| 1. |
- Brackmann, Christian, et al.
(författare)
-
Picosecond excitation for reduction of photolytic effects in two-photon laser-induced fluorescence of CO
- 2013
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34, s. 3541-3548
-
Tidskriftsartikel (refereegranskat)abstract
- Two-photon laser-induced fluorescence for detection of carbon monoxide (CO) frequently shows interferences by emission from photolytically produced C-2 radicals encountered under fuel-rich combustion conditions. Reduced C-2 interference for excitation with laser pulses in the picosecond regime is here demonstrated by comparison with excitation using nanosecond pulses for measurements in laminar premixed ethene-air flames. Compared with nanosecond pulses of 8 ns duration and 4 mJ pulse energy, picosecond pulses of 80 ps duration and around 0.5 mJ pulse energy gave similar to 10 times higher peak power, which allowed for efficient CO excitation and resulted in stronger signal with lower C-2 interference. CO fluorescence with picosecond excitation showed a linear to quadratic power dependence, indicating photoionization, whereas a more quadratic dependence was found for the C-2 interference. A sub-nanosecond effective lifetime of CO resulted in a rapid fluorescence decay compared with C-2 and allowed for efficient reduction in C-2 interference by minimizing the detection gate. In addition, interference compensation using time-resolved detection could be demonstrated. Altogether, picosecond pulses provide efficient two-photon excitation of CO in terms of signal strength as well as reduced C-2 interference. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Afzelius, Mikael, et al.
(författare)
-
Development of multipoint vibrational coherent anti-Stokes Raman spectroscopy for flame applications
- 2006
-
Ingår i: Applied Optics. - 2155-3165. ; 45:6, s. 1177-1186
-
Tidskriftsartikel (refereegranskat)abstract
- A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. In a multipass cavity the pump and Stokes laser beams are multiply reflected and refocused into a measurement volume with an adjustable number of separated points along a line. This optical arrangement was used in a vibrational CARS setup with planar BOXCARS phase-matching configuration. The CARS spectra from spatially separated points were recorded at different heights on a CCD camera. Measurements of temperature profiles were carried out in the burned gas zone of a premixed one-dimensional flame to demonstrate the applicability of this method for temperature measurements in high-temperature regions. The ability to measure in flames with strong density gradients was demonstrated by simultaneous measurements of Q-branch spectra of N-2 and CO in a Wolfhard-Parker burner flame. Interference phenomena found in multipoint spectra are discussed, and possible solutions are proposed. Merits and limitations of the technique are discussed. (c) 2006 Optical Society of America.
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| 3. |
- Aldén, Marcus, et al.
(författare)
-
Visualization and understanding of combustion processes using spatially and temporally resolved laser diagnostic techniques
- 2011
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 69-97
-
Forskningsöversikt (refereegranskat)abstract
- Laser diagnostic techniques have for more than 30 years added very valuable input for a deepened understanding of combustion processes. The present paper will focus on techniques developed for visualization of important parameters with the ability to get detailed information in space and time. The paper is not meant to be a complete review of the entire research field but rather a survey with a majority of the contributions from the authors' laboratory. After a short introduction and background, essentially three techniques will be highlighted. Planar Laser-Induced Fluorescence, PLIF, will be described in terms of further development directed towards the use of a high-power Alexandrite laser, in some cases yielding a substantially increased sensitivity as compared to the more conventional Nd:YAG and Excimer lasers. There will also be a description of possibilities for high-speed visualization, 3D imaging and the potential to visualize species requiring a two-photon excitation process. Regarding thermometry, the paper will describe and exemplify the use of Thermographic Phosphors, mainly for surface measurements but also in droplets and gas flows. The third area to be described is the use of Polarization Spectroscopy, PS, for spatially and temporally resolved measurements in the IR spectral region, e. g. CO2 and H2O, various hydrocarbons, HCl and HCN. The last chapter focuses on present challenges and future applications. A major challenge is diagnostics in environments with limited optical access. Two methods that are applicable under such conditions are discussed, namely endoscopic approaches and picosecond LIDAR. Another challenging area addressed is application of diagnostics in optically dense environments, e. g. sprays, where a new technique, Structured Laser Illumination Planar Imaging, SLIPI, has been developed for suppressing the influence of multiple scattering. Finally, approaches for measurements of "new" species, challenges associated with LIF quantification, as well as measurements in environments possessing strong background radiation are discussed. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 4. |
- Arndt, Christoph, et al.
(författare)
-
Laser Applications to Chemical, Security, and Environmental Analysis : introduction to the feature issue
- 2023
-
Ingår i: Applied Optics. - 1559-128X. ; 62:6, s. 1-3
-
Tidskriftsartikel (refereegranskat)abstract
- The eighteenth topical meeting on Laser Applications to Chemical, Security, and Environmental Analysis (LACSEA) was held in Vancouver, Canada from 11–15 July 2022, as part of the Optica Optical Sensors and Sensing Congress in a hybrid format allowing on-site and online attendance. The meeting featured a broad range of distinguished papers focusing on recent advances in laser and optical spectroscopy. A total of 52 contributed and invited papers were presented during the meeting, including topics such as photo-acoustic spectroscopy, imaging, non-linear technologies, frequency combs, remote sensing, environmental monitoring, aerosols, combustion diagnostics, hypersonic flow diagnostics, nuclear diagnostics, fs/ps applications, and machine learning and computational sensing.
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| 5. |
- Bao, Yupan, et al.
(författare)
-
Improved temporal contrast of streak camera measurements with periodic shadowing
- 2021
-
Ingår i: Optics Letters. - 0146-9592. ; 46:22, s. 5723-5726
-
Tidskriftsartikel (refereegranskat)abstract
- Periodic shadowing, a concept used in spectroscopy for stray light reduction, has been implemented to improve the temporal contrast of streak camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.
