| 1. |
- Brackmann, Christian, et al.
(författare)
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Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics
- 2014
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 115:1, s. 25-33
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Tidskriftsartikel (refereegranskat)abstract
- Two-photon laser-induced fluorescence (LIF) of ammonia (NH3) with excitation of the C'-X transition at 304.8 nm and fluorescence detection in the 565 nm C'-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH3-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm(2). Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm(2), i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400-700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH3-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH3-detection in combustion diagnostics.
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| 2. |
- Brackmann, Christian, et al.
(författare)
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Strategy for improved NH2 detection in combustion environments using an Alexandrite laser
- 2017
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Ingår i: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425. ; 184, s. 235-242
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Tidskriftsartikel (refereegranskat)abstract
- A new scheme for NH2 detection by means of laser-induced fluorescence (LIF) with excitation around wavelength 385 nm, accessible using the second harmonic of a solid-state Alexandrite laser, is presented. Detection of NH2 was confirmed by identification of corresponding lines in fluorescence excitation spectra measured in premixed NH3-air flames and on NH2 radicals generated through NH3 photolysis in a nonreactive flow at ambient conditions. Moreover, spectral simulations allow for tentative NH2 line identification. Dispersed fluorescence emission spectra measured in flames and photolysis experiments showed lines attributed to vibrational bands of the NH2 A2A1 ← X2B1 transition but also a continuous structure, which in flame was observed to be dependent on nitrogen added to the fuel, apparently also generated by NH2. A general conclusion was that fluorescence interferences need to be carefully considered for NH2 diagnostics in this spectral region. Excitation for laser irradiances up to 0.2 GW/cm2 did not result in NH2 fluorescence saturation and allowed for efficient utilization of the available laser power without indication of laser-induced photochemistry. Compared with a previously employed excitation/detection scheme for NH2 at around 630 nm, excitation at 385.7 nm showed a factor of ~ 15 higher NH2 signal. The improved signal allowed for single-shot NH2 LIF imaging on centimeter scale in flame with signal-to-noise ratio of 3 for concentrations around 1000 ppm, suggesting a detection limit around 700 ppm. Thus, the presented approach for NH2 detection provides enhanced possibilities for characterization of fuel-nitrogen combustion chemistry.
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| 3. |
- Brackmann, Christian, et al.
(författare)
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Structure of premixed ammonia plus air flames at atmospheric pressure: Laser diagnostics and kinetic modeling
- 2016
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 163, s. 370-381
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Tidskriftsartikel (refereegranskat)abstract
- The structure of premixed ammonia air flames, burning at atmospheric pressure under strain-stabilized conditions on a porous-plug burner, has been investigated using laser-diagnostic methods. Profiles of OH, NH, and NO were acquired by laser-induced fluorescence (LIF) and quantitative concentrations of OH and NO were retrieved using a concept for calibration versus absorption utilizing the LIF-signal itself, whereas NH concentrations were evaluated employing a saturated fluorescence signal. In addition, temperatures and relative oxygen concentrations were measured by rotational Coherent Anti-stokes Raman Spectroscopy (CARS). The new experimental data for flames with equivalence ratios of 0.9, 1.0, and 1.2 were used to validate and rank the performance of four contemporary detailed kinetic models. Simulations were carried out using experimental temperature profiles as well as by solving the energy equation. Two models of the same origin, developed by Mendiara and Glarborg (2009) and by Klippenstein et al. (2011), in most cases showed good agreement in terms of radical concentrations, however, the model of Mendiara and Glarborg had better prediction of temperatures and flame front positions. The model by Shmakov et al. (2010) had comparable performance concerning radical species, but significant discrepancies appeared in the prediction of flame front positions. The model of Duynslaegher et al. (2012), in addition to the flame front positions, deviated from experiments or other models in terms of NH and NO concentrations. A sensitivity analysis for the Mendiara-Glarborg mechanism indicated that remaining uncertainties of the rate constants implemented in the recent H/N/O models are difficult to scrutinize unambiguously due to experimental uncertainties. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 4. |
- Wang, Zhenkan, et al.
