| 1. |
|
|
| 2. |
- Bohlin, Alexis, et al.
(författare)
-
Direct measurement of S-branch N(2)-H(2) Raman linewidths using time-resolved pure rotational coherent anti-Stokes Raman spectroscopy.
- 2012
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 137:7
-
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.
|
|
| 3. |
- Bohlin, Alexis, et al.
(författare)
-
On the sensitivity of rotational O2 CARS thermometry to the Herman-Wallis factor
- 2012
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 599-603
-
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.
|
|
| 4. |
- Bohlin, Alexis, et al.
(författare)
-
Pure rotational CARS measurements of temperature and O2-concentration in a low swirl turbulent premixed flame
- 2013
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3629-3636
-
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.
|
|
| 5. |
- Bohlin, Alexis, et al.
(författare)
-
Validation of a rotational coherent anti-Stokes Raman scattering model for N2O at temperatures from 295K to 796K
- 2012
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 43:5, s. 604-610
-
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.
|
|
| 6. |
- Brackmann, Christian, et al.
(författare)
-
Structure of premixed ammonia plus air flames at atmospheric pressure: Laser diagnostics and kinetic modeling
- 2016
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 163, s. 370-381
-
Tidskriftsartikel (refereegranskat)abstract
- The structure of premixed ammonia air flames, burning at atmospheric pressure under strain-stabilized conditions on a porous-plug burner, has been investigated using laser-diagnostic methods. Profiles of OH, NH, and NO were acquired by laser-induced fluorescence (LIF) and quantitative concentrations of OH and NO were retrieved using a concept for calibration versus absorption utilizing the LIF-signal itself, whereas NH concentrations were evaluated employing a saturated fluorescence signal. In addition, temperatures and relative oxygen concentrations were measured by rotational Coherent Anti-stokes Raman Spectroscopy (CARS). The new experimental data for flames with equivalence ratios of 0.9, 1.0, and 1.2 were used to validate and rank the performance of four contemporary detailed kinetic models. Simulations were carried out using experimental temperature profiles as well as by solving the energy equation. Two models of the same origin, developed by Mendiara and Glarborg (2009) and by Klippenstein et al. (2011), in most cases showed good agreement in terms of radical concentrations, however, the model of Mendiara and Glarborg had better prediction of temperatures and flame front positions. The model by Shmakov et al. (2010) had comparable performance concerning radical species, but significant discrepancies appeared in the prediction of flame front positions. The model of Duynslaegher et al. (2012), in addition to the flame front positions, deviated from experiments or other models in terms of NH and NO concentrations. A sensitivity analysis for the Mendiara-Glarborg mechanism indicated that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to experimental uncertainties. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
|
|
| 7. |
|
|
| 8. |
- Carlsson, Henning, et al.
(författare)
-
Numerical and experimental study of flame propagation and quenching of lean premixed turbulent low swirl flames at different Reynolds numbers
- 2015
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:6, s. 2582-2591
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 9. |
- Hosseinnia, Ali, et al.
(författare)
-
Ethane thermometry using rotational coherent anti-Stokes Raman scattering (CARS)
- 2017
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4461-4468
-
Tidskriftsartikel (refereegranskat)abstract
- The complete rotational CARS (coherent anti-Stokes Raman scattering) spectrum of ethane (C2H6) has for the first time been recorded instantaneously under high spectral resolution, and the potential for thermometry has been investigated. Experiments were performed in the temperature range 292-650K in pure ethane and in binary mixtures with nitrogen. A polarization technique was used to suppress the non-resonant contribution to the CARS signal. The ethane RCARS spectra show both S- and R-branch lines, which are more closely spaced than for the well-known nitrogen spectrum and located at much smaller Raman shifts. The peak signal strength was found to be around 240 times lower for ethane than for nitrogen (at 292K). Two main approaches for ethane thermometry are evaluated, which both show high potential. The first is a method in which a spectrum with unknown temperature is fitted using a library of experimental spectra recorded at various temperatures. The second is a method based on ratios of integrated signals in different spectral regions. A theoretical model for simulation of theoretical spectra is under development.
|
|
| 10. |
- Hosseinnia, Ali, et al.
