Enhancement of turbulent jet diffusion flame blowout limits by annular counterflow

The effects of a proposed combustion technique, named as annular counterflo w, on the enhancement of jet diffusion flame blowout limits were investigat ed by a series of experiments conducted for the present study. Annular coun terflow was formed in a concentric annulus, in which fuel jet was ejected f rom a nozzle and air was sucked into an outer cylinder encompassing the noz zle. Three fuel nozzles and outer cylinders of different sizes were utilize d to perform the experiments. Schlieren technique and normal video filming were employed for the visualization of diverse flame morphologies triggered by the said flow. Gas samplings were taken and scrutinized by the use of a gas chromatograph. Results showed that the blowout limits can be enhanced dramatically by an increase in volume flow rates of air-suction. Mixing enh ancement is achieved with frequent and strong outward ejection of fluids fr om the cold jet when this technique is applied. The blowout limits are furt her extended when the diameter of outer cylinders becomes smaller and/or th at of the fuel nozzle becomes larger. The base widths of lifted flames were found to be narrower in the interim of annular counterflow application. Th e rates of increase in flame lift-off heights and base widths along with an increase in fuel flow velocities become sluggish when the volume flow rate s of air are increased. The amount of fuel that was sucked into the outer c ylinder was found to be negligible and trivial. A model based on annular an d coaxial jet was developed to predict the lifted flame base width and blow out limits. The coincidence between the prediction and experimental results unambiguously validates that the momentum of air-suction dominates the ben eficial effect. Copyright (C) 2001 John Wiley & Sons, Ltd.