The Role of Local Base Cavities in an Augmentor Bluffbody Flameholder

Abstract

This study consists of two parts: liftoff and blowout studies. In the liftoff study, the improvement of liftoff height of bluffbody stabilized partially premixed methane flames and the change of flow field in the recirculation zone of bluffbodies, of variously modified base geometries, are investigated in a high temperature (~ 1300 K) vitiated flow. The basic geometry of the bluffbody consists of a two-dimensional rectangular body with a rounded nose with fuel jets being discharged from the body at several locations upstream of the base. Flame liftoff height measurements are characterized by CH chemiluminescence, while the three-dimensional flow field is determined using stereo particle image velocimetry (PIV). The lowest liftoff height is observed when the geometric modifications from the original rectangular bluffbody base are carried out such that the base has three-dimensional local cavities together with two-dimensionally modified geometries. PIV measurements show that multi-dimensional vortex structures and intense recirculations are induced in the presence of the two and three dimensionally modified base. In the second part of the study, the improvement of the lean blowout limit of bluffbody stabilized methane flames is investigated. The flame configurations consist of fully and partially premixed flames and a hybrid combination of the two. The hybrid flame is produced by injecting methane jets from the streamlined shaped bluffbody into a fully premixed methane/air crossflow. We observe that the blowout limit of the hybrid configuration is extended by up to ~12 % (in terms of the equivalence ratio of the crossflow) with the modified geometries. No noticeable improvement was observed in the fully and partially premixed flame configurations. Gas chromatographic sampling and particle image velocimetry (PIV) show that high fuel mole fraction regions coexist with regions of low speed flow for the modified geometries. Further PIV analysis shows that the downstream flow fields of the modified bases generally have a larger number of incoherent vortices and lower strain rate in comparison with those of the unmodified base.