Experimental Study on the Flow and Heat Transfer Characteristics of TiO 2 -Water Nanofluids in a Spirally Fluted Tube

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1 Qi et l. Nnoscle Reserch Letters (2017) 12:516 DOI /s NANO EXPRESS Experimentl Study on the Flow nd Het Trnsfer Chrcteristics of TiO 2 -Wter Nnofluids in Spirlly Fluted Tue Cong Qi *, Chunyng Li nd Guiqing Wng Open Access Astrct The flow nd het trnsfer chrcteristics of TiO 2 -wter nnofluids with different nnoprticle mss frctions in spirlly fluted tue nd smooth tue re experimentlly investigted t different Reynolds numers. The effects of ph vlues nd doses of dispersnt gent on the stility of TiO 2 -wter nnofluids re discussed. The effects of nnoprticle mss frctions nd Reynolds numers on Nusselt numers nd frictionl resistnce coefficients in the spirlly fluted tue nd the smooth tue re lso investigted. It is found tht TiO 2 -wter nnofluids in the spirlly fluted tue hve lrger enhncement thn tht in the smooth tue. The het trnsfer enhncement nd the increse in frictionl resistnce coefficients of TiO 2 -wter nnofluids in the spirlly fluted tue nd the smooth tue for lminr flow nd turulent flow re compred. It is found tht there re lrger increse in het trnsfer nd smller increse in frictionl resistnce coefficients for turulent flow thn tht for lminr flow of TiO 2 -wter nnofluids in the spirlly fluted tue. The comprehensive evlutions for the thermo-hydrulic performnce of TiO 2 -wter nnofluids in the smooth tue nd spirlly fluted tue re lso discussed. Keywords: Nnofluids, Spirlly fluted tue, Het trnsfer enhncement, Frictionl resistnce coefficient Bckground Nnofluids re type of medium fluids with excellent het trnsfer performnce (for exmple ZnO-EG nnofluid [1], Cu-CTAC/NSl nnofluid [2], MWCNTs- CTAC/NSl nnofluid [3]), which re pplied in vrious fields, such s clen wter genertion [4], solr phototherml conversion [5], nd oiling het trnsfer [6]. Convection het trnsfer of nnofluids is n importnt het trnsfer process including nturl convection nd forced convection het trnsfer. Mny reserchers hve investigted the nturl convection het trnsfer of nnofluids. Li et l. [7] experimentlly investigted the nturl convection of squre enclosure filled with ZnO-EG/DW nnofluids nd otined conclusion tht the high EG queous solution concentrtion is disdvntgeous to het trnsfer enhncement. Hu et l. [8] experimentlly nd numericlly investigted the nturl convection of Al 2 O 3 -wter nnofluids in squre enclosure, nd it ws found tht nnofluids with the * Correspondence: qicong@cumt.edu.cn School of Electricl nd Power Engineering, Chin University of Mining nd Technology, Xuzhou , Chin highest nnoprticle frction worsen the het trnsfer. He et l. [9] numericlly studied the nturl convection of Al 2 O 3 -wter nnofluids in squre enclosure y lttice Boltzmnn method, nd the results showed tht the het trnsfer performnce decreses with the nnoprticle volume frction. Qi et l. numericlly studied the nturl convection of Cu-Gllium nnofluids in different spect rtio enclosures y single-phse model [10] nd two-phse lttice Boltzmnn model [11]; they [12] lso studied the nturl convection of Al 2 O 3 -wter nnofluids using two-phse lttice Boltzmnn model, nd the results showed tht nnofluids in smller spect rtio enclosure hve higher het trnsfer enhncement rtio. In conclusion, it is oserved tht some fctors, such s high heting power nd nnoprticle frction, re dvntgeous to het trnsfer enhncement, while some other fctors, such s the ig spect rtio of enclosure nd the se fluid with low therml conductivity, my led to reduction in nturl convection het trnsfer. Although nturl convection of nnofluids is widely pplied in mny fields, it cnnot meet the high The Author(s) Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution 4.0 Interntionl License ( which permits unrestricted use, distriution, nd reproduction in ny medium, provided you give pproprite credit to the originl uthor(s) nd the source, provide link to the Cretive Commons license, nd indicte if chnges were mde.

