Measurements on flame structure of bluff body and swirl stabilized premixed flames close to blow-off

Abstract

The structure of unconfined lean premixed CH4/air flames stabilized with an axisymmetric bluff-body and a swirler was studied for conditions far from and close to blow-off. Blow-off was approached by gradually reducing the equivalence ratio while keeping bulk velocity constant at about 20 m/s. The objective is to examine the critical regions of flame structure that initiate and develop to blow-off for relatively short flames. OH-PLIF and CH* chemiluminescence were adopted to capture the instantaneous flame front and time-averaged flame structure. Statistics of the flame root-lift-off, flame front curvature and flame surface density (FSD) were obtained. Local burning velocity was estimated based on the FSD field. The burning intensity was also demonstrated with CH* chemiluminescence. As blow-off was approaching, the flame structure was seen to be more compacted. Lift-off at the flame roots was observed to be more frequent and the lift-off height was increased at leaner conditions. The local burning intensity was significantly decreased at upstream stages including the flame root close to blow-off. The wrinkling ratio was growing obviously at downstream, with finer flame structure observed through the short and intensive probability density function (PDF) of curvatures. The enhanced wrinkling by the recirculating flow at downstream is assumed to be favorable for the flame stabilization prior to the final blow-off. This indicates minor effects of downstream on blow-off. Comparatively, upstream of the flame including the main shear layer and the flame root is more unstable and unfavorable for ignition and flame stabilization, which dominantly promotes the blow-off. The measurements in present study are useful for model validation in terms of flame structures, and also significant for exploring the blow-off behavior of shorter flames as proposed in previous studies.