Influence factors on thermal conductivity of ammonia-water nanofluids

J. Cent South UDkv. (2012) 19: 1622-1628DOI: 10. 1007/s11771-012-1185-0包SpringerInfluence factors on thermal conductivity of ammonia-water nanofluidsYANG Li(杨柳), DU Kai(杜垲), ZHANG Xiao- song(张小松)School of Energy and Environment, Southeast University, Nanjing 210096, China口Central South University Press and Springer-Verlag Berlin Heidelberg 2012Abstract: In order to investigate the mecbarism of raoparticles enhancing the heat and mass tansfer of the amonia-waterabsorption process, several oypes of binary nanofluids were prepared by mixing Al2O3 nanoparticles with polyacrylic acid (PAA),TIO2 with polyethylene glycol (PEG 1000), and TIN, siC, bydroxyapatite (noodle-like) with PEG 10000 to ammonia water solution,respectively. The thermal conductivities were measured by using a KD2 Pro thermal propertics analyzer. The infuencs of surfactantand ammonia on the dispersion stabilitics of tbe binary nanfluids were investigated by the light absorbency atio index metbods. Theresuls show that the type, coatent and size of nanoparticles, the temperature as well as the dispersion stability are the key parametersthat afet the thermal conductivity of nanfluids. For the given nanoparicle material and the base fluid, the thermal conductivityratio of the nanofluid to the ammonia-water liquid increases as the nanoparticle content and the temperature are incteased, and thediameter of nanoparticle is decreased. Furthemore, the thermal conductivity ratio increascs significantly by improving the stabiltiesofnaponuof nanofluids, which is achieved by adding sufactants or performing the proper ammonia content in the fluid.Key words: binary nanofluids; ammonia-water; thermal conductivity; size efect dispersion stabilitfound that the absorption rate and heat transfer rate of1 Introductionammonia-water nanofluid with 0.002% Al2O3 nano-particles were 29% aod 18% higher than those of theAmmonia-water absorptionrefrigerator hasammonia-water without nanoparticles, respectively, andsuperiority in utilizing the waste-heat and low-grade heatthe ammonia-water nanofluid with 0.002% Al2Ozsource and they bave drawo renewed attention with thenanoparticles was the optimal candidate for ammonia-growing awareness of the dual threats of global warmingwater absoption performance enbancement. YANG et aland ozone depletion. However, the drawbacks of[6] carried out the comparative experiments on theammonia-water absorption refrigeration, such as thefalling flm absorption between ammonia-water andheavy equipmeat, the low cofficicat of perfomanceammonia-water with various kinds of nanoparticles. I(COP) and the hazard in safety, restricted its broadwas found that the effective absorption ratio can beapplication. Therefore, many researches have beenincreased by 70% and 50% with Fe2O3 and ZnFe2Onperformed to improve its performance. Generally, therenanofluid, respectively, when the initial ammonia massare three methods to enhance the efficiency of heat andfraction is 15%.mass transfer: the mechanical treatnent, the chemicalThe enhancement of nanoparticles on the heattreatment and nanotechnology [1].transfer of single- species fluid has been studied by manyOver the past several decades, anoparticles haveresearchers. CHOI [7] reported that the thermaltriggered an explosion of scientific and industrial interestconductivity of the base fluid inacreased 中to 40% bydue to their outanding characteristics. It has long beenadding a lttle amount of nanoparticles and nanotubes. Totecognized that the suspensions of solid particles inexplain CHOI's experimental results, KEBLINSKI et alliquids provide useful advantages in bheat transfer fluid[8] suggested the potential mechanisms for thermal[2- 3]. The working fluids have the limitation of bheatconductivity enhancement such as Brownian motion,transfer perfomance, thus solid particles are dispersed iniquid layering and nanoparticle clustering. Moreover,the working fiuids to improve their thermal properties orYOU et al [9] reported that the criticai heat flux in poolheat transfer characteristics [4]. Consequently, manyboiling of Al2O3 nanofuids increased dranatically (byexperiments have been performed on the application ofabout 200%) compared to the pure water case. Recently,nanofluid in ammonia-water absorption. LEE ct al [5]KIM中国煤化工isabiliyy drivenFoundatioa ite: Przca(51176029, 50876020) supported by tbe National Natural Scie0 -03B0) spported by咖12也Five-Year National Science and Technology Support Key ProYHCNMHGabyheooofGraduate School of Southeas1 University. ChinaRecehred date: 2011-07- 26; Accepted date: 2011-11-14Corresponding stbor: DU Kai, Professor, Tel: +86 25-83792314; E-ail: dukai@scu.odu.ca1624J. Ccot. South Univ. (2012) 19: 1622-162830 mm in diameter whose cap is equipped wit a sensorof the particles in suspension. Higber absorbency meanspositioning device through which the sensor needle washigher concentration of nanoparticles in the solution andinscrted. For accurate measurements, the sensorbetter dispersion of nanofluid.positioning device was square- shaped wbose diagonal isequal to the inside diarneter of the cuvette. Thus, the3 Results and discussionsensor was inserted fully into the fuid, and orientedvertically and centrally inside the vial without touchingThe influences of the type, content and size ofthe side walls of the vial. Insertion of the sensor needlenanoparticles, the temperature and the stability ofprobe into the fluid in this orientation will minimizenanofluid on the thermal conductivities were measurederrors from free convection [20]. n addition, the sensorby using a KD2 Pro thermal properties analyzer, Theneedle was placed in the middle section of the nanofluidinfluences of surfactant and ammonia on the dispersionin the vial as sthown in Fig I, so that any bubbles in thestabilities of the binary nanofluids were investigated byfluid would float to the top and the sedimentation wouldthe light absorbency ratio index methods.drop to the bottom away from the needle.The thermal conductivity ratio is defined to examineThe following asunptions were madc for thethe efcct of tbe addition of nanoparticles on tbe thermalthermal conductivity measurement: 1) the long heatconductivity of ammonia water:source can be treated as an infnitely long heat source; 2)i= Ken/Kp(3)the medium is both homogeneous and isotropic, and aunifom initial temperature. Although these assumptionswhere i is the fective thermal cooductivity ratio; Kerare not true in strict sense, they are adequate for accurateand K: are the thermal conductivity of nanofluid andthermal properties mcasurements. The sensor needleammonia-water basefluid, respectively.used was KS-I which is made of stainless steel having alength of 60 mm and a diameter of 1.3 mm, and closely3.1 Influences of type and content of nanoparticles ouapproximates the infinite line heat source which givesthermal conductivity ratioleast disturbance to the sample during measurements.Figure 2 shows the experimental tesults of theEach measurement cycle lasts for 90 s. During the firstthermal conductivity ratio of different types of nanofuid30 s, the instrument will equilibrate which is thenin 25% ammonia-water at 20 °C. It can be seen that thefollowed by beating and cooling of sensor needle for 30 sthermal conductivity ratios of all kinds of nanofluidseach. At the end of the reading, the controller computesincrease approximately linearly with the increase of tbethe thermal conductivity using the change in temperaturecontent of nanoparticles. However, the rate of the(O7) and time data from Ref. [20]:increase of thermal conductivity ratio is varying andrelated to the type of nanoparticles. When the volumek。q(ntz -14)(1)fraction of Al2O3 (40 mm) nanoparticles increases ftom4m(O72 - AT)0.01 to 0.04, the thermal conductivity ratio increaseswhere q is constant heat rate applied to an infnitely longfrom 1.044 to 1.176 accordingly, whose increasing rate isand small“line" source; AT and△Tz are the changes inmuch higher than that of SiC nanoparticles of the samethe temperature at time 1 and 12 respectively.size. Similarly, the increasing rate of TiO2 (15 nm)nanoparticles is higher than that of the same sized TiN2.3 Measurement of dispersion stabilityIn order t如investigate the influence of dispersion1.225% ammonia-water (20°C)■- - 40 om Al2O3 (with 0.25% PAA).stability on the tbermal conductivity of binary nanofluid,步1.16。