The physics of primary breakup of a liquid jet in crossflow: a study based on interfacial characteristics
Tianyi Wang, Yannis Hardalupas
Abstract
The detailed interfacial characteristics of the liquid jet in crossflow (LJIC) are studied using Optical Connectivity (OC) across a range of gas Weber numbers (14.9 to 112.6) and liquid-to-gas momentum ratios (2.1 to 36.4). The liquid jet undergoes primary breakup in three regimes: bag, multimode and shear breakup, depending on the flow parameters. OC enables the visualisation of interfacial features of LJIC from two different viewing angles, trying to reconstruct the instantaneous three-dimensional characteristics of the liquid jet surface during breakup. The strengths of the OC technique compared to conventional shadowgraphy allow more accurate characterisation of the breakup parameters and capture informative images of the cross-sectional liquid jet view, leading to a better understanding of the breakup process of LJIC. Additionally, the dynamic features of the liquid jet, including the oscillating motion and windward convective interfacial waves, are extracted using Proper Orthogonal Decomposition (POD). From the POD modes, the dominant wavelength of the interfacial instability waves varies under different flow conditions and may increase with distance from the nozzle exit. The interfacial waves are believed to be dominated by a shear-type instability associated with the liquid jet trajectory. Meanwhile, Optical Flow Velocimetry (OFV) is used to quantify the interfacial motion of the LJIC from the OC images reconstructed by selected POD modes. In this way, the velocities of the interfacial structures identified by the selected POD modes are measured. It was found that interfacial structures with different wavelengths exhibit different propagation velocities along the interface, which allow them to form constructive interference within the breakup length and generate randomly high amplitude wave structures. The formation of such structures suggests a new non-linear ‘rogue wave’ type mechanism that explains the random nature of the primary breakup of a LJIC and the deviation of the measured breakup characteristics from the classical Kelvin-Helmholtz and Rayleigh-Taylor instabilities.