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| 6. |
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| 7. |
- Bood, Joakim
(författare)
-
Development of Dual-broadband Rotational CARS for Combustion Diagnostics
- 2000
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis concerns development and application of dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) for temperature and species concentration measurements in combustion processes. Both fundamental development of the technique, including experimental as well as modelling results, and measurements in practical combustion devices were conducted. A code for calculation of rotational CARS spectra of pure acetylene as well as mixtures of acetylene and nitrogen was developed. Using this code, temperatures and relative acetylene to nitrogen concentrations were evaluated from DB-RCARS measurements in pure acetylene and different acetylene/nitrogen mixtures. Moreover, rotational CARS spectra of dimethyl-ether (DME) have been analyzed. A powerful tool for simultaneous temperature and multiple species concentration measurements was developed by combining rotational CARS with vibrational CARS. The concept was demonstrated for measurements of temperature, oxygen, and carbon monoxide concentrations simultaneously in a premixed sooting ethene/air flame. Rotational CARS spectra of nitrogen at very high pressures (0.1-44 MPa) at room temperature were investigated. The experimental spectra were compared with calculated spectra using different Raman linewidth models. The results indicate some shortcomings in the present model, basically the density calculation and neglecting overlapping effects between adjacent spectral lines. A new method for CARS measurements in several spatially separated points simultaneously was developed. By using DB-RCARS the method was demonstrated for quantitative measurements of profiles of temperatures and oxygen concentrations. An atomic filter for rejection of stray light was developed. The filter was shown to efficiently reject stray light from the narrowband laser without affecting the shape of the rotational CARS spectrum or causing any signal losses. Within an interdisciplinary project intended to increase the fundamental knowledge about engine knock, DB-RCARS was used for the first time for thermometry in a spark-ignition engine running on a fuel consisting of a mixture of higher hydrocarbons (iso-octane and n-heptane). Cycle-resolved temperature measurements were successfully performed in the unburned gas mixture (the end-gas) ahead of the propagating flame front at pressures below 3.0 MPa and temperatures below 1000 K. In order to investigate the performance of a catalytic combustor, simultaneous temperature and relative oxygen-to-nitrogen concentrations were measured by DB-RCARS in the exhaust gas of the combustor. From a theoretical sensitivity analysis it was found that the accuracy of temperature and oxygen concentration could be improved by fitting the nonresonant susceptibilty in the evaluation process.
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| 8. |
- Bood, Joakim, et al.
(författare)
-
Diagnostic properties of two-photon-pumped stimulated emission in atmospheric species
- 2018
-
Ingår i: Springer Series in Optical Sciences. - Cham : Springer International Publishing. - 1556-1534 .- 0342-4111. ; 208, s. 1-17
-
Bokkapitel (refereegranskat)abstract
- Two-photon excitation provides access to atoms and molecules with absorption resonances in the vacuum ultraviolet (VUV), i.e., to the species otherwise inaccessible for probing under atmospheric conditions, since the atmosphere strongly absorbs VUV radiation. In addition, for high enough laser intensities, two-photon pumping may create population inversion between the pumped energy state and a lower-lying intermediate state, resulting in stimulated emission. In this chapter such an emission is discussed in terms of its diagnostic capacity. The method has primarily been investigated for the detection of a number of atomic species, such as oxygen and nitrogen, and a few small molecules, for example, CO and NH3. The major benefits of the technique are that the signal propagates in a laser-like beam, the backward-directed beam allows single-ended diagnostics, strong signals allow trace-level detection, and the optical setup is relatively simple. The main disadvantages are the poor and sometimes ambiguous spatial resolution and the difficulties with modeling the process due to the nonlinear dependence of the signal on the concentration of the active species and the integrative-growth nature of the signal. Besides exploring the potential for diagnostics, early works on two-photon-induced stimulated emission have been imperative for today’s development of air-lasing concepts based on backward-directed stimulated emission. This chapter is devoted to a review and summary of these pioneering studies.
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| 9. |
- Bood, Joakim, et al.
(författare)
-
Knock in spark-ignition engines : End-gas temperature measurements using rotational CARS and detailed kinetic calculations of the autoignition process
- 1997
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
-
Konferensbidrag (refereegranskat)abstract
- Cycle-resolved end-gas temperatures were measured using dual-broadband rotational CARS in a single-cylinder spark-ignition engine. Simultaneous cylinder pressure measurements were used as an indicator for knock and as input data to numerical calculations. The chemical processes in the end-gas have been analysed with a detailed kinetic mechanism for mixtures of iso-octane and n-heptane at different Research Octane Numbers (RON'S). The end-gas is modelled as a homogeneous reactor that is compressed or expanded by the piston movement and the flame propagation in the cylinder. The calculated temperatures are in agreement with the temperatures evaluated from CARS measurements. It is found that calculations with different RON'S of the fuel lead to different levels of radical concentrations in the end-gas. The apperance of the first stage of the autoignition process is marginally influenced by the RON, while the ignition delay of the second stage is increased with increasing RON.
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| 10. |
- Bood, Joakim, et al.
(författare)
-
Multipoint temperature and oxygen-concentration measurements using rotational coherent anti-Stokes Raman spectroscopy
- 2000
-
Ingår i: Optics Letters. - 0146-9592. ; 25:20, s. 1535-1537
-
Tidskriftsartikel (refereegranskat)abstract
- A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. With a system of cylindrical lenses, each laser beam is split into several focused beams, yielding separate planar boxcars configurations. Spectrally resolved CARS signals are detected at different heights on the CCD chip. With dual-broadband rotational CARS the setup is demonstrated for quantitative measurements of temperature- and oxygen-concentration profiles. The technique was demonstrated for three points only, but it can be extended to more points by use of special optics; this choice must be based on a sufficient signal-to-noise ratio in all points for the actual measurement condition.
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| 11. |
- Bood, Joakim, et al.
(författare)
-
Non-intrusive temperature and oxygen concentration measurements in a catalytic combustor using : Rotational coherent anti-stokes raman spectroscopy
- 1999
-
Ingår i: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations. - 9780791878590 ; 2
-
Konferensbidrag (refereegranskat)abstract
- Rotational CARS (Coherent anti-Stokes Raman Spectroscopy) was used to measure temperature and relative oxygen concentrations in the exhaust gas of a catalytic combustor. This laser technique has the general advantages of such techniques, i.e. possibility of performing in-situ measurements with high spatial and temporal resolution, and non-intrusiveness. Profiles of temperature and oxygen concentrations are presented for a full-load condition, and the measured temperatures are compared with calculated values. Of great importance for the results is the accuracy of the technique, and a sensitivity analysis is performed to test the temperature and oxygen concentration dependence on uncertainties in experimental parameters. It was shown that the accuracy of temperature and oxygen concentration could be improved by fitting the nonresonant susceptibility in the evaluation procedure. The measurements were performed as a project in the European gas turbine program AGATA with the aim to develop a catalytic combustor with ceramic structural components and producing low emissions of pollutants.
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| 12. |
- Bood, Joakim, et al.
