A review on nanofluid stability: preparation and application

Due to excellent thermal performance and application prospects, nanofluids are attracting many researchers to pay more attention to new types of heat transfer fluids. However, previous studies have focused on the effects of nanofluids on thermophysical properties without considering the limitation of dispersion stability in practical applications. Excellent nanofluid stability is judged with the zeta potential value above 30 mV. Unstable nanofluids block the fluid flow in heat exchanger channels, which reduces the system performance by over 23 %. Based on a systematic review of improving nanofluid stability, this research discusses the preparation, characterisation, influencing factor, dispersion mechanism, and dispersion method on nanofluid stability. Four methods for improving nanofluid stability are summarised. The nanofluid is stable at pH values between 4 and 9 b y controlling its ionic concentration. The nanofluid concentration is required below 2 % to improve the repulsion between nanoparticles. The stability of the nanofluid is affected by the type and amount of the surfactant, which fails to improve the nanofluid stability at temperature above 60 °C. In addition, the molecular forces between the mixed nanofluids enhance the stability of the nanofluid. This review examines the variation patterns of nanofluid stability and the effect of stability on heat transfer. It is expected to identify some opportunities and demonstrate future challenges in both the in-lab research and the commercialisation of nanofluids.