Effect of nanoparticles concentration on the characteristics of nanofluid sprays for cooling applications

This study addresses the effect of nanofluid synthesis on the rheological properties of the resulting fluid and their consequent effect on the characteristics (size and velocity distribution of droplets, spray cone angle, etc.) of the sprayed nanofluids. The results are discussed in the light of how the spray characteristics affect the use of the resulting nanofluid spray for cooling purposes. Nanoparticles of alumina (Al2O3) and zinc oxide (ZnO) are mixed in water-based solutions, for concentrations varying between 0.5% and 2 mass% for alumina and between 0.01% and 0.1 mass% for the zinc oxide particles. FeCl2·4H2O (0.1 mass%) was also used to infer on the effect of the nature (material) of the particles in the physicochemical properties of the resulting solutions. Among the various surfactants tested, citric acid (0.15%) was chosen for the final working mixtures, as it assured a stable behaviour of the solutions prepared during the entire study. The nanoparticles were characterized in detail, and the physicochemical properties of the fluid were measured before and after atomization, to evaluate any possible particle loss in the liquid feeding system or retention in the atomizer. The nanofluids were sprayed using a pressure-swirl atomizer at 0.5 MPa injection pressure. Droplet size and velocity in the spray were probed using phase Doppler anemometry. For the range of experimental conditions covered here, the results show that liquid viscosity is an important parameter in predetermining the spray characteristics of nanofluids, as it affects the primary liquid breakup. Despite this, only a mild increase is observed in the nanofluids viscosity, mainly for higher concentrations of alumina, which was not sufficient to significantly affect the spray characteristics, except for a small decrease in the spray cone angle and the size of the atomized droplets. Hence, for cooling purposes, the atomization mechanisms are not compromised by the addition of the nanoparticles and their using is beneficial, as they enhance the thermal properties without a significant deterioration of other fluid properties such as viscosity and spray characteristics. Present spray characteristics promote liquid adhesion to the cooling surfaces and droplet size and velocity are kept within a range that is appropriate for spray cooling, following the literature recommendations and our analysis