July 27th, 2017

Droplets, bubbles and particles

        Many industrial applications, as well as many common situations in our daily lives, are characterised by the presence of gas and liquid interacting with each other. This is the case for example, with  rain droplets, ocean waves, fuel being injected in an engine, and with aerosols inhaled by patients   in hospital. The shape and the volume occupied by the two phases can be described through the evolution of the interface. The movement of the interface is quite a fascinating  problem, and involves several aspects of fluid motion, from inertia and compressibility forces, that can be described at the “macroscopic” level, with the molecular interactions at the interface resulting in surface tension forces and curvature. If the interface develops on a solid surface, the forces at the contact line between the solid, gas and liquid, become a critical aspect  for the overall evolution of the gas and liquid phases, determining the wettability of the surface. This is a relevant topic for atomising systems, and in particular the prefilming airblast injectors typically used in aero-engines applications. In these injectors, the quality of the fuel preparation largely depends on the capability of placing the fuel in the right position. All  aspects related to the surface tension and contact angle are fundamental to determine the evolution of the liquid film over the prefilming surface. The breakup of the liquid into tiny droplets is eventually achieved through the interaction with gas streams. Both the velocity of the air flows and the surface tension of the liquid are relevant parameters for the final size distribution of the cloud of droplets that is generated.

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