P. Lee, U.S. Choi, S. Li, J.A. Eatman, Meauring thermal conductivity o luid containing oxide nanoparticle, J. Heat Traner, 121, (1999). 5. X. Wang, X. Xu, U.S. Choi, Thermal conductivity o nanoparticle-luid mixture, J. Thermophyic and Heat Traner, 13, (1999). 6. S.K. Da, N. Putra, W. Roetzel, Pool boiling characteritic o nanoluid, Int. J. Heat and M Traner, 46, (2003). 7. S.P. Lee, U.S. Choi, Application o metallic nanoparticle upenion in advanced cooling ytem, Recent Advance in Solid/Structure and Application o Metallic Material (edited by Y. Kwon, D. Davi and H. Chung), PP - ol. 342/MD ol. 72, ASME, New Yor, (1996). 8. B.C. Pa and Y.I. Cho, Hydrodynamic and heat traner tudy o dipered luid with ubmicron metallic oxide particle, Experimental Heat Traner, 11, , Y. Xuan and W. Roetzel, Conception or heat traner correlation o nanoluid, Int. J. Heat and Ma Traner, ol. 43, pp , Y.M. Xuan and Q. Li, Invetigative heat traner and low eature o nanoluid, ASME J. Heat Traner, 125, , D. Wen and Y. Ding, Experimental invetigation into convective heat traner o nanoluid at the entrance region under laminar low condition, Int. J. Heat and Ma Traner, ol. 47, pp , Y. Ding, H. Alia, D. Wen and R.A. William, Heat traner o aqueou upenion o carbon nanotube (CNT nanoluid), Int. J. Heat and Ma Traner, ol. 49 pp , J. Lee and I. Mudawar, Aement o the eectivene o nanoluid or ingle-phae and two-phae heat traner in micro-channel, International Journal o Heat and Ma Traner, ol. 50, pp ,

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