Flame ion generation rate as a measure of the flame thermo-acoustic response

Similarly to chemi-luminescence, a chemi-ionization mechanism is imbedded in the chain of carbon oxidation (heat release) for hydrocarbons-air flames which involves fast reactions between radicals. This fact suggests to use this chemi-ionization rate as an alternative to the conventional OH*, CH* or C-2* emission method to characterize the dynamic response of the flame to flow perturbations. To evaluate the idea, a systematic comparative study of the dynamic response of premixed methane-air burner surface stabilized flat and Bunsen-type flames subjected to external acoustic perturbations was conducted. The flame response was determined using the emitted OH* or CH* chemi-luminescence and the ion generation rate of the flame, characterized by measuring the saturation current. The flame thermo-acoustic behavior was described using the flame transfer function concept. It is found that both gain and phase of the measured transfer functions using both methods show an excellent agreement for lean flames. For higher equivalence ratios a quantitative difference in gain was measured which was pronounced in the low frequency range. To elucidate a possible reason of this phenomenon a dedicated experimental study was conducted. Results suggests that the difference is caused by the shortcomings of the chemi-luminescence rather then the chemi-ionization signal as a measure of the dynamic response of near stoichiometric and rich flames. The obtained results indicate that the chemi-ionization process can be considered as a promising alternative indicator for thermo-acoustic instabilities and the dynamic flame response in combustion applications. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.