Analysis of Combustion Instability of Methane-Air Premixed Swirling Flame Based on OH-PLIF Measurements

A method for determining combustion instability using flame structure parameters is presented. A speaker is used to provide controllable external excitation for the combustion system. The experimental object is a methane-air swirl premixed flame. The flame structure parameters such as height, width, and flame surface density extracted from the hydroxyl planar laser-induced fluorescence image were used to analyze combustion instability at different equivalence ratios (0.8-1.2) and inlet flow rates. It is confirmed that the inflection point of the flame structure parameters corresponds to the evolution of combustion instability verified by the flame transfer function. The results show that with the increase of inlet velocity v, the flame aspect ratio h/b, average OH* concentration, and surface density Σ gradually decrease. The thickness δ of the flame brush shows an increasing trend under the same conditions. With the increase of equivalence ratio Φ, the average OH* concentration and flame surface density Σ increase continuously. The changing trend of flame brush thickness decreases first and then increases to a peak. Finally, it continues to decline after reaching the peak. The flame responds strongly to the sound field when the equivalence ratio is 0.9 and 1.0. In the range of 80-240 Hz, the flame response near 110 and 190 Hz is stronger at each equivalence ratio (0.8-1.0). When the equivalent ratio is 0.9 and 1.0, the amplitude fluctuations of the flame transfer function are much larger than those under other conditions. Meanwhile, the specific performances of the flame structure parameters are that the flame height, average OH* concentration, and flame surface density decrease, and the flame brush thickness increases. These results can be used as a basis for judging combustion instability. This method proves that the parameter information monitored during the flame combustion process can be used to judge the changes in combustion conditions and can adjust the corresponding conditions more accurately and quickly.