| 1. |
- Bladh, Henrik, et al.
(author)
-
Optical soot characterization using two-color laser-induced incandescence (2C-LII) in the soot growth region of a premixed flat flame
- 2011
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 641-648
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Journal article (peer-reviewed)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
(author)
-
Development and Application of Pure Rotational CARS for Reactive Flows
- 2012
-
Doctoral thesis (other academic/artistic)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.
(author)
-
Direct measurement of S-branch N(2)-H(2) Raman linewidths using time-resolved pure rotational coherent anti-Stokes Raman spectroscopy.
- 2012
-
In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:7
-
Journal article (peer-reviewed)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.
(author)
-
Effective suppression of stray light in rotational coherent anti-stokes Raman spectroscopy using an angle-tuned short-wave-pass filter.
- 2010
-
In: Applied Spectroscopy. - 1943-3530. ; 64:8, s. 964-966
-
Journal article (peer-reviewed)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.
(author)
-
Improvement of rotational CARS thermometry in fuel-rich hydrocarbon flames by inclusion of N-2-H-2 Raman line widths
- 2009
-
In: Journal Of Raman Spectroscopy. - : Wiley. - 0377-0486 .- 1097-4555. ; 40:7, s. 788-794
-
Conference paper (peer-reviewed)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-CO2, N-2-H2O, 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 Phi = 2.5, the use of available line-width data for all major species gives a temperature raise of 72 K at a temperature of similar to 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 (C) 2009 John Wiley & Sons, Ltd.
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| 6. |
- Bohlin, Alexis, et al.
(author)
-
On the sensitivity of rotational CARS N-2 thermometry to the Herman-Wallis factor
- 2011
-
In: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 42:10, s. 1843-1847
-
Journal article (peer-reviewed)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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| 7. |
- Bohlin, Alexis, et al.
(author)
-
On the sensitivity of rotational O2 CARS thermometry to the Herman-Wallis factor
- 2012
-
In: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 599-603
-
Journal article (peer-reviewed)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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| 8. |
- Bohlin, Alexis, et al.
(author)
-
Pure rotational CARS measurements of temperature and O2-concentration in a low swirl turbulent premixed flame
- 2013
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3629-3636
-
Journal article (peer-reviewed)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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| 9. |
- Bohlin, Alexis, et al.
(author)
-
Rotational CARS N-2 thermometry: validation experiments for the influence of nitrogen spectral line broadening by hydrogen
- 2010
-
In: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486. ; 41:8, s. 875-881
-
Conference paper (peer-reviewed)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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| 10. |
- Bohlin, Alexis, et al.
(author)
-
Rotational CARS thermometry in diffusion flames: On the influence of nitrogen spectral line-broadening by CH4 and H-2
- 2011
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 823-830
-
Journal article (peer-reviewed)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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| 11. |
- Bohlin, Alexis, et al.
(author)
-
Validation of a rotational coherent anti-Stokes Raman scattering model for N2O at temperatures from 295K to 796K
- 2012
-
In: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 604-610
-
Journal article (peer-reviewed)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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| 12. |
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| 13. |
- Carlsson, Henning, et al.
(author)
-
Numerical and experimental study of flame propagation and quenching of lean premixed turbulent low swirl flames at different Reynolds numbers
- 2015
-
In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:6, s. 2582-2591
-
Journal article (peer-reviewed)abstract
- This paper presents a joint experimental and large eddy simulation (LES) study of lean premixed low swirl stabilized methane/air flames at different Reynolds numbers (Re similar to 20,000-100,000). The aims are to investigate the sensitivity of the structures and dynamics of low swirl flames to the inflow boundary conditions and to evaluate the capability of an LES flamelet model in predicting the stabilization and local extinction of the flames. Chemiluminescence measurements are carried out for Re - 20,000-50,000 and further detailed oxygen concentration and temperature fields are measured using rotational coherent anti-Stokes Raman spectroscopy (RCARS) for Re - 20,000 and 30,000 along the centerline of the burner and at various radial positions at different heights above the burner. The data are used first for validation of the combustion LES model employed in the numerical simulations, and then the RCARS and LES results are used to delineate the effect of ambient air entrainment on the flame structure at various burner exit velocities. A three-scalar flamelet model based on a level-set G-equation shows excellent predictions of the lift-off positions and the structures of the flames, including quenching at the trailing edge of the flame. The results show that the flame lift-off height varies only slightly when the burner exit velocity is increased, which is consistent with a shear-layer flame stabilization mechanism reported previously. The volume of the flame decreases substantially with increasing burner exit velocity at relatively low Reynolds numbers, as a result of flame quenching at the trailing edge of the flame caused by entrainment of the ambient air into the fuel/air stream and the flame itself. At high Reynolds numbers the flame structures become fairly self-similar with the flame volume nearly independent of the Reynolds number. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 14. |
- Castellanos, Leonardo, et al.
