| 1. |
- Cui, Yanqing, et al.
(författare)
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Optical diagnostics of misfire in partially premixed combustion under low load conditions
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 329
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Tidskriftsartikel (refereegranskat)abstract
- To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion.
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| 2. |
- Li, Bo, et al.
(författare)
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Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence
- 2017
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Ingår i: Optics Express. - 1094-4087. ; 25:21, s. 25809-25818
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique for sensitive CO detection, using a 230 nm pulse of 9 µJ and 45 fs. The advantages of fs-TPLIF in excitation of molecular species were analyzed. Spectra of CO fs-TPLIF were recorded in stable laminar flames spatially resolved across the flame front. A hot band (1, n) together with the conventional band (0, n) of the B→A transitions were observed in the burned zone and attributed to the broadband nature of the fs excitation. The CO fs-TPLIF signal recorded across the focal point of the excitation beam shows a relatively flat intensity distribution despite of the steep laser intensity variation, which is beneficial for CO imaging in contrast to nanosecond and picosecond TPLIF. This phenomenon can be explained by photoionization, which over the short pulse duration dominates the population depletion of the excited B state due to the high peak power, but only contributes in total a negligible X state depletion due to the low pulse energy. Single-shot CO fs-TPLIF images in methane/air flames were recorded by imaging the broadband fluorescence. The results indicate that fs-TPLIF is a promising tool for CO imaging in flames.
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| 3. |
- Li, Bo, et al.
(författare)
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Methyl Radical Imaging in Methane-Air Flames Using Laser Photofragmentation-Induced Fluorescence
- 2015
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Ingår i: Applied Spectroscopy. - : SAGE Publications. - 1943-3530 .- 0003-7028. ; 69:10, s. 1152-1156
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Tidskriftsartikel (refereegranskat)abstract
- Imaging detection of methyl radicals has been performed in laminar premixed methane-air flames at atmospheric pressure. A nanosecond Q-switched neodymium-doped yttrium aluminum garnet (Nd : YAG) laser was employed to provide the fifth-harmonic-generated 212.8 nm laser beam. The intense ultraviolet (UV) laser pulse was sent through the flame front to photodissociate The methyl (CH3) radicals in the reaction zone of the flames stabilized on a piloted jet flame burner. The emission spectra from the photodissociated fragments were collected using an imaging spectrometer with the flame-front structure spatially resolved. Combining the spatial and spectral information, we recognized that the emission from the (A-X) methine (CH) transitions located at 431 nm was generated from the CH3 photolysis and could be used to visualize the distribution of CH3 radicals. With proper filtering, the high-power UV laser (around 15 mJ/pulse) provided by the compact Nd : YAG laser makes it possible to visualize CH3 distribution naturally generated in the reaction zone of laminar methane-air premixed flames.
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| 4. |
- Li, Bo, et al.
(författare)
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Strategy for single-shot CH3 imaging in premixed methane/air flames using photofragmentation laser-induced fluorescence
- 2017
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; , s. 4487-4495
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Tidskriftsartikel (refereegranskat)abstract
- Single-shot imaging of methyl radical (CH3) in premixed methane/air flames is demonstrated using photofragmentation laser-induced fluorescence (PF-LIF) technique. A pump-probe strategy was adopted with the pump laser at 212.8 nm photolyzing CH3, and with the probe laser at 426.8 nm detecting the photolyzed CH (X 2Π) fragments. Spatially resolved spectrograph of the PF-LIF signal from a stable laminar flame was recorded across the reaction zone to investigate potential interferences. The results indicate that the single-photon channel, CH3 + 212.8 nm → CH (X 2Π) + H2, dominates the photofragmentation process. The CH2 radical was excluded from being an interfering precursor of the CH (X 2Π) fragments owing to its relatively low concentration and small absorption cross section. Naturally present CH in the flame was identified as the main interference, but was conservatively estimated to account for only less than 4% of the total PF-LIF signal. Signal-to-noise ratio of around 10 was realized for single-shot imaging of natural CH3 in turbulent jet flames.
