A Schlieren flow visualization system has been developed for the study of the injection of jets in a supersonic cross-flow, such as used in supersonic-combustion ramjets. Both single-jet in-jec¬tion and cascaded, dual, tandem, injection have been considered in a M∞ supersonic cross-flow in a continuous supersonic wind tunnel. The study included the development of a high-performance, but cheap, light source for the Schlieren system. An earlier study has demonstrated that LEDs are suitable light sources for Schlieren imaging of supersonic flow, though LEDs have to be pushed to their limits in terms of requirements of small pulse width and high optical output. Recent developments in power Ver¬tical Cavity Surface Emitting Lasers (VCSELs) have resulted in a far more efficient light source. Small emission angles, high power and nanosecond pulses enable Schlieren imaging of un¬steady flows in the high-Mach-number regime. Though the design of drive electronics for VCSELs and LEDS are very similar, in order to obtain (much) smaller pulse widths, the de-sign needs an upgrade. The present paper demonstrates Schlieren imaging using an in-house de¬veloped driver emitting pulses of (effective) width down to 12.9 ns, which is at least 10 times faster than required for Mach 1.6 flows. The Schlieren set-up has been used for the investigation of the injection of dual, tandem, un-der-expanded sonic jets into a supersonic cross-flow. The effects of tandem dual injection on the mixing is considered as function of the jet-to-free-stream momentum ratio J and the di-men¬sionless distance S between the two jets. The Schlieren images have been used to extract the location of the upper boundary of the jet shear layer. In order to compare mixing cha¬rac-ter¬istics a three-parameter power-law least-squares fit has been developed to describe the time-averaged location of the upper boundary of the jet shear layer. The fit is able to describe the upper shear layer; however, some data strings show a dip in the penetration height, which sug¬gests that an alternative description should be defined. The calculated averages of the penetration height of the upper boundary of the jet shear layer, de¬fined as a measure for the mixing performance of the tandem-jet system, show that, for each value of the momentum ratio, there is an optimal distance between the injection orifices. It is ob¬served that for a higher momentum ratio the optimal distance between the jets increases. Spe¬cific spacings can improve the average penetration height of the jet shear layer by 30% com¬pared to that of single injection at the same momentum ratio.
|Name||AIAA Scitech 2020 Forum|
|Conference||AIAA Scitech Forum 2020|
|Period||6/01/20 → 10/01/20|