Detection of OH in an atmospheric flame at 1.5 um using optical frequency comb spectroscopy

Abstract

Abstract-We report broadband detection of OH in a premixedCH4/air flat flame at atmospheric pressure using cavity-enhanced absorption spectroscopy based on an Er:fiber femtosecond laser and a Fourier transform spectrometer. By taking ratios of spectra measured at different heights above the burner we separate twenty OH transitions from the largely overlapping water background. We retrieve from fits to the OH lines the relative variation of OH concentration and flame temperature with a height above the burner and compare them with the 1D simulations of flame structure.Absorption spectroscopy enables calibration-free detection of intermediate combustion products, such as the OH radical, which is important for the understanding and modelling of flame chemistry. The OH radical has been detected using cavity ring-down spectroscopy (CRDS) in the ultraviolet spectral range [1], both in low pressure [2] and atmospheric [3] flames. However, performing OH detection in the telecom near-infrared (NIR) wavelength range is more attractive because of the availability of laser sources and optical components. Until now, various techniques have been used for OH detection in flames around 1.5µm, such as CRDS [4], cavityenhanced absorption spectroscopy [5], wavelength modulation spectroscopy (WMS) [6] and intra-cavity laser absorption spectroscopy [7]. One of the main problems with OH detection at this wavelength is the strong interference of water absorption lines. The selectivity can be increased by lowering the flame pressure, which decreases the pressure broadening and spectral overlap [4,7]. However, many industrial applications require measurements under atmospheric or higher pressures [8]. The lack of accurate parameters of high-temperature water transitions around 1.5µm prevents unambiguous identification of the water lines [9,10]. A way to circumvent this problem has been to measure NIR water spectra at different temperatures and subtract them from the spectra measured in the flame [5,6]. This, however, often requires extrapolation towards the flame * E-mail: aleksandra.foltynowicz@umu.se temperature and introduces uncertainties and distortion in the OH spectra.Here we use NIR cavity-enhanced optical frequency comb spectroscopy to detect broadband OH spectra in a premixed CH 4 /air flat flame of a stabilized burner at atmospheric pressure without the interference of water. We isolate the OH lines from the water background by taking ratios of spectra measured at different heights above the burner (HABs). This works because water is a final combustion product and therefore its concentration is almost constant above the production zone. We retrieve from our spectra the relative variation of the OH concentration and flame temperature with HAB and compare them to a 1-D model of the flame.The experimental setup is an improved version of that described in [10]. It is based on an optical frequency comb generated by an Er:fiber femtosecond laser, a 60 cm long open-air cavity containing the flame with a 3.8 cm diameter, and a fast-s...

An experimental water line list at 1950 K in the 6250–6670 cm−1 region

“…The comb beam probes the line of sight in the flame (flame diameter of 3.8 cm) at atmospheric pressure and at a height above the burner (HAB) of 2.5 mm. At this HAB the temperature and species are rather homogeneously distributed along the line of sight [40,43], the average flame temperature is 1950 ± 50 K, the average water concentration is 17 ± 1% (both characterized using tunable diode laser absorption spectroscopy [40]), and the average hydroxide (OH) concentration is 0.28% [41].…”

Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame

“…The Lorentzian half width of all H 2 O absorption lines is set to 0.810 GHz, and the Doppler half-width to 0.713 GHz. The OH line list is taken from the HITRAN 2016 database [37], with the Lorentzian half width set to 2.04 GHz, according to an estimation from a previous work [38] using the same flame, and the Doppler half-width to 0.732 GHz. The VO number is set to −40, the flame diameter to 3.8 cm, and the flame temperature to 1950 K, the value determined from WMS measurements at HAB = 3.0 mm [35].…”

Optical frequency comb Faraday rotation spectroscopy

“…Note that the dependence on I 0 e −̄L is removed by the normalization procedure of Eq. (15), which infers that any variations in intensity that are common for the two signals are suppressed. This includes interferences from diamagnetic (10)…”

“…The difficulties may be further exacerbated by drifting optical fringe backgrounds that prohibit long-term averaging. Congested absorption spectra are commonly encountered in, e.g., combustion diagnostics, and are especially troublesome in wavelength regions where sufficiently detailed spectral models for water vapor are not yet available [15].…”