Characterization of the reaction zone structures in a laboratory combustor using optical diagnostics: from flame to flameless combustion

Flame emission spectra and OH* imaging are carried out together with planar laser-induced fluorescence (PLIF) of OH and CH2O to characterize the reaction zone structures of three combustion modes, i.e. a flame mode, a transition mode and a flameless mode, established in a laboratory combustor. Spectroscopic measurements indicate that the invisibility of the flameless mode can be attributed to a moderate suppression of the CH* and C-2* emissions as well as a significant increase of the continuous background from the CO2* emission. PLIF measurements of OH and CH2O show that all studied cases experience a premixing of unburnt reactants with the recirculated oxygen-containing hot burnt gases in the near-inlet region. It is found that the interplay between turbulence intensity and the availability of oxygen from both the inlet air jet and recirculation is important in establishing flameless combustion in the present combustor. Consistent with a previous chemical kinetic study, the present experimental results suggested that the flameless condition is in favor of converting a highly diluted CH4/air mixture into CO and H-2, which will be further oxidized. For the flameless case, CH2O is detected slightly before the OH* region, where a relatively high level of minimal OH is observed. The reaction zone region marked by the OH* emission for the flameless case is characterized by a uniform instantaneous OH distribution with small standard deviation, suggesting a distributed oxidization of intermediate species such as CO and H-2 for this combustion mode. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.