Untitled

Funder

Organizational Unit

Publications

Insights into Droplet-Surface Collisions for Aeronautical Applications

Publication . Ribeiro, Daniela Filipa Santo; Silva, André Resende Rodrigues da

The droplet impact can seem a simple subject but the originated mechanisms are really complex and still not completely understood by the scientific community. These studies are important for a large variety of areas, however, this thesis is focused on aeronautical applications, such as fuel injection in aero-engines and coatings. In this way and in order to reduce fossil fuel consumption, it is proposed to introduce biofuels in the civil aviation sector. This thesis is divided into three main sections: single droplet impact upon smooth and microstructured surfaces with variable wettability, single droplet impact upon thin liquid films, and single droplet impact upon heated smooth surfaces. For single droplet impact upon dry smooth and micro-structured surfaces with variable wettability, three perspectives of the phenomenon were acquired: top, lateral, and bottom perspective. The surface wettability was varied by applying plasma treatments, which do not alter the roughness of the surface. The structured surfaces were composed of four different arrangements of regular micro-structures. Deposition, splashing, and rebound were observed both on the impact with smooth and structured surfaces, and the results were compared with spreading/splashing thresholds proposed in the literature. The impact morphologies were identified and analyzed and the spreading phase was studied. For single droplet impact on thin liquid films, first, the universality of deposition/splashing boundaries available in the literature was evaluated. Secondly, the study focused on a seldomknown phenomenon called bubble encapsulation. In this phenomenon, after prompt and crown splash, the crown sheet walls start to bend inwards and eventually close forming an empty bubble above the liquid film. The occurrence and repeatability of this phenomenon were observed as well as their characteristics, formation mechanisms, and phenomenologies. Finally, an empirical correlation was proposed to predict the onset of bubble encapsulation. Finally, for the study of droplet impact upon heated smooth surfaces, a heating device was used to heat a thin stainless steel plate. All four heat transfer regimes were observed and it was possible to identify differences in the outcomes depending on the amount of biofuel presented in the mixture. The evaporation time was also evaluated.

2024-04-26Doctoral thesis

Open access