On the Hysteresis Phenomenon of Turbulent Lifted Diffusion Methane Flame

This paper reports an experimental investigation on the flow characteristics upstream of a lifted turbulent diffusion flame in the presence of a coflow. Three fuel nozzles made of a long pipe with different outlet geometry were examined. One pair of these nozzles has the same orifice diameter but different normalized lip thickness, and another pair has the same normalized lip thickness but different orifice diameter. The strength of the co-airflow was also varied to assess its impact on the liftoff height of the jet diffusion flame. Previously published studies reported the existence of a hysteresis phenomenon in the liftoff height of a turbulent diffusion flame in the presence of a high co-airflow. That is, as the fuel velocity decreases, the lifted flame base would first move upstream (toward the burner) to a local minima followed by a downstream movement before its reattachment. The results of the present study, however, showed that such a phenomenon does not appear for a fuel pipe having a very small lip thickness. The present results also revealed that in the hysteresis region, the flame base sits where the turbulence intensity experiences its local maxima in the upcoming unburnt mixture. This corroborates the premixed stability theory which is based on turbulence intensity. Based on this, a correlation was found between the flame liftoff height in the hysteresis region and the fuel and co-airflow velocity at the nozzle exit. This relationship predicts successfully the liftoff height trend as a function of the fuel jet and co-airflow velocity and nozzle geometry. Away from the hysteresis region, however, the flame base location tends more toward the outside of the local turbulence intensity maxima. This indicates the limitations of the premixed stability theory in predicting the flame behavior in this region where the effect of the flow large-scale structures becomes important.