Digital holography for exploring instabilities and breakup of liquid jets in supersonic crossflows

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Johnson, Joshua Antonio
Mazumdar, Ellen Y.C.
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Direct injection studies of liquid jets in supersonic crossflows (JICFs) are critical for understanding combustion in scramjet engines. Exploring these fluid dynamic interactions is not only an important step towards characterizing fundamental liquid breakup properties but also key for improving engine design and increasing efficiency. Current engine designs lack precise injector optimization and, therefore deliver inefficient fuel sprays. To remedy this, previous studies in the literature have examined how supersonic crossflows affect gaseous and liquid jet breakup characteristics using backlit imaging or schlieren techniques. In this work, we aim to study jet instabilities and droplet breakup characteristics in JICFs for the first time using digital in-line holography techniques. Experiments are conducted in a heated Mach 1.71 crossflow with a transitional regime liquid jet (slenderness ratio L/D of 19) with a diameter of 0.5 mm. High-speed and high-resolution digital in-line holography techniques are utilized to spatially resolve the jet breakup characteristics near the injection point. Results show that the front-edge instability wavelength spacing ranges from 68.3 to 104.5 microns, decreasing as the injected liquid pressure increases from 100 to 500 psi. These results show an inverse relationship between these instabilities and the injected pressure. Both windward and leeward droplet velocities and sizes are also measured using digital holography and analyzed to determine trends. Findings show a clear relationship between the liquid jet injection pressure and the velocity profile of the droplets on the windward side of the jet in the streamwise direction. Droplet size distributions showed small droplet diameters ranging from 3.8 to 25 microns. The unique experimental results acquired in this work can be used to understand entrainment effects, improve mathematical multiphase flow breakup models, optimize injector geometry, and refine future scramjet engine designs.
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