| 1. |
- Coskun, Gökhan, et al.
(författare)
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Analysis of in-cylinder H2O2 and HO2 distributions in an HCCI engine - Comparison of laser-diagnostic results with CFD and SRM simulations
- 2015
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:9, s. 3131-3139
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Tidskriftsartikel (refereegranskat)abstract
- By applying a novel approach to evaluate photofragmentation laser-induced fluorescence (PFLIF) imaging, experimental quantitative information on the temporal in-cylinder distribution of hydrogen peroxide (H2O2) in a homogeneous charge compression ignition (HCCI) engine was extracted. The results from PFLIF were then compared to those obtained from chemical kinetics simulations using computational fluid dynamics (CFD) and a stochastic reactor model (SRM). For the CFD simulations, a sector mesh was applied using Reynolds-averaged Navier-Stokes (RANS) equations together with a reduced chemical kinetic model. These simulations provided detailed information on the spatial distribution of H2O2, HO2 as well as other important species and temperature. The SRM, which offers substantially reduced simulation times but no spatial information, was used with the same reduced kinetic model. Two-dimensional images from PFLIF and CFD show a fair temporal agreement, while details of the spatial distributions disagree. The CFD images show that the combustion chemistry is affected by the interaction with the cylinder walls with, for instance, a local delay of the formation and consumption of H2O2. By using probability density functions (PDFs) of H2O2 and HO2 mass fractions, comparisons could be made between experimental data and both the CFD and SRM simulations. In general the range of mass fractions show good agreement but the experimental distributions are wider. Possible reasons for this discrepancy are actual heterogeneities in the H2O2/HO2 concentration distributions not predicted by the model, spatial temperature variations, which will influence the strength of the PFLIF signal, spatial variations in the laser profiles, not accounted for in the data processing, and photon noise. The good agreement between the CFD and SRM shows the relevance of fast PDF based simulation tools. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Duwig, Christophe, et al.
(författare)
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High resolution imaging of flameless and distributed turbulent combustion
- 2012
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 159:1, s. 306-316
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Tidskriftsartikel (refereegranskat)abstract
- Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner - DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH(2)O PLIF image with 50 mu m spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH(2)O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH(2)O signals. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 3. |
- Gao, Jinlong, et al.
(författare)
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Planar laser-induced imaging of CH3 for high resolution single-shot reaction-zone visualization in premixed methane/air flames over broad stoichiometric ratios
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 243
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Tidskriftsartikel (refereegranskat)abstract
- We report a novel approach for single-shot planar imaging of CH3 radicals in premixed methane and air flames. A 213 nm beam from the 5th harmonic of an Nd:YAG laser was resonantly absorbed by the CH3 radicals, which were excited to the highly pre-dissociative upper level and dissociated to H2 and CH (X), as the main dissociation channel. The CH radicals were consequently excited by a 388 nm beam from an alexandrite laser, and the fluorescence from the excited CH radicals was collected off-resonant at 431 nm. Using this Photo-Fragmentation Planar Laser-Induced Fluorescence (PF-PLIF) technique, instantaneous flame front structures, represented by CH3 radicals, can be visualized with high spatial resolution over a broad range of stoichiometric ratios. Signal-to-noise ratios up to 50 were observed for premixed methane/air flame with stoichiometric ratio as low as 0.26. The CH radicals naturally presented in flame front are more than 400 times lower in concentration than the CH3 radicals in premixed methane/air flames even at the conditions close to stoichiometric or slightly fuel rich cases where the highest CH concentrations exist, and the CH3/CH concentration ratios increase dramatically moving towards fuel lean conditions. By adopting a structured illumination of the 213 nm pump beam, the naturally presented CH radicals were visualized simultaneously with CH3 at slightly fuel rich laminar flames, where the CH signal intensity was 5 times lower than that from CH3. The results indicate that the CH3 PF-PLIF technique can provide much stronger signal than the CH PLIF and presented a much promising potential for applications in fuel-lean flames. Finally, the CH3 PF-PLIF was performed in premixed turbulent flames to demonstrate the feasibility of this technique for flame front visualization in turbulent premixed flames.
