Near-limit behavior of high-Lewis number premixed flames in tubes at normal and low gravity

Abstract

The high-Lewis number (Le) diffusive-thermal instability in premixed gas combustion is studied experimentally in tubes using lean C4H10–O2–He mixtures (Le≊3.0). Two self-excited instabilities are observed for flames propagating in a tube open at the ignition end and closed at the other end: (1) a combined spiral wave and radial pulsation and (2) a pure radial pulsation. These modes are observed at both earth gravity (1 g) and microgravity (μg) and therefore are believed to be a direct manifestation of the diffusive-thermal instability rather than a buoyancy-induced instability. Moreover, the frequencies of the observed modes are lower than the fundamental acoustic frequencies of the tubes which renders unlikely the possibility that they are acoustically-induced instabilities. A critical fuel concentration for the onset of the combined spiral wave and radial pulsation, as well as the pure radial pulsation is found. The transition between spiral waves and radial pulsations is also observed. These observations may be analogous to the propagating chemical wave and rotating spiral wave solutions found in reaction–diffusion systems other than premixed gas combustion.