(author)
-
Water vapor in hydrogen flames measured by time-resolved collisional dephasing of the pure-rotational N2 CARS signal
- 2023
-
In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 39:1, s. 1279-1287
-
Journal article (peer-reviewed)abstract
- We present a novel diagnostic technique to probe water vapor (H2O) concentration in hydrogen (H2) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N2). The rotational Raman coherence of the N2 molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H2 flames where H2O is present in appreciable amount, it introduces a faster dephasing of the N2 coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H2O mole fraction from the dephasing effect of its inelastic collisions with N2. The proof-of-principle is demonstrated in a laminar H2/air diffusion flame, performing sequential measurements of the collisional dephasing of the N2 CARS signal up to 360 ps. We measure the temperature and the relative O2/N2 and H2/N2 concentrations at a short probe delay, and input the results in the time-domain model to extract the H2O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical ‘H3 flame’ performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at ∼20 and ∼250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H2 flames, thus providing a full characterization of the flow composition, as well as of the temperature field.
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| 15. |
- Cha, Jihyoung, 1988-, et al.
(author)
-
A Numerical Approach to Optimize the Design of a Pintle Injector for LOX/GCH4 Liquid-Propellant Rocket Engine
- 2023
-
In: Aerospace. - : MDPI. - 2226-4310. ; 10:7
-
Journal article (peer-reviewed)abstract
- This study presents an optimal design approach of a pintle injector for a deep throttlable liquid-propellant rocket engine (LPRE). Even though the pintle injector is used in rocket engines, it has become more important since reusable launch vehicles (RLVs) recently became a trend due to their economic and environmental benefits. However, since many variables must be determined to design a pintle injector, optimizing the pintle injector design is complicated. For this, we design a pintle injector to optimize the performance parameters; the spray angle, vaporization distance, and Sauter mean diameter (SMD). To confirm the approach, we design a pintle injector using an optimization method based on convex quadratic programming (CQP) for a 1000 N thrust and a throttle ability of 5 to 1 LPRE with liquid oxygen and gaseous methane. Then, we verify the performance using a numerical simulation. Through this work, we check the effectiveness of the optimization method for a pintle injector design.
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| 16. |
- Gao, Yi, et al.
(author)
-
In situ determination of N-2 broadening coefficients in flames for rotational CARS thermometry
- 2013
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3637-3644
-
Journal article (peer-reviewed)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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| 17. |
- Gelang, Magnus, et al.
(author)
-
Bayesian inference of total biogeographic history under an event-based model
-
Other publication (other academic/artistic)abstract
- We propose a Bayesian approach to infer the historical biogeography under an event-based model, where the total biogeographic histories are sampled from its posterior probability distribution using Metropolis-coupled Markov chain Monte Carlo. Total histories are stochastically mapped on a phylogeny followed by invoking a biogeographical model, which defines the biogeographical events dispersal, vicariance, persistence and extinction. A hypothetic order of probabilities for these events to happen is applied in the priors of the analysis, where a stick-breaking process is used to pick variables from a flat Dirichlet distribution. In comparison to the two most commonly used methods, the proposed method delivers relatively similar reconstructions, albeit with some differences such as favouring extinctions more. These differences are linked to either the treatment of total histories, or to the fundamental statistical differences of the three approaches. In conculsion, this method favours extinctions more than the compared methods, but the main difference is more complex and is instead linked to the treatment of total histories.
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| 18. |
- Kliewer, C. J., et al.
(author)
-
Time-domain measurements of S-branch N-2-N-2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy
- 2012
-
In: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 108:2, s. 419-426
-
Journal article (peer-reviewed)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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| 19. |
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| 20. |
- Mazza, Francesco, et al.
