187 results on '"Andreas Ehn"'
Search Results
2. Holistic analysis of a gliding arc discharge using 3D tomography and single-shot fluorescence lifetime imaging
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Sebastian Nilsson, David Sanned, Adrian Roth, Jinguo Sun, Edouard Berrocal, Mattias Richter, and Andreas Ehn
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Engineering (General). Civil engineering (General) ,TA1-2040 - Abstract
Abstract Gliding arc plasmas, a versatile form of non-thermal plasma discharges, hold great promise for sustainable chemical conversion in electrified industrial applications. Their relatively high temperatures compared to other non-thermal plasmas, reactive species generation, and efficient energy transfer make them ideal for an energy-efficient society. However, plasma discharges are transient and complex 3D entities influenced by gas pressure, mixture, and power, posing challenges for in-situ measurements of chemical species and spatial dynamics. Here we demonstrate a combination of innovative approaches, providing a comprehensive view of discharges and their chemical surroundings by combining fluorescence lifetime imaging of hydroxyl (OH) radicals with optical emission 3D tomography. This reveals variations in OH radical distributions under different conditions and local variations in fluorescence quantum yield with high spatial resolution from a single laser shot. Our results and methodology offer a multidimensional platform for interdisciplinary research in plasma physics and chemistry.
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- 2024
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3. The Space-Charge Problem in Ultrafast Diagnostics: An All-Optical Solution for Streak Cameras
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Vassily Kornienko, Yupan Bao, Joakim Bood, Andreas Ehn, and Elias Kristensson
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Physics ,QC1-999 ,Applied optics. Photonics ,TA1501-1820 - Abstract
The field of ultrafast science is dependent on either ultrashort laser pulse technology or ultrafast passive detection. While there exists a plethora of sub-picosecond laser pulse solutions, streak cameras are singular in providing sub-picosecond passive imaging capabilities. Therefore, their use in fields ranging from medicine to physics is prevalent. Streak cameras attain such temporal resolutions by converting signal photons to electrons. However, the Coulomb repulsion force spreads these electrons spatiotemporally aggravating streak cameras’ temporal resolution and dynamic range—an effect that increases in severity in ultrafast applications where electrons are generated nearly instantaneously. While many electro-optical solutions have been proposed and successfully implemented, this issue remains as a challenge for all sub-picosecond streak camera technology. Instead of resorting to electro-optical solutions, in this work, we present an all-optical approach based on the combination of photon tagging and spatial lock-in detection with a technique called periodic shadowing—that is directly applicable to all generations of streak cameras. We have demonstrated that this accessible all-optical solution, consisting of a single externally applied optical component, results in (a) a >3× improvement in dynamic range, (b) a 25% increase in temporal resolution, and (c) a reduction of background noise levels by a factor of 50, which, when combined, allows for a markedly improved accuracy in the measurement of ultrafast signals.
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- 2024
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4. Effect of a single nanosecond pulsed discharge on a flat methane–air flame
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Yupan Bao, Chengdong Kong, Jonas Ravelid, Jinguo Sun, Sebastian Nilsson, Elias Kristensson, and Andreas Ehn
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Plasma-assisted combustion ,Non-thermal plasma ,Nanosecond discharge ,Flat flame ,Laser diagnostics ,Atmospheric pressure plasma ,Fuel ,TP315-360 ,Energy industries. Energy policy. Fuel trade ,HD9502-9502.5 - Abstract
Successful implementation of plasma-assisted combustion in applied thermal processes heavily relies on how the plasma can be formed as it interacts with the reactive flow and what the effects are of such a plasma on the combustion process. The current study is an experimental investigation of a plasma-assisted lifted flat methane–air flame by a nanosecond pulsed discharge at atmospheric pressure. The nanosecond pulsed discharge, with a pulse duration of 4 ns and an amplitude of 30 kV to 50 kV, is used to stimulate the flame with a repetition rate of 1 Hz. The flame/plasma interactions are investigated with electrical and optical/laser diagnostics to study plasma-formation and its effect on the temperatures and formaldehyde formation. The flame speed seems to be accelerated for tens of milliseconds after the plasma stimulation, without noticeable gas temperature increase at the flame front and in the post-flame region. Formaldehyde is formed in the unburnt region while there is a slight increase in formaldehyde signal in the preheat zone. These results show that a volumetric effect of plasma-assisted combustion can be achieved with a short nanosecond plasma from a single excitation.
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- 2023
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5. Fluorescence lifetime imaging through scattering media
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Sebastian Nilsson, Elias Kristensson, Marcus Aldén, Joakim Bood, and Andreas Ehn
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Medicine ,Science - Abstract
Abstract Fluorescence lifetime determination has proven to be useful, e.g. identification of molecules, quantitative estimation of species concentration and determination of temperatures. Lifetime determination of exponentially decaying signals is challenging if signals of different decay rates are being mixed, resulting in erroneous results. Such issues occur when the contrast of the measurement object is low, which can be limiting in applied measurements due to spurious light scattering. A solution is presented here where structured illumination is used to enhance image contrast in fluorescence lifetime wide-field imaging. Lifetime imaging determination was carried out using Dual Imaging Modeling Evaluation (DIME), and spatial lock-in analysis was used for removing spurious scattered signal to enable fluorescence lifetime imaging through scattering media.
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- 2023
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6. Snapshot multicolor fluorescence imaging using double multiplexing of excitation and emission on a single detector
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Karolina Dorozynska, Simon Ek, Vassily Kornienko, David Andersson, Alexandra Andersson, Andreas Ehn, and Elias Kristensson
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Medicine ,Science - Abstract
Abstract Fluorescence-based multispectral imaging of rapidly moving or dynamic samples requires both fast two-dimensional data acquisition as well as sufficient spectral sensitivity for species separation. As the number of fluorophores in the experiment increases, meeting both these requirements becomes technically challenging. Although several solutions for fast imaging of multiple fluorophores exist, they all have one main restriction; they rely solely on spectrally resolving either the excitation- or the emission characteristics of the fluorophores. This inability directly limits how many fluorophores existing methods can simultaneously distinguish. Here we present a snapshot multispectral imaging approach that not only senses the excitation and emission characteristics of the probed fluorophores but also all cross term combinations of excitation and emission. To the best of the authors’ knowledge, this is the only snapshot multispectral imaging method that has this ability, allowing us to even sense and differentiate between light of equal wavelengths emitted from the same fluorescing species but where the signal components stem from different excitation sources. The current implementation of the technique allows us to simultaneously gather 24 different spectral images on a single detector, from which we demonstrate the ability to visualize and distinguish up to nine fluorophores within the visible wavelength range.
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- 2021
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7. Beyond MHz image recordings using LEDs and the FRAME concept
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Vassily Kornienko, Elias Kristensson, Andreas Ehn, Antoine Fourriere, and Edouard Berrocal
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Medicine ,Science - Abstract
Abstract Many important scientific questions in physics, chemistry and biology rely on high-speed optical imaging techniques for their investigations. These techniques are either passive, relying on the rapid readout of photoactive elements, or active, relying on the illumination properties of specially designed pulse trains. Currently, MHz imaging speeds are difficult to realize; passive methods, being dictated by electronics, cause the unification of high spatial resolution with high frame rates to be very challenging, while active methods rely on expensive and complex hardware such as femto- and picosecond laser sources. Here we present an accessible temporally resolved imaging system for shadowgraphy based on multiplexed LED illumination that is capable of producing four images at MHz frame rates. Furthermore as the LEDs are independent of each other, any light burst configuration can be obtained, allowing for instance the simultaneous determination of low- and high speed events in parallel. To the best of the authors’ knowledge, this is the fastest high speed imaging system that does not rely on pulsed lasers or fast detectors, in this case reaching up to 4.56 MHz.
