A Novel Approach For The Evaluation Of Local And Global Flame Time Lag From Oh* Filtered Images

The response of the flame to acoustic perturbations is commonly described in terms of the flame transfer function (FTF), which has been experimentally characterized in many previous works. In many cases, the results reveal that instantaneous heat release rate displays a constant time lag with respect to velocity fluctuations, which is commonly interpreted as an 'average' convective time, used by the fuel to travel from the injection to the combustion zone. However, the definitions of characteristic flame length and convective velocity to be used to describe the FTF time lag are not well established. The approaches and results reported in different previous works are not fully coincident and further work is still needed to propose the most adequate definition of these important magnitudes for the analysis of thermoacoustic instabilities. In this context, this work explores different approaches based on OH* filtered flame images which could provide a different point of view to assess the characteristic flame length related to the FTF time lag. A first, important difference is the use of heat release rate fluctuations, while all previous works are focused on either peak or mean values. The cross correlation of global and local heath flux reveals a particular pattern related to the local time lag for different flame areas. The analysis was applied to premixed CH4 flame images at different equivalent ratios and forced at different excitation frequencies. Experimental results have shown a good agreement of the image patterns with the theoretical formulation and the results allowed assessing local time lags for different flame zones.