Thermal conductivity and colloidal stability of nanofluids

A nanofluid is a fluid containing suspended solid particles, with sizes of the order of nanometers. The nanofluids are better conductors of heat than the base fluid itself. Therefore it is of interest to know more about the effective thermal conductivity of such a nanofluid. In the present work, the temperature oscillation technique has been used to measure the thermal conductivity of nanofluids.; The temperature oscillation technique used to measure the thermal diffusivity of a fluid was introduced by Santucci and co-workers nearly two decades ago, yet its application is still limited, perhaps because of the perceived difficulties in obtaining accurate results. Here the author attempts to clarify this approach by first estimating the maximum size of the liquid's cylindrical volume, performing a systematic series of experiments to find the allowable amplitude and frequency of the imposed temperature oscillations, and then validating the experimental setup and the characterization method by measuring the thermal conductivity of pure water at different temperatures and comparing the results with previously published work. The purpose of this study is to evaluate the feasibility of applying this approach to measure the thermal conductivity of nanofluids.; The thermal conductivity of nanofluid was then measured and its dependence on various parameters such as particle size, particle volume fraction and temperature was investigated. It is observed that the thermal conductivity of the aluminum oxide---water nanofluid has a maximum at 20 nm particle diameter at 40°C, 55°C, 70°C and 85°C. Also, the enhancement is relatively higher at higher temperatures. The thermal conductivity increases with increasing particle volume fraction. The enhancement starts out increasing sharply at low particle volume fractions and slows down afterwards.; Due to the possibility of nanofluids replacing conventional heat transfer fluids in the near future, one needs to investigate the colloidal stability of the nanofluids, as their long-term stability is crucial for applications. A computational technique to predict the sedimentation time of a nanofluid is developed in the present work. Using this technique the author then determines that sedimentation is more severe with metallic and higher density particle material, greater particle size, higher particle volume fraction and closer to neutral electrostatic nature of the suspension.; In summary, the temperature oscillation technique to measure the thermal conductivity of fluids is characterized. Using this technique the behavior of the thermal conductivity of nanofluids is investigated experimentally. Then finally, the colloidal stability of the nanofluid was studied computationally. Thus, the present work contributes to the larger objective of using nanofluids in heat transfer applications.