Observations on the stabilization region of lifted non-premixed methane transverse jet flames

Abstract

Several experiments are conducted to investigate the structure of the stabilization region of lifted nonpremixed methane jet flames at high jet-to-crossflow velocity ratio. Visible emission and CH chemiluminescence from coannular-piloted flames show that the crossflow sweeps the pilot flame around to the lee side of the jet, where it ignites methane along the lee side. OH planar laser-induced fluorescence images show both diffuse and sheetlike reaction zones at the flame base and show that OH radicals reach the jet centerline often, at times spanning the entire jet width. This presents a view of the flame base as a partially premixed flame propagating through a fluctuating velocity field. The velocity field is measured in the lee and windward flame-base regions and is conditionally sampled at the instantaneous location of the fluctuating flame base. In both regions, the mean conditional velocity component normal to the mean flame base is three times the laminar flame speed, SL. This result is in general agreement with recent measurements in coflowing jet flames but is somewhat surprising because the crossflow velocity is 4.8 SL. On the lee side, the velocity component in the crosswind direction is distributed between −3 SL and 4 SL, with a mean of only 0.37 SL, clearly demonstrating that the flame front lies in a recirculation zone. Thus, it appears that low-velocity regions created by jet-crossflow interaction play a key role in flame stabilization.