(författare)
-
Rotational CARS thermometry and concentration measurements in ethane-nitrogen mixtures using Fourier analysis
- 2018
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486. ; 49:7, s. 1096-1108
-
Tidskriftsartikel (refereegranskat)abstract
- Rotational coherent anti-Stokes Raman scattering (CARS) is an established diagnostic technique for thermometry and species concentration measurements of di-atomics and tri-atomics in combustion processes; however, studies on hydrocarbon fuel molecules have been scarce. In this work, we have investigated the diagnostic potential for rotational CARS on ethane with emphasis on concentration measurements in mixtures with nitrogen. Rotational CARS spectra have periodic line structures, and because ethane and nitrogen spectra partially overlap, a novel Fourier analysis approach was developed and applied to facilitate evaluation of relative concentrations and temperatures. Instead of the standard procedure of spectral fitting using libraries of theoretically calculated spectra, the fitting was based on an experimental database of temperatures and relative ethane/nitrogen concentrations. The method was successfully demonstrated in an ethane diffusion flame where ethane, initially at room temperature, is heated downstream as well as mixed with increasing amounts of nitrogen. The evaluated temperatures and concentrations are in good agreement with computational fluid dynamic model simulations.
|
|
| 11. |
- Kliewer, C. J., et al.
(författare)
-
Time-domain measurements of S-branch N-2-N-2 Raman linewidths using picosecond pure rotational coherent anti-Stokes Raman spectroscopy
- 2012
-
Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 108:2, s. 419-426
-
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.
|
|
| 12. |
- Kristensson, Elias, et al.
(författare)
-
Stray light suppression in spectroscopy using periodic shadowing
- 2014
-
Ingår i: Optics Express. - 1094-4087. ; 22:7, s. 7711-7721
-
Tidskriftsartikel (refereegranskat)abstract
- It is well known that spectroscopic measurements suffer from an interference known as stray light, causing spectral distortion that reduces measurement accuracy. In severe situations, stray light may even obscure the existence of spectral lines. Here a novel general method is presented, named Periodic Shadowing, that enables effective stray light elimination in spectroscopy and experimental results are provided to demonstrate its capabilities and versatility. Besides its efficiency, implementing it in a spectroscopic arrangement comes at virtually no added experimental complexity. (C) 2014 Optical Society of America
|
|
| 13. |
- Kristensson, Elias, et al.
(författare)
-
Stray light suppression in spectroscopy using periodic shadowing
- 2014
-
Ingår i: Optics Express. - : Optica Publishing Group. - 1094-4087. ; 22:7, s. 7711-7711
-
Tidskriftsartikel (refereegranskat)abstract
- It is well known that spectroscopic measurements suffer from an interference known as stray light, causing spectral distortion that reduces measurement accuracy. In severe situations, stray light may even obscure the existence of spectral lines. Here a novel general method is presented, named Periodic Shadowing, that enables effective stray light elimination in spectroscopy and experimental results are provided to demonstrate its capabilities and versatility. Besides its efficiency, implementing it in a spectroscopic arrangement comes at virtually no added experimental complexity.
|
|
| 14. |
- Marrocco, Michele, et al.
(författare)
-
Sensitivity of coherent anti-Stokes Raman lineshape to time asymmetry of laser pulses
- 2014
-
Ingår i: Optics Letters. - 0146-9592. ; 39:19, s. 5748-5751
-
Tidskriftsartikel (refereegranskat)abstract
- We show that coherent anti-Stokes Raman lineshapes do not follow known spectral profiles if the time asymmetry of realistic laser pulses is taken into account. Examples are given for nanosecond and picosecond laser pulses commonly employed in frequency-resolved coherent anti-Stokes Raman scattering. More remarkably, the analysis suggests an effect of line narrowing in comparison to the customary approach, based primarily on the Voigt lineshape. (C) 2014 Optical Society of America
|
|
| 15. |
- Nordström, Emil, et al.
