High Frequency Pulsed Injection into a Supersonic Duct Flow

A study is presented of the effect of pulsation (100% modulation) of a sonic jet of helium into a supersonic (ducted) crossflow on its mixing. An injector was developed to provide a high-speed high-frequency (up to 13 kHz) pulsed jet. The injector nozzle is formed between fixed internal passages and a three- or four-sided wheel embedded within the device that rotates due to the flow in the nozzle. For a given geometry the pulsation frequency is repeatable, constant above a certain pressure ratio, and scales with the speed of sound of the gas. The pulsed jet in the crossflow was visualized in a side view by schlieren photography. The plume of injected helium was visualized in a cross section, 69 effective jet diameters downstream of injection, by seeding the helium with a small amount of ethanol, which condensed as tiny particles and illuminated with a laser light sheet. Results indicate a modest reduction in mean plume cross-sectional size (and, therefore, reduction of mixing) with pulsation and an increase in mean helium penetration. The visualization results are consistent with a transitional state between the turbulent puffs and vortex rings previously observed in low-speed experiments on subsonic pulsed jets in crossflow.