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- 2020
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8. Non-thermal gliding arc discharge assisted turbulent combustion (up to 80 kW) at extended conditions: phenomenological analysis
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Chengdong Kong, Qingshuang Fan, Xin Liu, Arman Ahamed Subash, Tomas Hurtig, Andreas Ehn, Marcus Aldén, and Zhongshan Li
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Fuel Technology ,General Chemical Engineering ,General Physics and Astronomy ,Energy Engineering and Power Technology ,General Chemistry - Published
- 2022
9. Numerical Simulation of Microwave-Enhanced Low Swirl Methane-Air Flames
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Christer Fureby, Elna Nilsson, Daniel Lörstad, Kevin Nordin Bates, Tomas Hurtig, Niklas Zettervall, Rasmus Robertsson, and Andreas Ehn
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- 2023
10. Understanding the characteristics of non-equilibrium alternating current gliding arc discharge in a variety of gas mixtures (air, N2, Ar, Ar/O2, and Ar/CH4) at elevated pressures (1-5 atm)
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Chengdong Kong, Jinlong Gao, Andreas Ehn, Marcus Aldén, and Zhongshan Li
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Atom and Molecular Physics and Optics ,Energy Engineering ,Condensed Matter Physics ,Fusion, Plasma and Space Physics - Abstract
This work aims at clarifying the fundamental mechanisms of non-equilibrium alternating current gliding arc discharge (GAD) by investigating effects of gas compositions and pressures on the GAD characteristics with electrical and optical methods. Interestingly, the glow-to-spark transition was found by adding O2 or CH4 into the argon or modulating the power supply. This transition occurs attributed to the fact that the discharge mode is largely affected by the effective electron decay time (τ) as well as the feedback response of the power supply to the free electron density in the GAD. Short τ or low free electron density tends to result in the spark-type discharge. It further implies that the power supply characteristics is crucial for discharge mode control. The pressure effects on the GAD characteristics were found to vary with gas composition when the same alternating current power supply was used. In N2 or air, the emission intensity from the plasma column increases with pressure while the mean electric field strength ( E) along the plasma column decreases with pressure. Differently, in Ar, the emission intensity and E do not change much with pressure. It can be explained by the different energy partition and transfer pathways between monatomic and molecular species. The molecular gases have vibrational excitation pathways to facilitate the electronic excitation and ionization that is different from the monatomic gas.
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- 2022
11. Plasma Effects on Swirl Flames in a Scaled Siemens Energy Dry Low Emission Burner
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Xin Liu, Arman Ahamed Subash, Yupan Bao, Zhongshan Li, Andreas Ehn, Tomas Hurtig, Jenny Larfeldt, Daniel Lörstad, Thommie Nilsson, Christer Fureby
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- 2022
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12. Improved temporal contrast of streak camera measurements with periodic shadowing
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Yupan, Bao, Vassily, Kornienko, David, Lange, Wolfgang, Kiefer, Tina, Eschrich, Matthias, Jäger, Joakim, Bood, Elias, Kristensson, and Andreas, Ehn
- 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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- 2021
13. Infrared Spectroscopy as Molecular Probe of the Macroscopic Metal-Liquid Interface
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Johannes Kiefer, Johan Zetterberg, Andreas Ehn, Jonas Evertsson, Gary Harlow, and Edvin Lundgren
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hydrogen bonding ,ATR-FTIR ,adsorption ,Technology ,Engineering (General). Civil engineering (General) ,TA1-2040 ,Biology (General) ,QH301-705.5 ,Physics ,QC1-999 ,Chemistry ,QD1-999 - 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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- 2017
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14. Single-shot, spatially-resolved stand-off detection of atomic hydrogen via backward lasing in flames
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Pengji Ding, Joakim Bood, Marcus Aldén, Maria Ruchkina, and Andreas Ehn
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Materials science ,Hydrogen ,Atom and Molecular Physics and Optics ,General Chemical Engineering ,Physics::Optics ,chemistry.chemical_element ,Energy Engineering ,Ultrashort nonlinear optics ,01 natural sciences ,7. Clean energy ,Signal ,Combustion diagnostics ,law.invention ,010309 optics ,Hydrogen atom ,Optics ,law ,Multiphoton processes ,0103 physical sciences ,Backward lasing technique ,Physical and Theoretical Chemistry ,010306 general physics ,Image resolution ,Streak camera ,business.industry ,Mechanical Engineering ,Laser ,chemistry ,Temporal resolution ,Femtosecond ,business ,Lasing threshold - Abstract
We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH4/O2 flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH3.
- Published
- 2019
15. Characteristics of a Gliding Arc Discharge Under the Influence of a Laminar Premixed Flame
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Andreas Ehn, Jinlong Gao, Zhongshan Li, Chengdong Kong, Jiajian Zhu, and Marcus Aldén
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Premixed flame ,Nuclear and High Energy Physics ,Materials science ,Electrical breakdown ,Plasma ,Condensed Matter Physics ,Combustion ,01 natural sciences ,010305 fluids & plasmas ,Electric arc ,Electric field ,0103 physical sciences ,Breakdown voltage ,Plasma channel ,Atomic physics - Abstract
The effect of combustion on a gliding arc (GA) discharge is investigated using simultaneous measurements of current and voltage waveforms, as well as imaging and spectroscopic analysis of plasma and flame luminescence. Attributed to the existence of flame, the breakdown voltage and current peak are reduced and the bright sparks during breakdown are dampened. The intrinsic reason is largely owing to the thermal effect of flame. Electrical breakdown is mainly determined by the reduced electric field strength (E/N), which is inversely proportional to temperature. Assuming a constant E/N for breakdown, the combustion-induced temperature increment gives rise to a reduction of the breakdown voltage. The gas composition seems to have less impact on the breakdown voltage. However, the addition of CH 4 can induce more radicals (e.g., H atoms) that enhance the intensity of relevant spectral emissions, especially from OH*. Due to the transport of relatively long-lived radicals, the width of the plasma column of the GA discharge is broadened to form a local reaction zone, serving as a flame holder. Interestingly, the plasma channel moves more smoothly as the flame is present. It implies that the flow field is less turbulent owing to combustion.
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- 2019
16. Fiber-based stray light suppression in spectroscopy using periodic shadowing
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Miaoxin, Gong, Haisol, Kim, Jim, Larsson, Torsten, Methling, Marcus, Aldén, Elias, Kristensson, Christian, Brackmann, Tina, Eschrich, Matthias, Jäger, Wolfgang, Kiefer, and Andreas, Ehn
- Abstract
Stray light is a known strong interference in spectroscopic measurements. Photons from high-intensity signals that are scattered inside the spectrometer, or photons that enter the detector through unintended ways, will be added to the spectrum as an interference signal. A general experimental solution to this problem is presented here by introducing a customized fiber for signal collection. The fiber-mount to the spectrometer consists of a periodically arranged fiber array that, combined with lock-in analysis of the data, is capable of suppressing stray light for improved spectroscopy. The method, which is referred to as fiber-based periodic shadowing, was applied to Raman spectroscopy in combustion. The fiber-based stray-light suppression method is implemented in an experimental setup with a high-power high-repetition-rate laser system used for Raman measurements in different room-temperature gas mixtures and a premixed flame. It is shown that the stray-light level is reduced by up to a factor of 80. Weak spectral lines can be distinguished, and therefore better molecular species identification, as well as concentration and temperature evaluation, were performed. The results show that the method is feasible and efficient in practical use and that it can be employed as a general tool for improving spectroscopic accuracy.
