Experimental investigation of the flow field during combustion in narrow circular ducts

Combustion of methane-oxygen mixtures inside small diameter quartz tubes yielded cycles of ignition, flame propagation, and extinction. This phenomenon was studied with the intent of determining the flow structure that underlay the observed oscillatory motion. The fluid motion during the oscillations was visualized by seeding TiO2 micron-sized particles. Particle Streak Velocimetry was used to obtain flow-field measurements during the oscillations. In addition, acoustic emissions from full-tube oscillations were analyzed using velocity data. To my knowledge, the current study is the first attempt to experimentally determine the structure of these oscillatory reactive flows. It was established that the flow was essentially one-dimensional with little curving of the flow tube only near the flame front. Entropy generation from vorticity was negligible when compared with the entropy produced from combustion of methane in oxygen, as it was determined through an application of Crocco’s theorem. This confirmed previous computational studies indicating that flame oscillations are thermally, not fluid mechanically, driven. An acoustic wave propagation analysis also yielded a sound pressure level consistent with audio perception during flame oscillations with acoustic emissions