(författare)
-
Rotational coherent anti-Stokes Raman spectroscopy (CARS) in nitrogen at high pressures (0.1-44 MPa) : Experimental and modelling results
- 2000
-
Ingår i: Journal of Raman Spectroscopy. - 0377-0486. ; 31:8-9, s. 703-710
-
Tidskriftsartikel (refereegranskat)abstract
- Pure rotational coherent anti-Stokes Raman scattering (CARS) has for the first time been investigated for pressures up to 44 MPa in nitrogen gas at room temperature. An atomic filter consisting of a heated cell with sodium vapour was successfully applied for suppression of stray light originating from the narrowband CARS pump laser beam. With increasing pressure the rotational CARS spectrum is smoothed gradually, and above around 10 MPa spectral lines are no longer resolvable. Experimental data were compared using Raman linewidths calculated with three different models: the energy corrected sudden scaling law (ECS), the modified exponential gap model (MEG), and a semi-classical ab initio model. All three models resulted in bad spectral fits when a linear scaling with pressure was employed. By using additional scale factors for the Raman linewidths the quality of the spectral fits and also the temperature accuracy were significantly improved. The resulting scale factors indicate a non-linear pressure dependence of the linewidths, and visualize a narrowing of the spectral envelope at the highest pressure, i.e. 44 MPa. The results indicate shortcomings in the present isolated line models and also emphasize the need for new experimental data on pure rotational Raman linewidths at high density. Copyright (C) 2000 John Wiley and Sons, Ltd.
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| 13. |
- Bood, Joakim, et al.
(författare)
-
Stray light rejection in rotational coherent anti-Stokes Raman spectroscopy by use of a sodium-seeded flame
- 1998
-
Ingår i: Applied Optics. - 1559-128X. ; 37:36, s. 8392-8396
-
Tidskriftsartikel (refereegranskat)abstract
- A common experimental problem with rotational coherent anti-Stokes Raman spectroscopy (CARS) is undesired spectral interference that is due to stray light from the primary laser beams. Also, for the most developed approach, dual-broadband rotational CARS, practical measurements often suffer from stray light interference from the narrow-band laser, inasmuch as the CARS signal is produced inherently in the spectral vicinity of the narrow-band laser beam. An optical filter does not provide a sufficiently sharp transmission profile, thus leading to signal loss and spectral distortion of the rotational CARS signal. An atomic filter consisting of a sodium-seeded flame is presented here as a solution to the problem, and its usefulness was demonstrated in dual-broadband rotational CARS experiments.
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| 14. |
- Brackmann, Christian, et al.
(författare)
-
Quantitative measurements of species and temperature in a DME-air counterflow diffusion flame using laser diagnostic methods
- 2006
-
Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 178:6, s. 1165-1184
-
Tidskriftsartikel (refereegranskat)abstract
- A diffusion flame of dimethyl ether (DME) and air in a counterflow burner has been investigated experimentally by means of different laser-based methods. Quantitative measurements of temperature, O-2 concentration, and OH concentration have been carried out using dual broadband rotational coherent anti-Stokes Raman Spectroscopy (CARS), Rayleigh scattering and laser-induced fluorescence (LIF). Furthermore a qualitative formaldehyde profile has been measured using LIF. The measured profile has been compensated for fluorescence signal quenching and the Boltzmann distribution to obtain a representative qualitative formaldehyde profile for the flame.
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| 15. |
- Brackmann, Christian, et al.
(författare)
-
Quantitative picosecond laser-induced fluorescence measurements of nitric oxide in flames
- 2017
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4533-4540
-
Tidskriftsartikel (refereegranskat)abstract
- Quantitative concentrations measurements using time-resolved laser-induced fluorescence have been demonstrated for nitric oxide (NO) in flame. Fluorescence lifetimes measured using a picosecond Nd:YAG laser and optical parametric amplifier system have been used to directly compensate the measured signal for collisional quenching and evaluate NO concentration. The full evaluation also includes the spectral overlap between the ∼15cm-1 broad laser pulse and multiple NO absorption lines as well as the populations of the probed energy levels. Effective fluorescence lifetimes of 1.2 and 1.5ns were measured in prepared NO/N2/O2 mixtures at ambient pressure and temperature and in a premixed NH3-seeded CH4/N2/O2 flame, respectively. Concentrations evaluated from measurements in NO/N2/O2 mixtures with NO concentrations of 100-600ppm were in agreement with set values within 3% at higher concentrations. An accuracy of 13% was estimated by analysis of experimental uncertainties. An NO profile measured in the flame showed concentrations of ∼1000ppm in the post-flame region and is in good agreement with NO concentrations predicted by a chemical mechanism for NH3 combustion. An accuracy of 16% was estimated for the flame measurements. The direct concentration evaluation from time-resolved fluorescence allows for quantitative measurements in flames where the composition of major species and their collisional quenching on the probed species is unknown. In particular, this is valid for non-stationary turbulent combustion and implementation of the presented approach for measurements under such conditions is discussed.
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| 16. |
- Brackmann, Christian, et al.
(författare)
-
Simultaneous vibrational and pure rotational coherent anti-Stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames
- 2002
-
Ingår i: Applied Optics. - 2155-3165. ; 41:3, s. 564-572
-
Tidskriftsartikel (refereegranskat)abstract
- The potential of measuring temperature and multiple species concentrations (N-2, O-2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS, spectra, O-2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique fur probing of additional species are discussed. (C) 2002 Optical Society of America.
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| 17. |
- Brackmann, Christian, et al.
(författare)
-
Thermometry in internal combustion engines via dual-broadband rotational coherent anti-Stokes Raman spectroscopy
- 2004
-
Ingår i: Measurement Science & Technology. - 0957-0233. ; 15:3, s. 13-25
-
Forskningsöversikt (refereegranskat)abstract
- Rotational coherent anti-Stokes Raman spectroscopy (CARS) has since the beginning of the 1980s been developed as a non-intrusive tool for temperature measurements in combustion. Since the introduction of the dual-broadband concept in 1986, the quality of the technique has been much improved, and application to practical combustion situations facilitated. Since the first demonstration of its use in spark-ignition engines in 1993, several measurement campaigns in engines have been accomplished. These campaigns concerned temperature measurements in the unburned gas mixture before combustion as part of a larger project with the aim of improving the knowledge on the phenomenon of engine knock. In this paper, the results of this work are reviewed with a focus on the characteristics of the technique and the quality of the evaluated temperatures. Re-evaluations of data using an improved theoretical model are presented and compared with previous results. Moreover, the treatment of large data sets on single shots from spatial regions with conditions varying from unburned to burned gas is discussed. It is demonstrated that dual-broadband rotational CARS probing nitrogen and oxygen has a high potential for thermometry at the conditions in the unburned gas mixture. Merits and limitations of the technique are discussed and the issues treated are, among others, experimental problems, data evaluation, and single-shot temperature accuracy.
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| 18. |
- Brydegaard, Mikkel, et al.
(författare)
-
Short-Wave infrared atmospheric scheimpflug lidar
- 2018
-
Ingår i: EPJ Web of Conferences. - : EDP Sciences. ; 176
-
Konferensbidrag (refereegranskat)abstract
- Atmospheric dual-band Scheimpflug lidar is demonstrated at 980 and 1550 nm. Signals are compared during three weather conditions, and the spatio-temporal resolution of the atmospheric structure is considered. The potential for aerosol classification is evaluated, and future directions are discussed.