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- 2021
17. Correction: Experimental Investigation of Plasma Discharge Effect on Swirl Flames at a Scaled Siemens Dry Low Emission Burner
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Xin Liu, Arman A. Subash, Yupan Bao, Tomas Hurtig, Zhongshan Li, Andreas Ehn, Jenny Larfeldt, Daniel Lörstad, Thommie Nilson, and Christer Fureby
- Published
- 2021
18. Experimental Investigation of Plasma Discharge Effect on Swirl Flames at a Scaled Siemens Dry Low Emission Burner
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Andreas Ehn, Arman Ahamed Subash, Xin Liu, Yupan Bao, Thommie Nilsson, Christer Fureby, Daniel Lörstad, Jenny Larfeldt, Zhongshan Li, and Tomas Hurtig
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Materials science ,Low emission ,Analytical chemistry ,Combustor ,High voltage electrode ,Exhaust gas ,Plasma ,Combustion ,Energy engineering ,NOx - Abstract
The effect of a Rotating Gliding Arc (RGA) plasma discharge on the flame in a scaled Siemens Dry Low Emission (DLE), SGT-750, burner was experimentally investigated under atmospheric combustion conditions. The central pilot section of the burner, named RPL (rich pilot lean), was redesigned with an integrated high voltage electrode to generate an RGA. The exhaust gas was sampled and analysed in terms of CO and NOx emissions, and the CO emission data show that the RGA extends the lean blow-out limit (LBO). High-speed OH chemiluminescence imaging was employed to understand the transient behaviour of the flame in both conditions with and without RGA and also to study the process of flame re-stabilization by the assistance of the RGA. A flame kernel, initiated around the RGA channel, was observed to play an important role in the re-stabilizing process of the flame. Although the NOx emission for the flame with RGA was found to be higher than that without RGA, it was still less than what previous data show for operating conditions with the RPL flame. (Less)
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- 2021
19. Single-shot 3D imaging of hydroxyl radicals in the vicinity of a gliding arc discharge
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Chengdong Kong, Sebastian Pfaff, Elias Kristensson, Tomas Hurtig, Johan Zetterberg, Andreas Ehn, Karolina Dorozynska, Mattias Richter, Panagiota Stamatoglou, and Yupan Bao
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010302 applied physics ,Three-dimentional molecular distribution ,Radical ,Plasma ,Condensed Matter Physics ,Fusion, Plasma and Space Physics ,01 natural sciences ,Molecular physics ,010305 fluids & plasmas ,Arc (geometry) ,Electric arc ,chemistry.chemical_compound ,chemistry ,Laser-induced fluorescence ,Frequency Recognition Algorithm for Multiple Exposures ,0103 physical sciences ,Plasma channel ,Hydroxyl radical ,structured illumination ,Ground state ,Hydroxyl radical (OH) - Abstract
Plasma-related studies in gas phase are challenging to carry out due to plasma’s transient and unpredictable behavior, excessive luminosity emission, 3D complexity and aggressive chemistry and physiochemical interactions that are easily affected by external probing. Laser-induced fluorescence is a robust technique for non-intrusive investigations of plasma-produced species. In this letter, we present 3D distributions of ground state hydroxyl radicals (OH) radicals in the vicinity of a glow-type gliding arc plasma. Such radical distributions are captured instantaneously in one single camera acquisition by combining structured laser illumination and a lock-in based imaging analysis method called FRAME. The interference of plasma emission is automatically subtracted by the FRAME technique. In addition, the orientation of the plasma discharge can be reconstructed from the 3D data matrix, which can then be used to calculate 2D distributions of ground state OH radicals in a plane perpendicular to the orientation of the plasma channel. Our results indicate that OH distributions around a gliding arc are strongly affected by gas dynamics. We believe that the ability to instantaneously capture 3D transient molecular distributions in a plasma discharge, with minimal plasma emission interference, will have a strong impact on the plasma community for in situ investigations of plasma-induced chemistry and physics.
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- 2021
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20. Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane–air flame
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Pengji Ding, Maria Ruchkina, Davide Del Cont-Bernard, Andreas Ehn, Deanna A Lacoste, Joakim Bood
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- 2021
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21. Simultaneous multispectral imaging of flame species using Frequency Recognition Algorithm for Multiple Exposures (FRAME)
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Edouard Berrocal, Mattias Richter, Andreas Ehn, Marcus Aldén, Zheming Li, Elias Kristensson, and Jesper Borggren
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Fluorescence-lifetime imaging microscopy ,Computer science ,Atom and Molecular Physics and Optics ,General Chemical Engineering ,Multispectral image ,General Physics and Astronomy ,Energy Engineering and Power Technology ,Energy Engineering ,Structured illumination ,02 engineering and technology ,Combustion ,01 natural sciences ,Signal ,Multispectral imaging ,010309 optics ,Simultaneous detection ,Planar ,Laser-induced fluorescence ,0103 physical sciences ,Species detection ,Envelope (mathematics) ,Frame (networking) ,General Chemistry ,021001 nanoscience & nanotechnology ,Fuel Technology ,0210 nano-technology ,Biological system - Abstract
Imaging the interaction between different combustion species under turbulent flame conditions requires methods that both are extremely fast and provide means to spectrally separate different signals. Current experimental solutions to achieve this often rely on using several cameras that are time-gated and/or equipped with different spectral filters. In this work we explore a technique called Frequency Recognition Algorithm for Multiple Exposures (FRAME) as an alternative solution for instantaneous multispectral imaging of flame species. The method is based on exciting different species with different spatial “codes” and to separate each signal component using a spatial frequency-sensitive lock-in algorithm. This methodology permits the signal from several different species to be recorded at the exact same time with a single camera. Furthermore, since the signals are recognized based on the superimposed spatial codes, there is no need for spectral separation prior to detection. The entire fluorescence envelope from each species can thus, in principle, be detected. In the current work, we present simultaneous planar laser-induced fluorescence imaging of OH and CH2O in a turbulent dimethyl ether (DME)/air flame.
- Published
- 2018
22. Quantitative Imaging of Ozone Vapor Using Photofragmentation Laser-Induced Fluorescence (LIF)
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Wubin Weng, Dina Hot, Marcus Aldén, Kajsa Larsson, Andreas Ehn, Joakim Bood, and Andreas Lantz
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Ozone ,Atom and Molecular Physics and Optics ,medicine.medical_treatment ,Analytical chemistry ,02 engineering and technology ,01 natural sciences ,Fluence ,law.invention ,010309 optics ,chemistry.chemical_compound ,Optics ,law ,0103 physical sciences ,medicine ,Absorption (electromagnetic radiation) ,Laser-induced fluorescence ,Instrumentation ,Spectroscopy ,Detection limit ,Excimer laser ,LIF ,business.industry ,imaging ,021001 nanoscience & nanotechnology ,Laser ,photofragmentation ,Fluorescence ,laser-induced fluorescence ,chemistry ,13. Climate action ,0210 nano-technology ,business - Abstract
In the present work, the spectral properties of gaseous ozone (O3) have been investigated aiming to perform quantitative concentration imaging of ozone by using a single laser pulse at 248 nm from a KrF excimer laser. The O3 molecule is first photodissociated by the laser pulse into two fragments, O and O2. Then the same laser pulse electronically excites the O2 fragment, which is vibrationally hot, whereupon fluorescence is emitted. The fluorescence intensity is found to be proportional to the concentration of ozone. Both emission and absorption characteristics have been investigated, as well as how the laser fluence affects the fluorescence signal. Quantitative ozone imaging data have been achieved based on calibration measurements in known mixtures of O3. In addition, a simultaneous study of the emission intensity captured by an intensified charge-coupled device (ICCD) camera and a spectrograph has been performed. The results show that any signal contribution not stemming from ozone is negligible compared to the strong fluorescence induced by the O2 fragment, thus proving interference-free ozone imaging. The single-shot detection limit has been estimated to ∼400 ppm. The authors believe that the presented technique offers a valuable tool applicable in various research fields, such as plasma sterilization, water and soil remediation, and plasma-assisted combustion.
- Published
- 2017
23. Skeletal Methane–Air Reaction Mechanism for Large Eddy Simulation of Turbulent Microwave-Assisted Combustion
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Elna J.K. Nilsson, Christer Fureby, Jenny Larfeldt, N. Zettervall, P. Petersson, Marcus Aldén, Tomas Hurtig, Andreas Ehn, and Anders Larsson
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Reaction mechanism ,Chemistry ,Turbulence ,General Chemical Engineering ,Nuclear engineering ,Energy Engineering and Power Technology ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Combustion ,01 natural sciences ,7. Clean energy ,010305 fluids & plasmas ,Chemical kinetics ,Fuel Technology ,Biofuel ,0103 physical sciences ,0210 nano-technology ,Microwave ,NOx ,Large eddy simulation - Abstract
Irradiating a flame via microwave radiation is a plasma-assisted combustion (PAC) technology that can be used to modify the combustion chemical kinetics in order to improve flame stability and to delay lean blow-out. One practical implication is that combustion engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. Furthermore, this technology may assist in reducing thermoacoustic instabilities, which is a phenomenon that may severely damage the engine and increase NOX production. To further understand microwave-assisted combustion, a skeletal kinetic reaction mechanism for methane–air combustion is developed and presented. The mechanism is detailed enough to take into account relevant features, but sufficiently small to be implemented in large eddy simulations (LES) of turbulent combustion. The mechanism consists of a proposed skeletal methane–air reaction mechanism accompanied by subsets for ozone, singlet oxygen, chemionization, and...