2 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 2 of 12 efficient het dissiption under the condition of high power density. Compred with nturl convection, forced convection het trnsfer hs higher het trnsfer coefficient. Reserchers dopted different experimentl methods to investigte the forced convection het trnsfer chrcteristics of nnofluids. Sun et l. [13, 14] experimentlly investigted the flow nd het trnsfer chrcteristics of Cu-wter, Al-wter, Al 2 O 3 -wter, Fe 2 O 3 -wter, nd Cuwter nnofluids in uilt-in twisted elt externl thred tues, nd it ws found tht Cu-wter nnofluids show the est het trnsfer performnce. Yng et l. [15] experimentlly investigted the flow nd het trnsfer chrcteristics of Cu-wter nd Cu-viscoelstic fluid nnofluids in smooth tue, nd the results showed tht Cu-viscoelstic fluid nnofluids hve higher het trnsfer performnce thn viscoelstic se fluid ut lower flow resistnce thn Cu-wter nnofluids. Adolqi et l. [16] experimentlly studied the het trnsfer enhncement of TiO 2 -BioGlycol/wter nnofluids in flt tues nd estlished new correltion etween the het trnsfer enhncement nd the friction fctor, nd the results showed tht the het trnsfer performnce of nnofluids is pproximtely 28.2% greter thn the se fluid. Nphon [17] experimentlly studied the het trnsfer chrcteristics of TiO 2 -wter nnofluids in horizontl spirlly coiled tues, nd it ws found tht het trnsfer performnce of nnofluids increses with the decresing curvture nd the incresing nnoprticle frction. Shhrul et l. [18] nd Kumr nd Sonwne [19] experimentlly investigted the het trnsfer chrcteristics of three kinds of nnofluids (Al 2 O 3 -wter, SiO 2 -wter, nd ZnO-wter) nd two kinds of nnofluids (Fe 2 O 3 - wter nd Fe 2 O 3 -EG) in shell nd tue het exchnger, nd it ws found tht ZnO-wter nd Fe 2 O 3 -wter nnofluids show the est het trnsfer performnce in their respective reserch. El-Mghlny et l. [20] experimentlly investigted the het trnsfer chrcteristics nd pressure drop of Cu-wter nnofluids in horizontl doule-tue het exchnger, nd the results showed tht het trnsfer enhncement of nnofluids increses with the nnoprticle frction. Sundr et l. [21] experimentlly studied the flow nd het trnsfer chrcteristics of Fe 3 O 4 -wter nnofluids in horizontl plin tue with return end nd wire coil inserts, nd the results showed tht het trnsfer performnce increses with the incresing nnoprticle frction nd decresing p/d rtio of wire coil inserts. Aove studies minly focused on the het trnsfer performnce of nnofluids in the smooth tue, flt tue, spirlly coiled tue, or tue with wire coil inserts. In ddition to ove experimentl studies, the forced convection het trnsfer chrcteristics of nnofluids in spirlly corrugted tues re lso investigted. Drzi et l. [22, 23] experimentlly nd numericlly studied the turulent het trnsfer of Al 2 O 3 -wter nnofluids in heliclly corrugted tue, nd the results showed tht etter het trnsfer performnce is otined thn tht in plin tue. Drzi et l. [24] experimentlly investigted the turulent het trnsfer chrcteristics of SiO 2 -wter nnofluids in heliclly corrugted tues nd discussed the effects of five pitches of corrugtion on the het trnsfer of corrugted tues, nd the results showed tht the smll pitch of corrugtions cn ugment the het trnsfer performnce significntly. Prk et l. [25] studied the het trnsfer of thermochromic liquid crystls in spirlly fluted tue, nd the results showed tht the het trnsfer enhncement rtio etween the spirlly fluted tue nd the smooth tue t the low Reynolds numer (30,000) is higher thn tht t high Reynolds numers (50,000 nd 70,000). Aove reserches minly investigted the het trnsfer nd flow chrcteristics of nnofluids in spirlly corrugted tues. However, the comprehensive nlysis for the thermo-hydrulic performnce of nnofluids in the smooth tue nd spirlly fluted tue needs to e discussed further. Aove studies mde gret contriution to the flow nd het trnsfer chrcteristics in the smooth tue, smooth tue with wire coil inserts, het exchnger, spirlly corrugted tue, nd so on. The min novelty of this mnuscript minly includes the following: (1) new method of testing the stility of nnofluids (trnsmittnce method) is estlished y n ultrviolet spectrophotometer, which is different from the precipittion method widely dopted y the pulished references. The results of trnsmittnce method re quntifile while the results of precipittion method re less quntifile; nd (2) the comprehensive evlutions for the thermohydrulic performnce of TiO 2 -wter nnofluids in the smooth tue nd spirlly fluted tue re discussed, which re less investigted. On n interesting note, it is found tht nnofluids t the highest Reynolds numer my not hve the est thermo-hydrulic performnce in the spirlly fluted tue nd there is criticl Reynolds numer for the est thermo-hydrulic performnce. Methods Nnofluid Preprtion nd Stility Study TiO 2 is chosen s the nnoprticle, nd wter is selected s the se fluid. Figure 1 shows the TiO 2 nnoprticles. TiO 2 -wter nnofluids in the experiment re prepred y two-step method, nd Fig. 2 presents the detils of the preprtion process. For ech of the su-steps, mechnicl stirring time is hlf n hour nd the soniction time is 40 min. The mss frction of the dispersnt gent in the wter is 6 wt%, nd the ph vlue of nnofluid is 8. Tle 1 presents the informtion of some mterils in the preprtion process of nnofluids. From Fig. 1, it is found tht the nnoprticles esily ggregte

3 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 3 of 12 Fig. 1 Morphology of TiO 2 nnoprticles. TEM imges of TiO 2 nnoprticles: 20 nm, 50 nm, nd c 100 nm together. Hence, the stility of nnofluids is investigted using the precipittion method widely dopted y the pulished references. The stility of TiO 2 -wter nnofluids with vrious mss frctions (0.1, 0.3, nd 0.5 wt%) t different quiescent times is studied in Fig. 3, which shows tht the stility of nnofluids 72 h lter is still good. In order to further check the stility of nnofluids, new method of testing the stility of nnofluids (trnsmittnce method) is estlished y n ultrviolet spectrophotometer in this pper. Figure 4 shows the trnsmittnce (τ) chnges of TiO 2 -wter nnofluids (ω = 0.3%) with the quiescent time. The effects of different doses (M) of dispersnt gent nd different ph vlues on the stility of nnofluids re investigted. As we know, if nnoprticles uniformly distriute in the wter, nnofluids will reflect the most light, resulting in nnofluids hving high reflectnce nd low trnsmittnce. It cn e found from Fig. 4 tht nnofluids (ω = 0.3%) with M = 6 wt% nd ph = 8 hve the lowest trnsmittnce. Nnofluids with other mss frctions (ω = 0.1% nd ω = 0.5%) re ll prepred t M = 6 wt% nd ph = 8 in this pper, nd the trnsmittnce chnge trends of nnofluids with ω = 0.1% nd ω = 0.5% re the sme with the nnofluids with ω = 0.3%. Hence, the good stility of nnofluids prepred in this pper cn e gurnteed. In ddition, following the investigtion of the effects of dispersnt gent nd ph on the therml conductivity nd viscosity of wter, smll influence on them due to the little dispersnt gent nd NOH is found. Figure 5 shows the therml conductivities nd dynmic viscosities of TiO 2 -wter nnofluids t different tempertures nd sher rtes. It is found tht the therml conductivity of wter in this pper hs good greement with Mxwell [26]. It cn e found tht the therml conductivity increses with the nnoprticle mss frction nd the therml conductivity of nnofluids increses y % compred with wter due to the high therml conductivity of nnoprticles. Also, it is found tht the therml conductivity increses with the temperture, ecuse high temperture enhnces the Brownin motion of nnoprticles nd improves the therml conductivity of nnofluids. In ddition to the conclusions of therml conductivity, it cn e found tht the dynmic viscosity increses with the sher rte t the initil stge nd keeps constnt with the incresing sher rte nd the viscosity of nnofluids increses y % compred with wter. It is ecuse tht smll sher force dded in the nnofluids t the initil stge reks the equilirium of the flow field nd cuses n increse in dynmic viscosity (sher-thickening ehvior). The dynmic viscosity is constnt when the flow field reches