| Carbon nanotubes (CNTs) nanofluids refers to a new class of liquid working fluids in the heat transfer exist stably within a certain time through putting the carbon nanotubes into different groups by certain way and ratio. Due to the excellent thermal conductivity of carbon nanotubes, carbon nanotubes nanofluids has broad application prospects in many fields, such as energy, electricity, chemicals, etc. From aspects of the CNTs nanofluids preparation, stability, dispersion, thermal properties (thermal conductivity and viscosity) and convective heat transfer, the article studies on carbon nanotubes nanofluids enhancement and performance of heat transfer, laying a foundation for the research and application of carbon nanotubes nanofluids.By adding gum arabic (GA) dispersant and using the method of two-step, prepared different concentrations of CNTs/water, CNTs/glycol solution, CNTs/lithium bromide solution, Fe3O4-CNTs/water magnetic nanofluids, and formed the carbon nanotubes nanofluids that having good dispersion and stability. Through analyzing the stability mechanism of carbon nanotubes nanofluids, found that it has a close relationship with the properties of carbon concentration and the nature of base liquid, and also with the amount of gum arabic dispersant.The article measured the thermal characteristics and viscosity of different mass concentration of CNTs/water, CNTs/glycol solution, CNTs/lithium bromide solution nanofluids. Experimental results show that the addition of carbon nanotubes improves the thermal conductivity of different liquids, but it is different with the effection on the viscosity, when CNTs (diameter 10-20nm, length>5μm) at the maximum aspect ratio, the thermal conductivity of different base CNTs nanofluids increased more than 8%. The aspect ratio, straightness, mass concentration of carbon nanotubes, the nature of the base fluid are different factors affecting their thermal properties. In addition, the article analyzed the mechanism of carbon nanotubes to enhance the thermal performance of nanofluids from the aspects of brownian motion, heat radiation, particle dispersion, etc.The experiment prepared magnetic nanofluids by adding gum arabic that can be stabilized in the presence of a strong magnetic field by putting the Fe3O4 coat on the carbon nanotubes. The experiment studied the variation with the thermal conductivity of the base liquid concentration, reaction time and variation of magnetic field direction in a strong magnetic field. Experimental results show that Fe3O4-CNTs nano-particles into chains and CNTs orientation has played a leading role on increasing thermal conductivity of magnetic Fe3O4-CNTs nanofluids with increasing magnetic field strength, when the magnetic field strength was 200mT, the thermal conductivity of magnetic fluid which Fe3O4-CNTs concentration was 0.4g/L increased 9.73%; As a result of the anisotropy of carbon nanotubes, in a certain magnetic field direction, Fe3O4-CNTs thermal network chain makes a significant increase in the thermal conductivity of magnetic nano-fluid; Due to carbon nanotubes’s large aspect ratio, Fe3O4 coated carbon nanotubes can effectively reduce the Fe3O4-CNTs chain lengths in a magnetic field and a chain velocity,further improve the thermal conductivity of the base liquid.The experiment tested convective heat transfer coefficient of water-based nanotubes nanofluids that has different concentrations within the range Re=5600-7000. Experimental results show that the addition of carbon nanotubes increases inner tube convective heat transfer coefficient of the the water-based fluid, increasing the heat transfer characteristics of the water. Meanwhile, the mass concentration, flow rate, and inlet temperature of carbon nanotubes is an important factor in strengthening its convective heat transfer characteristics. When the flow rate was 160L/h, inlet temperature was 32.5℃, the convective heat transfer coefficient of nanofluid CNTs concentration was 1.2g/L increased 14.35%. |
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