- Published
- 2017
24. Stabilization of a turbulent premixed flame by a plasma filament
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Chengdong Kong, Marcus Aldén, Zhongshan Li, and Andreas Ehn
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Atmospheric pressure discharge ,Materials science ,General Chemical Engineering ,General Physics and Astronomy ,Energy Engineering and Power Technology ,Spatial separation ,02 engineering and technology ,Combustion ,01 natural sciences ,law.invention ,Physics::Fluid Dynamics ,Laser diagnostics ,020401 chemical engineering ,law ,Physics::Plasma Physics ,0103 physical sciences ,0204 chemical engineering ,Physics::Chemical Physics ,Plasma assisted combustion ,Premixed flame ,Jet (fluid) ,010304 chemical physics ,Fluid Mechanics and Acoustics ,Turbulence ,Turbulent flame stabilization ,General Chemistry ,Plasma ,Mechanics ,Fusion, Plasma and Space Physics ,Ignition system ,Fuel Technology ,Atmospheric-pressure discharge ,Physics::Space Physics ,Plasma channel - Abstract
The mechanism of stabilizing a turbulent premixed methane-air flame using warm filamentary plasma is investigated by using laser diagnostics. First, stabilization of a turbulent jet flame is demonstrated in a setup using a pin-to-pin plasma discharge. The coupled plasma-flame structures were visualized utilizing planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and methylidyne radicals (CH), as well as laser Rayleigh scattering thermometry imaging. The results show that the plasma channel and the flame front are spatially separated by a layer of hot burning products attributed to the flame propagation from the plasma core. Because of this spatial separation, the impacts of plasma on combustion are primarily thermal since the energetic radical species (such as O, H), produced by the discharge, have short equilibration time and cannot spread far away from the discharge channel before reaching the equilibrium state. From this point of view, turbulence would be beneficial for promoting the transport of plasma-produced radicals and thus bridge the gap between the plasma and the flame front. The plasma is still able to stabilize the flame. Based upon the experimental results, a frequent ignition-flame propagation (FIFP) model is proposed to explain the flame stabilization process. For the contracted plasma filament, the local power density is high enough to initialize the flame kernel that propagates away from the plasma channel until extinction. The propagation process is, however, strongly affected by turbulence. Local extinction is highly probable and thus the flame front has to be close to the ignition source at strong turbulence. At such conditions, the stabilized flame can be regarded as a large number of flame pockets, repeating the three phases of ignition, propagation and extinction, which can be summarized as the FIFP model. It infers that the flame propagation phase is important for sustaining the flame to complete combustion. Hence, this phase should be extended, which is more probable to achieve if the plasma ignition pilot is located in a section of limited turbulence.
- Published
- 2019
25. Layered structure around an extended gliding discharge column in a methane-nitrogen mixture at high pressure
- Author
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Marcus Aldén, Chengdong Kong, Zhongshan Li, Jinlong Gao, and Andreas Ehn
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010302 applied physics ,Exothermic reaction ,Materials science ,Physics and Astronomy (miscellaneous) ,Atom and Molecular Physics and Optics ,Mixing (process engineering) ,Analytical chemistry ,chemistry.chemical_element ,Plasma ,Combustion ,01 natural sciences ,Nitrogen ,Methane ,Spectral line ,010305 fluids & plasmas ,chemistry.chemical_compound ,chemistry ,13. Climate action ,0103 physical sciences ,Layer (electronics) - Abstract
The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in N2-CH4 gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH4 in nitrogen, compared to that in pure N2. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic species, surrounded by the relatively cold N2-CH4 mixture. At the mixing layer between the high-temperature zone and the N2-CH4 mixture, some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column.
- Published
- 2019
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26. Detection of atomic oxygen in a plasma-assisted flame via a backward lasing technique
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Joakim Bood, Deanna A. Lacoste, Andreas Ehn, Pengji Ding, Davide Del Cont-Bernard, and Maria Ruchkina
- Subjects
Materials science ,business.industry ,Atom and Molecular Physics and Optics ,Physics::Optics ,02 engineering and technology ,Plasma ,Nanosecond ,021001 nanoscience & nanotechnology ,Laser ,01 natural sciences ,Signal ,Atomic and Molecular Physics, and Optics ,law.invention ,010309 optics ,Optics ,law ,0103 physical sciences ,Femtosecond ,Spontaneous emission ,Atomic physics ,Physics::Chemical Physics ,0210 nano-technology ,business ,Laser-induced fluorescence ,Lasing threshold - 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.
- Published
- 2019
27. A setup for studies of laminar flame under microwave irradiation
- Author
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Andreas Ehn, Christer Fureby, Tomas Hurtig, and Elna J.K. Nilsson
- Subjects
Physics::Fluid Dynamics ,Materials science ,Heat flux ,Combustor ,Pulse duration ,Laminar flow ,Mechanics ,Plasma ,Physics::Chemical Physics ,Combustion ,Instrumentation ,Microwave ,Microwave cavity - Abstract
Plasma assisted combustion is a very active research field due to the potential of using the technology to improve combustion efficiency and decrease pollutant emission by stabilizing lean burning flames. It has been shown in a number of studies that a small amount of electrical energy can be deposited in the flame by applying microwaves, resulting in enhanced flame propagation and thus improved flame stabilization and delayed lean blow-out. However, the effects have not yet been properly quantified since there are significant experimental challenges related to the determination of both the laminar burning velocity and the electric field strength. In the present work, a novel setup is described, where a well-defined burner system is coupled to a microwave cavity. The burner is of heat flux type, where a flat laminar flame is stabilized on a perforated burner head. The advantage of this burner for the current use is that the method and related uncertainties are well studied and quantified, and the geometry is suitable for coupling with the microwave cavity. The setup, experimental procedure, and data analysis are described in detail in this article. Laminar burning velocity for a methane-air flame at ϕ = 0.7 is determined to certify that the burner works properly in the microwave cavity. The flame is then exposed to pulsed microwaves at 1 kHz with a pulse duration of 50 µs. The laminar burning velocity at these conditions is determined to be 18.4 cm/s, which is an increase by about 12% compared to the laminar burning velocity that is measured without microwave exposure. The setup shows potential for further investigations of lean flames subjected to various microwave pulse sequences. The data are of high quality with well-defined uncertainties and are therefore suitable to use for validation of chemical kinetics models.
- Published
- 2019
28. Backward lasing for range-resolved detection of atomic hydrogen in a methane-oxygen flame
- Author
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Andreas Ehn, Pengji Ding, Maria Ruchkina, Marcus Aldén, and Joakim Bood
- Subjects
Materials science ,Hydrogen ,Streak camera ,Physics::Optics ,chemistry.chemical_element ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Laser ,01 natural sciences ,7. Clean energy ,Methane ,law.invention ,010309 optics ,chemistry.chemical_compound ,chemistry ,law ,Excited state ,0103 physical sciences ,Femtosecond ,Physics::Atomic Physics ,Physics::Chemical Physics ,Atomic physics ,0210 nano-technology ,Laser-induced fluorescence ,Lasing threshold - Abstract
We demonstrate range-resolved detection of atomic hydrogen in methane/oxygen flames based on 2-photon excited backward lasing using 205-nm femtosecond laser pulses. Range resolution is achieved by temporally resolving the backward emission with a streak camera.
- Published
- 2018
- Full Text
- View/download PDF
29. Spatially-resolved hydrogen atom detection in flames using backward lasing
- Author
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Marcus Aldén, Joakim Bood, Andreas Ehn, Pengji Ding, and Maria Ruchkina
- Subjects
Materials science ,Hydrogen ,020209 energy ,Physics::Optics ,chemistry.chemical_element ,02 engineering and technology ,Hydrogen atom ,Laser ,Combustion ,01 natural sciences ,7. Clean energy ,Signal ,3. Good health ,law.invention ,010309 optics ,chemistry ,law ,0103 physical sciences ,Femtosecond ,0202 electrical engineering, electronic engineering, information engineering ,Physics::Atomic Physics ,Atomic physics ,Image resolution ,Lasing threshold - Abstract
We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration. The lasing signal in a backward direction is obtained by two-photon pumping with 205-nm femtosecond laser pulses.
