Abstract
We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H2O and OH radical in a premixed CH4/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H2O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 × 10-7 cm-1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.
Highlights
Fast and simultaneous detection of multiple species in combustion is essential for a deeper understanding of chemical reaction dynamics, as well as for obtaining invaluable information and feedback for the design of combustion systems with improved fuel efficiency or decreased pollutant emissions [1,2]
The detection bandwidth of absorption techniques based on continuous wave lasers such as wavelength modulation spectroscopy (WMS) [11,12] and cavity ring-down spectroscopy [13,14,15,16] is usually limited to 1–2 cm−1 [17,18]
The results reveal changes in H2O and OH concentrations of 0.05% and 0.003% per Vernier orders (VO), respectively
Summary
Fast and simultaneous detection of multiple species in combustion is essential for a deeper understanding of chemical reaction dynamics, as well as for obtaining invaluable information and feedback for the design of combustion systems with improved fuel efficiency or decreased pollutant emissions [1,2]. Much larger detection bandwidth is provided by techniques based on broadband sources such as optical frequency combs [19,20,21], supercontinuum sources [22,23] and Fourier-domain mode-locked lasers [24] Spectrometers employing these sources have been used to determine water concentration and/or temperature in combustion environments with time resolution of the order of hundreds of microseconds or milliseconds [21,23,24,25]. Time-resolved measurements of species concentrations in dynamically changing combustion environments have not been realized using a broadband cavity-enhanced system Such system would open up for sensitive multispecies detection in applications where single pass through the combustion process does not provide sufficient sensitivity because of the low analyte abundance or weak line strengths. To verify the accuracy of the results, the retrieved concentrations of both analytes in the steady-state regions of each modulation cycle are compared to simulated values of a static flame model
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