Separation of microscale aggregates from nanofluids using a mini-hydrocyclone: Influence of flow rate, concentration, and temperature

• The 10 mm mini-hydrocyclone effectively separates silica microparticles from water and nanofluid (cut size of 4–6 µm). • The fish-hook effect is clearly observed for particles in the range 0.1–5 µm. • The presence of nanoparticles complicates the separation due to increased viscosity and altered particle–particle interactions. • The fish-hook effect intensifies with increased solid concentration and feed rate or decreased temperatures. • The hydrocyclone performance and fish-hook prominence can be optimized by adjusting operating parameters. Nanofluids have shown significant potential to advance various processes in different technological sectors, yet their applications and commercial use are hindered due to intrinsic challenges such as instability and agglomeration of nanoparticles. Given the mini-hydrocyclones potential in the separation of microparticles from liquids, an experimental investigation is conducted to assess the capability of the cyclonic separator as a micro-nano classifier, aiming to remove microscale aggregates from a nanofluid stream in any application. A mini-hydrocyclone with a diameter of 10 mm is applied for this objective. To resemble an agglomerated and unstable nanofluid, silica microparticles with a size distribution in the range of 0.5–63 µm are dispersed in silica–water nanofluid with a size distribution of 1–198 nm. The impact of different operating parameters, including suspension flow rate, microparticle concentration, and temperature, is assessed. The results are compared with the suspension of microparticles in water to determine the impact of nanoparticles on hydrocyclone performance. An overall separation efficiency of 61–79 % was achieved for separation of silica microparticles from water, and clear features of fish-hook phenomenon were observed in grade efficiency curves. Adding nanoparticles to the suspension caused a slight decrease by ∼2 % in the separation efficiency of microscale particles, while the overall separation efficiency decreased significantly. The former is attributed to the higher effective viscosity, which leads to weaker flow rotation and, consequently, a reduced centrifugal force on the particles, while the latter is due to the reduced particle size in the suspension. A higher solid concentration demonstrated a similar influence on the separation performance. In addition, it was found that a low suspension temperature and a high microparticle concentration lead to reduced separation efficiency, while increasing the feed flow rate improves separation efficiency up to a certain point, beyond which further increases result in a decline in efficiency. The range of the fish-hook effect is specified, indicating that particles in the range of 0.1–5 µm are influenced and involved in the fish-hook phenomenon. The analysis of grade efficiency curves suggests that the mini-hydrocyclone is an effective tool for separating microparticles from water and nanofluids. It stands out as a promising device for nano-micro particles classification by offering high separation efficiency for target microscale particles while allowing nanoparticles to pass along with the dispersion medium. Furthermore, operating conditions such as temperature, flow rate, and microparticle concentration affect hydrocyclone performance and the extent of the fish-hook phenomenon, and can therefore may be leveraged to adjust the separation/classification quality depending on the operational objectives.