| 1. |
- Ehn, Andreas, et al.
(författare)
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Advanced laser-based techniques for gas-phase diagnostics in combustion and aerospace engineering
- 2017
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 71:3, s. 341-366
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Forskningsöversikt (refereegranskat)abstract
- Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.
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| 2. |
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| 3. |
- Kazemi, Mehdi Mohammad, et al.
(författare)
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Influence of the alkyl side-chain length on the ultrafast vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide (C(n)mimNTf(2)) ionic liquids
- 2017
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 19:24, s. 15988-15995
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Tidskriftsartikel (refereegranskat)abstract
- Probing the vibrational dynamics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnmimNTf2) ionic liquids (ILs) using femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) has indicated the ultrafast vibrational energy transfer between counter ions which is governed by interionic interactions and facilitated by hydrogen bonds. In this study, fs-CARS is used to investigate the ultrafast dynamics of the vibrational modes of the CnmimNTf2 ILs with n = 6, 8, 10, and 12 in a spectral region, which involves the imidazolium ring and the alkyl side-chain vibrations. The vibrational Raman modes with wavenumbers around 1418 cm−1 are excited through the CARS process and the ultrafast time evolution of the consequently excited vibrational modes is monitored. The investigation of the life times of the fs-CARS transient signals indicates that the time scale of the dynamics becomes much faster when the alkyl side-chain length of the CnmimNTf2 is longer than n = 8. This observation suggests an increase in the hydrogen bonding interactions due to the nano-structuring of the ionic liquids, which became evident with an increasing length of the alkyl side-chain. This behavior is also found in molecular dynamics simulations. There, an increase of the oxygen density around the C(2)–H moiety of the imidazolium ring, which is the predominant site for hydrogen bond formation, is observed. In other words, the longer the alkyl side-chain, the more reorganization of the ionic liquid into polar and non-polar domains occurs and the higher the probability of finding interionic hydrogen bonds at the C(2)–H position becomes.
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| 4. |
- Kiefer, Johannes, et al.
(författare)
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Gas-Phase and Combustion Diagnostics by Infrared Laser-Induced Grating Spectroscopy
- 2016
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Ingår i: Optics and Photonics for Energy and the Environment, E2 2016. - 9781557528209
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Konferensbidrag (refereegranskat)abstract
- Infrared laser-induced grating spectroscopy (IR-LIGS) is a powerful method for the detection of molecules in atmospheric and combustion chemistry by exploiting their strong and species-specific fundamental vibrations. This work discusses potentials and limitations of the method.
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| 5. |
- Kiefer, Johannes, et al.
(författare)
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Impact of plasma dynamics on equivalence ratio measurements by laser-induced breakdown spectroscopy
- 2015
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Ingår i: Applied Optics. - 2155-3165. ; 54:13, s. 4221-4226
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Tidskriftsartikel (refereegranskat)abstract
- The systematic errors introduced by triggering a USB spectrometer for laser-induced breakdown spectroscopy equivalence ratio measurements are studied. We analyze the temporal behavior of laser-induced plasma in a nonreacting methane/air mixture and investigate the influence of the dynamics on equivalence ratio measurements with gated and ungated detection. For use of gated detectors, optimal delay times were found to be between 500 and 2000 ns to allow effective suppression of interferences while maintaining sufficient signal-to-noise levels. Good precision was found for short and long exposure time intervals when an intensified CCD camera was employed. On the other hand, the jitter of an externally triggered ungated spectrometer leads to high uncertainties. Running the ungated spectrometer freely, the single-shot uncertainty can be reduced by more than 1 order of magnitude. (C) 2015 Optical Society of America
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| 6. |
- Kiefer, Johannes, et al.
