Size, interface and temperature effects on specific heat capacities of Cu-water nanofluid and Cu nanoparticle: A molecular analysis

Accurate estimation of specific heat capacity (C-p) is of essential importance for characterization of the heat transfer performance of nanofluids in many applications. In this study, the particle size, interface, and temperature effects on the C-p of Cu-water nanofluid and Cu nanoparticle are systematically studied. Also, the TIP4P rigid and the SPC/Fw flexible water models are compared to demonstrate their influence on the estimation of C-p. The investigation is performed by using the molecular dynamics simulation method, at the mean temperatures of 300 K, 350 K, and 400 K. The results show that the C-p increases with increasing nanoparticles size, but increases with decreasing temperature. The increase is attributed to the interface effect demonstrated by the vibrational density of state (VDOS). The VDOS mismatch is close to zero with the increase in the C-p of the Cu-water nanofluids. In contrast, there is a substantial divergence of the C-p of the nanoparticles from the theoretical values. The reason can be owing to the effect of particle size and the interaction with the surrounding water molecules. Compared to the TIP4P model, the SPC/Fw model shows an increase in the Cp. It is attributed to the intramolecular degrees of freedom that existed in the model, since they provide a small, but probably significant, contribution to intermolecular interactions. This work is helpful for understanding the enhancement mechanisms of specific heat capacity for nanofluids and the suspended nanoparticles used in, e.g., solar thermal applications.