Experimental investigation of flame propagation in long, narrow tubes
Combustion of premixed propane-air flames propagating in horizontal quartz tubes of small diameter (6.4 mm) was studied using high-speed photography and Particle Streak Velocimetry (PSV). Several regions of propagation were established, including an initial region of uniform propagation, a subsequent region of vibratory propagation characterized by high-amplitude axial flame oscillations and acceleration of the flame, and regions of small-amplitude oscillations during which the net propagation was nearly uniform. The regions of highest instability and acceleration were found to occur most prevalently in stoichiometric and slightly rich cases (with fuel-air equivalence ratios of 1.0 and 1.2, respectively). The effect of tube length was also studied, using lengths of 59.1 cm and 104 cm. It was found that after a period of stable, uniform propagation, every case would inevitably transition to an unstable, oscillatory mode of propagation, with the exception of lean mixtures in the shorter tube, in which flames propagated uniformly to the end. Flame structure and phenomenology were also investigated. The flames in all cases began by assuming a stable, parabolic shape during uniform propagation. During regions of instability and oscillation, a large variety of flame morphologies was observed, including highly elongated shapes, partial or complete inversion of the flame front, and flames which lost a coherent shape altogether. Finally, flow visualization was accomplished by seeding the test section with aluminum oxide (Al2O3) particles, in order to investigate the flow structures produced by the propagating flame. It was definitively established that flames propagating in tubes do produce a flow in the unburned gases ahead of the flame front. Quantitative velocity data were extracted using Particle Streak Velocimetry, which revealed that the nature of this oscillatory flow produced by the flame was unsteady and possibly 3-dimensional, defying characterization as a fully-developed laminar flow.
Laminar flames, flame propagation in tubes, flame oscillations, particle streak velocimetry, flow visualization
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign