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Study On Enhanced Convective Heat Transfer Of Nanoparticle Suspension

Posted on:2008-01-14Degree:DoctorType:Dissertation
Country:ChinaCandidate:A H QiangFull Text:PDF
GTID:1101360245490912Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Responding to the need for industry process intensification and more efficient heat transfer systems, many efforts have been devoted to heat transfer enhancement techniques in the past few decades. With the rapid development of nanotechnology, some scholars attempted to enforce heat transfer process with nanoparticles, which introduced a new method into heat transfer enhancement. The purpose of this paper is to demonstrate experimentally and theoretically the convective heat transfer characteristics of nanofluids, so as to lay a foundation for the farther investigation on heat transfer of nanofluids.CuO nanoparticle suspension has been prepared by directly mixing nanoparticles and base fluids. Some auxiliary dispersants and ultrasonic oscillation were necessary to obtain even distributed and stabilized suspensions. Sedimentation experiment, zeta potential, granularity test and infrared spectrum were given to illustrate the stability of suspensions. Some factors that affect the stability and evenness of suspension, such as the property and concentration of dispersants, pH of base fluid were discussed.An experimental system was designed and built up to investigate convective heat transfer characteristics of CuO nanofluids flowing in a tube. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on heat transfer were discussed in detail. The convective heat transfer performance of water and dispersant solution were also tested to compare with nanofluids. The experimental results show that nanoparticles remarkably increase the convective heat transfer coefficient of base fluid. The heat transfer feature of nanofluids increases with the volume fraction of nanoparticles.By considering factors affecting the convective heat transfer of nanofluids, a convective heat transfer model for nanofluids under laminar flow was established. It is according to effective continuum theory, which supposes a common velocity for two phases and merely considers the mass flux of particles migration. The tatal flux of particle migration takes into account the effects of shear-induced and viscosity-gradient-induced, Brownian motion, as well as thermophoresis-induced particle migration. With the model the effects of particle size, K c/Kμand mean concentration of particle on concentration distribution were analyzed. The migration of particles leads to non-uniform viscosity and thermal conductivity distribution of suspension. Increases in blunting of velocity profile influences the fluid temperature grads close to the wall and thus intensifies heat transfer of nanofluids.Comparison between experimental data and calculated results indicates that the model describes correctly the trend of experimental value and simulation results are higher than those of even thermal conductivity distribution. The capability of model is improved after considering the migration of thermophoresis. The particle migration model is reasonable for energy transport of nanoparticle suspension.
Keywords/Search Tags:nanoparticle suspension, stability, convective heat transfer, enhancement, particle migration model
PDF Full Text Request
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