- Published
- 2018
30. Improved spectral sensitivity by combining periodic shadowing and high dynamic range imaging
- Author
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Elias Kristensson and Andreas Ehn
- Subjects
Physics ,Spectrometer ,Stray light ,Dynamic range ,business.industry ,Detector ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Analytical Chemistry ,010309 optics ,Spectral sensitivity ,Optics ,High-dynamic-range imaging ,0103 physical sciences ,Emission spectrum ,0210 nano-technology ,Spectroscopy ,business - Abstract
Most emission spectra are characterized by lines of various intensities, a feature making them difficult to probe in their entirety - the limited dynamic range of the detector prohibits the simultaneous observation of both weak and intense spectral features. A further known complication in spectroscopy concerns the generation and detection of stray light, which is an undesired contribution of light, often associated with unavoidable imperfections in the spectrometer. Stray light leads to an offset that often exceeds the intensity of weak lines, especially those that were barely detectable in the absence of stray light. This problem is well-known in, for example, laser-induced Raman spectroscopic measurements. In this paper, we describe a methodology to solve both the stray light problem and that associated with the limited dynamic range of the detector. The method is based on combining the high dynamic range imaging concept commonly employed in digital photography with the periodic shadowing technique, where the former is used to boost the dynamics and the latter to suppress stray light. The capabilities of the approach, which is suitable for investigation of temporally stable sources, are demonstrated and the results are compared with measurements performed with both conventional equipment and the periodic shadowing technique. (Less)
- Published
- 2015
31. Investigation of formaldehyde enhancement by ozone addition in CH4/air premixed flames
- Author
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Wubin Weng, Zhihua Wang, Elna Heimdal Nilsson, Andreas Ehn, Marcus Aldén, Yajun Zhou, Zhongshan Li, Kefa Cen, and Jiajian Zhu
- Subjects
Ozone ,Chemistry ,General Chemical Engineering ,Analytical chemistry ,Formaldehyde ,General Physics and Astronomy ,Energy Engineering and Power Technology ,Laminar flow ,General Chemistry ,Combustion ,Photochemistry ,Decomposition ,Methane ,law.invention ,chemistry.chemical_compound ,Fuel Technology ,law ,Bunsen burner ,Stoichiometry - Abstract
The ozone (O3) decomposition in the pre-heat zone of flames can initiate and accelerate the chain-branching reactions. In the present study, formaldehyde (CH2O) was investigated by both experiment and simulation methods in methane/air laminar premixed flames under atmospheric conditions. The formaldehyde concentration profiles in the flames were measured with CH2O-PLIF. When 4500 ppm of ozone was added, the formaldehyde concentration in Bunsen type laminar flame was enhanced by 58.5% at fuel-rich condition (I = 1.4) and 15.5% at stoichiometric condition. In the simulation work, the most recent ozone sub-mechanism was coupled with GRI-mech 3.0 kinetic mechanism. It showed that with 4500 ppm ozone addition, the formaldehyde concentration was enhanced by about 48.1% at rich condition (I = 1.4) and about 14.7% in stoichiometric mixture. The simulation suggested an early production of CH2O with ozone addition, especially in rich conditions. These reactions occurred at relatively low temperature, around 500 K. In order to isolate these reactions from the flame, experiments with preheated unburned mixtures were carried out. A larger amount of formaldehyde was produced in the zone far from the flame as the preheating temperature was increased. It indicated that the combustion enhancement with ozone could be caused by the additional reactions of ozone at relatively low temperature. Simulations showed that methoxy radical (CH3O) is the key specie for production of formaldehyde at lower temperatures. Early in the pre-heat zone of the laminar flame, formaldehyde occurs via decomposition of CH3O while in the pre-heated gas mixture via reaction of CH3O with O2. Furthermore, the O3 effect on turbulent flames was investigated showing a greater enhancement in formaldehyde signal than that in the laminar cases. This difference in formaldehyde signal enhancement could be attributed to the expansion of the preheat zone, due to turbulence. (Less)
- Published
- 2015
32. Plasma assisted combustion: Effects of O3 on large scale turbulent combustion studied with laser diagnostics and Large Eddy Simulations
- Author
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Jenny Larfeldt, Per Petersson, N. Zettervall, Jiajian Zhu, Andreas Ehn, Anders Larsson, Zhongshan Li, Marcus Aldén, Christer Fureby, and Tomas Hurtig
- Subjects
Reaction mechanism ,Laminar flame speed ,Chemistry ,Mechanical Engineering ,General Chemical Engineering ,Analytical chemistry ,Combustion ,Methane ,Adiabatic flame temperature ,Chemical kinetics ,chemistry.chemical_compound ,Planar laser-induced fluorescence ,Physical and Theoretical Chemistry ,Laser-induced fluorescence - Abstract
In plasma-assisted combustion, electric energy is added to the flame where the electric energy will be transferred to kinetic energy of the free electrons that, in turn, will modify the combustion chemical kinetics. In order to increase the understanding of this complex process, the influence of one of the products of the altered chemical kinetics, ozone (O3), has been isolated and studied. This paper reports on studies using a low-swirl methane (CH4) air flame at lean conditions with different concentrations of O3 enrichment. The experimental flame diagnostics include Planar Laser Induced Fluorescence (PLIF) imaging of hydroxyl (OH) and formaldehyde (CH2O). The experiments are also modeled using Large Eddy Simulations (LES) with a reaction model based on a skeletal CH4-air reaction mechanism combined with an O3 sub-mechanism to include the presence of O3 in the flame. This reaction mechanism is based on fundamental considerations including reactions between O3 and all other species involved. The experiments reveal an increase in CH2O in the low-swirl flame as small amounts of O3 is supplied to the CH4-air stream upstream of the flame. This increase is well predicted by the LES computations and the relative radical concentration shift is in good agreement with experimental data. Simulations also reveal that the O3 enrichment increase the laminar flame speed, su, with ∼10% and the extinction strain-rate, Iext, with ∼20%, for 0.57% (by volume) O3. The increase in Iext enables the O3 seeded flame to burn under more turbulent conditions than would be possible without O3 enrichment. Sensitivity analysis indicates that the increase in Iext due to O3 enrichment is primarily due to the accelerated chain-branching reactions H 2 + O â OH + H , H 2 O + O â OH + OH and H + O 2 â OH + O . Furthermore, the increase in CH2O observed in both experiments and simulations suggest a significant acceleration of the chain-propagation reaction CH 3 + O â CH 2 O + H . (Less)
- Published
- 2015
33. Internationella Biologiolympiaden 1999 i Uppsala.
- Author
-
Broman, Christina and Svensson, Monica
- Published
- 2023
34. Observation of gliding arc surface treatment
- Author
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Andreas Ehn, Yukihiro Kusano, Mirko Salewski, Frank Leipold, Marcus Aldén, Alexander Bardenshtein, Niels Krebs, Zhongshan Li, and Jiajian Zhu
- Subjects
Quantitative Biology::Biomolecules ,Materials science ,Atmospheric pressure ,Airflow ,Analytical chemistry ,Surfaces and Interfaces ,Condensed Matter Physics ,Surfaces, Coatings and Films ,law.invention ,Condensed Matter::Soft Condensed Matter ,Arc (geometry) ,Contact angle ,Plasma arc welding ,Boundary layer ,law ,Materials Chemistry ,Surface modification ,Composite material ,Alternating current ,Astrophysics::Galaxy Astrophysics - Abstract
An alternating current (AC) gliding arc can be conveniently operated at atmospheric pressure and efficiently elongated into the ambient air by an air flow and thus is useful for surface modification. A high speed camera was used to capture dynamics of the AC gliding arc in the presence of polymer surfaces. A gap was observed between the polymer surface and the luminous region of the plasma column, indicating the existence of a gas boundary layer. The thickness of the gas boundary layer is smaller at higher gas flow-rates or with ultrasonic irradiation to the AC gliding arc and the polymer surface. Water contact angle measurements indicate that the treatment uniformity improves significantly when the AC gliding arc is tilted to the polymer surface. Thickness reduction of the gas boundary layer, explaining the improvement of surface treatment, by the ultrasonic irradiation was directly observed for the first time.
