| 1. |
- Bladh, Henrik, et al.
(författare)
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Optical soot characterization using two-color laser-induced incandescence (2C-LII) in the soot growth region of a premixed flat flame
- 2011
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 641-648
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Tidskriftsartikel (refereegranskat)abstract
- The soot growth region of a premixed one-dimensional ethylene/air flame has been investigated using two-color laser-induced incandescence (2C-LII) with focus on optical soot properties. From the 2C-LII technique, primary soot particle sizes were deduced together with the temperature from pyrometry of the laser-heated particles, while the gas temperatures were obtained from pure rotational CARS nitrogen thermometry. Soot particle sizes were also measured from thermophoretically sampled soot particles analyzed by transmission electron microscopy (TEM), and the results showed growing isolated primary soot particles up to a height of 10 mm, after which strong soot aggregation occurred and the increase in soot primary particle size ceased. The measured data was used in the evaluation of the soot absorption function, E(m), and a strong increase by a factor of two was observed from the lowest heights to the highest for assumed constant values of soot density and specific heat at all heights. By comparing the 2C-LII particle sizes with the sizes obtained from TEM, differences were observed. Part of the explanation is that the 2C-LII evaluation did not take aggregation into account, but it is additionally speculated that the thermal accommodation coefficient decreases with height above burner. These results are interesting in view of morphological and composition changes of the soot during the growth process and have implications for the use of 2C-LII as a soot diagnostic technique. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Bohlin, Alexis
(författare)
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Development and Application of Pure Rotational CARS for Reactive Flows
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis deals with the further development of pure rotational coherent anti-Stokes Raman spectroscopy (RCARS) for improving the capabilities of gas phase thermometry. The main effort has been to make the technique more robust when employed under a wide range of temperatures and operational conditions. A primary aim has been to investigate the impact of collisional broadening on N2 RCARS thermometry, especially in an environment in which N2 is perturbed by H2. Since an interaction of this sort is species-specific and temperature-dependent, it plays a very critical role in RCARS thermometry. It was found that in a sequence of implementation, validation and application, thermometric accuracy could be improved by the implementation of N2-H2 line-broadening coefficients. Investigation of these topics involved exploring a novel technique of time-resolved picosecond RCARS for direct measurements of S-branch N2-N2/N2-H2 Raman linewidths. The N2 and O2 Herman-Wallis factors, used to quantify vibration-rotation interaction and breakdown of the rigid rotor model were also investigated. This correction affects the line-intensities, and also has an impact on RCARS thermometry. Conclusions regarding the sensitivity related to this factor could be achieved by employing different expressions available in the specialized literature. A theoretical code for N2O concerned with thermometric accuracy in a set of temperature-calibrated cell experiments was developed and was validated. This work expands the list of RCARS molecules previously developed, and points to interesting possibilities such as that of improving the thermometric precision. The technique was also applied to flame diagnostics. Temperatures were mapped along the centerline of a one-dimensional flame provided on a McKenna burner, this serving as important input data for other related optical techniques. The homogeneity of this flame was investigated for two different operational shielding co-flows, those of N2 and of air. Measurements were also performed in a low-swirl turbulent premixed flame, for validating existing models of large eddy simulations. Probability density functions for a large range of temperatures (300 K to 1700 K) and relative O2 concentrations were provided. The simultaneous measurements of these quantities provided a better understanding than possessed earlier of air entrainment from the surroundings.
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| 3. |
- Bohlin, Alexis, et al.
(författare)
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Direct measurement of S-branch N(2)-H(2) Raman linewidths using time-resolved pure rotational coherent anti-Stokes Raman spectroscopy.
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:7
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Tidskriftsartikel (refereegranskat)abstract
- S-branch N(2)-H(2) Raman linewidths have been measured in the temperature region 294-1466 K using time-resolved dual-broadband picosecond pure rotational coherent anti-Stokes Raman spectroscopy (RCARS). Data are extracted by mapping the dephasing rates of the CARS signal temporal decay. The J-dependent coherence decays are detected in the time domain by following the individual spectral lines as a function of probe delay. The linewidth data set was employed in spectral fits of N(2) RCARS spectra recorded in binary mixtures of N(2) and H(2) at calibrated temperature conditions up to 661 K using a standard nanosecond RCARS setup. In this region, the set shows a deviation of less than 2% in comparison with thermocouples. The results provide useful knowledge for the applicability of N(2) CARS thermometry on the fuel-side of H(2) diffusion flames.