stedy stte gin, which hs good greement with the chrcteristics of Newtonin fluid. Experimentl System An experimentl system for the flow nd het trnsfer chrcteristics of TiO 2 -wter nnofluids in spirlly fluted tue is estlished. Figure 6 represents the schemtic digrm of the experimentl system. The experimentl system minly includes the het trnsfer test section, flow resistnce test section, temperture control sink, nd pump. The spirlly fluted tue is heted y resistnce wire connected to DC power. Outer wll temperture of the spirlly fluted tue is otined y ten Dispersnt gent (TDL-ND1) NOH Deionized wter Stir Stir Stir Stle nnofluids TiO2 nnoprticles Ultrsonic virting Fig. 2 Preprtion of nnofluids. Preprtion process of TiO 2 -wter nnofluids y two-step method

4 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 4 of 12 Tle 1 Informtion of mterils. Informtion of some mterils in the preprtion of nnofluids Mterils Mnufcturer Properties TiO 2 nnoprticles Bse fluid (deionized wter) Dispersnt gent T-type thermocouples which re uniformly distriuted in the surfce of the spirlly fluted tue. Outlet temperture nd inlet temperture of nnofluids of the spirlly fluted tue re mesured y two K-type thermocouples. All thermocouples re connected to dt cquisition instrument (Agilent 34972A). The flow resistnce is mesured y differentil pressure instrument. The detiled digrm of the spirlly fluted tue is shown in Fig. 7. For the smooth tue nd spirlly fluted tue, the mterils re ll stinless steel, the equivlent dimeters re the sme, the lengths re ll 1200 mm, the test sections re ll the middle section 1000 mm of the tue, nd 100 mm section is left t ech end of the tue in order to void the entrnce effect. Clcultion Equtions The heting power is supplied y DC power: Q 0 ¼ UI Nnjing Tnsil Advnced Mterils Co., Ltd. Prepred y ultrpure wter device Nnjing Tnsil Advnced Mterils Co., Ltd. Type: TTP-A10; Crystl form: ntse; Prticle dimeter: 10 nm Resistivity: MΩ cm@25 C Type: TDL-ND1; Element: mcromolecule polymers; Scope of ppliction: wter or solvent (se fluid) ð1þ where Q is the heting power, U is the voltge, nd I is 0 the electric current. The het sored y fluid is clculted s follows: Q f ¼ c p q m ðt out T in Þ ð2þ where Q f is the het sored y fluid, c p is the specific het of fluid, q m is the mss flow rte, nd T out nd T in re the outlet temperture nd inlet temperture of fluid. Het cpcity is given s follows: c p ¼ ð1 φþ ρc p f þ φρc p p ð3þ ð1 φþρ f þ φρ p where c p is the het cpcity of nnofluids, φ is the nnoprticle volume frction, the suscript f represents the se fluid, nd the suscript p represents the nnoprticles. The verge temperture of fluid is clculted s follows: T f ¼ ðtout þ TinÞ=2 ð4þ where T f is the verge temperture of fluid in the tue. The outer wll verge temperture of the tue is shown s follows: " # T ow ¼ X10 i¼1 TwðÞ i =10 ð5þ where T ow is the outer wll verge temperture of the tue, Tw(i) is the temperture of thermocouples ttched to the outer wll of tue, nd there re ten thermocouples ttched uniformly to the outer wll of tue. The internl wll verge temperture of the tue cn e clculted s follows: T iw ¼ T ow Q f lnðro=riþ ; ði ¼ 1; 2; 3 10Þ ð6þ 2πλl where T iw is the internl wll verge temperture of the tue, rondri re the externl rdius nd internl rdius of tue, λ is the therml conductivity of tue, nd l is the length of tue. The convective het trnsfer coefficient is clculted s follows: Q f h f ¼ ð7þ πd e lt ð iw T f Þ where h f is the convective het trnsfer coefficient nd d e is the equivlent dimeter of the tue. The Nusselt numer is clculted s follows: Fig. 3 Stility oservtion of nnofluids. TiO 2 -wter nnofluids t different quiescent times: t =0h, t = 48 h, nd c t =72h