- Published
- 2014
35. Translational, rotational, vibrational and electron temperatures of a gliding arc discharge
- Author
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Zhongshan Li, Andreas Ehn, Chengdong Kong, Mirko Salewski, Jinlong Gao, Jiajian Zhu, Frank Leipold, Marcus Aldén, and Yukihiro Kusano
- Subjects
010302 applied physics ,Materials science ,Atmospheric pressure ,Rotational temperature ,Electron ,Condensed Matter Physics ,7. Clean energy ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,010305 fluids & plasmas ,Electric arc ,symbols.namesake ,Physics::Plasma Physics ,Electric field ,0103 physical sciences ,symbols ,Electron temperature ,Atomic physics ,Rayleigh scattering ,Vibrational temperature - Abstract
Translational, rotational, vibrational and electron temperatures of a gliding arc discharge in atmospheric pressure air were experimentally investigated using in situ, non-intrusive optical diagnostic techniques. The gliding arc discharge was driven by a 35 kHz alternating current (AC) power source and operated in a glow-type regime. The two-dimensional distribution of the translational temperature (Tt) of the gliding arc discharge was determined using planar laser-induced Rayleigh scattering. The rotational and vibrational temperatures were obtained by simulating the experimental spectra. The OH A–X (0, 0) band was used to simulate the rotational temperature (Tr) of the gliding arc discharge whereas the NO A–X (1, 0) and (0, 1) bands were used to determine its vibrational temperature (Tv). The instantaneous reduced electric field strength E/N was obtained by simultaneously measuring the instantaneous length of the plasma column, the discharge voltage and the translational temperature, from which the electron temperature (Te) of the gliding arc discharge was estimated. The uncertainties of the translational, rotational, vibrational and electron temperatures were analyzed. The relations of these four different temperatures (Te>Tv>Tr >Tt) suggest a high-degree non-equilibrium state of the gliding arc discharge.
- Published
- 2017
36. Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering
- Author
-
Andreas Ehn, Xuesong Li, Johannes Kiefer, and Jiajian Zhu
- Subjects
Materials science ,Technische Fakultät ,02 engineering and technology ,Combustion ,01 natural sciences ,law.invention ,Physics::Fluid Dynamics ,010309 optics ,symbols.namesake ,law ,0103 physical sciences ,Physics::Atomic Physics ,Rayleigh scattering ,Aerospace engineering ,Instrumentation ,Spectroscopy ,Scattering ,business.industry ,Pulse duration ,Laser Doppler velocimetry ,021001 nanoscience & nanotechnology ,Laser ,Particle image velocimetry ,symbols ,ddc:620 ,0210 nano-technology ,business ,Raman scattering - 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.
- Published
- 2017
37. High dynamic spectroscopy using a digital micromirror device and periodic shadowing
- Author
-
Elias Kristensson, Andreas Ehn, and Edouard Berrocal
- Subjects
Physics ,Stray light ,Dynamic range ,business.industry ,Atom and Molecular Physics and Optics ,010401 analytical chemistry ,01 natural sciences ,Signal ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Digital micromirror device ,law.invention ,010309 optics ,Optics ,law ,0103 physical sciences ,Charge-coupled device ,Image sensor ,business ,Adaptive optics ,High dynamic range - Abstract
We present an optical solution called DMD-PS to boost the dynamic range of 2D imaging spectroscopic measurements up to 22 bits by incorporating a digital micromirror device (DMD) prior to detection in combination with the periodic shadowing (PS) approach. In contrast to high dynamic range (HDR), where the dynamic range is increased by recording several images at different exposure times, the current approach has the potential of improving the dynamic range from a single exposure and without saturation of the CCD sensor. In the procedure, the spectrum is imaged onto the DMD that selectively reduces the reflection from the intense spectral lines, allowing the signal from the weaker lines to be increased by a factor of 28 via longer exposure times, higher camera gains or increased laser power. This manipulation of the spectrum can either be based on a priori knowledge of the spectrum or by first performing a calibration measurement to sense the intensity distribution. The resulting benefits in detection sensitivity come, however, at the cost of strong generation of interfering stray light. To solve this issue the Periodic Shadowing technique, which is based on spatial light modulation, is also employed. In this proof-of-concept article we describe the full methodology of DMD-PS and demonstrate – using the calibration-based concept – an improvement in dynamic range by a factor of ~100 over conventional imaging spectroscopy. The dynamic range of the presented approach will directly benefit from future technological development of DMDs and camera sensors.
- Published
- 2017
38. Investigations of Microwave Stimulation of Turbulent Flames with Implications to Gas Turbine Combustors
- Author
-
Christer Fureby, P. Petersson, Marcus Aldén, Zhongshan Li, N. Zettervall, Jenny Larfeldt, Elna J.K. Nilsson, Tomas Hurtig, and Andreas Ehn
- Subjects
Flexibility (engineering) ,Waste management ,Hydrogen ,business.industry ,Chemistry ,Electric potential energy ,Fossil fuel ,chemistry.chemical_element ,02 engineering and technology ,Combustion ,medicine.disease_cause ,01 natural sciences ,7. Clean energy ,Energy engineering ,Soot ,010305 fluids & plasmas ,Electricity generation ,020401 chemical engineering ,13. Climate action ,0103 physical sciences ,medicine ,0204 chemical engineering ,business ,Process engineering - Abstract
Efficient and clean production of electrical energy and mechanical (shaft) energy for use in industrial and domestic applications, surface- and ground transportation and aero-propulsion is currently of significant general concern. Fossil fuels are mainly used for transportation and aero-propulsion, but also for power generation. Combustion of fossil fuels typically give rise to undesired emissions such as unburned hydrocarbons, carbon dioxide, carbon monoxide, soot and nitrogen oxides. The most widespread approach to minimize these is to apply various lean-burn technologies, and sometimes also dilute the fuel with hydrogen. Although efficient in reducing emissions, lean-burn often results in combustion instabilities and igniteon issues, and thus become challenging itself. Another desired aspect of today’s engines is to increase the fuel flexibility. One possible technique that may be useful for circumventing these issues is plasma-assisted combustion, i.e. to supply a small amount of electric energy to the flame to stimulate the chemical kinetics. Although not new, this approach has not yet been fully explored, partly because of it’s complexity, and partly because of apparent sufficiency. Recently, however, several research studies of this area have emerged. This paper attempts to provide a brief summary of microwave-assisted combustion, in which microwaves are utilized to supply the electrical energy to the flame, and to demonstrate that this method is useful to enhance flame stabilization, delay lean blow-off, and to increase combustion efficiency. The main effect of microwaves (or electrical energy) is to enhance the chemical kinetics, resulting in increased reactivity and laminar and turbulent flame speeds. Here we will demonstrate that this will improve the performance of gas turbine combustors. (Less)
- Published
- 2017
39. Investigations of microwave stimulation of a turbulent low-swirl flame
- Author
-
Jiajian Zhu, Jenny Larfeldt, Anders Larsson, Tomas Hurtig, Elna J.K. Nilsson, P. Petersson, N. Zettervall, Zhongshan Li, Marcus Aldén, Christer Fureby, and Andreas Ehn
- Subjects
Turbulent combustion ,Other Physics Topics ,020209 energy ,General Chemical Engineering ,Nozzle ,Analytical chemistry ,Energy Engineering ,02 engineering and technology ,Combustion ,01 natural sciences ,7. Clean energy ,Methane ,010305 fluids & plasmas ,chemistry.chemical_compound ,Laser-induced fluorescence ,0103 physical sciences ,0202 electrical engineering, electronic engineering, information engineering ,Physical and Theoretical Chemistry ,Large eddy simulations ,Premixed flame ,Mechanical Engineering ,Diffusion flame ,Laminar flow ,Mechanics ,Plasma-assisted combustion ,Adiabatic flame temperature ,chemistry ,13. Climate action ,Combustor - Abstract
Irradiating a flame by microwave radiation is one of several plasma-assisted combustion (PAC) technologies that can be used to modify the combustion chemical kinetics in order to improve flame-stability and to delay lean blow-out. One practical implication is that engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. In addition, this technology may assist in reducing thermoacoustic instabilities that may severely damage the engine and increase emission production. To examine microwave-assisted combustion a combined experimental and computational study of microwave-assisted combustion is performed for a lean, turbulent, swirl-stabilized, stratified flame at atmospheric conditions. The objectives are to demonstrate that the technology increases both the laminar and turbulent flame speeds, and modifies the chemical kinetics, enhancing the flame-stability at lean mixtures. The study combines experimental investigations using hydroxyl (OH) and formaldehyde (CH2O) Planar Laser-Induced Fluorescence (PLIF) and numerical simulations using finite rate chemistry Large Eddy Simulations (LES). The reaction mechanism is based on a methane (CH4)–air skeletal mechanism expanded with sub-mechanisms for ozone, singlet oxygen, chemionization, electron impact dissociation, ionization and attachment. The experimental and computational results show similar trends, and are used to demonstrate and explain some significant aspects of microwave-enhanced combustion. Both simulation and experimental studies are performed close to lean blow off conditions. In the simulations, the flame is gradually subjected to increasing reduced electric field strengths, resulting in a wider flame that stabilizes nearer to the burner nozzle. Experiments are performed at two equivalence ratios, where the leaner case absorbs up to more than 5% of the total flame power. Data from experiments reveal trends similar to simulated results with increased microwave absorption.