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| 4. |
- Bohlin, Alexis, et al.
(författare)
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Effective suppression of stray light in rotational coherent anti-stokes Raman spectroscopy using an angle-tuned short-wave-pass filter.
- 2010
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Ingår i: Applied Spectroscopy. - 1943-3530. ; 64:8, s. 964-966
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Tidskriftsartikel (refereegranskat)abstract
- Stray light interference is a common problem in spontaneous rotational Raman spectroscopy and rotational coherent anti-Stokes Raman spectropscopy (CARS). The reason is that the detected spectrum appears in the spectral vicinity of the probe beam wavelength, and stray light at this wavelength from optics and surfaces is hard to suppress. In this Note, efficient suppression of stray light is demonstrated for rotational CARS measurements using a commercially available short-wave-pass filter. By angle-tuning this filter with a specified cut-off wavelength at 561 nm, the cut-off wavelength could be tuned to a desired spectral position so that more than 80% transmission is achieved as close as 15 cm(-1) (approximately 0.4 nm) from the probe beam wavelength of 532.0 nm, while the intensity at this wavelength is suppressed by two orders of magnitude.
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| 5. |
- Bohlin, Alexis, et al.
(författare)
-
On the sensitivity of rotational CARS N-2 thermometry to the Herman-Wallis factor
- 2011
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 42:10, s. 1843-1847
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Tidskriftsartikel (refereegranskat)abstract
- Purely rotational spectral signals of coherent anti-Stokes Raman scattering (CARS) from nitrogen molecules are studied as a function of the vibration-rotation interaction that weakens the rigid rotor approximation under which the dominant terms of the Raman cross section are calculated. The effect of the vibration-rotation interaction is quantified by means of the Herman-Wallis (HW) factor, and different approaches to its determination are evaluated in terms of their relative contribution to the CARS intensity and thermometric measurements made in a fuel-rich hydrocarbon flame. Known HW factors are contrasted with more complete expressions of recent derivation, and it is found that relative line strength adjustments amount to about a few percent. Such differences result in temperature corrections of less than 1%. This value should be considered for the definition of the ideal thermometric accuracy of the technique but it is of minor importance in comparison with other sources of uncertainty (e. g. Raman line widths) that emerge from the complexity typical of reactive gas mixtures. Copyright (C) 2011 John Wiley & Sons, Ltd.
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| 6. |
- Bohlin, Alexis, et al.
(författare)
-
On the sensitivity of rotational O2 CARS thermometry to the Herman-Wallis factor
- 2012
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 599-603
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Tidskriftsartikel (refereegranskat)abstract
- Contributions of vibrationrotation interaction to line intensities of pure rotational coherent anti-Stokes Raman scattering (CARS) of oxygen are here considered in the perspective of what was remarked by us recently for nitrogen. The interaction is conventionally evaluated by means of the so-called HermanWallis (HW) factor, the need of which is conceptually clear but the theoretical expression is debatable, as shown in the literature of this field. In this work, the various solutions for the anisotropic HW factor of pure rotational Raman transitions are compared in light of the implications for the corresponding CARS spectral line intensities. The results show that neglecting the use of HW factors in rotational N2 and O2 CARS thermometry leads to systematic overestimation of temperatures; on the order of 1.5% in the temperature range 300-2100K in comparison with the use of the most plausible HW factors. The results also suggest that, in general, oxygen is more sensitive than nitrogen to the HW correction, and in particular, different S-branch HW expressions for oxygen are responsible for thermometric uncertainties between 0.5% and 1% at flame temperatures. Copyright (c) 2012 John Wiley & Sons, Ltd.
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| 7. |
- Bohlin, Alexis, et al.