- 15 nm TiO2 (with 0.3% PEG1000)the stability of nanofluids was studicd by measuring theabsorbency of nanofluid with and without surfactants in一40 nm siC (without sufactant)diferent mass fractions of ammonia-water basefluid after1.12standing for 48 h. The absorbency was measured byultraviolet-visible pectropbotometer, and the absorbency1.08-[21] of the suspension was defined byA=1g(Io/I)= sbc .(2)1.0要互where A is absorbency; Lo is the incident light intensity; Iis the transmitted light intensity; e is molar absoptivity中国煤化工(/molcm); b is optical path (cm); c is molarMHCN M H Grparticles%concentration (mol/L). It can be included from Eq. (1)Fig.IDermal conoucuviy auo ot diferent types ofthat the absorbency is proportional to the concenatrationnanofuid in 25% ammocia waterJ. Cent. South Univ. (2012) 19: 1622-16281625nanoparticles. Hence, the type and the content ofsuspensions have higher thermal conductivity ratio cannanoparticles are the two key influencing factors on thebe explained as follows. When the size of nanoparticlesthermal conductivity ratio of ammonia-water nanofluids.is smaller, the effect of van der Waals force is stronger,and the movements of particles are more frequent and3.2 Influence of size of nanoparticles on thermalsevere, the interactions between nanoparticles andconductivity ratiobetween nanoparticles and liquid are stronger and theThe experimental results of the thermal conductivityenergy transmission is further, which can induce a higherratio of nanofluid with different sized Al2O3 nano-thermal conductivity ratio in macroscopic view.particles in 25% ammonia water are shown in Fig. 3. Itcan be seen that the thermal conductivity ratios of3.3 Influence of temperature on thermal conductivitynanofluid with 20 nm Al2O3 nanoparticles is higher thanratlothat with 40 nm Al2O3 nanoparticles. In the sameFigures 5 and 6 show the experimental results of thecondition, the thermal conductivity ratio of nanofluidthermal conductivity ratio of Al2O3 (40 nm) and TiO2with 40 nm bydroxylapatite nanoparticles is higher than(20 nm) nanofluid at 20 °C and 40 °C, respectively. Itthat with 60 nm particles, as shown in Fig.4. It seemscan be seen that for both Al2O3 and TiO2 nanofluids, thethat the thermal conductivity ratios of noodle likthermal conductivity ratio increase obviously when thenanoparticles have the same variation tendency on thetemperature of the nanofluid increases from 20。C tosize dependence with the spherical nanoparticles. Thi40 °C. The temperature dependence of thermalexperimental result is in accordance with the researchconductivity ratio can be explained as follows. When theresults of KIM et al [22]， in which the thermaltemperature of the nanofluid increases, the thermalconductivity ratios between nanofluid and basefluidmotion of nanoparticles will beenhanced. Besides, theincrease nonlinearly with the decrease of the size of thermal agitations of water and ammonia molecules arenanoparticles.strengthened, and the knock-on effects [23] on theThe possible reasons that smaller sized nanoparticles particles by the liquid molecules are stronger, which will25% ammonia-water (20"C)25% ammonia-water0一- 20nm Al2O (with 0.3% PAA)1.2●- 40nm Al2O3 (20°C, with 0.3% PAA)P1.20- ●- 40nm Al2O3(with 0.3% PAA) 8- 40 nm Al2O3 (40"C, with 0.3% PAA)g 1.16-1.16会1.12-1.12|1.08-1.04-1.00Volume fraction of nano particles/%Volume fraction of nano-particles/%Fg 3 Thermal conductivity ratio of different sized AlzO3Fig.5 Thermal conductivity ratio of Al2O3 nanofluid ananofluids in 25% ammonia-waterdifferent temperatures in 25% ammonia-water1.125% ammonia- water■- I5 nm TiO2 (20°C, with 0.25% PEG1000)一400m hydroxyapatite._ Cgooudewunstite。PEG10000)0一15 nm TiO2 (40°C, with 0.25% PEG1000)(noodle like, with 0.5% PEG10000)号1.08-. 1.12-国1.02中国煤化工Volune fraction of nano-particles/%YHC N M H Crpricteislg. 4 Thermal conductity ratio of difcrent sizedFig. 6 Thermal conductivity ratio of TiO2 nanofluid at differenthydroxyapatite nanofluid in 25% ammonia-watertemperatures in 25% ammonia-water1626J. Cent South Univ. (2012) 19: 1622-1628strengthen the energy tasmission and improve thethermal conductivity ratio of nanofluids.