- Published
- 2017
40. FRAME: femtosecond videography for atomic and molecular dynamics
- Author
-
Elias Kristensson, Edouard Berrocal, Marcus Aldén, Andreas Ehn, Joakim Bood, and Zheming Li
- Subjects
Measurement point ,videography ,business.industry ,ultrafast photonics ,Physics::Optics ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,imaging techniques ,Electronic, Optical and Magnetic Materials ,010309 optics ,ultrafast spectroscopy ,Molecular dynamics ,Wavelength ,Optics ,Temporal resolution ,0103 physical sciences ,Femtosecond ,High temporal resolution ,Original Article ,0210 nano-technology ,Videography ,business ,Ultrashort pulse - 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.
- Published
- 2017
- Full Text
- View/download PDF
41. Simultaneous one-dimensional fluorescence lifetime measurements of OH and CO in premixed flames
- Author
-
Joakim Bood, Moah Christensen, Malin Jonsson, Marcus Aldén, and Andreas Ehn
- Subjects
Materials science ,Quenching (fluorescence) ,Physics and Astronomy (miscellaneous) ,business.industry ,Streak camera ,General Engineering ,General Physics and Astronomy ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Laser ,7. Clean energy ,01 natural sciences ,Fluorescence ,Optical parametric amplifier ,Molecular physics ,law.invention ,010309 optics ,Wavelength ,Optics ,law ,0103 physical sciences ,0210 nano-technology ,Adiabatic process ,business ,Laser-induced fluorescence - Abstract
A method for simultaneous measurements of fluorescence lifetimes of two species along a line is described. The experimental setup is based on picosecond laser pulses from two tunable optical parametric generator/optical parametric amplifier systems together with a streak camera. With an appropriate optical time delay between the two laser pulses, whose wavelengths are tuned to excite two different species, laser-induced fluorescence can be both detected temporally and spatially resolved by the streak camera. Hence, our method enables one-dimensional imaging of fluorescence lifetimes of two species in the same streak camera recording. The concept is demonstrated for fluorescence lifetime measurements of CO and OH in a laminar methane/air flame on a Bunsen-type burner. Measurements were taken in flames with four different equivalence ratios, namely ϕ = 0.9, 1.0, 1.15, and 1.25. The measured one-dimensional lifetime profiles generally agree well with lifetimes calculated from quenching cross sections found in the literature and quencher concentrations predicted by the GRI 3.0 mechanism. For OH, there is a systematic deviation of approximately 30 % between calculated and measured lifetimes. It is found that this is mainly due to the adiabatic assumption regarding the flame and uncertainty in H2O quenching cross section. This emphasizes the strength of measuring the quenching rates rather than relying on models. The measurement concept might be useful for single-shot measurements of fluorescence lifetimes of several species pairs of vital importance in combustion processes, hence allowing fluorescence signals to be corrected for quenching and ultimately yield quantitative concentration profiles.
- Published
- 2013
42. Temporal filtering with fast ICCD cameras in Raman studies
- Author
-
M. Levenius, Andreas Ehn, Joakim Bood, Marcus Aldén, and Malin Jonsson
- Subjects
Background subtraction ,Chemistry ,business.industry ,010401 analytical chemistry ,ComputingMethodologies_IMAGEPROCESSINGANDCOMPUTERVISION ,Streak ,Laser ,Polarization (waves) ,01 natural sciences ,Fluorescence ,0104 chemical sciences ,law.invention ,010309 optics ,symbols.namesake ,Optics ,law ,0103 physical sciences ,symbols ,General Materials Science ,Raman spectroscopy ,business ,Spectroscopy ,Raman scattering ,Jitter - 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. Copyright (c) 2013 John Wiley & Sons, Ltd. (Less)
- Published
- 2013
43. Investigation of Ozone Stimulated Combustion in the SGT-800 Burner at Atmospheric Conditions
- Author
-
Jiajian Zhu, Elna J.K. Nilsson, Zhongshan Li, Andreas Ehn, Marcus Aldén, Jenny Larfeldt, Arman Ahamed Subash, and Andreas Lantz
- Subjects
Pressure drop ,Atmospheric pressure ,Chemistry ,business.industry ,Analytical chemistry ,Industrial gas ,Combustion ,7. Clean energy ,law.invention ,13. Climate action ,Natural gas ,law ,Bunsen burner ,Combustor ,Seeding ,business - Abstract
The effect of ozone (O3) in a turbulent, swirl-stabilized natural gas/air flame was experimentally investigated at atmospheric pressure conditions using planar laser-induced fluorescence imaging of formaldehyde (CH2O PLIF) and dynamic pressure monitoring. The experiment was performed using a dry low emission (DLE) gas turbine burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. CH2O PLIF imaging was carried out for four different seeding gas compositions and seeding injection channel configurations. Two seeding injection-channels were located around the burner tip while the other two were located along the center axis of the burner at different distances upstream the burner outlet. Four different seeding gas compositions were used: nitrogen (N2), oxygen (O2) and two ozone/oxygen (O3/O2) mixtures with different O3 concentration. The results show that the O3 clearly affects the combustion chemistry. The natural gas/air mixture is preheated before combustion which is shown to kick-start the cold combustion chemistry where O3 is highly involved. The CH2O PLIF signal increases with O3 seeded into the flame which indicates that the pre-combustion activity increases and that the cold chemistry starts to develop further upstream. The small increase of the pressure drop over the burner shows that the flame moves upstream when O3 is seeded into the flame, which confirms the increase in pre-combustion activity.
- Published
- 2016
- Full Text
- View/download PDF
44. Setup for microwave stimulation of a turbulent low-swirl flame
- Author
-
Jenny Larfeldt, Per Petersson, Tomas Hurtig, Andreas Ehn, Marcus Aldén, Christer Fureby, Zhongshan Li, Anders Larsson, and Jiajian Zhu
- Subjects
microwave enhancement ,Acoustics and Ultrasonics ,Laminar flame speed ,Atom and Molecular Physics and Optics ,020209 energy ,Nozzle ,Analytical chemistry ,02 engineering and technology ,01 natural sciences ,7. Clean energy ,Cylinder (engine) ,law.invention ,Physics::Fluid Dynamics ,010309 optics ,Resonator ,Optics ,law ,0103 physical sciences ,0202 electrical engineering, electronic engineering, information engineering ,Physics::Chemical Physics ,Premixed flame ,plasma-assisted combustion ,business.industry ,Chemistry ,energetically enhanced combustion ,Plasma ,turbulent flames ,Condensed Matter Physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Combustor ,laser diagnosticsvplanar laser-induced fluorescence ,business ,Microwave - Abstract
An experimental setup for microwave stimulation of a turbulent flame is presented. A low-swirl flame is being exposed to continuous microwave irradiation inside an aluminum cavity. The cavity is designed with inlets for laser beams and a viewport for optical access. The aluminum cavity is operated as a resonator where the microwave mode pattern is matched to the position of the flame. Two metal meshes are working as endplates in the resonator, one at the bottom and the other at the top. The lower mesh is located right above the burner nozzle so that the low-swirl flame is able to freely propagate inside the cylinder cavity geometry whereas the upper metal mesh can be tuned to achieve good overlap between the microwave mode pattern and the flame volume. The flow is characterized for operating conditions without microwave irradiation using particle imaging velocimetry (PIV). Microwave absorption is simultaneously monitored with experimental investigations of the flame in terms of exhaust gas temperature, flame chemiluminescence (CL) analysis as well as simultaneous planar laser-induced fluorescence (PLIF) measurements of formaldehyde (CH2O) and hydroxyl radicals (OH). Results are presented for experiments conducted in two different regimes of microwave power. In the high-energy regime the microwave field is strong enough to cause a breakdown in the flame. The breakdown spark develops into a swirl-stabilized plasma due to the continuous microwave stimulation. In the low-energy regime, which is below plasma formation, the flame becomes larger and more stable and it moves upstream closer to the burner nozzle when microwaves are absorbed by the flame. As a result of a larger flame the exhaust gas temperature, flame CL and OH PLIF signals are increased as microwave energy is absorbed by the flame.
