Local characteristics of fragments in atomizing sprays

Dual-angle particle tracking velocimetry (PTV) developed previously by the authors Pham et al. (2017) has been extended in this study to simultaneously measure volume and velocity of both irregular and sub-ranged spherical fragments generated in the near-field of atomizing sprays, enabling measurements of local Weber number. Dual-angle PTV is an imaging based technique where fragment morphologies are captured using two cameras oriented at 90 degrees to each other. Both cameras operate under PIV (particle imaging velocimetry) mode to enable tracking of fragment locations. In this contribution, fragments are sub-ranged, depending on their size, into 3 different groups: larger fragments (d10 > 150 μm), medium fragments (150 μm > d10 > 15 μm) and small droplets (d10 < 15 μm). The dual-angle PTV technique is used to track the larger fragments only given the difficulty in tracking the medium and small counterparts between two viewing angles due to spray dispersion. The medium and small drops, being ellipsoidal-like shapes, are imaged with a single-view under PIV mode. Combining the dual-angle PTV (3D) and single-view processing (2D) approach has enabled one of the first direct measurements of the local Weber number of arbitrarily shaped fragments. The results and the associated analysis suggest that larger fragments in the near-field zone generally do not deviate significantly from a nominal square root of local Weber number of 0.8. This is in direct agreement with the Direct Numerical Simulations (DNS) of Shinjo and Umemura (2010) and the experiments by Sallam et al. (2002) which employ pressure atomization. Using local Weber numbers has generally been absent and the results here shed more light into air-blast atomization as well as standard pressure atomization, where shear is relevant.