(författare)
-
Edge effects on decomposition in Sphagnum bogs : Implications for carbon storage
- 2022
-
Ingår i: Ecosphere. - : John Wiley & Sons. - 2150-8925 .- 2150-8925. ; 13:9
-
Tidskriftsartikel (refereegranskat)abstract
- Peatlands provide multiple ecosystem services, including extensive carbon sequestration and storage, yet many peatlands have been degraded or destroyed. Peatlands' carbon storage capacity is connected to inherently low decomposition rates, causing the buildup of organic matter. This pattern could be explained by waterlogged conditions that reduce the amount of available oxygen for the decomposer community, a low pH that inhibits bacterial decomposition, or colder temperatures lowering metabolic rates. This study focused on edge effects on decomposition in the transition zone (lagg) between Sphagnum bogs and the surrounding forest, with the expectation that decomposition is lowest in the bog and highest in the forest but with a mix of factors causing intermediate decomposition rates near the bog edge. Transitional decomposition rates were measured across six bogs in central Sweden during the summer of 2021, following the Tea Bag Index. Three 20-m transects, each containing seven pairs of tea bags, were buried across the margins of each bog, centered at the edge of the Sphagnum moss. Soil moisture content, pH, and plant composition were also recorded at each burial site, and temperature loggers placed evenly among four of the bogs. Our results confirmed our hypothesis regarding edge effects, with soil moisture levels showing a strong negative interaction with decomposition rate. The interaction between pH and decomposition rate was significant, but with an unexpected negative relation, most likely due to low pH in the surrounding forest. Temperature displayed no significance, and plants indicative of low decomposition rates included Vaccinium oxycoccos, Drosera rotundifolia, and Sphagnum species. In contrast to other studies, we did not find an increase in decomposition with increased species richness among the studied bog ecosystems. In conclusion, there is an edge effect on decomposition, and maintaining, or restoring, the hydrology of a peatland is the most important factor for continued carbon storage, with a rough estimation of an area decomposition rate possible to be estimated based on its vegetation.
|
|
| 16. |
- Nordström, Emil, et al.
(författare)
-
Fourier-based approach to overcome anomalies in high-resolution spectra of vibrational CARS measurements of gases
- 2014
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 45:7, s. 489-494
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we report on an unconventional experimental procedure useful for thermometric measurements on the basis of high-resolution vibrational coherent anti-Stokes Raman scattering (CARS) generated using conventional lasers. The high spectral dispersion of 0.02 cm(-1)/pixel is achieved by means of some specific spectral arrangements (single-mode pump laser and diffraction grating providing spectra at the sixth order) and is further strengthened by the use of a relay lens system mounted before the charge-coupled device camera. Surprisingly, at the high spectral dispersion of our measurements, a significant and persistent thermometric inaccuracy is observed. The effect arises from an inevitable spectral modulation of defined frequency hidden in the main CARS signal of the gas under study, and to secure a good thermometric evaluation, we describe a Fourier approach that is experimentally demonstrated for the typical nitrogen CARS spectrum measured at room temperature. Copyright (C) 2014 John Wiley & Sons, Ltd.
|
|
| 17. |
- Nordström, Emil, et al.