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| 4. |
- Gao, Qiang, et al.
(författare)
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Gas Temperature Measurement Using Differential Optical Absorption Spectroscopy (DOAS)
- 2018
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 0003-7028 .- 1943-3530. ; 72:7, s. 1014-1020
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Tidskriftsartikel (refereegranskat)abstract
- A nonintrusive method for flow gas temperature measurement using differential optical absorption spectroscopy (DOAS) was demonstrated. A temperature-dependent spectra (TDS) originated from the DOAS spectra of sulfur dioxide (SO2) in the wavelength range of 276–310 nm was introduced, and the relationship between the TDS and the temperature was built through experimental calibration process. This relationship is found to be independent of SO2 concentration and can be used for temperature measurements. The experimental results indicated that the precision of the TDS method is < ± 0.3% for SO2 concentrations higher than 150 ppm with the optical path length of 170 mm. For lower concentrations, the precision is estimated to be ± 0.4% at 1 ppm. The relative deviation between the temperature measured by the TDS method and that measured by a thermocouple is within 3% in the temperature range of 298–750 K, and the TDS method has a quicker response to the fast-changing temperature than the thermocouple.
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| 5. |
- He, Yong, et al.
(författare)
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In-situ Measurement of Sodium and Potassium Release during Oxy-Fuel Combustion of Lignite using Laser-Induced Breakdown Spectroscopy: Effects of O-2 and CO2 Concentration
- 2013
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:2, s. 1123-1130
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced breakdown spectroscopy (LIBS) was used in this study to measure quantitatively the sodium (Na) and potassium (K) release from burning coal particles under oxy-fuel combustion environments. A specially designed laminar premixed burner was employed to provide a postflame environment with different O-2 and CO2 concentrations, in which the effects of O-2 and CO2 on the release of Na and K during coal oxy-fuel combustion were studied systematically. For the devolatilization stage, neither O-2 nor CO2 had significant influence on the Na and K release. The release of Na and K during the char stage, however, changed significantly at different O-2 and CO2 concentrations. Under these experimental conditions, when the O-2 concentration increased from 3.9% to 10.6%, the peak concentration of Na at the char stage increased from 15.2 mg/m(3) to 33.7 mg/m(3), and the maximum concentration of K increased from 6.2 mg/m(3) to 11.7 mg/m(3). When the CO2 concentration increased from 35.8% to 69.4%, the release of Na and K was inhibited during the char stage, with the peak concentration decreasing from 8.9 mg/m(3) to 6.9 mg/m(3) for Na and from 3.7 mg/m(3) to 2.4 mg/m(3) for K. During the ash stage, the release of Na and K decreased with the O-2 concentration, whereas it increased with the CO2 concentration.
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| 6. |
- Johansson, Olof, et al.
(författare)
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Photofragmentation laser-induced fluorescence imaging in premixed flames
- 2011
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 158:10, s. 1908-1919
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional measurements of primarily hydroperoxyl radicals (HO2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H-2/O-2. CH4/O-2, and CH4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q(1)(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO2, but also smaller contributions from H2O2 and, for the methane flames, CH3O2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO2, from which 0 atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H2O and H-2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 7. |
- Kiefer, J., et al.
(författare)
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Laser-induced breakdown spectroscopy in gases using ungated detection in combination with polarization filtering and online background correction
- 2010
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Ingår i: Measurement Science & Technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 21:6
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative and fast analysis of gas mixtures is an important task in the field of chemical, security and environmental analysis. In this paper we present a diagnostic approach based on laser-induced breakdown spectroscopy (LIBS). A polarization filter in the signal collection system enables sufficient suppression of elastically scattered light which otherwise reduces the dynamic range of the measurement. Running the detector with a doubled repetition rate as compared to the laser online background correction is obtained. Quantitative measurements of molecular air components in synthetic, ambient and expiration air are performed and demonstrate the potential of the method. The detection limits for elemental oxygen and hydrogen are in the order of 15 ppm and 10 ppm, respectively.
