Effect of Excitation on Supersonic Jet Afterburning

Abstract

High-temperature Mach 2 jets that consisted of ethylene-oxygen combustion products diluted with nitrogen were used in an experimental study of supersonic jet excitation. Afterburning reaction was observed under certain conditions in which excess fuel was present, as a result of the fuel-rich gas mixing with fresh oxygen in the surrounding air. By fitting an adjustable-geometry cavity at the nozzle exit, the jets were passively excited at various frequencies via a flow-induced cavity resonance mechanism. Mie-scattering flow visualization and chemiluminescence measurements were used to measure the response of the excited jets under both afterburning and non-afterburning conditions. The effects on supersonic mixing and afterburning reaction were quantified in terms of changes in initial shear layer growth rate and chemiluminescence from the flames. Three cases having similar compressibility conditions but with different exit temperature and composition were investigated ; the shear-flow convective Mach numbers ranged between 1.3 and 1.4 for all cases while the exit temperature ranged between 1300 K and 1510 K. Mie-scattering images revealed that the excited shear layers in the near-jet region rolled up into organized large-scale structures, which are extraordinary at such a high convective Mach number. The dynamics of the large coherent structures was manipulated by varying the excitation frequencies. The results showed that flow excitation increased the initial shear layer growth rate by an amount that was sensitive to excitation frequency. For the present compressibility condition, the increase of as much as 50 % was observed when the frequency was at or near the jet preferred mode. However, the net effect on afterburning reaction was determined by a delicate balance between afterburning suppression due to large-scale cold-flow entrainment that caused quenching and afterburning enhancement which was a result of faster mixing. The results showed that global emission from the afterburning flames could be either reduced or increased from the natural state by as much as 80% depending on the excitation frequency and flow conditions. While the effect of excitation was strongest when the frequency was within the jet-preferred-mode bandwidth, the afterburning reaction in general appeared to be intensified at higher frequencies.