Regimes of the length of a laminar liquid jet fragmented by a gas co-flow

Atomization occurs when a high-speed gas jet breaks a low-speed liquid jet coaxial to it, forming a spray via the production of ligaments, sheets, and drops. This mechanism has received a lot of attention and different fragmentation regimes have been identified qualitatively using imaging in the vicinity of the nozzle's exit plane, as a proxy for the change of underlying break-up mechanisms. As the Weber number increases, i.e. as the gas aerodynamic stresses become larger with respect to the liquid surface tension, the liquid-gas interface suffers from instabilities that are governed by either capillarity, shear, or acceleration (or a combination of these). Several fragmentation regimes, in the low Weber number range, are typically deemed undesirable for applications such as propulsion or additive manufacturing, but no quantitative regime map is readily available. Using back-lit high-speed imaging in a multiscale approach, coaxial two-fluid fragmentation is studied in a broad range of gas Weber numbers that spans five fragmentation regimes. A fixed laminar liquid injection rate is maintained while the gas jet exit velocity is varied widely. The liquid core length, i.e. the extent of the liquid jet that is still fully connected to the nozzle, is an important metric of fragmentation, but most research has been focused on its average value in the bag break-up and fiber-type atomization regimes. The scaling laws of the first three statistical moments, the minimum value, and the correlation time of the liquid core length are reported across fragmentation regimes. The changes in scaling laws appear to be good indicators for several of the transitions explored.