How Nanofluids Influence Droplet impact on a Solid Substrate and Surface-to-Droplet Heat Transfer

The aim of this thesis is to understand the effect of added nanoparticles on both the hydrodynamics and heat transfer of droplets impacting on solid substrate. Therefore, in the first part, nanofluid drop impact onto a smooth sapphire substrate is experimentally investigated over wide ranges of Reynolds (100 < $\Re$ < 10000) and Weber (50 < $\We$ < 500) numbers for three \Al~nanofluid mass concentrations of 0.01~wt.\%, 0.1~wt.\%, and 1~wt.\% using high-speed photography in order to study the effect of nanoparticles on droplet spreading and spreading-to-splashing transition. In the final part, another set of experiments is conducted with \Ti~nanofluids with the mass concentrations of 0.2~wt.\%, 0.5~wt.\%, and 1~wt.\% using both high-speed photography and high speed thermography. The nanofluid properties density, viscosity, surface tension, thermal conductivity, stability, and particle size distribution are experimentally characterized and used to describe and model both parts of this work.

In the first part of the thesis, a new maximum spreading model based on energy balance is introduced with a new model of maximum spreading time incorporating fluid viscosity. A large portion in the $\We$ vs. $\Re$ map is covered with seven different concentrations of aqueous glycerol solutions. This new maximum spreading model is compared to others from the literature to evaluate the effect of nanoparticle existence in the corresponding base fluids. Afterwards, the effect on spreading-to-splashing transition is evaluated. It is demonstrated that even a small fraction of nanoparticles in fluids affects the splashing behavior of a droplet upon impact on a smooth surface. Nanofluids influence this transition boundary by promoting splashing at low Reynolds numbers. We explain this behavior through the increased lamella spreading speed and lift during the lamella spreading stage. Finally, we develop an empirical correlation that describes the splashing threshold dependence on nanoparticle concentration for the first time.

In the second and final part, we investigate the effect of \Ti~nanofluids and their concentrations on the early stages of droplet spreading on a hot surface below boiling point (80~\deg). For this purpose, water and three water-based \Ti~nanofluids having different concentrations, i.e., 0.2~wt.\%, 0.5~wt.\%, and 1~wt.\%, are tested at four impact speeds on a smooth TiAlN-coated sapphire substrate. In order to achieve systematical results to see the effect of only nanoparticle existence in the fluid, the thermal and rheological properties of the nanofluids are experimentally characterized and considered in the calculations. Experimental results show that the nanofluids show marginally smaller spreading than in the case of water. The cooled area also does not change with the nanoparticle addition. Moreover, the temperature decrease at the impact point is also independent of both impact speed and nanoparticle concentration. Furthermore, the nanofluid with the largest concentration (1~wt.\%) yields less than 9~\% enhancement in the average cooling performance during the first 5~ms of the droplet spreading, which barely passes the uncertainty level ($\approx6~\%$) of the measurements.