| 1. |
- Andrae, Johan, 1973-, et al.
(author)
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A Numerical Study of Sidewall Quenching with Propane/Air Flames
- 2002
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In: Proceedings of the Combustion Institute. - 0082-0784 .- 1878-027X. ; 29, s. 789-795
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Journal article (peer-reviewed)abstract
- The head-on (i.e., stagnation) configuration has generally been used to numerically and experimentally characterize the flame-wall interaction with complex chemistry and multicomponent transport. Other studies have treated the transient case of a flame propagating toward a wall, and combustion in a boundary layer has also been dealt with. In this paper, a two-dimensional stationary model has been used to study the sidewall quenching,of laminar propane/air flames in a boundary-layer flow. This geometry may be described as a flame parallel to the wall that is swept away with a laminar boundary-layer flow while propagating toward and interacting with the wall. The main purpose has been to examine the extent to which the flame can propagate toward the cooled wall for lean flames compared to stoichiometric flames. A detailed kinetic model is used to examine the oxidation of both the fuel and the intermediate hydrocarbons (IHCs). For stoichiometric and near stoichiometric mixtures, the thermal coupling between the flame and the wall is small but significant. However, for very lean flames, the thermal coupling between the flame and the wall is found to be very significant. The intermediate hydrocarbons are the dominant emissions for stoichiometric and near-stoichiometric flames in contrast to the leaner flames in which the fuel becomes more significant. This implies that the IHCs are very important for the overall hydrocarbon emissions from flame quenching; as a result detailed kinetics of complex fuels should be used when determining the unburned hydrocarbon emissions.
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| 2. |
- Demirgok, Berk, et al.
(author)
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Effect of thermal expansion on flame propagation in channels with nonslip walls
- 2015
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In: Proceedings of the Combustion Institute. - New York : Elsevier. - 0082-0784 .- 1878-027X .- 1540-7489. ; 35:1, s. 929-936
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Journal article (peer-reviewed)abstract
- Propagation of premixed flames in narrow channels is investigated by means of extensive numerical simulations of a complete system of combustion and hydrodynamic equations, incorporating transport properties (thermal conduction, diffusion and viscosity) and Arrhenius chemical kinetics. The system includes mass conservation and Navier–Stokes equations as well as those for the energy and species balance. A flame propagates from the closed end of a channel to the open one. An initially planar flame front gets corrugated due to wall friction and thereby accelerates. It is shown that a flame exhibits an exponential state of acceleration only when the thermal expansion coefficient Θ exceeds a certain critical value Θ>Θc. The quantity Θc is tabulated as a function of the Reynolds number related to the flame propagation, Re, being Θc≈6 for Re=5∼20. The major flame characteristics such as the flame propagation speed and acceleration rate are scrutinized. It is demonstrated that the acceleration promotes with Θ but weakens with Re. In this respect, the present computational results support the theoretical prediction of Bychkov et al . Physical Review E 72 (2005) 046307 in a wide range of Θ and Re. While very good quantitative and qualitative agreement between numerical and theoretical results is found for realistically large thermal expansion, Θ>=8, agreement deteriorates with decreasing Θ. Specifically, while the theory and modeling do not quantitatively agree for Θc<Θ<8, they nevertheless demonstrate a qualitative resemblance (the exponential state of acceleration). Finally, no exponential acceleration at Θ<Θc denotes that the theory completely breaks in that case, but this fits other works in the field and thereby allows reconciling various formulations on the flame acceleration.
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| 3. |
- Gruber, Andrea, et al.
(author)
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Modeling of mean flame shape during premixed flame flashback in turbulent boundary layers
- 2014
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 0082-0784 .- 1878-027X .- 1540-7489.
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Journal article (peer-reviewed)abstract
- Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary layer of fully-developed turbulent channel flows are used for a priori validation of a new model that aims to describe the mean shape of the turbulent flame brush during flashback. Comparison with the DNS datasets, for both fuel-lean and fuel-rich mixture conditions and for Damköhler numbers lower and larger than unity, shows that the model is able to capture the main features of the flame shape. Although further a priori and a posteriori validation is required, particularly at higher Reynolds numbers, this new simple model seems promising and can potentially have impact on the design process of industrial combustion equipment.
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| 4. |
- Kagan, L., et al.