(författare)
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Pure rotational CARS measurements of temperature and O2-concentration in a low swirl turbulent premixed flame
- 2013
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3629-3636
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Tidskriftsartikel (refereegranskat)abstract
- Abstract in UndeterminedTemperature and relative O-2-concentrations have been measured using pure rotational coherent anti-Stokes Raman spectroscopy (RCARS) in a low swirl turbulent premixed flame. This lean flame burning methane has previously been investigated using laser diagnostic techniques and numerical modeling of flame characteristics. In the present work, single-shot RCARS measurements were performed and analysed statistically to obtain temperature and O-2-concentration probability density functions. The results were compared with large eddy simulations (LES) showing fairly good agreement. The rotational CARS technique was successfully applied in the whole range of temperatures (300-1700 K) and relative O-2-concentrations (9-21%), and based on the simultaneous measurements of temperature and O-2-concentration an improved understanding of the reaction progress and air entrainment from the surroundings can be achieved. From the evaluated O-2-concentration, the H2O mole fraction could be estimated, which subsequently made an important thermometric correction possible of up to similar to 40 K at 1700 K by inclusion of N-2-H2O line-broadening coefficients in the theoretical spectral calculations. Spatial averaging effects were observed in a fraction of spectra where both hot and cold spectral envelopes were detected, most often with corresponding peaks displaced relative to each other. The large range of CARS signal intensities (about 3 orders of magnitude) when intermittently probing low and high temperature gases was dealt with by using a multi-track function of the CCD camera that increased the dynamic range. Merits and limitations of pure rotational CARS for diagnostics in turbulent flames are discussed and comparison is made with vibrational CARS. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 8. |
- Bohlin, Alexis, et al.
(författare)
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Rotational CARS N-2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen
- 2010
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486. ; 41:8, s. 875-881
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Konferensbidrag (refereegranskat)abstract
- Rotational coherent anti-Stokes Raman spectroscopy (CARS) in fuel-rich hydrocarbon flames, with a large content of hydrogen in the product gases (similar to 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-2 Raman line widths was more successful. The difference in evaluated temperatures between the two different sets increases approximately linearly, reaching 20 K (at T similar to 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. Copyright (C) 2010 John Wiley & Sons, Ltd.
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| 9. |
- Bohlin, Alexis, et al.
(författare)
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Rotational CARS thermometry in diffusion flames: On the influence of nitrogen spectral line-broadening by CH4 and H-2
- 2011
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 823-830
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Tidskriftsartikel (refereegranskat)abstract
- Rotational CARS N-2 thermometry has been developed for improved accuracy in spatial flame regions with high concentrations of either H-2 or CH4, which are typical conditions on the fuel side of diffusion flames. In initial validation experiments up to 700 K, rotational CARS measurements were performed in binary mixtures of H-2 and N-2. Subsequent spectral evaluation with commonly used N-2-N-2 Raman line widths clearly underestimated the real temperatures. Inclusion of recently calculated N-2-H-2 spectral line-broadening coefficients weighted by the concentration of hydrogen resulted in CARS temperatures in much better agreement with the true temperatures. Based on this validation work, improved temperature accuracy was achieved in flame measurements on the fuel side of a hydrogen diffusion flame by probing nitrogen simultaneously with the detection of the hydrogen S-0-transition at 354 cm(-1), and weighting the species specific Raman line widths with the information of relative concentration. The validation experiments were repeated for binary mixtures of nitrogen and methane up to 500 K. It was shown that N-2-N-2 Raman line widths gave reproducible temperatures independent of methane concentration in binary mixtures with nitrogen, thus no explicit knowledge on N-2-CH4 Raman line widths is needed for this temperature region. In general, the impact of Raman line widths for accurate rotational CARS thermometry is demonstrated. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 10. |
- Bohlin, Alexis, et al.
(författare)
-
Validation of a rotational coherent anti-Stokes Raman scattering model for N2O at temperatures from 295K to 796K
- 2012
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 604-610
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Tidskriftsartikel (refereegranskat)abstract
- Pure rotational coherent anti-Stokes Raman scattering (CARS) spectra of N2O was recorded in a series of temperature calibrated cell measurements in the region 295-796K. A theoretical rotational CARS code for N2O was implemented and temperature analysis was performed by fitting the modeled theoretical spectra to the experimental spectra. Excellent agreement between the experimental and modeled spectra was obtained and the thermometric accuracy for the measurements was evaluated to be better than 1%. Also, rotational N2 CARS spectra were recorded at the same measurement conditions and the temperature analysis performed on these spectra resulted in the same accuracy. The peak signal strength was found to be similar to 5 times stronger for N2O than for N2 at ambient temperature and pressure. The temperature precision was evaluated to a relative standard deviation of 2.0%-2.7% in the studied temperature range, about half of the values for N2, which is attributed to the larger number of spectral lines for N2O. Rotational N2O CARS thermometry show great potential, because of a high Raman cross-section and large number of populated rotational states at any temperature. Copyright (c) 2012 John Wiley & Sons, Ltd.
