Laser-induced fluorescence detection of HCO in a low-pressure flame

doi:10.1016/S0082-0784(06)80465-0

Symposium (International) on Combustion

Volume 23, Issue 1, 1991, Pages 1847-1854

https://doi.org/10.1016/S0082-0784(06)80465-0 Get rights and content

We report the first laser-induced fluorescence measurement of the formyl radical, HCO, in a flame environment. A methane/oxygen flame =1.03, was burned at 5.8 Torr. The (0,0,2) vibrational level of thestate (which in emission forms the hydrocarbon flame bands) was excited at 245 nm. Dispersed fluorescence spectra confirmed the identification. Profiles of HCO, OH and temperature were determined using LIF. Spectroscopic aspects, including line assignments, interferences from OH and O₂, and further information needed for quantitative measurements, are discussed. A flame model shows the correct ordering for the appearance of these radicals (as well as CH and singlet CH₂) as a function of height, but predicts all species to arise later in the flame than measured.

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The formyl radical (HCO) has for a long time been recognized as a good indicator of the flame front, as its concentration was found to be temporally and spatially correlated with the local HR rate [5,11]. The first HCO fluorescence detection in a combustion environment was performed in a methane/oxygen flame (ϕ = 1.02) at low pressure (5.8 Torr) by excitating the QR0 (J = 9) line in the (0, 0, 2)–(0, 0, 0) band of the B–X system near 244 nm, and the fluorescence emission from 276 nm to 284 nm was collected [12]. In further studies [11,13], two-dimensional HCO PLIF detection has been reported under various flame conditions by exciting the rotational transitions in the B–X (0, 0, 0)–(0, 0, 0) band near 258 nm with a conventional Nd:YAG/Dye laser system.
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.
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Time resolved data are more difficult to obtain due to the limited repetition rates and limited energies delivered per pulse from the lasers, although OH and CH concentration measurements at a few kHz were recently reported in flames to characterize transient phenomena in turbulent reacting flows (see for example [17–19]). Quantitative spatially resolved heat release rate measurements deduced from LIF signals are still challenging [20]. This has motivated a series of studies based on LIF measurements of different species concentrations to estimate the local distribution of heat release rate in flow configurations of increasing complexity [10,21–24].
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Laser-induced fluorescence detection of HCO in a low-pressure flame

Prog. Energy Comb. Sci.

Comb. Flame

Comb. Flame

Comb. Flame

Chem. Phys. Lett.

Chem. Phys. Lett.

J. Mol. Struct.

Chem. Phys. Lett.

Chem. Phys.