(author)
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Detonation initiation by a hot corrugated wall
- 2007
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 0082-0784 .- 1878-027X .- 1540-7489. ; 31, s. 2415-2420
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Journal article (peer-reviewed)abstract
- A numerical simulation of premixed gas combustion spreading from a hot corrugated wall is performed. It is found that sufficiently deep corrugations may trigger an abrupt transition from deflagrative to detonative combustion. The effect is found to be highly sensitive to the flame's normal speed and the reaction rate pressure dependence, favoring fast flames and high-order reactions.
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| 5. |
- Nygren, Jenny, et al.
(author)
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Three-dimensional laser induced fluorescence of fuel distributions in an HCCI engine
- 2002
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In: Proceedings of the Combustion Institute. - 0082-0784 .- 1878-027X. ; 29, s. 679-685
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Conference paper (peer-reviewed)abstract
- Three-dimensional imaging of fuel tracer planar laser-induced fluorescence in a homogeneous charge compression ignition (HCCI) engine is presented. A high-speed multiple Nd:YAG laser and detection system, in combination, with a scanning mirror, are used to collect eight images, with an equidistant separation of 0.5 mm. Three-dimensional isoconcentration surfaces calculated from the data are visualized. Three-dimensional imaging offers new opportunities to study different combustion events, specifically the topology of flame structures. For example, it is possible to distinguish if separate islands in a fluorescence image really are separate or if it is an effect from wrinkling in and out of the laser sheet. The PLIF images were also analyzed by identifying five intensity ranges corresponding to increasing degrees of reaction progress. The gradual fuel consumption and thus combustion was then analyzed by calculating the volumetric fraction of these intensity ranges for different crank angle positions. The occurrence of multiple isolated ignition spots and the observed gradual decrease in fuel concentration indicates that HCCI combustion relies on distributed reactions and not flame propagation.
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| 6. |
- Bengtsson, Per Erik, et al.
(author)
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Dual-broadband rotational cars measurements in an IC engine
- 1994
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In: Symposium (International) on Combustion. - 0082-0784. ; 25:1, s. 1735-1742
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Journal article (peer-reviewed)abstract
- This is the first report of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) measurementsin an internal combustion (IC) engine. Single-shot, dual-broadband rotational CARS (DB-RCARS) spectra were recorded both prior to ignition and in the postcombustion gases. From these spectra, both temperature and relative oxygen concentrations were evaluated. The pressure was registered simultaneously with the CARS measurements in the spark-ignition engine burning natural gas and air. Prior to ignition, normally at temperatures below 1000 K and pressures below 2 MPa, a rotational CARS spectrum is very temperature sensitive, and the technique can be used for temperature measurements with high accuracy. Evaluated temperatures show a negligible dependence on uncertainties in parameters such as the nonresonant susceptibility of the gas and the slit width. Moreover, no collisional narrowing of the lines has to be taken into account. The relative standard deviation of evaluated temperatures and of relative oxygen concentrations from single-shot measurements were as low as 1, and 1.4-1.9%, respectively. In the postcombustion gases at temperatures above 2000 K and pressures above 1.5 MPa, the nonresonant CARS background gave a large contribution to the total spectrum. In this temperature and pressure range, the evaluated values of temperature and nonresonant susceptibility are not independent, and the nonresonant susceptibility had to be fixed at a precalculated value to get a reliable temperature evaluation. The avantages and disadvantages of rotational CARS in comparison with vibrational CARS for IC engine measurements are discussed.
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| 7. |
- Gashi, S, et al.
(author)
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Curvature and wrinkling of premixed flame kernels - comparisons of OHPLIF and DNS data
- 2005
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In: Symposium (International) on Combustion. - : Elsevier BV. - 0082-0784. ; 30, s. 809-817
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Journal article (peer-reviewed)abstract
- The effects of curvature and wrinkling on the growth of turbulent premixed flame kernels have been investigated using both 2D OH planar laser-induced fluorescence (PLIF) and 3D direct numerical simulation (DNS). Comparisons of results between the two approaches show a high level of agreement, providing confidence in the simplified chemistry treatment employed in the DNS, and indicating that chemistry may have only a limited influence on the evolution of the freely propagating flame. This is in contrast to previous studies of the very early flame development where chemistry may be dominant. Statistics for curvature and wrinkling are presented in the form of probability density functions, and there is good agreement with previous findings. The limitations of 2D PLIF measurements of curvature are quantified by comparison with full 3D information obtained from the DNS. The usefulness of PLIF in providing data over a wide parameter range is illustrated using statistics obtained from both CH4/air and H-2/air mixtures, which show a markedly different behaviour due to their different thermo-diffusive properties. The results provide a demonstration of the combined power of PLIF and DNS for flame investigation. Each technique is shown to compensate for the weaknesses of the other and to reinforce the strengths of both.
