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Study On Stability And Thermal Conductivity And Characteristics Of Convective Heat Transfer Using Nanofluids

Posted on:2017-08-22Degree:MasterType:Thesis
Country:ChinaCandidate:R LiuFull Text:PDF
GTID:2311330503492840Subject:Power Engineering and Engineering Thermophysics
Abstract/Summary:PDF Full Text Request
With the rapid development of electronic equipment, the high heat transfer loading and intensity of heat exchanger apparatus have been the major problem on miniaturization and integration. Nanofluids is a new kind of energy transport working medium which could help improve the thermal conductivity of refrigerant and enhance the heat transfer performance of heat dissipating system. So, study on the thermal conductivity and heat transfer enhancement of nanofluids could help to learn more about the feasibility in practical application. It could also help to improve the performance of heat exchange system, which has broad application prospects and potential economic value.In this paper, different nanofluids were prepared by two-step, both the experimental and simulative methods were used to give an overall analysis. Firstly, in order to watch the settlement condition of nanoparticles and give an evaluation on it, the stability of nanofluids was test using sedimentation and light absorption method, the ultrasonic oscillation and surfactant were considered. Secondly, the thermal conductivity of nanofluids were studied using the hot disk 2500 S thermal constant analyzer and the results were compared with DI-water. The influence of based fluids, temperature, variety and concentration of nanoparticles were analyzed. Finally, the nanofluids were used in the rectangular and fan-shaped microchannels heat sinks to investigate the convective heat transfer and the temperature distribution gained by experiment and two-phase method simulation was compared. Besides, an Image IR 3350 was used to observe the temperature distribution on substrate directly, which could reflect the heat convection improvement more visually.The results show that ultrasonic oscillation could provide a high micro-energy environment with micro-jet and strong shock which could help to broke agglomeration among particles. But with the time passing, the particle accumulation occur following the rise in temperature. Different surfactants have diverse effect on nanoparticles, so that the absorbance and thermal conductivity of nanofluids may be changed. So, the standing time should be considered to observe the settlement rate of solid particles.The addition of ethylene glycol(EG) could increase the viscosity of based fluids and handicap the settlement of nanoparticles, but the thermal conductivity of based fluids is also decreased. Compared with the Si O2, Ti O2 and Al2O3 nanofluids, Al2O3 nanofluids had the better heat conduct performance due to the difference of particle density and thermophysical property specific heat. In traditional concepts, the thermal conductivity of nanofluids will increased with the decrease of particles size, but an opposite phenomenon had been discovered because the Brown movement will be more drastic when the size is less than 50 nm. With the increasing of temperature and particles density, the particles thermal motion would be promoted, the ability of heat transfer was also enhanced, but the surfactants might lose effectiveness and the heat conduct would be decreased.Compared with the rectangular microchannel heat sink, the periodic change on equivalent diameter of fan-shaped microchannel heat sink could contributed to the turbulence. The addition of nanoparticles increased the pressure drop and the frictional resistance coefficient. Therefore the pump power was also increased. The two-phase mixture model was applied to simulate the heat transfer of nanofluids in fan-shaped microchannel heat sink, and the results were in good agreement with the experimental data. With the increasing of particles density and the volume flowrate, the temperature distribution on substrate will would be more gently flat, the thermal resistance would be decreased, and the heat transfer performance of microchannel heat sink using nanofluids was strengthened.
Keywords/Search Tags:nanofluids, thermal conductivity, microchannel heat sink, convective heat transfer
PDF Full Text Request
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