Srikrishna Sahu

The aim of this paper is to experimentally characterize the liquid jet breakup unsteadiness in a coaxial air-blast atomizer. The current research focuses on the measurement of the fluctuations of the jet breakup length and the flapping instability of the liquid jet, which contribute to the downstream fluctuations of the spray characteristics. The optical connectivity technique was used to measure the instantaneous breakup length of the water jet. Also, time resolved shadowgraph images of the primary jet breakup process were captured by high-speed imaging to characterize the jet instabilities at different axial locations from the atomizer exit. Experiments were performed for a wide range of air-to-liquid momentum flux ratio (M) and aerodynamic Weber number (Weg) corresponding to membrane- and/or fiber breakup mode of the jet disintegration process. The mean jet breakup length was found to vary inversely with M through a power law relation in agreement with the literature, while the breakup length fluctuations were found to first decrease and then increase with M. In order to capture the unsteady dynamics of the jet breakup process, the proper orthogonal decomposition analysis of the optical connectivity images was performed. The jet flapping and the fluctuations of the jet breakup length were identified as the second and the third spatial proper orthogonal decomposition modes, respectively, for all operating conditions of the atomizer. The amplitude and the frequency of the instabilities were measured by temporal tracking of the liquid–air interface on the shadowgraph images. The disturbance close to the injector exit corresponds to the Kelvin–Helmholtz instability, while close to the jet breakup point the jet exhibits the flapping instability, which is characterized by lateral oscillation of the jet about the atomizer axis. The influence of the liquid jet Reynolds number and momentum flux ratio on the KH and the flapping instabilities are examined.

Detail characterization of the liquid jet core in a model cross-stream airblast atomizer by application of the Optical Connectivity (OC) technique is the goal of the present paper. The unique ability of the technique in imaging the fluorescence signal exclusively from the jet core region facilitates simultaneous visualization of the liquid jet from the side- and the front-view with high background contrast. This allows measurement of some important primary breakup characteristics. The jet penetration and breakup length are obtained from the side-view images, while the front view images provides the jet width and the spreading angle. The above parameters are measured for a wide range of injector operating conditions. The measurement of mean jet trajectory and mean penetration/breakup length indicated large difference in comparison to the past studies based on experiments conducted in wind-tunnels. This is attributed to air flow confinement within the atomizer in the present case and also accurate measurement of breakup location in the OC technique. The range of the measured mean width of the jet and spreading angle indicated ’sheet breakup mode’ of the liquid jet for all injector operating conditions. Correlations are obtained to describe the evolution of different mean quantities with respect to relevant non-dimensional numbers. The fluctuations of jet breakup parameters are also measured. Some interesting correlations are obtained between fluctuations of different primary breakup characteristics which are discussed in the paper.

This paper aims to investigate the mechanism of breakup of a transversely injected liquid jet due to swirling cross-stream flow of air in a model airblast injector. The focus is especially on examining the differences in the jet atomization in the absence and presence of air swirl within the injector. Multi-directional imaging of the liquid jet core is achieved by application of the Optical Connectivity (OC) technique for simultaneous visualization of the jet core from the side- and front-view. In addition, visualization of the jet core from the bottom view is also possible. Apart from the jet penetration and breakup length (obtained from the side-view image), the present technique facilitates measurement of jet width and spread rate (from the corresponding front-view image) at the same time. The airblast injector is operated for a wide range of operating conditions. The results highlight significant influence of swirling the air flow on the jet breakup morphology as well as primary breakup parameters. The measurements provided some new insight into the physics of the primary atomization within an atomizer, where the air flow is confined, in contrast to the widely studied jet-in-crossflow in wind-tunnels. In the present case, prior to its breakup into ligaments and droplets, the liquid jet undergoes jet-to-sheet transformation. Also, the liquid sheet deflects in the direction of air swirl and disintegrates in a complex manner. The addition of swirl to the air flow leads to reduction in the mean jet breakup parameters, while the fluctuations are higher.

This paper reports application of the optical connectivity (OC) technique for visualization and measurement of primary liquid jet core in a coaxial air-blast atomizer with annular swirling air flow. Experiments were conducted for various injector operating flow conditions corresponding to Weber number (We g ) and air-to-liquid momentum flux ratio (M) in the range 80–300 and 1–8, respectively. The air swirl number, S, was varied in the range 0–0.8 using different air swirlers within the atomizer. The image processing methods for characterization of jet breakup length and jet instabilities were described in detail. Visualization of the instantaneous images of the jet core captured by the OC technique in the absence and presence of air swirl were presented. Both mean and fluctuations of liquid jet breakup lengths were calculated. An experimental correlation was obtained for mean breakup length, which was found to bear a nonlinear inverse relation with both M and S. The air swirl was found to increase the fluctuations of jet breakup length signifying higher unsteadiness in the jet breakup process. Preliminary results on characterization of jet instabilities based on identification of liquid–air interface location on either side of the injector axis on the OC images were presented. The differences due to the effect of air swirl on jet instabilities close to the injector exit and near the mean jet breakup point were discussed.