Abstract
The first supercontinuum (SC) absorption spectroscopy measurements showing the feasibility of quantitative temperature evaluation are presented to the best of the authors' knowledge. Temperature and multi-species measurements were carried out at a detection rate of ~2 MHz in a high-temperature flow cell within a temperature range from 450 K to 750 K at 0.22 MPa, representing conditions during the suction and compression stroke in an internal combustion (IC) engine. The broadband SC pulses were temporally dispersed into fast wavelength sweeps, covering the overtone absorption bands 2ν(1), 2ν(3), ν(1) + ν(3) of H2O and 3ν(3) of CO2 in the near-infrared region from 1330 nm to 1500 nm. The temperature information is inferred from the peak ratio of a temperature sensitive (1362.42 nm) and insensitive (1418.91 nm) absorption feature in the ν(1) + ν(3) overtone bands of water. The experimental results are in very good agreement with theoretical intensity ratios calculated from absorption spectra based on HiTran data.
Highlights
For the investigation and characterization of technical combustion systems such as internal combustion (IC) engines, gas turbines or industrial furnaces it is essential to measure scalar quantities like temperature and species concentrations with high temporal resolution, i.e., in the order of tens of microseconds for IC engines [1]
The experimental results are in very good agreement with theoretical intensity ratios calculated from absorption spectra based on HiTran data
The spectral resolution ∆λ of a dispersion-swept SC spectrometer is given by ∆λ = τm/|D|, where D is the total group-velocity dispersion of the employed dispersive element [9]. τm is the temporal resolution of the setup, determined by the optical pulse-width of the light source or the response time of the detection system, consisting of photodiode and oscilloscope, respectively
Summary
For the investigation and characterization of technical combustion systems such as internal combustion (IC) engines, gas turbines or industrial furnaces it is essential to measure scalar quantities like temperature and species concentrations with high temporal resolution, i.e., in the order of tens of microseconds for IC engines [1]. Employing a strongly dispersive fiber, for example within a dispersion compensating module (DCM) for optical communication systems, the SC pulses can be dispersed into wavelength sweeps due to the group-velocity dispersion of the different spectral components This offers the possibility of time-domain based broadband spectroscopy by detection of the dispersed SC pulses with a high bandwidth photodiode and oscilloscope. The detection of the broadband absorption spectrum and multiple absorption transitions offers the possibility to determine the temperature from the ratio of at least two absorption features, exhibiting a different temperature sensitivity according to Boltzmann statistics, which describes the population of energy levels as a function of temperature Another approach is to fit a broad span of the theoretical to the experimental spectrum as shown elsewhere [29]. Both effects have to be balanced against each other
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