OH-PLIF Imaging of the Reaction Zone in Swirled, Strongly-Pulsed Jet Diffusion Flames with a Low Reynolds Number

Planar laser-induced fluorescence (PLIF) imaging of the OH radical was used to study the reaction-zone structure in the near-nozzle region of strongly-pulsed jet diffusion flames in the presence of a swirling coflow. The nominal steady jet velocity at the nozzle exit leads to a Reynolds number of Re-jet = 5000. Ethylene-fueled flames were investigated over a wide range of injection conditions and different swirl levels. OH PLIF imaging was successfully employed in these highly sooting flames by careful selection of the laser fluence and camera gate width to minimize soot-related signal interference. The reaction-zone structure, as indicated by OH PLIF, is broadly consistent with the observation of luminous flame structure for these types of flames under the same injection conditions. The overall area of the reaction zones, as indicated by statistics generated from the OH PLIF signals, is generally higher for strongly-pulsed flames compared to steady flames. In addition, the measured reaction-zone area generally increases with increased swirl level, consistent with a broader and more convoluted reaction-zone structure for flames with swirl. The root mean square of the reaction-zone area, which is broadly indicative of the degree to which the reaction zones are impacted by the local flow fluctuations, is generally higher for flames with swirl than for those without swirl, suggesting a relatively stronger impact of flow turbulence on the flame structure of swirled flames. In many cases, the reaction zones were observed to exhibit discontinuities (i.e., non-ignition of fuel) at the instantaneous flame tip in the early period of fuel injection. The discontinuities are less evident for the case of flames with swirl.