Kambiz Vafai
University of California, Riverside
USA
Title: The anomalous behavior of nano fluids thermal properties: Do we really have an answer for this dilemma?
Biography
Biography: Kambiz Vafai
Abstract
A large number of research work related to the heat transfer enhancement using nanofluids both experimentally and theoretically was conducted by a number of investigators. Although various studies have shown that nanofluids illustrate higher heat transfer enhancement than those of base fluids, contradictory results on nanofluids behavior were also reported. Many researchers observed the phenomenon of higher thermal conductivity of various nanofluids compared to that of the base fluids. However, differences between the results were observed, i.e., some showed that the increase of thermal conductivity of nanofluids is an anomaly that cannot be predicted by the existing conventional equation; while some others showed that the increase is not an anomaly and can be predicted by using the existing conventional equation. Recently, a number of studies showed superior thermal conductivity of grapheme (single two-dimensional layer of carbon atoms bound in a hexagonal lattice structure) much higher than the value observed in all other materials (> 5000 W•m−1•K−1). This value should be verified since the measurements at the atomic scale may exhibit a large error when determining the temperature gradient across the atomic layer. While many studies in literature have reported enhanced effective thermal conductivity of nanofluids, specific heat capacity measurements have shown controversial results. The effects of temperature and nanoparticles volume fraction on the specific heat capacity of conventional nanofluids are in agreement in all studies cited in this study. Disagreement was reported for the results of the specific heat capacity of molten salt-based nanofluids. A number of studies showed an enhancement in the specific heat capacity of nanofluids using 1% concentration of nanoparticles by weight only. However, other studies have shown deterioration in the specific heat capacity of nanofluids compared with the base mixture using various volume concentrations of nanoparticles. Therefore, additional theoretical and experimental research studies are required to clarify the mechanisms responsible for specific heat capacity enhancement or deterioration in nanofluids.