“After standing for 48hBy comparing Fig. 3 with Fig. 5, for Al2O3.- 15 nm TiO2 with 0.25% PEG10004一15 nm TiO2 without surfactantnanofhuid, increasing temperature scems to be more●- 40nm Al2O; with 0.3% PAAeffective than decreasing the particle size. This0一40 nm Al2O3 without surfactantcircurmstance agrees with the experimental result of DASet al [17], in which the thermal condutivity ratios of■Al2O3 nanofluid to water increase greatly when thetemperature increases frorm 21 °C to 36 °C. It can beconcluded that for ammonia-water nanofluid (binarynanofluid)，the temperature dependence of thermalconductivity ratio is in accordance with that of0Csingle- species nanofluid.Volumpe fraction of nano-particles/%3.4 Infuence of stabllity on thermal conductivityFig. 8 Absorbency of binary nanofluids with and without aflerratiostanding for 48hIn order to investigate the influence of dispersionstability on the thernal conductivity ratio of binaryabsorbency approaches to zero, which means that almostnanofluid, the comparative experiments on the thermalall the Al2O3 nanoparticles drop on the bttom of the testconductivity ratio of the binary nanofluid with antube after 2 d static storage. This is the reason that thewithout sufactant, and nanofluid with different contentsthermal conductivity ratio will decrease greatly withoutof ammonia were carried out.surfactant.The comparative experiment results on the thermalThe nanoparticles have high specific surface areaconductivity ratio of the Al2O3 nanofluid with 0.25%and intense surface activity, which directly results inPAA and without surfactant, TiO2 nanofluid with 0.3%colliding and reuniting among the nanoparticles.PEG 1000 and without surfactant are shown in Fig. 7. ItTherefore, it is dificult for them to be dispersed steadilycan be seen that, especially for Al2Oz nanofluid, theand uniformnly in the ammonia-water solution withoutthermal conductivity ratio of the nanofluid withsurfactant. The effect of surfactants on improving thsurfactant is obviously higher than that withoutdispersion can be explained as follows. An interfacialsurfactant. The surfactants have lttle influences on thefilm surrounding nanoparticles will be formned as a resultthermnal conductity ratio, which will be proved later inof the absorption of dispersant on the interface. When thethis work. The reason of surfactants performing positiveintensity and thickness of interfacial flm are enough, itroles in the thermal conductivities of binary nanofluidcan protect the nanoparticles from aggregating (24]. Thecan be attributed to the improvements of surfactants topoor stabilized nanoparticles will collide and drop on thethe dispersion stabilities of nanofluid, as shown in Fig, 8.bottom of the cuvette， and they cannot makeIt can be seen that after standing for 48 h, the absorbencycontributions to the thermal conductivity of fluid. Even ifof nanofluid with surfactant is much higher than thatthe poor stabilized nanoparticles suspend in the fluidwithout surfactant. Especially for Al2O3 nanofluid, thetemporarily, the particles are reunited to large sizedparticles (micrometer-size or even larger), thus the1.2025% ammonia water (20°C)relatively large reunion cannot achieve some superiorproperties of the nanoparticles, such as the micro-●一15 nm TiO2 with 0.3% PEG1000安1.16convection and high specific surface area.enhancement of themmal conductivity of nanoparticles。一40 nm Al2O3 without surfactant王cannot be fully functioned in the Danofluid of poor15 nm TiO2 without surfactantstability. So the thermal conductivity of poorly stabilizednanofluid is much less than that of stable nanofluid withadding proper surfactant.The influence of ammonia content on the thermal1.04-conductivity ratio of binary nanofluid is also studied andthe1.00l中国煤化IO TO2x sic 0TiNnzction of 4% in 0%,Volume fraction of nano particles%12.5%HCNM H Gvm inFig.9. ItcanFlg 7 Thermal conductivity ratio of binary nanofuids with andbe found that the thermal conductivity ratio of ammonia-without surfactantwater basefluid decreases with the rising of ammoniaJ. Ceat. South Uaiv (2012) 19: 