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| 8. |
- Hult, Johan, et al.
(author)
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Experimental analysis of local flame extinction in a turbulent jet diffusion flame by high repetition 2-D laser techniques and multi-scalar measurements
- 2005
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In: Symposium (International) on Combustion. - : Elsevier BV. - 0082-0784. ; 30:1, s. 701-709
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Journal article (peer-reviewed)abstract
- In this paper, we present a detailed experimental study of turbulence chemistry interactions in the "DLR_B" turbulent jet diffusion flame. The flame operates on mixtures of CH4, H-2, and N-2 in the fuel stream at Re = 22,800 and is a target flame within the TNF workshop. Extinction and re-ignition events can be tracked in real time and related to the underlying flow field phenomena and temperature fields. Time resolved measurements of OH radical concentration fields are performed in combination with temperature and velocity field measurements. For this purpose, we combined high repetition rate (33 kHz) PLIF imaging with stereoscopic PIV and double pulse Rayleigh imaging techniques. Comparisons are made with results from multi-scalar Raman/Rayleigh/LIF point measurements that reveal the thermochemical state of the flame. The large deviations from equilibrium observed on resulting OH/temperature joint pdfs could be related to strain rate and Damkohler number variations caused by turbulent flow structures leading to frequent extinctions. The 2D measurement series uniquely reveal the underlying mechanism that can lead to such events. Finally, comparisons are made to strained laminar flame calculations, which are generally found to be in good agreement with the measured data. (c) 2004 The Combustion Institute.
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| 9. |
- Kohse-Hoinghaus, K, et al.
(author)
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Combustion at the focus: laser diagnostics and control
- 2005
-
In: Symposium (International) on Combustion. - : Elsevier BV. - 0082-0784. ; 30:1, s. 89-123
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Journal article (peer-reviewed)abstract
- Fifty years after the foundation of the Combustion Institute and almost 150 years after Michael Faraday's famous lectures on the combustion of a candle, combustion diagnostics have come a long way from visual inspection of a flame to detailed analysis of a combustion process with a multitude of sophisticated techniques, often using lasers. The extended knowledge on combustion phenomena gained by application of these diagnostic techniques, combined with equally advanced numerical simulation of the process, has been instrumental in designing modern combustion devices with efficient performance and reduced pollutant emission. Also, similar diagnostic techniques are now employed to develop sensors for process control in combustion. This article intends to give a perspective on the potential of combustion diagnostics by highlighting selected application examples and by guiding the reader to recent literature. In particular, techniques are emphasized, which permit measurement of important features of the chemical composition, sometimes in conjunction with flow field parameters. Although a complete image of present research and applications in combustion diagnostics and control is beyond the scope of this article, this overview may be a starting place where ideas may be found to solve specific combustion problems with the aid of diagnostics.
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| 10. |
- Lövås, Terese, et al.
(author)
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Comparison of automatic reduction procedures for ignition chemistry
- 2002
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In: Proceedings of the Combustion Institute. - 0082-0784. ; 29, s. 1387-1393
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Conference paper (peer-reviewed)abstract
- In this paper, we present a comparison between the reduced mechanisms obtained through a computational singular perturbation method (CSP) and the reduced mechanisms obtained through a lifetime analysis based only on the diagonal elements of the Jacobian matrix and a species sensitivity The two methods are used for the analysis of autoignition, which is an interesting test situation because of the sensitivity of ignition to the radical pool and the smaller range of timescales expected. It is found that the steady-state species selected by the two methods are in good agreement. The mechanisms are reduced to a 10-step mechanism when CSP is applied and an 11-step mechanism in the case of the simpler lifetime analysis. Both mechanisms are compared with the detailed mechanism and experimental data and are found to reproduce the physical and chemical parameters very well. This shows that for a large part of the timescale range, the system is close to linear. The comparison shows the advantage of the CSP method as being somewhat more accurate. However, the simpler lifetime analysis is of sufficient accuracy and of more convenience when applied to a system requiring a considerable reduction in computational time, as is the case when applying online reduction.
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