Phenomenological Observations of Thick Liquid Sheet Breakup Mechanisms in a Transient Flat-fan Water Spray at High Frame Rates

This study investigates the transient breakup processes during liquid atomisation, which is key for applications that require control of droplet size and velocity distributions, such as drug delivery or fuel sprays. Using high-speed imaging, the breakup of a liquid flat fan spray under controlled conditions has been observed. Water was injected vertically at 20 bar pressure and 15 ms injection duration through a slit nozzle (effective orifice diameter of 0.48 mm) into quiescent air at ambient conditions. An ultra high-speed camera (Kirana, Specialised Imaging Ltd), equipped with an industrial lens and laser background illumination were utilised to capture detailed images of the spray at various stages of the breakup process. The data was collected at frame rates ranging from 20 kHz to 5 MHz, with images of the liquid sheet, breakup, and dispersed droplets captured at different times during the injection event. The results revealed three distinct stages in the breakup process. In the first stage, a laminar liquid sheet with thick rims forms, and Squire waves develop. In the second stage, the spray stabilises, with low-amplitude wave fluctuations. In the third stage, the sheet undergoes kinematic thinning, causing instabilities and the formation of bag-like structures and perforations. These create small satellite droplets, which interact with vortical flows, moving upstream, opposite to the main flow. Some droplets glide or splash, forming capillary patterns, while others puncture the sheet, leading to further breakup. The study categorises these phenomena, contributing to a better understanding of spray breakup that can aid in the development of predictive models for improving spray atomisation system design.