| 1. |
- Akyuz, M, et al.
(författare)
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Characteristics of laser-triggered electric discharges in air
- 2005
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1070-9878. ; 12:5, s. 1060-1070
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Tidskriftsartikel (refereegranskat)abstract
- An experiment of laser-triggered electric discharges has been conducted in a uniform background electric field. By focusing the beam of a pulsed Nd-YAG laser (wavelength 532 nm, pulse duration 5 ns) in a narrow (15 mm) plane-parallel air gap exposed to a dc voltage, streamer discharges and disruptive discharges were triggered.. The laser energy and the background electric field strength were varied in the ranges 0-180 mJ and 0.8-1.4 MV/m, respectively, and the laser plasma was induced either close to the anode, close to the cathode or mid-gap. From images of the electrode gap and from current measurements, the occurrence of and time to the streamer discharge and the disruptive discharge were determined. No distinct level was found of either the laser energy or the plasma energy for the transition from one discharge case to another, even if the general trend was that the energy required for a certain discharge case was reduced when increasing the background electric field. These and other observations make the method of laser triggering a less suitable method for studies of streamer initiation and streamer propagation, but revealed several interesting features of laser-triggered electric discharges.
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| 2. |
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| 3. |
- Ferry, Anders, et al.
(författare)
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Spectroscopic studies of luminescent and ionically conducting Eu[N(CF3SO2)2]3-PPG4000 complexes
- 1998
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 109:7, s. 2921-2928
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Tidskriftsartikel (refereegranskat)abstract
- Alternating current impedance, Fourier transform Raman/infrared (IR), and luminescence excitation (continuous and time-resolved) measurements have been conducted on solutions of poly(propylene glycol) (MW 4000) complexed with Eu[N(CF3SO2)2]3 salt, EuTFSI3, along with differential scanning calorimetry (DSC) studies. From observed frequency shifts of characteristic internal anionic vibrational modes (Raman and IR), we conclude that the salt is solvated by the polymer host. The TFSI anions, however, interact extensively with Eu3+ cations at all concentrations investigated. Ion-polymer interactions are manifested as changes in characteristic vibrational modes of the polymer. Continuous and time-resolved site-selective luminescence data give, respectively, evidence for two different types of chemical environments for solvated Eu3+ cations. In particular, the strongly forbidden non-degenerate 5D0 - 7F0 transition exhibits a structured two-component profile in the spectra. DSC data show that the glass transition temperature, Tg, is only marginally affected by the introduction of a relatively high concentration of salt into the host matrix, whereas the resulting polymer-salt complex is of rubbery character, distinctly different from the pure host polymer, which is a viscous liquid at room temperature. The present findings are interpreted in terms of a phase-segregated microstructure. This conjecture is supported by previous studies on PPG4000-based electrolytes indicating microscopic phase anomalies.
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| 4. |
- Franke, Axel, et al.
(författare)
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Analysis of the Ionization Equilibrium in the Post-Flame Zone
- 2003
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Ingår i: SAE Technical Papers.
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Tidskriftsartikel (refereegranskat)abstract
- A combined experimental and theoretical effort has been made to identify the most important contributors to equilibrium ionization in the post-flame gas. In the past, nitric oxide (NO) has always been assumed to be the main electron donor in the compressed hot post-flame gases. However, correlations observed between the amount of NO in the exhaust gases and the current amplitude may be deceiving due to the fact that both the formation of NO and the ionization process are strongly temperature dependent. The temperature-current relationship in data from various experiments in constant volume combustion chambers and engines was utilized to check the hypothesis that NO acts as the major contributor to ionization. Based on a well-motivated model for the current, the effect of temperature and electron donor concentration has been separated. The results indicate that species with much lower ionization energy than NO make a significant contribution to the conductivity of the gas in the post- flame zone. Assuming realistic concentrations of the most abundant alkali metals in air, an analysis of the ionization equilibrium has been performed. The results substantiate the experimental findings.
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| 5. |
- Franke, Axel, et al.
(författare)
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Application of advanced laser diagnostics for the investigation of the ionization sensor signal in a combustion bomb
- 2005
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 81:8, s. 1135-1142
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Tidskriftsartikel (refereegranskat)abstract
- The ionization sensor is an electrical probe for diagnostics in internal combustion engines. Laser-induced fluorescence (LIF) imaging of fuel, hydroxyl (OH), and nitric oxide (NO) distributions has been employed to extend our knowledge about the governing processes leading to its signal. By monitoring the flame propagation in quiescent and turbulent mixtures, the cycle-to-cycle variations in the early sensor signal was attributed to the stochastic contact between flame front and electrodes. An analysis of the relationship between gas temperature and sensor current in the post-flame gas suggests a dominant role of alkali traces in the ionization process at the conditions under study. Significant cooling of the burned gas in the vicinity of the electrodes was observed in quiescent mixtures. Imaging of the post-flame gas in turbulent combustion revealed moving structures with varying NO and OH concentrations, which were identified as sources of variation in the sensor current.
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| 6. |
- Franke, Axel
(författare)
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Characterization of an Electrical Sensor for Combustion Diagnostics
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ionization sensor is an electrical probe for diagnostics in internal combustion engines. The combustion process affects the electrical properties of the gas in the cylinder. Thus the sensor signal contains copious information about the conditions in the combustion chamber. A thorough characterization of the sensor makes it possible to take advantage of a larger portion of this information for feedback control of the engine. The present work focuses on the identification of the basic mechanisms governing the functioning of the ionization sensor and their interaction. Optical diagnostics, equilibrium analysis and an elementary model have been employed to characterize the sensor. It was found that the contact between flame front and cathode as well as the mixture composition along the main current path governs the sensor signal during early combustion. On the basis of these findings, a zone-based model for the sensor was suggested. Imaging of the flame propagation revealed that turbulence distorts the shape of the first current peak by affecting the contact between the cathode and the wrinkled flame front. Experimental data and an analysis of the ionization equilibrium in the post-flame gas showed that traces of alkali metals in the atmosphere make a major contribution to thermal ionization at temperatures characteristic of the combustion of diluted mixtures. An investigation of the relationship between in-cylinder pressure and ionization sensor signal under various gas flow conditions indicated that high gas flow impairs this relationship. Imaging of nitric oxide and hydroxyl radicals in the post-flame gas supplied experimental evidence that the flow of cold, possibly unburned gas from the edge of the combustion chamber to the region of the electrode gap can explain this impaired relationship. The knowledge obtained will hopefully help to improve algorithms to derive information from the sensor signal and to use this information to monitor and optimize the combustion process.
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| 7. |
- Franke, Axel, et al.
(författare)
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The effect of in-cylinder gas flow on the interpretation of the ionization sensor signal
- 2003
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Ingår i: SAE Technical Papers. - Warrendale : S A E Inc..
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Konferensbidrag (refereegranskat)abstract
- The location of the peak pressure can serve as a control parameter to adjust ignition timing and optimize engine performance. The ionization sensor, an electrical probe for combustion diagnostics, can provide information about the peak pressure location. However, the reliability of such information is rather poor. In-cylinder gas flow at the electrodes may be one reason for this. We present results from an investigation of the relationship between ionization sensor current and pressure under various gas flow conditions. The gas flow velocity in the vicinity of the electrode gap was measured by LDA. From the results one may infer how the in-cylinder gas flow affects the reliability of the prediction of pressure peak location from the ionization sensor signal. One finding is that high bulk gas flow impairs the precision of the prediction in certain configurations.
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| 8. |
- Franke, Axel, et al.
(författare)
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The Role of the Electrodes for the Ionization Sensor Signal
- 2003
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Ingår i: SAE Technical Papers.
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Tidskriftsartikel (refereegranskat)abstract
- The effect of the electrodes on the early signal of the ionization sensor has been studied experimentally and with a model for the sensor. Experiments in a constant-volume combustion chamber with a generic electrode configuration allowed to investigate the role of electrode contact and the main path of the current. A framework for a model allowing the inclusion of electrode processes is introduced, and a first implementation of this model is presented. The results from the simulations are in good qualitative agreement with the experimental observations. Our conclusion is that electrode processes can limit the current during early combustion, and that the geometry of the electrodes, and especially the cathode, govern the characteristic shape of the first current peak.