5 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 5 of 12 c Fig. 4 Trnsmittnce (τ) of nnofluid (ω = 0.3%). Trnsmittnce chnges with quiescent time of TiO 2 -wter nnofluid (ω = 0.3%) with different doses (M) of dispersnt gent: M = 5 wt%, M = 6 wt%, c M = 7 wt%, nd d M = 8 wt% d Nu ¼ h fd e ð8þ λ f where Nu is the Nusselt numer nd λ f is the therml conductivity of fluid in the tue mesured y therml conductivity mesuring instrument. The Reynolds numer is shown s follows: Re ¼ ρud e =μ f ð9þ where Re is the Reynolds numer, ρ is the density of fluid, u is the velocity of fluid, nd μ f is the dynmic viscosity of fluid mesured y super rottionl rheometer. Density of nnofluids is shown s follows: ρ ¼ ð1 φþρ f þ φρ p ð10þ where ρ is the density of the nnofluids, φ is the volume frction of the nnoprticles, ρ f is the density of wter, nd ρ p is the density of the nnoprticles. Frictionl resistnce coefficient of fluid is presented s follows: Fig. 5 Therml conductivities nd dynmic viscosities. Therml conductivities nd dynmic viscosities of TiO 2 -wter nnofluids t different tempertures nd sher rtes. Therml conductivities Dynmic viscosity

6 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 6 of 12 Fig. 6 Experimentl system. Schemtic digrm of experimentl system f ¼ 2de ρu 2 Δp ð11þ Δl where f is the frictionl resistnce coefficient nd Δp Δl is the pressure drop per unit length. The eqution of the comprehensive evlution etween het trnsfer nd flow resistnce is shown s follows [27]: ς ¼ Nu Nu ðfþsmooth tueþ = f f ðfþsmooth tueþ where ς is the comprehensive evlution index.!1 3 ð12þ Uncertinty Anlysis Experimentl errors re cused y the ccurcies of the equipment in the experimentl system. The corresponding error equtions re shown s follows: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! δnu u 2 Nu ¼ δq f þ δt 2 t ð13þ Q f T δf f s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi δp 2 ¼ þ δl 2 þ δqm 2 p l qm ð14þ where the ccurcy of DC power is ± 5.0%, the ccurcy of thermocouple is ± 0.1%, nd the error of the Nusselt numer cn e otined from Eq. (13) ndis pproximtely ± 5.0%. The ccurcy of pressure trnsducer is ± 0.5%, the ccurcy of length is ± 0.1%, the ccurcy of mss flow rte is ± 1.06%, nd the error of frictionl resistnce coefficient cn e otined from Eq. (14) nd is pproximtely ± 1.29%. Results nd Discussions Experimentl System Vlidtion Before the experimentl study on nnofluids, the experimentl system vlidtion is necessry. Wter is chosen s the het trnsfer medium. Nusselt numers nd frictionl resistnce coefficients etween the experimentl results of this pper nd the results of pulished litertures re shown in Figs. 8 nd 9. It cn e found from Figs. 8 nd 9 tht Nusselt numers nd frictionl resistnce coefficients t different Reynolds numers hve good greement with the results of the pulished litertures [28, 29] nd [30, 31] respectively. The mx errors for Nusselt numers nd frictionl resistnce coefficients t lminr flow nd turulent flow re pproximtely 3.5, 2.8, 2.1, nd 2.1%, respectively, which verify the ccurcy nd reliility of the experimentl system. Also, it is found tht the results of Dittus-Boelter in Fig. 8 re higher thn the rel results under the trnsitionl flow Fig. 7 Spirlly fluted tue. Detiled digrm of the spirlly fluted tue

7 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 7 of 12 Fig. 8 Het trnsfer chrcteristics vlidtion. Comprison of Nusselt numers etween the experimentl results nd the results of litertures. Lminr flow Turulent flow ecuse the empiricl formul only cn e pplied to the strong turulence zone, which grees with the results of literture [28]. It proves the vlidity of the results in this pper further. Experimentl Results nd Discussions The flow nd het trnsfer chrcteristics of TiO 2 -wter nnofluids in the smooth tue re investigted. Figure 10 presents the Nusselt numers of the smooth tue filled with nnofluids t different Reynolds numers. For lminr flow nd turulent flow, the Nusselt numer increses with the Reynolds numer nd nnoprticle mss frction. The turulivity of fluid increses with the Reynolds numer, which reduces the lminr oundry lyer nd improves the het trnsfer. Adding more nnoprticles into the se fluid cuses the increse of whole therml conductivity, which lso improves the het trnsfer. In ddition, it is suggested [32, 33] tht other fctors including the increse of Brownin motion of nnoprticles, reduction of the contct ngles, nonuniform sher rte, prticle shpe, nd