Conduction heat transfer characteristics and dispersion behaviour of carbon nanofluids as a function of different parameters.

Thermal conductivity enhancements in aqueous media in the presence of carbon nanotubes (SWNTs) are investigated. The SWNTs nanofluids are prepared using a two-step method. The concentration of SWNTs suspensions is below 1 wt%. The thermal conductivities of the SWNTs suspensions are measured by a modified transient hot wire method (KD2 thermal property meter). Zeta potential, turbidity measurements and sedimentation photographs have been used to characterise quantitatively colloidal stability of the dispersions. Different parameters such as mass fraction, temperature, effect of various surfactants and pH, and effect of various physical treatment techniques are discussed. The results show that SWNTs suspensions have noticeably higher thermal conductivities than the base fluid without SWNTs. For SWNTs-water suspensions at 1wt%, thermal conductivity is enhanced by 30% at 25°C. The application of a suitable surfactant (such as GA and SDS), leads to better dispersion behaviour in nanofluids and increases the settling time from 24 h to several months. At pH 9.5, a good dispersion was obtained which is attributed to charge build-up on the surface of SWNTs due to adding the dispersants (for GA and SDS). This also indicates that, among various physical treatment techniques employed in this study, the ultrasonic disruptor was the most effective method to break down the agglomerated nanoparticles suspended in base fluids. [ABSTRACT FROM AUTHOR]