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| 5. |
- Li, Bo, et al.
(författare)
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Strategy of interference-free atomic hydrogen detection in flames using femtosecond multi-photon laser-induced fluorescence
- 2017
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 42:6, s. 3876-3880
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen atoms are key species in combustion of hydrogen/hydrocarbon fuels. Interference-free detection of hydrogen atoms natively generated in flames using femtosecond laser-induced fluorescence (LIF) was investigated employing two colors, i.e., 243 nm and 486 nm, as excitation source: two-photon excitation followed by a relay one-photon excitation. This strategy was compared with another commonly adopted two-photon LIF strategy using 205 nm for excitation. The potential interferences were investigated, and a direct verification method was proposed to prove this strategy be interference-free, and imaging of hydrogen atoms natively generated in methane/air flames was achieved.
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| 6. |
- Liu, Haifeng, et al.
(författare)
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Effects of Flame Temperature on PAHs and Soot Evolution in Partially Premixed and Diffusion Flames of a Diesel Surrogate
- 2019
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Ingår i: Energy and Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 33:11, s. 11821-11829
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, polycyclic aromatic hydrocarbons (PAHs) and soot formation in rich partially premixed flames and nonpremixed flames were studied using a blend of n-heptane and toluene. The flames were diluted with Ar, N2, and CO2 to control the flame temperature. Laser-induced fluorescence, laser-induced incandescence, and two-color pyrometry were used to study the effects of flame temperature on the PAHs and soot evolution. Results show that temperature distributions are similar for different gas dilutions at low flow rates. However, the higherature area increases dramatically in size at high flow rates and it depends on diluents in the order Ar > N2 > CO2 with regard to flame temperature. With an increase in the flow rate of Ar, the higher growth rate of flame temperature and bigger region of higherature region can lead to a higher growth rate from small to large PAHs and higher soot volume fraction. However, for CO2 dilution, the increased flow rate results in the increase of formation of large PAHs, but soot formation is reduced due to the fact that lower flame temperature suppresses the soot formation. Therefore, it can be concluded that the evolution of PAHs and soot strongly depends on flame temperature.
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| 7. |
- Liu, Haifeng, et al.
(författare)
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Laser diagnostics and chemical kinetic analysis of PAHs and soot in co-flow partially premixed flames using diesel surrogate and oxygenated additives of n-butanol and DMF
- 2018
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 188, s. 129-141
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Tidskriftsartikel (refereegranskat)abstract
- Effects of oxygenated fuels on soot reduction strongly depend on the base fuel. Interesting candidates from oxygenated fuels in this respect include both n-butanol and 2,5-dimethylfuran (DMF), because they have already been used in diesel engines recently. However, information is rather limited on n-butanol and DMF added into a diesel fuel surrogate in fundamental flames to investigate the mechanism of soot reduction. In the current work, both n-butanol and DMF was successively added into diesel surrogate (80% n-heptane and 20% toluene in volume, named as T20) in co-flow partially premixed flames. The effects of different oxygenated structures on polycyclic aromatic hydrocarbons (PAHs) and soot were investigated at the same oxygen weight fractions of 4% and the same volume fractions of 20%. The diagnostics on PAHs, soot volume fractions and soot sizes were conducted by using both laser-induced fluorescence (LIF) and two-color laser-induced incandescence (2C-LII). A combined detailed kinetic model (n-heptane/toluene/butanols/DMF/PAHs) has been obtained in order to clarify the chemical effects of the different oxygenated fuels on PAHs formation. Results show that the reduced toluene content due to the addition of oxygenated fuels is the dominant factor for the reduction of soot, as compared with the base fuel of T20. The oxygenated structure of n-butanol has a higher ability to reduce PAHs and soot as compared with the addition of DMF. This is due to the fact that the consumption of DMF leads to much formation of C5H5 which enhances the formation of PAHs and subsequent soot. However, the formation of PAHs can be inhibited remarkably as blending n-butanol because only small hydrocarbons like C2H2 and C3H3 etc. are formed. The formation rate of A4 is more similar to that of soot in comparison with the smaller ring aromatics. For the size of soot particles, the distribution range is shrunk from 19–70 nm for T20 to 20–40 nm for the addition of oxygenated fuels. As compared to the effects of oxygenated structures, DMF20 presents a little wider distribution on soot sizes than that of B16.8. Some larger soot particles are detected in DMF20 flame but cannot be found in B20 flame.