ggregtion lso hve gret influence in the het trnsfer enhncement. In the previous pulished pper [11], the effects of Brownin force nd prticle size on het trnsfer enhncement were discussed. It ws found tht Brownin force is the iggest force of the interction forces etween nnoprticles, which is dvntgeous to the het trnsfer enhncement, nd the smll prticle size is lso dvntgeous to the het trnsfer enhncement. It is found from Fig. 10 tht the het trnsfer enhncement rtio from wter to ω =0.1wt% nnofluids shows the lrgest one, ut the het trnsfer enhncement rtio of nnofluids from ω =0.1wt% to ω = 0.3 wt% egins to decline, nd the het trnsfer enhncement rtio of nnofluids from ω =0.3wt% to ω = 0.5 wt% witnesses the smllest one. As Fig. 5 shows, the therml conductivity nd viscosity of nnofluids increse y % nd % compred with wter, respectively. For the lminr flow, the effects of viscosity on het trnsfer re smll due to the low velocity nd few nnoprticles, nd then the therml conductivity plys mjor role from wter to ω =0.1 wt% nnofluids. However, with n increse in nnoprticle frction, it shows more drmtic increse in viscosity compred with the increse in the therml conductivity, which cuses the het trnsfer enhncement Fig. 9 Flow chrcteristics vlidtion. Comprison of frictionl resistnce coefficients etween the experimentl results nd the results of litertures. Lminr flow Turulent flow

8 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 8 of 12 Fig. 10 Nusselt numers in the smooth tue. Nusselt numers of the smooth tue filled with nnofluids t different Reynolds numers. Lminr flow Turulent flow rtio to decrese. For the turulent flow, it is found tht the het trnsfer enhncements of nnofluids with different nnoprticle mss frctions re close. This is ecuse the turulence plys mjor role in the het trnsfer enhncement, nd the effect of nnoprticle mss frction ecomes smll. Also, it cn e found tht nnofluids show lrger het trnsfer enhncement rtio in lminr flow compred with tht in turulent flow. Nnoprticle mss frction plys mjor role in the het trnsfer enhncement in lminr flow, nd it shows lrge het trnsfer enhncement with the incresing nnoprticle mss frction. However, the effect of nnoprticle mss frction on het trnsfer enhncement ecomes smll in turulent flow, nd the turulence intensity plys mjor role; hence, it shows smller het trnsfer enhncement rtio with the incresing nnoprticle mss frction in turulent flow compred with tht in lminr flow. Bsed on the dt of Fig. 10, Fig. 11 shows the Nusselt numer rtios of nnofluids to the wter in the smooth tue. It cn e found tht TiO 2 -wter nnofluids with ω = 0.5 wt%, ω = 0.3 wt%, nd ω =0.1wt% enhnce the het trnsfer y 11.2, 7.4, nd 4.5% for lminr flow nd 16.1, 13.9, nd 11.9% for turulent flow t est compred with wter in the smooth tue, respectively. In ddition to the study on the het trnsfer chrcteristics of TiO 2 -wter nnofluids in the smooth tue, the flow chrcteristics re lso investigted. Figure 12 presents the frictionl resistnce coefficients nd pressure drop of the smooth tue filled with nnofluids. From Fig. 12, it is found tht the frictionl resistnce coefficient decreses with the Reynolds numer ecuse the incresing Reynolds numer cuses the increse of velocity, which is inversely proportionl to the frictionl resistnce coefficient ccording to the Eqs. (9) nd (11). It is found tht the pressure drop decreses with the frictionl resistnce coefficient ecuse the pressure drop is proportionl to the Reynolds numer ut the frictionl resistnce coefficient is inversely proportionl to the Reynolds numer. Hence, the pressure drop is inversely proportionl to the frictionl resistnce coefficient. Also, it cn e found from Fig. 12 tht the frictionl resistnce coefficient increses with nnoprticle mss frction ut the increse is smll etween different nnoprticle mss frctions. For TiO 2 -wter nnofluids with ω =0.5 wt%, ω =0.3wt%,ndω = 0.1 wt% in the smooth tue, mximum enhncement of 7.9, 5.2, nd 3.0% t lminr flow nd 2.5, 1.5, nd 0.6% t turulent flow occurs in the frictionl resistnce coefficients compred with wter in the smooth tue, respectively. Adding nnoprticles into wter cuses the increse of viscosity which is proportionl to the frictionl resistnce coefficient. However, the frictionl resistnce is minly cused y the screw structure of the spirlly fluted tue, nd the effect of nnoprticles on the frictionl resistnce is much smller thn tht of the screw structure, which cuses smll difference etween different nnoprticle mss frctions. Fig. 11 Nusselt numer rtios in the smooth tue. Nusselt numer rtios etween nnofluids nd se fluid in the smooth tue