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| 19. |
- Buldyreva, J, et al.
(författare)
-
Linewidth modelling of C2H2-N-2 mixtures tested by rotational CARS measurements
- 2006
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 1097-4555 .- 0377-0486. ; 37:6, s. 647-654
-
Tidskriftsartikel (refereegranskat)abstract
- The acetylene-nitrogen system is investigated in a combined theoretical and experimental effort in view of its thermometry applications. Accurate values of the rotational Raman linewidths of acetylene are of crucial importance for the computation of spectra from rotational coherent anti-Stokes Raman scattering (CARS) used for temperature and concentration evaluations. Since the direct measurements of these linewidths are not available, they are rigorously calculated by a semi-classical Robert-Bonamy formalism employing exact trajectories. The computed linewidths are used as input parameters in the theoretical spectra simulation, which is also improved by the inclusion of an interbranch interference with coherent Stokes Raman scattering (CSRS). These theoretical spectra are used to evaluate the temperature from experimental CARS spectra of acetylene-nitrogen mixtures recorded in the pressure range 0.1-0.6 MPa. The new linewidths (NLWs) and the CSRS interference inclusion result in a clear improvement of spectral fitting as well as in reasonable values of evaluated temperatures. Copyright (c) 2006 John Wiley & Sons, Ltd.
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| 20. |
- Cont-Bernard, Davide Del, et al.
(författare)
-
Femtosecond two-photon laser-induced fluorescence imaging of atomic hydrogen in a laminar methane-air flame assisted by nanosecond repetitively pulsed discharges
- 2020
-
Ingår i: Plasma Sources Science and Technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 29:6
-
Tidskriftsartikel (refereegranskat)abstract
- Sustainable and low-emission combustion is in need of novel schemes to enhance combustion efficiency and control to meet up with new emission standards and comply with varying quality of renewable fuels. Plasma actuation is a promising candidate to achieve this goal but few detailed experiments have been carried out that target how specific combustion and plasma related species are affected by the coupling of plasma and combustion chemistry. Atomic hydrogen is such a species that here is imaged by using the two-photon absorption laser induced fluorescence (TALIF) technique as an atmospheric pressure methane-air flame is actuated by nanosecond repetitively pulsed (NRP) discharges. Atomic hydrogen is observed both in the flame and in the discharge channel and plasma actuation results in a wide modification of the flame shape. A local 50% increase of fluorescence occurs at the flame front where it is crossed by the discharge. Atomic hydrogen in the discharge channel in the fresh-gases is found to decay with a time constant of about 2.4 μs. These results provide new insights on the plasma flame interaction at atmospheric pressure that can be further used for cross-validation of numerical calculations.
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| 21. |
- Coskun, Gökhan, et al.
(författare)
-
Analysis of in-cylinder H2O2 and HO2 distributions in an HCCI engine - Comparison of laser-diagnostic results with CFD and SRM simulations
- 2015
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:9, s. 3131-3139
-
Tidskriftsartikel (refereegranskat)abstract
- By applying a novel approach to evaluate photofragmentation laser-induced fluorescence (PFLIF) imaging, experimental quantitative information on the temporal in-cylinder distribution of hydrogen peroxide (H2O2) in a homogeneous charge compression ignition (HCCI) engine was extracted. The results from PFLIF were then compared to those obtained from chemical kinetics simulations using computational fluid dynamics (CFD) and a stochastic reactor model (SRM). For the CFD simulations, a sector mesh was applied using Reynolds-averaged Navier-Stokes (RANS) equations together with a reduced chemical kinetic model. These simulations provided detailed information on the spatial distribution of H2O2, HO2 as well as other important species and temperature. The SRM, which offers substantially reduced simulation times but no spatial information, was used with the same reduced kinetic model. Two-dimensional images from PFLIF and CFD show a fair temporal agreement, while details of the spatial distributions disagree. The CFD images show that the combustion chemistry is affected by the interaction with the cylinder walls with, for instance, a local delay of the formation and consumption of H2O2. By using probability density functions (PDFs) of H2O2 and HO2 mass fractions, comparisons could be made between experimental data and both the CFD and SRM simulations. In general the range of mass fractions show good agreement but the experimental distributions are wider. Possible reasons for this discrepancy are actual heterogeneities in the H2O2/HO2 concentration distributions not predicted by the model, spatial temperature variations, which will influence the strength of the PFLIF signal, spatial variations in the laser profiles, not accounted for in the data processing, and photon noise. The good agreement between the CFD and SRM shows the relevance of fast PDF based simulation tools. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 22. |
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| 23. |
- Ding, Pengji, et al.
(författare)
-
Bidirectional Flame Lasing
- 2018
-
Ingår i: Northern Optics & Photonics 2018 12-14 September 2018 Lund University, Lund, Sweden : Conference Proceedings - Conference Proceedings. - 9789163964886
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 24. |
- Ding, Pengji, et al.
(författare)
-
Detection of atomic oxygen in a plasma-assisted flame via a backward lasing technique
- 2019
-
Ingår i: Optics Letters. - 0146-9592. ; 44:22, s. 5477-5480
-
Tidskriftsartikel (refereegranskat)abstract
- In this Letter, we have investigated 845 nm lasing generation in atomic oxygen, present in a lean methane-air flame, using two-photon pumping with femtosecond 226 nm laser pulses, particularly focusing on the impact of nanosecond repetitively pulsed glow discharges forcing on the backward lasing signal. Characterizations of the backward lasing pulse, in terms of its spectrum, beam profile, pump pulse energy dependence, and divergence, were conducted to establish the presence of lasing. With plasma forcing of the flame, the backward lasing signal was observed to be enhanced significantly, ∼50%. The vertical concentration profile of atomic oxygen was revealed by measuring the backward lasing signal strength as a function of height in the flame. The results are qualitatively consistent with results obtained with two-dimensional femtosecond two-photon-absorption laser-induced fluorescence, suggesting that the backward lasing technique can be a useful tool for studies of plasma-assisted combustion processes, particularly in geometries requiring single-ended standoff detection.
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| 25. |
- Ding, Pengji, et al.
(författare)
-
Femtosecond laser-induced quantum-beat superfluorescence of atomic oxygen in a flame
- 2021
-
Ingår i: Physical Review A. - 2469-9926. ; 104:3
-
Tidskriftsartikel (refereegranskat)abstract
- Among different approaches to generate mirrorless lasing, resonant multiphoton pumping of gas constituents by deep-UV laser pulses exhibits so far the highest efficiency and produces measurable lasing energies, but the underlying mechanism was not yet fully settled. Here, we report lasing generation from atomic oxygen in a methane-air flame via femtosecond two-photon excitation. Temporal profiles of the lasing pulses were measured for varying concentrations of atomic oxygen, which shows that the peak intensity and time delay of the lasing pulse approximately scales as N and 1/N, respectively, where N represents the concentration. These scaling laws match well with the prediction of oscillatory superfluorescence (SF), indicating that the lasing we observed is essentially SF rather than amplified spontaneous emission. In addition, the quantum-beating effect was also observed in the time-resolved lasing pulse. A theoretical simulation based on nonadiabatic Maxwell-Bloch equations well reproduces the experimental observations of the temporal dynamics of the lasing pulses. These results on fundamentals should be beneficial for the better design and applications of lasing-based techniques.
