Thermal and rheological behaviour of stearate-based phase change nanofluids

•The first-order solid–liquid transition of PT8 is not significantly altered by the dispersion of different nanomaterials.•Reductions in sub-cooling for GnP/PT8 NePCMs are higher than for MgO/PT8 ones.•Thermal diffusivity enhancements are higher for GnP/PT8 than for MgO/PT8.•MgO/PT8 nanofluids are viscoplastic (solid at rest), while GnP/PT8 are viscoelastic (liquid at rest).•The fractal structure of MgO forms a strong gel network; the sheet-like shape of GnP favour liquid-like behaviour. A comprehensive study has been performed on the thermal and rheological behaviour of two sets of isooctyl stearate (PureTemp 8, PT8) based nanofluids with different concentrations of spherical MgO nanoparticles (up to 15 wt%) and graphene nanoplatelets (up to 2 wt%). The results show that the presence of the nanomaterials does not significantly alter the phase change temperatures of the base fluids, although the latent heat decreases by 27% and 7.6% for 15 wt% MgO and 1.0 wt% GnP-based nanofluids, respectively. Both nanomaterials are found to increase the thermal diffusivity of the nanofluids. GnP-based suspensions have a higher thermal diffusivity than the MgO ones at similar nanomaterial concentrations. Rheological analyses show that MgO-PT8 nanofluids behave as a viscoplastic material (solid at zero shear rate); whereas the GnP-PT8 samples are viscoelastic (liquid at zero shear rate). MgO nanoparticles promote the formation of large fractal agglomerates, leading to a strong gel network of particles within PT8, while platelet-shaped GnP gives a fluid-like behaviour.