Study On Natural Convection Heat Transfer Of MWCNT Nanofluids In An Enclosure

In recent twenty years, carbon nanotubes have caused widespread concern in domestic and foreign researchers because of its good thermal performance which provided new ideas and methods to find high thermal conductivity medium. In this paper, the experimental and numerical simulation study on the natural convection heat transfer characteristics of the water-based multi-walled carbon nanotubes(MWCNT) nanofluids in the closed enclosure was carried out.Using electron microscopy(Hitachi SU-70) and laser particle size analyzer(LS-230 Coulter), the particle morphology and size distribution was measured, and stability of MWCNT nanofluids were analyzed. The thermal conductivity and viscosity of MWCNT nanofluids were measured by employing the transient plane source method and rotation method respectively. And the effect of fluid temperature, MWCNT volume fraction and the incubation time on thermal conductivity and viscosity of nanofluids were discussed. Results show that: the thermal conductivity increases with the increase of temperature and MWCNT volume fraction, and decreases with the increase of incubation time; but the viscosity of nanofluids decreases with the temperature increasing, and increases with the raise of volume fraction and incubation time.Based on PID control principle and Lab VIEW2012 platform, the temperature control system was developed to realize real-time monitoring and data storage for natural convection heat transfer experiment. This system can meet the requirements of MWCNT nanofluids natural convection heat transfer experiment in the Rayleigh number range of 8.94×106~7.15×107. The experimental results show that: The intensity of natural convection heat transfer of nanofluids in an enclosure enhances with the increase of temperature difference of the hot and cold walls, but is no enhanced with the increase of MWCNT volume fraction. The main reason causing the reduction of nanofluids convection intensity is owing to that the addition of MWCNT leads the rise of nanofluids viscosity is greater than the increase of thermal conductivity.Lattice Boltzmann method was employed to investigate the natural convection heat transfer characteristic of MWCNT nanofluids in an enclosure. According to the coupled double-distribution-function LBM-TD2G9 model, a Lattice Boltzmann simulation program for natural convection heat transfer of nanofluids in an enclosure was compiled, and the accuracy of numerical simulation results were verified. The results show that: The intensity of natural convection heat transfer of nanofluids enhances with the increase of volume fraction and Rayleigh number. The heat transfer process in an enclosure at low Rayleigh number mainly depends on the heat transfer between the hot and cold walls. This simulation program is no longer applicable for natural convection heat transfer of nanofluids at higher Rayleigh number, all aspects of the physical changes need to be considered.