(författare)
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Infrared spectroscopy as molecular probe of the macroscopic metal-liquid interface
- 2017
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Ingår i: Applied Sciences (Switzerland). - : MDPI AG. - 2076-3417. ; 7:12
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Tidskriftsartikel (refereegranskat)abstract
- Metal-liquid interfaces are of the utmost importance in a number of scientific areas, including electrochemistry and catalysis. However, complicated analytical methods and sample preparation are usually required to study the interfacial phenomena. We propose an infrared spectroscopic approach that enables investigating the molecular interactions at the interface, but needing only minimal or no sample preparation. For this purpose, the internal reflection element (IRE) is wetted with a solution as first step. Second, a small plate of the metal of interest is put on top and pressed onto the IRE. The tiny amount of liquid that is remaining between the IRE and the metal is sufficient to produce an IR spectrum with good signal to noise ratio, from which information about molecular interactions, such as hydrogen bonding, can be deduced. Proof-of-concept experiments were carried out with aqueous salt and acid solutions and an aluminum plate.
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| 7. |
- Kiefer, Johannes, et al.
(författare)
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Misalignment Effects in Laser-Induced Grating Experiments
- 2016
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 70:12, s. 2025-2028
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced grating spectroscopy (LIGS) is an experimental method in which two pulsed laser beams and a continuous-wave laser beam have to be superimposed under well-defined angles to generate a coherent signal beam. In this Note, the possible effects of different forms of misalignment are examined. This includes the overlap of the pump lasers as well as the influence of the probe laser alignment on the temporal profile of the signal.
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| 8. |
- Sahlberg, Anna Lena, et al.
(författare)
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Mid-infrared laser-induced thermal grating spectroscopy in flames
- 2017
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 36:3, s. 4515-4523
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Tidskriftsartikel (refereegranskat)abstract
- For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3. μm is demonstrated as a versatile diagnostic tool. This spectral region is of particular interest in combustion diagnostics as many relevant species such as hydrocarbons and water exhibit fundamental vibrational modes and hence can be probed with high sensitivity. Another benefit of the IR-LITGS is that it allows performing spectroscopy in the infrared combined with signal detection in the visible. Hence, the strong thermal radiation inherent in flames does not represent an interference. As the first step, we present the application of IR-LITGS to cold gas flows, where traces of ethylene and water vapor are detected. The time-resolved LITGS signals, which can be acquired in a single laser shot, are rich in information and allow deriving temperature and to some extend chemical composition. In the second step, the IR-LITGS technique is applied to ethylene/air flames stabilized on a flat flame burner. A proof-of-concept study is carried out, in which the temperature is determined in the burned region of flames with systematically varied equivalence ratio (0.72 < Φ <. 2.57). Moreover, in a highly sooty flame, LITGS signals were recorded as a function of height above the burner and allowed the determination of the temperature profile. The proposed IR-LITGS method has the potential for enabling single-shot measurements of several parameters at a time. Its applicability to sooty flame environments opens up new opportunities to study the complex formation of carbonaceous particles in flames.
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| 9. |
- Sahlberg, Anna Lena, et al.
(författare)
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Mid-infrared pumped laser-induced thermal grating spectroscopy for detection of acetylene in the visible spectral range
- 2016
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 70:6, s. 1034-1043
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Tidskriftsartikel (refereegranskat)abstract
- We present mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) using excitation radiation around 3 μm generated by a simple broadband optical parametric oscillator (OPO). Acetylene as a typical small hydrocarbon molecule is used as an example target species. A mid-infrared broadband OPO pumped by the fundamental output of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to generate the pump beams, with pulse energies of 6-10 mJ depending on the wavelength. The line width of the OPO idler beam was ∼5 cm-1, which is large enough to cover up to six adjacent acetylene lines. The probe beam was the radiation of a 532 nm cw solid state laser with 190 mW output power. Signals were generated in atmospheric pressure gas flows of N2, air, CO2 and Ar with small admixtures of C2H2. A detection limit of less than 300 ppm was found for a point measurement of C2H2 diluted in N2. As expected, the oscillation frequency of the IR-LITGS signal was found to have a large dependency on the buffer gas, which allows determination of the speed of sound. Moreover, the results reveal a very strong collisional energy exchange between C2H2 and CO2 compared to the other gases. This manifests as significant local heating. In summary, the MIR-LITGS technique enables spectroscopy of fundamental vibrational transitions in the infrared via detection in the visible spectral range.
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