(författare)
-
Local gas heating in sooting flames by heat transfer from laser-heated particles investigated using rotational CARS and LII
- 2015
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35, s. 3707-3713
-
Tidskriftsartikel (refereegranskat)abstract
- Soot particles strongly absorb radiation in the visible and infrared spectral regions, and the soot interaction with laser light during laser diagnostic interrogation leads to particle heating and often to subsequent sublimation. Consequently, laser-heated particles transfer heat to the ambient gas leading to local gas heating, a process that has received minor attention so far in the diagnostic community. In the present work, this specific local gas heating is measured in a pump-probe-type experiment. A 1064-nm laser beam heated the soot particles in an ethylene/ air diffusion flame (on a Gulder-burner) with known soot volume fraction, and a two-beam rotational coherent anti-Stokes Raman spectroscopy (CARS) setup was used to probe the local gas temperature on time scales from nanoseconds to milliseconds. The temperature of the heated particles was simultaneously probed using a two-color laser-induced incandescence (2C-LII) detection system. The results show that laser heating of soot particles from flame temperatures to sublimation temperatures leads to local gas heating of similar to 100 K at a soot volume fraction of 4 ppm, in good agreement with theoretical predictions. The implication of these results to the application of laser diagnostic techniques is briefly discussed. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
|
|
| 18. |
- Nordström, Emil, et al.
(författare)
-
Pure rotational Coherent anti-Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics
- 2013
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 44:10, s. 1322-1325
-
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.
|
|
| 19. |
- Nordström, Emil, et al.
(författare)
-
Raman linewidth measurements using time-resolved hybrid picosecond/nanosecond rotational CARS
- 2015
-
Ingår i: Optics Letters. - 0146-9592. ; 40:24, s. 5718-5721
-
Tidskriftsartikel (refereegranskat)abstract
- We report an innovative approach for time-domain measurements of S-branch Raman linewidths using hybrid picosecond/nanosecond pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS). The Raman coherences are created by two picosecond excitation pulses and are probed using a narrow-band nanosecond pulse at 532 nm. The generated RCARS signal contains the entire coherence decay in a single pulse. By extracting the decay times of the individual transitions, the J-dependent Raman linewidths can be calculated. Self-broadened S-branch linewidths for nitrogen and oxygen at 293 K and ambient pressure are in good agreement with previous time-domain measurements. Experimental considerations of the approach are discussed along with its merits and limitations. The approach can be extended to a wide range of pressures and temperatures and has potential for simultaneous single-shot thermometry and linewidth determination. (C) 2015 Optical Society of America
|
|
| 20. |
- Nordström, Emil
(författare)
-
Rotational Coherent Anti-Stokes Raman Spectroscopy - Development and Applications
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the last decades, coherent anti-Stokes Raman spectroscopy (CARS) has been an important tool in combustion research. The focus of this thesis is the improvement of the capability of the pure rotational CARS (RCARS) technique for this type of applications. The technique is used to measure temperatures and relative species concentrations in the gas phase and thermometry is performed by fitting theoretically modelled spectra to measured ones. In a first approximation, the thermometric information in the RCARS signal lies in the relative RCARS peaks intensities, reflecting the relative populations of the rotational states of the probed molecules. However, for accurate RCARS thermometry, a number of additional factors must be taken into account. The most important of these is the Raman linewidths. This parameter, which is governed by collisional energy transfer between the probed molecules and the surrounding gas, can have large impact on evaluated temperature if it is not properly included in the RCARS modelling. Therefore, for many of the articles on which this thesis is based, much effort has been made to investigate the thermometric impact of the Raman linewidths. There are also a number of works based on reporting Raman linewidths and developing a new method of measuring them. The RCARS technique can also be sensitive to different kinds of spectral interference, either in the form of scattered light from the laser sources used to generate the RCARS signal, or from unwanted contributions to the coherent RCARS signal itself. For several of the included papers, the application of polarization techniques was used to reduce this type of interference. This enabled more accurate thermometry and in some cases detection of the resonant signal was entirely dependent on this technique. The application of the RCARS technique in the papers included in this thesis has mainly been flame measurements. Both laminar and turbulent flames have been studied in collaborations with other experimental research groups and theoretical modelling groups. Spatial temperature scans in two dimensions have been performed in turbulent flames and the RCARS evaluations have been used to improve large eddy simulations of turbulent flames. RCARS measurements have also been performed in a heavily sooting flame in order to characterize how the laser induced incandescence (LII) technique influences the flame during measurement.
|
|
