Analysis of liquid break up and droplet distribution in a flat fan spray using image processing

The study is concerned with the experimental validation of mathematical models for sprays based on the recently proposed Fully Lagrangian Approach (FLA). The FLA can track individual droplets and simultaneously compute continuous number density values along droplet trajectories. This has recently been generalised for the case of polydisperse evaporating droplets (gFLA). The current work focuses on the development of an image processing algorithm applied to microscopic, high-speed image sequences, in order to characterise the liquid break-up process and the formation of droplets and their distribution in sprays. The study takes into consideration the stochastic nature of these sprays, the data is collected in a systematic grid-wise manner, throughout the spray field and for all timeframes. The proposed method is designed to reconstruct spatial droplet size distribution required for the validation of the FLA. The experimental setup consists of a slot type nozzle, that generates a flat flan spray, and a common rail injection system capable of generating steady-state as well as pulsating sprays. The spray is visualised using a high-speed camera at 5000 Hz and a long-distance microscope fitted with an objective lens; a pulsating LED light source for backlit illumination is synchronised with the camera. The focal plane of the camera is oriented parallel to the spray sheet to ensure fluctuations in the spray are within its depth of field. In the near-nozzle area, an analysis is performed to identify and differentiate between structures in the spray using an in-house image processing algorithm. We include analysis of the mechanisms leading to the break-up and atomisation. In the downstream, dilute atomised area, the position and size of individual particles, are identified in consecutive image sequences and then extracted across all time frames. This statistical data is then used to obtain the particle size distribution in various parts of the spray.