- Published
- 2016
- Full Text
- View/download PDF
45. Fluorescence lifetime imaging in a flame
- Author
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Joakim Bood, Bo Li, Marcus Aldén, Olof Johansson, Andreas Ehn, and Andreas Arvidsson
- Subjects
Fluorescence-lifetime imaging microscopy ,Quenching (fluorescence) ,business.industry ,Chemistry ,Mechanical Engineering ,General Chemical Engineering ,Analytical chemistry ,Combustion ,Fluorescence ,Optics ,Fluorescence cross-correlation spectroscopy ,Physical and Theoretical Chemistry ,Time-resolved spectroscopy ,business ,Laser-induced fluorescence ,Excitation - 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. (Less)
- Published
- 2011
46. Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence
- Author
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Andreas Ehn, Zhiwei Sun, Kefa Cen, Bo Li, Marcus Aldén, Zhongshan Li, Zhihua Wang, and Joakim Bood
- Subjects
Flue gas ,Ozone ,Chemistry ,General Chemical Engineering ,Organic Chemistry ,Analytical chemistry ,Energy Engineering and Power Technology ,Laminar flow ,Flue-gas desulfurization ,chemistry.chemical_compound ,Fuel Technology ,Planar laser-induced fluorescence ,Laser-induced fluorescence ,Body orifice ,Sulfur dioxide - Abstract
Direct ozone (O-3) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO2, NO3, N2O5, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO2. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO2 was utilized to investigate the reaction structures between a turbulent O-3 jet (dry air with 2000 ppm O-3) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O-3 jet with an influence width of about 6d in radius. The NO2 PLIF results support the conclusions deduced from the NO-PLIF measurements. (C) 2010 Elsevier Ltd. All rights reserved. (Less)
- Published
- 2010
47. Re-igniting the afterglow plasma column of an AC powered gliding arc discharge in atmospheric-pressure air
- Author
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Jinlong Gao, Chengdong Kong, Andreas Ehn, Zhongshan Li, Jiajian Zhu, and Marcus Aldén
- Subjects
010302 applied physics ,Materials science ,Physics and Astronomy (miscellaneous) ,Atmospheric pressure ,Airflow ,Energy Engineering ,Astrophysics::Cosmology and Extragalactic Astrophysics ,AC power ,01 natural sciences ,7. Clean energy ,010305 fluids & plasmas ,Afterglow ,law.invention ,Electric arc ,Ignition system ,Physics::Plasma Physics ,13. Climate action ,law ,0103 physical sciences ,Plasma diagnostics ,Atomic physics ,Alternating current ,Astrophysics::Galaxy Astrophysics - Abstract
The stability and re-ignition characteristics of the plasma column of an alternating current (AC) powered gliding arc discharge operating in atmospheric-pressure air were investigated for better plasma-mode controlling and optimized applications. By modulating the AC power supply and the air flow field, the states of afterglow plasma column were varied. When pulsating the AC power supply sequence, re-ignitions of the afterglow columns were introduced and their characteristics were studied using simultaneous high-speed photography and electrical measurements. Two re-ignition types were observed in the afterglow column with different decay times (the temporal separation of two sequential pulsed AC power trains). For a short decay time (120 kV/m), the electron impact ionization becomes dominant to trigger the spark re-ignition event.
- Published
- 2018
48. Effect of turbulent flow on an atmospheric-pressure AC powered gliding arc discharge
- Author
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Chengdong Kong, Jiajian Zhu, Marcus Aldén, Zhongshan Li, Andreas Ehn, and Jinlong Gao
- Subjects
010302 applied physics ,Materials science ,Atmospheric pressure ,Turbulence ,General Physics and Astronomy ,Energy Engineering ,Plasma ,Mechanics ,01 natural sciences ,010305 fluids & plasmas ,law.invention ,Vortex ,Volumetric flow rate ,Electric arc ,Physics::Plasma Physics ,law ,0103 physical sciences ,Plasma diagnostics ,Alternating current - Abstract
A high-power gliding arc (GA) discharge was generated in a turbulent air flow driven by a 35 kHz alternating current electric power supply. The effects of the flow rate on the characteristics of the GA discharge were investigated using combined optical and electrical diagnostics. Phenomenologically, the GA discharge exhibits two types of discharge, i.e., glow type and spark type, depending on the flow rates and input powers. The glow-type discharge, which has peak currents of hundreds of milliamperes, is sustained at low flow rates. The spark-type discharge, which is characterized by a sharp current spike of several amperes with duration of less than 1 μs, occurs more frequently as the flow rate increases. Higher input power can suppress spark-type discharges in moderate turbulence, but this effect becomes weak under high turbulent conditions. Physically, the transition between glow- and spark-type is initiated by the short cutting events and the local re-ignition events. Short cutting events occur owing to the twisting, wrinkling, and stretching of the plasma columns that are governed by the relatively large vortexes in the flow. Local re-ignition events, which are defined as re-ignition along plasma columns, are detected in strong turbulence due to increment of the impedance of the plasma column and consequently the internal electric field strength. It is suggested that the vortexes with length scales smaller than the size of the plasma can penetrate into the plasma column and promote mixing with surroundings to accelerate the energy dissipation. Therefore, the turbulent flow influences the GA discharges by ruling the short cutting events with relatively large vortexes and the local re-ignition events with small vortexes.
- Published
- 2018
49. Instantaneous imaging of ozone in a gliding arc discharge using photofragmentation laser-induced fluorescence
- Author
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Chengdong Kong, Kajsa Larsson, Zhongshan Li, Jinlong Gao, Joakim Bood, Dina Hot, Andreas Ehn, and Marcus Aldén
- Subjects
Materials science ,Acoustics and Ultrasonics ,Atom and Molecular Physics and Optics ,medicine.medical_treatment ,02 engineering and technology ,7. Clean energy ,01 natural sciences ,010305 fluids & plasmas ,law.invention ,Electric arc ,law ,0103 physical sciences ,medicine ,Laser-induced fluorescence ,plasma ,Excimer laser ,Photodissociation ,imaging ,Plasma ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Laser ,photofragmentation ,laser-induced fluorescence ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,gliding arc ,ozone ,13. Climate action ,Excited state ,Atomic physics ,0210 nano-technology ,Excitation - Abstract
Ozone vapor, O3, is here visualized in a gliding arc discharge using photofragmentation laser-induced fluorescence. Ozone is imaged by first photodissociating the O3 molecule into an O radical and a vibrationally hot O2 fragment by a pump photon. Thereafter, the vibrationally excited O2 molecule absorbs a second (probe) photon that further transits the O2-molecule to an excited electronic state, and hence, fluorescence from the deexcitation process in the molecule can be detected. Both the photodissociation and excitation processes are achieved within one 248 nm KrF excimer laser pulse that is formed into a laser sheet and the fluorescence is imaged using an intensified CCD camera. The laser-induced signal in the vicinity of the plasma column formed by the gliding arc is confirmed to stem from O3 rather than plasma produced vibrationally hot O2. While both these products can be produced in plasmas a second laser pulse at 266 nm was utilized to separate the pump- from the probe-processes. Such arrangement allowed lifetime studies of vibrationally hot O2, which under these conditions were several orders of magnitude shorter than the lifetime of plasma-produced ozone. (Less)
- Published
- 2018
50. Single-shot photofragment imaging by structured illumination
- Author
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Andreas Ehn, Joakim Bood, Marcus Aldén, Malin Jonsson, Elias Kristensson, Kajsa Larsson, and Jesper Borggren
- Subjects
Materials science ,Stray light ,business.industry ,Laminar flow ,Laser ,Signal ,Atomic and Molecular Physics, and Optics ,Intensity (physics) ,law.invention ,Optics ,law ,Spatial frequency ,Laser-induced fluorescence ,business ,Beam (structure) - Abstract
A laser method to suppress background interferences in pump-probe measurements is presented and demonstrated. The method is based on structured illumination, where the intensity profile of the pump beam is spatially modulated to make its induced photofragment signal distinguishable from that created solely by the probe beam. A spatial lock-in algorithm is then applied on the acquired data, extracting only those image components that are characterized by the encoded structure. The concept is demonstrated for imaging of OH photofragments in a laminar methane/air flame, where the signal from the OH photofragments produced by the pump beam is spatially overlapping with that from the naturally present OH radicals. The purpose was to perform for the first time, to the best of our knowledge, single-shot imaging of HO(2) in a flame. These results show an increase in signal-to-interference ratio of about 20 for single-shot data.
- Published
- 2015
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