| 1. |
- Nilsson, Per, et al.
(author)
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Effects of flame stretch and wrinkling on CO formation in turbulent premixed combustion
- 2002
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In: Proceedings of the Combustion Institute. - 1540-7489 .- 1873-2704. ; 29:2, s. 1873-1879
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Conference paper (peer-reviewed)abstract
- This paper presents an investigation of CO formation in a lean premixed propane/air turbulent flame in an afterburner configuration. A previous experiment showed that a high amount of CO was formed in the mean turbulent flame brush. To explain the high CO concentration in the flame zone, the effects of flame stretch and flame wrinkling are studied, based on an ensemble averaged laminar flamelet library approach. It is shown that the flame stretch decreases the laminar burning velocity by 20% under the studied flame conditions, and the stretched flamelet model predicts the non-equilibrium CO concentration in the postflame zone. However, the high CO concentration in the mean flame brush cannot be predicted by a stretched flamelet library model alone. A flamelet model accounting for wrinkled flamelets in the mean turbulent flame brush, and the effect of flame stretch (mainly strain rate), is tested. The model is based on a level-set G-equation for the mean position of the turbulent flame brush and an ensemble average of strained laminar flamelet libraries. A comparison of the numerical results with the experimental data and a previous translating flamelet model clearly shows that the wrinkled flamelet model predicts the intermediate species, such as CO, more accurately. The major species such as O-2 and CO2, as well as temperature, are found to be not sensitive to the flame wrinkling.
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| 2. |
- Castellanos, Leonardo, et al.
(author)
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Water vapor in hydrogen flames measured by time-resolved collisional dephasing of the pure-rotational N2 CARS signal
- 2023
-
In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 39:1, s. 1279-1287
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Journal article (peer-reviewed)abstract
- We present a novel diagnostic technique to probe water vapor (H2O) concentration in hydrogen (H2) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N2). The rotational Raman coherence of the N2 molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H2 flames where H2O is present in appreciable amount, it introduces a faster dephasing of the N2 coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H2O mole fraction from the dephasing effect of its inelastic collisions with N2. The proof-of-principle is demonstrated in a laminar H2/air diffusion flame, performing sequential measurements of the collisional dephasing of the N2 CARS signal up to 360 ps. We measure the temperature and the relative O2/N2 and H2/N2 concentrations at a short probe delay, and input the results in the time-domain model to extract the H2O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical ‘H3 flame’ performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at ∼20 and ∼250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H2 flames, thus providing a full characterization of the flow composition, as well as of the temperature field.
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| 3. |
- Chen, Canruo, et al.
(author)
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On the stabilization mechanism of high-speed deflagrations in narrow channels with heat loss
- 2024
-
In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 40:1-4
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Journal article (peer-reviewed)abstract
- Statistically steady supersonic deflagrations are numerically investigated in narrow channels with strong thermal expansion and heat loss. Four modes of flame propagation are observed, namely, extinction, low-speed deflagration, high-speed deflagration, and DDT. It is determined that larger thermal expansion facilitates initiation of high-speed deflagrations while the heat loss can suppress the transition to detonation. The high-speed deflagration mode is shown to be the result of the dynamic balance between thermal expansion and wall heat loss. The limits of high-speed deflagration in terms of the thermal expansion and heat loss coefficients are determined. The statistically steady oscillatory high-speed deflagrations propagate at average velocities close to half of the CJ detonation velocity. The dynamics of the flame front and shock waves are visualized using numerical schlieren. Periodic acceleration and deceleration of the leading shock are identified, and the mechanism of DDT suppression is elucidated.
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| 4. |
- Dai, P., et al.
(author)
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Autoignition and detonation development from a hot spot inside a closed chamber : Effects of end wall reflection
- 2021
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In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 38:4, s. 5905-5913
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Journal article (peer-reviewed)abstract
- The advancement of highly boosted internal combustion engines (ICEs) with high thermal efficiency is mainly constrained by knock and super-knock, respectively, due to the end gas autoignition and detonation development. The pressure wave propagation and reflection in a small confined space may strongly interact with local end gas autoignition, leading to combustion characteristics different from those in a large chamber or open space. The present study investigates the transient autoignition process in an iso-octane/air mixture inside a closed chamber under engine-relevant conditions. The emphasis is given to the assessment of effects of the pressure wave-wall reflection and the mechanism of extremely strong pressure oscillation typical for super-knock. It is found that the hot spot induced autoignition in a closed chamber can be greatly affected by shock/pressure wave reflection from the end wall. Different autoignition modes respectively from the hot spot and the end wall reflection are identified. A non-dimensional parameter quantifying the interplay between different length and time scales is introduced, which helps to identify different autoignition regimes including detonation development near the end wall. It is shown that detonation development from the hot spot may cause super-knock with devastating pressure oscillation. However, the detonation development from the end wall can hardly produce pressure oscillation strong enough for the super-knock. The obtained results provide a fundamental insight into the knocking mechanism in engines under highly boosted conditions.
