Combustion of premixed fuel and air downstream of a plane sudden-expansion

Experiments have been performed to quantify the isothermal and combusting f lows downstream of a plane sudden-expansion. The detailed measurements corr espond to an area expansion ratio of 2.86 and a Reynolds number of 20000, a nd the combusting flows comprised premixed methane and air over a range of equivalence ratios with emphasis on values of 0.72 and 0.92 which gave rise to smooth and rough combustion, respectively. The results show that the extent of asymmetry of the isothermal flows was r educed by coupling the pressures between the two recirculation regions, and by imposing oscillations at the half-wave or full-wave frequency of the du ct, and by combustion. Periodic variations of flame shape, velocities, acce leration, and temperature were observed in sympathy with the dominant press ure oscillation of rough combustion, and the length of the recirculation zo nes varied from less than 0.5 to 3 step heights. Rich and lean Limits were established for combustion within the duct and, w hereas the flame blew off at the lean limit, it detached from the expansion at the rich limit and stabilised on the flange at the duct exit. Within th ese limits, there were ranges of equivalence ratios over which the flame st abilised on one of the two steps with incomplete combustion. The imposition of oscillations narrowed the range of equivalence ratios over which the fl ame could be stabilised but reduced the equivalence ratio of the lean limit at which the flame could be stabilised on both steps and the effect increa sed with amplitude and was greatest when the frequency of the imposed oscil lations corresponded to that of the half-wave in the duct. An increase in t he amplitude of flow oscillations, natural or imposed, caused the concentra tions of NOx measured at the duct exit to decrease. Active control of flows with high amplitude of oscillations produced the ex pected reductions, but not over the entire measured range of equivalence ra tio, and the imposition of pressure oscillations at the second harmonic of the halfwave frequency had a greater effect and over a wider range.