Perforation induced ligament evolution in low-temperature swirl spray via off-axis holography

Abstract

Ligaments generated from a perforated sheet are commonly observed in the primary breakup process and have a fundamental influence on spray characteristics. Ignition and combustion instabilities of aeroengines at high altitudes raise the demand for dynamic measurement of ligaments in a low-temperature spray. Picosecond pulsed digital off-axis holography (DOH) system with 65 kHz high-speed and 4.5μm equivalent pixel size high-resolution setups is established. Three-dimensional (3D) evolution of ligaments in a pressure swirl spray under fuel temperature of 245–250 K and fuel pressure of 0.2–0.8 MPa is characterized. The stringy object extraction method optimized by FMCs cross correlation algorithm successfully extracts a 3D arc-shaped ligament in the spray at 0.2 MPa. The transition of ligaments into droplets during two modes of sheet perforation as hole–edge and two-hole interactions at 0.5 MPa is quantified with time resolution up to 15.4μs. Both the nucleation and growth of perforations are found involved with surface waves, and their early diameter growth velocities remain stable in the short term, consistent with Taylor–Culick velocity characteristics. The 3D velocity of a pair of ligaments in the spray at 0.5 MPa is obtained based on the spatial displacement of their daughter droplets within double frames, indicating a downward helical movement trend after separation from the conical swirling sheet. This work demonstrates DOH a powerful tool for 3D morphology extraction and dynamic analysis of ligaments and perforations in the pressure swirl atomization scenario.