Nanofluids have stimulated immense research interest due to their superior thermophysical properties, heat transfer features and potential applications in numerous important fields. Role of nanostructures in heat transfer mechanisms and thermal properties particularly thermal conductivity of nanofluids has been presented and relevant studies are critically reviewed in this study. Research demonstrated that nanofluids exhibit anomalous thermal conductivity (generally higher than their base fluids) which increases with the loading of nanoparticles. Despite of some findings on positive impact of agglomeration or clustering of nanoparticles on thermal conductivity, contrary findings (negative) and argumentations are still widely accepted in the nanofluids research community. Literature results showed that while cluster size increases with concentration of nanoparticles, thermal conductivity of nanofluids decreases with increasing the cluster size. However, it is not yet well-understood how to control the morphology of the clusters of nanoparticles and how do they play role in changing the thermal properties of nanofluids. Furthermore, studies revealed that the primary shape or structures of nanomaterials also influence the properties of nanofluids. Nanofluids containing nanotubes of large aspect ratio exhibit superior thermal conductivity compared to nanofluids having nanoparticles of any other shapes. Nanorods (cylinder)-laden nanofluids showed slightly higher thermal conductivity than that of nanosphere-based nanofluids. Nevertheless, the structures of agglomerated or individual nanoparticles and their nano- or molecular-level activities in the host fluids are mainly responsible for the anomalous thermal conductivity of nanofluids.

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