Stability of nanofluids – a critical review

Removing high heat fluxes from small volumes is one of many problems that came up with the developments in nanotechnology. Nanofluids as suspensions of nanoparticles in base fluids are suggested for use in heat sinks, heat exchangers or even in complete refrigeration cycles for a variety of applications including cooling of car engines, laser systems, and electronic and optical devices. The theoretical and experimental studies on nanofluids in the literature are mainly focused on nanofluid thermal conductivity and viscosity which directly affect heat transfer and pressure drop characteristics. The influence of parameters such as the concentration, temperature, nanoparticle and base fluid material on the thermal and hydrodynamic performance is also examined extensively. Although significant improvement is reported in the nanofluid heat removal performance, the nanofluid stability issues remain unsolved. The enhanced thermal conductivity of nanofluids may decrease with elapsed time due to the decrease in dispersion stability. Nanoparticle settlement causes the nanofluids not to preserve their superior thermal characteristics. In addition, a decrease in dispersion stability may damage system components as it causes clogging on the channel walls. Hence, the long term stability of nanofluids is critical for sustainable heat transfer augmentation, and enlarged application areas. In this study, a state-of-the-art review on the stability of nanofluids with emphasis on thermal and hydrodynamic performance is presented. Stability assessment techniques and stability indicators are discussed in detail. Some future prospects are given as guidelines for contemporary researchers to take the lead in systematic research in this field.