Thermal conductivity enhancement in micro- and nano-particle suspensions

It has been recognized that the addition of highly conductive particles can significantly increase the thermal conductivity of heat-transfer fluids. Particles in the micro- and nano-size range have attracted the most interest because of their enhanced stability against sedimentation and, as a result, reduction in potential for clogging a flow system. Additional interest has been drawn by recent reports of anomalous enhancements in thermal conductivity above predictions of classical theory for suspensions containing nano-particles. In this research we report on an experimental study of the effective thermal conductivity of suspensions containing micro- and nano-particles. First, we investigated the effect of the particle aspect ratio on heat transfer in fluids. Spherical and cylindrical silicon-carbide particles were dispersed in ethylene glycol, and multi-walled nanotubes were suspended in water with surfactant. To carry out a detailed analysis, size and geometry of the particles were determined from optical and transmission-electron-microscopy imaging. Provided the volume-averaged aspect ratio was used in theoretical calculations, the conductivity of the silicon carbide suspensions was found to be in excellent agreement with effective-medium theory. Experimental data on the thermal conductivity of multi-walled nanotubes dispersions could also be interpreted in terms of the aspect-ratio dependence predicted by effective medium theory if the additional nanoscale effect of interfacial resistance was considered. The aspect ratio of dispersed particles was changed through further processing of both micro- and nano-fluids. As the result, the obtained thermal conductivities doubtlessly revealed that aspect ratio is a key factor affecting conductive heat transport in suspensions. For nanofluids, despite the promise of enhanced stability due to the nanoscale size of particles, particle agglomeration state can have a profound effect on the resulting thermal conductivity of the suspension. This was investigated by coupling thermal conductivity experiments with optical-absorbance and zeta-potential measurements of the stability of carbon nanotube-based nanofluids. An optimal surfactant-to-carbon nanotube mass ratio was found that resulted in both maximum thermal conductivity enhancement and suspension stability. Comparison of thermal conductivities for well-dispersed and agglomerated suspensions was carried out with the aid of ethanol de-stabilization. The experimental data indicated that only individualized nanotubes contribute appreciably to the thermal conductivity enhancement and, as a result, suspension stability is another essential parameter affecting the thermal conductivity of nanoparticle suspensions.