Oscillatory ignitions and cool flames in the oxidation of butane in a jet-stirred reactor

Abstract

The oxidation of butane ([C 4 H 10 ] : [ O 2 ] = 1.13:1.00) has been studied over the temperature and pressure ranges 371 ⩽ T/ K ⩽ 675, 226 ⩽ P /Torr ⩽ 489 in a jet stirred reactor with a residence time of 9.4 s (1 Torr ≈ 133.3 Pa), The gas temperature and pressure were probed and phase diagrams constructed delineating regions of oscillatory ignitions and cool flames, and high- and low -temperature stationary states. On heating at an initial pressure of 400 Torr from 570 K sharp transitions were observed, first to an oscillatory ignition and then to an oscillatory cool flame region, followed by a smooth transition to a high-temperature stationary state via a supercritical Hopf bifurcation. On cooling from this high - temperature stationary state, oscillatory cool flames were observed with a sharp extinction at 542 K, without any entry to the oscillatory ignition region. The latter could be entered, however, by suddenly cooling the system from the oscillatory cool flame region by temporarily substituting nitrogen for oxygen in the gas streams. Complex waveforms, consisting of bursts of oscillatory cool flames interspersed with periods of monotonic cooling, were also observed at lower pressures. A Nd : YAG pumped dye laser was used to probe laser induced fluorescence from form aldehyde in the oscillatory ignition region. Variations in the internal surface of the reactor demonstrated the significance of surface reactions. An outline mechanism, based on detailed numerical simulations, is presented to account for the shape of the ignition profiles and the transition from multiple ignitions to oscillatory cool flames.