Transverse Acoustic Forcing of Gaseous Hydrogen/Liquid Oxygen Turbulent Shear Coaxial Flames

Gaseous hydrogen/liquid oxygen turbulent shear coaxial flames were subjected experimentally to both pressure antinode (PAN) and pressure node (PN) transverse acoustic forcing as a function of the gas-to-liquid momentum flux ratio. The response of the flow was recorded using simultaneous high-speed imaging of backlit shadowgraphs and OH* chemiluminescence emission. It was observed that a wave amplification mechanism, defined in Sec. III, previously observed for unforced flows, remained present in the forced flows but was modified by the forcing. The response tended to be axisymmetric for PAN forcing, where the pressure fluctuations are symmetric, and asymmetric for PN forcing, where the velocity fluctuations are asymmetric. Additional analyses performed included dynamic mode decomposition (DMD) and an analysis of the OH* wave trajectories as a function of time. Beyond the main observation that the wave amplification mechanism remained operative not only for unforced flows but also for both kinds of transverse forcing, secondary observations included a greater relative effect on the flow for PN forcing than for PAN forcing and a reduced general response of the flow to the acoustics at higher momentum flux ratios. The velocity of waves observed in the OH* emission were found to depend mainly on the shear layer velocity and not directly on the acoustics.