Experimental investigation of heat transfer enhancement using ionic liquid-Al2O3 hybrid nanofluid in a cylindrical microchannel heat sink

Recent research on the preparation and application of nanofluids focuses on requirements such as durability, even and stable suspension of particles, no chemical change of particles or fluid, and negligible agglomeration of particles. Having such properties is more important for nanofluids employed in miniature heat transfer devices like microchannels, which are more likely to be blocked due to the small size of the channels. The present study aims to investigate the thermal performance and stability of a novel nanofluid containing ionic liquid-alumina nanohybrids. For preparation of the hybrid system, Al2O3 (alumina) nanoparticles at different sizes (20, 50, 80, 135 nm) were coated by ionic liquid [BMIM]PF6. Then, the hybrid particles (at nominal sizes of 20, 50, 80, 135 nm) were dispersed in water to form stable nanofluids to be used as the working fluid in a cylindrical microchannel heat sink. The stability of the as-prepared nanofluids were evaluated by zeta potential analysis and sedimentation photograph. The particle size distributions were assessed through dynamic light scattering technique. It was demonstrated that the hybrid nanofluid can enhance Nusselt number up to 26% at 50 nm relative to water. It was also found that in the Reynolds range of more than 700, the ability of the 20 nm nanohybrid to enhance Nusselt number increased, while the 50 nm nanohybrid appeared even weaker than water. The results of this research were also compared with that of alumina nanofluid previously tested in the same device, based on the alumina particle size. At 20 and 50 nm, thermal performance of the hybrid nanofluid was better than that of alumina nanofluid such that by using hybrid nanofluid, Nusselt number was almost doubled compared to alumina nanofluid at 50 nm. But at other sizes which were beyond the nanoscale range (>100 nm), Nusselt number of the hybrid system was lower than alumina nanofluid. Finally, thermal performances of alumina nanofluids and hybrid nanofluids at 20 nm and 50 nm were re-tested after 500 min of operation in the flow loop and the results were compared with the initial conditions. It was found that the alumina nanofluid has been faced with a stability reduction while the hybrid nanofluid was rather stable during this time period. This study reveals that ionic liquid-alumina hybrid nanofluids can be a suitable option as a working fluid especially in small scale heat transfer devices thanks to their high stability, durability and facile preparation method.