(author)
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Coherent N2+ emission mediated by coherent Raman scattering for gas-phase thermometry
- 2022
-
In: Optics Letters. - : Optica Publishing Group. - 0146-9592 .- 1539-4794. ; 47:23, s. 6105-6108
-
Journal article (peer-reviewed)abstract
- We report on the generation of coherent emission from femtosecond (fs) laser-induced filaments mediated by ultrabroadband coherent Raman scattering (CRS), and we investigate its application for high-resolution gas-phase thermometry. Broadband 35-fs, 800-nm pump pulses generate the filament through photoionization of the N2 molecules, while narrowband picosecond (ps) pulses at 400 nm seed the fluorescent plasma medium via generation of an ultrabroadband CRS signal, resulting in a narrowband and highly spatiotemporally coherent emission at 428 nm. This emission satisfies the phase-matching for the crossed pump-probe beams geometry, and its polarization follows the CRS signal polarization. We perform spectroscopy on the coherent N2+ signal to investigate the rotational energy distribution of the N2+ ions in the excited B2Σu+ electronic state and demonstrate that the ionization mechanism of the N2 molecules preserves the original Boltzmann distribution to within the experimental conditions tested.
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| 21. |
- Mazza, Francesco, et al.
(author)
-
Coherent Raman spectroscopy on hydrogen with in-situ generation, in-situ use, and in-situ referencing of the ultrabroadband excitation
- 2022
-
In: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 30:20, s. 35232-35245
-
Journal article (peer-reviewed)abstract
- Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H2). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H2 and the non-resonant CRS spectra in a laminar H2/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H2 CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.
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| 22. |
- Mazza, Francesco, et al.
(author)
-
High-temperature rotational-vibrational O2CO2 coherent Raman spectroscopy with ultrabroadband femtosecond laser excitation generated in-situ
- 2022
-
In: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 237
-
Journal article (peer-reviewed)abstract
- We present ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO2 and O2, applied for multispecies thermometry and relative concentration measurements in a standard laminar premixed hydrocarbon flame. The experimental system employs fs-laser-induced filamentation to generate the compressed supercontinuum in-situ, resulting in a ∼24 fs full-width-at-half-maximum pump/Stokes pulse with sufficient bandwidth to excite all the ro-vibrational Raman transitions up to 1600 cm-1. We report the simultaneous recording of the ro-vibrational CO2 Q-branch and the ro-vibrational O2 O-, Q- and S-branch coherent Stokes Raman spectra (CSRS) on the basis of a single-laser-shot. The use of filamentation as the supercontinuum generation mechanism has the advantage of greatly simplifying the experimental setup, as it avoids the use of hollow-core fibres and chirped mirrors to deliver a near-transform-limited ultrabroadband pulse at the measurement location. Time-domain models for the ro-vibrational Q-branch spectrum of CO2 and the ro-vibrational O-, Q- and S-branch spectra of O2 were developed. The modelling of the CO2 Q-branch spectrum accounts for up to 180 vibrational bands and for their interaction in Fermi polyads, and is based on recently available, comprehensive calculations of the vibrational transition dipole moments of the CO2 molecule: the availability of spectroscopic data for these many vibrational bands is crucial to model the high-temperature spectra acquired in the flue gases of hydrocarbon flames, where the temperature can exceed 2000 K. The numerical code was employed to evaluate the CSRS spectra acquired in the products of a laminar premixed methane/air flame provided on a Bunsen burner, for varying equivalence ratio in the range 0.6–1.05. The performance of the CO2 spectral model is assessed by extracting temperatures from 40-laser-shots averaged spectra, resulting in thermometry accuracy and precision of ∼5% and ∼1%, respectively, at temperatures as high as 2220 K.
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| 23. |
- Mazza, Francesco, et al.
(author)
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The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy
- 2023
-
In: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 158:9
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Journal article (peer-reviewed)abstract
- We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm−1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.
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| 24. |
- McKenna, Jessica, et al.
(author)
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RAVEN: Test Bench and Test Plan Development
- 2022
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In: IAC 2022 Congress Proceedings, 73<sup>rd</sup> International Astronautical Congress (IAC), Paris, France. - : International Astronautical Federation, IAF.
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Conference paper (peer-reviewed)
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| 25. |
- Nordström, Emil, et al.
(author)
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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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In: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 44:10, s. 1322-1325
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Journal article (peer-reviewed)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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| 26. |
- Olofsson, Nils-Erik, et al.
(author)
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Are Sooting Premixed Porous-Plug Burner Flames One-Dimensional? A Laser-Based Experimental Investigation
- 2013
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In: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 185:2, s. 293-309
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Journal article (peer-reviewed)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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