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The efficient dissipation of high heat loads is a major challenge in many industrial applications such as microelectronics and more recently in thermal management in electric vehicles. Spray cooling is still pointed as one of the cooling techniques with highest potential, given the large heat transfer coefficients that it can dissipate. Furthermore, strategies have been explored to further enhance heat transfer mechanisms, such as the addition of nanoparticles to the base fluid to enhance their thermal properties. However, while many researchers have focused on the effect of adding the nanoparticles in the bulk properties of the nanofluids, the actual modification in the fluid flow and in the local heat transfer processes occurring at spray/wall interactions are yet to be described. Particularly, the nature and concentration of the particles locally affect the wettability at droplet-wall interactions, thus influencing the spreading dynamics and heat transfer processes. In line with this, the present study addresses the detailed characterization of the effect of the nature and concentration of the nanoparticles in the dynamics and heat transfer processes occurring at the impact of nanofluid droplets on a solid heated surface. Gold and silver nanoparticles are dissolved in water DD in concentrations ranging between 0.1wt% and 5wt%. Millimetric droplets with a fixed initial diameter of 3mm are generated and impact on a smooth stainless steel surface with velocities varying between 0.8m/s and 2m/s. The surface is heated by Joule effect, from ambient temperature up to 120ºC. Droplet dynamics (spreading diameter, droplet height, etc) is evaluated together with the temperature field on the heated surface and with the heat fluxes exchanged during droplet spreading, using synchronized analysis of high-speed video and high-speed thermography. The results show that the heat transfer is indeed enhanced by the presence of the nanoparticles, for low impact velocities (bellow 2m/s). Instead, for higher impact velocities, the heat fluxes removed by the nanofluid particles are similar or even lower than those removed by water droplets. This is due to specific modifications in the spreading process of the naonofluid droplets, which are significant at lower spreading velocities. These modifications are partially explained by the rheological modifications in the resulting nanofluids, but also due to local wetting variations, which affect the fluid flow within the lamella and consequently the heat transfer mechanisms. Laser Scanning Confocal Microscopy will show details on these local wetting modifications.