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| 5. |
- Fan, Chuan Gang, et al.
(author)
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Experimental study of sidewall effect on flame characteristics of heptane pool fires with different aspect ratios and orientations in a channel
- 2017
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489 .- 1873-2704. ; 36:2, s. 3121-3129
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Journal article (peer-reviewed)abstract
- A series of small scale tests was conducted to investigate the influence of sidewall on flame characteristics of heptane pool fires in a channel, considering pool shape (aspect ratio: 1, 2, 4 and 8) and pool orientation relative to sidewall. Distance between fire and sidewall was changed systematically. Both transverse flame development (along the direction of channel width) and longitudinal flame development (along the direction of channel length) were recorded by digital video. Results show that for a fixed fire location, the heat release rate increases with the increasing pool aspect ratio (namely a larger pool perimeter), which indicates more air entrainment and more intense combustion. In wall fire cases, when the long pool rim is perpendicular with channel sidewall, the flame can obtain more air entrainment with a weaker boundary restriction from the sidewall, compared to the case with the long pool rim being parallel with sidewall. Comparison of some previously established correlations based on various experimental conditions with our test results is made. Due to the fact without considering sidewall effect and fuel shape on the air entrainment of fire plume, the classic correlations need to be further improved. Therefore, an integral flame length model considering both sidewall effect and fuel shape is developed, which correlates well with all the data from cases with various pool positions, orientations and aspect ratios. © 2016 The Combustion Institute. Published by Elsevier Inc.
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| 6. |
- Kruger, Jonas, et al.
(author)
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The effect of turbulent clustering on particle reactivity
- 2017
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In: Proceedings of the Combustion Institute. - : ELSEVIER SCIENCE INC. - 1540-7489 .- 1873-2704. ; 36:2, s. 2333-2340
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Journal article (peer-reviewed)abstract
- The effect of turbulence on the heterogeneous (solid-fluid) reactions of solid particles is studied numerically with Direct Numerical Simulations (DNS). A simplified reaction system is used, where the solid-fluid reaction is represented by a single isothermal reaction step. It is found that, due to the clustering of particles by the isotropic turbulence, the overall reaction rate is entirely controlled by the turbulence for large Damkohler numbers. The particle clustering significantly slows down the reaction rate for increasing Damkohler numbers which reaches an asymptotic limit that can be analytically derived. This implies that the effect of turbulence on heterogeneously reacting particles should be included in models that are used in CFD simulations of e. g. char burnout in combustors or gasifiers. Such a model, based on the chemical and turbulent time scales, is here proposed for the heterogeneous reaction rate in the presence of turbulence.
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| 7. |
- Li, Zhongshan, et al.
(author)
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Development of improved PLIF CH detection using an Alexandrite laser for single-shot investigation of turbulent and lean flames
- 2007
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489 .- 1873-2704. ; 31 I, s. 727-735
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Conference paper (peer-reviewed)abstract
- We report on the development of planar laser-induced fluorescence (PLIF) for CH imaging with improved detection sensitivity for single-shot investigations of turbulent, lean, premixed flames. A ringcavity, pulsed Alexandrite laser was frequency-doubled to excite the lines in the R-branch band-head of the B-X (0,0) band and broadband fluorescence from the B-X (0,1), A-X (1,1) and (0,0) bands, overlapping in the spectral range around 431 nm, was collected. The employed Alexandrite laser, which is characterized by its long pulse duration (150 ns), gives a tunable laser beam around 775 nm with a pulse energy for the second harmonic at the CH absorption wavelength of about 70 mJ. Moreover, the laser has the possibility to be operated in narrow bandwidth (100 MHz) or broad bandwidth (8 cm-1). An introductory high resolution excitation scan over the R-branch band-head was performed and, in addition, saturated excitation with the broadband option of the laser was investigated. By simultaneous excitation of several rotational transitions and to bring these transitions close to saturation, high signal-to-noise ratios were reached over a wide range of equivalence ratios. A sharp and thin CH layer was observed in single-shot PLIF images from laminar premixed methane/air flames from Φ = 0.6 to Φ = 1.5. Finally, the impact of the developed CH PLIF technique is demonstrated in a highly turbulent, lean, partially premixed methane/air flame established on a co-axial jet flame burner. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 8. |
- Li, Zhongshan, et al.
