An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of aqueous suspensions of multi-walled carbon nanotubes

Abstract

Through past research, it is known that carbon nanotubes have the potential of enhancing the thermal performance of heat transfer fluids. The research is of importance in electronics cooling, defense, space, transportation applications and any other area where small and highly efficient heat transfer systems are needed. However, most of the past work discusses the experimental results by focusing on the effect of varying concentration of carbon nanotubes (CNTs) on the thermal performance of CNT nanofluids. Not much work has been done on studying the effect of processing variables. In the current experimental work, accurate measurements were carried out in an effort to understand the impact of several key variables on laminar flow convective heat transfer. The impact of ultrasonication energy on CNT nanofluids processing, and the corresponding effects on flow and thermal properties were studied in detail. The properties measured were viscosity, thermal conductivity and the convective heat transfer under laminar conditions. Four samples of 1 wt % multi walled carbon nanotubes (MWCNT) aqueous suspensions with different ultrasonication times were prepared for the study. Direct imaging was done using a newly developed wet-TEM technique to assess the dispersion characteristics of CNT nanofluid samples. The results obtained were discussed in the context of the CNT nanofluid preparation by ultrasonication and its indirect effect on each of the properties. It was found that the changes in viscosity and enhancements in thermal conductivity and convective heat transfer are affected by ultrasonication time. The maximum enhancements in thermal conductivity and convective heat transfer were found to be 20 % and 32 %, respectively, in the sample processed for 40 minutes. The thermal conductivity enhancement increased considerably at temperatures greater than 24 °C. The percentage enhancement in convective heat transfer was found to increase with the axial distance in the heat transfer section. Additionally, the suspensions were found to exhibit a shear thinning behavior, which followed the Power Law viscosity model.