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| 8. |
- Li, Bo, et al.
(författare)
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Accurate measurements of laminar burning velocity using the Heat Flux method and thermographic phosphor technique
- 2011
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 939-946
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Tidskriftsartikel (refereegranskat)abstract
- The Heat Flux method was further developed to significantly reduce its experimental uncertainty and used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. Measurements of the adiabatic burning velocity of methane air flames at initial mixture temperatures of 318 K are presented. Previously, tiny thermocouples in the thin burner plate were used to evaluate when the heat flux of the flame to the burner is zero. Related errors limit the accuracy of the method so far. A new experimental procedure based on thermographic phosphors is described which avoids these errors. The new experimental procedure is described. An UV thermographic phosphor ZnO:Zn was selected and used to sensitively control the temperature uniformity on the burner plate to within 60 mK. Uncertainties of the measurements were analyzed and assessed experimentally. A more accurate evaluation of the gas velocities, using mass weighting, was introduced to increase the accuracy further. The uncertainty of the measured adiabatic burning velocities due to the temperature scattering can be reduced from typically +/-1.5 cm s at Phi = 0.7 methane air flame to +/-0.25 cm s. The overall accuracy of the burning velocities including the uncertainty from the employed mass flow controllers was evaluated to be better than +/-0.35 cm s. Possibilities to further improve the measurement accuracy are discussed with practical considerations. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 9. |
- Li, Bo, et al.
(författare)
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Experimental and numerical study of a conical turbulent partially premixed flame
- 2009
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 32, s. 1811-1818
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Tidskriftsartikel (refereegranskat)abstract
- The structure and dynamics of a turbulent partially premixed methane/air flame in a conical burner were investigated using laser diagnostics and large-eddy simulations (LES). The flame structure inside the cone was charecterized in detail using LES based on a two-scalar flamelet model, with the mixture fraction for the mixing field and level-set G-function for the partially premixed flame front propagation. In addition, planar laser induced florescence (PLIF) of CH and chemiluminiscence imaging with high speed video were performed through a glass cone. CH and CH2O PLIF were also used to examine the flame structures above the cone. It is shown that in the entire flame the CH layer remains very thin, whereas the CH2O layer is rather thick. The flame is stabilized inside the cone a short distance above the nozzle. The stabilization of the flame can be simulated by the triple-flame model but not the flamelet-quenching model. The results show that flame stabilization in the cone is a result of premixed flame front propagation and flow reversal near the wall of the cone which is deemed to be dependent on the cone angle. Flamelet based LES is shown to capture the measured CH structures whereas the predicted CH2O structure is some-what thinner than the experiments.
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| 10. |
- Li, Bo, et al.
(författare)
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Post-flame gas-phase sulfation of potassium chloride
- 2013
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 160:5, s. 959-969
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Tidskriftsartikel (refereegranskat)abstract
- The sulfation of KCl during biomass combustion has implications for operation and emissions: it reduces the rates of deposition and corrosion, it increases the formation of aerosols, and it leads to higher concentrations of HCl and lower concentrations of SO2 in the gas phase. Rigorously homogeneous systems are required to characterize the gas-phase formation of alkali sulfates. We have measured the temperature and gas-phase concentrations of KCl and HCl, and detected the presence of aerosols in the post-flame region of a range of hydrocarbon flames seeded with KCl, with and without the addition of SO2. Dilution of the flame products with different amounts of N-2 ensured post-flame temperatures in the range 950-1400 K. In the absence of SO2, KCl levels were constant in the post-flame zone and no aerosols were formed, even at the lowest temperatures. In the presence of SO2, KCl was consumed and HCl and aerosols formed, most pronounced in flames with the lowest post-flame temperatures. This shows that KCl is sulfated in the gas phase to K2SO4, and this is followed by homogeneous nucleation of K2SO4 to form aerosols. Predictions from a kinetic model of the S/Cl/K chemistry agreed well with the experimental results. The model showed that the rate-limiting steps in the sulfation process are the oxidation of SO2 to SO3 and the homogeneous nucleation of K2SO4. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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