Role of convective acceleration in the interfacial instability of liquid-gas coaxial jets

Abstract

Interfacial instabilities play a major role in breakup events in turbulent multiphase flow. Their role has been clearly identified for two-fluid atomization, and is of paramount importance in spray formation. In planar geometries, Kelvin-Helmholtz instabilities are the main mechanism of creation of a two-phase mixing layer, and information such as wavelengths and frequencies is available in the literature. In cylindrical geometries, the instabilities quickly become three-dimensional and thorough characterization is lacking, despite a wide range of applications using coaxial atomization. We conduct an experimental study of how the interfacial instabilities of a liquid jet surrounded by a turbulent gas co-flow accelerate and develop, before break-up and spray formation. We use high-speed shadowgraphy over a wide range of gas Reynolds numbers to compute the velocity of interfacial perturbations, using Lagrangian tracking, followed by a Eulerian conditioning to obtain local statistics. We identified two regimes of the gradient of the longitudinal mean velocity as a function of the gas Reynolds number: a quadratic scaling at low gas Reynolds numbers and a linear scaling at higher gas Reynolds number. In contrast, the transverse velocity gradients show a linear scaling with gas Reynolds number throughout the studied range.

Spatial characterization of the flapping instability of a laminar liquid jet fragmented by a swirled gas co-flow

“…Once again, one can note that there are no significant changes between SR = 0 and SR = 0.25, for a constant M . The increased flapping frequency in the presence of swirl beyond a critical swirl ratio is compatible with previous findings concerning faster timescales of the interfacial instability dynamics [21]. Higher flapping frequencies in the presence of swirl were also reported recently in a wider range of dynamic pressure ratios and several liquid Reynolds numbers [17].…”

Statistics and dynamics of a liquid jet under fragmentation by a gas jet

“…In an almost identical configuration, Ref. [33] reported a change of regime in the spatial gradient of the interfacial perturbation velocity around Re g = 45 000, alternatively We g = 190. This supports 044304-13 FIG.…”

“…Note that the Weber number is defined as We g = ρ g U 2 g d l /σ in this figure . the idea that a transition in the properties of the gas flow, thus in the interface instabilities, can participate to the change of regimes in the behavior of L B . Reference [33] suggested that this transition is linked to a change in breakup regimes from membrane-breakup to fiber-type atomization. We investigate this by looking at a qualitative phase diagram, obtained by visual inspections of the breakup phenomena over the range of explored parameters.…”

Droplet breakup in airflow with strong shear effect

“…The swing breakup process is similar to the flapping breakup of coaxial liquid jets, during which the liquid jet with a coaxial airflow becomes unstable and deviates from its axis to flap (Delon, Cartellier & Matas 2018; Ricard et al. 2021). However, different from the flapping breakup in coaxial liquid jets, which originates from the shear instability, the swing breakup in this study is induced by the flattening deformation of the droplet body (the detailed comparison is presented in Appendix B).…”