1622-16281627content. However, the thermal conductivity ratios of thethe surface of the grains and the external plane oftwo kinds of Al2O3, TiO2 and SiC binary nanofluid withHelmboltz. The Zeta potential is thus defined as theammonia-water as basefluid are much larger than thoseelectrical potential developed at the solid/liquid interfacewith distiled water as basefluid, which means thatin response to the relative movement of solid particlesincreasing the ammonia content can improve the thermalnd liquid or as the strength of the particle electricalconductivity ratios of the three kinds of nanofluid.barrier. The higher the potential with the same polarity is,However, for TiN nanofluid, the thermal conductivitythe higber the electrostatic repulsion between particlesratio increases firstly, and then sharply decreases with thebecomes. On the other hand, when tbe suspension isincrease of ammonia content. The influence of ammoniaclose to the iso electric point, the particles tend tocontent on the thermal conductivity ratio of binaryflocculate. The Zeta potential of nanoparticles is relatednanofluid can be attributed to the influence of ammoniato the pH value of the basefluid. The dispersion stabilitycontent on the dispersion stability, as shown in Fig.10.of Al2O3 and TiO2 nanofluids with distilled water poorerthan that in ammonia water can be explained as follows:Volume fraction of nanoparticles: 4% (20°C)According to Refs. [19, 25- -26], the pH values of1.20-0一40nm Al2O3 (with 0.3% PAA).iso-lectric points of Al2O3, TiO2 and SiC are below oro一15 nm TiO2 (with 0.25% PEG1000)close to 7. With the increases of mass fractions of一40mm sicC名1.16- *-15mTIN 百ammonia-water base solution, the pH value of solutionsincreases, the Zeta potential of nanofuids is far away一个from iso-electric point and bigher, and the high Zeta.12potential of nanofluids can keep a high electrostaticrepulsion which can keep the dispersion more stable anduniform. However, for TiN nanofluid, according to1.04-Ref. [27], although the pH value of iso eletric point isbclow 7, the Zcta potential increases firstly and then1.00412.525.0decreases with the increase of pH value of basefluid,which induces a poor dispersion stability of TiNMass fraction of ammonia%FIg. 9 Thbermal conductivity ratio of binary nanfluids withnanofluid in too higher pH value fluid. So, the dispersionstability of TiN nanofluid is improved firstly and thendifferent contents of ammoniadeteriorated with the increase of pH value related to theVolure fraction of nanoparicles: 4% (20°C)content of ammonia.After standing for 48 hThe dispersion stabilities of nanofluids are greatly5-一40 am Al2O3 (with 0.3% PAA)。improved by obtaining proper ammonia contents duc to15 nm TiO2 (with 0.25%PEG1000)the high Zeta potentials. Thus, the thermal conductivity●一40m siCrenhancement of the nanofluid can be fully functioned ina uniformly dispersion condition.3.5 Influence of surfactant on thermal conductivity2Figure 11 shows the influences of surfactants on the毋= Distiled water(20°C) Io●12.5% ammonia-water。 25% ammonia-waterMass fraction of ammonia/%! 0.57Fig. 10 Absorbency of binary nanofluid with differcnt contcatsof ammonia after standing for 48 he 0.56-王0.55-[1.44%Figure 10 shows thabsorbencies of the binarynanofluids with the ammonia content after standing for0.54-48 h. It can be seen that the absorbency of Al2O, TiO2and siC nanofuid with distilled water (DW) was far less2 0.535than that of nanofuids with ammonia-water and the中国煤化工%一05%-absorbency will increase with the increase of the massPEG10000fraction of ammonia-water basefuid. SuspensionMHCNMHGoniawaterstability is directly dependent on the Zeta potential of theFig. 11 Thermnal conductivity of ammonia-water base fluid withpowder that represents the potential difference between different kinds of surfactant1628J. Cent. South Uaiv. (2012) 19: 1622-1628thermal conductivity of basefluid. It can be seen that the[7] CHOI s U s. Enhancing tbermal conductivity of fluids withoano-particles M/ Siginer D A. Wang H P. Develthree kinds of surfactants in low concentration haveApplications of Non-Newtonian Flows, EFD. 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