9 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 9 of 12 c Fig. 12 Frictionl resistnce coefficients nd pressure drop in the smooth tue. Frictionl resistnce coefficients nd pressure drop of the smooth tue filled with nnofluids. Frictionl resistnce coefficients: lminr flow nd turulent flow; pressure drop: c lminr flow nd d turulent flow d Aove studies re on smooth tue, nd the flow nd het trnsfer chrcteristics of wter nd TiO 2 -wter nnofluids in the spirlly fluted tue will e investigted in the following text. Figure 13 presents the Nusselt numers of the spirlly fluted tue filled with TiO 2 - wter nnofluids t different Reynolds numers. It otins similr conclusion in the spirlly fluted tue (Fig. 13) similr to tht in the smooth tue (Fig. 10). It is found tht the Nusselt numer increses with the Reynolds numer nd nnoprticle mss frction. The differences etween the spirlly fluted tue nd smooth tue re tht there is lrger het trnsfer enhncement in the spirlly fluted tue thn tht in the smooth tue, which is due to the screw structure of the spirlly fluted tue. Bsed on the dt of Fig. 13, Fig. 14 shows the Nusselt numer rtios of nnofluids to the wter in the spirlly fluted tue. Figure 14 shows tht TiO 2 -wter nnofluids with ω = 0.5 wt%, ω = 0.3 wt%, nd ω = 0.1 wt% cn enhnce the het trnsfer y 14.7, 12.6, nd 11.3% for lminr flow nd 42.8, 35.4, nd 24.6% for turulent flow t est compred with wter in the spirlly fluted tue, respectively. There is lrger increse in het trnsfer for turulent flow thn tht for lminr flow. Fig. 13 Nusselt numers in the spirlly fluted tue. Nusselt numers of the spirlly fluted tue filled with nnofluids t different Reynolds numers. Lminr flow Turulent flow

10 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 10 of 12 Fig. 14 Nusselt numer rtios in the spirlly fluted tue. Nusselt numer rtios etween nnofluids nd se fluid in the spirlly fluted tue The flow chrcteristics of TiO 2 -wter nnofluids in the spirlly fluted tue re lso studied. Figure 15 presents the frictionl resistnce coefficients nd pressure drop of the spirlly fluted tue filled with nnofluids, which shows tht the frictionl resistnce coefficient decreses with the Reynolds numer nd increses with the nnoprticle mss frction, nd the pressure drop decreses with the frictionl resistnce coefficient. The resons re similr to tht in the smooth tue (Fig. 12c, d). TiO 2 -wter nnofluids with ω = 0.5 wt%, ω =0.3wt%, nd ω = 0.1 wt% cn enhnce the frictionl resistnce coefficients y 20.2, 16.5, nd 12.5% for lminr flow nd 10.5, 7.7, nd 2.0% for turulent flow t est compred with wter in the spirlly fluted tue, respectively. There is smller increse in frictionl resistnce coefficients for turulent flow thn tht for lminr flow. The het trnsfer chrcteristics of TiO 2 -wter nnofluids in the smooth tue nd spirlly fluted tue re investigted in this pper seprtely. Figure 16 shows the comprison of Nusselt numers etween the smooth tue nd the spirlly fluted tue filled with nnofluids t different Reynolds numers. It cn e found tht TiO 2 -wter nnofluids with ω = 0.5 wt%, ω = 0.3 wt%, nd ω = 0.1 wt% in the spirlly fluted tue cn enhnce the het trnsfer y 257.9, 245.1, nd 240.7% t est compred with TiO 2 -wter nnofluids in the smooth tue, respectively. Also, TiO 2 -wter nnofluids with ω = 0.5 wt%, ω =0.3wt%, nd ω =0.1 wt% in the spirlly fluted tue cn enhnce the het trnsfer y 291.3, 268.8, nd 253.1% t est compred with wter in the smooth tue, respectively. TiO 2 - wter nnofluids in the spirlly fluted tue hve lrger enhncement thn tht in the smooth tue. In order to syntheticlly nlyze the thermo-hydrulic performnce of TiO 2 -wter nnofluids in the smooth c Fig. 15 Frictionl resistnce coefficients nd pressure drop in the spirlly fluted tue. Frictionl resistnce coefficients of the spirlly fluted tue filled with nnofluids. Frictionl resistnce coefficients: lminr flow nd turulent flow; pressure drop: c lminr flow nd d turulent flow d

