Simultaneous multi-species and temperature visualization of premixed flames in the distributed reaction zone regime

Structures of turbulent premixed flames, operating in the thin and distributed reaction zone regimes, were investigated for stoichiometric premixed methane/air jet flames with jet Reynolds number up to 40,000 and corresponding Karlovitz number up to 286. Multi-species planar laser-induced fluorescence with high spatial resolution was applied to simultaneously image combinations of CH/OH/CH2O and HCO/OH/CH2O. In addition, OH/CH2O imaging was performed in combination with simultaneous Rayleigh scattering thermometry. The CH and HCO layers showed progressive broadening along the axial distance for flames with Reynolds number above 21,000 and the corresponding Karlovitz number above 126. At Reynolds number 40,000 and the corresponding Karlovitz number of 286, a mean CH layer thickness more than 10 times larger than that under laminar condition was observed, providing a clear experimental evidence of distributed reaction zone owing to turbulence/flame interaction. Additionally, spatial correlations between species show that OH and CH2O locate at mutually exclusive regions. In contrast, both CH and HCO can overlap substantially with CH2O. The regions of strong CH/HCO signals correspond to regions with weak CH2O signals. Moreover, CH and HCO are shown to be able to penetrate deeper into the OH layer than CH2O. Regions where CH and HCO appear distributed show a rather homogeneous temperature distribution with reduced maximum temperature compared with non-distributed conditions.