Improvement of rotational CARS thermometry in fuel‐rich hydrocarbon flames by inclusion of N₂‐H₂ Raman line widths

Abstract

In rotational coherent anti-Stokes Raman spectroscopy (CARS) thermometry applied to air-fed flames, the temperature sensitivity mainly depends on the intensity distribution of the nitrogen spectral lines. Temperatures are estimated by numerical fitting of theoretical spectra to experimental ones, and one uncertainty in the calculation of theoretical CARS spectra for specific flame conditions is the accuracy in utilized line-broadening coefficients. In a previous article, self-broadened N 2 -N 2 line widths were considered in the spectral calculations as well as those of N 2 -CO, N 2 -CO 2 , N 2 -H 2 O, and N 2 -O 2 . In the present article, we also include N 2 -H 2 line widths calculated from a newly developed model, and it is shown that the evaluated temperature from flame spectra increases with increasing mole fractions of hydrogen. For example, in a very rich flame at = 2.5, the use of available line-width data for all major species gives a temperature raise of 72 K at a temperature of ∼1700 K, in comparison with using self-broadened N 2 -N 2 line widths only. Half of this temperature raise is related to the inclusion of N 2 -H 2 line widths. This article emphasizes the importance of using adequate line-broadening models for rotational CARS thermometry in flames. Copyright

Rotational CARS N2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen

“…Further, it was observed that with the use of self-broadened N 2 -N 2 Raman line widths only, the evaluated CARS temperatures systematically underestimated the reference temperature by as much as 6-9% at the highest hydrogen concentration (90%). These results are in qualitative agreement with previous results, [13] where interchanging the Raman line widths from N 2 -N 2 with N 2 -H 2 raised the evaluated flame temperatures by about 2%, when probing nitrogen in the product gases of a rich hydrocarbon flame ( = 2.5) with a hydrogen concentration of about 20%.…”

“…Bohlin et al included in evaluations of rotational CARS spectra from sooting premixed ethylene/air flames. [13] It was demonstrated that at an equivalence ratio of 2.5, at which the hydrogen concentration can be as high as 20%, the use of adequate N 2 -H 2 Raman line widths instead of assuming that N 2 -N 2 Raman line widths could be used also for this part, increased the temperature evaluation by 36 K.…”

Direct measurement of S-branch N2-H2 Raman linewidths using time-resolved pure rotational coherent anti-Stokes Raman spectroscopy

“…[5][6][7] When including these Raman linewidths 7 in evaluations of N 2 RCARS spectra, recorded in the product gases of a rich hydrocarbon flame containing 20% H 2 , the predicted temperature was systematically increased by about 2%. 8 This behavior was confirmed in a subsequent study, probing N 2 in binary mixtures with H 2 at calibrated temperature conditions below 800 K. 9 Here, it was shown that when neglecting the effects on the N 2 spectral lines in the environment dominated by collisions with H 2 , predicted temperatures from N 2 RCARS could be underestimated by as much as 6%-7%. However, by modifying the Raman linewidth model to incorporate each of the speciesspecific contributions to the broadening coefficient, weighted in accordance with their relative mole fractions, the thermometry was improved.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part IV

“…Gao et al [126] presented a new line shape model for molecular H 2 that was developed and tested for ambient pressure applications. In a related paper, Bohlin et al [127] published an improvement of rotational CARS thermometry for fuel-rich hydrocarbon flames by inclusion of N 2 -H 2 Raman line widths. Peng et al [128] presented the details of a simple setup for hybrid coherent Raman microspectroscopy.…”