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| 9. |
- Furlani, Maurizio, et al.
(författare)
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Time resolved luminescence and vibrational spectroscopic studies on complexes of poly(ethylene oxide) oligomers and EuTFSI3 salt
- 1998
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Ingår i: Solid State Ionics. - : Elsevier. - 0167-2738 .- 1872-7689. ; 113-115:1-2, s. 129-138
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Tidskriftsartikel (refereegranskat)abstract
- AC impedance, FT-Raman/IR, DSC, continuous and time resolved luminescence measurements have been conducted on solutions of poly(ethylene glycol) (PEG), MW 400, and poly(ethylene glycol)-dimethyl ether (DME), MW 425, complexed with Eu[N(CF3SO2)2]3 salt, EuTFSI3. Ion-polymer interactions are manifested as changes in characteristic vibrational modes of the polymer, including CH, and -OH stretching motions at ∼2700-3700 cm-1, and also in cation-induced polymer modes at ∼ 865-910 cm-1. Comparing the vibrational features of the TFSI anion (i.e., both Raman and IR), we find no modes that are substantially changing with increasing salt concentration, or upon change of cation (i.e., M = Li+, Na+ or Eu3+). This observation suggests that TFSI-salts are highly dissociated in PEO oligomer solvents even up to relatively high salt concentrations (i.e., O:M = 26: 1). Clear evidence of -OH end-group coordination in the PEG systems emerges from IR spectra and the strong dependence of T-g upon salt concentration, and also from the pronounced temperature dependence of the ionic conductivity. Despite of this, however, few distinct differences could be observed in the luminescence spectra between the PEG and the DME host materials. Luminescence spectra of Eu3+ show a relatively small distribution of energies (30 cm-1 FWHM in 5D0 - 7F0) in a low-symmetry site throughout the entire concentration range investigated for both PEG and DME solvents. The population decays of the 5D0 excited state, measured by exciting to the degenerate state 5D1 with a pulsed dye laser, are also very similar for the PEG and DME hosts (lifetimes = 0.35 ms).
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| 10. |
- Hultqvist, Anders, et al.
(författare)
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A Study of the Homogeneous Charge Compression Ignition Combustion Process by Chemiluminescence Imaging
- 1999
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Ingår i: SAE Technical Paper Series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
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Konferensbidrag (refereegranskat)abstract
- An experimental study of the Homogeneous Charge Compression Ignition (HCCI) combustion process has been conducted by using chemiluminescence imaging. The major intent was to characterize the flame structure and its transient behavior. To achieve this, time resolved images of the naturally emitted light were taken. Emitted light was studied by recording its spectral content and applying different filters to isolate species like OH and CH.Imaging was enabled by a truck-sized engine modified for optical access. An intensified digital camera was used for the imaging. Some imaging was done using a streak-camera, capable of taking eight arbitrarily spaced pictures during a single cycle, thus visualizing the progress of the combustion process. All imaging was done with similar operating conditions and a mixture of n-heptane and iso-octane was used as fuel.Some 20 crank angles before Top Dead Center (TDC), cool flames were found to exist. They appear with a faint structure, evenly distributed throughout the combustion chamber. There was no luminosity recorded between the end of cool flames and the start of the main heat release. Around TDC the main heat release starts. Looking at a macro scale, we find that the charge starts to burn simultaneously at arbitrary points throughout the charge. Since the thermal boundary layer is colder than the bulk of the charge, the local heat release is delayed close to the walls. As a result, the total heat release is slowed down. Ensemble averaged 1 images show this wall boundary effect clearly when plotted against CAD. The peak intensity at the main combustion event is one order of magnitude greater than that of the cool flame and the structure is a lot more protruding.Since spontaneous emission imaging is a line-of-sight integration, the flame structure appears a bit smeared. The micro scale structure is very similar from one cycle to another, but there are large variations between cycles on the macro scale.
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