Acoustic emission measurements for characterizing hydrocarbon/oxygen diffusion flame behavior

An investigation was performed to characterize the behavior of a hydrocarbon/oxygen diffusion flame by acoustic emission measurements. Experiments were performed in a horizontally mounted, 2-D axisymmetric stainless steel chamber with a single shear coaxial injector and an adjustable graphite nozzle at the chamber exit for maintaining a constant chamber pressure. Methane gas flowed through the annular injector port while oxygen exited the center port. Ignition of the diffusion flame was achieved by a C₂H ₆/O₂ torch. Four acoustic transducers placed circumferentially around the diffusion flame at various locations were used in combination with a Pyrex viewing window to observe and classify the diffusion flame as either an anchored flame to the injector, a detached flame with small amplitudes of oscillation, or a near-blowout flame with large amplitudes of oscillation. Results showed that there was a noticeable difference in diffusion flame acoustic emission spectra based on flame stability behavior. Anchored diffusion flames showed no significant trace of acoustic emission at fuel flow rates below 0.65 g/s. Stable, small oscillating detached flames showed minimal amplitude acoustic levels, whereas near-blowout diffusion flames covered a wide range of acoustic frequencies. As the diffusion flame exhibited greater instability behavior, more frequency hits (a measure of the number of times an acoustic signal exceeded a preset threshold) at higher amplitudes were generated until eventual flame blowout occurred. Any increase in the (O/F)_mass resulted in an increase in acoustic frequency amplitude and absolute energy. Narrower frequency ranges at higher amplitude values also accompanied this increase in (O/F)_mass.