Advanced Laser Diagnostics Investigating the Fluid Mechanics of Primary Breakup

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Spray systems are widely used in various industries, from fuel injection to pharmaceuticals, and improving their efficiency requires a deeper understanding of the primary breakup process. When a liquid core exits a nozzle, it breaks up into larger, primary drops, which then further divide into smaller droplets in a process called primary breakup. However, this mechanism is the least well-understood area of spray systems, and current numerical models have to rely on significant simplifications. The lack of knowledge about primary breakup is largely attributed to insufficient experimental data of sprays at higher Reynolds (Re) and Weber (We) numbers, this is due to the high optical density in the near-field region of a spray where this mechanism occurs. Thus, making imaging the liquid-gas interfaces (LGIs) at primary breakup difficult. To address this challenge, this PhD project aims to use state-of-the-art laser diagnostic tools, specifically two-photon light-induced fluorescence (2p-LIF) and ballistic imaging. Both ballistic imaging and 2p-LIF have been successfully employed to visualise high optical density regions The efficacy of breakup can be determined by resolving the forces acting on primary drops, which can be achieved using wavelet-based optical flow (wOF) techniques. The objective of this project is to develop an accurate wOF platform to be used with advanced laser diagnostic tools to determine the velocity and acceleration components at LGIs of the primary drops. These sprays will be generated from various isolated breakup mechanisms, including shear, turbulence, and cavitation, using a specially designed spray facility. The facility will flow liquid through custom-designed nozzles at different injection pressures and will be capable of providing a wide variety of Re, We, and cavitation (Ca) numbers. The PhD project comprises three sections, with the first two laying the groundwork for the third. Firstly, the development of a wOF platform to accurately estimate the velocity and acceleration of atomising sprays will be undertaken, using synthetic data from direct numerical simulation of atomising spray databases supplied by partners at RWTH Aachen University. Secondly, a spray facility will be designed to isolate the various breakup mechanisms highlighted earlier, with various liquids and nozzle geometries. Finally, the spray facility will be used to produce mildly atomising to highly atomising sprays for each breakup configuration, visualising the primary breakup process using 2p-LIF or ballistic imaging and coupled with the wOF platform developed to resolve the velocity and acceleration components to determine the efficacy of each breakup mechanism. The project aims to address the challenge of insufficient experimental data of the primary breakup of sprays at higher Reynolds and Weber numbers, providing new insights and data for the wider spray community to further aid in the modelling of spray systems. The data from the fundamental spray facility will serve as a basis of understanding for further research in the PRIME project proposed by Dr Brian Peterson.