Interfacial characteristics of a perturbed liquid jet in quiescent air

Abstract

The physical understanding of liquid jet breakup in quiescent air remains incomplete due to the complex interactions among influencing parameters and limitations in current measurement techniques. In this study, a needle pin was positioned at the nozzle exit of a liquid jet pressure atomiser to introduce an artificial perturbation of controlled magnitude, enabling an investigation on the influence of flow disturbances on the breakup process. This perturbation is introduced to model potential flow disturbances that may occur inside the nozzle of an atomiser, such as liquid flow separation or cavitation. The interfacial characteristics of the liquid jet, including surface morphology and interfacial motion, were analysed to assess the impact of the imposed perturbation on the breakup process. Optical Connectivity (OC), which transmits a laser beam through the intact liquid core, was employed to capture detailed interface geometry. The instantaneous interfacial characteristics were tracked in time using Optical Flow Velocimetry (OFV) to measure the interfacial velocity. Proper Orthogonal Decomposition (POD) was applied to extract the dominant interfacial wave structures, which were subsequently correlated with interfacial motion to provide a comprehensive assessment of the perturbation effects. The consistency between the dominant interfacial geometry extracted from POD and the measured interfacial velocity further validates the reliability of the OC-OFV technique. The findings reveal that introducing artificial perturbations and adjusting their amplitude can alter the interfacial motion and geometry of the liquid jet by modifying internal flow patterns, which in turn influence the liquid breakup process and the velocity of the resulting liquid fragments. This highlights the significant impact of nozzle disturbances, such as cavitation or manufacturing defects, on atomisation performance. Moreover, the results suggest that applying controlled artificial perturbations could serve as an effective strategy for controlling the breakup process and optimising the resulting spray droplet velocity.