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Preparation, Stability And Thermal Conductivity Of Nanofluids

Posted on:2006-06-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:H T ZhuFull Text:PDF
GTID:1101360155966244Subject:Materials science
Abstract/Summary:PDF Full Text Request
Nanofluid is a new class of heat transfer fluid which containing suspended metallic or non-metallic nanoparticles in traditional fluid. Compared with conventional thermal fluids and fluids containing micrometer-sized particles, nanofluids have the advantages of higher thermal conductivity, superior stability, and better lubricating properties. Nanofluids have potential application in many industrial sectors, including transportation, energy production and supply, motor cooling, and electronics cooling, so they become the hot points of material, physics, chemistry and thermal physics.Now, there are two kind of methods to prepare nanofluids: dispersing method and one-step evapor-condensation. Dispersing method, dispersing nanoparticles in base fluid, has the advantage of simple process, and disadvantage of lower thermal conductivity and stability of nanofluids. One-step evapor-condensation, combining the physical synthesis of nanoparticles with the preparation of nanofluid, has advantage of higher thermal conductivity and stability of nanofluids, and has difficulties to prepare nanofluid in large scale and lower cost.The theoretic and experimental research on the thermal conductivity of nanofluids has been done in the past years, only a few theoretic formulas and models were obtained. Those formulas and models were derived from merely one or few nanofluid systems, they only used in a very small number of systems accurately. The theoretic formulas and models should derive from many nanofluids system. Therefore, many other nanofluids are needed to be prepared, and their thermal conductivity should be further studied.One-step liquid chemical method to prepare nanofluid was first brought forward in this dissertation by the author, which combines the liquid chemical synthesis of nanoparticles with the preparation of nanofluids. Four kinds of nanofluids were prepared in this dissertation: thermal conductivity and stability of those nanofluids were also being studied. Following are the main contents:1.Cu/EG (ethylene glycol) nanofluid was synthesized through a novel microwave -polyol process in which sodium hypophosphite or hydrazine was added as reducer and microwaveirradiation was used as heating source. The particles size of copper in the nanofluid is very small (about lOnm) and the stability of this nanofluid is very well.The influences of concentration of reducer and PVP, react time, and heating time on reduction rate, particles size and phase component were studied. The result showed that the use of microwave irradiation and strong reducer accelerated the reaction rate and benefited the dispersion and the particles' size distribution of copper nanoparticles.The influences of microwave irradiation time, pH value, concentrations of PVP and dispersant OP on the stability of nanofluid were investigated by size and zeta potential analysis. Cu nanofluid with good stability can be obtained under the conditions of optimal microwave irradiation time, optimal concentrations of PVP and dispersant OP.2. Using non-crystal Cu(OH)2 as precursor, CUO/H2O nanofluids were synthesized by ultrasonic chemical method. The studies were focused on the influence of ultrasonic time, reaction medium and disposal method of precursor on the structure and shape of CuO nanoparticles. Non-crystal Cu(OH)2 could be converted to CuO nanoparticles in water under ultrasonic condition. With the increasing of ultrasonic time, the crystal degree increased, and the conglomeration of CuO became spherical nanoparticles. With the increasing of water volume percentage, the reaction rate increased. The roles of ultrasonic cavitations and EG in precursor were benefited to the dispersion of nanoparticles.We studied the influences of concentration and sort of dispersant on the stability of CuO/H2O nanofluids. The zeta potential increased with the adding of CTAB dispersant. When the concentration of CTAB was 0.35%, the stability of nanofluid was best. The effect of nonionic dispersant PEG to CuO nanoparticals' facial modification is slight. With the adding of dispersant ACT or PMAA, the zeta potential decreased. The nanofluids were very well dispersed when adding one of these dispersants, and the stable effect of PMAA was better.3. Fe3O4/H2O nanofluids were prepared by coprecipitation method. The size of Fe3O4 nanoparticles in nanofluids was about 15nm, the stability of nanofluid was excellent.The influences of concentration of reactant, sort and adding quantity of dispersant were investigated in this dissertation. When the concentration of reactant was 0.2M, the particles size of Fe3O4 was smaller and the distribution was narrow. Adding the dispersant in the reaction process could make the particles size easy to control. Among dispersants of DP-518, MN, PEG and ACT,the last one avail to get nanofluid with smaller nano particles.The influences of pH value ^ ultrasonic time, dispersant sort and weight percentage on nanofluids' stability were studied. Appropriate pH value was the key to get stabilized nanofluid. Proper ultrasonic time was avail to break up the agglomeration in washing process. The addition of MN shortened the stable range of nanofluids in acid, while widened the range in basic condition. The addition of FeCl3 made nanofluids stable in a broader range.4. Using natural micro-crystal graphite as raw material, nano-graphite was prepared by ball milling/floatation process. The influences of milling medium, pH value, milling time and surface agent on powder size were investigated. The results showed that adopting of ZrO2 ball, adding surface agent and adjusting pH value to 9.5 were benefited to improve milling effect. Flake nano graphite was gotten with three times of milling/floatation.The influences of dispersant sort, pH value, and combination of dispersants on the stability of nanofluids were studied. Graphite/fyO nanofluids with perfect stability were prepared by using combined dispersant (ACT combined with PVP.or OP) under the basic condition.5. The hot-wire device for measuring thermal conductivity of nanofluids was designed and assembled. The precision of the device was checked by using ethylene glycol and distilled water as standards. The results proved the accuracy of the device was enough to test the thermal conductivity of nanofluids.The thermal conductivity of four kinds of nanofluids (Cu/EG, CuO/F^O, FesCj/fyO, graphite/ H2O) were measured by the hot-wire device. The results showed that when the volume fraction of copper nanoparticle was 0.5%, the thermal conductivity enhancement of Cu/EG was 43%; when the loading of nano-CuO was O.35vol%, the enhancement of CuO/ H2O was 19.9%; when the loading of Fe3O4 was 1.0%, the enhancement of Fe3O4/H2O was 48%; when the loading of graphite was 2.0%, the enhancement was 30%. Compared with nanofluids produced by other methods, these nanofluids, prepared by one-step wet chemical method, have higher conductivity.Using the MaxwelK H&C^ Yu&Choi and Kumar model, we analyzed the thermal conductivities of our four nanofluids. The above models did not explain the thermal conductivity enhancement of those nanofluids. So further investigation on the heat conductivity mechanism of nanofluids are needed.Following are the creative points:1) One-step wet chemical method to prepare nanofluids was brought forward for the first time. Nanofluids prepared through this method had higher thermal conductivity.2) Cu/EG nanofluids were prepared by a novel microwave/polyol method (a one-step wet chemical method). The adoption of microwave irradiation and strong reducer accelerated the reaction rate and benefited the dispersion and the particles size distribution of copper nanoparticles. It also improved the stability and thermal conductivity of Cu nanofluids.3) Using amorphous Cu(0H)2 as precursor, CuO/H2O nanofluids were synthesized by ultrasonic chemical method. The stable conditions of CuO nanofluids were studied for the first time.4) The thermal conductivities of Fe3O4/H2O and graphite/H2O nanofluids were investigated for the first time. Fe3O4/H2O and graphite/H2O nanofluids prepared by one-step wet chemical method had superior thermal conductivity.The researches of this dissertation supply fundamental theory for preparing nanofluid by one-step wet chemical method, which are the directors of controlling nanoparticles' size, structure and shape, and directors of modulating stability of nanofluids. These studies on the thermal conductivity of nanofluids indicate the direction of nanofluids studies. The studies of this dissertation promote the industrialization of nanofluids.
Keywords/Search Tags:nanofluids, one-step wet chemical method, preparation, stability, thermal conductivity.
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