Atomization Mechanisms: from Low to High-velocity Sprays – Cryospray

The destabilization of a liquid jet into droplets by a fast gas stream is at the heart of many applications, in particular related to propulsion (turboreactors, rocket engines). However, existing models and numerical simulations used to dimension these systems have been validated far from the conditions of applications, and are unable to describe fragmentation mechanisms in fully turbulent conditions or for low surface tension fluids. This project aims at experimentally and numerically studying jet instabilities and droplet formation in cryogenic conditions, where relevant dimensionless numbers (namely Reynolds number Re for turbulence impact, and Weber number We for surface tension role) are at least two orders of magnitude larger than in the fragmentation literature and previous laboratory experiments.

The experimental approach will be carried out at CEA SBT with liquid/gaseous helium, and at Institut Néel with liquid/gaseous nitrogen. CEA is already equipped with a suitable cryostat for the helium experiments, where few modifications are necessary. Significantly larger We, Re and dynamic pressure ratios can be reached in Helium for selected conditions. The second nitrogen cryostat will be built during the project at I. Néel, and will be more versatile, allowing for a large number of experiments at Weber and Reynolds numbers much larger than in the air water case. The nozzle geometry retained for the experiments will be defined in agreement with CNES, which has agreed to share its expertise on this project. Theses geometries will include a swirled liquid central jet, as is in LOX/CH4 spatial applications. Experiments will use a vapour overheating/liquid undercooling strategy, in order to avoid phase change issues encountered in previous works. The cryogenic spray will be characterized with advanced experimental methods: Phase Doppler anemometry, optical fiber probes and holography, methods that the present partners are familiar with.

In parallel, we will use one of the best fluid mechanics codes existing for industrial applications including two phase flows, YALES2, which has already been used with success at LEGI. We will also use the Lattice Boltzmann Method LBM (LMFA), which is totally different. The complementarity of LBM resides in the fact that it introduces modelling at smaller scales than classical fluid mechanics approaches. This method has been recently tested with success on a liquid fragmentation configuration. Note that the lower density ratio of our experiments will be favorable compared to the large density ratio limit of the usual laboratory experiments in the air water case. These codes, once adapted and validated by our new experimental data, can then be used to reliably predict fragmentation in multiphase conditions relevant to propulsion systems.

This will directly lead to optimization of atomization processes, but also to a significant decrease of the environmental impact caused by unburnt droplets (NOX emissions). Covering a much wider range of physical parameters will provide an opportunity to either unify or invalidate the various existing scaling laws proposed in models for jet instability and drop formation. In order to reach these objectives, we have constituted a consortium with experts in atomization and jet instabilities (PI at LMFA, but also LEGI), cryogenics (CEA and Institut Néel), optics (L. Mées at LMFA), experimental and numerical methods for two phase flows (LMFA and LEGI).