Pure rotational coherent anti-Stokes Raman spectroscopy in mixtures of CO and N_2

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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

Section: Evaluation Of Flame Temperaturesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Bohlin¹,

Vestin²,

Joubert³

et al. 2009

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“…The need for knowledge on Raman line widths in the CARS community working with combustion diagnostics stimulated research on the N 2 -H 2 O and N 2 -CO 2 system already around 20 years ago. [6,7] It was not until few years ago work on the N 2 -CO spectral line broadening was performed, [8,9] but at that time there was still a lack in knowledge on N 2 -H 2 spectral line broadening. Recent investigations have shown theoretical as well as experimental work on establishing the N 2 -H 2 Raman line widths' dependence of temperature and J quantum number.…”

Section: Introductionmentioning

confidence: 99%

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

Bohlin

Vestin

Bonamy

et al. 2010

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Rotational coherent anti-Stokes Raman spectroscopy (CARS) in fuel-rich hydrocarbon flames, with a large content of hydrogen in the product gases (∼20%), has in previous work shown that evaluated temperatures are raised several tens of Kelvin by taking newly derived N 2 -H 2 Raman line widths into account. To validate these results, in this work calibrated temperature measurements at around 300, 500 and 700 K were performed in a cell with binary gas mixtures of nitrogen and hydrogen. The temperature evaluation was made with respect to Raman line widths either from self-broadened nitrogen only, N 2 -N 2 [energy-corrected-sudden (ECS)], or by also taking nitrogen broadened by hydrogen, N 2 -H 2 [Robert-Bonamy (RB)], Raman line widths into account. With increased amount of hydrogen in the cell at constant temperature, the evaluated CARS temperatures were clearly lowered with the use of Raman line widths from self-broadened nitrogen only, and the case with inclusion of N 2 -H 2Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T ∼ 300 K), 43 K (at T = 500 K) and 61 K (at T = 700 K) at the highest hydrogen concentration (90%). The results from this work further emphasize the importance of using adequate Raman line widths for accurate rotational CARS thermometry.

“…The present results demonstrate that a weighting procedure is sometimes necessary if low concentrations are to be accurately evaluated from flame spectra. Also, in a recent study 11 we showed that accurate CO concentration measurements in ethylene-air flames were only possible using the SSW, owing to the weak spectral signature of CO.…”

Section: Discussionmentioning

confidence: 99%

“…Another matter of importance for accurate concentration measurements, as discussed in Ref. 11, is the theoretical knowledge about parameters such as Raman linewidths and anisotropic polarizabilities. Moreover, for very low concentrations, the resonant part of the CARS signal will disappear into the baseline of the non-resonant signal, hence the noise in this background sets a lower limitation for detection.…”

Section: Introductionmentioning

confidence: 99%

Improved species concentration measurements using a species‐specific weighting procedure on rotational CARS spectra

Vestin

Afzelius

Bengtsson

2005

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Rotational coherent anti-Stokes Raman spectroscopy (CARS) is used for measurements of temperaturesand relative species concentrations. The evaluation of a rotational CARS spectrum is normally performed using a least-squares fitting algorithm to find the best-fit theoretical spectrum in a library of spectra calculated at different temperatures and relative concentrations. A general problem is that species with weak spectral features have a minor influence on the results in the standard evaluation procedure. A species-specific weighting (SSW) procedure enhancing the influence of weak spectral features in the spectral fitting algorithm has therefore been developed. The SSW procedure was tested on the evaluation of low concentrations of O 2 in mixtures with N 2 , and it is shown that both the accuracy and precision of single-shot spectra concentration measurements are improved by use of the SSW evaluation procedure. Also, a comparison with previously used weighting procedures shows that this approach is superior. The application of the technique to oxygen concentration measurements in the product gas of a fuel-lean flame is demonstrated. In addition, a test was made using the developed weighting procedure for thermometry, by applying spectral weighting to the rotational spectrum originating from the first thermally excited vibrational state. The precision of the temperature measurements was then slightly improved.