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| 26. |
- Ding, Pengji, et al.
(författare)
-
Femtosecond two-photon-excited backward lasing of atomic hydrogen in a flame
- 2018
-
Ingår i: Optics Letters. - 0146-9592. ; 43:5, s. 1183-1186
-
Tidskriftsartikel (refereegranskat)abstract
- We report on an observation of bi-directional 656 nm lasing action of atomic hydrogen in a premixed CH4∕air flame induced by resonant femtosecond 205 nm two-photon excitation. In particular, the backward-propagating lasing pulse is characterized in the spatial and temporal domains for the sake of a single-ended diagnostic. Its picosecond-scale duration and smooth temporal profile enable spatially resolved detection of hydrogen atoms in the millimeter range, which is successfully demonstrated using two narrow welding flames.
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| 27. |
- Ding, Pengji, et al.
(författare)
-
Gain mechanism of femtosecond two-photon-excited lasing effect in atomic hydrogen
- 2019
-
Ingår i: Optics Letters. - 1539-4794. ; 44:9, s. 2374-2377
-
Tidskriftsartikel (refereegranskat)abstract
- By aiming to establish single-ended standoff combustion diagnostics, bidirectional lasing emissions of atomic hydrogen at 656 nm wavelength have been generated via two-photon resonant excitation by focusing 205 nm femtosecond laser pulses into a premixed CH4/O2 flame. The forward lasing strength is approximately one order of magnitude stronger than that of the backward one, due to the geometry of traveling wave excitation over a 2-mm-long pencil-shaped gain volume and the short gain lifetime of 3.5 ps. The gain coefficient of hydrogen lasing was determined to approximate 52/cm. As for the underlying physics of hydrogen lasing, amplified spontaneous emission (ASE) occurs simultaneously with four-wave mixing (FWM), and ASE dominates in the forward direction, whereas the backward lasing is virtually only ASE.
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| 28. |
- Ding, Pengji, et al.
(författare)
-
Picosecond backward-propagating lasing of atomic hydrogen via femtosecond 2-photon-excitation in a flame
- 2018
-
Ingår i: CLEO : QELS_Fundamental Science, CLEO_QELS 2018 - QELS_Fundamental Science, CLEO_QELS 2018. - 9781557528209 ; Part F93-CLEO_QELS 2018
-
Konferensbidrag (refereegranskat)abstract
- We report on the observation of backward-propagating 656-nm lasing of atomic hydrogen in flame using 205-nm femtosecond laser pulses. It shows a donut-shaped spatial mode and smooth temporal profile, suggesting spatially-resolved measurements in few-millimeters resolution.
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| 29. |
- Ding, Pengji, et al.
(författare)
-
Signature of femtosecond laser-induced superfluorescence from atomic hydrogen
- 2022
-
Ingår i: Physical Review A. - 2469-9926. ; 105:1
-
Tidskriftsartikel (refereegranskat)abstract
- Cavity-free lasing generation from gas constituents has been studied in the past decade since it promises great potentials in remote sensing and optical diagnostics techniques. Here we report on experimental investigations of temporal characteristics of H-atom lasing emission at 656 nm by examining the dependences of its durations and delays on the pump-laser-pulse energies. An indirect measurement was also performed to test the delays of the lasing pulse for varying H-atom concentrations. The results show that the lasing pulse exhibits considerable superfluorescence signatures. Analysis based on experimental parameters by using deductive expressions of superfluorescence theory shows good agreement to this conjecture. Our investigations on fundamentals could pave the way to a better understanding of the lasing generation and further applications of lasing-based optical diagnostics.
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| 30. |
- Ding, Pengji, et al.
(författare)
-
Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane-air flame
- 2021
-
Ingår i: Journal of Physics D: Applied Physics. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 54:27
-
Tidskriftsartikel (refereegranskat)abstract
- The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane-air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane-air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air.
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| 31. |
- Dominguez, Armand, et al.
(författare)
-
A compact Scheimpflug lidar imaging instrument for industrial diagnostics of flames
- 2023
-
Ingår i: Measurement science and technology. - : Institute of Physics. - 0957-0233 .- 1361-6501. ; 34:7
-
Tidskriftsartikel (refereegranskat)abstract
- Scheimpflug lidar is a compact alternative to traditional lidar setups. With Scheimpflug lidar it is possible to make continuous range-resolved measurements. In this study we investigate the feasibility of a Scheimpflug lidar instrument for remote sensing in pool flames, which are characterized by strong particle scattering, large temperature gradients, and substantial fluctuations in particle distribution due to turbulence. An extinction coefficient can be extracted using the information about the transmitted laser power and the spatial extent of the flame. The transmitted laser power is manifested by the intensity of the ‘echo’ from a hard-target termination of the beam located behind the flame, while the information of the spatial extent of the flame along the laser beam is provided by the range-resolved scattering signal. Measurements were performed in heptane and diesel flames, respectively. © 2023 The Author(s).
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| 32. |
- Ehlers, Patrick, 1981-
(författare)
-
Further development of NICE-OHMS : – an ultra-sensitive frequency-modulated cavity-enhanced laser-based spectroscopic
technique for detection of molecules in gas phase
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, NICE-OHMS, is a laser-based spectroscopic detection technique that comprises the concepts of frequency modulation (FM, for reduction of 1/f-noise by detecting the signal at a high frequency) and cavity enhancement (CE, for a prolongation of the optical path length) in a unique way. Properly designed, this gives the technique an intrinsic immunity against the frequency-to-noise conversion that limits many other types of CE techniques. All this gives it an exceptionally high sensitivity for detection of molecular species. Although originally developed for frequency standard purposes in the late 1990s, soon thereafter development of the technique towards molecular spectroscopy and trace gas detection was initiated. This thesis focuses on the further development of Doppler- broadened NICE-OHMS towards an ultra-sensitive detection technique. A number of concepts have been addressed. A few of these are: i) The detection sensitivity of fiber-laser-based NICE- OHMS has been improved to the 10−12 cm−1 range, which for detection of C2H2 corresponds to a few ppt (parts-per-trillion, 1:1012) in gas phase, by improving the locking of the laser to a cavity mode by use of an acousto-optic modulator. ii) It is shown that the system can be realized with a more compact footprint by implementation of a fiber-optic circulator. iii) A systematic and thorough investigation of the experimental conditions that provide maximum signals, referred to as the optimum conditions, e.g. modulation and demodulation conditions and cavity length, has been performed. As a part of this, an expression for the NICE-OHMS line shape beyond the conventional triplet formalism has been proposed and verified. iv) To widen the applicability of NICE-OHMS for detection of pressure broadened signals, also a setup based upon a distributed-feedback (DFB) laser has been realized. v) In this regime, the Voigt profile cannot model signals with the accuracy that is needed for a proper assessment of analyte concentrations. Therefore, the thesis demonstrates the first implementations of line profiles encompassing Dicke narrowing and speed-dependent effects to NICE-OHMS. While such profiles are well-known for absorption, there were no expressions available for their dispersion counterparts. Such expressions have been derived and validated by accompanying experiments. vi) The applicability of the technique for elemental detection, then referred to as NICE-AAS, has been prophesied.
