Flame-Acoustic Interaction

in Turbulent Diffusion Flames

A. Ghosh1, and K. H. Yu¹

Even after many decades of research, problems of combustion instabilities still remain unsolved in rocket propulsion.  As these problems are often encountered in a late stage of system development, they can be potential “show-stoppers” presenting possibly serious risks associated with chemical propulsion system.  This investigation is motivated by the desire to understand the basic physics of the transverse mode instabilities that could affect large liquid rocket engines, particularly in the vicinity of the injector element. A sectional model of the shear-coax injector flowfield was constructed and hydrogen-oxygen diffusion flames were established over both laminar and turbulent flow ranges.  Transverse instabilities were simulated by controlled acoustic forcing across the flow direction at various frequencies.  Under certain frequency settings, it was observed that significant flame-acoustic interaction could occur leading to large-scale flame movement.  Some of these cases were characterized in detail using OH* chemiluminescence and schlieren images that were obtained under phase-locked conditions.  In general, it was observed that low frequency forcing had minimal effects on the flame wrinkling, while the high frequency case showed large movement of the flames including periodic wrinkling of the flame surface.  The results shed a new light into possible interaction and coupling mechanism in a near field of shear-coaxial injectors.  In particular, density gradient appears to play an important role.