The role of gas turbulence in liquid fragmentation – FragTurb

The fragmentation of a liquid phase by a gas flow happens through a cascade of coupled mechanisms. This process is ubiquitous to engineering applications that aim at producing a high-quality spray (cloud of droplets) and can be found in many situations in nature. Beyond a lack of understanding of the couplings, the role of the gas turbulence in these mechanisms has not been studied in detail. The project will study gas-assisted fragmentation at variable gas turbulence intensities over a wide range of operating conditions. The role of turbulence and the couplings in this ill-understood cascade of mechanisms will be established. Work packages (WP) 1-3 will study the liquid destabilization, the initial detached liquid structures, and the final droplet populations respectively. WP4 will work to identify relevant non-dimensional parameters for different regimes to extract physics, scaling laws, and models pertaining to the fragmentation processes. Two atomization configurations, a planar and a coaxial atomizer, will be used to study the fragmentation cascade with and without large-scale instabilities. In addition, a fundamental experiment will be developed to study the fragmentation of liquid sheets and drops in a turbulent box, from gas flows with low turbulence to flows where the turbulent fluctuations are solely responsible for the break-up. This will establish a unique framework to decipher the coupled physics of fragmentation and the critical role played by turbulence in that context. Comparisons between both atomization configurations and the turbulent fragmentation experiment will be performed to inform the extraction of the physics of individual mechanisms involved in the fragmentation cascade as well as their couplings.

Due to the strong dependence of scales with operating parameters, back-lit imaging will be used with temporal and spatial resolutions varying in a wide range. The inherent multiscale nature of the process will also be tackled by multi-resolution approaches using ultra-high-speed cameras and variable magnification lens arrangements. In addition, X-ray high-speed radiography will be leveraged to study the dense two-phase flow formed by turbulent gas-assisted fragmentation, complementing visible light measurements. The final droplet populations will be studied with a combination of tools, namely laser interferometry, Doppler optical phase detection probes, and imaging. Existing predictions and scaling laws are often contradictory since they are only based on one or a few of the fragmentation mechanisms. This project makes it possible to fully investigate the full process and the broad range of temporal and spatial scales involved in this turbulent two-phase flow, relying on a multiscale, multi-method experimental approach.

The project will provide a comprehensive open-source database of fragmentation metrics that can serve for numerical validation and aid in the development of future experimental investigations. The improved understanding of turbulent liquid fragmentation will lead to new reliable scaling laws and advanced physical models that will provide a faithful description of the turbulent fragmentation processes for numerical simulations. These tools could then be coupled with the available knowledge of the remaining physics involved in fragmentation applications, as well as open new pathways for the implementation of control strategies for fragmentation. This will not only increase process efficiency, but also ensure better reliability, with economic, environmental, and safety impacts. Additionally, spray feedback control is a necessary tool for applications such as propulsion, where it offers a way to ensure engine restart at high altitude in the case of a failure.