Flame stability limits and near blowout characteristics of CNG inverse jet flame

This paper reports the stability limits and near blowout characteristics of unrecessed and recessed compressed natural gas (CNG) inverse jet flames (IJFs) by varying the central air jet velocity for a constant fuel jet velocity. The experimental results indicate that unrecessed IJF has higher flame stability as compared to the recessed case for the same fuel jet velocity. In order to understand the reason for the enhanced flame stability in unrecessed IJF as compared to the recessed IJF, the visible appearance of these two inverse jet flame configurations are examined for understanding the various transient events taking place towards blowout. In the case of unrecessed IJF, initiation of local extinction and reignition phenomena on the flame surface ushers the onset of blowout. In contrast, this phenomenon is not observed in recessed IJF, rather the flame gets partially lifted from the burner rim prior to blowout. The enhanced flame stability of unrecessed IJF is attributed to the presence of base flame near the burner rim that acts as a pilot flame. On the other hand, the absence of base flame in recessed IJF is the reason for its lower flame stability limit as compared to the unrecessed IJF. Based on the nature of blowout events, blowout mechanisms for both unrecessed and recessed IJFs are proposed in the present work.