Spray Interaction with Supersonic Crossflow

Abstract

The flow residence time in a scramjet combustor is of the order of a millisecond (10−3s). High energy density liquid fuels are the energy carriers of choice in scramjet engines, however liquid fuels must be atomized, evaporated and mixed before heat release by combustion can occur. Atomization, mixing and ignition require fnite time. Therefore it is important to study spray formation and its atomization in supersonic flows. Experiments are performed to study the spray formation from a water jet injected through a plain orifce atomizer with an exit diameter (d) of 1 mm, into a Mach 2.2 supersonic crossflow. High-speed shadowgraphy is performed using high-frequency nano-pulsed LASER as well as LED, to capture the local structures and spray features. The high-speed camera and nano-pulsed LASER is synchronized using a delay pulse generator. The pulse width of the LASER is kept at 8 and 14 ns, such that to freeze the spray features in time. The bow shock profles are observed to overlap between different momentum flux ratios (q) of the injected jet when shifted to the sonic point. The penetration height of the spray is evaluated using the upper spray boundary. The average spray trajectories are compared for three different momentum flux ratios. Wavelike disturbances are observed on the windward side of spray, which further develops into ligaments. Fundamental questions like ligament origin, speed and their breakup are addressed. The ligaments are present at spray boundary and observed to move with free stream flow near the injector. Further ligaments are tracked in successive shadowgraph images using cross-correlation technique to fnd their speed. The ligament speed is also compared for different momentum flux ratio and found to vary inversely with the momentum flux ratio. The wavelength associated with ligaments or surface waves is observed to increase linearly along the spray boundary irrespective of the momentum flux ratio. The ligament breakup is characterized using shear Weber number and wavelength is used as characteristic length scale. It is found that ligaments break due to shear from free stream flow. Small shocks (shocklets) are formed ahead of ligaments which are noted to increase the residence time. Shocklets also delay atomization of the ligaments.