(författare)
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Structure and burning velocity of turbulent premixed methane/air jet flames in thin-reaction zone and distributed reaction zone regimes
- 2019
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:2, s. 2537-2544
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Tidskriftsartikel (refereegranskat)abstract
- A series of turbulent premixed methane/air jet flames are studied using simultaneous planar lase diagnostic imaging of OH/CH/temperature and CH/OH/CH2O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH2O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH2O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor.
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| 5. |
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| 6. |
- Zhou, Bo, et al.
(författare)
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Distributed reactions in highly turbulent premixed methane/air flames Part I. Flame structure characterization
- 2015
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:7, s. 2937-2953
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous planar laser-induced fluorescence (PLIF) measurements of a series of reactive scalars and Rayleigh scattering measurements of temperature, i.e. CH/CH2O/OH, HCO/CH2O/OH and T/CH2O/OH, and laser Doppler anemometry (LDA) measurements are carried out to characterize the flame/turbulence interaction in various regimes of turbulent combustion, including the laminar flamelet regime, the thin reaction zone (TRZ) regime, and the distributed reaction zone (DRZ) regime. A series of turbulent pre-mixed methane/air jet flames with different jet speeds and equivalence ratios are studied. The jet Reynolds number ranges from 6000 to 40,000 and the Karlovitz number (Ka) of the studied flames varies from 25 to 1470. It is shown that in the TRZ regime CH/HCO layer remain thin but the layer of CH2O and temperature gradient are broadened owing to the rapid turbulence transport. In the DRZ regime the CH and HCO layers are also broadened owing to the rapid transport of reactive species such as OH radicals from the high temperature regions where these radicals are formed to the low temperature region. In the DRZ regime CH and HCO are found to coexist with OH or CH2O owing to the rapid turbulence eddy interaction, which differs fundamentally from that in the TRZ regime and the laminar flamelet regime. For the present investigated flames, the temperature range for the distributed reaction to occur is found to be between 1100 K and 1500 K. It is shown that the structures of flames in different regimes can affect the turbulence field differently. In the DRZ regime the temperature gradient is lower than that in the laminar flamelet and the TRZ regimes, which results in a lower peak of turbulence intensity owing to the retarded velocity gradient across the flames and thereby a lower rate of turbulence production.
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| 7. |
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| 8. |
- Zhou, Bo, et al.
(författare)
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Thin reaction zone and distributed reaction zone regimes in turbulent premixed methane/air flames : Scalar distributions and correlations
- 2017
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 175, s. 220-236
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Tidskriftsartikel (refereegranskat)abstract
- A series of premixed turbulent methane/air jet flames in the thin reaction zone (TRZ) and distributed reaction zone (DRZ) regimes were studied using simultaneous three-scalar high-resolution imaging measurements, including HCO/OH/CH2O, CH/OH/CH2O, T/OH/CH2O and T/CH/OH/. These scalar fields offer a possibility of revisiting the structures of turbulent premixed flames in different combustion regimes. In particular, CH2O provides a measure of the preheat zone, CH/HCO a measure of the inner layer of the reaction zone, and OH a measure of the oxidation zone. Scalar correlations are analyzed on both single-shot and statistical basis, and resolvable correlated structures of ∼100 µm between scalars are captured. With increasing turbulence intensity, it is shown that the preheat zone and the inner layer of the reaction zone become gradually broadened/distributed, and the correlation between HCO and [OH]LIF×[CH2O]LIF decreases. A transition from the TRZ regime to the DRZ regime is found around Karlovitz number of 70–100. The physical and chemical effects on the broadening of the flame are investigated. In the TRZ regime the inner layer marker CH and HCO remains thin in general although occasional local broadening of CH/HCO could be observed. Furthermore, there is a significant probability of finding CH and HCO at rather low temperatures even in the TRZ regime. In the DRZ regime, the broadening of CH and HCO are shown to be mainly a result of local reactions facilitated by rapid turbulent transport of radicals and intermediate reactants in the upstream of the reaction paths. Differential diffusion is expected to have an important effect in the DRZ regime, as H radicals seemingly play a more important role than OH radicals.
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