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| 8. |
- Tang, Qinglong, et al.
(författare)
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Study on ignition and flame development in gasoline partially premixed combustion using multiple optical diagnostics
- 2017
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 177, s. 98-108
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Tidskriftsartikel (refereegranskat)abstract
- Gasoline partially premixed combustion (PPC) is a potential strategy to achieve high engine efficiency, as well as low NOx and soot emissions. But the in-cylinder combustion process of PPC is not well understood. In this paper, multiple optical diagnostics are applied to investigate the PPC ignition and flame development in a light-duty optical engine under single-injection condition. For the injection timing of −25 CA after top dead center (ATDC), the results indicate that the combustion process of gasoline PPC can be basically divided into four stages: 1) multiple auto-ignition kernels emerging in fuel-rich regions; 2) flame front propagation of the ignition kernels towards fuel-lean regions; 3) auto-ignition in the end-gas of fuel-lean regions; 4) a “burnout” stage in the whole combustion chamber after the main heat release process ends. The natural flame emission spectra from these four stages in PPC are analyzed. Distinct flame front propagation is verified during the early stages of the flame development process by both formaldehyde and OH planar laser-induced fluorescence (PLIF) imaging. The wide spread and late persistence of OH radicals after the main heat release process may account for the low soot emissions of gasoline PPC. The flame expansion speeds, determined by monitoring the flame fronts extracted from the combustion images, are much higher than that in SI (spark ignition) or SACI (spark-assisted compression ignition) combustion. With earlier fuel injection timing of −90 CA ATDC, the flame propagation process is less pronounced, and the sequential auto-ignition process prevails. Variation of the fuel stratification degree caused by the different fuel injection timings is responsible for this transformation in the flame development pattern for gasoline PPC.
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| 9. |
- Xu, Leilei, et al.
(författare)
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LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine
- 2023
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 39:4, s. 4851-4860
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.
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| 10. |
- Zhong, Shenghui, et al.
(författare)
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Structure and propagation of n-heptane/air premixed flame in low temperature ignition regime
- 2020
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 275
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a large eddy simulation of n-heptane/air turbulent premixed combustion in a reactor assisted turbulent slot (RATS) burner under different preheating conditions. N-heptane/air mixture at an equivalence ratio of 0.6, pressure of 1 atm and temperature of 600, 650 and 700 K is considered to investigate the effect of low temperature chemistry on turbulent burning velocities and flame regimes, including chemically frozen (CF) regime where the fuel/air mixture inside the burner is chemically frozen, low temperature ignition (LTI) regime where the fuel/air mixture inside the burner undergoes LTI reactions, and transition regime from CF to LTI. The results show that the flame in the LTI regime exhibits the highest turbulent burning velocity. Differential diffusion is found to play an important role in the LTI regime whereas it is less important in the CF regime. To investigate the effect of LTI reactions on the flame, a series of two-dimensional laminar flames are simulated, in which the effect of turbulence on the flames is eliminated. The results show that in the LTI regime, the laminar burning velocity is drastically enhanced and the heat release zone is broadened. Budget term analysis shows that the enhanced rate of production and diffusion towards the preheat zone of the flames and the smaller gradient of reactant mass fraction are the main reasons behind the increased laminar burning velocity in the LTI regime.
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