Strategy for PLIF single-shot HCO imaging in turbulent methane/air flames

Formyl (HCO) has since long been recognized as a common intermediate species and a potential local indicator of the major heat release in hydrocarbon combustion. Consequently, the detection of HCO is desirable especially in turbulent flames of practical relevance. However, due to the low concentration and low fluorescence quantum yield, single-shot based detection of HCO with planar laser-induced fluorescence (PLIF) has been a real challenge for experimentalists. In the present paper, a series of systematic investigations have been performed in order to develop a strategy for single-shot HCO PLIF detection in methane/air premixed flames. Potential spectral interference and applicable combustion conditions were analyzed in stable laminar flames employing fluorescence detection with high spectral and spatial resolution for different laser wavelengths. The wavelength 259.004 nm was identified as optimum in giving the maximum signal and minimum spectral interference from other species (e.g., OH and hot O2). Photolytically generated HCO from formaldehyde (CH2O) was also observed, which restricts the applicable laser fluence to below 2.5 J/cm2 in order to diminish the influence of CH2O down to 5%. Besides, large hydrocarbon species generated in rich flames were found to contribute a considerable interference which can hardly be screened out. This limits the application of the HCO PLIF technique to lean premixed flames. Finally, by employing an optimized alexandrite laser system, single-shot HCO PLIF imaging in a turbulent methane/air flame is demonstrated, indicating the feasibility of further application of this technique to turbulent combustion systems.