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| 33. |
- Ehn, Andreas, et al.
(författare)
-
Development of a temporal filtering technique for suppression of interferences in applied laser-induced fluorescence diagnostics.
- 2009
-
Ingår i: Applied Optics. - 2155-3165. ; 48:12, s. 2373-2387
-
Tidskriftsartikel (refereegranskat)abstract
- A temporal filtering technique, complementary to spectral filtering, has been developed for laser-induced fluorescence measurements. The filter is applicable in cases where the laser-induced interfering signals and the signal of interest have different temporal characteristics. For the interfering-signal discrimination a picosecond laser system along with a fast time-gated intensified CCD camera were used. In order to demonstrate and evaluate the temporal filtering concept two measurement situations were investigated; one where toluene fluorescence was discriminated from interfering luminescence of an aluminum surface, and in the other one Mie scattering signals from a water aerosol were filtered out from acetone fluorescence images. A mathematical model was developed to simulate and evaluate the temporal filter for a general measurement situation based on pulsed-laser excitation together with time-gated detection. Using system parameters measured with a streak camera, the model was validated for LIF imaging of acetone vapor inside a water aerosol. The results show that the temporal filter is capable of efficient suppression of interfering signal contributions. The photophysical properties of several species commonly studied by LIF in combustion research have been listed and discussed to provide guidelines for optimum use of the technique.
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| 34. |
- Ehn, Andreas, et al.
(författare)
-
Fluorescence lifetime imaging in a flame
- 2011
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 807-813
-
Tidskriftsartikel (refereegranskat)abstract
- A novel method for two-dimensional fluorescence lifetime imaging is presented. The technique is demonstrated on averaged planar laser-induced fluorescence (PLIF) signals recorded in a flame. Although demonstrated on averaged images, the concept applies equally well to single-shot images. Formaldehyde was probed through pico-second excitation at 355 nm in a rich methane/oxygen flame (Phi = 2.6). Images were recorded with a dual ICCD camera detection setup with different gate characteristics of the two cameras. The recorded images were analyzed using simulated values of LIF signal detection to generate two-dimensional images of effective lifetimes. Measured lifetimes range from roughly 1 to 4.5 ns. The lifetime image data were used for quenching correction of the LIF images, which, after correction, showed better qualitative agreement compared to a formaldehyde concentration profile simulated with the GRI 3.0 mechanism. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 35. |
- Ehn, Andreas, et al.
(författare)
-
FRAME: femtosecond videography for atomic and molecular dynamics : Femtosecond videography
- 2017
-
Ingår i: Light, science & applications. - : Springer Science and Business Media LLC. - 2047-7538. ; 6:9, s. 1-7
-
Tidskriftsartikel (refereegranskat)abstract
- Many important scientific questions in physics, chemistry and biology require effective methodologies to spectroscopically probe ultrafast intra- and inter-atomic/molecular dynamics. However, current methods that extend into the femtosecond regime are capable of only point measurements or single-snapshot visualizations and thus lack the capability to perform ultrafast spectroscopic videography of dynamic single events. Here we present a laser-probe-based method that enables two-dimensional videography at ultrafast timescales (femtosecond and shorter) of single, non-repetitive events. The method is based on superimposing a structural code onto the illumination to encrypt a single event, which is then deciphered in a post-processing step. This coding strategy enables laser probing with arbitrary wavelengths/bandwidths to collect signals with indiscriminate spectral information, thus allowing for ultrafast videography with full spectroscopic capability. To demonstrate the high temporal resolution of our method, we present videography of light propagation with record high 200 femtosecond temporal resolution. The method is widely applicable for studying a multitude of dynamical processes in physics, chemistry and biology over a wide range of time scales. Because the minimum frame separation (temporal resolution) is dictated by only the laser pulse duration, attosecond-laser technology may further increase video rates by several orders of magnitude.
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| 36. |
- Ehn, Andreas, et al.
(författare)
-
Quantitative oxygen concentration imaging in toluene atmospheres using Dual Imaging with Modeling Evaluation
- 2013
-
Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 1432-1114 .- 0723-4864. ; 54:1, s. 1433-1433
-
Tidskriftsartikel (refereegranskat)abstract
- Fluorescence lifetimes of toluene as a function of oxygen concentration in toluene/nitrogen/oxygen mixtures have been measured at room temperature using picosecond-laser excitation of the S-1-S-0 transition at 266 nm. The data satisfy the Stern-Volmer relation with high accuracy, providing an updated value of the Stern-Volmer slope. A newly developed fluorescence lifetime imaging scheme, called Dual Imaging with Modeling Evaluation (DIME), is evaluated and successfully demonstrated for quantitative oxygen concentration imaging in toluene-seeded O-2/N-2 gas mixtures.
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| 37. |
- Ehn, Andreas, et al.
(författare)
-
Single-laser shot fluorescence lifetime imaging on the nanosecond timescale using a Dual Image and Modeling Evaluation algorithm
- 2012
-
Ingår i: Optics Express. - 1094-4087. ; 20:3, s. 3043-3056
-
Tidskriftsartikel (refereegranskat)abstract
- A novel technique, designated dual imaging and modeling evaluation (DIME), for evaluating single-laser shot fluorescence lifetimes is presented. The technique is experimentally verified in a generic gas mixing experiment to provide a clear demonstration of the rapidness and sensitivity of the detector scheme. Single-laser shot fluorescence lifetimes of roughly 800 ps with a standard deviation of similar to 120 ps were determined. These results were compared to streak camera measurements. Furthermore, a general fluorescence lifetime determination algorithm is proposed. The evaluation algorithm has an analytic, linear relationship between the fluorescence lifetime and detector signal ratio. In combination with the DIME detector scheme, it is a faster, more accurate and more sensitive approach for rapid fluorescence lifetime imaging than previously proposed techniques. Monte Carlo simulations were conducted to analyze the sensitivity of the detector scheme as well as to compare the proposed evaluation algorithm to previously presented rapid lifetime determination algorithms. (C) 2012 Optical Society of America
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| 38. |
- Ehn, Andreas, et al.
