A critical investigation of the anomalous behavior of molten salt-based nanofluids

A critical investigation is presented in this work to study the effect of nanoparticle addition, temperature, and nanoparticle size-dependence on the specific heat capacity of both conventional and molten salt-based nanofluids. The effects of temperature and nanoparticle volume fraction on the specific heat capacity of conventional nanofluids are in agreement in all studies cited in this review. Different correlations based on the available data were developed as a function of temperature and volume concentration only. However, the effect of nanoparticle size-dependence was ignored in these correlations. A general correlation for Al2O3–water nanofluids, one of the most commonly studied nanofluids, that takes into account the effect of temperature, volume fraction, and nanoparticle size-dependence was developed and verified in this review. 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. Moreover, very few studies have demonstrated the effect of nanoparticle size-dependence on the specific heat capacity of molten salt nanofluids and disagreement in the results was reported in these studies. Few models based on the conventional specific heat model were developed to determine the specific heat capacity of molten salt nanofluids. These models suffer from the lack of knowledge of many terms in these equations which make them impractical. Different mechanisms were assumed in the literature to explain the abnormal behavior of molten salt nanofluids. Additional theoretical and experimental research studies are required to clarify the mechanisms responsible for specific heat capacity enhancement or deterioration in nanofluids.