Effect of ignition position on the combustion instability of premixed methane-air in a semiopen duct

Abstract

Experiments were conducted in a semiopen chamber to investigate the combustion instabilities of methane-air mixtures. The effect of ignition position (IP for short, i.e., the distance between ignition source and the closed end of a channel) on the characteristics of the pressure waveform of premixed stoichiometric methane-air mixture combustion is investigated in detail. The results reveal that the pressure waveforms demonstrate periodic oscillations when the ignition position is at the half-part of the duct on the side of the open end. The maximum pressure for the oscillating cases is 2.38 times that for the nonoscillating cases, and the maximum rate of pressure rise for the oscillating cases is one order of magnitude higher than that for the nonoscillating cases. The instant of burnout of the methane-air mixture escaping earlier from the open end coincides with the onset of the periodic oscillation. Then, the pressure oscillation is amplified with the exponentially growing amplitude through the flame-pressure interaction. The oscillation amplitude increases as the ignition position shifts toward the open end of the duct. The flame trajectory shows an intimate link with the pressure profile. The oscillating flame velocity precedes the oscillating pressure signal by a 180° phase shift. It is corroborated that the oscillation period depends on the length of the unburnt gas column located between the closed-end and the flame front, irrespective of the ignition position. The pressure oscillation amplitude is a linear function of the frequency. The growth rate of pressure oscillation is calculated through the velocity coupling mechanism. It is shown that the experimental growth rate is compatible with the calculated growth rate for premixed methane combustion. Front ignition produces the highest acoustic energy and thus a higher maximum combustion pressure.