11 Qi et l. Nnoscle Reserch Letters (2017) 12:516 Pge 11 of 12 screw structure. The conclusions of Fig. 17 re very importnt for the choices of tues nd Reynolds numers in the het-exchnge equipment considering the comprehensive evlution of the thermo-hydrulic performnce. For the smooth tue, the higher Reynolds numer cn e chosen due to the fctor tht the thermohydrulic index lwys increses with the Reynolds numer. While for the spirlly fluted tue, the pproprite Reynolds numer for the highest thermo-hydrulic index is out Fig. 16 Comprison of Nusselt numers in two tues. Comprison of Nusselt numers etween the smooth tue nd spirlly fluted tue filled with nnofluids t different Reynolds numers tue nd spirlly fluted tue, Fig. 17 presents the comprehensive nlysis of nnofluids in the smooth tue nd the spirlly fluted tue sed on the Eq. (12). It cn e found tht the highest comprehensive evlution index ξ for spirlly fluted tue is out t Re = 2300 which is the criticl Reynolds numer etween lminr flow nd turulent flow. The increses of the Nusselt numer nd frictionl resistnce coefficients re minly due to the nnoprticles, the Reynolds numer, nd the screw structure of spirlly fluted tue. For spirlly fluted tue, due to the screw structure, the increse of the Nusselt numer is lrger thn the increse of frictionl resistnce coefficients t smll Reynolds numer (Re 2300); conversely, the increse of the Nusselt numer is smller thn the increse of frictionl resistnce coefficients t ig Reynolds numer (Re > 2300). Also, the comprehensive evlution index ξ for the smooth tue increses with the Reynolds numer. The increse of the Nusselt numer is lwys lrger thn the increse of frictionl resistnce coefficients ecuse the smooth tue hs no Conclusions The flow nd het trnsfer chrcteristics of TiO 2 -wter nnofluids in spirlly fluted tue re experimentlly studied. Some conclusions re otined s follows: (1) TiO 2 -wter nnofluids with different nnoprticle mss frctions re prepred, nd TiO 2 -wter nnofluids with M = 6 wt% nd ph = 8 hve the lowest trnsmittnce nd show the est stility. (2) For TiO 2 -wter nnofluids in the spirlly fluted tue,thereislrgerincreseinhettrnsfernd smller increse in frictionl resistnce coefficients for turulent flow thn tht for lminr flow. (3) TiO 2 -wter nnofluids in the spirlly fluted tue hve lrger enhncement thn tht in the smooth tue. TiO 2 -wter nnofluids in the spirlly fluted tue cn enhnce the het trnsfer y 257.9% t est compred with tht in the smooth tue. (4) The highest comprehensive evlution indexes of TiO 2 -wter nnofluids in the spirlly fluted tue nd smooth tue re different. For the spirlly fluted tue, the highest comprehensive evlution index ξ is t Re = 2300 which is the criticl Reynolds numer etween the lminr flow nd the turulent flow. For the smooth tue, the comprehensive evlution index ξ increses with the Reynolds numer. Acknowledgements This work is finncilly supported y the Ntionl Nturl Science Foundtion of Chin (Grnt No ) nd the Fundmentl Reserch Funds for the Centrl Universities (Grnt No. 2015XKMS063). Authors Contriutions CQ prticipted in the design of the experiment set, crried out the experiment of nnofluid, nd drfted the mnuscript. CYL crried out the experiment of nnofluid nd processed the dt. GQW crried out the experiment of nnofluid. All uthors red nd pproved the finl mnuscript. Competing Interests The uthors declre tht they hve no competing interests. Fig. 17 Comprehensive nlysis of the two tues. Comprehensive nlysis of nnofluids in the smooth tue nd the spirlly fluted tue Pulisher s Note Springer Nture remins neutrl with regrd to jurisdictionl clims in pulished mps nd institutionl ffilitions.

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