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This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute. The current work experimentally investigates the thermal and electrical conductivities of nanofluids containing graphene sheets that have very high thermal conductivity. Here, the graphene is prepared from natural graphite by oxidation-reduction process through a single step method. The graphene nanofluid thus obtained exhibited greater stability even after six months of preparation without addition of any surfactants. The detailed characterization process involving TEM, UV absorption and DLS measurements revealed the well dispersed nature of nanofluid with sheets appropriately interconnected and entangled. The DLS measurement indicates a trimodal size distribution of graphene sheet ranging from 5nm to 1500nm. It was also found that the absorption peak of the sample was 269 nm. This reveals the complete reduction of graphene oxide to graphene and the value is in good agreement with the literature. The thermal conductivity is measured using the traditional Transient Hot Wire (THW) method and enhancements are substantial even at lower concentrations while such behaviour is not predicted by the classical Maxwell theory. The thermal conductivity of graphene nanofluids are measured for different concentrations of 0.01 - 0.2 volume % at different temperatures. It is observed that the thermal conductivity increases with increase in concentration of grapheme, which is as expected. The maximum enhancement obtained is 27% at 0.2% concentration. The enhancement also shows a strong temperature dependence which is unlike that of its carbon predecessors like CNT and graphene oxide nanofluids. Electrical conductivity is measured using a 4 cell conductivity meter with inbuilt automatic temperature compensation. Electrical conductivity enhancement is found to be linear with increase in graphene volume fraction. |