(författare)
-
Temporal filtering with fast ICCD cameras in Raman studies
- 2013
-
Ingår i: Journal of Raman Spectroscopy. - : Wiley. - 0377-0486 .- 1097-4555. ; 44:4, s. 622-629
-
Tidskriftsartikel (refereegranskat)abstract
- A common problem when applying Raman scattering in applied research is spectral interference from laser-induced fluorescence. Extensive work has been invested in developing spectral and polarization filters as well as modulation schemes to refine spontaneous Raman signals. This current work, however, focuses on utilizing the temporal domain using a picosecond laser system and ICCD cameras with relatively short decay of the camera gate to prevent the fluorescence tail from being captured in Raman experiments. Further, the approach of using an ICCD camera to perform temporal filtering is compared to earlier proposed detection schemes using streak cameras or Kerr gates. The temporal-filtering scheme is evaluated in a spectroscopic investigation where a background subtraction algorithm is presented. The temporal-filtering scheme was also evaluated for Raman imaging of a levitated water droplet surrounded by fluorescing toluene vapor. Furthermore, the temporal-filter detection scheme was simulated in order to provide straight forward evaluation tools to estimate the potential of performing temporal filtering with a laser/camera system considering: laser-pulse duration, time jitter, camera-gate characteristics, gate delay times, fluorescence lifetimes, and relative signal strength between the Raman and fluorescence signal. The fluorescence signal was modeled with a closed two-level system, and the simulated results were compared to results from an investigation of the rising slope of toluene fluorescence. These evaluation tools and experimental investigations may serve as guidelines for planning and performing Raman measurements in situations where traditional filter-rejection schemes are insufficient.
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| 39. |
- Grandin, Börje, et al.
(författare)
-
A study of the influence of exhaust gas recirculation and stoichiometry on the heat release in the end-gas prior to knock using rotational coherent anti-Stokes-Raman spectroscopy thermometry
- 2002
-
Ingår i: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 3:4, s. 209-221
-
Tidskriftsartikel (refereegranskat)abstract
- Heat release in the end-gas prior to autoignition was investigated using different experimental methods including transducers for heat flux and pressure as well as rotational coherent anti-Stokes-Raman spectroscopy, which is a laser-based method for non-intrusive instantaneous thermometry of the gas. The time history was examined in the cases of mixtures of various stoichio-metries, where some were diluted with exhaust gas recir-culation (EGR). The measured temperature history was compared with the isentropic temperature calculated from the cylinder pressure trace. This comparison revealed a difference in heat release from low-temperature reactions in the end-gas for the various mixtures tested at a constant indicated mean effective pressure and a fixed position of 50 per cent burnt charge. It is shown that lean mixtures tend to exhibit the highest knock intensity, mainly due to a decrease in specific heat, as compared to the richer mixtures, which result in an earlier knock onset and as a consequence higher knock intensity. Furthermore, the comparison of temperatures indicates that the rich mixtures have a high heat release from low-temperature chemistry, which to some extent negates the higher specific heat of the charge. As a consequence, a slight enrichment of the charge can lead to higher knock intensity in comparison with a stoichiometric mixture. In spite of the lower specific heat of the charge when a stoichiometric charge was diluted with cooled EGR, these mixtures showed a very low tendency to knock.
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| 40. |
- Grandin, Börje, et al.
(författare)
-
Heat Release in the End-Gas Prior to Knock in Lean, Rich and Stoichiometric Mixtures With and Without Egr
- 2002
-
Ingår i: SAE Technical Papers. - 0148-7191.
-
Tidskriftsartikel (refereegranskat)abstract
- SI Engine knock is caused by autoignition in the unburnt part of the mixture (end-gas) ahead of the propagating flame. Autoignition of the end-gas occurs when the temperature and pressure exceeds a critical limit when comparatively slow reactions - releasing moderate amounts of heat - transform into ignition and rapid heat release. In this paper the difference in the heat released in the end-gas - by low temperature chemistry - between lean, rich, stochiometric, and stoichiometric mixtures diluted with cooled EGR was examined by measuring the temperature in the end-gas with Dual Broadband Rotational CARS. The measured temperature history was compared with an isentropic temperature calculated from the cylinder pressure trace. The experimentally obtained values for knock onset were compared with results from a two-zone thermodynamic model including detailed chemistry modelling of the end-gas reactions.
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| 41. |
- Grandin, Börje, et al.
(författare)
-
The effect of knock on the heat transfer in an SI engine : Thermal boundary layer investigation using CARS temperature measurements and heat flux measurements
- 2000
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
-
Konferensbidrag (refereegranskat)abstract
- It is generally accepted that knocking combustion influences the heat transfer in SI engines. However, the effects of heat transfer on the onset of knock is still not clear due to lack of experimental data of the thermal boundary layer close to the combustion chamber wall. This paper presents measurements of the temperature in the thermal boundary layer under knocking and non-knocking conditions. The temperature was measured using dual-broadband rotational Coherent anti-Stokes Raman Spectroscopy (CARS). Simultaneous time-resolved measurements of the cylinder pressure, at three different locations, and the heat flux to the wall were carried out. Optical access to the region near the combustion chamber wall was achieved by using a horseshoe-shaped combustion chamber with windows installed in the rectangular part of the chamber. This arrangement made CARS temperature measurements close to the wall possible and results are presented in the range 0.1-5 mm from the wall. The engine was run with constant fuel flow under near stoichiometric conditions. Knocking and non-knocking conditions were achieved by using different mixtures of n-heptane and iso-octane.
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| 42. |
- 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.
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| 43. |
- Hosseinnia, Ali, et al.
(författare)
-
Femtosecond rotational CARS with quantum control of molecular rotation
- 2022
-
Ingår i: Optical Sensors and Sensing Congress 2022 (AIS, LACSEA, Sensors, ES). - 9781557528209
-
Konferensbidrag (refereegranskat)abstract
- Quantum control is implemented in a 2-beam fs/ns rotational CARS setup via a second control pulse with variable delay. The potential of the concept for improved diagnostics is demonstrated by species-selective enhancement/annihilation via field-free alignment.
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| 44. |
- 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.
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| 45. |
- Hosseinnia, Ali, et al.