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| 11. |
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| 12. |
- Gao, Yi, et al.
(författare)
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In situ determination of N-2 broadening coefficients in flames for rotational CARS thermometry
- 2013
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3637-3644
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Tidskriftsartikel (refereegranskat)abstract
- Total N-2 S-branch broadening coefficients have been measured in situ as a function of radial position in a highly sooting ethylene diffusion flame by the use of time-resolved dual-broadband picosecond pure rotational CARS (RCARS). Time-domain measurements of the J-dependent N-2 rotational coherence decays in the flame were used to determine the broadening coefficients, and these coefficients were then used in the spectral fitting routine for the determination of temperature and [O-2]/[N-2] ratio in the flame. Corrections of up to 125 K are found when compared to spectral fitting using an ECS self-broadened N-2 linewidth model. The presented technique effectively takes into account contributions to the N-2 linewidths from all collision partners without any a priori knowledge of relative species concentrations. Published by Elsevier Inc. on behalf of The Combustion Institute.
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| 13. |
- Kliewer, C. J., et al.
(författare)
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Time-domain measurements of S-branch N-2-N-2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy
- 2012
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 108:2, s. 419-426
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved dual-broadband picosecond pure rotational CARS has been applied to measure self-broadened S-branch N-2-N-2 Raman linewidths in the temperature range 294-1466 K. The coherence decays were detected directly in the time domain by following the J-dependent CARS signal decay as a function of probe delay. The rotational Raman N-2-N-2 linewidths were derived from these time-dependent decays and evaluated for thermometric accuracy. Comparisons were made to the energy-corrected sudden (ECS) and modified exponential gap (MEG) dynamical scaling laws, and the results were used to quantify the sensitivity of nanosecond rotational CARS thermometry to the linewidth model employed. The uncertainty based on the linewidth model used in pure N-2 was found to be 2 %. The merits and limitations of this rapid method for the determination of accurate Raman linewidths are discussed.
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| 14. |
- Nordström, Emil, et al.
(författare)
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Pure rotational Coherent anti-Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics
- 2013
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Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 44:10, s. 1322-1325
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we report for the first time the rotational coherent anti-Stokes Raman spectroscopy (CARS) spectrum of water vapor, recorded through experiments at 400K and 670K for Raman shifts up to 600cm(-1). Using the standard dual-broadband pure rotational CARS approach, the resonant structure was overwhelmed by a strong non-resonant background. By employing a polarization technique, this background could be suppressed and the resonant structure discerned. The spectral line structure matched spectra from spontaneous rotational Raman scattering found in the literature. The CARS signal intensity at the highest peak from water vapor at 400K was more than five orders of magnitude weaker than for N-2 because of a low Raman cross section and a larger number of transitions. Due to the weak resonant signal, water vapor is an unsuitable candidate for CARS thermometry and relative concentration measurements in combustion. Still, rotational CARS spectra from product gases in combustion will be affected by the presence of water, which is briefly discussed. Copyright (c) 2013 John Wiley & Sons, Ltd.
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| 15. |
- Olofsson, Nils-Erik, et al.
(författare)
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Are Sooting Premixed Porous-Plug Burner Flames One-Dimensional? A Laser-Based Experimental Investigation
- 2013
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 185:2, s. 293-309
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Tidskriftsartikel (refereegranskat)abstract
- The one-dimensional character of sooting premixed porous-plug (McKenna) burner flames has been examined, motivated by recent investigations where the one-dimensionality has been questioned. The examination employs laser diagnostic techniques to measure different characteristics: temperature using rotational coherent anti-Stokes Raman spectroscopy (CARS) and soot properties using laser-induced incandescence and elastic light scattering. Special attention has been paid to the influence of an outer shroud gas flow of either nitrogen or air. The nitrogen shroud gas leads to flame cooling at the outer edge, whereas the air shroud results in heating through the oxidation of CO/H-2/soot. This generally results in a more inhomogeneous spatial particle size profile for the nitrogen shroud flame, with smaller particle sizes at the flame edge. The present results emphasize the need to characterize burner and operating conditions accurately, and also, regarding parameters that at first glance seem less relevant, such as the shroud gas, to provide useful data for comparison between laboratories.
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