Mixing characteristics and emissions of strongly-forced non-premixed and partially-premixed jet flames in crossflow

The effects of forcing and partial premixing on the mixing characteristics and pollutant emissions of jet flames in crossflow (JFICF) are studied experimentally for 70% methane/30% hydrogen fuel blend and propane. For forced non-premixed JFICF, the jet fuel flow is modulated using an acoustic speaker system, which results in a drastic decrease in flame length and soot luminosity. Simultaneous planar laser Mie scattering and CH* chemiluminescence imaging is used to detail the flame structure and dynamics of the reacting forced non-premixed JFICF, whereas acetone planar laser-induced fluorescence is used to quantify the near-field mixing of the non-reacting case. Forced JFICF are characterized by periodic ejections of high-momentum, deeply penetrating vortical structures, which draw air into the jet nozzle and enhance mixing in the near-field region of the jet. The mixing enhancement effectively partially premixes the jet fluid below the flame base. Flame luminosity images and exhaust gas emissions measurements show that forced non-premixed JFICF exhibit similar characteristics to unforced partially-premixed JFICF. Both strong forcing and partial premixing result in net reductions in NOx, but increases in CO and unburned hydrocarbons. NOx scaling analysis is presented for both forced non-premixed and unforced partially premixed flames. Using flame volume arguments, EI NO x scales with amplitude ratio, but does not scale with partial premixing. The difference in scaling behavior is attributed to differences in flame structure.