(author)
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Mid-infrared polarization spectroscopy of C2H2: Non-intrusive spatial-resolved measurements of polyatomic hydrocarbon molecules for combustion diagnostics
- 2007
-
In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489 .- 1873-2704. ; 31 I, s. 817-824
-
Conference paper (peer-reviewed)abstract
- Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection of acetylene molecules making use of the asymmetric C-H stretching vibration at around 3 μm. The infrared laser pulses were produced through difference frequency generation in a LiNbO3 crystal pumped by a Nd:YAG and dye laser system. By directly probing the ro-vibrational transitions with IRPS, sensitive detection of molecules with otherwise inaccessible electronic states was realized with high temporal and spatial resolution by using a pulsed laser and a cross-beam geometry. Detection sensitivities of 2 × 1013 molecules/cm3 (10 ppm in 70 mbar gas mixture) of C2H2 were achieved using the P(11) line of the (010(11)0)-(0000000) band. The dependence of the IRPS signal on the pump laser fluence, acetylene mole fraction, and buffer gas pressure of Ar, N2, H2, and CO2 has been studied experimentally. The investigation demonstrates the quantitative nature of IRPS for sensitive detection of polyatomic IR active molecules. In order to fully demonstrate the technique for combustion applications, nascent acetylene molecules were measured in a low pressure methane/oxygen flame. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 9. |
- Magri, Luca, et al.
(author)
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Multiple-scale thermoacoustic stability analysis of a coaxial jet combustor
- 2017
-
In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 36:3, s. 3863-3871
-
Journal article (peer-reviewed)abstract
- In this paper, asymptotic multiple-scale methods are used to formulate a mathematically consistent set of thermo-acoustic equations in the low-Mach number limit for linear stability analysis. The resulting sets of nonlinear equations for hydrodynamics and acoustics are two-way coupled. The coupling strength depends on which multiple scales are used. The double-time-double-space (2T-2S), double-time-single-space (2T-1S) and single-time-double-space (1T-2S) limits are revisited, derived and linearized. It is shown that only the 1T-2S limit produces a two-way coupled linearized system. Therefore this limit is adopted and implemented in a finite-element solver. The methodology is applied to a coaxial jet combustor. By using an adjoint method and introducing the intrinsic sensitivity, (i) the interaction between the acoustic and hydrodynamic subsystems is calculated and (ii) the role of the global acceleration term, which is the coupling term from the acoustics to the hydrodynamics, is analyzed. For the confined coaxial jet diffusion flame studied here, (i) the growth rate of the thermo-acoustic oscillations is found to be more sensitive to small changes in the hydrodynamic field around the flame and (ii) increasing the global acceleration term is found to be stabilizing in agreement with the Rayleigh Criterion.
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| 10. |
- Miao, Chengxi, et al.
(author)
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On the role of hydrodynamic instability and flame symmetry in flame-acoustic coupling in narrow channels
- 2024
-
In: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 40:1-4
-
Journal article (peer-reviewed)abstract
- In the present study, thermoacoustic oscillations of a flame propagating from an open to a closed endof an narrow channel with adiabatic walls are studied numerically. The study revisits the importance ofhydrodynamic instability and flame symmetry for both primary and secondary acoustic instabilities. For a nonsymmetric slanted flame, the primary instability is linked to the development of the hydrodynamic instability,resulting in a corrugated flame front. Spectral Proper Orthogonal Decomposition (SPOD) was employed todemonstrate that the mode at the fundamental frequency is closely associated with the nonlinear behavior ofthe hydrodynamic cells at the flame front. For the secondary instability of a non-symmetric flame, resonancebetween the acoustic waves at the fundamental mode and the hydrodynamic modes was observed, without aclear emergence of the parametric instability. For a symmetric flame, the parametric acoustic instability canbe more easily observed in a narrow channel. For a symmetric flame, SPOD of the fundamental frequencysuggests the formation of a mixture pocket at the centerline, that can potentially invert the flame front, whilethe first harmonic is related to the flame front inversion process.
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