(författare)
-
Simultaneous temporally and spectrally resolved Raman coherences with single-shot fs/ns rotational CARS
- 2020
-
Ingår i: Optics Letters. - 0146-9592. ; 45:2, s. 308-311
-
Tidskriftsartikel (refereegranskat)abstract
- A novel technique for studies of the dynamics of molecular coherences has been developed. The concept is based on prompt excitation using broadband femtosecond (fs) laser pulses, whereupon a narrowband nanosecond (ns) laser pulse probes the fast dynamics of the coherences. Detection of the spectrally dispersed coherent signal using a streak camera allows simultaneous spectrally and temporally resolved studies of all excited coherences in a single-shot acquisition. Here we demonstrate the high capacity and versatility of this hybrid fs/ns coherent anti-Stokes Raman scattering (CARS) technique through some illustrative examples, all single-shot measurements, namely time-resolved studies of rotational Raman coherences in nitrogen and air, dynamics of Stark effect on rotational lines, and beating phenomena originating from close-lying rotational lines.
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| 46. |
- Hosseinnia, Ali, et al.
(författare)
-
Single-shot coherent control of molecular rotation by fs/ns rotational coherent anti-Stokes Raman spectroscopy
- 2022
-
Ingår i: Optics Express. - 1094-4087. ; 30:18, s. 32204-32214
-
Tidskriftsartikel (refereegranskat)abstract
- We present a novel method, to our knowledge, to control the shape of the spectra using 2-beam hybrid femtosecond (fs)/nanosecond (ns) coherent anti-Stokes Raman scattering (RCARS). The method is demonstrated experimentally and theoretically by utilizing a species-selective excitation approach via a field-free molecular alignment as an illustrative example. Two non-resonant fs laser pulses with proper delay selectively create and then annihilate N2 resonances in a binary mixture with O2 molecules. The RCARS signal is simultaneously resolved in spectral and temporal domains within a single-shot acquisition. The method requires very low pulse energies for excitation, hence minimizing multiphoton ionization probability, allowing for coherent control at various temperatures and pressures, with spectroscopic applications in non-stationary and unpredictable reacting flows.
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| 47. |
- Hosseinnia, Ali, et al.
(författare)
-
Single-shot fs/ns rotational CARS for temporally and spectrally resolved gas-phase diagnostics
- 2021
-
Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 38:1, s. 1843-1850
-
Tidskriftsartikel (refereegranskat)abstract
- We report thermometry and new Raman linewidth data using a novel method for time-domain rotational coherent anti-Stokes Raman spectroscopy (RCARS) on single-shot basis. The coherences are generated by two fs pulses and probed by a ns pulse. The resulting signal is detected using a combined spectrograph/streak camera setup with high spectral and temporal resolution. Rotational CARS spectrograms of nitrogen (N2), ethylene (C2H4), ethane (C2H6), and argon (Ar) are demonstrated at ambient condition. Self-broadened N2–N2 S-branch Raman linewidths were measured on single-shot basis and show excellent agreement with literature values. Furthermore, Raman linewidths of N2 S-branch perturbed by C2H4 and C2H6 were measured for the first time. The time-resolved approach was also utilized to separate complex spectra from a mixture of species through their difference in coherence lifetimes. Since the Raman coherences decay much faster for both ethane and ethylene than for nitrogen, pure nitrogen spectra could be obtained by analyzing only the temporal tail of the signal. Moreover, a model for time-resolved RCARS of N2 was developed, which employs the single-shot measured Raman linewidths to calculate the corresponding spectro-temporally resolved spectrograms at different temperatures. The model was used to evaluate the temperature from single-shot data of an N2/C2H6 mixture, showing very high accuracy. The demonstrated method constitutes an essential improvement for single-shot thermometry as it does not require pre-knowledge about collider species concentrations and their impact on linewidths.
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| 48. |
- Johansson, Olof, et al.
(författare)
-
Detection of hydrogen peroxide using photofragmentation laser-induced fluorescence.
- 2008
-
Ingår i: Applied Spectroscopy. - : SAGE Publications. - 1943-3530 .- 0003-7028. ; 62:1, s. 66-72
-
Tidskriftsartikel (refereegranskat)abstract
- Photofragmentation laser-induced fluorescence (PF-LIF) is for the first time demonstrated to be a practical diagnostic tool for detection of hydrogen peroxide. Point measurements as well as two-dimensional (2D) measurements in free-flows, with nitrogen as bath gas, are reported. The present application of the PF-LIF technique involves one laser, emitting radiation of 266 nm wavelength, to dissociate hydrogen peroxide molecules into OH radicals, and another laser, emitting at 282.25 nm, to electronically excite OH, whose laser-induced fluorescence is detected. The measurement procedure is explained in detail and a suitable time separation between photolysis and excitation pulse is proposed to be on the order of a few hundred nanoseconds. With a separation time in that regime, recorded OH excitation scans were found to be thermal and the signal was close to maximum. The PF-LIF signal strength was shown to follow the same trend as the vapor pressure corresponding to the hydrogen peroxide liquid concentration. Thus, the PF-LIF signal appeared to increase linearly with hydrogen peroxide vapor-phase concentration. For 2D single shot measurements, a conservatively estimated value of the detection limit is 30 ppm. Experiments verified that for averaged point measurements the detection limit was well below 30 ppm.
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| 49. |
- Johansson, Olof, et al.
(författare)
-
Hydroxyl radical consumption following photolysis of vapor-phase hydrogen peroxide at 266 nm: Implications for photofragmentation laser-induced fluorescence measurements of hydrogen peroxide
- 2009
-
Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 97:2, s. 515-522
-
Tidskriftsartikel (refereegranskat)abstract
- The decay of OH concentration following photolysis of room-temperature vapor-phase hydrogen peroxide is studied as a function of photolysis fluence at 266 nm in an open air environment. The rate of decay is found to increase with increasing photolysis fluence, i.e., with increasing number of photodissociated H2O2(g) molecules. Single-exponential functions approximate the OH concentration decay rather well, even for higher photolysis levels, and the decay time is shown to be inversely proportional to the H2O2(g) concentration. For fluences of about 450 mJ/cm(2) the difference between a single-exponential decay and measured data is becoming evident after approximately 150 mu s. Calculations based on a chemical kinetics model agree well with experimental data also for times > 150 mu s. By combining the model with measurements, the actual photolysis levels used in experiments are estimated. The best fit between measured data and the model suggests that about 1.1% of the H2O2(g) molecules are dissociated with a photolysis fluence of similar to 450 mJ/cm(2), in reasonable agreement with a Beer-Lambert based estimation. Excitation scans did not unfold any differences between OH spectra recorded at different photolysis fluences.
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| 50. |
- Johansson, Olof, et al.
(författare)
-
Photofragmentation laser-induced fluorescence imaging in premixed flames
- 2011
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 158:10, s. 1908-1919
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional measurements of primarily hydroperoxyl radicals (HO2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H-2/O-2. CH4/O-2, and CH4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q(1)(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO2, but also smaller contributions from H2O2 and, for the methane flames, CH3O2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO2, from which 0 atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H2O and H-2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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