Structure of Supersonic Flames Imaged Using OH/Acetone Planar Laser-Induced Fluorescence

Abstract

The general structure of a supersonic flame was identified by obtaining images of the laser-induced fluorescence from hydroxyl (OH) radicals and from acetone, which was used to mark the hydrogen fuel in the low-temperature region upstream of the lifted flame base. Results are compared to the numerical simulations of Roy and Edwards (Roy, C. J., and Edwards, J. R., Numerical Simulation of a Three-Dimensional Flame/Shock Wave Interaction, AIAA Journal, Vol. 38, No. 5, 2000, pp. 745-754). The flame properties are significantly affected by the liftoff of the flame base and by the intense fuel/air mixing in the liftoff region, both of which are driven by the supersonic coflow. The lifted supersonic flame has the following two features that are associated with premixed flames: 1) there is significant mixing of air into the fuel stream upstream of the flame base (as evidenced by the rapid decrease in the fuel marker intensity in the low temperature liftoff region); 2) OH radicals (but not fuel) exist on the centerline everywhere downstream of the flame base. This condition is significantly different from the conditions within attached jet flames, which typically contain fuel near the centerline from the flame base to the flame tip. The results indicate that precautions should be taken in the attempt to model lifted flames, so that the premixed nature of the flame base is simulated correctly.