High-speed 3D imaging of liquid jets, surfaces and respiratory droplets

Sprays are commonly found in, among other, combustion, agriculture and food processing. For each of these applications, the understanding of spray liquid dynamics is crucial for optimization of efficiency, accuracy, and robustness of the spray­-system in use. Sprays are also found as a collection of respiratory droplets ejected when people are speaking, yelling, coughing etc. that is one of the main transmission routes for viral disease in the recent COVID­19 pandemic. The experimental research performed on these sprays is often in 2D and not seldom on average data. However, the spray dynamics of interest acts in 3D space, during very short timescales and are stochastically unique. Here, instantaneous high-­speed 3D imaging is required to fully characterize these events.

This thesis applies and analyses three different laser­-based instantaneous high­-speed 3D imaging techniques on three different liquid dynamics. These include, (1) volumetric Laser Induced Fluorescence (LIF) imaging of liquid jets, (2) LIF structured illumination for surface 3D reconstruction of a liquid hollow cone sheet and (3) stereoscopic particle tracking velocimetry of respiratory droplets. The volumetric imaging was found to be challenging because of refractive effects at the liquid­-air interface. The structured illumination 3D reconstruction technique managed to reconstruct a transient 3D event where liquid breakups, ruptures, surface ­waves, and ejection angles were extracted. Simulations found that the used reconstruction was accurate to below 1% of the structure and could resolve small surface waves with a height up to 65% of the theoretical limit. Finally, the stereoscopic imaging extracted 3D tracks of respiratory droplets with found experimental average speed uncertainties around 0.3 m/s. In addition, this experiment enabled simultaneous estimation of speed and size of respiratory droplets that give valuable information on the risks of disease spreading.

The presented instantaneous high­-speed 3D reconstruction techniques can provide data that paves the way towards a deeper understanding of liquid dynamics in general and sprays in particular. The data is advantageous partly since it can be directly applied by modellers to improve and validate their simulations. In the future, both more validation and application of the presented techniques are required which is enabled by the open-­sourced software and data that this thesis provides.